Microsomal metabolism of hydroxyl radical scavenging agents: relationship to the microsomal oxidation of alcohols

Distinct effects of form selective cytochrome P450 inhibitors on cytochrome P450-mediated monooxygenase and hydrogen peroxide generating NADPH oxidase

A characteristic of cytochrome P450 (CYP) enzymes is their ability to generate H₂O₂, either directly or indirectly via superoxide anion, a reaction referred to as "NADPH oxidase" activity. H₂O₂ production by CYPs can lead to the accumulation of cytotoxic reactive oxygen species which can compromise cellular functioning and contribute to tissue injury. Herein we determined if form selective CYP inhibitors could distinguish between the activities of the monooxygenase and NADPH oxidase activities of rat recombinant CYP1A2, CYP2E1, CYP3A1 and CYP3A2 and CYP1A1/2-enriched β-naphthoflavone-induced rat liver microsomes, CYP2E1-enriched isoniazide-induced rat liver microsomes and CYP3A subfamily-enriched dexamethasone-induced rat liver microsomes. In the presence of 7,8-benzoflavone (2.0 μM) for CYP1A2 and 4-methylpyrazole (32 μM) or DMSO (16 mM) for CYP2E1, monooxygenase activity was blocked without affecting NADPH oxidase activity for both the recombinant enzymes and microsomal preparations. Ketoconazole (1.0 μM), a form selective inhibitor for CYP3A subfamily enzymes, completely inhibited monooxygenase activity of rat recombinant CYP3A1/3A2 and CYP3A subfamily in rat liver microsomes; it also partially inhibited NADPH oxidase activity. 7,8-benzoflavone is a type I ligand, which competes with substrate binding, while 4-methylpyrazole and DMSO are type II heme binding ligands. Interactions of heme with these type II ligands was not sufficient to interfere with oxygen activation, which is required for NADPH oxidase activity. Ketoconazole, a type II ligand known to bind multiple sites on CYP3A subfamily enzymes in close proximity to heme, also interfered, at least in part, with oxygen activation. These data indicate that form specific inhibitors can be used to distinguish between monooxygenase reactions and H₂O₂ generating NADPH oxidase of CYP1A2 and CYP2E1. Mechanisms by which ketoconazole inhibits CYP3A NADPH oxidase remain to be determined.

Use of hepatocytes from Hoplias malabaricus to characterize the toxicity of a complex mixture of lipophilic halogenated compounds

Organisms are continuously exposed to a plethora of anthropogenic toxicants daily released to the environment. In the present study, the effects of a mixture of halogenated organic compounds (HOCs) extracted from hepatic lipids were evaluated on the primary hepatocyte culture from fish Hoplias malabaricus. Cells were isolated through non-enzymatic perfusion protocol and cultured during 3 days to allow attachment. Two concentrations of the mixture of HOCs (10 ng ml(-1) [Mix10] and 50 ng ml(-1) [Mix50]) were tested in cells for 2 days by medium replacement. The control groups, with and without solvent (DMSO) were run in the same conditions. Both tested concentrations of HOCs increased the catalase and GST activities, but only the Mix50 increase the DNA damage and decreased the GSH concentration and cell viability. Lipid peroxidation increased in the Mix10 group, but it seems to be more a consequence of DMSO presence than the HOCs themselves. The DMSO at 0.1% increased the lipid peroxidation, GSH concentration, apoptosis and DNA damage. The present data suggest that DMSO interferes with the hepatocytes of H. malabaricus in culture and that the mixture of HOCs tested alters the redox state of the hepatocytes.

The role of glutamine transaminase K (GTK) in sulfur and alpha-keto acid metabolism in the brain, and in the possible bioactivation of neurotoxicants

Glutamine transaminase K (GTK), which is a freely reversible glutamine (methionine) aromatic amino acid aminotransferase, is present in most mammalian tissues, including brain. Quantitatively, the most important amine donor in vivo is glutamine. The product of glutamine transamination (i.e., alpha-ketoglutaramate; alphaKGM) is rapidly removed by cyclization and/or conversion to alpha-ketoglutarate. Transamination is therefore "pulled" in the direction of glutamine utilization. Major biological roles of GTK are to maintain low levels of phenylpyruvate and to close the methionine salvage pathway. GTK also catalyzes the transamination of cystathionine, lanthionine, and thialysine to the corresponding alpha-keto acids, which cyclize to ketimines. The cyclic ketimines and several metabolites derived therefrom are found in brain. It is not clear whether these compounds have a biological function or are metabolic dead-ends. However, high-affinity binding of lanthionine ketimine (LK) to brain membranes has been reported. Mammalian tissues possess several enzymes capable of catalyzing transamination of kynurenine in vitro. Two of these kynurenine aminotransferases (KATs), namely KAT I and KAT II, are present in brain and have been extensively studied. KAT I and KAT II are identical to GTK and alpha-aminoadipate aminotransferase, respectively. GTK/KAT I is largely cytosolic in kidney, but mostly mitochondrial in brain. The same gene codes for both forms, but alternative splicing dictates whether a 32-amino acid mitochondrial-targeting sequence is present in the expressed protein. The activity of KAT I is altered by a missense mutation (E61G) in the spontaneously hypertensive rat. The symptoms may be due in part to alteration of kynurenine transamination. However, owing to strong competition from other amino acid substrates, the turnover of kynurenine to kynurenate by GTK/KAT I in nervous tissue must be slow unless kynurenine and GTK are sequestered in a compartment distinct from the major amino acid pools. The possibility is discussed that the spontaneous hypertension in rats carrying the GTK/KAT I mutation may be due in part to disruption of glutamine transamination. GTK is one of several pyridoxal 5'-phosphate (PLP)-containing enzymes that can catalyze non-physiological beta-elimination reactions with cysteine S-conjugates containing a good leaving group attached at the sulfur. These elimination reactions may contribute to the bioactivation of certain electrophiles, resulting in toxicity to kidney, liver, brain, and possibly other organs. On the other hand, the beta-lyase reaction catalyzed by GTK may be useful in the conversion of some cysteine S-conjugate prodrugs to active components in vivo. The roles of GTK in (a) brain nitrogen, sulfur, and aromatic amino acid/kynurenine metabolism, (b) brain alpha-keto acid metabolism, (c) bioactivation of certain electrophiles in brain, (d) prodrug targeting, and (e) maintenance of normal blood pressure deserve further study.

In vitro studies on the activity of japanese kampo herbal medicines Oren-Gedoku-To (TJ-15) and Toki-Shakuyaku-San (TJ-23) as scavengers of free radicals

Oren-Gedoku-To (TJ-15) inhibited enzymatically (NADPH or CumOOH) and non-enzymatically (Fe-ascorbate) induced lipid peroxidation in rat liver microsomes as assessed by TBA-reactive product accumulation. Toki-Shakuyaku-San (TJ-23) had little effect on either system. The protective effect of TJ-15 against lipid peroxidation was not dependent upon the presence of microsomal drug-metabolizing activity and could not be fully accounted for by its action on microsomal electron transfer, as evaluated by studying the kinetics of cytochrome c reduction. Both TJ-15 and TJ-23 reduced the stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH), indicating their free radical scavenging ability. The reactivity of TJ-15 was comparable with that of standard ascorbate, while the activity of TJ-23 was approximately 10 times lower. In a free solution TJ-15 and TJ-23 effectively scavenged OH* radicals, as indicated by the inhibition of ethylene production from 2-keto-4-methiolbutyric acid (KMBA), and O2*- anion radicals, as assessed by the inhibitory effect on the rate of nitro blue tetrazolium (NBT) reduction by the O2*- generating system xanthine/xanthine oxidase.

The catalase inhibitor sodium azide reduces ethanol-induced locomotor activity

The involvement of brain catalase in modulating the psychopharmacological effects of ethanol was investigated by examining ethanol-induced locomotor activity in sodium azide-treated mice. Mice were pretreated with i.p. injections of the catalase inhibitor sodium azide (5, 10, or 15 mg/kg) or saline. Following this treatment, animals received i.p. injections of ethanol (0.0, 1.6, 2.4, or 3.2 g/kg). Ten minutes after ethanol administration, locomotor activity was recorded during a 10-min testing period in open-field chambers. The time effect between the two treatments (0, 30, 60, or 90 min) was also evaluated. Results indicated that sodium azide alone did not change spontaneous locomotor activity. However, this catalase inhibitor significantly reduced ethanol-induced locomotor activity when it was injected simultaneously or 30 min before ethanol injections. Moreover, perfused brain homogenates of mice treated with sodium azide also showed a significant reduction of catalase activity. No differences in blood ethanol levels were observed between sodium azide and saline pretreated animals. Results of an additional experiment showed that sodium azide (10 mg/kg, at 30 min) did not produce an effect on d-amphetamine- (2 mg/kg) or tert-butanol- (0.5 g/kg) induced locomotor activities. A specific interaction between ethanol and sodium azide at the level of the central nervous system is suggested. These results provide further support for the involvement of brain catalase in ethanol-induced behavioral effects. They also support the notion that acetaldehyde may be produced directly in the brain by catalase and that it may be an important regulator of ethanol's locomotor effects.

Quantification of total oxidant scavenging capacity of antioxidants for peroxynitrite, peroxyl radicals, and hydroxyl radicals

We have extended the application of our previously reported total oxidant scavenging capacity (TOSC) assay (Winston et al., Free Radical Biol. Med. 24, 480-493, 1998) to permit facile quantification of the absorbance capacity of antioxidants toward three potent oxidants, i.e., hydroxyl radicals, peroxyl radicals, and peroxynitrite. Respectively, these oxidants were generated by the iron plus ascorbate-driven Fenton reaction, thermal homolysis of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (ABAP), and 3-morpholinosydnonimine N-ethylcarbamide (SIN-1). Each of these oxidants reacts with alpha-keto-gamma-methiolbutyric acid (KMBA), which is oxidized and yields ethylene. The antioxidant capacity of the compounds tested is quantified from their ability to inhibit ethylene formation relative to a control reaction. Assay conditions were established in which control reactions give comparable yields of ethylene with each of the oxidants studied. Thus, the relative efficiency of various antioxidants could be compared under conditions of quantitatively similar KMBA oxidizing capability by the three oxidants. Reduced glutathione was an efficient scavenger of peroxyl radicals, but scavenged peroxynitrite and hydroxyl radicals relatively poorly. Uric acid, Trolox, and ascorbic acid were comparable scavengers of peroxynitrite and peroxyl radicals. Uric acid and Trolox were approximately an order of magnitude less efficient as scavengers of hydroxyl radicals. The classical hydroxyl radical scavenging agents mannitol, dimethyl sulfoxide, and benzoic acid had much higher TOSC values with hydroxyl than with peroxyl radicals or peroxynitrite. The very different chemical reactivity toward KMBA by the SIN-1- and iron-ascorbate-generated oxidants indicates that hydroxyl radical is not a major oxidant produced by the SIN-1 system. The data show that the TOSC assay is useful and robust in distinguishing the reactivity of various oxidants and the relative capacities of antioxidants to scavenge these oxidants.

Liver nuclear ethanol metabolizing systems (NEMS) producing acetaldehyde and 1-hydroxyethyl free radicals

Biotransformation of ethanol by liver nuclei was studied. The formation of acetaldehyde was determined by GC/FID. The 1-hydroxyethyl (1HEt) formation was established by spin trapping of the radical with N-t-butyl-alpha-phenylnitrone (PBN) followed by GC/MS. Liver nuclei, free of endoplasmic reticulum, cytosol or mitochondria, were able to biotransform ethanol to acetaldehyde in the presence of NADPH under air. Only 22% activity was observed in the absence of the cofactor. Twenty-six percent of the NADPH-dependent activity and 47% of the NADPH-independent activity were observable under nitrogen. Aerobic biotransformation was inhibited by CO, SKF 525A, 4-methylpyrazole and by diethyldithiocarbamate. This suggests that CYP2E1 is involved in the process. However, the formation of acetaldehyde was able to proceed under a pure CO atmosphere. The lack of inhibitory effects of 2-mercapto-1-methylimidazol and thiobenzamide excludes the potential participation of the NADPH flavin monooxigenase system. The formation of hydroxyl radicals in the process is suggested by the partial inhibitory effect of 5 mM mannitol and 5 mM sodium benzoate and by the fact that the 1HEt was detected. The NADPH-dependent anaerobic ethanol biotransformation pathway was stimulated by FAD and inhibited to some extent by iron chelators. The relevance of a liver nuclear ethanol biotransformation, generating reactive metabolites, such as acetaldehyde and free radicals, nearby DNA, nuclear proteins and lipids is discussed.

Altered methional homoeostasis is associated with decreased apoptosis in BAF3 bcl2 murine lymphoid cells

Methional is a potent inducer of apoptosis in an interleukin 3-dependent murine lymphoid cell line BAF3 b0 when it is added to the culture medium. In these cells transfected with the bcl2 gene, BAF3 bcl2, the apoptotic-inducing activity of methional is dramatically reduced. The addition of disulfiram (an inhibitor of aldehyde dehydrogenase) in order to reduce methional oxidation brought about an increase in apoptosis in BAF3 b0 but not in BAF3 bcl2 cells. In contrast, the addition of quercetin (an inhibitor of aldehyde reductase) in an attempt to diminish methional reduction increased apoptosis in both BAF3 b0 and BAF3 bcl2 cells. The extent of DNA fragmentation in BAF3 bcl2 cells approached that in BAF3 b0 cells in the presence of quercetin and exogenous methional, suggesting a defect in methional biosynthesis in BAF3 bcl2 cells. Direct evidence for this was obtained by measuring labelled methional in cells incubated with the sodium, salt of [U-14C]4-methylthio-2-oxobutanoic acid (MTOB), the precursor of methional. The 80% decrease in labelled methional in BAF3 bcl2 compared with BAF3 b0 cells was accompanied by a concomitant rise in the transamination of [14C]MTOB to [14C]methionine in BAF3 bcl2 cells. Inhibition of the transaminase, however, by a synthetic transition-state-type compound, pyridoxal-L-methionine ethyl ester, induced apoptosis in BAF3 b0 but not in BAF3 bcl2 cells, confirming that the defect in BAF3 bcl2 cells was not in the transaminase itself but rather in the oxidative decarboxylation step MTOB --> methional. In addition, no evidence was obtained for the synthesis of [14C]malondialdehyde from [14C]methional in BAF3 bcl2 cells. As these cells show no deficiency in their content of reactive oxygen species compared with that of BAF3 b0 cells, they may possess some other defect in the beta-hydroxylase enzyme system itself.

Production of reactive oxygen species by hemocytes from the marine mussel, Mytilus edulis: lysosomal localization and effect of xenobiotics

Hemolymph of M. Edulis is rich in phagocytic hemocytes. Hemocytes contain numerous lysosomes which, in turn, contain various hydrolytic enzymes. Phagocytic activity of M. edulis hemocytes is thought to be associated with NAD(P)H-oxidase activity of the plasma membrane. The laser dye, dihydrorhodamine 123 (DHR), was used for cytochemical and biochemical detection of the generation of reactive oxygen species (ROS) by isolated M. edulis hemocytes. Hemocytes readily take up DHR from the suspension medium and selectively concentrate it in the lysosomes, wherein DHR is oxidized to fluorescent rhodamine 123. Concomitant uptake of DHR with superoxide dismutase or the spin-trap, tert-phenylbutyl nitrone, but not catalase markedly reduced fluorescence in the lysosomes implicating superoxide anion (O2-) but not hydrogen peroxide (H2O2) in DHR oxidation. Uptake of the anthraquinone, purpurin, and FeEDTA with DHR greatly amplified fluorescence within the lysosomes. These data are consistent with uptake of xenobiotics by hemocytes and their concentration in lysosomes wherein, ROS are generated in response to their accumulation. The rate of DHR oxidation by hemocytes was not stimulated by zymosan, a known stimulator of the oxidative burst. In vitro studies using the xanthine oxidase/hypoxanthine reaction to generate O2- and selective inhibitors of ROS production indicated that DHR is oxidized by O2- and H2O2 but not by .OH and that iron can participate in the reaction. Incubating isolated hemocytes promoted low-level, SOD-sensitive, FeEDTA-stimulated production of ethylene from alpha-keto-gamma-methiolbutyric acid, indicating the in situ formation of .OH via production of O2-. The above suggest that enhanced production of ROS in M. edulis hemocytes by xenobiotic accumulation within the lysosomal compartment should be considered in the toxic sequelae of exposure of marine molluscs to chemical pollutants.

Methional derived from 4-methylthio-2-oxobutanoate is a cellular mediator of apoptosis in BAF3 lymphoid cells

4-Methylthio-2-oxobutanoic acid is the direct precursor of methional, which is a potent inducer of apoptosis in a BAF3 murine lymphoid cell line which is interleukin-3 (IL3)-dependent. Cultures treated for 8 h with methional in the presence of IL3 show extensive DNA double-strand breaks on flow cytometric analysis, increases in DNA fragmentation as measured by the amount of non-sedimentable DNA present in the 30,000 g supernatant of cell lysates and the typical laddering pattern of multiples of 180 bp seen upon agarose gel electrophoresis. No such features of apoptosis were found in cells treated with 4-methylthio 2-oxobutanoic acid or propanal, suggesting that the simultaneous presence of the methylthio group on the propanal moiety is essential for apoptosis to take place. Methional is further metabolized in cells by two reactions: oxidation via aldehyde dehydrogenase to (methylthio)propionic acid or beta-hydroxylation to malondialdehyde. The formation of malondialdehyde from methional in vitro by chemical hydroxylation under the conditions of the Fenton reaction provides a mechanism for the beta-hydroxylation which takes place in vivo. During apoptosis induced by IL3 deprivation, the ratio of 2,4-DNPH MDA to 2,4-DNPH methional is 0.94 in cells in IL3- medium compared with 0.54 in cells in IL3+ medium. These results support a role of cellular methional and malondialdehyde in apoptosis.

Fetal alcohol syndrome: the vulnerability of the developing brain and possible mechanisms of damage

Fetal alcohol exposure has multiple deleterious effects on brain development, and represents a leading known cause of mental retardation. This review of the effects of alcohol exposure on the developing brain evaluates results from human, animal and in vitro studies, but focuses on key research issues, including possible mechanisms of damage. Factors that affect the risk and severity of fetal alcohol damage also are considered.

Towards the physiological function of uric acid

Uric acid, or more correctly (at physiological pH values), its monoanion urate, is traditionally considered to be a metabolically inert end-product of purine metabolism in man, without any physiological value. However, this ubiquitous compound has proven to be a selective antioxidant, capable especially of reaction with hydroxyl radicals and hypochlorous acid, itself being converted to innocuous products (allantoin, allantoate, glyoxylate, urea, oxalate). There is now evidence for such processes not only in vitro and in isolated organs, but also in the human lung in vivo. Urate may also serve as an oxidisable cosubstrate for the enzyme cyclooxygenase. As shown for the coronary system, a major site of production of urate is the microvascular endothelium, and there is generally a net release of urate from the human myocardium in vivo. In isolated organ preparations, urate protects against reperfusion damage induced by activated granulocytes, cells known to produce a variety of radicals and oxidants. Intriguingly, urate prevents oxidative inactivation of endothelial enzymes (cyclooxygenase, angiotensin converting enzyme) and preserves the ability of the endothelium to mediate vascular dilatation in the face of oxidative stress, suggesting a particular relationship between the site of urate formation and the need for a biologically potent radical scavenger and antioxidant.

High-performance liquid chromatographic determination of methanesulphinic acid as a method for the determination of hydroxyl radicals

For the determination of hydroxyl radicals, dimethyl sulphoxide was used as a molecular probe and the methanesulphinic acid produced was determined by high-performance liquid chromatography of its Fast Yellow GC salt derivative. The results for hydroxyl radicals formed using the Fenton and hypoxanthine-xanthine oxidase systems agreed well with the theoretical values. Interferences from phenols, aromatic amines and amino acids, which give coloured substances by reaction with the diazonium salt, could be avoided. The recovery of methanesulphinic acid added to liver homogenates and incubated for 1 h at 37 degrees C was 70.2 +/- 2.1%. The detection limit for methanesulphinic acid in a sample solution was ca. 8 ng/ml.

A new assay for lignin-type peroxidases employing the dye azure B

The discovery in 1983 of fungal "ligninases" capable of catalyzing the peroxidation of nonphenolic aromatic lignin components has been seen as a major advance in understanding how certain basidiomycete fungi can completely degrade lignin. The ability of these lignin-type peroxidases to covert millimolar concentrations of veratryl alcohol to veratraldehyde, indicated by a change in the A310 of veratraldehyde, has become the standard assay for routine quantitation of LP activity. A new assay based on the oxidation of micromolar concentrations of the dye Azure B is presented. Although it is as simple and rapid as the veratryl alcohol assay, it appears to overcome some of the shortcomings of that assay. In particular, interference from UV- and short-wavelength visible-light-absorbing materials is greatly reduced and assay specificity is improved.

Lignin Peroxidase Activity Is Not Important in Biological Bleaching and Delignification of Unbleached Kraft Pulp by Trametes versicolor

The discovery in 1983 of fungal lignin peroxidases able to catalyze the oxidation of nonphenolic aromatic lignin model compounds and release some CO 2 from lignin has been seen as a major advance in understanding how fungi degrade lignin. Recently, the fungus Trametes versicolor was shown to be capable of substantial decolorization and delignification of unbleached industrial kraft pulps over 2 to 5 days. The role, if any, of lignin peroxidase in this biobleaching was therefore examined. Several different assays indicated that T. versicolor can produce and secrete peroxidase proteins, but only under certain culture conditions. However, work employing a new lignin peroxidase inhibitor (metavanadate ions) and a new lignin peroxidase assay using the dye azure B indicated that secreted lignin peroxidases do not play a role in the T. versicolor pulp-bleaching system. Oxidative activity capable of degrading 2-keto-4-methiolbutyric acid (KMB) appeared unique to ligninolytic fungi and always accompanied pulp biobleaching.

Pyridoindole stobadine is a potent scavenger of hydroxyl radicals

Stobadine is a potent scavenger of OH. radicals generated chemically in a free solution with kappa 2 higher than 10(10).M-1.s-1 as determined by two independent methods, namely destruction of deoxyribose and oxidation of 2-keto-4-methiolbutyric acid (KMBA). The high efficacy of stobadine to prevent ethylene production from KMBA was observed also in enzymatic (xanthine-xanthine oxidase-driven Fenton) and membrane-bound (NADPH-dependent microsomal electron transfer) sources of OH. radicals.

Inhibition of the oxidation of hydroxyl radical scavenging agents after alkaline phosphatase treatment of rat liver microsomes

Treatment of rat liver microsomes with alkaline phosphatase results in a loss in the FMN but not the FAD flavin prosthetic group of NADPH-cytochrome P-450 reductase (Taniguchi, H. and Pyerin, W. (1987) Biochim. Biophys. Acta 912, 295-307). Experiments were carried out to evaluate the effect of preventing electron transfer from the FADH2 to FMN component of the reductase, and subsequent mixed function oxidase activity, on reduction of ferric chelates, production of H2O2, and the generation of .OH-like species by microsomes. Treatment with alkaline phosphatase was confirmed to decrease NADPH-cytochrome c, but not NADPH-ferricyanide, reductase activity by microsomes and by purified NADPH cytochrome P-450 reductase. The oxidation of hydroxyl radical scavenging agents by microsomes and reductase was decreased by the alkaline phosphatase treatment in accordance with the decline in cytochrome c reductase activity. This decrease in hydroxyl radical production occurred in the presence of various ferric chelate catalysts. Rates of microsomal reduction of the ferric chelates were also inhibited after alkaline phosphatase treatment. Production of H2O2 was decreased in accordance to the fall in cytochrome c reductase activity and .OH production. Rates of H2O2 production appeared to be rate-limiting for the overall generation of .OH as the addition of an external H2O2-generating system stimulated .OH production as well as prevented the decline in .OH production caused by the alkaline phosphatase treatment. These results suggest that both the FAD and FMN flavin prosthetic groups of the reductase contribute towards the reduction of various ferric chelates. However, loss of the FMN component and activities dependent on electron transfer from this prosthetic group result in a decrease in H2O2 production, which appears to be responsible for the decline in the generation of .OH-like species by microsomes after treatment with alkaline phosphatase.

The involvement of free radicals in the mechanisms of monooxygenases

The occurrence of free radicals in the mechanisms of monooxygenases reflects the chemistry of dioxygen and the inertness of typical substrates. Thus, oxidation of such substrates requires attack by reduced dioxygen-derived free radicals. Consequently, a molecule of NAD(P)H must be invested for each substrate molecule oxidized. Furthermore, since free radicals are difficult to control, deviations from the intended reaction course are frequent. These considerations are illustrated by examination of the generation and fate of enzyme- and substrate-derived free radicals at various stages in the catalytic cycles of two monooxygenases important in xenobiotic biotransformation, dopamine beta-hydroxylase and cytochrome P-450.

Hydroxyl radical generation by postischemic rat kidney slices in vitro

To quantitate the formation of hydroxyl radicals (HO.) in ischemia and reoxygenation, dimethyl sulfoxide (DMSO) was added to "trap" evolving HO. in normal, in ischemic, and in ischemic and reoxygenated rat kidney slices, incubated in short-term organ culture in vitro. Hydroxyl radical generation was measured as the accumulation of the specific product of DMSO oxidation by HO., methane sulfinic acid (MSA) in the kidney tissue and surrounding medium using a new colorimetric assay. A mean difference of 7 nmol cumulative HO./gram tissue was detected in rat kidney slices subjected to ischemia and reoxygenation. This amount of HO. generation was not significantly greater than that found in nonischemic or in ischemic but not reoxygenated control tissues, and does not appear to represent the highly toxic burst of HO. radicals implied in current theoretical discussions of reperfusion injury. However, the addition of EDTA chelated iron (1:1) to the incubation medium led to marked postischemic HO. generation. We conclude that clearly toxic numbers of HO. radicals are not formed during reoxygenation in rat kidney slices, either because there is insufficient iron, because only a small fraction of cells in the kidney tissue make oxygen radicals, or because cellular defenses against HO. formation are more powerful than currently appreciated.

Interaction of ferric complexes with rat liver nuclei to catalyze NADH-and NADPH-Dependent production of oxygen radicals

The production of potent oxygen radicals by microsomal reaction systems has been well characterized. Relatively little attention has been paid to generation of oxygen radicals by liver nuclei, or to the interaction of nuclei with different ferric complexes to catalyze NADH- or NADPH-dependent production of reactive oxygen intermediates. Intact rat liver nuclei were capable of catalyzing an iron-dependent production of .OH as reflected by the oxidation of .OH scavenging agents such as 2-keto-4-thiomethylbutyrate, dimethyl sulfoxide, and t-butyl alcohol. Inhibition of .OH production by catalase implicates H2O2 as the precursor of .OH generated by the nuclei, whereas superoxide dismutase had only a partially inhibitory effect. The production of .OH with either cofactor was striking increased by addition of ferric-EDTA or ferric-diethylenetriamine-pentaacetic acid (DTPA) whereas ferric-ATP and ferric-citrate were not effective catalysts. All these ferric complexes were reduced by the nuclei in the presence of either NADPH or NADH. The pattern of iron chelate effectiveness in catalyzing lipid peroxidation by nuclei was opposite to that of .OH production; with either NADH or NADPH, nuclear lipid peroxidation was increased by the addition of ferric ammonium sulfate, ferric-ATP, or ferric-citrate, but not by ferric-EDTA or ferric-DTPA. NADPH-dependent nuclear lipid peroxidation was insensitive to catalase, superoxide dismutase, or .OH scavengers; the NADH-dependent reaction showed a partial sensitivity (30 to 40%) to these additions. The overall patterns of .OH production and lipid peroxidation by the nuclei are similar to those shown by microsomes, e.g., effect of ferric complexes, sensitivity to antioxidants; however, rates with the nuclei are less than 20% those of microsomes, which reflect the lower activities of NADPH- and NADH-cytochrome c reductase in the nuclei. The potential for nuclei to reduce ferric complexes and catalyze production of .OH-like species may play a role in the susceptibility of the genetic material to oxidative damage under certain conditions since such radicals would be produced site-directed and not exposed to cellular antioxidants.

Temperature dependence of the microsomal oxidation of ethanol by cytochrome P450 and hydroxyl radical-dependent reactions

The temperature dependence and activation energies for the oxidation of ethanol by microsomes from controls and from rats treated with pyrazole was evaluated to determine whether the overall mechanism for ethanol oxidation by microsomes was altered by the pyrazole treatment. Arrhenius plots of the temperature dependence of ethanol oxidation by pyrazole microsomes were linear and exhibited no transition breaks, whereas a slight break was observed at about 20 +/- 2.5 degrees C with control microsomes. Energies of activation (about 15-17 kcal/mol) were identical for the two microsomal preparations. Although transition breaks were noted for the oxidation of substrates such as dimethylnitrosamine and benzphetamine, activation energies for these two substrates were similar for control microsomes and microsomes from the pyrazole-treated rats. The addition of ferric-EDTA to the microsomes increased the rate of ethanol oxidation by a hydroxyl radical (.OH)-dependent pathway. Arrhenius plots of the .OH-dependent oxidation of ethanol by both microsomal preparations were linear with energies of activation (about 7 kcal/mol) that were considerably lower than values found for the P450-dependent pathway. These results suggest that, at least in terms of activation energy, the increase in microsomal ethanol oxidation by pyrazole treatment is not associated with any apparent change in the overall mechanism or rate-limiting step for ethanol oxidation but likely reflects induction of a P450 isozyme with increased activity toward ethanol. The lower activation energy for the .OH-dependent oxidation of ethanol suggests that different steps are rate limiting for oxidation of ethanol by .OH and by P450, which may reflect the different enzyme components of the microsomal electron transfer system involved in these reactions.

In vivo formation of hydroxyl radicals following intragastric administration of ferrous salt in rats

Accidental poisoning by oral iron preparations is a serious problem in young children. We investigated the formation of hydroxyl radicals (.OH) in rats after intragastric instillation of ferrous sulfate. .OH was detected via its reaction with intragastrically administered 2-keto-4-methylthiobutyrate to generate ethylene gas. Ascorbic acid is typically present in oral iron preparations in order to facilitate absorption by maintaining iron in the reduced state. However, ascorbate possesses two properties that can affect .OH, recycling of oxidized iron to the ferrous state augments .OH production, while ascorbate in high concentration scavenges .OH. In experiments conducted in vitro, both actions were evident, depending upon the concentration of ascorbate. In parallel experiments conducted in vivo, the scavenging action of ascorbate was more prominent. Experiments in vitro with .OH-scavengers (dimethylsulfoxide, ethanol) and with the enzyme, catalase, confirmed both the presence of .OH and its dependence upon generated hydrogen peroxide during the oxidation of ferrous salt by molecular oxygen. Hydroxyl radicals (and/or reactive higher oxidation states of iron) may play a role in tissue damage after accidental overdose of oral iron.

Production of ethane by rats treated with the colon carcinogen, 1,2-dimethylhydrazine

Ethane was exhaled by rats treated with the colon carcinogen, 1,2-dimethylhydrazine (DMH). At 1 hr, ethane production (mean +/- SD) was 0.2 +/- 0.2 nmol/kg (controls) and 5.2 +/- 1.3, 13.7 +/- 3.4, and 27.7 +/- 9.6, respectively, for DMH injections of 0.15 mmol/kg (20 mg/kg of the dihydrochloride salt), 0.45 mmol/kg, and 1.35 mmol/kg. Rates of ethane evolution tapered off after 2 hr, but persisted for up to 3 hr at the lower dose, and up to 5-6 hr at the higher dose. Although ethane is produced in vivo during lipid peroxidation, experiments with vitamin E, a potent lipid antioxidant, indicated that lipid peroxidation was unlikely to be the source of ethane in DMH-treated rats: pretreatment with vitamin E had no effect on ethane formation from DMH but did suppress ethane production from rats treated with carbon tetrachloride, an inducer of hepatic lipid peroxidation. When rats were injected with 1,2-diethylhydrazine in place of DMH, large amounts of ethane and ethylene were produced (9800 and 5600 nmol/kg/hr). The hydrocarbon gases exhaled by rats may arise from dimerization of methyl radicals (.CH3) generated during the metabolism of DMH, and from ethyl radicals (.CH2CH3) generated during the metabolism of 1,2-diethylhydrazine. Previously, it was shown that methane and ethane are formed from methyl radicals in vitro. Other investigators have observed formation of hydrocarbon gases during the in vitro metabolism of monoalkylhydrazines by microsomes, and ethyl radicals, derived from monoethylhydrazine, have been detected by electron spin-resonance spectroscopy. The results presented here suggest that in vivo metabolism of DMH may produce methyl radicals. Methyl radicals are capable of interacting with biomolecules. Their indiscriminate reaction with tissue constituents may be a contributory factor in DMH-induced carcinogenesis.

Association between alcoholism and increased hepatic iron stores

Although alcoholic liver disease is often associated with some increase in hepatic iron stores, it is now established that when gross iron overload is present, this is due to genetic hemochromatosis. Furthermore, there appears to be a critical iron concentration necessary for the induction of hepatic fibrosis. Lipid peroxidation induced by ethanol and/or iron would appear to play a major role in hepatic damage in both humans and experimental animals. Although the exact mechanism(s) of induction of lipid peroxidation by ethanol and iron remains to be elucidated, both toxins can exert a synergistic effect upon hepatic lipid peroxidation. Iron overload has also been shown to stimulate directly hepatocyte and hepatic procollagen mRNA expression, which is further stimulated by ethanol. The observed synergism between iron and alcohol with respect to both hepatic lipid peroxidation and collagen biosynthesis offers a possible explanation of the apparent early onset of fibrosis and cirrhosis in patients with iron overload who have an excessive alcohol intake.

Chemiluminescence studies on the generation of oxygen radicals from the interaction of NADPH-cytochrome P-450 reductase with iron

The ability of NADPH-cytochrome P-450 reductase to interact with iron and generate oxygen radicals was evaluated by assaying for low level chemiluminescence. The basic reaction system which contained the reductase, an NADPH-generating system, ferric-EDTA as an electron acceptor, and t-butyl hydroperoxide as the oxidant acceptor, resulted in the production of chemiluminescence. Omission of any of these components resulted in a complete loss of chemiluminescence. The light emission was completely sensitive to inhibition by glutathione and butylated hydroxytoluene, partially sensitive (about 60% decrease) to catalase and hydroxyl radical scavengers, and relatively insensitive (about 20% decrease) to superoxide dismutase. The ability of other ferric chelates to replace ferric-EDTA in catalyzing the reductase-dependent chemiluminescence was evaluated. Ferric-citrate, -ADP, -ATP, and ferric-ammonium sulfate were ineffective in promoting chemiluminescence, whereas ferric-diethylenetriaminepentaacetic acid was even more effective than ferric-EDTA. Thus, the ferric chelates, which catalyze reductase-dependent chemiluminescence, are those which are efficient electron acceptors from the reductase and were previously shown to be those capable of catalyzing hydroxyl radical production by microsomes and the reductase. It is suggested that chemiluminescence results from (a) the direct interaction of the reduced iron chelate with the hydroperoxide (Fenton-type of reaction) to generate alkoxyl and peroxyl radicals, and (b) the generation of hydroxyl radicals, which subsequently react with the hydroperoxide to generate secondary radicals. The latter, but not the former, would be sensitive to inhibition by catalase and competitive hydroxyl radical scavengers, whereas both would be sensitive to antioxidants such as butylated hydroxytoluene. Chemiluminescence appears to be a versatile tool for studying the reductase-dependent generation of oxygen radicals and for the interaction of reductase with iron.

Production of 4-hydroxypyrazole from the interaction of the alcohol dehydrogenase inhibitor pyrazole with hydroxyl radical

Pyrazole, an effective inhibitor of alcohol dehydrogenase, was previously shown to be a scavenger of the hydroxyl radical. 4-Hydroxypyrazole is a major metabolite in the urine of animals administered pyrazole in vivo. Experiments were conducted to show that 4-hydroxypyrazole was a product of the interaction of pyrazole with hydroxyl radical generated from three different systems. The systems utilized were the iron-catalyzed oxidation of ascorbate, the coupled oxidation of hypoxanthine by xanthine oxidase, and NADPH-dependent microsomal electron transfer. Ferric-EDTA was added to all the systems to catalyze the production of hydroxyl radicals. A HPLC procedure employing either uv detection or electrochemical detection was utilized to assay for the production of 4-hydroxypyrazole. The three systems all supported the oxidation of pyrazole to 4-hydroxypyrazole by a reaction which was sensitive to inhibition by competitive hydroxyl radical scavengers such as ethanol, mannitol, or dimethyl sulfoxide and to catalase. The sensitivity to catalase implicates H2O2 as the precursor of the hydroxyl radical by all three systems. Superoxide dismutase inhibited production of 4-hydroxypyrazole only in the xanthine oxidase reaction system. In the absence of ferric-EDTA (and azide), microsomes catalyzed the oxidation of pyrazole to 4-hydroxypyrazole by a cytochrome P-450-dependent reaction which was independent of hydroxyl radicals. This latter pathway may be primarily responsible for the in vivo metabolism of pyrazole to 4-hydroxypyrazole. The production of 4-hydroxypyrazole from the interaction of pyrazole with hydroxyl radicals may be a sensitive, rapid technique for the detection of these radicals in certain tissues or under certain conditions, e.g., increasing oxidative stress.

Hydroxyl radical generation by microsomes after chronic ethanol consumption

The effect of iron and other compounds known to be toxic because of the production of oxygen radicals, e.g., paraquat and menadione on the generation of hydroxyl radicals (.OH) by microsomes from chronic ethanol-fed rats and their pair-fed controls was determined. In the absence of any additions, or in the presence of ferric-chloride, -ADP or -EDTA, microsomes from the ethanol-fed rats showed a 2-fold increase in the production of .OH. Paraquat and menadione increased the generation of .OH by microsomes from the ethanol-fed and the pair-fed controls to an identical extent and thus these promoters of oxidative stress were not any more effective in interacting with microsomes after ethanol treatment. Under all conditions, .OH generation was sensitive to inhibition by catalase, implicating H2O2 as the precursor of .OH, whereas superoxide dismutase was without any significant effect. A working scheme to accommodate aspects of the interaction of iron, menadione and paraquat with microsomes with the subsequent production of .OH is described. The fact that .OH generation by microsomes in the presence of several sources of iron such as unchelated iron or ferric-ADP is elevated after chronic ethanol consumption could contribute to the hepatotoxic effects of ethanol. Studies on iron metabolism by liver cells and the effect of ethanol on the disposition of this critical trace metal are needed to further evaluate the role of oxygen radicals in the actions of ethanol.

NADPH- and linoleic acid hydroperoxide-induced lipid peroxidation and destruction of cytochrome P-450 in hepatic microsomes

Temporal aspects of the effects of inhibitors on hepatic cytochrome P-450 destruction and lipid peroxidation induced by NADPH and linoleic acid hydroperoxide (LAHP) were compared. In the absence of added Fe2+, NADPH-induced lipid peroxidation in hepatic microsomes exhibited a slow phase followed by a fast phase. The addition of Fe2+ eliminated the slow phase, thus demonstrating that iron is a rate-limiting component in the reaction. EDTA, which complexes iron, and p-chloromercurobenzoate (pCMB), which inhibits NADPH-cytochrome P-450 reductase, inhibited both phases of the reaction. Catalase as well as scavengers of hydroxyl radical, inhibited NADPH-induced lipid peroxidation almost completely. GSH also inhibited the NADPH-dependent reaction but only when added at the beginning of the reaction. In contrast with NADPH-dependent lipid peroxidation, the autocatalytic reaction induced by LAHP was not biphasic, NADPH-dependent or iron-dependent, nor was it inhibited by hydroxyl radical scavengers, catalase or GSH. A synergistic effect on lipid peroxidation was observed when both NADPH and LAHP were added to microsomes. It is concluded that both the fast and slow phases of NADPH-dependent microsomal lipid peroxidation are catalyzed enzymatically and are dependent upon Fe2+, whereas LAHP-dependent lipid peroxidation is autocatalytic. Since the fast phase of enzymatic lipid peroxidation occurred during the fast phase of destruction of cytochrome P-450, it is postulated that iron made available from cytochrome P-450 is sufficient to promote optimal lipid peroxidation. Since catalase and hydroxyl radical scavengers inhibited NADPH-dependent but not LAHP-dependent lipid peroxidation, it is concluded that the hydroxyl radical derived from H2O2 is the initiating active-oxygen species in the enzymatic reaction but not in the autocatalytic reaction.

Lipid peroxidation and mechanisms of toxicity

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

Free radical metabolism of alcohols by rat liver microsomes

By using e.s.r. spectroscopy coupled with the spin trapping technique we have detected the formation of free radical intermediates by rat liver microsomes incubated with either ethanol, 2-propanol or 2-butanol in the presence of a NADPH regenerating system and 4-pyridyl-l-oxide-t-butyl nitrone (4-POBN) as spin trap. The e.s.r. spectra have been identified as due to the hydroxyalkyl free radical adducts of 4-POBN. The free radical formation depends upon the activity of the microsomal monoxygenase system and is blocked by omitting NADP+ from the incubation mixture, by anaerobic incubation or by enzyme denaturation. The involvement of hydroxyl radicals (OH.) produced through a Fenton-type reaction from endogenously formed hydrogen peroxide is suggested by the opposite effects exerted on the e.s.r. signal intensity by azide and catalase. Consistently, iron chelation by desferrioxamine inhibits the free radical formation, while the supplementation of EDTA-iron increases it by several fold. Inhibitors of cytochrome P450-dependent monoxygenase system reduce to various extents the production of free radical intermediates suggesting that reactive oxygen species might be formed at the active site of cytochrome P450 where they react with alkyl alcohol molecules. The data presented support the hypothesis that free radical species are generated during the microsomal metabolism of alcohols and suggest the possibility that ethanol-derived radicals might play a role in the pathogenesis of the liver lesions consequent upon alcoholic abuse.

Prevention of microsomal production of hydroxyl radicals, but not lipid peroxidation, by the glutathione-glutathione peroxidase system

The glutathione-glutathione peroxidase system is an important defense against oxidative stress. The ability of this system to protect against iron-catalyzed microsomal production of hydroxyl radicals [oxidation of 4-methylmercapto-2-oxo-butyrate (KMBA)] and lipid peroxidation was evaluated. When rat liver cytosol was added to microsomes, strong inhibition against KMBA oxidation was observed. No protection was found when the cytosol was boiled or dialyzed. In the latter case, the addition of 0.5 mM glutathione restored almost complete protection, whereas in the former case protection could be restored by the addition of both glutathione and glutathione peroxidase. Cysteine could not replace glutathione, nor could glutathione S-transferase replace glutathione peroxidase. The glutathione-glutathione peroxidase system was also very effective in decreasing production of hydroxyl radicals stimulated by the addition of menadione or paraquat to microsomes. In the absence of cytosol, the addition of glutathione plus glutathione peroxidase was also effective; however, 5 mM glutathione was necessary to protect against KMBA oxidation. The effective concentration of glutathione required for protection was lowered when glutathione reductase was added to the system, to regenerate reduced glutathione. These results indicate that low concentrations of glutathione in conjunction with glutathione peroxidase plus reductase can be very effective in preventing microsomal formation of hydroxyl radicals catalyzed by iron and other toxic compounds. Microsomal lipid peroxidation was decreased 40% by glutathione alone, and this decrease was potentiated in the presence of glutathione reductase. In contrast to KMBA oxidation, the combination of glutathione plus glutathione peroxidase was not any more effective than glutathione alone in preventing lipid peroxidation. The differences in sensitivities of microsomal lipid peroxidation and KMBA oxidation to glutathione peroxidase suggest that these two processes can be distinguished from each other, and that free H2O2 and hydroxyl radicals are involved in KMBA oxidation, but not lipid peroxidation.

Complex-formation and reduction of ferric iron by 2-oxo-4-thiomethylbutyric acid, and the production of hydroxyl radicals

2-Oxo-4-thiomethylbutyric acid (OMBA) is a widely used oxygen-radical-scavenging agent and has been used for the detection of .OH-like species in a variety of systems. This scavenger reacts with other radicals and is therefore not specific for .OH. Since iron is required in most systems for the generation of OH-like species, studies were carried out to investigate the possible interaction of OMBA with iron. Fe3+ reacted with OMBA to produce complexes that gave rise to discrete spectra. Intense purple complexes, with broad absorbance maxima of 525-550 nm, were found at OMBA/Fe3+ ratios of up to 1:1, whereas red complexes with a prominent shoulder between 440 and 480 nm were found at higher OMBA/Fe3+ ratios. OMBA caused reduction of ferric iron to the ferrous state, as detected with 2,2'-bipyridyl as the indicator. This reduction occurs in the dark, can be photo-accelerated especially by light with wavelengths near the absorbance maximum of the respective complexes, and is increased as the OMBA/Fe3+ ratio is elevated. The presence of phosphate buffer quenches the purple and red ferric-ion-OMBA complexes and lowers the rate of reduction of Fe3+ by OMBA about 10-fold. The resulting ferrous-ion-OMBA-phosphate complex is very stable against autoxidation. Both the ferrous-ion-OMBA and ferric-ion-OMBA complexes reacted with H2O2, with the subsequent production of ethylene gas from OMBA. The interaction with H2O2 resulted in discrete spectral changes of both the ferrous-ion-OMBA and ferric-ion-OMBA complexes. The ferrous-ion-OMBA/H2O2 or ferric-ion-OMBA/H2O2 system appeared to produce .OH free radicals via a Fenton-type of reaction since ethylene production was inhibited by competitive OH scavengers. Ferrous-ion-OMBA complex reacted with H2O2 not only to produce ethylene from the OMBA, but also to promote the oxidation of another scavenger, ethanol. The ability of OMBA to chelate iron, to promote reduction of ferric iron and to react with H2O2 to produce potent oxidizing radicals may play a role in the lack of specificity of OMBA as a scavenger of oxygen radicals.

Enhanced formation of ethane and n-pentane by rat hepatocytes in the presence of dimethyl sulfoxide

Incubation of rat hepatocytes with 14 mM dimethyl sulfoxide (DMSO) produced an increase in the formation of ethane, measured by capillary column gas chromatography, to 18.0 pmoles/hr/10(7) cells from 11.2 pmoles/hr/1-(7) cells from 5.6 pmoles/hr/10(7) cells in control hepatocytes. This was about one-third the stimulation of ethane and n-pentane formation produced by incubation of hepatocytes with 13 mM carbon tetrachloride. DMSO-stimulated ethane and n-pentane formation was inhibited up to 63% by 0.1 microM alpha-tocopherol and up to 89% by N2. Formation of dimethylsulfide from DMSO by hepatocytes was the same in air and N2. DMSO increased methane production by hepatocytes to 31.3 pmoles/hr/10(7) cells from 6.9 pmoles/hr/10(7) cells in control hepatocytes. Although DMSO apparently stimulated lipid peroxidation by hepatocytes, as measured by ethane and n-pentane formation, there was no increase in the formation of thiobarbituric acid reactive material. DMSO was not toxic to hepatocytes, measured by release of cytosolic lactate dehydrogenase, over a 2-hr incubation. Possible mechanisms for the increase in alkane formation by DMSO are discussed.

Inhibition of mitochondrial aldehyde dehydrogenase and acetaldehyde oxidation by the glutathione-depleting agents diethylmaleate and phorone

Experiments were carried out to study the effect of two commonly used glutathione-depleting agents, diethylmaleate and phorone, on the oxidation of acetaldehyde and the activity of aldehyde dehydrogenase. The oxidation of acetaldehyde by intact hepatocytes was inhibited when the cells were incubated with diethylmaleate. Washing and resuspending the cells in diethylmaleate-free medium afforded protection against the inhibition of acetaldehyde oxidation. The oxidation of acetaldehyde by isolated rat liver mitochondria as well as by disrupted mitochondria in the presence of excess NAD+ was inhibited by diethylmaleate or phorone, indicating inhibition of the low-Km aldehyde dehydrogenase. In addition, diethylmaleate inhibited oxidation of acetaldehyde by the high-Km cytosolic aldehyde dehydrogenase. Significant accumulation of acetaldehyde occurred when ethanol was oxidized by hepatocytes in the presence, but not in the absence, of diethylmaleate. Thus, diethylmaleate blocks the oxidation of added or metabolically generated acetaldehyde, analogous to results with other inhibitors of the low-Km aldehyde dehydrogenase such as cyanamide. These results suggest that caution should be used in interpreting the effects of diethylmaleate or phorone on metabolic reactions, especially those involving metabolism of aldehydes such as formaldehyde, because, in addition to depleting glutathione, these agents inhibit the low-Km aldehyde dehydrogenase.

Microsomal interactions between iron, paraquat, and menadione: effect on hydroxyl radical production and alcohol oxidation

The addition of menadione or paraquat to rat liver microsomes resulted in about a threefold increase in the production of hydroxyl radical (.OH) as reflected by the increased oxidation of 2-keto-4-thiomethylbutyric acid (KMBA) to ethylene. This increase was not sensitive to superoxide dismutase but was blocked by catalase. The increase occurred in the absence of added iron and was not affected by the potent iron chelating agent, desferrioxamine, which suggests the possibility that .OH was produced from an interaction between H2O2 and the paraquat or menadione radical. Menadione and paraquat were especially effective in stimulating the oxidation of KMBA in the presence of certain iron chelates such as ferric-ADP, -ATP, or -EDTA, but not ferric-desferrioxamine, -citrate, or -histidine, or unchelated iron. In fact, ferric-ADP or -ATP only stimulated .OH production in the presence of menadione or paraquat. In the presence of ferric-EDTA, the greater than additive increase of .OH production was sensitive to catalase, but not to superoxide dismutase, suggesting the possibility of reduction of ferric-EDTA by paraquat or menadione radical. The interactions with ferric adenine nucleotides may increase the catalytic effectiveness of menadione or paraquat in producing potent oxidants such as the hydroxyl radical, and thus play a role in the toxicity associated with these agents. Paraquat and menadione had little effect on the overall oxidation of ethanol by microsomes. Microsomal drug metabolism was decreased by menadione or paraquat. As a consequence, the effect of these agents on the microsomal oxidation of ethanol was complex since it appeared that paraquat and menadione stimulated the oxidation of ethanol by a .OH-dependent mechanism, but inhibited the oxidation of ethanol by a cytochrome P-450-dependent oxidation pathway. Experiments with carbon monoxide, ferric-EDTA, and 2-butanol plus catalase tended to verify that microsomal oxidation of alcohols was increased by a .OH-dependent pathway when menadione or paraquat were added to microsomes.

Interaction of pyrazole and 4-methylpyrazole with hepatic microsomes: effect on cytochrome P-450 content, microsomal oxidation of alcohols, and binding spectra

Microsomes isolated from rats treated with either pyrazole or 4-methylpyrazole, potent inhibitors of alcohol dehydrogenase, catalyzed the oxidation of ethanol and 2-butanol at rates 2-3-fold higher than saline controls. Time course experiments and dose-response experiments indicated that an increase in the microsomal oxidation of alcohols could be observed 24 hr after a single treatment with 200 mg/kg body weight of either pyrazole or 4-methylpyrazole, and after 2 or 3 days of treatment with 50 mg/kg of either of these compounds. The pyrazole treatment did not change the activity of NADPH-cytochrome P-450 reductase, the content of cytochrome P-450, or the oxidation of aminopyrine. Hence, microsomal oxidation of alcohols was increased by the pyrazole treatment whether results were expressed "per mg of protein" or "per nmol of P-450." Microsomes from the pyrazole-treated rats displayed an increase in binding spectrum with ethanol as the substrate as compared to controls, as well as type 2 binding spectrum with dimethyl sulfoxide and 2-butanol. These results suggest the possibility that pyrazole may induce an alcohol-preferring P-450 isozyme. By contrast, the 4-methylpyrazole treatment, besides increasing the oxidation of alcohols, also increased the oxidation of aminopyrine and the content of cytochrome P-450. The increase in the oxidation of alcohols and aminopyrine was primarily due to the increase in content of P-450 produced by the 4-methylpyrazole treatment. Binding spectra with dimethyl sulfoxide and 2-butanol were also observed after 4-methylpyrazole treatment; however, the 2-butanol-binding spectrum was a modified type 1 spectrum, not type 2.(ABSTRACT TRUNCATED AT 250 WORDS)