Optimization of WEDM Parameters While Machining Biomedical Materials Using EDAS-PSO

In the present work, an attempt has been made to study the influence of process parameters of the wire electric discharge machining (WEDM) process on the machining characteristics. The commercially pure titanium is machined by WEDM using brass wire as an electrode. The input parameters in this work were pulse on-time (A_on), pulse off-time (A_off), servo voltage (SV) and wire tension (WT). On the other hand, dimensional accuracy (DA), average surface roughness (R_a) and maximum surface roughness (R_z) were chosen as the response parameters. The empirical relations developed for response characteristics were solved collectively using Evaluation Based on Distance from Average Solution (EDAS) and Particle Swarm Optimization (PSO). The optimized setting for minimizing the surface irregularities while machining titanium alloy on WEDM is predicted as A_on: 8 μs; A_off: 13 μs; SV: 45 V; and WT: 8 N. Moreover, the predicted solution at the optimized parametric settings came out as DA: 95%; Ra: 3.163 μm; Rz: 22.99 μm; WL: 0.0182 g; and DR: 0.1277 mm. The validation experiments at the optimized setting showed the close agreement between predicted and experimental values. The morphological study by scanning electron microscopy (SEM) at the optimized setting revealed a significant reduction in surface defects such as micro cracks, micro cavities, globules and sub-surfaces, etc. In a nutshell, the study justified the effectiveness of EDAS-PSO in efficiently predicting the results for machining of pure titanium (Grade 2) using the WEDM process.

Rice straw burning: a review on its global prevalence and the sustainable alternatives for its effective mitigation

Being one of the most important staple crops of the world, rice has played a vital role in slaking the calorie requirements of the masses in all the inhabitable continents of our planet. Regardless of this fact, there are many environmental concerns related to the rice production systems across the globe. One of the major worries is the emission of lethal greenhouse gases as a result of the different steps and procedures concerned with rice production and their contribution towards global warming. This study presents the status quo of the rice straw burning practice across the globe. It focuses on the greenhouse gas emissions as a result of the open field burning of rice residues and its direct effect on the environment, eventually contributing towards climate change. The study evidently shortlists the most profound regions contributing towards the open burning dilemma and the socio-political reasons associated with it. The study additionally discusses the different alternatives to straw burning with a clear-cut motive of throwing light on the opportunities that lie in the efficacious and sustainable utilization of homogeneous agricultural wastes. Different in-field straw management techniques related to the farmers and off-field methods related to the industry have been discussed. Predicated upon a survey of the life cycle assessment (LCA) studies across the globe, it is concluded that soil incorporation and electricity generation are the most environment friendly alternatives with an enormous scope of improvement in the coming future.

Impact of Cryogenic Treatment on HCF and FCP Performance of β-Solution Treated Ti-6Al-4V ELI Biomaterial

The poor fatigue strength of Ti-6Al-4V ELI is a main cause of failure in structural implants. In this work, Ti-6Al-4V ELI was subjected to β-solution treatment to obtain martensite microstructure and further subjected to −196 °C for 24 h. Significant improvement in high cycle fatigue performance of martensite Ti-6Al-4V ELI was observed on exposure to cryogenic cycle. Resistance to fatigue crack growth of alloy was augmented in martensite structure as compared with mill annealed sample and the same was retained even after exposure to cryogenic treatment. The variation observed in fatigue behavior due to cryogenic treatment was correlated with fractography and metallurgical investigations. Improvement in high cycle fatigue performance can be attributed to a combined effect of a decrease in the size of prior β grain, formation of massive α patch and its subsequent transformation into ultra-fine α and β during the soaking period at −196 °C.

Investigations of Machining Characteristics in the Upgraded MQL-Assisted Turning of Pure Titanium Alloys Using Evolutionary Algorithms

Environmental protection is the major concern of any form of manufacturing industry today. As focus has shifted towards sustainable cooling strategies, minimum quantity lubrication (MQL) has proven its usefulness. The current survey intends to make the MQL strategy more effective while improving its performance. A Ranque–Hilsch vortex tube (RHVT) was implemented into the MQL process in order to enhance the performance of the manufacturing process. The RHVT is a device that allows for separating the hot and cold air within the compressed air flows that come tangentially into the vortex chamber through the inlet nozzles. Turning tests with a unique combination of cooling technique were performed on titanium (Grade 2), where the effectiveness of the RHVT was evaluated. The surface quality measurements, forces values, and tool wear were carefully investigated. A combination of analysis of variance (ANOVA) and evolutionary techniques (particle swarm optimization (PSO), bacteria foraging optimization (BFO), and teaching learning-based optimization (TLBO)) was brought into use in order to analyze the influence of the process parameters. In the end, an appropriate correlation between PSO, BFO, and TLBO was investigated. It was shown that RHVT improved the results by nearly 15% for all of the responses, while the TLBO technique was found to be the best optimization technique, with an average time of 1.09 s and a success rate of 90%.

Early retinal differentiation of human pluripotent stem cells in microwell suspension cultures

Objective

To develop a microwell suspension platform for the adaption of attached stem cell differentiation protocols into mixed suspension culture.

Results

We adapted an adherent protocol for the retinal differentiation of human induced pluripotent stem cells (hiPSCs) using a two-step protocol. Establishing the optimum embryoid body (EB) starting size and shaking speed resulted in the translation of the original adherent process into suspension culture. Embryoid bodies expanded in size as the culture progressed resulting in the expression of characteristic markers of early (Rx, Six and Otx2) and late (Crx, Nrl and Rhodopsin) retinal differentiation. The new process also eliminated the use of matrigel, an animal-derived extracellular matrix coating.

Conclusions

Shaking microwells offer a fast and cost-effective method for proof-of-concept studies to establish whether pluripotent stem cell differentiation processes can be translated into mixed suspension culture.

Electronic supplementary material

The online version of this article (doi:10.1007/s10529-016-2244-7) contains supplementary material, which is available to authorized users.

Comparing hand hygiene measures in a neonatal ICU: a randomized crossover trial

OBJECTIVE

To compare plain soap, alcohol hand rub and iodophors as hand hygiene measures in a neonatal intensive care unit (NICU).

DESIGN

Randomized, crossover, three-armed, controlled trial with blinded outcome measurement.

SETTING

Level III NICU.

PARTICIPANTS

35 NICU nurses.

INTERVENTION

Participants were assigned to plain soap hand washing, alcohol hand rub and povidone-iodine hand scrub by a random cross-over design. Interventions were preceded by 14-day neutral periods. Cultures from hands were taken before and after each hand-hygiene use, prior to 5 patient-care activities.

MAIN OUTCOME MEASURE

The primary outcome was mean post-hygiene colony forming unit count (CFU-C).

RESULTS

There were differences between soap, alcohol and povidone groups vis a vis posthygiene CFUC [median: 60, 8 and 10.5, respectively (P<0.001)], absolute reduction in CFU-C [median: 15, 100 and 40, respectively (P<0.001)], percent reduction in CFUC [median: 33.3, 92 and 87, respectively (P<0.001)] and proportion with low CFU-C [47%, 71% and 72%, respectively (P<0.001)]. Alcohol [Adjusted OR 3.2 (95% CI 1.9, 5.3)], povidone-iodine [AOR 3.1 (95% CI 1.8, 5.3)] and high prehygiene CFU-C (>300) [AOR 0.18 (95% CI 0.1, 0.3)] were independently associated with low CFU-C.

CONCLUSION

After a 2 minute hand wash at entry into NICU, alcohol hand rub and povidone-iodine scrub are superior to plain soap hand wash for subsequent decontamination of hands of nurses working in NICU.

Nitric oxide inhibits methionine synthase activity in vivo and disrupts carbon flow through the folate pathway

Many of nitric oxide's biological effects are mediated via NO binding to the iron in heme-containing proteins. Cobalamin (vitamin B(12)) is structurally similar to heme and is a cofactor for methionine synthase, a key enzyme in folate metabolism. NO inhibits methionine synthase activity in vitro, but data concerning NO binding to cobalamin are controversial. We now show spectroscopically that NO reacts with all three valency states of cobalamin and that NO's inhibition of methionine synthase activity most likely involves its reaction with monovalent cobalamin. By following incorporation of the methyl moiety of [(14)C]methyltetrahydrofolic acid into protein, we show that NO inhibits methionine synthase activity in vivo, in cultured mammalian cells. The inhibition of methionine synthase activity disrupted carbon flow through the folate pathway as measured by decreased incorporation of [(14)C]formate into methionine, serine, and purine nucleotides. Homocysteine, but not cysteine, attenuated NO's inhibition of purine synthesis, providing further evidence that NO was acting through methionine synthase inhibition. NO's effect was observed both when NO donors were added to cells and when NO was produced physiologically in co-culture experiments. Treating cells with an NO synthase inhibitor increased formate incorporation into methionine, serine, and purines and methyl-tetrahydrofolate incorporation into protein. Thus, physiological concentrations of NO appear to regulate carbon flow through the folate pathway.

Kinetics of CO and NO ligation with the Cys(331)-->Ala mutant of neuronal nitric-oxide synthase

Nitric-oxide synthases (NOS) catalyze the conversion of l-arginine to NO, which then stimulates many physiological processes. In the active form, each NOS is a dimer; each strand has both a heme-binding oxygenase domain and a reductase domain. In neuronal NOS (nNOS), there is a conserved cysteine motif (CX(4)C) that participates in a ZnS(4) center, which stabilizes the dimer interface and/or the flavoprotein-heme domain interface. Previously, the Cys(331) --> Ala mutant was produced, and it proved to be inactive in catalysis and to have structural defects that disrupt the binding of l-Arg and tetrahydrobiopterin (BH(4)). Because binding l-Arg and BH(4) to wild type nNOS profoundly affects CO binding with little effect on NO binding, ligand binding to the mutant was characterized as follows. 1) The mutant initially has behavior different from native protein but reminiscent of isolated heme domain subchains. 2) Adding l-Arg and BH(4) has little effect immediately but substantial effect after extended incubation. 3) Incubation for 12 h restores behavior similar but not quite identical to that of wild type nNOS. Such incubation was shown previously to restore most but not all catalytic activity. These kinetic studies substantiate the hypothesis that zinc content is related to a structural rather than a catalytic role in maintaining active nNOS.

Activation of soluble guanylate cyclase by carbon monoxide and nitric oxide: a mechanistic model

Soluble guanylate cyclase (GC) from bovine lung is activated 4-fold by carbon monoxide (CO) and 400-fold by nitric oxide (NO). Spectroscopic and kinetic data for ligation of CO and NO with GC are summarized and compared with similar data for myoglobin (Mb), hemoglobin (Hb), and heme model compounds. Kinetic, thermodynamic, and structural data form a basis on which to construct a model for the manner in which the two ligands affect protein structure near the heme for heme proteins in general and for GC in particular. The most significant datum is that although association rates of ligands with GC are similar to those with Mb and Hb, their dissociation rates are dramatically faster. This suggests a delicate balance between five- and six-coordinate heme iron in both NO and CO complexes. Based on these and other data, a model for GC activation is proposed: The first step is formation of a six-coordinate species concomitant with tertiary and quaternary structural changes in protein structure and about a 4-fold increase in enzyme activity. In the second step, applicable to NO, the bond from iron to the proximal histidine ruptures, leading to additional relaxation in the quaternary and tertiary structure and a further 100-fold increase in activity. This is the main event in activation, available to NO and possibly other activators or combinations of activators. It is proposed, finally, that the proximal base freed in step 2, or some other protein base suitably positioned as a result of structural changes following ligation, may provide a center for nucleophilic substitution catalyzing the reaction GTP --> cGMP. An example is provided for a similar reaction in a derivatized protoheme model compound. The reaction mechanism attempts to rationalize the relative enzymatic activities of GC, heme-deficient GC, GC-CO, and GC-NO on a common basis and makes predictions for new activators that may be discovered in the future.

Kinetics and equilibria of soluble guanylate cyclase ligation by CO: effect of YC-1

Previous work has proved that the enzyme-soluble guanylate cyclase, GC, is activated several 100-fold by the combination of carbon monoxide plus a benzylindazole derivative called YC-1. That is about the same as activation by nitric oxide, which has a well-established role both in vivo and in vitro. This report addresses several spectroscopic, equilibrium, and kinetic effects wrought by YC-1 on carboxyl guanylate cyclase, including the following: a shift in the Soret absorption band by 4 nm to shorter wavelength; an increase in CO affinity by an order of magnitude; a dramatic change in the kinetics of CO association. After photolytic dissociation of CO, the majority, but not all, of bimolecular ligand recombination occurs with a time constant about 1000-fold faster than in the absence of YC-1, while a smaller fraction recombines almost, but not quite, the same as usual. This is reminiscent of the kinetics of NO association with GC, which also shows two prominent phases. The results just listed pertain in the presence of GTP/cGMP, which would be present during enzyme catalysis. Qualitatively similar, but smaller, effects occur in the absence of GTP/cGMP. Measurements are reported to characterize other changes in buffer conditions. The results are consistent with a mechanistic model that attributes a crucial role to the proximal bond that connects the heme iron to a histidine side chain in GC but also requires protein control of the distal environment.

Kinetics of NO ligation with nitric-oxide synthase by flash photolysis and stopped-flow spectrophotometry

Nitric-oxide synthase (NOS) catalyzes conversion of L-arginine to nitric oxide, which subsequently stimulates a host of physiological processes. Prior work suggests that NOS is inhibited by NO, providing opportunities for autoregulation. This contribution reports that NO reacts rapidly (ka congruent with 2 x 10(7) M-1 s-1) with neuronal NOS in both its ferric and ferrous oxidation states. Association kinetics are almost unaffected by L-arginine or the cofactor tetrahydrobiopterin. There is no evidence for the distinct two phases previously reported for association kinetics of CO. Small amounts of geminate recombination of NO trapped in a protein pocket can be observed over nanoseconds, and a much larger amount is inferred to take place at picosecond time scales. Dissociation rates are also very fast from the ferric form, in the neighborhood of 50 s-1, when measured by extrapolating association rates to the zero NO concentration limit. Scavenging experiments give dissociation rate constants more than an order of magnitude slower: still quite fast. For the ferrous species, extrapolation is not distinguishable from zero, while scavenging experiments give a dissociation rate constant near 10(-4) s-1. Implications of these results for interactions near the heme binding site are discussed.

Soluble guanylate cyclase: effect of YC-1 on ligation kinetics with carbon monoxide

Recently it has been reported that in the presence of YC-1, a benzyl indazole derivative, carbon monoxide activates soluble guanylate cyclase, GC, to about the same extent as its best known activator, nitric oxide. Kinetic studies utilizing flash photolysis of GC complexed with CO in the presence and absence of YC-1 show, in contrast to another recent report of a mixing experiment, that YC-1 has a profound effect on bimolecular association kinetics and a smaller, but significant, effect on ligand affinity. Most prominent is the appearance of a major, new phase in the bimolecular recombination kinetics in the presence of 200 microM YC-1: This major fraction rebinds CO approximately 1000-fold more rapidly than in the absence of YC-1. Another portion, considerably less than half, exhibits kinetics that are almost exactly the same as in the absence of YC-1. It is now clear that both YC-1 and CO have a strong synergistic effect on enzyme activity and also a dramatic effect on ligand binding behavior. It is, therefore, a reasonable inference that ligand binding at the heme iron atom is intimately connected with enzyme activation, a hypothesis that would have been difficult to maintain if the earlier report, that YC-1 has no effect on CO binding, were correct. Possible reasons for the discrepancy between the two measurements are suggested.

Dissociation of nitric oxide from soluble guanylate cyclase

Kinetic studies of soluble guanylate cyclase complexed with nitric oxide prove that NO dissociation in the presence of the substrate GTP and Mg2+ is as much as 50 times faster than in their absence. In the presence of those two reagents the dissociation rate constant is k(obs) = 0.04 +/- 0.01 s-1 at 20 degrees C, which is by far the fastest NO dissociation rate constant ever reported for a ferrous heme protein. Extrapolated to 37 degrees C, this corresponds to a half life of about 5 s for NO dissociation from soluble guanylate cyclase at physiological conditions, which is presumably fast enough to account for deactivation of the enzyme in biological systems. Dissociation rate constants are also reported for a variety of other reagent conditions.

Kinetics of nitric oxide dissociation from five- and six-coordinate nitrosyl hemes and heme proteins, including soluble guanylate cyclase

Kinetics of NO dissociation were characterized for three five-coordinate systems, heme-NO, HSA-heme-NO (human serum albumin), GC-NO (soluble guanylate cyclase), and for the six-coordinate system, Im-heme-NO. Nitrosyl myoglobin was redetermined for comparison. Previously known, six-coordinate R and T state nitrosyl hemoglobins are also included in the comparison. The data indicate that NO dissociates more than 1000 times faster from five-coordinate model heme than it does from the six-coordinate analog. Such a negative trans-effect between NO and a proximal base is in sharp contrast to carboxy heme derivatives, in which ligand dissociation rates are greatly slowed in when a trans base is present. As a result of opposite trans-effects, six-coordinate carboxy and nitrosyl derivatives have comparable dissociation rates, even though the five-coordinate species are very different. In proteins, five- and six-coordinate forms do not show a large difference in dissociation rates. Part of the reason may be due to different probabilities for geminate recombination in the different proteins, but this cannot explain all the facts. There must also be influences of the protein structure on bond-breaking rate constants themselves. With the exception of hemoglobin in the T state, nitrosyl guanylate cyclase shows the highest NO dissociation rate constant, k(obs) = 6 x 10(-4) s(-1). This would yield a half-life of about 2 min at 37 degrees C for dissociation of NO from GC-NO, a number that has implications for the mechanism of regulation of the activity of this key heme enzyme.

Kinetics of CO ligation with nitric-oxide synthase by flash photolysis and stopped-flow spectrophotometry

Interaction of CO with hemeproteins has physiological importance. This is especially true for nitric-oxide synthases (NOS), heme/flavoenzymes that produce .NO and citrulline from L-arginine (Arg) and are inhibited by CO in vitro. The kinetics of CO ligation with both neuronal NOS and its heme domain module were determined in the presence and absence of tetrahydrobiopterin and Arg to allow comparison with other hemeproteins. Geminate recombination in the nanosecond time domain is followed by bimolecular association in the millisecond time domain. Complex association kinetics imply considerable heterogeneity but can be approximated with two forms, one fast (2-3 x 10(6) M-1 s-1) and another slow (2-4 x 10(4) M-1 s-1). The relative proportions of the two forms vary with conditions. For the heme domain, fast forms dominate except in the presence of both tetrahydrobiopterin and Arg. In the holoenzyme, slow forms dominate except when both reagents are absent. Geminate recombination is substantial, approximately 50%, only when fast forms predominate. Stopped-flow mixing found dissociation constants near 0.3 s-1. These data imply an equilibrium constant such that very little CO should bind at physiological conditions unless large CO concentrations are present locally.

Evidence for a slow tertiary relaxation in the reaction of tert-butyl isocyanide with horseradish peroxidase

The kinetics of tert-butyl isocyanide binding to the heme protein horseradish peroxidase (HRP) at 22 degrees C was examined on all time scales, from minutes to picoseconds, in aqueous borate buffer at pH 9.08. Unlike myoglobin (Mb) or hemoglobin, HRP shows two bimolecular ligand binding processes. For comparison, binding of the same ligand with Mb was measured under identical conditions. Ligand entry into the protein from the solvent in a mixing experiment is extremely slow in HRP: the bimolecular association constant is 0.04 M-1 s-1, while in Mb it is 4 x 10(3) M-1 s-1. Surprisingly, in view of that difference, picosecond and nanosecond photolyses reveal that once the ligand has reached the iron(II) site there is no difference in cage return or escape from the protein. The rate for the fastest cage return (from the contact pair) is close to 6 x 10(10) s-1 in both proteins. The rates of escape from the contact pair to form a secondary protein-caged pair are also similar: for Mb, 10 x 10(10) s-1, and for HRP, 8.5 x 10(10) s-1. The rate of rebinding from the protein-separated cage is near 4 x 10(6) s-1 in both proteins, and the rate of escape from protein to solvent is close to 3.7 x 10(6) s-1 in both. The difference between the two proteins lies in the low-millisecond time domain. After flash photolysis of HRP, there is a concentration-dependent recombination not seen in mixing experiments. This bimolecular rate constant varies slightly for different HRP preparations, being 2.6 x 10(4) or 4.0 x 10(4) M-1 s-1 in two cases, both of which are much faster than is observed in mixing experiments, namely, 0.04 M-1 s-1. In Mb, photolysis and mixing experiments consistently give the same combination rate, which is somewhat slower than the faster part of the HRP recombination. Similar measurements for the smaller ligand methyl isocyanide revealed no anomalous behavior. The interpretation proposed involves tertiary relaxation after ligand escape, which is significant in blocking the return of the large t-BuNC, but has no apparent effect on smaller ligands. Thus, HRP-t-BuNC reveals in dramatic fashion a phenomenon merely hinted at in earlier work involving the T-state binding kinetics of hemoglobin.

Kinetics of nitrosation of thiols by nitric oxide in the presence of oxygen

Nitrosothiols are powerful vasodilators. They act by releasing nitric oxide, which activates the heme protein guanylate cyclase. We have studied the kinetics of nitrosothiol formation of glutathione, cysteine, N-acetylcysteine, human serum albumin, and bovine serum albumin upon reaction with nitric oxide (NO) in the presence of oxygen. These studies have been made at low pH as well as at physiological pH. At pH 7.0, contrary to published reports, nitric oxide by itself does not react with thiols to yield nitrosothiol. However, formation of nitrosothiols is observed in the presence of oxygen. For all thiols studied, the rates of nitrosothiol formation were first order in O2 concentration and second order in NO concentration and at lower concentrations (< 5 mM thiol) also depended on thiol concentrations. Analysis of the kinetic data indicated that the rate-limiting step was the reaction of NO with oxygen. Analysis of the reaction products suggest that the main nitrosating species is N2O3: RSH+N2O3-->RSNO+NO2- + H+. Rate constants for this reaction for glutathione and several other low molecular weight thiols are in the range of 3-1.5 x 10(5) M-1 s-1, and for human and bovine serum albumins 0.3 x 10(5) M-1 s-1 and 0.06 x 10(5) M-1 s-1, respectively. The data further indicate that the reaction rate of the nitrosating species N2O3 with thiols is competitive with its rate of hydrolysis. At physiological concentrations nitrosoglutathione formation represents a significant metabolic fate of N2O3, and at glutathione concentrations of 5 mM or higher almost all of N2O3 formed is consumed in nitrosation of glutathione. Implications of these results for in vivo nitrosation of thiols are discussed.

Basis of guanylate cyclase activation by carbon monoxide

Kinetics of CO association with guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] and dissociation from carboxy guanylate cyclase have been studied at pH 7.5 by flash photolysis, yielding rate constants at 23 degrees C of 1.2 +/- 0.1 x 10(5) M-1.sec-1 and 28 +/- 2 sec-1, respectively. While the CO combination rate constant is the same as for the T state of hemoglobin, the CO dissociation rate constant is much higher than expected for a six-coordinate carboxyheme protein; yet the absorption spectrum is indicative of a six-coordinate heme. The two observations are reconciled by a reaction mechanism in which CO dissociation proceeds via a five-coordinate intermediate. This intermediate is structurally very similar to the five-coordinate nitrosyl heme derivative of guanylate cyclase and is presumably responsible for the observed 4-fold activation of guanylate cyclase by CO. Thus, we provide a model that explains enzyme activities of the nitrosyl and carboxy forms of the enzyme on the basis of a common mechanism.

Myoglobin-NO at low pH: free four-coordinated heme in the protein pocket

In either sperm whale or horse heart myoglobin, binding of NO and lowering of solution pH work together to weaken, and ultimately break, the bond between iron and the proximal histidine. This is reminiscent of the reaction observed at neutral pH in the case of guanylate cyclase, the heme enzyme that catalyzes the conversion of GTP to cGMP. Bond breaking is characterized by a spectral change from a nine-line to a three-line ESR signal and accompanied by a shift from 420 to 387 nm in the UV-vis spectrum of the Soret band maximum. Analysis of the pH-dependent spectral changes shows that they are reversible, at least within a few hours, that the transition is cooperative, involving six protons during pH lowering but only two as it is raised, and that the pK is about 4.7. Different proteins exhibit different pK values, which are generally lower than that for "chelated" protoheme. The pK differences reflect the extra bond stability afforded by the protein structure. Investigations of thermal and photochemical NO displacement by CO suggest that the local pocket around the ligand, although significantly altered (according to circular dichroism investigations), nonetheless still imposes a barrier against the outward diffusion of ligand into the solvent. Nanosecond and picosecond flash photolysis shows that in proteins at low pH there is an extremely efficient geminate recombination of the ligand with the four-coordinated species through a single-exponential process. This occurs to a significantly larger extent than for the case of NO-"chelated" protoheme (where no distal barrier for ligand is present). At neutral pH, when the proximal histidine bond is intact, the geminate recombination for NO takes longer and displays multiexponential kinetics. Altogether, these results suggest that, even though distal effects probably also play a role, proximal effects make an important contribution in modulating ligand-iron bond formation.

Geminate recombination of diatomic ligands CO, O2, and NO with myoglobin

The kinetics of geminate recombination of horse heart myoglobin with the diatomic ligands carbon monoxide, dioxygen, and nitric oxide have been reexamined. The new measurements are distinguished from previous studies by (1) consideration of the complete time range longer than 1 ps, (2) inclusion of the effect of temperature changes near ambient, (3) attention to the relation between recombination kinetics and the yield of dissociated partners on the millisecond time scale, and (4) use of singular value decomposition in the analysis. These picosecond results, together with earlier nanosecond data, for O2 prove that models incorporating one, two, or even three discrete intermediates are not sufficient to account for all features of geminate recombination kinetics. Instead, a continuous evolution of the geminate pair distribution is preferred.

Kinetics of nitric oxide autoxidation in aqueous solution

A kinetic study of the reaction of NO with O2 in aqueous solution has been carried out using a colorimetric method based on a pH indicator and a stopped-flow spectrophotometer. The reaction is second order in NO concentration and first order in O2 concentration; the overall third order rate constant is 6.3 (+/- 0.4) x 10(6) (M-1)2 s-1. Model calculations for the reaction at estimated physiological concentrations of NO and O2 indicate that the simple autoxidation of NO will not limit its diffusion from the site of production in endothelial cells to a spatially removed target molecule such as guanylate cyclase in myocytes and platelets.

Gastrointestinal hemorrhage in AIDS: arteriographic diagnosis and transcatheter treatment

The usefulness of arteriography and transcatheter treatment was studied in nine patients with gastrointestinal hemorrhage and acquired immunodeficiency syndrome (AIDS). Selective arteriography was performed in all patients; transcatheter treatment was performed by means of embolotherapy or selective vasopressin infusion. Medical records were reviewed to determine the cause of hemorrhage and clinical outcome. Arteriography enabled identification of the site of hemorrhage in seven patients. Hemorrhage was caused by Kaposi sarcoma (n = 2), cytomegalovirus colitis (n = 1), lymphoma (n = 2), or unknown causes (n = 4). Neovascularity and dense parenchymal stain were present in patients with Kaposi sarcoma. Transcatheter treatment consisted of embolization (n = 3), vasopressin infusion (n = 2), or both (n = 1). Hemorrhage was controlled in six cases in which transcatheter treatment was administered. Complications included thrombosis of the femoral artery in a 7-month-old infant and formation of a pseudoaneurysm of the femoral artery, which was treated successfully with ultrasound-guided compression. In patients with AIDS and profuse gastrointestinal hemorrhage, arteriography often enables identification of a specific site of hemorrhage, which can be stopped with transcatheter treatment.

Quaternary structure and geminate recombination in hemoglobin: flow-flash studies on alpha 2CO beta 2 and alpha 2 beta 2CO

The kinetics of geminate recombination for the diliganded species alpha 2CO beta 2 and alpha 2 beta 2CO of human hemoglobin were studied using flash photolysis. The unstable diliganded species were generated just before photolysis by chemical reduction in a continuous flow reactor from the more stable valency hybrids alpha 2CO beta 2+ and alpha 2+ beta 2CO, which could be prepared by high pressure liquid chromatography. Before the flash photolysis studies, the hybrids had been characterized by double-mixing stopped-flow kinetics experiments. At pH 6.0 in the presence of inositol hexaphosphate (IHP) both of the diliganded species show second order kinetics for overall addition of a third CO that is clearly characteristic of the T state (l' = 1-2 x 10(5) M-1 s-1), whereas at higher pH and in the absence of IHP they show combination rates characteristic of an R state. The kinetics of geminate recombination following photolysis of a bound CO, however, showed little dependence on pH and IHP concentration. This surprising observation is explained on the basis that the kinetics of geminate recombination of CO primarily depends on the tertiary structure of the ligand binding site, which apparently does not differ much between the R state and the liganded T state formed on adding IHP in this system. Since this explanation requires distinguishing different tertiary structures within a particular quaternary structure, it amounts to a contradiction to the two-state allosteric model.

A new mode for heme-heme interactions in hemoglobin associated with distal perturbations

The distal side of the heme pocket, known to regulate ligand affinity, is shown to be directly involved in subunit interactions. Valency hybrids with oxygen or carbon monoxide bound to the reduced chain are used to model R-state hemoglobin with different distal perturbations. Electron paramagnetic resonance of the oxidized chains shows that the carbon monoxide perturbation is transmitted between subunits to the distal histidine and the oxidized iron center. A comparison of hybrids with only one type of chain oxidized and hybrids with a single alpha beta dimer oxidized is consistent with this perturbation being transmitted across the alpha 1 beta 1 interface. This represents a new mode of subunit interactions in hemoglobin.

Studies on cytosolic guanylate cyclase from human placenta

We have purified the soluble form of guanylate cyclase from human placenta greater than 2400-fold. The enzyme shared several characteristics with the enzyme purified from other sources including molecular mass and subunit composition, activation by divalent cations, inhibition by ATP and Michaelis constants. The enzyme, however, had a lower absorption maximum in the Soret region (417 +/- 1 nm) than the enzyme from other sources and was activated only one-fifth as much by nitric oxide as the bovine lung enzyme. It appears that the heme prosthetic group in the human placental enzyme may be hexa-coordinate and in the bovine lung enzyme the heme group may be penta-coordinate.

Double-mixing kinetic studies of the reactions of monoliganded species of hemoglobin: alpha 2(CO)1 beta 2 and alpha 2 beta 2(CO)1

The kinetics of CO association to and dissociation from the two isomers of monoliganded species alpha ICO beta I(alpha II beta II) and alpha I beta I (alpha II beta COII) has been studied by double-mixing stopped-flow and microperoxidase methods. The monoliganded species were generated by hybridization between excess ferric Hb and alpha CO2 beta +2 or alpha +2 beta CO2 prepared by high-pressure liquid chromatography (HPLC). The results indicated that: 1) there were no significant differences in the reactivities of alpha and beta chains in the first step of ligation; 2) in the second step of ligation there was significant cooperativity in the reaction of deoxyhemoglobin with 0.05 or 0.1 equivalent of CO. Diliganded species were therefore formed in significant amounts. The double-mixing HPLC results suggested that in the second step of ligation alpha chains reacted faster than the beta chains, and the main diliganded species formed was alpha I beta ICO (alpha IICO beta II) or its isomer alpha ICO I(alpha II beta IICO). These results seem to indicate that the reaction of the first CO is mostly random and in the second step of ligation CO binds more to the tetramers in which one beta chain is already ligated: alpha I beta I (alpha II beta II) + CO----alpha ICO beta I (alpha II beta II) and alpha I beta ICO (alpha II beta II) + CO----alpha I beta ICO (alpha IICO beta II).

Double mixing stopped-flow method for the study of equilibria and kinetics of dimer-tetramer association of hemoglobins: studies on Hb Carp, Hb A, and Hb Rothschild

A double mixing stopped-flow method is described for studying the dimer-tetramer equilibria of oxyhemoglobins and the kinetics of association of unliganded dimers. The three hemoglobins studied were: Hb Carp, Hb A, and Hb Rothschild (Trp beta 37 (C3)----Arg). The new method reproduces the data obtained for oxyHb A by other established methods. In agreement with previous studies, the new method indicates little, if any, dissociation of oxyHb carp into dimers even in 2 M urea solutions (0.1 M Bis-Tris pH 7.0). OxyHb Rothschild, on the other hand, is extensively dissociated into dimers (K(Hb4L4 in equilibrium with 2Hb2) = 37.3 x 10(-6) M) and the rate constant for the association of deoxy dimers of Hb Rothschild is about one-tenth of the value for Hb A indicating that the deoxy tetramer of Hb Rothschild is at least 10 times more dissociated into dimers than deoxyHb A.

Double-mixing kinetic studies of the reactions of methyl isocyanide and CO with diliganded intermediates of hemoglobin: alpha 2CO beta 2 and alpha 2 beta 2CO

Kinetics of the reactions of CO and methyl isocyanide with two diliganded intermediates of hemoglobin, alpha 2CO beta 2 and alpha 2 beta 2CO, have been studied by double-mixing and microperoxidase methods. The valency hybrids were prepared by high-pressure liquid chromatography. The reaction time courses of ligand combination and dissociation with both of the ligands were biphasic, and in CO combination reaction the zero-time amplitudes of the two phases were independent of the protein concentration. In the presence of 2 M urea the reaction time course was clearly dependent on protein concentration, as the zero-time amplitude of the fast phase increased at lower protein concentrations. These two observations indicate that little dissociation of tetramers into dimers occurs in the absence of urea. Consistent with this, the kinetic data for the reactions of CO best fit a reaction model consisting of two tetrameric species not in rapid equilibrium with each other. Various considerations, however, suggest that the reaction model is more appropriately described as 2D in equilibrium R in equilibrium T. The reaction of triliganded species (Hb4(CO)2Me1) with methyl isocyanide was monophasic, and the reaction model suggested a fast T in equilibrium R structural change after the binding of the third ligand. Although the precise structural nature of the two species remains undefined, it is concluded that the biphasicity in the reactions of the two hybrids is characteristic of the diliganded species only and is independent of the nature of the ligand.

Quaternary structure and the geminate recombination of carp hemoglobin with methylisocyanide

The kinetics of geminate recombination were studied for the methylisocyanide derivative of carp hemoglobin. Carp hemoglobin is of interest because it has been established that the fully liganded form switches between a high affinity R state at pH 9 and a low affinity T state at pH 6 in the presence of IHP. Geminate recombination was observed on both the picosecond and the nanosecond time scales under all conditions; however, only a small variation is observed in the rates and the yields of geminate recombination as the protein switches from the R to the T state. Taken together with overall "on" and "off" rates, the data indicate that the change from the R to the T configuration affects bond breaking most, but also influences subsequent escape from the protein as well as both entry into the protein and bond formation. There is some reason to postulate tertiary conformational change in the T state on the microsecond time scale following ligand escape from the protein.

Kinetic studies on partially liganded species of carboxyhemoglobin: (alpha 1CO-beta 1CO)alpha 2 beta 2 or (alpha 2CO beta 2CO)alpha 1 beta 1

Kinetics of CO combination with and dissociation from isomer III, (alpha 1CO beta 1CO)alpha 2 beta 2 or alpha 1 beta 1 (alpha 2CO beta 2CO), and Hb Rothschild have been studied using the double mixing and microperoxidase methods. Isomer III was prepared in a manner so that it was the only reactive species in the reaction mixture. The biphasic reaction time course in both the "on" and "off" reactions of isomer III and the CO combination reaction of Hb Rothschild are attributed to slow relaxation between the fast and slow CO-reacting species in the two proteins: isomer III: l'f = 6 x 10(6) M-1 s-1, l'dimer = 1.7 x 10(6) M-1 s-1, l's = 2.2 x 10(5) M-1 s-1, lf = 0.15 s-1, ls = 0.01 s-1; Hb Rothschild: l'f = 2.8 x 10(6) M-1 s-1; l's = 2.7 x 10(5) M-1 s-1.

Kinetic studies on partially liganded species of carboxyhemoglobin: alpha 2 CO beta 2 and alpha 2 beta 2CO

The valency hybrids of Hb A, alpha 2CO beta 2+, and alpha 2+ beta 2CO have been prepared by a new high pressure liquid chromatography method, and the kinetics of their CO-combination and dissociation reactions have been studied by double mixing and microperoxidase methods. Both reactions are biphasic. The slow phase in CO-combination and the fast phase in CO-dissociation are due to the reactions of alpha CO2 beta T2 or alpha 2 beta 2CO,T. The fast phase in CO-combination reaction has two components, one due to the dimers of the hybrid and the other due to the R-state tetramer. Immediately after the reduction of the valency hybrids, the overall system is represented by the equation: 2 alpha CO beta in equilibrium alpha 2CO beta 2R in equilibrium alpha 2CO beta 2T or (formula: see text) If the solutions are aged for 3-11 s, the R-state population is reduced gradually to a very small size, and the main species after 11 s of aging are dimers and T-state tetramers. Analysis of the kinetic data indicates slow R in equilibrium T equilibria in the absence of phosphates and significant dissociation of the T-state tetramer. It is concluded that the subunit contacts alpha 1-beta 2 (or alpha 2-beta 1) are impaired seriously in the hybrids. Very slow R in equilibrium T relaxation makes these hybrids unlikely intermediates in the sequential binding of CO to Hb tetramer.

Reaction of nitric oxide with heme proteins and model compounds of hemoglobin

Rates for the reaction of nitric oxide with several ferric heme proteins and model compounds have been measured. The NO combination rates are markedly affected by the presence or absence of distal histidine. Elephant myoglobin in which the E7 distal histidine has been replaced by glutamine reacts with NO 500-1000 times faster than do the native hemoglobins or myoglobins. By contrast, there is no difference in the CO combination rate constants of sperm whale and elephant myoglobins. Studies on ferric model compounds for the R and T states of hemoglobin indicate that their NO combination rate constants are similar to those observed for the combination of CO with the corresponding ferro derivatives. The last observation suggests that the presence of an axial water molecule at the ligand binding site of ferric hemoglobin A prevents it from exhibiting significant cooperativity in its reactions with NO.

Picosecond geminate recombination of nitrosylmyoglobins

The kinetics of NO geminate recombination to sperm whale and elephant myoglobins has been studied on the picosecond time scale using an amplified colliding-pulse mode-locked ring dye laser. The dynamics of ligand rebinding are shown to be affected by the distal structure of the protein surrounding the heme pocket.

Hepatic transport and binding of rose bengal in the presence of albumin and gamma globulin

Gamma globulin and albumin are compared with respect to their effects on the hepatic transport of rose bengal and with respect to the rates and affinities with which they bind this dye. The apparent intrinsic clearance of rose bengal is greater in the presence of albumin than in the presence of gamma globulin, and this difference increases with the protein concentration. Because the binding affinities of these proteins also differ, however, it cannot be concluded decisively that the mechanisms of dye removal are distinct. For both proteins the binding reaction rates as measured by stopped-flow spectrophotometry are much faster than the rate of convection along the sinusoid or the rate of removal of free dye by liver cells. The transport data and the binding rate constants are the basis for an extended theoretical model developed and analyzed in an accompanying report.

Geminate recombination of CO in rabbit, opossum, and adult hemoglobins

The geminate recombination of CO with Hb following dissociation by a 10-ns laser pulse has been studied as a function of pH (9.2 and 7.0 without inositol hexaphosphate and 6.0 with inositol hexaphosphate) and temperature (5-35 degrees C). The hemoglobins studied included adult, Rothschild, rabbit, opossum, and carp. Despite significant differences in their structural and functional properties, the first four of these hemoglobins show similar trends in the yields, rates, and activation energies of the geminate recombination. The nature of the "cage recombination" in hemoglobin is discussed in the light of such findings. Neither a slow diffusion model nor a model based upon a specific non-heme binding site accounts for the observations.

Geminate recombination in carboxy hemoglobin A and its relation to overall carbon monoxide reactivity

The geminate recombination of CO with carboxy hemoglobin (Hb4(CO)3) following a ten nanosecond laser pulse and the overall combination of the fourth CO with Hb4(CO)3 has been studied as a function of pH in the presence and absence of inositol hexaphosphate. The results indicate that the kinetics of both reactions are independent of pH and phosphate concentration. The results are discussed in terms of a two-step mechanism: a pre-equilibrium step followed by heme--ligand bond formation. The latter is also known as the geminate recombination reaction (Hb + CO in equilibrium Hb X CO in equilibrium HbCO).

Reaction of nitric oxide with heme proteins: studies on metmyoglobin, opossum methemoglobin, and microperoxidase

Kinetic and EPR studies show that the first step in the reaction of NO with ferric myoglobin, opossum hemoglobin, and microperoxidase is the reversible formation of the H-NO complex: H + NO in equilibrium H-NO (where H = Mb+, or Hb+ OP, or MP+). The NO-combination rates are markedly affected by the presence or absence of the distal histidine. The distal histidine significantly reduces the NO-combination rates, perhaps by interaction between the distal histidine and the ferric iron. Thus the beta-chains of Hb+ OP and metmyoglobin show similar combination rates. In the absence of a distal histidine, the NO-combination rates in the alpha-chains of Hb+ OP are much faster and similar to those observed for the five-coordinate heme in microperoxidase. The loss of a water molecule from the six-coordination site is assumed to be the rate-limiting step.

Studies of human hemoglobin intermediates. The double mixing method for studying the reactions of the species Hb4(CO) and Hb4(CO)2

Using the double mixing method we have studied the reactions of the partially liganded species (Hb4, Hb4L1, Hb4L2, Hb4L3) of normal human hemoglobin with carbon monoxide. In the first mixing, oxygen is removed from the species Hb4(O2) chi (CO) gamma and at the second mixing the species Hb4(CO) gamma reacts with CO. At 90% saturation of oxyHb with CO the main intermediate species are Hb4(CO)3 and Hb4(CO)2, and at 10% saturation Hb4 and Hb4(CO). The four CO-combination rate constants determined are: l'1 = 1 X 10(5) M-1 S-1, l'2 = 7 X 10(5) M-1 S-1, l'3 = 2 X 10(5) M-1 S-1 and l'4 = 4.8 X 10(6) M-1 S-1. The results indicate that there is no monotonic increase in the successive CO-combination rate constants. It is difficult to explain these results on the basis of the two-state model (Monod et al., 1965) or the stereochemical model of Perutz (1970).

Functional studies on hemoglobin opossum. Conclusions drawn regarding the role of the distal histidine

In the hemoglobin of the opossum, the alpha chains have different residues at positions E7 and E11 than do most other mammalian hemoglobins. In the opossum, the usual histidine at alpha E7 is replaced by glutamine, the valine at alpha E11 by isoleucine, and the hemoglobin is known to have a low oxygen affinity and a low Hill coefficient. Comparison of kinetic studies of opossum hemoglobin with normal human hemoglobin shows that alpha chains in Hb opossum, despite the lack of distal histidine, do not differ significantly in CO-combination rates in either the T or R states. These rates are much slower than the rates reported for Hb Zurich, the hemoglobin from Chironomus thumi thumi, or the monomeric components of glycera hemoglobin, all of which also have a different residue at E7. As compared with Hb A, the changes in ligand affinities in the T and R states are small and cannot account for the unusually high values of p50 for Hb opossum. The equilibrium and kinetic data indicate that the L = (T)/(R) is about 100 times higher for Hb opossum than for Hb A; CCO = KR/KT approximately equal to 0.014. The kinetic data on l'4 and l also indicate that the R leads to T equilibrium for Hb4(CO)4 and Hb4(CO)2 can be shifted in either direction by adding inositol hexaphosphate or by changing the pH.

The interaction of heme-hemopexin with CO

The equilibria and kinetics of the reaction of heme-hemopexin with CO were studied. A stoichiometry of one CO/heme was determined, and the affinity of heme-hemopexin for CO was found to be pH-dependent. At pH 8.0, the affinity constant was 4.5 X 10(5) M-1 compared with 4 X 10(6) M-1 at pH 6.1. The kinetics of CO binding were also pH-dependent. A biphasic reaction at neutral pH could be resolved into a faster phase (kon = 2.2 X 10(3) M-1 s-1) solely found at pH 6.0, and a slower phase (kon = 2.0 X 10(2) M-1 s-1) solely found at pH 8.0. The dissociation reaction on the other hand was found to be independent of pH in the range examined (koff = 5 X 10(-4) s-1).

Interaction of sickle cell hemoglobin with erythrocyte membranes

The interactions of hemoglobin S with the erythrocyte membrane were compared with the corresponding interactions of hemoglobin A by measuring in both steady-state and kinetic experiments the quenching of the fluorescence of a probe embedded in erythrocyte membranes. Whereas hemoglobin A could be dissociated from membranes, a fraction of hemoglobin S was irreversibly bound even in the oxy state. Deoxyhemoglobin S interacted much more strongly with erythrocyte membranes than did deoxyhemoglobin A: a portion of the deoxyhemoglobin S was irreversibly bound, and the reversibly bound fraction of hemoglobin S dissociated more slowly than did deoxyhemoglobin A. It is suggested that the binding of deoxyhemoglobin S is a two-step reaction in which the first step involves electrostatic interaction with band III erythrocyte membrane protein and the second step involves a hydrophobic interaction with membrane lipids. The latter reaction reflects the greater hydrophobicity of hemoglobin S. The unique interaction of hemoglobin S with erythrocyte membranes may be important in the formation of irreversibly sickled cells.

Diffusion coefficients of hemoglobin by intensity fluctuation spectroscopy: effects of varying pH and ionic strength

Measurements of the mutual diffusion coefficients (D) of the liganded human hemoglobins (Hb) oxy-HbA and oxy-HbS were performed as a function of Hb concentration (CHb), pH, and ionic strength (tau) by intensity fluctuation spectroscopy (IFS). Average diffusion coefficients, (D), and normalized variances, ((D/(D) - 1)2), were recorded. Results are reported and select features are discussed quantitatively. (a) for tau = 0.15 M, the shape of the (d) vs. CHb curve is found to vary with pH. We developed a precise description of this effect in the form of an algebraic relationship between (D), CHb, and Z, the titration charge. (b) only slight differences between the (D) values of oxy-HbS and oxy-HbA are observed, at tau = 0.15 M, for CHb Less Than or Equal To 10 g%. These differences are explained by the theory of part a. (c) No evidence of aggregation is found in solutions of oxy-HbA or oxy-HbS, at tau = 0.15 M, for CHb Less Than or Equal To 10 g%. (d) Indications of aggregation appear in oxy-HbA solutions at very low concentrations of salt. An estimate is made of the extent of aggregation, and the average radius of a cluster is determined.

Kinetic study of the interaction of oxy- and deoxyhemoglobins with the erythrocyte membrane

Changes in fluorescence intensity of a membrane-embedded probe were used to study the kinetics of binding of oxy- and deoxyhemoglobin to erythrocyte membranes. For these studies, stopped-flow fluorimetric techniques were utilized. Both binding and dissociation of hemoglobin from membranes followed heterogeneous first-order kinetics. The rate constants for binding of oxyhemoglobin were about 10 times larger than those of deoxyhemoglobin; the dissociation rate constants of oxyhemoglobin were about one-quarter those of the unliganded form. The results are discussed in light of the steady-state binding constants previously derived for both oxy- and deoxyhemoglobin.