Design and Evaluation of PEGylated Liposomal Formulation of a Novel Multikinase Inhibitor for Enhanced Chemosensitivity and Inhibition of Metastatic Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) has one of the highest mortality rates among cancers. Chemotherapy is the standard first-line treatment, but only modest survival benefits are observed. With the advent of targeted therapies, epidermal growth factor receptor (EGFR) has been acknowledged as a prospective target in PDAC since it is overexpressed in up to 60% of cases. Similarly, the tyrosine-protein kinase Met (cMET) is also overexpressed in PDAC (27-60%) and is a prognostic marker for poor survival. Interestingly, EGFR and cMET share some common signaling pathways including PI3K/Akt and MAPK pathways. Small molecule inhibitors or bispecific antibodies that can target both EGFR and cMET are therefore emerging as novel options for cancer therapy. We previously developed a dual EGFR and cMET inhibitor (N19) that was able to inhibit tumor growth in nonsmall cell lung cancer models resistant to EGFR tyrosine kinase inhibitors (TKI). Here, we report the development of a novel liposomal formulation of N19 (LN19) and showed significant growth inhibition and increased sensitivity toward gemcitabine in the pancreatic adenocarcinoma orthotopic xenograft model. Taken together, our results suggest that LN19 can be valued as an effective combination therapy with conventional chemotherapy such as gemcitabine for PDAC patients.

Hyperemia-free delineation of epicardial and microvascular impairments using a basal index

The assessment of functional coronary lesion severity using intracoronary hemodynamic parameters like the pressure-derived fractional flow reserve and the flow-derived coronary flow reserve are known to rely critically on the establishment of maximal hyperemia. We evaluated a hyperemia-free index, basal pressure drop coefficient (bCDP), that combines pressure and velocity for simultaneous assessment of the status of both epicardial and microvascular circulations. In 23 pigs, simultaneous measurements of distal coronary arterial pressure and flow were performed using a dual-sensor tipped guidewire in the settings of both normal and abnormal microcirculation with the presence of epicardial lesions of area stenosis (AS) < 50% and AS > 50%. The bCDP, a parameter based on fundamental fluid dynamics principles, was calculated as the transtenotic pressure-drop divided by the dynamic pressure in the distal vessel, measured under baseline (without hyperemia) conditions. The group mean values of bCDP for normal (84 ± 18) and abnormal (124.5 ± 15.6) microcirculation were significantly different. Similarly, the mean values of bCDP from AS < 50% (72.5 ± 16.1) and AS > 50% (136 ± 17.2) were also significantly different (p < 0.05). The bCDP could significantly distinguish between lesions of AS < 50% to AS > 50% under normal microcirculation (52.1 vs. 85.8; p < 0.05) and abnormal microcirculation (84.9 vs. 172; p < 0.05). Further, the bCDP correlated linearly and significantly with the hyperemic parameters FFR (r = 0.42, p < 0.05) and CDP (r = 0.50, p < 0.05). The bCDP is a promising clinical diagnostic parameter that can independently assess the severity of epicardial stenosis and microvascular impairment. We believe that it has an immediate appeal for detection of coronary artery disease if validated clinically.

Effect of heart rate on hemodynamic endpoints under concomitant microvascular disease in a porcine model

Diagnosis of the ischemic power of epicardial stenosis with concomitant microvascular disease (MVD) is challenging during coronary interventions, especially under variable hemodynamic factors like heart rate (HR). The goal of this study is to assess the influence of variable HR and percent area stenosis (%AS) in the presence of MVD on pressure drop coefficient (CDP; ratio of transstenotic pressure drop to the distal dynamic pressure) and lesion flow coefficient (LFC; ratio of %AS to the CDP at the throat region). We hypothesize that CDP and LFC are independent of HR. %AS and MVD were created using angioplasty balloons and 90-μm microspheres, respectively. Simultaneous measurements of pressure drop (DP) and velocity were done in 11 Yorkshire pigs. Fractional flow reserve (FFR), CDP, and LFC were calculated for the groups HR < 120 and HR > 120 beats/min, %AS < 50 and %AS > 50, and additionally for DP < 14 and DP > 14 mmHg, and analyzed using regression and ANOVA analysis. Regression analysis showed independence between HR and the FFR, CDP, and LFC while it showed dependence between %AS and the FFR, CDP, and LFC. In the ANOVA analysis, for the HR < 120 beats/min and HR > 120 beats/min groups, the values of FFR (0.82 ± 0.02 and 0.82 ± 0.02), CDP (83.15 ± 26.19 and 98.62 ± 26.04), and LFC (0.16 ± 0.03 and 0.15 ± 0.03) were not significantly different (P > 0.05). However, for %AS < 50 and %AS > 50, the FFR (0.89 ± 0.02 and 0.75 ± 0.02), CDP (35.97 ± 25.79.10 and 143.80 ± 25.41), and LFC (0.09 ± 0.03 and 0.22 ± 0.03) were significantly different (P < 0.05). A similar trend was observed between the DP groups. Under MVD conditions, FFR, CDP, and LFC were not significantly influenced by changes in HR, while they can significantly distinguish %AS and DP groups.

Effect of changes in contractility on pressure drop coefficient and fractional flow reserve in a porcine model

OBJECTIVES AND BACKGROUND

Decisions based on invasive functional diagnostic measurements are often made in the setting of fluctuating hemodynamic variables that may alter resting or hyperemic measurements. The purpose of this investigation is to analyze the effect of myocardial contractility (CY) on invasive functional parameters. We hypothesize that the pressure drop coefficient (CDPe; ratio of pressure drop to distal dynamic pressure) and fractional flow reserve (FFR; ratio of average pressures distal and proximal to a stenosis) are not affected by fluctuations in CY and can distinguish between different severities of epicardial stenosis.

METHODS

Simultaneous measurements of distal coronary-arterial pressure and velocity were performed in 10 pigs using a dual-sensor tipped guidewire for heart rate (HR) <110 bpm and HR >110 bpm, in the presence of coronary lesions of <50% area stenosis (AS) and >50% AS. Variations in myocardial function and vascular resistance were induced by atrial pacing, papaverine and balloon obstruction, respectively. The maximum rate of rise of left ventricular pressure ([dp/dt]max) was the index of contractility. The contractile function of the heart was empirically defined as CY >900 mm Hg/sec (higher) and CY <900 mm Hg/sec (normal).

RESULTS

For CY >900 mm Hg/sec, under AS <50% and AS >50%, the mean values of FFR (0.91 ± 0.02 and 0.78 ± 0.02), and CDPe (15.6 ± 5.3 and 70.7 ± 24.7) were significantly different (P<.05). Similarly, for CY <900 mm Hg/sec, under AS <50% and AS >50%, the mean values of FFR (0.83 ± 0.04 and 0.63 ± 0.04), and CDPe (43.8 ± 14.9 and 191.8 ± 61.4) were also significantly different (P<.05).

CONCLUSIONS

Both FFR and CDPe could effectively distinguish between stenosis severity at normal and higher levels of myocardial contractility.

Influence of heart rate on fractional flow reserve, pressure drop coefficient, and lesion flow coefficient for epicardial coronary stenosis in a porcine model

A limitation in the use of invasive coronary diagnostic indexes is that fluctuations in hemodynamic factors such as heart rate (HR), blood pressure, and contractility may alter resting or hyperemic flow measurements and may introduce uncertainties in the interpretation of these indexes. In this study, we focused on the effect of fluctuations in HR and area stenosis (AS) on diagnostic indexes. We hypothesized that the pressure drop coefficient (CDP(e), ratio of transstenotic pressure drop and distal dynamic pressure), lesion flow coefficient (LFC, square root of ratio of limiting value CDP and CDP at site of stenosis) derived from fluid dynamics principles, and fractional flow reserve (FFR, ratio of average distal and proximal pressures) are independent of HR and can significantly differentiate between the severity of stenosis. Cardiac catheterization was performed on 11 Yorkshire pigs. Simultaneous measurements of distal coronary arterial pressure and flow were performed using a dual sensor-tipped guidewire for HR < 120 and HR > 120 beats/min, in the presence of epicardial coronary lesions of <50% AS and >50% AS. The mean values of FFR, CDP(e), and LFC were significantly different (P < 0.05) for lesions of <50% AS and >50% AS (0.88 ± 0.04, 0.76 ± 0.04; 62 ± 30, 151 ± 35, and 0.10 ± 0.02 and 0.16 ± 0.01, respectively). The mean values of FFR and CDP(e) were not significantly different (P > 0.05) for variable HR conditions of HR < 120 and HR > 120 beats/min (FFR, 0.81 ± 0.04 and 0.82 ± 0.04; and CDP(e), 95 ± 33 and 118 ± 36). The mean values of LFC do somewhat vary with HR (0.14 ± 0.01 and 0.12 ± 0.02). In conclusion, fluctuations in HR have no significant influence on the measured values of CDP(e) and FFR but have a marginal influence on the measured values of LFC. However, all three parameters can significantly differentiate between stenosis severities. These results suggest that the diagnostic parameters can be potentially used in a better assessment of coronary stenosis severity under a clinical setting.

In vitro measurement and calculation of drag force on iliac limb stentgraft in a compliant arterial wall model

Interventional treatment of aortic aneurysms using endovascular stentgrafting is a minimally invasive technique. Following device implantation, transient drag forces act on the stentgraft. When the drag force exceeds the fixation force, complications like stentgraft migration, endoleaks and stentgraft failure occur. In such a scenario the device becomes unstable, causing concern over the long-term durability of endovascular repairs. The objective of this study is: (1) to measure the drag force on iliac limb stentgraft, having a distal diameter that is half the size of the proximal end, in an in vitro experiment; (2) to calculate the drag force using blood flow-compliant arterial wall interaction model and compare it with the measured values on the stentgraft for the in vitro experiment; (3) to calculate drag force on the stentgraft using physiological flow conditions. Experimental data for a stentgraft within a silicon tubing, representing a compliant artery, shows a peak drag force of 2.79 N whereas the calculation predicts a peak drag force of 2.57 N; thus a percentage difference of 7.8% is observed. When physiological flow and pressure pulse are used for the blood flow-compliant arterial wall computations, a peak drag force of 0.59 N is obtained for the same stentgraft that was used in the experiment. The outer cavity between the distal end of the iliac limb stentgraft and the arterial wall reduces the drag force. These forces can be used as design guideline for determining the fixation force needed for the stentgraft under physiological pulsatile flow.

HIFU procedures at moderate intensities—effect of large blood vessels

A three-dimensional computational model is presented for studying the efficacy of high-intensity focused ultrasound (HIFU) procedures targeted near large blood vessels. The analysis applies to procedures performed at intensities below the threshold for cavitation, boiling and highly nonlinear propagation, but high enough to increase tissue temperature a few degrees per second. The model is based upon the linearized KZK equation and the bioheat equation in tissue. In the blood vessel the momentum and energy equations are satisfied. The model is first validated in a tissue phantom, to verify the absence of bubble formation and nonlinear effects. Temperature rise and lesion-volume calculations are then shown for different beam locations and orientations relative to a large vessel. Both single and multiple ablations are considered. Results show that when the vessel is located within about a beam width (few mm) of the ultrasound beam, significant reduction in lesion volume is observed due to blood flow. However, for gaps larger than a beam width, blood flow has no major effect on the lesion formation. Under the clinically representative conditions considered, the lesion volume is reduced about 40% (relative to the no-flow case) when the beam is parallel to the blood vessel, compared to about 20% for a perpendicular orientation. Procedures involving multiple ablation sites are affected less by blood flow than single ablations. The model also suggests that optimally focused transducers can generate lesions that are significantly larger (>2 times) than the ones produced by highly focused beams.

Assessment of the status of urban air pollution and its impact on human health in the city of Kolkata

Air pollution has significant effects on exacerbation of asthma, allergy and other respiratory diseases. Like many other magacities in the world the ambient air quality of Kolkata is also being deteriorated day by day. Automobile exhausts and certain industrial pollutants produce O(3) by photochemical reactions. The particulate matter, particularly less than 10 microm in size, can pass through the natural protective mechanism of human respiratory system and plays an important role in genesis and augmentation of allergic disorders. Sources of air pollution in the area and the unique problem arising out of the emission from the vehicles, industries, etc. have been described. Ambient air quality was monitored along with micrometeorological data and the results are discussed. The status of air pollution in the area has been evaluated and a questionnaire survey was conducted to estimate the allergic symptoms and exposure to assess the respiratory disorders. The data are analysed to evaluate the critical situation arising out of the emission of air pollutants and the impact on human health due to respirable diseases (RDs) to middle class sub-population (activity-wise) in the area are assessed. A strategic air quality management plan has been proposed. For the mitigation of air pollution problems in the city, the different measures to be adopted to maintain the balance between sustainable development and environmental management have been discussed.

Gastric toxicity and mucosal ulceration induced by oxygen-derived reactive species: protection by melatonin

Uncontrolled hydrochloric acid secretion and ulceration of the stomach mucosa due to various factors are serious global problems. Although the mechanism of acid secretion from the parietal cell is now well understood, the processes involved in gastric ulceration are still not clear. Among various causes of gastric ulceration, lesions caused by stress, alcohol consumption, Helicobacter pylori infection and due to use of nonsteroidal antiinflammatory drugs have been shown to be mediated largely through the generation of reactive oxygen species, especially the hydroxyl radical. A number of excellent drugs have proven useful in controlling hyperacidity and ulceration but their long-term use is associated with disturbing side-effects. Hence, the search is still on to find a compound possessing antisecretory, antiulcer and antioxidant properties which will serve as a therapeutic agent to reduce gastric hyperacidity and ulcers. This article describes the role of reactive oxygen species in gastric ulceration, drugs controlling them with their merits and demerits and, the role of melatonin, a pineal secretory product, in protecting against gastric lesions. In experimental studies, melatonin has been shown to be effective in reducing mucosal breakdown and ulcer formation in a wide variety of situations. Additionally, the low toxicity of melatonin supports further investigation of this molecule as a gastroprotective agent. Finally, we include a commentary on how melatonin research with respect to gastric pathophysiology can move forward with a view of eventually using this indole as a therapeutic agent to control gastric ulceration in humans.

Finite element model of antibody penetration in a prevascular tumor nodule embedded in normal tissue

We have developed a pharmacokinetic model for monoclonal antibodies (mAb) to aid in investigating protocols for targeting small primary tumors or sites of metastatic disease. The model describes the uptake of systemically-administered antibody by a prevascular spherical tumor nodule embedded in normal tissue. The model incorporates plasma kinetics, transcapillary transport, interstitial diffusion, binding reactions, and lymphatic clearance. Antigen internalization can easily be incorporated. Simulations obtained from a three-dimensional finite element analysis are used to assess errors in predictions from earlier models in which the influence of the normal tissue was collapsed into a boundary condition at the tumor surface. The model employing a Dirichlet boundary condition substantially overpredicted the mean total tumor mAb concentration at all times. Although the model with a concentration-dependent flux (composite) boundary condition underpredicted mAb concentration, the discrepancy with finite element results is only notable at early times. Sensitivity analyses were performed on mAb dose and on the coefficients for mAb diffusion in the tissue regions, since reported antibody diffusivity values have varied over 30-fold. The results of the study suggest that mAb diffusivity and mAb binding site density in tumors should have major influences on optimizing doses and scheduling of mAb administration in tumor targeting protocols.

An essential role of active site arginine residue in iodide binding and histidine residue in electron transfer for iodide oxidation by horseradish peroxidase

The objective of the present study is to delineate the role of active site arginine and histidine residues of horseradish peroxidase (HRP) in controlling iodide oxidation using chemical modification technique. The arginine specific reagent, phenylglyoxal (PGO) irreversibly blocks iodide oxidation following pseudofirst order kinetics with second order rate constant of 25.12 min(-1) M(-1). Radiolabelled PGO incorporation studies indicate an essential role of a single arginine residue in enzyme inactivation. The enzyme can be protected both by iodide and an aromatic donor such as guaiacol. Moreover, guaiacol-protected enzyme can oxidise iodide and iodide-protected enzyme can oxidise guaiacol suggesting the regulatory role of the same active site arginine residue in both iodide and guaiacol binding. The protection constant (Kp) for iodide and guaiacol are 500 and 10 microM respectively indicating higher affinity of guaiacol than iodide at this site. Donor binding studies indicate that guaiacol competitively inhibits iodide binding suggesting their interaction at the same binding site. Arginine-modified enzyme shows significant loss of iodide binding as shown by increased Kd value to 571 mM from the native enzyme (Kd = 150 mM). Although arginine-modified enzyme reacts with H2O2 to form compound II presumably at a slow rate, the latter is not reduced by iodide presumably due to low affinity binding. The role of the active site histidine residue in iodide oxidation was also studied after disubstitution reaction of the histidine imidazole nitrogens with diethylpyrocarbonate (DEPC), a histidine specific reagent. DEPC blocks iodide oxidation following pseudofirst order kinetics with second order rate constant of 0.66 min(-1) M(-1). Both the nitrogens (delta, epsilon) of histidine imidazole were modified as evidenced by the characteristic peak at 222 nm. The enzyme is not protected by iodide suggesting that imidazolium ion is not involved in iodide binding. Moreover, DEPC-modified enzyme binds iodide similar to the native enzyme. However, the modified enzyme does not form compound II but forms compound I only with higher concentration of H2O2 suggesting the catalytic role of this histidine in the formation and autoreduction of compound I. Interestingly, compound I thus formed is not reduced by iodide indicating block of electron transport from the donor to the compound I. We suggest that an active site arginine residue regulates iodide binding while the histidine residue controls the electron transfer to the heme ferryl group during oxidation.

Estimated flow resistance increase in a spiral human coronary artery segment

Coronary flow estimates were made for a spiral coronary artery segment (identified from a post-mortem replica casting) by using a modified Dean number based on the approximate coil radius of curvature, as suggested earlier. The estimates were found to correlate experimental pressure drop data for helical coiled tubes. Over a physiological range of mean Reynolds numbers from 100 to 400 for blood flow through main coronary arteries, estimates of the flow resistance increase relative to a straight lumen segment ranged from about 20 to 80 percent, and were of similar magnitude to those found in a flow study in a sinuous coronary vessel segment with no spiral.

Melatonin protects against stress-induced gastric lesions by scavenging the hydroxyl radical

The antiulcer effect of melatonin on gastric lesions caused by restraint-cold stress or by indomethacin (IMN) was studied with the intent of determining the mechanism of action of the indole. Melatonin dose-dependently prevents both stress and IMN-induced gastric damage with around 90% inhibition at a dose of 60 mg per kg BW. When compared with already-marketed antiulcer drugs, such as ranitidine and omeprazole, melatonin was found to be more effective than ranitidine but less effective than omeprazole in preventing stress ulcer. When compared with other antioxidants, melatonin was more potent than glutathione and essentially equipotent to alpha-tocopherol in blocking stress-induced ulcer. As stress-induced gastric lesions are mainly caused by oxidative damage due to hydroxyl radicals (*OH), the effect of melatonin in scavenging the *OH generated during stress conditions, as well as in an in vitro model system, was studied. The results indicate that melatonin at the dose of 60 mg per kg BW caused an 88% reduction of endogenous *OH during stress. Melatonin was also highly effective in scavenging *OH generated in vitro by a Cu2+-ascorbate system. In this case, melatonin at 100 microM reduced *OH by 80%. Melatonin was also found to be a more potent radical scavenger than benzoate, a known *OH scavenger. The results indicate that melatonin prevents stress-induced gastric lesions by scavenging the endogenous *OH. As it also protects against IMN-induced gastric damage, it probably also offers gastroprotection by maintaining endogenous prostaglandin levels.

Physiological flow simulation in residual human stenoses after coronary angioplasty

To evaluate the local hemodynamic implications of coronary artery balloon angioplasty, computational fluid dynamics (CFD) was applied in a group of patients previously reported by [Wilson et al. (1988), 77, pp. 873-885] with representative stenosis geometry post-angioplasty and with measured values of coronary flow reserve returning to a normal range (3.6 +/- 0.3). During undisturbed flow in the absence of diagnostic catheter sensors within the lesions, the computed mean pressure drop delta p was only about 1 mmHg at basal flow, and increased moderately to about 8 mmHg for hyperemic flow. Corresponding elevated levels of mean wall shear stress in the midthroat region of the residual stenoses, which are common after angioplasty procedures, increased from about 60 to 290 dynes/cm2 during hyperemia. The computations (Ree approximately equal to 100-400; alpha e = 2.25) indicated that the pulsatile flow field was principally quasi-steady during the cardiac cycle, but there was phase lag in the pressure drop-mean velocity (delta p - u) relation. Time-averaged pressure drop values, delta p, were about 20 percent higher than calculated pressure drop values, delta ps, for steady flow, similar to previous in vitro measurements by Cho et al. (1983). In the throat region, viscous effects were confined to the near-wall region, and entrance effects were evident during the cardiac cycle. Proximal to the lesion, velocity profiles deviated from parabolic shape at lower velocities during the cardiac cycle. The flow field was very complex in the oscillatory separated flow reattachment region in the distal vessel where pressure recovery occurred. These results may also serve as a useful reference against catheter-measured pressure drops and velocity ratios (hemodynamic endpoints) and arteriographic (anatomic) endpoints post-angioplasty. Some comparisons to previous studies of flow through stenoses models are also shown for perspective purposes.

Dexamethasone makes the gastric mucosa susceptible to ulceration by inhibiting prostaglandin synthetase and peroxidase--two important gastroprotective enzymes

The plausible mechanism by which dexamethasone makes the gastric mucosa susceptible to ulceration has been studied. As acid aggravates ulcer, the role of dexamethasone on acid secretion was first investigated. Dexamethasone stimulates both basal and drug (mercaptomethylimidazole)-induced gastric acid secretion by 100 and 50% respectively in male Wister rats 24 h after intramuscular administration at the dose of 1 mg/kg body wt. This stimulated acid secretion is 93% blocked by cimetidine indicating increased liberation of histamine in the process. Pretreatment of dexamethasone before 24 h produces ulcer in 30% of the pylorus- ligated rats and aggravates the ulcer index by 82% in both pylorus and esophagus ligated rats. The incidence of ulceration in the latter cases is also increased by 25%. As mucosal prostaglandin synthetase and peroxidase play an important role in gastroprotection through biosynthesis of prostaglandin and by scavenging endogenous H2O2 respectively, the effect of dexamethasone on the activities of these gastroprotective enzymes were studied. Prostaglandin synthetase and peroxidase activities of the mucosa are significantly inhibited by 87 and 83% respectively by 24-h pretreatment with dexamethasone. The results indicate that dexamethasone makes the mucosa prone to ulceration by inhibiting the activity of prostaglandin synthetase to block the gastroprotective action of prostaglandin and also by inhibiting the peroxidase, thereby elevating the endogenous H2O2 level to generate more reactive hydroxyl radical responsible for the mucosal damage.

Catheter obstruction effect on pulsatile flow rate--pressure drop during coronary angioplasty

The coupling of computational hemodynamics to measured translesional mean pressure gradients with an angioplasty catheter in human coronary stenoses was evaluated. A narrowed flow cross section with the catheter present effectively introduced a tighter stenosis than the enlarged residual stenoses after balloon angioplasty; thus elevating the pressure gradient and reducing blood flow during the measurements. For resting conditions with the catheter present, flow was believed to be about 40 percent of normal basal flow in the absence of the catheter, and for hyperemia, about 20 percent of elevated flow in the patient group. The computations indicated that the velocity field was viscous dominated and quasi-steady with negligible phase lag in the delta p(t)-u(t) relation during the cardiac cycle at the lower hydraulic Reynolds numbers and frequency parameter. Hemodynamic interactions with smaller catheter-based pressure sensors evolving in clinical use require subsequent study since artifactually elevated translesional pressure gradients can occur during measurements with current angioplasty catheters.

Mechanism of horseradish peroxidase catalyzed epinephrine oxidation: obligatory role of endogenous O2- and H2O2

Horseradish peroxidase (HRP) catalyzes cyanide sensitive oxidation of epinephrine to adrenochrome at physiological pH in the absence of added H2O2 with concurrent consumption of O2. Both adrenochrome formation and O2 consumption are significantly inhibited by catalase, indicating a peroxidative mechanism as a major part of oxidation due to intermediate formation of H2O2. Sensitivity to superoxide dismutase (SOD) also indicates involvement of O2- in the oxidation. Although SOD-mediated H2O2 formation should continue epinephrine oxidation through a peroxidative mechanism, low catalytic turnover, on the contrary, indicates that O2- takes part in a vital reaction to form an intermediate for adrenochrome formation and O2 consumption. Generation of O2- is evidenced by ferricytochrome c reduction sensitive to SOD. On addition of H2O2, both adrenochrome formation and O2 consumption are further increased due to reaction of molecular oxygen with some intermediate oxidation product. Peroxidative oxidation proceeds by one-electron transfer generating o-semiquinone and similar free radicals which when stabilized with Zn2+ or spin-trap, alpha-phenyl-tert-butylnitrone (PBN), inhibit adrenochrome formation and O2 consumption. The free radicals thus favor reduction of O2 rather than the disproportionation reaction. Spectral studies indicate that, during epinephrine oxidation in the presence of catalase, HRP remains in the ferric state absorbing at 403 nm. This suggests that HRP catalyzes epinephrine oxidation by its oxidase activity through Fe3+/Fe2+ shuttle consuming O2, where the rate of reduction of ferric HRP with epinephrine is slower than subsequent oxidation of ferrous HRP by O2 to form compound III. Compound III was not detected spectrally because of its quick reduction to the ferric state by epinephrine or its subsequent oxidation product. In the absence of catalase, peroxidative cycles predominate when HRP still remains in the ferric state through the transient formation of compounds I and II not detectable spectrally. Among various mono- and dihydroxyl aromatic donors tested, only epinephrine shows the oxidase reaction. Binding studies indicate that epinephrine interferes with the binding of CN-, SCN-, and guaiacol indicating that HRP preferentially binds epinephrine near the heme iron close to the anion or aromatic donor binding site to catalyze electron transfer for oxidation. HRP thus initiates epinephrine oxidation by its oxidase activity generating O2- and H2O2. Once H2O2 is generated, the peroxidative cycle continues with the consumption of O2, through the intermediate formation of O2- and H2O2 which play an obligatory role in subsequent cycles of peroxidation.

Inhibition of gastric mucosal prostaglandin synthetase activity by mercaptomethylimidazole, an inducer of gastric acid secretion--plausible involvement of endogenous H2O2

We have reported earlier that mercaptomethylimidazole (MMI), an antithyroid drug of thionamide group, induces gastric acid secretion at least partially through the liberation of histamine, sensitive to cimetidine. Now, we show that the drug has a significant inhibitory effect on the cyclooxygenase and peroxidase activity of the prostaglandin (PG) synthetase of the gastric mucosal microsomal preparation. The effect can also be mimicked by low concentrations of H2O2. While studying the possible intracellular effect of MMI on acid secretion, a cell fraction (F3) enriched in parietal cell was isolated by controlled digestion of the mucosa with protease. This cell fraction is activated by MMI as measured by increased O2 consumption. The activation is sensitive to omeprazole, a proton-pump inhibitor, indicating that the activation is due to increased acid secretion by MMI. MMI was also found to directly inhibit the peroxidase activity of the F3 cell fraction and may thus increase the intracellular level of H2O2. The cyclooxygenase activity of the PG synthetase of the F3 cell fraction is also inhibited by MMI and the effect can be reproduced by low concentrations of H2O2. Both MMI and H2O2 can also inhibit the peroxidase activity of the PG synthetase. We suggest that in addition to the activation of the parietal cell by MMI possibly through endogenous H2O2, MMI induces acid secretion in vivo by inactivating the PG synthetase thereby inhibiting the biosynthesis of PG and removing its inhibitory influence on acid secretion so that the histamine released by MMI can stimulate acid secretion with maximum efficiency.

Haem propionates control oxidative and reductive activities of horseradish peroxidase by maintaining the correct orientation of the haem

The role of haem propionates in oxidative and reductive reactions catalysed by horseradish peroxidase (HRP) was studied after successful reconstitution of ferric protoporphyrin IX dimethyl ester (PPDME) into the apoperoxidase. The reconstituted enzyme oxidizes neither guaiacol (aromatic electron donor) nor iodide or thiocyanate (inorganic donor). Although the reconstituted enzyme binds guaiacol with a similar Kd (13 mM) to that of the native enzyme (10 mM), the Kd for SCN- binding (5 mM) is decreased 20-fold compared with that of the native enzyme (100 mM). This indicates that haem propionates hinder the entry or binding of inorganic anion to the active site of the native HRP. However, the reconstituted enzyme is catalytically inactive as it does not form spectroscopically detectable compound II with H2O2. CD measurements indicate a significant loss of haem CD spectrum of the reconstituted enzyme at 409 nm, suggesting a loss of asymmetry of the haem-protein interaction. Thus the inability of the reconstituted enzyme to form catalytic intermediates results from the change in orientation of the haem due to loss of interactions via the haem propionates. HRP also catalyses reductive reactions such as reduction of iodine (I+) in the presence of EDTA and H2O2. The reconstituted enzyme cannot catalyse I+ reduction because of the loss of I+ binding to the haem propionate. Since I+ reduction requires formation of the catalytically active enzyme-I+-EDTA ternary complex, the loss of reductive activity is primarily due to the loss of active enzyme formation. Haem propionates thus play a vital role in the oxidative and reductive reactions of HRP by favouring the formation of catalytic intermediates with H2O2 by maintaining the correct orientation of the haem with respect to the surrounding residues.

Oxidative inactivation of gastric peroxidase by site-specific generation of hydroxyl radical and its role in stress-induced gastric ulceration

We have shown earlier that restraint-cold stress-induced gastric ulceration in rats is caused by metal ion-dependent generation of hydroxyl radical (OH.) and oxidative inactivation of the gastric peroxidase (GPO), an important H2O2 scavenging enzyme. To study the mechanism of the oxidative damage of GPO, the purified enzyme was exposed to an OH. generating system containing Cu2+, ascorbate, and H2O2. Kinetic studies indicate that the enzyme is inactivated in a time-dependent process showing saturation with respect to Cu2+ concentration. The enzyme specifically requires Cu2+ and is not inactivated by the same concentration of Fe2+, Mn2+, or Zn2+. Sensitivity to catalase indicates the critical role of H2O2 in the inactivation. Inactivation is insensitive to superoxide dismutase, suggesting no role of superoxide. The rate of inactivation is not increased in D2O excluding the involvement of singlet oxygen in the process. However, OH. scavengers such as benzoate or mannitol cannot prevent inactivation. The results indicate a plausible generation of OH. within the enzyme molecule as the cause of inactivation. Fragmentation of peptide linkage or intramolecular crosslinking, gross change of tertiary structure, or change in intrinsic tryptophan fluorescence which occurs in "global" oxidation are not evident. Inactivation is dependent on pH and from a plot of K(obs) of inactivation against pH, the controlling role of an ionizable group of the enzyme having a pka of 7.8 could be suggested, deprotonation of which favors inactivation. Amino acid analysis shows a specific loss of two lysine residues in the inactivated enzyme. Competitive kinetic studies indicate that pyridoxal phosphate, a specific modifier of the lysine residue, prevents inactivation by competing with Cu2+ for binding at the GPO. A Cu2+ binding motif consisting at least of two lysine residues exists in GPO, which specifically binds Cu2+ and generates OH.. The radical oxidizes the lysine residues and perturbs the heme environment to cause inactivation. We suggest that oxidative damage of GPO is mediated by site-specific generation of OH. and not by the OH. generated in the bulk phase.

Activation of parietal cell by mercaptomethylimidazole: a novel inducer of gastric acid secretion

Mercaptomethylimidazole (2-Mercapto-1-methylimidazole, MMI), an antithyroid drug of thionamide group, significantly activated the parietal cell for acid secretion, as evidenced by increased O2 consumption by more than 2.5-fold over the basal level. When compared, MMI-induced activation was similar to that of histamine but significantly higher than that of isobutylmethylxanthine or carbachol. Activation by MMI was not prevented by receptor blockers of the parietal cell, indicating that its effect was not mediated through the cell surface histamine-H2 receptor or the muscarinic receptor. However, the activation was almost completely blocked only by omeprazole, an established inhibitor of the terminal proton-pumping H+-K+-ATPase of the parietal cell. That MMI-induced activation was coupled with the H+ transport was further confirmed by significant increase in [14C]-aminopyrine uptake by MMI in rabbit gastric gland preparation. MMI-dependent activation of the parietal cell correlated well with the inhibition of the endogenous peroxidase activity. In vitro studies indicated that MMI irreversibly inactivated both peroxidase and catalase activity of the parietal cell in presence of H2O2. As inactivation of these H2O2-scavenging enzymes should increase accumulation of intracellular H2O2, the effect of latter was studied on acid secretion. H2O2 at a low concentration, stimulated acid secretion by sevenfold in isolated gastric mucosa, which was sensitive to omeprazole. It also significantly stimulated [14C]-aminopyrine uptake in gastric gland preparation. We suggest that MMI activated parietal cells to stimulate acid secretion by endogenous accumulation of H2O2 through inactivation of the peroxidase-catalase system.

Mechanism-based inactivation of lacrimal-gland peroxidase by phenylhydrazine: a suicidal substrate to probe the active site

Humans are exposed to various hydrazine derivatives for therapeutic control of several diseases, and mammalian peroxidases are implicated in the oxidative metabolism of many drugs. The results presented here indicate that lacrimal-gland peroxidase is irreversibly inactivated in a mechanism-based way by phenylhydrazine, which acts as a suicidal substrate in the presence of H2O2. The pseudo-first-order kinetic constants for inactivation at pH 5.5 are Ki=18 microM, kinact=0.25 min-1 and tau50=2.75 min, with a second-order rate constant of 0.75x10(4) M-1.min-1. Approx. 27 mol of phenylhydrazine and 54 mol of H2O2 are required per mol of enzyme for complete inactivation. The pH-dependent inactivation kinetics indicate the involvement of an ionizable group on the enzyme with a pKa value of 5.4, protonation of which favours inactivation. SCN-, the plausible physiological electron donor of the enzyme, protects it from inactivation. Binding studies by optical difference spectroscopy indicate that phenylhydrazine interacts with the enzyme with a KD value of 60 microM, and its binding is prevented by the presence of SCN-. The enzyme is also protected by 5, 5-dimethyl-1-pyrroline N-oxide, a free-radical trap, suggesting the involvement of a radical species in the inactivation. ESR studies indicate the formation of a spin-trapped phenyl radical (aN=15.9G and abetaH=24.8G) generated on incubation of phenylhydrazine with the enzyme and H2O2. A 75% loss of the Soret spectrum is observed when the enzyme is completely inactivated. However, in the presence of the spin trap, spectral loss is prevented and the enzyme compound II is readily reduced to the native state by phenylhydrazine. The phenylhydrazine-inactivated enzyme reacts with H2O2 or CN- to form compound II or the cyanide complex with a characteristic spectrum, indicating that haem iron is protected from attack by the radical species. The inactivated enzyme binds SCN- with a KD value similar to that of the native enzyme (15+/-3 mM), suggesting that the donor-binding site remains unaffected. CD studies of the inactive enzyme show complete disappearance of the Soret band at 409 nm with the appearance of a new band at 275 nm. This indicates that the haem environment of the enzyme is perturbed in the inactive form. As benzene, the end product of phenylhydrazine oxidation, has no effect on the enzyme, we suggest that the phenyl radical formed by one-electron oxidation by catalytically active enzyme inactivates it by incorporation in the vicinity of its haem moiety. The data support the use of phenylhydrazine as a probe for structural and mechanistic analysis of the active site of the lacrimal-gland peroxidase.

Low catalytic turnover of horseradish peroxidase in thiocyanate oxidation. Evidence for concurrent inactivation by cyanide generated through one-electron oxidation of thiocyanate

The catalytic turnover of horseradish peroxidase (HRP) to oxidize SCN- is a hundredfold lower than that of lactoperoxidase (LPO) at optimum pH. While studying the mechanism, HRP was found to be reversibly inactivated following pseudo-first order kinetics with a second order rate constant of 400 M-1 min-1 when incubated with SCN- and H2O2. The slow rate of SCN- oxidation is increased severalfold in the presence of free radical traps, 5-5-dimethyl-1-pyrroline N-oxide or alpha-phenyl-tert-butylnitrone, suggesting the plausible role of free radical or radical-derived product in the inactivation. Spectral studies indicate that SCN- at a lower concentrations slowly reduces compound II to native state by one-electron transfer as evidenced by a time-dependent spectral shift from 418 to 402 nm through an isosbestic point at 408 nm. In the presence of higher concentrations of SCN-, a new stable Soret peak appears at 421 nm with a visible peak at 540 nm, which are the characteristics of the inactivated enzyme. The one-electron oxidation product of SCN- was identified by electron spin resonance spectroscopy as 5-5-dimethyl-1-pyrroline N-oxide adduct of the sulfur-centered thiocyanate radical (aN = 15.0 G and abetaH = 16.5 G). The inactivation of the enzyme in the presence of SCN- and H2O2 is prevented by electron donors such as iodide or guaiacol. Binding studies indicate that both iodide and guaiacol compete with SCN- for binding at or near the SCN- binding site and thus prevent inactivation. The spectral characteristics of the inactivated enzyme are exactly similar to those of the native HRP-CN- complex. Quantitative measurements indicate that HRP produces a 10-fold higher amount of CN- than LPO when incubated with SCN- and H2O2. As HRP has higher affinity for CN- than LPO, it is concurrently inactivated by CN- formed during SCN- oxidation, which is not observed in case of LPO. This study further reveals that HRP catalyzes SCN- oxidation by two one-electron transfers with the intermediate formation of thiocyanate radicals. The radicals dimerize to form thiocyanogen, (SCN)2, which is hydrolyzed to form CN-. As LPO forms OSCN- as the major stable oxidation product through a two-electron transfer mechanism, it is not significantly inactivated by CN- formed in a small quantity.

Hydroxyl radical is the major causative factor in stress-induced gastric ulceration

The role of the metal-catalyzed production of hydroxyl radicals (OH.) on gastric ulceration caused by restraint-cold stress in rat was studied. Stress causes a 50% increase in the thiobarbituric acid reactive species (TBARS) as a measure of the lipid peroxidation, nearly 70% increase in protein oxidation as measured by its carbonyl content and about 40% decrease in the glutathione content of the fundic stomach, suggesting oxidative damage by stress. Stress also causes a time-dependent increase in the mitochondrial superoxide dismutase activity and a decrease in the peroxidase activity, both of which correlate well with the increase in the severity of ulceration as measured by the ulcer index. Specific OH. scavengers such as benzoate or dimethylsulfoxide (DMSO) and the free radical trap such as alpha-phenyl N-tert-butyl nitrone (PBN) significantly inhibit gastric ulceration suggesting the role of OH. in this oxidative damage. Desferrioxamine (DFO), a nontoxic transition metal ion chelator, protects the mucosa against stress-ulceration dose dependently. Increased level of TBARS and the inactivation of gastric peroxidase are also prevented by DFO or by antioxidants such as glutathione or vitamin E, suggesting the critical role of metal ion and OH. in the oxidative damage. A metal-catalyzed OH. generating system constituted by Cu2+, H2O2 and ascorbate (reducing equivalent of O2-) causes inactivation of the purified gastric peroxidase in vitro, which can be effectively prevented by DFO. The stress-induced activation of the superoxide dismutase is completely blocked by pretreatment with alpha-amanitin indicating an increased synthesis of the enzyme by increased transcription of its m-RNA. Quantitative measurement indicates that stress causes a fivefold increase in the generation of OH., which correlates well with the increase in ulcer index with the progress of stress. The results indicate that the stress-induced gastric ulceration is a consequence of the oxidative damage of the gastric mucosa. This is caused by the OH. generated through the metal-catalyzed Haber-Weiss reaction between O2- and H2O2, the latter being formed by the stimulation of the superoxide dismutase and inactivation of the gastric peroxidase.

Lactoperoxidase-catalysed oxidation of indomethacin, a nonsteroidal antiinflammatory drug, through the formation of a free radical

Lactoperoxidase (LPO, EC 1.11.1.7; donor-H2O2 oxidoreductase) catalyses the oxidation of indomethacin, a nonsteroidal antiinflammatory drug by H2O2 as measured by time-dependent decay of indo-methacin extinction at 280 nm and concurrent appearance of stable oxidation product(s) at 412 nm. From a plot of log Vmax against varying pH of indomethacin oxidation, involvement of an ionizable group of the enzyme having pka = 5.7 could be ascertained for controlling the oxidation process. Spectral studies revealed that LPO-compound II oxidises indomethacin through one-electron transfer and is reduced to the native ferric state as shown by its spectral shift from 430 nm to 412 nm through an isosbestic point at 421 nm. The one-electron oxidation product is a nitrogen-centered free radical detected as a 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adduct (alpha N = 15 G, alpha H beta = 16 G) in electron spin resonance spectroscopy. The free radical is scavenged by reaction with O2 as shown by O2 consumption sensitive to the free-radical trap, DMPO. Binding studies by optical difference spectroscopy indicate that indomethacin binds to LPO with an apparent KD value of 24.5 microM. The free energy change, delta G', for the binding is -26.3 KJ mol-1, suggesting that the interaction is favourable for oxidation. Indomethacin binding remains unaltered by a change of pH from 5.25 to 7.5, presumably because of hydrophobic interaction. The binding is competitive with resorcinol, an aromatic electron donor, showing the KD value to be as high as 100 microM. We suggest that indomethacin interacts at the aromatic donor binding site and is oxidised by one-electron transfer by LPO catalytic intermediates to stable oxidation product(s) through the formation of a free radical.

EDTA inhibits lactoperoxidase-catalyzed iodide oxidation by acting as an electron-donor and interacting near the iodide binding site

Ethylenediamine tetraacetate (EDTA) inhibits lactoperoxidase (LPO)-catalyzed rate of iodide oxidation in concentration and pH-dependent manner. A plot of log Kiapp values against various pH yields a sigmoidal curve from which an ionisable group of pKa value 6.0 could be ascertained for controlling the inhibition of catalytically active LPO by EDTA. Kinetic studies indicate that EDTA competitively inhibits iodide oxidation by acting as an electron donor. EDTA al so reduces LPO-compound-11 to the native ferric state by one-electron transfer as evidenced by the spectral shift from 428 to 412 nm. Optical difference spectroscopic studies indicate that EDTA binds to LPO with the apparent equilibrium dissociation constant (KD) of 12 +/- 2 mM at pH 6.5. A plot of log KD values against various pH produces a sigmoidal curve from which an ionisable group of LPO having pKa = 5.47 could be calculated, deprotonation of which favours EDTA binding. EDTA also binds to LPO-CN-complex indicating its binding site away from heme iron centre. The KD of LPO-EDTA complex is significantly increased (62 +/- 5 mM) by iodide suggesting that EDTA binds close to the iodide binding site. EDTA also increases the KD value of LPO-hydroquinone complex from 62 +/- 5 mM to 200 +/- 21 mM indicating that EDTA and aromatic donor binding sites are also close. We suggest that EDTA inhibits iodide oxidation competitively as an electron donor by interacting at or near the iodide binding site and these sites are close to the aromatic donor binding site.

Hamadryas baboons Papio hamadryas as maintenance hosts of Schistosoma mansoni in Saudi Arabia

Free ranging hamadryas baboons (Papio hamadryas) in four localities in the west and north of Saudi Arabia were examined for natural infection with Schistosoma mansoni. Faecal examination revealed infection with S. mansoni on four occasions within one year (at a prevalence rate of 2.5-4.0%) in only one locality, the Al-Baha area. The eggs were viable, as shown by miracidial hatching tests, and were recorded at a density of 140-280 eggs/g of faeces (7000-14,000 eggs/day). Post-mortem examination of 13-24 baboons from each locality revealed infection with S. mansoni (adult worms and eggs in tissue) in only one locality, the Al-Baha area, at a prevalence rate of 4.16%. Viable eggs were found in the faeces and tissue of the infected baboons. The low prevalence rate of S. mansoni in hamadryas baboons in Saudi Arabia is in accordance with the low prevalence rate of S. mansoni in humans in the area. This natural baboon isolate was highly infective to snail intermediate hosts and mammalian hosts under experimental conditions. The epidemiological significance of the role of P. hamadryas (considering their large overall population of 250,000) as maintenance hosts of S. mansoni in Saudi Arabia is discussed.

Probing the active site residues in aromatic donor oxidation in horseradish peroxidase: involvement of an arginine and a tyrosine residue in aromatic donor binding

The plausible role of arginine and tyrosine residues at the active side of horseradish peroxidase (HRP) in aromatic donor (guaiacol) oxidation was probed by chemical modification followed by characterization of the modified enzyme. The arginine-specific reagents phenylglyoxal (PGO), 2,3-butanedione and 1,2-cyclohexanedione all inactivated the enzyme, following pseudo-first-order kinetics with second-order rate contents of 24M(-1.)min(-1), 0.8M(-1.)min(-1) and 0.54M(-1.)min(-1) respectively. Modification with tetranitromethane, a tyrosine-specific reagent, also resulted in 50% loss of activity following pseudo-first-order kinetics with a second-order rate constant of 2.0M(-1.)min(-1). The substrate, H2O2, and electron donors such as I- and SCN- offered no protection against inactivation by both types of modifier, whereas the enzyme was completely protected by guaiacol or o-dianisidine, an aromatic electron donor (second substrate) oxidized by the enzyme. These studies indicate the involvement or arginine and tyrosine residues at the aromatic donor site of HRP. The guaiacol-protected phenylglyoxal-modified enzyme showed almost the same binding parameter (Kd) as the native enzyme, and a similar free energy change (deltaG')for the binding of the donor. Stoicheiometric studies with [7-14C]phenylglyoxal showed incorporation of 2 mol of phenylglyoxal per mol of enzyme, indicating modification of one arginine residue for complete activation. The difference absorption spectrum of the tetranitromethane-modified against the native enzyme showed a peak at 428 nm, characteristic of the nitrotyrosyl residue, that was abolished by treatment with sodium dithionite, indicating specific modification of a tyrosine residue. Inactivation stoicheiometry showed that modification of one tyrosine residue per enzyme caused 50% inactivation. Binding studies by optical difference spectroscopy indicated that the arginine-modified enzyme could not bind guaiacol at all, whereas the tyrosine-modified enzyme bound it with reduced affinity (Kd 35mM compared with 10mM for the native enzyme). Both the modified enzymes, however, retained the property of the formation of compound II (one-electron oxidation state higher than native ferriperoxidase) with H2O2, but reduction of compound II to native enzyme by guaiacol did not occur in the PGO-modified enzyme, owing to lack of binding. No non-specific change in protein structure due to modification was evident from circular dichromism studies. We therefore suggest that the active site of HRP for aromatic donor oxidation is composed of an arginine and an adjacent tyrosine residue, of which the former plays an obligatory role in aromatic donor binding whereas the latter residue plays a facilitatory role, presumably by hydrophobic interaction or hydrogen bonding.

Hemodynamic changes in recurrent intracranial terminal aneurysm after endovascular treatment

RATIONALE AND OBJECTIVES

Multiple cases of recurrence of aneurysms after endovascular treatment have been reported. The purpose of the current hemodynamic study was to identify changes in shear stress and pressure associated with the recurrence of terminal intracranial aneurysms after endovascular occlusion.

METHODS

Using a finite element method, a pulsed flow with a non-Newtonian viscosity of blood was simulated within the aneurysm cavity. A recurrent terminal intracranial aneurysm of a patient originally treated with balloon occlusion was then studied. This was based on a physiologic pulsatile flow, which was observed in the middle cerebral artery. Before and after the balloon occlusion, local maximum wall shear stress and pressure drop at the neck of the aneurysm were calculated and compared with the normal shear stress.

RESULTS

Although the maximum shear stress at the right neck of the aneurysm was significantly reduced after balloon insertion, it was still 2.5 times greater than the normal maximum shear stress. This was attributable to the presence of a portion of the aneurysmal neck (residual neck), which was not obliterated by the balloon. The balloon also helped to reduce the maximum pressure inside the aneurysm by approximately 15%. Hemodynamic changes in the residual aneurysm neck, where the shear stress and the pressure are high, and other factors may be responsible for the recurrence of aneurysms after balloon or coil occlusion.

CONCLUSION

Residual necks after balloon occlusion, coil occlusion, or both are attributable to the geometric orientation of the aneurysm with respect to parent and daughter vessels and the variety of configurations of the balloon and coils used to occlude the aneurysms. Inadequate reduction in local shear stress found in these residual necks is an important factor in the recurrence and rupture of the aneurysm after endovascular occlusion.

Characterization of sheep lacrimal-gland peroxidase and its major physiological electron donor

A soluble sheep lacrimal-gland peroxidase was purified to apparent homogeneity. It had a native molecular mass of 75 kDa with a subunit molecular mass of 82 kDa and an isoelectric point of 6.5. Western blotting showed that it shares some of the enzyme antigenic determinants in common with other soluble peroxidases. The enzyme exhibits a Soret peak at 410 nm which is shifted to 431 nm by 5 equiv. of H2O2 due to the formation of compound II. The latter is, however, unstable and gradually returns to the native state. The enzyme forms complexes with CN- and N3- and is reduced by dithionite showing a characteristic reduced peroxidase spectrum. Although the enzyme oxidizes I-, SCN- and Br- optimally at pH 5.5., 5.25 and 5.0 respectively, at physiological pH, it oxidizes I- and SCN- only. Since extracellular SCN- concentration is much higher than I-, SCN- may act as the major electron donor to the enzyme. The second-order rate constants for the reaction of the enzyme with H2O2 (k+1) and of compound I with SCN- (k+2) were 4 X 10(7) M-1 X s-1 and 8.1 X 10(5) M-1 X s-1 respectively. A plot of log Vmax against pH yields a sigmoidal curve consistent with a single ionizable group on the enzyme with a pK(a) value of 5.75, controlling thiocyanate oxidation. In a coupled system with the peroxidase, H2O2, SCN-, GSH, NADPH and glutathione reductase, peroxidase-catalysed SCN- oxidation by H2O2 could be coupled to NADPH consumption. The system is proposed to operate in vivo for the efficient elimination of endogenous H2O2.

Flow-pressure drop measurement and calculation in a tapered femoral artery of a dog

This study describes the in vivo measurement of pressure drop and flow during the cardiac cycle in the femoral artery of a dog, and the computer simulation of the experiment based on the use of the measured flow, vessel dimensions and blood viscosity. In view of the experimental uncertainty in obtaining the accurate velocity profile at the wall region, the velocity pulse at the center was measured and numerical calculations were performed for the center line instantaneous velocity and within the two limits of spatial distribution of inlet flow conditions: uniform and parabolic. Temporal and spatial variations of flow parameters, i.e., velocity profile, shear rate, non-Newtonian viscosity, wall shear stress, and pressure drop were calculated. There existed both positive and negative shear rates during a pulse cycle, i.e., the arterial wall experiences zero shear three times during a cardiac cycle. For the parabolic inlet condition, the taper of the artery not only increased the magnitude of the positive and negative shear rates, but caused a steep gradient in shear rate, a phenomenon which in turn affects wall shear stress and pressure. In contrast, for the uniform inlet condition, the flow through the tapered artery was predominantly the developing type, which resulted in reduction in magnitude of wall shear rate along the axial direction.

Concurrent reduction of iodine and oxidation of EDTA at the active site of horseradish peroxidase: probing the iodine binding site by optical difference spectroscopy and steady state kinetic analysis for the formation of active enzyme-I(+)-EDTA ternary complex for iodine reductase activity

Horseradish peroxidase (HRP) catalyzes the reduction of iodine to iodide by EDTA with pseudocatalatic degradation of H2O2 to O2 (Banerjee et al., (1986) J. Biol. Chem. 261, 10592-10597; and Banerjee (1989) J. Biol. Chem. 264, 9188-9194). The reduction of iodine (I+) is dependent on EDTA concentration and is blocked by spin trap, DMPO, indicating the involvement of free radical species in the reduction process. Incubation of EDTA with both HRP and H2O2 results in the appearance of triplet ESR signal of spin-trapped EDTA radical (aN = 15 G), indicating its one-electron oxidation to a nitrogen-centered monocation radical (N-N+). The latter oxidizes H2O2 to evolve O2 and regenerate EDTA. In the presence of I+, a ternary complex of compound I-I(+)-EDTA is formed, which generates compound II-I. complex and both nitrogen-centered dication radical (N(+)-N+) through intermolecular electron transfer from EDTA nitrogens. Compound II-I. complex is further reduced similarly by another molecule of EDTA to form ferric enzyme, I-, and (N(+)-N+).(N(+)-N+) the oxidation product of EDTA, which may be released from the active site and, being more reactive, oxidizes H2O2 to O2 at a faster rate to regenerate EDTA. The existence of (N(+)-N+) is suggested from the similarity of its ESR signal with that of single nitrogen-centered monocation radical (N-N+). EDTA degradation by oxidative decarboxylation due to two-electron oxidation from the same or both nitrogen, atoms is not evident, and EDTA concentration remains the same throughout the reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Irreversible inactivation of lactoperoxidase by mercaptomethylimidazole through generation of a thiyl radical: its use as a probe to study the active site

The mechanism of suicidal inactivation of lactoperoxidase (LPO) by mercaptomethylimidazole (MMI) has been studied. Analogue studies indicate a specific requirement for the thiol group of MMI for inactivation of LPO in the presence of H2O2. MMI is oxidized via one-electron transfer by LPO compound II as demonstrated by a spectral shift from 430 to 412 nm through an isosbestic point at 421 nm. A decrease in Soret absorbance at 412 nm and the appearance of visible peaks at 592 and 636 nm are the characteristics of the inactivated enzyme. The one-electron oxidation product of MMI was identified by e.s.r. spectroscopy as the 5,5'-dimethyl-l-pyrroline N-oxide (DMPO) adduct of the sulphur-centred thiyl radical. Both inactivation and spectral change are prevented by the radical trap DMPO, suggesting involvement of the thiyl radical in inactivation. pH-dependent inactivation kinetics indicate the involvement of an ionizable group on LPO (pKa 6.1), deprotonation of which favours inactivation. The enzyme is protected by iodide and not by guaiacol, suggesting that MMI interacts at or near the iodide-binding site which is away from the aromatic-donor-binding site. The inactive enzyme can form compound II and bind aromatic donor, indicating that the MMI oxidation product does not attack haem iron or aromatic-donor-binding site. We suggest that MMI interacts at the iodide-binding site for oxidation and the reactive product, probably the thiyl radical, is incorporated into the adjacent electron-rich site of haem porphyrin to cause inactivation.

Thiocyanate, a plausible physiological electron donor of gastric peroxidase

Gastric peroxidase (GPO) was purified to apparent homogeneity to characterize its major physiological electron donor. The enzyme (RZ = 0.7), with a subunit molecular mass of 50 kDa, is a glycoprotein, with a relative abundance of aspartic and glutamic acid over arginine and lysine. It has a Soret maximum at 412 nm, which is shifted to 426 nm by H2O2 due to formation of compound II. Although the physiological electron donors I-, Br- and SCN-, but not Cl-, are oxidized by GPO optimally at acid pH, only I- and SCN- are oxidized appreciably at physiological pH. Considering that the I- concentration in stomach is less than 1 microM, whereas the SCN- concentration is about 250 microM, SCN- may act as a major electron donor for GPO. Moreover, SCN- oxidation remains unaltered in the presence of physiological concentrations of other halides. The second-order rate constant for the reaction of GPO with H2O2 (k1) and compound I with SCN- (k2) at pH 7 was found to be 8 x 10(7) M-1.s-1 and 2 x 10(5) M-1.s-1 respectively. GPO has significant pseudocatalase activity also in the presence of I- or Br-, but it is blocked by SCN-. The SCN- oxidation product OSCN- may be reduced back to SCN- by cellular GSH, and GSSG may be reduced back to GSH by glutathione reductase and NADPH. In a system reconstituted with pure glutathione reductase, NADPH, GSH, SCN- and H2O2. GPO-catalysed SCN- oxidation could be coupled to NADPH oxidation. This system where GPO utilizes SCN- as the major physiological electron donor may operate efficiently to scavenge intracellular H2O2.

Mechanism of inhibition of horseradish peroxidase-catalysed iodide oxidation by EDTA

EDTA inhibits horseradish peroxidase (HRP)-catalysed iodide oxidation in a concentration and pH-dependent manner. It is more effective at pH 6 than at lower pH values. A plot of log Kiapp. values as a function of pH yields a sigmoidal curve from which a pKa value of 5.4 can be calculated for an ionizable group on the catalytically active HRP for EDTA inhibition. Among the structural analogues of EDTA, tetramethylethylenediamine (TEMED) is 80% as effective as EDTA, whereas the EDTA-Zn2+ chelate and EGTA are ineffective. Kinetic studies indicate that EDTA competitively inhibits iodide oxidation. Spectral studies show that EDTA can quickly reduce compound I to compound II, but reduction of preformed compound II to the native enzyme is relatively slow, as demonstrated by the time-dependent spectral shift from 417 nm to 402 nm through an isosbestic point at 408 nm. Under steady-state conditions, in a reaction mixture containing HRP, EDTA and H2O2, the enzyme remains in the compound-II form, with absorption maxima at 417, 527 and 556 nm. Direct evidence for one-electron oxidation of EDTA by HRP intermediates is provided by the appearance of an e.s.r. signal of a 5,5-dimethyl-1-pyrroline N-oxide (spin trap)-EDTA radical adduct [aN (hyperfine splitting constant) = 1.5 mT] in e.s.r. studies. The signal intensity, however, decreases in the presence of iodide. The KD of the HRP-EDTA complex obtained from optical difference spectroscopy increases with an increase in iodide concentration, and the double-reciprocal plot for EDTA binding indicates that EDTA and iodide compete for the same binding site for oxidation. We suggest that EDTA inhibits iodide oxidation by acting as an electron donor.

Mechanism-based inactivation of gastric peroxidase by mercaptomethylimidazole

The mechanism of inhibition of gastric peroxidase (GPO) activity by mercaptomethylimidazole (MMI), an inducer of gastric acid secretion, has been investigated. Incubation of purified GPO with MMI in the presence of H2O2 results in irreversible inactivation of the enzyme. No significant inactivation occurs in the absence of H2O2 or MMI, suggesting the involvement of peroxidase-catalysed oxidized MMI (MMIOX.) in the inactivation process. The inactivation follows pseudo-first-order kinetics consistent with a mechanism-based (suicide) mode. The pseudo-first-order kinetic constants at pH 8 are ki = 111 microM, k(inact.) = 0.55 min-1 and t1/2 = 1.25 min, and the second-order rate constant is 0.53 x 10(4) M-1 x min-1. Propylthiouracil also inactivates GPO activity in the same manner but its efficiency (k(inact./ki = 0.46 mM-1 x min-1) is about 10 times lower than that of MMI (k(inact./ki = 5 mM-1 x min-1). The rate of inactivation with MMI shows pH-dependence with an inflection point at 7.3, indicating the involvement in the inactivation process of an ionizable group on the enzyme with a pKa of 7.3. The enzyme is remarkably protected against inactivation by micromolar concentrations of electron donors such as iodide and bromide but not by chloride. Although GPO oxidizes MMI slowly, iodide stimulates it through enzymic generation of I+ which is reduced back to I- by MMI. Although MMIOX. is formed at a much higher rate in the presence of I-, a constant concentration of I- maintained via the reduction of I+ by MMI, protects the active site of the enzyme against inactivation. We suggest that MMI inactivates catalytically active GPO by acting as a suicidal substrate.

Chemical and kinetic evidence for an essential histidine residue in the electron transfer from aromatic donor to horseradish peroxidase compound I

Horseradish peroxidase, when incubated with diethyl pyrocarbonate (DEPC), a histidine-specific reagent, shows time-dependent inactivation to oxidize aromatic electron donor, guaiacol. The inactivation follows pseudo-first order kinetics with a second order rate constant of 0.67 min-1 M-1. The pH dependence of inactivation shows an inflection point at 6.02, indicating histidine derivatization by DEPC. A difference spectrum of modified versus native enzyme shows a peak at 244 nm for N-carbethoxyhistidine that is diminished by hydroxylamine. Stoichiometric studies indicate that out of 2 histidine residues modified, one is responsible for inactivation. A plot of log reciprocal half-time of inactivation against log DEPC concentration suggests that only 1 histidine is essential. From the computer-stimulated structure of horseradish peroxidase, we tentatively suggest that this critical histidine is most likely distal histidine 42. Binding studies show that this histidine is not involved in guaiacol binding. Modified enzyme forms compound I with H2O2 but not compound II, suggesting a block of electron transfer process. Modified compound I cannot oxidize guaiacol as evidenced by the absence of donor-induced spectral shift from 408 nm, suggesting a block of electron transfer from bound donor to compound I. We suggest that this tentatively identified distal histidine controls aromatic donor oxidation by regulating electron transport without affecting donor binding or compound I formation.

Effect of stress on the antioxidant enzymes and gastric ulceration

The effect of cold-restraint stress on the antioxidant enzymes of the rat gastric mucosa was studied with a view to finding out their role in stress induced gastric ulceration. Histological examination revealed stress induced extensive damage of the surface epithelial cell with lesions extending up to submucosa in some cases. Stress causes time-dependent increase in histamine and pepsin content but decrease in acid content of the gastric fluid with the progress of ulceration (ulcer index) for two hours. The tissue lipid peroxidation was significantly increased as evidenced by accumulation of malondialdehyde. Since lipid peroxidation results from the generation of reactive oxygen species, stress effect was studied on some antioxidant enzymes such as superoxide dismutase, peroxidases and prostaglandin synthetase as a function of time. The time dependent increase in stress ulcer correlates well with the concomitant increase in superoxide dismutase activity and decrease in peroxidase and prostaglandin synthetase activity. This creates a favourable condition for accumulation of endogenous H2O2 and more reactive hydroxyl radical (OH.). Administration of antioxidants such as reduced glutathione or sodium benzoate prior to stress causes significant decrease in ulcer index and lipid peroxidation and protection of gastric peroxidase activity suggesting the involvement of reactive oxygen species in stress induced gastric ulceration. This is supported by the in vitro observation that OH. can also inactivate peroxidase and induce lipid peroxidation. As prostaglandin is known to offer cytoprotection, stress-induced loss of prostaglandin synthetase activity appears to aggravate the oxidative damage caused by reactive oxygen species.

Numerical studies of three-dimensional arterial flows in reverse curvature geometry: Part I--Peak flow

A three-dimensional flow simulation at Repeak = 192 and 580 was made in a smooth reverse curvature model that conformed to the gentle "S" shape from a human left femoral artery angiogram. The objective of this numerical investigation was to find the changes in pressure, shear stress, velocity profile, and particle path occurring in the double-curved arterial vessel. Due to the impingement of blood at the outer wall in the first bend region, the wall shear stress approached 40 dyne/cm2--a value over twice as large as in the straight upstream segment. Conversely, at the inner wall in the first bend, a low shear stress region was found where the value of the shear stress was consistently smaller than that in the straight section. The initiation of centrifugal effects caused by the first bend could clearly be seen at Repeak = 580, but due to the close proximity of the reverse curvature segment, the momentum effect due to the second bend overshadowed the centrifugal effect. Hence, only near the end of the second bend did the centrifugal effect due to the second bend result in a double-spiral-secondary motion. In addition, the numerically calculated pressure drop data were in agreement with prior experimental values.

Effect of dexamethasone on the peptic activity of gastric lumen and mucosa

Dexamethasone (9 alpha-fluoro-16 alpha methyl-11 beta,17 alpha,21-trihydroxy-1,4-pregnadiene-3,20-dione-21-phosphate), a synthetic glucocorticoid, has a dual role on pepsinogen content of the gastric lumen and mucosa as measured by its peptic activity. Following stimulation the luminal peptic activity gradually decreases after 6 hr, then returns to basal levels at 18 hr and by 24 hr is inhibited by 50%. The luminal peptic activity induced by the secretory compound mercaptomethylimidazole (MMI) is also decreased. Dexamethasone effect on both basal and MMI-induced peptic activity can be reproduced by cycloheximide or puromycin, translational blockers of protein synthesis. This drug also has an independent time and dose-dependent inhibitory effect on gastric mucosal peptic activity which does not correlate with increased peptic activity of the lumen. Dexamethasone appears to be more effective than hydrocortisone and corticosterone in inhibiting the basal peptic activity of both lumen and mucosa. The inhibitory effect of this drug on tissue peptic activity is not mediated through induction of any inhibitory protein as evidenced by the insensitivity of the effect to actinomycin D. Studies on [14C]phenylalanine incorporation into gastric protein indicate that the effect of dexamethasone on tissue pepsinogen content is not due to a generalized block of protein synthesis.

Inhibition of intestinal peroxidase activity by nonsteroidal antiinflammatory drugs

The peroxidase activity of the mitochondrial fraction of rat intestine is inhibited in vitro by non-steroidal antiinflammatory drugs (NSAIDs), such as indomethacin (IMN) and acetylsalicylic acid (ASA), the former being more potent than the latter. The peroxidase was solubilised by cetab-NH4Cl extraction and purified to apparent homogeneity by Sephadex G-150 gel filtration and affinity chromatography on Con-A Sepharose. The purified enzyme activity was 80% inhibited by 150 microM IMN and 50% by 2.67 mM ASA. IMN could also inhibit lactoperoxidase activity to the same extent but not the horseradish peroxidase activity. The inhibition of peroxidase-catalysed iodide oxidation by IMN and ASA was optimal at pH 5.5 and 4.5, respectively. Kinetic studies revealed that the inhibition by IMN was competitive with respect to iodide or guaiacol, while the inhibition by ASA was noncompetitive and reversible in nature. Studies of some structural analogues showed that indole-3-acetic acid was as effective as IMN, while salicylic acid was more potent than ASA. Spectral studies showed a small bathochromic shift of the Soret band of the enzyme by IMN, suggesting its possible interaction at or near the heme moiety. The competitive nature of IMN may be explained as due to its oxidation by the peroxidase to a product absorbing at 412 nm, the formation of which is inhibited by iodide. We suggest that IMN inhibits intestinal peroxidase activity by acting as a competitive substrate for the enzyme. As intestinal peroxidase is mainly contributed by the invading eosinophils, NSAIDs may affect the host defence mechanism by inhibiting the activity of the enzyme.

Iodide modulation of the EDTA-induced iodine reductase activity of horseradish peroxidase by interaction at or near the EDTA-binding site

Horseradish peroxidase (HRP) catalyses the reduction of iodinium ion (I+) to iodide by H2O2 in the presence of EDTA. I+ reduction occurs optimally at pH 6 whereas the enzyme catalyses iodide oxidation optimally at pH 3.5. Thus the two activities reside on the same enzyme with two characteristic pH optima. Iodide modulates the expression of the reductase activity by EDTA. Higher concentrations of iodide inhibit the reductase activity by EDTA. Nitrite, an electron donor, acts similarly to iodide. Both EDTA and nitrite competitively inhibit iodide oxidation, indicating that they compete with iodide for the same binding site for electron flow to the haem iron group. However, unlike iodide, EDTA converts compound I, not into the native enzyme, but into a compound absorbing at 416 nm which reduces I+ and then returns to the native form. The apparent equilibrium dissociation constant, KD, for the formation of the EDTA-HRP complex (15 mM) is doubled in the presence of iodide, indicating interference with EDTA binding by iodide. EDTA binds away from the haem iron centre and not through intramolecular Ca2+. The pH-dependence of EDTA binding indicates that an ionizable group of the enzyme with pKa 5.8, presumably a distal histidine, controls the binding. The data suggest that iodide competes with EDTA for compound I and modulates the iodine reductase activity by limiting the formation of the 416 nm-absorbing active compound.

Localization of gastric peroxidase and its inhibition by mercaptomethylimidazole, an inducer of gastric acid secretion

Mercaptomethylimidazole (MMI) is a potent inducer of gastric acid secretion which is associated with significant inhibition of peroxidase activity of rat gastric mucosa in vivo. A time-dependent increase in acid secretion correlates well with time-dependent decrease in the peroxidase activity. In a chamber experiment in vitro using isolated gastric mucosa, MMI stimulates acid secretion, showing an almost linear response up to 600 microM. The time-dependent increase in acid secretion is also correlated with time-dependent inhibition of the peroxidase activity. This effect is not mediated through oxidation of MMI by flavin-containing mono-oxygenase, which is absent from gastric mucosa. The peroxidase has been localized mainly in parietal cells isolated and purified from gastric mucosa by controlled digestion with collagenase followed by Percoll-density-gradient centrifugation. Peroxidase activity was further localized in the outer membrane of the purified mitochondria of the parietal cell by some membrane-impermeant reagents, indicating outward orientation of the enzyme. MMI can inhibit the peroxidase activity of both the parietal cell and its mitochondria in a concentration-dependent manner. The possible involvement of the parietal-cell peroxidase-H2O2 system in MMI-induced acid secretion may be suggested.

Chemical and kinetic evidence for an essential histidine in horseradish peroxidase for iodide oxidation

Horseradish peroxidase (HRP), when incubated with diethylpyrocarbonate (DEPC), shows a time-dependent loss of iodide oxidation activity. The inactivation follows pseudo-first order kinetics with a second order rate constant of 0.43 min-1 M-1 at 30 degrees C and is reversed by neutralized hydroxylamine. The difference absorption spectrum of the modified versus native enzyme shows a peak at 244 nm, characteristic of N-carbethoxyhistidine, which is diminished by treatment with hydroxylamine. Correlation between the stoichiometry of histidine modification and the extent of inactivation indicates that out of 2 histidine residues modified, one is responsible for inactivation. A plot of the log of the reciprocal half-time of inactivation against log DEPC concentration further suggests that only 1 histidine is involved in catalysis. The rate of inactivation shows a pH dependence with an inflection point at 6.2, indicating histidine derivatization by DEPC. Inactivation due to modification of tyrosine, lysine, or cysteine has been excluded. CD studies reveal no significant change in the protein or heme conformation following DEPC modification. We suggest that a unique histidine residue is required for maximal catalytic activity of HRP for iodide oxidation.

Purification and characterization of a soluble peroxidase of rat preputial gland: comparison with lactoperoxidase

A highly active soluble peroxidase (donor: H2O2 oxidoreductase EC 1.11.1.7) has been purified from the preputial gland of the rat by hydroxylapatite chromatography, ammonium sulfate fractionation, Sephadex gel filtration and affinity chromatography on con A-Sepharose. The enzyme shows apparent homogeneity when analysed by acid and alkaline-PAGE. Its molecular, spectral, kinetic and catalytic properties were compared with those of bovine lactoperoxidase (LPO). Preputial gland peroxidase (PPO) is a glycoprotein of molecular weight of 70-73 kDa slightly lower (78 kDa) than that of LPO. Using isoelectric focussing, PPO was resolved into eight different closely spaced protein species spanning a pI range of 5.4 to 6.4, while LPO focuses into several closely spaced protein bands in the pI range 8.5-9.3. PPO is similar to LPO in its spectral (Soret) and some kinetic properties, but it differs significantly from LPO in substrate (H2O2) tolerance and substrate inactivation. PPO also differs from LPO in showing differential inactivation by SDS. Both enzymes are glycoproteins and although concanavalin A (con A) showed a variable interaction with both enzymes, wheat germ agglutinin interacted specifically with LPO only. We suggest that PPO, the secretory peroxidase of the preputial gland, differs significantly from LPO in its molecular and catalytic properties.

Dissociation of gastric acid and pepsinogen secretion in response to mercaptomethylimidazole--a new secretory compound

Mercaptomethylimidazole (MMI), a potent antithyroid drug of the thionamide group, induces both acid and pepsinogen secretion independently in control and pylorus ligated mice. The effect is dose dependent and the drug is more effective than histamine, carbachol or isoproterenol when administered by an intraperitoneal route. MMI-stimulated pepsinogen secretion could be dissociated from the acid secretion by the use of cimetidine and omeprazole which effectively block the acid secretion without affecting the pepsinogen output. Neither acid nor pepsinogen secretion by MMI is inhibited by atropine indicating a lack of muscarinic receptor involvement in both of the processes. Nifedipine and verapamil, the calcium antagonists, by inhibiting the MMI-induced acid secretion can also dissociate pepsinogen secretion from the acid secretion. Clonidine, an alpha 2-agonist, and hexobarbital, a membrane active barbiturate, also inhibit acid secretion without affecting the pepsinogen output. These data indicate that MMI induces pepsinogen secretion independent of acid secretion. Furthermore, MMI-stimulated acid secretion is not additive with that of the histamine indicating same site (H2-receptor) of action while its synergistic effect in presence of carbachol (muscarinic receptor) indicates different site of interaction of the two compounds. On the other hand, an additive effect of MMI and carbachol on pepsinogen secretion indicates that while the carbachol effect is mediated through the muscarinic receptor, MMI stimulates pepsinogen secretion through some still unknown mechanism.

Nonsteroidal anti-inflammatory drugs inhibit gastric peroxidase activity

The peroxidase activity of the mitochondrial fraction of rat gastric mucosa was inhibited with various nonsteroidal anti-inflammatory drugs (NSAIDs) in vitro. Indomethacin was found to be more effective than phenylbutazone (PB) or acetylsalicylic acid (ASA). Mouse gastric peroxidase was also very sensitive to indomethacin inhibition. Indomethacin has no significant effect on submaxillary gland peroxidase activity of either of the species studied. Purified rat gastric peroxidase activity was inhibited 75% with 0.15 mM indomethacin showing half-maximal inhibition at 0.04 mM. The inhibition could be withdrawn by increasing the concentration of iodide but not by H2O2. NSAIDs inhibit gastric peroxidase activity more effectively at acid pH (pH 5.2) than at neutral pH. Spectral studies showed a bathochromic shift of the Soret band of the enzyme with indomethacin indicating its interaction at or near the heme part of the enzyme.

EDTA inhibits peroxidase-catalyzed iodide oxidation through interaction at the iodide binding site

EDTA inhibits the formation of I3- from iodide catalysed by various pure peroxidases. The inhibition is concentration-dependent and chloroperoxidase (CPO) is more sensitive than horseradish peroxidase (HRP) and lactoperoxidase (LPO). EDTA is more active than EGTA or other biological chelators tested. Zn2+, Mn2+ and Co2+ are equally active in reversing the effect of EDTA on both CPO and HRP almost completely, but ineffective in the case of LPO. The effect of EDTA on HRP can be reversed by a higher concentration of iodide but not by H2O2. EDTA causes a hypsochromic change in the absorption of the Soret band of HRP at 402 nm, and iodide can reverse this effect. EDTA can effectively displace radioiodide specifically bound to HRP. It is suggested that EDTA inhibits iodide oxidation by interacting at the iodide binding site of the HRP.