One-electron transfer reactions in biochemical systems. IV. A mixed mechanism in the reaction of milk xanthine oxidase with electron acceptors

High dissociation rate constant of ferrous-dioxy complex linked to the catalase-like activity in lactoperoxidase

Heme reduction of ferric lactoperoxidase (LPO) into its ferrous form initially leads to the accumulation of the unstable form of LPO-Fe(II), which spontaneously converts to a more stable species, the two of which can be identified by Soret peaks at 440 and 434 nm, respectively. Our data demonstrate that both LPO-Fe(II) species are capable of binding O(2) at a similar rate to generate the ferrous-dioxy complex. Its formation with respect to O(2) was first order and monophasic and with rate constants of k(on) = 3.8 x 10(4) m(-1) s(-1) and k(off) = 11.2 s(-1). The dissociation rate constant for the formation of LPO-Fe(II)-O(2) is relatively high, in contrast to hemoprotein model compounds. This high dissociation rate can be attributed to a combination of effects that include the positive trans effect of the proximal ligand, the heme pocket environment, and the geometry of the Fe-O(2) linkage. Our results have also shown that the decay of the LPO-Fe(II)-O(2) complex occurs by two sequential O(2)-independent steps. The first step involves formation of a short-lived intermediate that can be characterized by its Soret absorption peak at 416 nm and may be attributed to the weakening of the Fe(II)-O(2) linkage with a rate constant of 0.5 s(-1). The second step is spontaneous conversion of this intermediate to generate the native enzyme and presumably superoxide as end products with a rate constant of 0.03 s(-1). A comprehensive kinetic model that links LPO-Fe(II)-O(2) complex formation to the LPO catalase-like activity, combined with the classic catalytic cycle, is presented here.

Mitochondria as subcellular targets for clinically useful anthracyclines

Due to the widespread use of anthracyclines as antitumor agents, a large number of investigations have been reported analyzing clinical and molecular aspects of these quinone antibiotics. While the high affinity of anthracyclines towards chromosomal DNA has been held responsible for their antitumor activity, an increasing amount of data is being accumulated showing that these drugs also target mitochondria thus interfering with major mitochondrial functions. Since this toxicity of anthracyclines towards mitochondria is associated with side effects significantly limiting their chemotherapeutic dose, the corresponding underlying mechanisms need to be understood. Bioenergetic failure, enzyme inhibitions, lipid peroxidations, induction of membrane disorders as well as the initiation of oxidative stress are being attributed to the accumulation of anthracyclines at or inside mitochondria. In this review the wide spectrum of possible mode of actions of these antibiotics leading to mitochondrial dysfunctions will be presented and discussed.

Oxidative stress: free radical production in neural degeneration

It is not yet established whether oxidative stress is a major cause of cell death or simply a consequence of an unknown pathogenetic factor. Concerning chronic diseases, as Parkinson's and Alzheimer's disease are assumed to be, it is possible that a gradual impairment of cellular defense mechanisms leads to cell damage because of toxic substances being increasingly formed during normal cellular metabolism. This point of view brings into consideration the possibility that, besides exogenous factors, the pathogenetic process of neurodegeration is triggered by endogenous mechanisms, either by an endogenous toxin or by inherited metabolic disorders, which become progressively more evident with aging. In the following review, we focus on the oxidative stress theory of neurodegeneration, on excitotoxin-induced cell damage and on impairment of mitochondrial function as three major noxae being the most likely causes of cell death either independently or in connection with each other. First, having discussed clinical, pathophysiological, pathological and biochemical features of movement and cognitive disorders, we discuss the common features of these biochemical theories of neurodegeneration separately. Second, we attempt to evaluate possible biochemical links between them and third, we discuss experimental findings that confirm or rule out the involvement of any of these theories in neurodegeneration. Finally, we report some therapeutic strategies evolved from each of these theories.

The effect of oxygen at physiological levels on the detection of free radical intermediates by electron paramagnetic resonance

It is well known that oxygen enhances the relaxation of free radical EPR probes through spin lattice and Heisenberg spin-spin interactions with consequent effect on the line height and width. The two relaxation processes have opposing effects on the signal heights and depend on the concentration of oxygen, the incident microwave power, and the presence of other paramagnetic species. During EPR studies of chemical, biochemical, and cellular processes involving free radicals, molecular oxygen has significant magnetic influence on the EPR signal intensity of the free radical species under investigation in addition to affecting the rates of production of the primary species and the stability of the spin adduct nitroxides. These effects are often overlooked and can cause artifacts and lead to erroneous interpretation. In the present study, the effects of oxygen and ferricyanide on the EPR signal height of stable and persistent spin adduct nitroxides at commonly employed microwave powers were examined. The results show that under commonly adopted EPR spectrometer instrumental conditions, artifactual changes in the EPR signal of spin adducts occur and the best way to avoid them is by keeping the oxygen level constant using a gas-permeable cell.

Xanthine oxidase-catalyzed reduction of estrogen quinones to semiquinones and hydroquinones

Metabolic redox cycling between the stilbene estrogen diethylstilbestrol (DES) and diethylstilbestrol-4',4"-quinone (DES Q) has been demonstrated previously. The xanthine and xanthine oxidase-catalyzed reduction of estrogen quinone has been studied in this work to understand the role of metabolic redox cycling in estrogen metabolism. Xanthine and xanthine oxidase catalyzed the reduction of DES Q to 44% Z-DES and 9% E-DES. This reaction was inhibited by the addition of superoxide dismutase or by a lack of oxygen (under anaerobic conditions). DES Q was also reduced in a non-enzymatic reaction by superoxide radicals generated by potassium superoxide and crown ether. The reaction between the O2-. and DES Q was also investigated by an electron spin resonance spin-trapping technique. The superoxide anion generated in an oxygen-saturated xanthine and xanthine oxidase system was detected as 5,5-dimethyl-1-pyrroline-1-oxide-superoxide adduct. The addition of DES Q or 2,3-estradiol quinone totally inhibited the formation of this adduct. The reduction of DES Q by superoxide radicals was taken as evidence that this reaction was one possible mechanism of xanthine and xanthine oxidase-mediated reduction. In addition, reduction of DES Q by direct electron transfer to quinone by the enzyme may also occur. The intermediate formation of semiquinone free radicals in the reduction is implied by the nature of the single electron transfer reactions and, in addition, has been demonstrated for the catechol estrogen by electron spin resonance measurements. It is concluded that the reduction of estrogen quinones to their hydroquinones by xanthine oxidase occurs by both one electron transfer to the quinone and by formation of superoxide which then reduces the quinone.

Molecular mechanisms of quinone cytotoxicity

Quinones are probably found in all respiring animal and plant cells. They are widely used as anticancer, antibacterial or antimalarial drugs and as fungicides. Toxicity can arise as a result of their use as well as by the metabolism of other drugs and various environmental toxins or dietary constituents. In rapidly dividing cells such as tumor cells, cytotoxicity has been attributed to DNA modification. However the molecular basis for the initiation of quinone cytotoxicity in resting or non-dividing cells has been attributed to the alkylation of essential protein thiol or amine groups and/or the oxidation of essential protein thiols by activated oxygen species and/or GSSG. Oxidative stress arises when the quinone is reduced by reductases to a semiquinone radical which reduces oxygen to superoxide radicals and reforms the quinone. This futile redox cycling and oxygen activation forms cytotoxic levels of hydrogen peroxide and GSSG is retained by the cell and causes cytotoxic mixed protein disulfide formation. Most quinones form GSH conjugates which also undergo futile redox cycling and oxygen activation. Prior depletion of cell GSH markedly increases the cell's susceptibility to alkylating quinones but can protect the cell against certain redox cycling quinones. Cytotoxicity induced by hydroquinones in isolated hepatocytes can be attributed to quinones formed by autoxidation. The higher redox potential benzoquinones and naphthoquinones are the most cytotoxic presumably because of their higher electrophilicty and thiol reactivity and/or because the quinones or GSH conjugates are more readily reduced to semiquinones which activate oxygen.

Cytosolic NAD(P)H:(quinone-acceptor)oxidoreductase in human normal and tumor tissue: effects of cigarette smoking and alcohol

NAD(P)H:(quinone-acceptor)oxidoreductase (QAO), previously known as DT-diaphorase, catalyzes the reduction of quinones to hydroquinones. Enhanced activity of the enzyme has been suggested to protect cells against the cellular toxicity and carcinogenicity of quinones, but may activate some cytotoxic anti-tumor quinones. Cytosolic levels of QAO, carbonyl reductase (CR) and total quinone reductase activity have been measured in normal and tumorous human tissues. QAO was the major component of the total cytosolic quinone reductase activity in all the tissues investigated. CR represented 10 to 28% of the total cytosolic quinone reductase activity in normal tissue. Normal tissue QAO was high in the stomach and kidney, and lower in the lung, liver, colon and breast. Primary tumor from lung, liver, colon and breast had elevated levels of QAO compared to normal tissue, while tumor from kidney and stomach had lower levels. CR was not significantly altered in tumor tissue, except in the case of lung and colon tumor which showed an increase compared to normal tissue. A major determinant of the variability of human lung tumor QAO was the cigarette-smoking history of the donor. Non-smokers and past smokers had high levels of tumor QAO compared to normal tissue. Smokers had levels of tumor QAO that were not significantly different from those of normal tissue QAO. Smokers had a small increase in normal lung QAO compared to non-smokers. Alcohol use was associated with an increase in lung tumor QAO but had no effect on QAO in normal lung. The function of QAO in tumors is not known but the elevated activity of QAO in some tumors and the apparent depressant effect of smoking could influence the response of these tumors to quinone drugs or toxic agents that are metabolized by QAO.

The reactions of horseradish peroxidase, lactoperoxidase, and myeloperoxidase with enzymatically generated superoxide

The formation and decay of intermediate compounds of horseradish peroxidase, lactoperoxidase, and myeloperoxidase formed in the presence of the superoxide/hydrogen peroxide-generating xanthine/xanthine oxidase system has been studied by observation of spectral changes in both the Soret and visible spectral regions and both on millisecond and second time scales. It is tentatively concluded that in all cases compound III is formed in a two-step reaction of native enzyme with superoxide. The presence of superoxide dismutase completely inhibited compound III formation; the presence of catalase had no effect on the process. Spectral data which indicate differences in the decay of horseradish peroxidase compound III back to the native state in comparison with compounds III of lactoperoxidase and myeloperoxidase are also presented.

Myeloperoxidase oxidation states involved in myeloperoxidase-oxidase oxidation of thiols

The changes in the oxidation state of the leucocyte enzyme myeloperoxidase, induced by buffer and thiols, were studied with visible-light-absorption spectroscopy. It was concluded that phosphate buffer contains small amounts of H2O2 and that thiols, when added to buffer, induce the generation of minute amounts of superoxide radical anion. These minute amounts of reduced oxygen species are suggested to account for the initiation of myeloperoxidase-oxidase oxidation of thiols. Myeloperoxidase was found to be in its Compound III oxidation state during myeloperoxidase-oxidase oxidation of thiols. However, myeloperoxidase-mediated oxidation of thiols with concomitant O2 consumption can also occur with myeloperoxidase in its Compound II oxidation state. These studies indicate that the ferro and Compound III oxidation states may not be essential intermediates in myeloperoxidase-oxidase oxidation of thiols, but rather that the formation of the Compound III oxidation state retards the reaction.

Metabolic activation of 1-naphthol and phenol by a simple superoxide-generating system and human leukocytes

Phenol and 1-naphthol, products of benzene and naphthalene biotransformation, are metabolized during O2- generation by xanthine oxidase/hypoxanthine and phorbol myristate acetate (PMA)-stimulated human neutrophils. The addition of 1-naphthol to xanthine oxidase/hypoxanthine incubations resulted in the formation of 1,4-naphthoquinone (1,4-NQ) whereas phenol addition yielded only small quantities of hydroquinone, catechol and a unidentified reducible product but not 1,4-benzoquinone. This formation of 1,4-NQ was dependent upon hypoxanthine, xanthine oxidase, and 1-naphthol and was inhibited by the addition of superoxide dismutase (SOD) demonstrating that the conversion was O2-mediated. During O2- generation by PMA-stimulated neutrophils, the addition of phenol interfered with luminol-dependent chemiluminescence and resulted in covalent binding of phenol to protein. Protein binding was 80% inhibited by the addition of azide or catalase to the incubations indicating that bioactivation was peroxidase-mediated. In contrast, the addition of 1-naphthol to PMA-stimulated neutrophils interfered with superoxide-dependent cytochrome c reduction as well as luminol-dependent chemiluminescence and also resulted in protein binding. Protein binding was only partially inhibited by azide or catalase. The addition of SOD in combination with catalase resulted in a significantly greater inhibition of binding when compared to that of catalase alone. The results of these experiments indicate that phenol and 1-naphthol are converted to reactive metabolites during superoxide generating conditions but by different mechanisms. The formation of reactive metabolites from phenol was almost exclusively peroxidase-mediated whereas the bioactivation of 1-naphthol could occur by two different mechanisms, a peroxidase-dependent and a direct superoxide-dependent mechanism.

Free radical mechanisms in enzyme reactions

Free radicals are formed in prosthetic groups or amino acid residues of certain enzymes. These free radicals are closely related to the activation process in enzyme catalysis, but their formation does not always result in the formation of substrate free radicals as a product of the enzyme reaction. The role of free radicals in enzyme catalysis is discussed.

The role of compound III in reversible and irreversible inactivation of lactoperoxidase

In the presence of iodide (I-, 10 mM) and hydrogen peroxide in a large excess (H2O2, 0.1-10 mM) catalytic amounts of lactoperoxidase (2 nM) are very rapidly irreversibly inactivated without forming compound III (cpd III). In contrast, in the absence of I- cpd III is formed and inactivation proceeds very slowly. Increasing the enzyme concentration up to the micromolar range significantly accelerates the rate of inactivation. The present data reveal that irreversible inactivation of the enzyme involves cleavage of the prosthetic group and liberation of heme iron. The rate of enzyme destruction is well correlated with the production of molecular oxygen (O2), which originates from the oxidation of excess H2O2. Since H2O2 and O2 per se do not affect the heme moiety of the peroxidase, we suggest that the damaging species may be a primary intermediate of the H2O2 oxidation, such as oxygen in its excited singlet state (1 delta gO2), superoxide radicals (O-.2), or consequently formed hydroxyl radicals (OH.).

The superoxide dismutase activity of myeloperoxidase; formation of compound III

The reaction of superoxide anions with myeloperoxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7), which results in the formation of Compound III of myeloperoxidase, was investigated. It is shown that myeloperoxidase has a high affinity for superoxide anions because formation of Compound III was only partially inhibited by high concentrations of superoxide dismutase. Furthermore, when superoxide anions were generated in a mixture of both cytochrome c and myeloperoxidase in the absence of Cl-, only Compound III was formed and reduction of cytochrome c was not observed. In the presence of Cl-, Compound III was also formed and reduction of cytochrome c was inhibited. From the results described in this paper we conclude that Compound III is able to react with superoxide anions, probably resulting in formation of an intermediate (Compound I) which is catalytically active in the oxidation of Cl- to yield hypochlorous acid (HOCl). Because Compound III of myeloperoxidase is formed in phagocytosing neutrophils (Winterbourn, C.C., Garcia, R.C. and Segal, A.W. (1985) Biochem. J. 228, 583-592) we propose that, in vivo, myeloperoxidase also acts as a superoxide dismutase, and via formation of Compound I uses superoxide anions in the formation of HOCl.

Doxorubicin-dependent lipid peroxidation at low partial pressures of O2

Doxorubicin semiquinone, produced by reduction of doxorubicin with xanthine oxidase or ferredoxin reductase, reacted with H2O2 to cause deoxyribose oxidation that was catalysed by sub-micromolar concentrations of complexed iron. Both the mechanism of deoxyribose oxidation and the yield of oxidation products depended on the chelator. With EDTA or diethylenetriamine penta-acetic acid (DTPA), the reactive species behaved like free . OH. However, when ADP or no chelator was present, oxidation of deoxyribose was inhibited by mannitol but not benzoate or formate and was apparently not due to free . OH. Doxorubicin semiquinone and H2O2 caused peroxidation of phospholipid liposomes when ADP or no chelator was present, but not in the presence of EDTA or DTPA. Lipid peroxidation was iron dependent over a 0.1 to 1 microM range and was maximal with a pO2 of approximately 1.5 mm Hg, when the inhibitory effect of O2 on initiation is balanced by its stimulatory effects on propagation. The results imply that H2O2 and the doxorubicin semiquinone at low iron and O2 concentrations are very effective at initiating lipid peroxidation.

Superoxide radical from xanthine oxidase acting upon lumazine

The univalent and divalent reductions of dioxygen were measured using lumazine as a low turnover substrate and both xanthine and acetaldehyde as high turnover substrates. These measurements were made in solutions equilibrated with air and with 100% O2. The univalent route of dioxygen reduction predominated with the low turnover substrate and was increased by raising pO2 and by lowering substrate concentration. These results support the view that electron egress from heavily reduced xanthine oxidase occurs by divalent transfers, while that from the partially reduced enzyme is by univalent transfers. Xanthine oxidase, acting as lumazine, is a convenient source of O2-.

Differential pulse polarographic determination of plasma menadione

A differential pulse polarographic assay for plasma vitamin K3 (menadione) has been developed. Details of the assay are (i) lipid-soluble material is extracted from plasma into ether by the method of Bjornsson et al. [(1978) Thromb. Haemostas. 2, 466-473]; (ii) ether is evaporated under nitrogen and the residue is dissolved in the supporting electrolyte, methanol: 0.2 M borate buffer (9:1), pH 6.8; (iii) current height is measured at -0.32 V vs SCE on the differential pulse polarogram. The lower sensitivity limit of this technique after addition of standard vitamin K3 to plasma is 0.3 microM; the calibration curve is linear from 0.6 through 10 microM. Two patients treated with a single dose of menadiol sodium diphosphate, 20 mg/M2 i.m., achieved measurable plasma vitamin K3 levels at 0.5 to 1.0 h ranging between 0.5 (0.08 micrograms/ml) and 2 microM (0.3 micrograms/ml).

Oxidase and oxygenase function of the microsomal cytochrome P450 monooxygenase system

The rates of the NADPH-dependent formation of superoxide radicals and hydrogen peroxide have been measured in liver microsomes from phenobarbital-pretreated rats. Correcting a quenching of O2(-) radicals by microsomes, a stoichiometry of O2(-) to H2O2 close to 2:1 was obtained. This, and the fact that pseudo-substrates of microsomal cytochrome P450 like perfluoro-n-hexane and perfluorinated cyclohexane did not increase H2O2 formation in a catalase-inhibited assay, rules out a two-electron reduced oxygen species as the source of H2O2. The rates of O2(-) as well as H2O2 generation in the presence of 7-ethoxycoumarin were equally inhibited by carbon monoxide (75%) and resulted in photochemical action spectra with a maximum reactivation at 450 nm. Using the same conditions the monooxygenation was inhibited to a high degree (83%) but without exogenous substrate the inhibition of H2O2 formation dropped to 55%. It was concluded that most of the O2(-) originated from the oxycomplex of cytochrome P450 and that substrates can modify the rates of its decomposition and sensitivity to carbon monoxide. No correlation of H2O2 formation or of substrate monooxygenation with the optical substrate binding spectra could be observed. From the pH dependence a proton-assisted decomposition of oxy-cytochrome P450 appears likely. H2O2 formation was only slightly decreased at 20 microM dioxygen suggesting that H2O2 formation via cytochrome P450 should also occur in vivo.

Reactions of Adriamycin with haemoglobin. Superoxide dismutase indirectly inhibits reactions of the Adriamycin semiquinone

The Adriamycin semiquinone produced by the reaction of xanthine oxidase and xanthine with Adriamycin has been shown to reduce both methaemoglobin and cytochrome c. In air, but not N2, both reactions were inhibited by superoxide dismutase. With cytochrome c, superoxide formed by the rapid reaction of the semiquinone with O2, was responsible for the reduction. However, even in air, methaemoglobin was reduced directly by the Adriamycin semiquinone. Superoxide dismutase inhibited this reaction by removing superoxide and hence the semiquinone by displacing the equilibrium: Semiquinone + O2 in equilibrium or formed from quinone + O2-. to the right. This ability to inhibit indirectly reactions of the semiquinone could have wider implications for the protection given by superoxide dismutase against the cytotoxicity of Adriamycin. Oxidation of haemoglobin by Adriamycin has been shown to be initiated by a reversible reaction between the drug and oxyhaemoglobin, producing methaemoglobin and the Adriamycin semiquinone. Reaction of the semiquinone with O2 gives superoxide and H2O2, which can also react with haemoglobin. Catalase, by preventing this reaction of H2O2, inhibits oxidation of oxyhaemoglobin. Superoxide dismutase, however, accelerates oxidation, by inhibiting the reaction of the semiquinone with methaemoglobin by the mechanism described above. Although superoxide dismutase has a detrimental effect on haemoglobin oxidation, it may protect the red cell against more damaging reactions of the Adriamycin semiquinone.

Dioxygen activation by putidamonooxin. The oxygen species formed and released under uncoupling conditions

In the presence of substrates not favourable for hydroxylation, more than 80% of the dioxygen consumed by purified, reconstituted 4-methoxybenzoate monooxygenase appears in the reaction mixture as hydrogen peroxide. We have investigated whether under these conditions (a) reduced putidamonooxin, the oxygenase of this enzyme system, either autoxidizes in the presence of dioxygen, with liberation of superoxide anion radicals which then disproportionate to H2O2 and O2, or (b) dioxygen is reduced by two sequential single-electron steps leading to the active oxygen species that forms hydrogen peroxide directly when inactivated by protonation. Quantitative estimation of O-2 radicals, with either succinylated ferricytochrome c or epinephrine used as O-2 scavengers, revealed that only about 6% of the total electron flux channelled via putidamonooxin to dioxygen led to the monovalent reduction on dioxygen. This means that not more than 3% of the hydrogen peroxide found under uncoupling conditions arises from the rapid bimolecular disproportionation of initially formed O-2 radicals. Inconsistent results were obtained when lactoperoxidase was used as an O-2 trap. Our measurements indicate that the conversion of lactoperoxidase into compound III is an inappropriate method of detecting any O-2 radicals that may be found by the uncoupled 4-methoxybenzoate monooxygenase. The stoichiometry of about 1:1 for O2 uptake: H2O2 formation indicates that under uncoupling conditions H2O is virtually not formed. The role of [FeO2]+ as the active oxygenating species of putidamonooxin is discussed.

Biochemical effects on radioprotective agents on the liver microsomal hydroxylating system: in vitro studies

The action of two radioprotectors--cysteamine and cystamine--on the liver microsomal multi-enzyme hydroxylating system, a key stem in drug and biological compounds metabolism, has been studied. Their effects have been systematically analysed at the level of individual enzyme activities and global functions. The two compounds are quite inactive on NADPH and NADH cytochrome c reductase activities, but slightly denature the cytochrome P450 into cytochrome P420. Furthermore, they do inhibit to some extent (30 per cent at 10(-2) M) the rate of codeine hydroxylation and totally suppress ((at 10(-2) M) the NADPH-induced lipid peroxidation which occurs during enzymatic functioning. These results are discussed in the light of the toxicity of radioprotectors.

Lactoperoxidase-catalyzed lipid peroxidation of microsomal and artificial membranes

Lactoperoxidase, in the presence of H2O2, I-, and rat liver microsomes, will peroxidize membrane lipids, as evidenced by malondialdehyde formation. Fe3+ assists in the formation of malondialdehyde. Fe3+ can be added at the end of the reaction period as well as at the beginning with equal effectiveness, suggesting that it only acts to assist in the conversion of lipid peroxides, previously formed by lactoperoxidase, to malondialdehyde. The addition of EDTA to the microsomal reaction mixture results in a 40% decrease in malondialdehyde formation. The antioxidant butylated hydroxytoluene will completely block the formation of malondialdehyde. Malondialdehyde formation is not dependent upon the production of superoxide, singlet oxygen, or hydroxyl radicals. Peroxidation of membrane lipids by this system is equally effective in both intact microsomes and in liposomes, indicating that iodination of microsomal protein is not required for lipid peroxidation to occur.

The mechanism of liver microsomal lipid peroxidation

In the presence of Fe-3+ and complexing anions, the peroxidation of unsaturated liver microsomal lipid in both intact microsomes and in a model system containing extracted microsomal lipid can be promoted by either NADPH and NADPH : cytochrome c reductase or by xanthine and xanthine oxidase. Erythrocuprein effectively inhibits the activity promoted by xanthine and xanthine oxidase but produces much less inhibition of NADPH-dependent peroxidation. The singlet-oxygen trapping agent, 1, 3-diphenylisobenzofuran, had no effect on NADPH-dependent peroxidation but strongly inhibited the peroxidation promoted by xanthine and xanthine oxidase. NADPH-dependent lipid peroxidation was also shown to be unaffected by hydroxyl radical scavengers.. The addition of catalase had no effect on NADPH-dependent lipid peroxidation, but it significantly increased the rate of malondialdehyde formation in the reaction promoted by xanthine and xanthine oxidase. The results demonstrate that NADPH-dependent lipid peroxidation is promoted by a reaction mechanism which does not involve either superoxide, singlet oxygen, HOOH, or the hydroxyl radical. It is concluded that NADPH-dependent lipid peroxidation is initiated by the reduction of Fe-3+ followed by the decomposition of hydroperoxides to generate alkoxyl radicals. The initiation reaction may involve some form of the perferryl ion or other metal ion species generated during oxidation of Fe-2+ by oxygen.

Distribution of reducing equivalents on xanthine oxidase molecules and the rates of the intramolecular electron-transfer reactions

When xanthine oxidase turns over 1-methylxanthine aerobically at pH8.2, the time-sequence in development of its electron-paramagnetic-resonance signals is not primarily due to slow intramolecular reactions among its centres. It derives instead from gross differences in electron distribution within enzyme molecules reduced by the substrate in comparison with those that have subsequently been partly reoxidized by one-electron reaction with oxygen.