Oxidative decarboxylation of benzoate to carbon dioxide by rat liver microsomes: a probe for oxygen radical production during microsomal electron transfer

In Vitro and In Vivo Metabolism of a Novel Antimitochondrial Cancer Metabolism Agent, CPI-613, in Rat and Human

CPI-613, an inhibitor of pyruvate dehydrogenase (PDH) and α-ketoglutarate dehydrogenase (KGDH) enzymes, is currently in development for the treatment of pancreatic cancer, acute myeloid leukemia, and other cancers. CPI-613 is an analog of lipoic acid, an essential cofactor for both PDH and KGDH. Metabolism and mass balance studies were conducted in rats after intravenous administration of [14C]-CPI-613. CPI-613 was eliminated via oxidative metabolism followed by excretion of the metabolites in feces (59%) and urine (22%). β-Oxidation was the major pathway of elimination for CPI-613. The most abundant circulating components in rat plasma were those derived from β-oxidation. In human hepatocytes, CPI-613 mainly underwent β-oxidation (M1), sulfur oxidation (M2), and glucuronidation (M3). The Michaelis-Menten kinetics (Vmax and Km) of the metabolism of CPI-613 to these three metabolites predicted the fraction metabolized leading to the formation of M1, M2, and M3 to be 38%, 6%, and 56%, respectively. In humans, after intravenous administration of CPI-613, major circulating species in plasma were the parent and the β-oxidation derived products. Thus, CPI-613 metabolites profiles in rat and human plasma were qualitatively similar. β-Oxidation characteristics and excretion patterns of CPI-613 are discussed in comparison with those reported for its endogenous counterpart, lipoic acid. SIGNIFICANCE STATEMENT: This work highlights the clearance mechanism of CPI-613 via β-oxidation, species differences in their ability to carry out β-oxidation, and subsequent elimination routes. Structural limitations for completion of terminal cycle of β-oxidation is discussed against the backdrop of its endogenous counterpart lipoic acid.

Degradation mechanisms and kinetic studies for the treatment of X-ray contrast media compounds by advanced oxidation/reduction processes

The presence of iodinated X-ray contrast media compounds (ICM) in surface and ground waters has been reported. This is likely due to their biological inertness and incomplete removal in wastewater treatment processes. The present study reports partial degradation mechanisms based on elucidating the structures of major reaction by-products using gamma-irradiation and LC-MS. Studies conducted at concentrations higher than observed in natural waters is necessary to elucidate the reaction by-product structures and to develop destruction mechanisms. To support these mechanistic studies, the bimolecular rate constants for the reaction of OH and e(-)(aq) with one ionic ICM (diatrizoate), four non-ionic ICM (iohexol, iopromide, iopamidol, and iomeprol), and the several analogues of diatrizoate were determined. The absolute bimolecular reaction rate constants for diatrizoate, iohexol, iopromide, iopamidol, and iomeprol with OH were (9.58 +/- 0.23)x10(8), (3.20 +/- 0.13)x10(9), (3.34 +/- 0.14)x10(9), (3.42 +/- 0.28)x10(9), and (2.03 +/- 0.13) x 10(9) M(-1) s(-1), and with e(-)(aq) were (2.13 +/- 0.03)x10(10), (3.35 +/- 0.03)x10(10), (3.25 +/- 0.05)x10(10), (3.37 +/- 0.05)x10(10), and (3.47 +/- 0.02) x 10(10) M(-1) s(-1), respectively. Transient spectra for the intermediates formed by the reaction of OH were also measured over the time period of 1-100 micros to better understand the stability of the radicals and for evaluation of reaction rate constants. Degradation efficiencies for the OH and e(-)(aq) reactions with the five ICM were determined using steady-state gamma-radiolysis. Collectively, these data will form the basis of kinetic models for application of advanced oxidation/reduction processes for treating water containing these compounds.

Chemical interactions between additives in foodstuffs: a review

This paper critically reviews the key literature on food additive-additive chemical interactions published over the last 30 years together with appropriate relevant information on food additive-food component interactions. Five main classes of food additive are included, reflecting the research effort to date: the sulfur (IV) species of preservatives, synthetic food colouring materials, nitrate and nitrite, ascorbic acid, and sorbic acid. Within each class, aspects of the chemistry (reactivity), functionality, stability, use and reactions with other specific food additives are reviewed. Where appropriate, the importance of interactions of food additives with other components of food (i.e. nutrients and non-nutrients) has been assessed and certain aspects of toxicology included. The practical outcome of this review is presented as a set of recommendations for future research in this area. The use of the data in this review is proposed as a training set to develop the framework into a diagnostic tool. This might be used ultimately for the development of a multilevel framework, operating systematically, to understand the important parameters that dictate the outcome of additive interactions.

Decarboxylation of [1-(13)C]leucine by hydroxyl radicals

The decarboxylation of [1-13C]leucine by hydroxyl radicals was studied by using gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to follow the production of 13CO2. A Fenton reaction between a (Fe2+)-porphyrin and hydrogen peroxide under aerobic conditions yielded hydroxyl radicals. The decarboxylation rates (VLeu) measured by GC-IRMS were dependent on [1-13C]leucine, porphyrin and hydrogen peroxide concentrations. The 13CO2 production was also dependent on bicarbonate or carbon dioxide added in the reaction medium. Bicarbonate facilitated 13CO2 production, whereas carbon dioxide decreased 13CO2 production. Proton effects on some decarboxylation intermediates could explain bicarbonate or carbon dioxide effects. No effect on the decarboxylation rates was observed in the presence of the classical hydroxyl radicals scavengers dimethyl sulfoxide, mannitol, and uric acid. By contrast, a competitive effect with a strong decrease of the decarboxylation rates was observed in the presence of various amino acids: unlabeled leucine, valine, phenylalanine, cysteine, lysine, and histidine. Two reaction products, methyl-4 oxo-2 pentanoate and methyl-3 butanoate were identified by gas chromatography-mass spectrometry in comparison with standards. The present results suggest that [1-13C]leucine can participate to the coordination sphere of (Fe2+)-porphyrin, with a caged process of the hydroxyl radicals which cannot get out of the coordination sphere.

Oxidative decarboxylation of benzoic acid by peroxyl radicals

A chemical model based on the thermal decomposition of AAPH (2,2'-azobis(2-amidinopropane) dihydrochloride is used for the production of peroxyl radicals. Peroxyl radicals induces the decarboxylation of [7-13C]benzoic acid and the production of 13CO2, which is measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). The decarboxylation depends on temperature, AAPH, and benzoic acid concentrations. The decarboxylation also depends on the presence of oxygen. Electron spin resonance studies are performed to confirm the presence of peroxyl radicals under oxygen and of carbon-centered radicals in the absence of oxygen. Decarboxylation rates are measured in the presence of various antioxidants: ascorbate, dimethylsulfoxide, mannitol, and uric acid. It turns out that the decarboxylation is inhibited by each of these antioxidants. The ratio of decarboxylation rates, with and without the antioxidant, varies linearly with the antioxidant concentration. HPLC and GC-MS analyses of reaction products between benzoic acid and AAPH-derived radicals do not detect the presence of radical substitution products on the aromatic ring or the products derived from benzoic acid. There is no doubt that GC-IRMS is a powerful technique to investigate the effects of peroxyl radicals on benzoic acid. In addition, it is possible to follow the degradation of 13C-labeled chemical targets exposed to peroxyl radicals through the production of 13CO2.

Determination of hydroxyl radicals using salicylate as a trapping agent by gas chromatography-mass spectrometry

OBJECTIVES

To establish a sensitive method for measuring hydroxyl radical formation in biological systems using salicylate as probe.

METHODS

Salicylate hydroxylation products and related aromatic compounds were extracted and converted to trimethylsilyl (TMS) derivatives. The derivatives were analyzed by gas chromatography-mass spectrometry (GC-MS). Quantitation was achieved by selected-ion-recording (SIR) with benzoic acid (ring-D5) as an internal standard.

RESULTS

All compounds were well separated and specifically quantitated by a GC-MS procedure. Standard curves were linear in the concentration ranges investigated (0.1-10 nmol) for all individual compounds. Recovery from human plasma was in the range of 90-102%. The detection limit was between 50 fmol-1 pmol per 1 microL injection. The within-run and between-run coefficients of variation were between 4.6-9.1%. We were able to detect the baseline levels of hydroxylation products in human fibroblasts after incubation with salicylate.

CONCLUSIONS

The GC-MS assay presented here can specifically identify and quantitate salicylate hydroxylation products and related aromatic compounds, which can be used as an in vivo marker of oxidative stress. This sensitive method has broad applications, both in the area of free radical medicine and in the pharmacological study of aspirin and its metabolites.

Application of chemical cytochrome P-450 model systems to studies on drug metabolism--VIII. Novel metabolism of carboxylic acids via oxidative decarboxylation

The oxidative decarboxylation of carboxylic acids by the chemical cytochrome P-450 model and rat liver microsomal systems was investigated. In the chemical system using meso-tetrakis(pentafluorophenyl)porphyrin iron chloride [Fe(TPFPP)Cl] with iodosylbenzene (PhIO), alpha-arylcarboxylic acids and alpha,alpha,alpha-trisubstituted acetic acids are converted to the corresponding one-carbon-reduced alcohol (I) and carbonyl derivatives (II) via oxidative decarboxylation. These products were then used as standards to identify the metabolites in vivo and in vitro. Biliary excretion of Ia and IIa in bile duct-cannulated rats after oral administration of ketoprofen amounted to 0.22 and 0.03% of the dose, respectively. In the case of indomethacin, Ib and IIb were detected as metabolites in the rat liver microsomal system, in yields of 2.8 and 0.29%, respectively. Further, the yields of Ib and IIb were decreased in the presence of SKF-525A. Thus, these metabolites were formed by cytochrome P-450-dependent reactions. Metabolites Ia, Ib, IIa and IIb had moderate to strong inhibitory activities on arachidonic acid-induced platelet aggregation and cyclooxygenase activity in vitro, comparable to those of the parent compounds.

Biotransformation of para-aminobenzoic acid and salicylic acid by PMN

Para-aminobenzoic acid (PABA) is an essential cofactor for the production of folic acid in bacteria and has mild anti-inflammatory activity. We have recently reported that salicylic acid and benzoic acid are oxidized by stimulated granulocytes Polymorphonuclear Neutrophils (PMN). The oxidation of salicylate appears mediated by a potent oxygen metabolite generated during the respiratory burst which is dependent primarily on superoxide (O2-) for its production. These background studies with the salicylate group of drugs suggested that PABA might be similarly metabolized by PMN. In these studies, we demonstrate that PABA is metabolized by stimulated PMN. However, in contrast to the biochemical mechanism involved in the metabolism of salicylate, our scavenger studies indicate that PABA is metabolized primarily by the myeloperoxidase pathway. Our results may explain the mild anti-inflammatory actions of the drug and suggest that the degradation of PABA by PMN at an inflammatory site may limit the availability of PABA for bacterial growth.

Hydroxyl radical-mediated conversion of morphine to morphinone

1. The hydroxyl radical-mediated conversion of morphine to morphinone (MO) was examined as an alternative to the enzymic reaction. 2. Hydroxyl radicals were generated by autoxidation of ascorbate in the presence of iron and EDTA. This system oxidized morphine to MO which was identified by h.p.l.c. and t.l.c. The reaction was dependent on the concentration of added Fe2+ and required the addition of ascorbate when Fe3+ was used. 3. Catalase inhibited production of MO whereas superoxide dismutase (SOD) had no effect. Addition of a large amount of H2O2 to the system resulted in a significant decrease in production of MO. No MO production was initiated by H2O2 itself. The oxidation of morphine was inhibited by typical hydroxyl radical-scavenging agents. These results indicate that morphine undergoes oxidation to MO by hydroxyl radical.

Noninvasive measures of oxidative stress status in humans

Although oxidative stress is thought to be involved in the pathophysiology of several diseases and aging, it is not routinely measured in clinical diagnosis. This is at least partly because accepted and standardized methods for measuring oxidative stress in humans are not yet established. One of the greatest needs in the field of free radical biology is the development of reliable methods for measuring oxidative stress status (OSS) in humans. A listing of some analytical approaches to measuring oxidative stress is provided as well as a listing of some noninvasive techniques that have been used in humans.

The hydroxylation of the salicylate anion by a Fenton reaction and T-radiolysis: a consideration of the respective mechanisms

The yield of 2,3- and 2,5-dihydroxybenzoates (dHB's) from the reaction of .OH radicals with salicylate (SA) ions has been measured as a function of pH and in the presence of oxidants. Under steady-state radiolysis conditions, the production of these products occurs via the reactions .OH + SA----HO-SA. (radical adduct) HO-SA. H+.OH+----2-carboxyphenoxyl radical (SA.) + H2O HO-SA. + SA.----2,3-/2,5-dHB + SA The addition of the oxidants O2, Fe3+ edta, or Fe(CN)63- increases the relative yield of 2,5-dHB/2,3-dHB from about 0.2 to 1. A model to account for this effect is presented. Steady-state radiolyses of 3- and 4-hydroxybenzoate give dihydroxybenzoate products consistent with the phenol group being an ortho-para director in the electrophilic attack of the hydroxyl radical on the aromatic ring. A comparison of product distributions from the reaction of ferrous edta with hydrogen peroxide using salicylate as a scavenger strongly suggests that the same hydroxyl radical adducts are formed as in the radiation experiments.

Hydroxylation of salicylate by activated neutrophils

Salicylates are metabolized in vivo to hydroxylated compounds, including 2,3-dihydroxybenzoic acid and 2,5-dihydroxybenzoic acid (gentisic acid). The present study hypothesized that activated neutrophils represent one pathway for salicylate hydroxylation. Human neutrophils were incubated in medium containing 10 mM salicylate and stimulated with phorbol myristate acetate (PMA) for 1 hr. The cell-free supernatant fractions were analyzed by HPLC. Neutrophils (1 x 10(6) cells) produced 55 +/- 11 ng of gentisic acid. Neutrophils also produced smaller quantities of 2,3-dihydroxybenzoic acid. Antioxidant inhibitor experiments indicated that superoxide dismutase (SOD), heme protein inhibitors, and glutathione blocked gentisic acid formation, whereas catalase, mannitol, and deferoxamine failed to inhibit. Experiments with the reagent hypochlorous acid (HOCl) and the model myeloperoxidase (MPO) enzyme system did not support a role for the MPO pathway in gentisic acid formation. These findings demonstrate that activated neutrophils can hydroxylate salicylate by an unknown pathway. This pathway may contribute to the increased recovery of hydroxylated salicylates in patients with inflammatory disorders.

Oxygen radical generation by microsomes: role of iron and implications for alcohol metabolism and toxicity

Experiments were carried out to evaluate whether the molecular mechanism for ethanol oxidation by microsomes, a minor pathway of alcohol metabolism, involved generation of hydroxyl radical (.OH). Microsomes oxidized chemical .OH scavengers (KMB, DMSO, t-butyl alcohol, benzoate) by a reaction sensitive to catalase, but not SOD. Iron was required for microsomal .OH generation in view of the potent inhibition by desferrioxamine; however, the chelated form of iron was important. Microsomal .OH production was effectively stimulated by ferric EDTA or ferric DTPA, but poorly increased with ferric ATP, ferric citrate, or ferric ammonium sulfate. By contrast, the latter ferric complexes effectively increased microsomal chemiluminescence and lipid peroxidation, whereas ferric EDTA and ferric DTPA were inhibitory. Under conditions that minimize .OH production (absence of EDTA, iron) ethanol was oxidized by a cytochrome P-450-dependent process independent of reactive oxygen intermediates. Under conditions that promote microsomal .OH production, the oxidation of ethanol by .OH becomes more significant in contributing to the overall oxidation of ethanol by microsomes. Experiments with inhibitors and reconstituted systems containing P-450 and NADPH-P-450 reductase indicated that the reductase is the critical enzyme locus for interacting with iron and catalyzing production of reactive oxygen species. Microsomes isolated from rats chronically fed ethanol catalyzed oxidation of .OH scavengers, light emission, and inactivation of added metabolic enzymes at elevated rates, and displayed an increase in ethanol oxidation by a .OH-dependent and a P-450-dependent pathway. It is possible that enhanced generation of reactive oxygen intermediates by microsomes may contribute to the hepatotoxic effects of ethanol.

Do humans neutrophils form hydroxyl radical? Evaluation of an unresolved controversy

Hydroxyl radical is a potent oxidizing agent of potential importance in human pathobiology. Since neutrophilic phagocytes make superoxide and hydrogen peroxide during phagocytosis, it has been proposed that hydroxyl radical is also formed. In this paper we review the literature which supports or refutes formation of hydroxyl radical by neutrophils and the mechanism(s) by which this radical might be formed. We conclude that there is no definitive proof for hydroxyl radical formation by neutrophils. In fact, neutrophil release of lactoferrin and myeloperoxidase appears to limit formation of this radical. Future studies are likely to determine whether superoxide released by neutrophils interacts with target substrates to allow formation of hydroxyl radical.

Ferrous-salt-promoted damage to deoxyribose and benzoate. The increased effectiveness of hydroxyl-radical scavengers in the presence of EDTA

Hydroxyl radicals (OH.) in free solution react with scavengers at rates predictable from their known second-order rate constants. However, when OH. radicals are produced in biological systems by metal-ion-dependent Fenton-type reactions scavengers do not always appear to conform to these established rate constants. The detector molecules deoxyribose and benzoate were used to study damage by OH. involving a hydrogen-abstraction reaction and an aromatic hydroxylation. In the presence of EDTA the rate constant for the reaction of scavengers with OH. was generally higher than in the absence of EDTA. This radiomimetic effect of EDTA can be explained by the removal of iron from the detector molecule, where it brings about a site-specific reaction, by EDTA allowing more OH. radicals to escape into free solution to react with added scavengers. The deoxyribose assay, although chemically complex, in the presence of EDTA appears to give a simple and cheap method of obtaining rate constants for OH. reactions that compare well with those obtained by using pulse radiolysis.

Aromatic hydroxylation as a potential measure of hydroxyl-radical formation in vivo. Identification of hydroxylated derivatives of salicylate in human body fluids

Attack by .OH radicals, generated by a Fenton system, upon salicylate produces 2,3-dihydroxybenzoate and 2,5-dihydroxybenzoate as major products and catechol as a minor product. H.p.l.c. separation combined with electrochemical detection was used to identify and quantify 2,3-dihydroxybenzoate and 2,5-dihydroxybenzoate in human plasma and synovial fluid. We propose that conversion of salicylate into 2,3-dihydroxybenzoate, or of other aromatic compounds into specific hydroxylated products, may be a useful assay for .OH formation in the human body.

Myeloperoxidase oxidation of sulfur-centered and benzoic acid hydroxyl radical scavengers

Myeloperoxidase (MPO) oxidizes sulfur-centered and benzoate hydroxyl radical scavengers through formation of HOCl. Sulfur-centered hydroxyl radical scavengers compete with benzoate as antioxidants of HOCl. We conclude from these observations that competition experiments between benzoate and sulfur-centered hydroxyl radical scavengers are not sufficiently specific to infer participation of hydroxyl radicals in oxidative reactions mediated by neutrophils because of the unique action of MPO in affecting oxidation of the test radical scavengers.

Ethanol oxidation by hydroxyl radicals: role of iron chelates, superoxide, and hydrogen peroxide

Oxygen-derived free radicals such as the hydroxyl radical (.OH) have been shown to mediate the oxidation of ethanol by a variety of oxy radical-generating systems. Among these are microsomal electron transport systems (both intact and purified, reconstituted systems), the coupled oxidation of hypoxanthine or xanthine by xanthine oxidase, and the model iron-ascorbate system. The sequence of reactions leading to the oxy radical-dependent oxidation of ethanol as well as other hydroxyl radical-scavenging agents by these various systems is believed to proceed through the formation of a common intermediate, namely, hydrogen peroxide (H2O2), after dismutation of the superoxide anion radical (O2-.). The presence of iron, especially chelated iron, greatly enhances the production of .OH by serving as an oxidant for O2-. or a reductant for H2O2. Experiments were carried out to evaluate the role of iron, the chelating agent, O2-., and H2O2 in the oxidation of ethanol by a variety of in vitro systems (chemical, enzymatic, and intact membrane bound) that can produce oxy radicals via different mechanisms. The generation of .OH by all the systems studied was sensitive to catalase, which indicates that H2O2 is the precursor of .OH. Superoxide radical appears to be the reducing agent in the hypoxanthine-xanthine oxidase system, indicating an iron-catalyzed Haber-Weiss reaction. In the ascorbate, reductase, and microsomal systems, superoxide radical does not appear to be the reducing agent. However, superoxide radical probably is the precursor of H2O2. While iron plays an important role in the production of .OH by the various systems, the effect of iron depends on the nature of the iron chelate.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of the quinone type drugs on hydroxyl radical (OH.) production by rat liver microsomes

The quinone drugs are known to be metabolized to semiquinone free-radical intermediates and to enhance NADPH oxidation in microsomal system. The effect of adriamycin and mitomycin C on the decarboxylation of [14C] carboxyl benzoate via hydroxyl radical (OH.) production in the microsomal system was examined. The activity of these drugs was compared to 5-fluorouracil, cyclophosphamide, and methotrexate, which are inactive in oxygen consumption experiments and are non-quinone-type drugs. Adriamycin and mitomycin C stimulated decarboxylation of benzoate 100 and 50% above the controls, respectively, while 5-fluorouracil, cyclophosphamide, and methotrexate were not different from controls. Addition of superoxide dismutase increased benzoate decarboxylation with or without the drugs present, while catalase was inhibitory in both circumstances. These results suggest that the quinone drugs enhanced hydroxyl radical (OH.) production by liver microsomes, and offer a possible mechanism of cellular toxicity by these agents.

Effects of mitomycin C on metabolism in a rat liver preparation

Quinone drugs are used extensively as anti-neoplastic agents. The mechanism of their actions and the reasons for their unfavorable side effects are not well understood. Mitomycin C (MC) is an N-heterocyclic quinone with chemotherapeutic action against solid tumors. Previous research has led to the development of a model for drug activation involving NADPH reduction of the drug via microsomal mixed-function oxidases. We tested the possibility that NADPH is provided from the hexose monophosphate shunt (HMPS). The MC did indeed increase HMPS activity aerobically, while not affecting Kreb's cycle activity. Anaerobic stimulation of the shunt is also predicted by the model. However, under hypoxic conditions no HMPS or Kreb's activity was observed in MC-treated or untreated samples. Other investigators have documented the involvement of reactive oxygen species in microsomal systems in vitro. The oxygen requirement for MC stimulation of HMPS suggests oxygen radical involvement. We carried out experiments using [14C]-formate as a scavenger for hydrogen peroxide. There was no apparent change in H2O2 production when MC was added. Catalase is known to be involved in peroxide metabolism in vivo; however, addition of the catalase inhibitor sodium azide did not alter endogenous or MC-stimulated shunt activity. The microsomal inhibitor SKF-525A (10(-3) M) prevented MC stimulation of the HMPS, which is consistent with the model implicating microsomal enzymes in MC metabolism. Overall, we have shown the oxygen dependence of endogenous and MC-stimulated shunt activity, and the results provide evidence for MC activation of oxidative metabolism by a mechanism which involves microsomes.

The role of iron chelates in hydroxyl radical production by rat liver microsomes, NADPH-cytochrome P-450 reductase and xanthine oxidase

Uninduced rat liver microsomes and NADPH-Cytochrome P-450 reductase, purified from phenobarbital-treated rats, catalyzed an NADPH-dependent oxidation of hydroxyl radical scavenging agents. This oxidation was not stimulated by the addition of ferric ammonium sulfate, ferric citrate, or ferric-adenine nucleotide (AMP, ADP, ATP) chelates. Striking stimulation was observed when ferric-EDTA or ferric-diethylenetriamine pentaacetic acid (DTPA) was added. The iron-EDTA and iron-DTPA chelates, but not unchelated iron, iron-citrate or iron-nucleotide chelates, stimulated the oxidation of NADPH by the reductase in the absence as well as in the presence of phenobarbital-inducible cytochrome P-450. Thus, the iron chelates which promoted NADPH oxidation by the reductase were the only chelates which stimulated oxidation of hydroxyl radical scavengers by reductase and microsomes. The oxidation of aminopyrine, a typical drug substrate, was slightly stimulated by the addition of iron-EDTA or iron-DTPA to the microsomes. Catalase inhibited potently the oxidation of scavengers under all conditions, suggesting that H2O2 was the precursor of the hydroxyl radical in these systems. Very high amounts of superoxide dismutase had little effect on the iron-EDTA-stimulated rate of scavenger oxidation, whereas the iron-DTPA-stimulated rate was inhibited by 30 or 50% in microsomes or reductase, respectively. This suggests that the iron-EDTA and iron-DTPA chelates can be reduced directly by the reductase to the ferrous chelates, which subsequently interact with H2O2 in a Fenton-type reaction. Results with the reductase and microsomal systems should be contrasted with results found when the oxidation of hypoxanthine by xanthine oxidase was utilized to catalyze the production of hydroxyl radicals. In the xanthine oxidase system, ferric-ATP and -DTPA stimulated oxidation of scavengers by six- to eightfold, while ferric-EDTA stimulated 25-fold. Ferric-desferrioxamine consistently was inhibitory. Superoxide dismutase produced 79 to 86% inhibition in the absence or presence of iron, indicating an iron-catalyzed Haber-Weiss-type of reaction was responsible for oxidation of scavengers by the xanthine oxidase system. These results indicate that the ability of iron to promote hydroxyl radical production and the role that superoxide plays as a reductant of iron depends on the nature of the system as well as the chelating agent employed.

The generation of hydroxyl and alkoxyl radicals from the interaction of ferrous bipyridyl with peroxides

Reaction conditions by which the iron-chelate ferrous bipyridyl can be used as a Fenton reagent to generate specifically alkoxyl radical (.OR) from its corresponding alkyl hydroperoxide (ROOH) without producing hydroxyl radical (.OH) as a result of autoxidation are described. In this manner, the relative ability of common .OH-scavenging agents to react with .OH and various .OR species could be assessed. When .OH was generated from H2O2, 4-methylmercapto-2-oxobutyrate, ethanol and benzoate all were oxidized. When .OR (cumoxyl radical, t-butoxyl radical or ethoxyl radical) was generated specifically, each was found to oxidize 4-methylmercapto-2-oxobutyrate and ethanol. In contrast with .OH, however, none of the .OR radicals mediated the decarboxylation of benzoate. Cross-competition studies with the scavengers showed that, in contrast with the .OH-dependent reaction, the .OR-dependent oxidation of 4-methylmercapto-2-oxobutyrate and ethanol was not inhibited by benzoate. Rate constants for ferrous bipyridyl oxidation by ROOH and by H2O2 were found to be essentially the same, and therefore the differential oxidation of the various scavengers was not a reflection of iron-peroxide interaction, but rather an interaction between generated oxy radicals and the scavengers. In contrast with the H2O2 system, catalase did not inhibit the oxidation of 4-methylmercapto-2-oxobutyrate or ethanol by either the cumene hydroperoxide or the t-butyl hydroperoxide system, suggesting that the oxidizing species was not derived from H2O2. These results suggest that benzoate decarboxylation might serve as a more specific probe to detect the presence of .OH than either 4-methylmercapto-2-oxobutyrate or ethanol, which react readily with .OR.

Organic hydroperoxide-dependent oxidation of ethanol by microsomes: lack of a role for free hydroxyl radicals

Organic hydroperoxides can replace NADPH in supporting the oxidation of ethanol by liver microsomes. Experiments were carried out to evaluate the role of hydroxyl radicals in the organic hydroperoxide-catalyzed reaction. Maximum rates of ethanol oxidation occurred in the presence of either 0.5 mM cumene hydroperoxide or 2.5 mM t-butyl hydroperoxide and were linear for 2 to 4 min. The Km for ethanol was about 12 mM and Vmax was about 8 nmol ethanol oxidized/min/mg microsomal protein. Besides ethanol, the organic hydroperoxides supported the oxidation of longer-chain alcohols (1-butanol), and secondary alcohols (isopropanol). The organic hydroperoxide-supported oxidation of alcohols was not affected by several hydroxyl-radical scavengers such as dimethylsulfoxide, mannitol, or 2-keto-4-thiomethylbutyrate which blocked NADPH-dependent oxidation of alcohols by 50% or more. Iron-EDTA, which increases the production of hydroxyl radicals, increased the NADPH-dependent oxidation of ethanol, whereas desferrioxamine, which blocks the production of hydroxyl radicals, inhibited the NADPH-dependent oxidation of ethanol. Neither iron-EDTA nor desferrioxamine had any effect on the organic hydroperoxide-supported oxidation of ethanol. Cumene-and t-butyl hydroperoxide did not support microsomal oxidation of hydroxyl-radical scavengers. These results suggest that, in contrast to the NADPH-dependent oxidation of ethanol, free-hydroxyl radicals do not play a role in the organic hydroperoxide-dependent oxidation of ethanol by microsomes. Ethanol appears to be oxidized by two pathways in microsomes, one which is dependent on hydroxyl radicals, and the other which appears to be independent of these oxygen radicals.

Evidence for the involvement of activated oxygen in fungal degradation of lignocellulose

Oxygen has been shown to be necessary as a cosubstrate for the fungal degradation of lignins. In this work, the active forms of oxygen were tentatively identified in three ways: --effect of chemically generated active radicals and molecular species on lignocellulosic complexes, --use of activated oxygen scavengers in culture media of ligninolytic fungi, --characterization of active forms of oxygen by specific reactions. The data obtained strongly suggest that two main oxygen species are involved, namely OH radical and singlet oxygen (1O2). Chemical or enzymic scavengers inhibit the degradation of lignocelluloses by Phanerochaete chrysosporium. The fungus has been demonstrated to synthesize OH.

Inhibition of microsomal oxidation of alcohols and of hydroxyl-radical-scavenging agents by the iron-chelating agent desferrioxamine

Rat liver microsomes (microsomal fractions) catalyse the oxidation of straight-chain aliphatic alcohols and of hydroxyl-radical-scavenging agents during NADPH-dependent electron transfer. The iron-chelating agent desferrioxamine, which blocks the generation of hydroxyl radicals in other systems, was found to inhibit the following microsomal reactions: production of formaldehyde from either dimethyl sulphoxide or 2-methylpropan-2-ol (t-butylalcohol); generation of ethylene from 4-oxothiomethylbutyric acid; release of 14CO2 from [I-14C]benzoate; production of acetaldehyde from ethanol or butanal (butyraldehyde) from butan-1-ol. Desferrioxamine also blocked the increase in the oxidation of all these substrates produced by the addition of iron-EDTA to the microsomes. Desferrioxamine had no effect on a typical mixed-function-oxidase activity, the N-demethylation of aminopyrine, nor on the peroxidatic activity of catalase/H2O2 with ethanol. H2O2 appears to be the precursor of the oxidizing radical responsible for the oxidation of the alcohols and the other hydroxyl-radical scavengers. Chelation of microsomal iron by desferrioxamine most likely decreases the generation of hydroxyl radicals, which results in an inhibition of the oxidation of the alcohols and the hydroxyl-radical scavengers. Whereas desferrioxamine inhibited the oxidation of 2-methylpropan-2-ol, dimethyl sulphoxide, 4-oxothiomethylbutyrate and benzoate by more than 90%, the oxidation of ethanol and butanol could not be decreased by more than 45-60%. Higher concentrations of desferrioxamine were required to block the metabolism of the primary alcohols than to inhibit the metabolism of the other substrates. The desferrioxamine-insensitive rate of oxidation of ethanol was not inhibited by competitive hydroxyl-radical scavengers. These results suggest that primary alcohols may be oxidized by two pathways in microsomes, one dependent on the interaction of the alcohols with hydroxyl radicals (desferrioxamine-sensitive), the other which appears to be independent of these radicals (desferrioxamine-insensitive).

Evidence for two ethanol oxidizing pathways in reconstituted mixed-function oxidase systems

The oxidation of ethanol and typical hydroxyl radical scavengers by NADPH-cytochrome P-450 reductase and cytochrome P-450 purified from phenobarbital-treated rats were studied. Ethanol and the scavengers could be oxidized by the reductase system itself. This system was inhibited by superoxide dismutase, competing hydroxyl radical scavengers and desferrioxamine, but stimulated by either EDTA or iron. These results suggest that an iron-catalyzed Haber-Weiss reaction might be involved in the mechanism by which the reductase mediates the oxidation of typical hydroxyl radical scavengers and ethanol. The addition of cytochrome P-450 had no effect on the oxidation of the scavengers, whereas the oxidation of ethanol was enhanced two-to three-fold over the reductase-dependent rate. The oxidation of ethanol was dependent on both the amount of reductase and P-450. There was no effect of competing scavengers, superoxide dismutase or desferrioxamine on the increased rate of ethanol oxidation produced upon addition of cytochrome P-450. Organic hydroperoxides supported the oxidation of ethanol, but not that of the scavengers when added directly to cytochrome P-450. These results suggest that two independent pathways are operative in supporting NADPH-dependent microsomal oxidation of ethanol. One pathway involves hydroxyl radicals which can be generated by the reductase, whereas the other pathway requires the combined presence of both the reductase and cytochrome P-450, and appears to be independent of oxygen radicals.