Cytochrome P-450 as a microsomal peroxidase utilizing a lipid peroxide substrate

Oxygen Surrogate Systems for Supporting Human Drug-Metabolizing Cytochrome P450 Enzymes

Oxygen surrogates (OSs) have been used to support cytochrome P450 (P450) enzymes for diverse purposes in drug metabolism research, including reaction phenotyping, mechanistic and inhibition studies, studies of redox partner interactions, and to avoid the need for NADPH or a redox partner. They also have been used in engineering P450s for more cost-effective, NADPH-independent biocatalysis. However, despite their broad application, little is known of the preference of individual P450s for different OSs or the substrate dependence of OS-supported activity. Furthermore, the biocatalytic potential of OSs other than cumene hydroperoxide (CuOOH) and hydrogen peroxide (H₂O₂) is yet to be explored. Here, we investigated the ability of the major human drug-metabolizing P450s, namely CYP3A4, CYP2C9, CYP2C19, CYP2D6, and CYP1A2, to use the following OSs: H₂O₂, tert-butyl hydroperoxide (tert-BuOOH), CuOOH, (diacetoxyiodo)benzene, and bis(trifluoroacetoxy)iodobenzene. Overall, CuOOH and tert-BuOOH were found to be the most effective at supporting these P450s. However, the ability of P450s to be supported by OSs effectively was also found to be highly dependent on the substrate used. This suggests that the choice of OS should be tailored to both the P450 and the substrate under investigation, underscoring the need to employ screening methods that reflect the activity toward the substrate of interest to the end application. SIGNIFICANCE STATEMENT: Cytochrome P450 (P450) enzymes can be supported by different oxygen surrogates (OSs), avoiding the need for a redox partner and costly NADPH. However, few data exist comparing relative activity with different OSs and substrates. This study shows that the choice of OS used to support the major drug-metabolizing P450s influences their relative activity and regioselectivity in a substrate-specific fashion and provides a model for the more efficient use of P450s for metabolite biosynthesis.

The relationships between cytochromes P450 and H2O2: Production, reaction, and inhibition

In this review we address the relationship between cytochromes P450 (P450) and H2O2. This association can affect biology in three distinct ways. First, P450s produce H2O2 as a byproduct either during catalysis or when no substrate is present. This reaction, known as uncoupling, releases reactive oxygen species that may have implications in disease. Second, H2O2 is used as an oxygen-donating co-substrate in peroxygenase and peroxidase reactions catalyzed by P450s. This activity has proven to be important mainly in reactions involving prokaryotic P450s, and investigators have harnessed this reaction with the aim of adaptation for industrial use. Third, H2O2-dependent inhibition of human P450s has been studied in our laboratory, demonstrating heme destruction and also the inactivating oxidation of the heme-thiolate ligand to a sulfenic acid (-SOH). This reversible oxidative modification of P450s may have implications in the prevention of uncoupling and may give new insights into the oxidative regulation of these enzymes. Research has elucidated many of the chemical mechanisms involved in the relationship between P450 and H2O2, but the application to biology is difficult to evaluate. Further studies are needed reveal both the harmful and protective natures of reactive oxygen species in an organismal context.

Intranasal Toxicity of BMS-181885, A Novel 5-HT1 Agonist

One-month intranasal toxicity studies were conducted with BMS-181885 at doses of 1.5, 9, or 15 mg/animal/day in rats and 4, 24, or 40 mg/animal/day in monkeys. A 1-month intermittent intranasal toxicity study was also conducted in monkeys at doses of 3, 6, and 12 mg/animal 3 days per week. BMS-181885 was generally well tolerated in rats but resulted in dose-dependent nasal mucosal injury, primarily characterized by subacute inflammation of the nasal mucosa, and degeneration, single-cell necrosis, and/or erosion of the olfactory epithelium and, to a lesser extent, the respiratory epithelium. In monkeys, daily BMS-181885 administration was well tolerated and produced similar dose-dependent nasal injury primarily characterized by subacute inflammation of the nasal mucosa with degeneration and erosion of the olfactory epithelium. In a separate experiment, intermittent administration also resulted in dose-dependent nasal injury. In cultured rat nasal mucosal cells, BMS-181885 was toxic to olfactory epithelial cells with a range of mean IC50s between 44 and 291 μM. In contrast, BMS-181885 had no effect on respiratory epithelial cells up to its maximum solubility. Cytochrome P450 inhibition had no effect on the toxicity of BMS-181885 in olfactory epithelial cells but produced dose-dependent toxicity in respiratory epithelial cells, which was not present previously. The in vitro data suggest that parent drug, rather than a toxic metabolite, caused the drug-associated nasal mucosal injury.

Involvement of Cytochrome P450 in Reactive Oxygen Species Formation and Cancer

This review examines the involvement of cytochrome P450 (CYP) enzymes in the formation of reactive oxygen species in biological systems and discusses the possible involvement of reactive oxygen species and CYP enzymes in cancer. Reactive oxygen species are formed in biological systems as byproducts of the reduction of molecular oxygen and include the superoxide radical anion (∙O2-), hydrogen peroxide (H2O2), hydroxyl radical (∙OH), hydroperoxyl radical (HOO∙), singlet oxygen ((1)O2), and peroxyl radical (ROO∙). Two endogenous sources of reactive oxygen species are the mammalian CYP-dependent microsomal electron transport system and the mitochondrial electron transport chain. CYP enzymes catalyze the oxygenation of an organic substrate and the simultaneous reduction of molecular oxygen. If the transfer of oxygen to a substrate is not tightly controlled, uncoupling occurs and leads to the formation of reactive oxygen species. Reactive oxygen species are capable of causing oxidative damage to cellular membranes and macromolecules that can lead to the development of human diseases such as cancer. In normal cells, intracellular levels of reactive oxygen species are maintained in balance with intracellular biochemical antioxidants to prevent cellular damage. Oxidative stress occurs when this critical balance is disrupted. Topics covered in this review include the role of reactive oxygen species in intracellular cell signaling and the relationship between CYP enzymes and cancer. Outlines of CYP expression in neoplastic tissues, CYP enzyme polymorphism and cancer risk, CYP enzymes in cancer therapy and the metabolic activation of chemical procarcinogens by CYP enzymes are also provided.

Monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 enzymes

This review examines the monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 (CYP) enzymes in bacterial, archaeal and mammalian systems. CYP enzymes catalyze monooxygenation reactions by inserting one oxygen atom from O2 into an enormous number and variety of substrates. The catalytic versatility of CYP stems from its ability to functionalize unactivated carbon-hydrogen (C-H) bonds of substrates through monooxygenation. The oxidative prowess of CYP in catalyzing monooxygenation reactions is attributed primarily to a porphyrin π radical ferryl intermediate known as Compound I (CpdI) (Por•+FeIV=O), or its ferryl radical resonance form (FeIV-O•). CYP-mediated hydroxylations occur via a consensus H atom abstraction/oxygen rebound mechanism involving an initial abstraction by CpdI of a H atom from the substrate, generating a highly-reactive protonated Compound II (CpdII) intermediate (FeIV-OH) and a carbon-centered alkyl radical that rebounds onto the ferryl hydroxyl moiety to yield the hydroxylated substrate. CYP enzymes utilize hydroperoxides, peracids, perborate, percarbonate, periodate, chlorite, iodosobenzene and N-oxides as surrogate oxygen atom donors to oxygenate substrates via the shunt pathway in the absence of NAD(P)H/O2 and reduction-oxidation (redox) auxiliary proteins. It has been difficult to isolate the historically elusive CpdI intermediate in the native NAD(P)H/O2-supported monooxygenase pathway and to determine its precise electronic structure and kinetic and physicochemical properties because of its high reactivity, unstable nature (t½~2 ms) and short life cycle, prompting suggestions for participation in monooxygenation reactions of alternative CYP iron-oxygen intermediates such as the ferric-peroxo anion species (FeIII-OO-), ferric-hydroperoxo species (FeIII-OOH) and FeIII-(H2O2) complex.

Induction of microsomal cytochrome P450s by tire-leachate compounds, habitat components of Aedes albopictus mosquito larvae

Benzothiazole (BZT) and its derivatives are the major leachate compounds of automobile tires, the principal breeding habitat of Aedes albopictus, particularly in the United States. Effects of the compounds on insecticide toxicity, and activities and expression of microsomal cytochrome P450s (P450s) in the mosquito larvae were examined. Mosquito larvae were more tolerant to carbaryl, rotenone, and temephos when they were pre-exposed to tire-leachate compounds, particularly BZT. There was no change in toxicity from the aldrin treatment by BZT. The effect of BZT was reversed when a P450 inhibitor, piperonyl butoxide, was applied in admixture with the insecticides. Microsomes from BZT-treated larvae had increased peroxidation activity of tetramethylbenzidine. This correlated with increased intensity of SDS-PAGE protein bands corresponding to molecular weights of 59 and 62 kD, which were detected as heme-containing proteins, a characteristic of P450s. The results suggest that BZT induces P450s, which detoxify insecticides and thus cause insecticide tolerance in the mosquito larvae. Arch.

Iron ore mines leachate potential for oxyradical production

The ecotoxicological effects of mining effluents is coming under much greater scrutiny. It appears necessary to explore possible health effects in association with iron ore mining effluents. The present results clearly demonstrate that iron-ore leachate is not an inert media but has the potential to induce lipid peroxidation. Peroxidation was assessed by measuring oxygen consumption in the presence of a reducing agent such as ascorbate or NADPH and a chelator such as EDTA. Labrador iron ore is an insoluble complex crystalline material containing a mixture of metals (Fe, Al, Ti, Mn, Mg,ellipsis, ) in contrast to the iron sources used for normal lipid peroxidation studies. The metal of highest percentage is iron (59. 58%), a metal known to induce oxyradical production. Iron ore powder initiated ascorbic acid-dependent lipid peroxidation (nonenzymatic) in liposomes, lipids extracted from rat and salmon liver microsomes, and intact salmon liver microsomes. It also revealed an inhibitory effect of NADPH-dependent microsomes lipid peroxidation as well as on NADPH cytochrome c reductase activity. However, nonenzymatic peroxidation in rat liver microsomes was not significantly inhibited. Cytochrome P450 IA1- and IIB1-dependent enzymatic activities as well as P450 levels were not affected. The inhibition could be due to one of the other components of iron ore leachate (Mn, Al,ellipsis, ). These effects of iron-ore leachate indicate that a potential toxicity could be associated with its release into lakes. Further studies are necessary to explore in vivo effects on aquatic animals.

Regulation of prostaglandin biosynthesis by nitric oxide is revealed by targeted deletion of inducible nitric-oxide synthase

We investigated the effects of targeted deletion of the inducible NO synthase (iNOS) gene on the formation of prostaglandins in vivo and ex vivo. Peritoneal macrophages were obtained from control and iNOS-deficient mice, and prostaglandin E(2) (PGE(2)) was quantified after stimulation with gamma-interferon and lipopolysaccharide to induce COX-2. Total nitrate and nitrite production was completely abolished in cells from iNOS-deficient animals compared with control cells. PGE(2) formation by cells from iNOS-deficient animals was decreased compared with cells from control animals 80% at 12 h (0.85 +/- 0.90 ng/10(6) cells versus 15.4 +/- 2.1 ng/10(6) cells, p < 0.01) and 74% at 24 h (9.4 +/- 4.3 ng/10(6) cells versus 36.8 +/- 4.1 ng/10(6) cells, p < 0.01). COX-2 protein expression was not significantly different in cells from control or knockout animals. Levels of PGE(2) in the urine of iNOS-deficient mice were decreased 78% (0.24 +/- 0.14 ng/mg of creatinine versus 1.09 +/- 0.66 ng/mg of creatinine, p < 0.01) compared with control animals. In addition, the levels of urinary F(2)-isoprostanes, an index of endogenous oxidant stress, were significantly decreased in iNOS-deficient animals. In contrast, the levels of thromboxane B(2) derived from platelets allowed to aggregate ex vivo were significantly increased in iNOS-deficient mice compared with wild-type mice. These studies support the hypothesis that NO and/or NO-derived species modulate cyclooxygenase activity and eicosanoid production in vivo.

Experimental pancreatitis induced by synthetic prooxidant tert-butyl hydroperoxide

The purpose of this study was to verify whether injection of tert-butyl hydroperoxide (Bu(t)OOH, a well-known prooxidant agent) into the bile-pancreatic duct can induce acute pancreatitis. A rapid blockade of the secretion was observed in the majority of the animals after 3 hours of observation. After 6 hours, the secretion reached a very low level, significantly different compared with controls. In groups of rats injected with Bu(t)OOH, pancreatic weight gain was observed compared with the rats injected with physiologic saline. Histology of pancreata removed 3 hours after injection of Bu(t)OOH showed acinar cell vacuolization, interstitial edema, focal necrosis of pancreatic acini, fat-tissue necrosis, and leukocyte infiltration of the organ. These changes were considerably greater after the 6-hour observation period. Electron-microscopic inspection revealed profound morphologic changes 3 hours after Bu(t)OOH injection. The control rats receiving physiologic saline alone had well-preserved pancreatic tissue structure. In conclusion, injection of the prooxidant agent, tert-butyl hydroperoxide, into common bile-pancreatic duct induces acute necrotizing pancreatitis, which indicates the crucial role of free radical reactions in pathogenesis of this disease.

A simple and efficient method for hemoglobin removal from mammalian tissue cytosol by zinc sulfate and its application to the study of lipoxygenase

A simple and efficient method is described to remove hemoglobin (Hb) from human term placental cytosol to study dioxygenase and co-oxidase activities of lipoxygenase. In the untreated samples, 70%-80% of the linoleic acid-dependent dioxygenase and co-oxidase activities were found to be associated with the pseudo-lipoxygenase activity of Hb. Zinc sulfate (0.5 mM) precipitated >97% of the Hb present in the cytosol. The dioxygenase activity of the ZnSO4 treated cytosol exhibited a Vmax value of 313 nmoles linoleic acid hydroperoxide formed/min/mg protein and a K(M) of 1.4 mM for linoleic acid. The ZnSO4 treated cytosol displayed co-oxidase activity toward benzidine, dimethoxybenzidine, guaiacol, pyrogallol, tetramethylbenzidine and tetramethyl-p-phenylenediamine. Nordihydroguaiaretic acid, 5,8,11-eicosatriynoic acid, butylated hydroxyanisole, butylated hydroxytoluene and gossypol caused concentration dependent inhibition of dioxygenase and co-oxidase activities. These results suggest ZnSO4 precipitation of Hb from cytosol does not alter the functional characteristics of the human term placental lipoxygenase.

Inhibitory effect of free radicals derived from organic hydroperoxide on progesterone synthesis in human term placental mitochondria

Different natural and synthetic organic hydroperoxides have been found to stimulate TBARS formation in human term placental mitochondria. The levels of TBARS were lower than arising from NADPH-dependent lipid peroxidation. BHT, Mn2+ and DMPO counteracted TBARS formation in the presence of cumene hydroperoxide implicating involvement of free radicals in this process. On the other hand superoxide dismutase, catalase and EDTA while being inhibitory in NADPH-dependent lipid peroxidation did not inhibit cumene hydroperoxide-dependent TBARS formation. Amphenone B and SKF-525A, inhibitors of cytochrome P-450, strongly inhibit both NADPH- and cumene hydroperoxide-dependent lipid peroxidation. These data provide evidence that cytochrome P-450SCC is involved in both these processes. However NADPH-dependent lipid peroxidation and the cumene hydroperoxide have been found to inactivate placental mitochondrial cytochrome P-450SCC. The presence of cumene hydroperoxide resulted in a more rapid inactivation of cytochrome P-450SCC and consequently inhibited NADPH-dependent lipid peroxidation. It has been observed for the first time that progesterone biosynthesis can be inhibited by cumene hydroperoxide. Protective effect of Mn2+ and DMPO on progesterone biosynthesis indicates the importance of free radicals as transient products of cytochrome P-450SCC-dependent cumene hydroperoxide metabolism. In contrast to progesterone formation from cholesterol, the conversion of pregnenolone to progesterone was not affected by cumene hydroperoxide. This suggests that inhibition of progesterone synthesis from cholesterol by hydroperoxide may be ascribed to its effect on the desmolase activity of cytochrome P-450SCC in placental mitochondria. On the basis of the results obtained, we propose that the inhibition of progesterone biosynthesis by naturally occurring hydroperoxides may contribute to the development of preeclampsia.

Cytochrome P450 dependent xenobiotic activation by physiological hydroperoxides in intact hepatocytes

Xenobiotic metabolic activation by intact hepatocytes was recently shown to be enhanced by the addition of nontoxic concentrations of t-butyl hydroperoxide and prevented by cytochrome P450 inhibitors. Furthermore, H2O2 (Km = 103 microM) was found to be highly effective in supporting the human microsomal CYP1A2 catalyzed metabolic activation of the heterocyclic aromatic amine 2-amino-3-methylimidazo (4,5-f) quinoline (IQ) to mutagenic metabolites and the DNA adduct formed was the same as that formed by the mixed-function oxidase catalyzed activation system. In the following, it is shown that the cytotoxicity of other xenobiotics including carcinogenic arylamines and their N-hydroxyarylamine metabolites were markedly enhanced by hydroperoxide addition but not in the presence of cytochrome P450 inhibitors. The CYP1A2 dependent O-demethylation of methoxyresorufin in 3-methylcholanthrene induced hepatocytes was also markedly enhanced when intracellular H2O2 was generated by the mitochondrial monoamine oxidase (MAO) substrates tyramine or kynurenamine. Linoleic acid hydroperoxide also dramatically enhanced the cytotoxicity of phenelzine towards isolated hepatocytes and the microsomal metabolism of phenelzine to form ethylbenzene. The P450 inhibitors phenylimidazole, benzylimidazole prevented the metabolic activation of phenelzine but not lipid peroxidation. These results suggest that linoleic acid hydroperoxide can activate hydrazines via a cytochrome P450 peroxidase catalyzed one electron oxidation to form highly cytotoxic reactive intermediates. Furthermore, increased hydrogen peroxide formation, e.g. as a result of oxidative stress, would also be expected to enhance the metabolic activation of carcinogenic arylamines via the peroxygenase function of CYP1A2.

Mechanism-based inactivation of bovine cytochrome P-450(11beta) by 18-unsaturated progesterone derivatives

Two 18-unsaturated progesterone derivatives, 18-vinylprogesterone (18-VP) and 18-ethynylprogesterone (18-EP) have proved to be potent inhibitors of the bovine cytochrome P-450(11beta), the enzyme involved in the last steps of aldosterone biosynthesis [Delorme, C., Piffeteau, A., Viger, A. & Marquet, A. (1995) Eur. J. Biochem. 232, 247-256]. In the present study, we demonstrate that these two compounds exhibit the characteristics of mechanism-based inactivators of this enzyme. Inactivation followed pseudo-first-order and saturation kinetics. The kinetic parameters of inactivation were k(i) = 0.11 min(-1) and Ki = 4 microM for 18-VP, and k(i) = 0.12 min(-1) and 22 microM for 18-EP. Inactivation of P-450(11beta) activity was strictly dependent on the presence of NADPH. Protection by the substrate deoxycorticosterone was observed, demonstrating a selective modification at the substrate-binding site. With radiolabeled 18-VP, inactivation was shown to be irreversible with a stoichiometry of 1.4 mol bound [3H]18-VP/mol inactivated cytochrome P-450(11beta). SDS/PAGE analysis of the [3H]18-VP-inactivated enzyme showed that, under conditions preventing heme dissociation, the P-450(11beta) band was labeled, while no labeling of the apoprotein was observed under denaturating conditions. Furthermore, the loss of catalytic activity could be correlated with the destruction of the P-450 chromophore evaluated by the FeII-CO versus FeII difference spectra. These arguments led us to propose that 18-vinylprogesterone inactivates cytochrome P-450(11beta) by heme destruction rather than by protein modification.

Effect of vitamin E dietary intake on in vitro activation of aflatoxin B1

The molecular mechanism of action of vitamin E on mammalian cells remains to be elucidated. In this study, vitamin E dietary intake was assessed for its effects on the initiation phase of carcinogenesis. We have conducted a dose-effect relationship between vitamin E dietary intake and aflatoxin B1 (AFB1) genotoxicity measured in vitro. Thus AFB1 induced mutagenesis in Salmonella typhimurium TA98 was investigated and compared to effect of vitamin E dietary intake on hepatic microsomal P-450 content and specific activities involved in AFB1 metabolism. Rats were fed ad libitum a diet containing 0, 0.05, 0.5 or 5 IU of alpha-tocopherol for 8 weeks. Modulation of vitamin E level in postmitochondrial and microsomal fractions resulted in nutritional effects. Cytochrome P-450 content was not modified by the level of vitamin E in the diet. The microsomal P-450 activities, P-450 IIB1 and IIIA, were decreased in the deficient group to -35% and -16%, respectively, as compared with control diet (0.05 IU). Diet supplemented with 0.5 IU of vitamin E increased P-450 IIB and IIIA activities (+28% and +37%, respectively) whereas a diet highly supplemented in vitamin E (5 IU) reduced these specific P-450 activities. Lipid peroxidation, estimated by the formation of thiobarbituric acid reactive products, increased in the dietary vitamin E free diet (+20%) and strongly decreased in the supplemented group (-99%). This study establishes that in vivo, dietary vitamin E protects directly membrane against damage induced by lipid peroxidation and indirectly hepatic microsomal monooxygenase activities. However, vitamin E accumulation seems to alter membrane structure and function. The nutritional effect of vitamin E on hepatic microsomal cytochrome P-450 activities modified the AFB1 genotoxicity measured in vitro.

Metabolic N-oxide formation by rabbit-liver microsomal cytochrome P-4502B4: involvement of superoxide in the NADPH-dependent N-oxygenation of N,N-dimethylaniline

NADPH-sustained N-oxygenation of N,N-dimethylaniline (DMA) was investigated with the aid of a reconstituted membranous cytochrome P-4502B4 system. The N-oxidative process did not appear to be supported by hydroxyl radicals or products arising from lipid peroxidation. However, superoxide dismutase was a very potent scavenger of N-oxide formation, while catalase was ineffective. Superoxide by itself did not bring about N-oxygenation of DMA. Therefore, O2- was presumed to serve as a source of the actual proximate oxidant. The reconstituted hemoprotein system catalyzed N-oxygenation of DMA when excess H2O2 substituted for NADPH/O2. This 'peroxygenase' process was entirely dependent on the presence of native enzyme and was not inhibited by CO or metyrapone. By contrast, cyanide severely blocked metabolic transformation. Among some other hemeproteins tested, only horseradish peroxidase was efficient in producing appreciable amounts of N-oxide in the presence of H2O2. Peroxidatic DMA N-oxygenation in intact liver microsomes fortified with cumene hydroperoxide was 2-fold stimulated by pretreatment of rabbits with phenobarbital, whereas administration of 3-methylcholanthrene or ethanol decreased turnover. Studies with uninduced hepatic microsomes, in which the activity of the flavin-containing monooxygenase had been partially suppressed by thermal treatment, revealed pronounced susceptibility of the NADPH-dependent N-oxide formation to the inhibitory action of both superoxide dismutase and antibody to NADPH-cytochrome P-450 reductase. These findings were interpreted to mean that at least 23% of the total amount of N-oxide produced in these preparations resulted from superoxide-dependent conversion of DMA by the P-450 system.

Characteristics of the glutathione/glutathione-S-transferase detoxification system in melphalan resistant human prostate cancer cells

Glutathione (GSH) and glutathione-S-transferases (GST) have been implicated in resistance of tumor cells to certain alkylating agents, including melphalan. Glutathione levels and GST activities were determined in melphalan-resistant sublines of the human prostate carcinoma cell lines DU 145, PC-3 and LNCaP produced by serial treatment with melphalan at progressively increasing concentrations. The resistant sublines M4.5DU145, M5DU145, M6DU145, M6PC-3 and M6LNCaP were 27-, 7-, 3-, 6- and 2-fold more resistant to melphalan than the parental lines. The melphalan-resistant DU 145 and PC-3 lines showed cross-resistance to cisplatin and tetraplatin, but retained sensitivity to vinblastine, colchicine and etoposide. Interestingly, both sublines were also resistant to methotrexate and adriamycin. The melphalan-resistant LNCaP line showed slight resistance to cisplatin and adriamycin, but remained sensitive to tetraplatin and methotrexate. This line also retained sensitivity to vinblastine while developing resistance to colchicine. Intracellular GSH levels were increased 2.8 fold for M5DU145, 1.7 fold for M6PC-3 and 2.1 fold for M6LNCaP compared to the parental lines, whereas GST activity using chlorodinitro-benzene as a substrate was comparable for all lines. When cumene hydroperoxide was used as a substrate, an increase in GST activity was noted only in the M6PC-3 line as compared with the parent line. Western blot analysis showed no change in GST isozyme profile between parent and resistant DU 145 lines; however a mu class isoenzyme was detected in the resistant, but not in the parent PC-3 line, using a Yb1 antibody. M5DU145 cells maintained in the absence of melphalan for seven months maintained their resistance to melphalan. Depletion of GSH, with buthionine sulfoximine, to control levels reversed melphalan resistance to control levels.

Differential cumene hydroperoxide sensitivity of cytochrome P-450 enzymes IA1 and IIB1 determined by their way of membrane incorporation

The cytochrome P-450-dependent O-dealkylation of alkoxyresorufins was used to study the effect of cumene hydroperoxide on cytochrome P-450 IIB1 and IA1 in microsomal and reconstituted systems. In liver microsomal systems from respectively phenobarbital and 3-methylcholanthrene pretreated male Wistar rats, cytochrome P-450 IIB1-dependent pentoxyresorufin-O-dealkylation appeared to be more sensitive to cumene hydroperoxide treatment than cytochrome P-450 IA1-dependent ethoxyresorufin-O-dealkylation. This phenomenon was also observed when the cumene hydroperoxide sensitivity of P-450 IIB1 and IA1 was studied in an isosafrole pretreated rat liver microsomal system. The decrease in alkoxy-O-dealkylating activities appeared to proceed by destruction of the cytochrome P-450 component of the enzyme system. Purification and reconstitution of the enzyme system components in a system in which the isolated proteins were not incorporated into a membrane resulted in the disappearance of the difference in sensitivity between the two P-450 enzymes. However, in a reconstituted system with membrane incorporated proteins, again cytochrome P-450 IIB1 expressed a higher sensitivity towards cumene hydroperoxide than cytochrome P-450 IA1. From this it was concluded that the differential cumene hydroperoxide sensitivity of cytochrome P-450 IIB1 and IA1 is not caused by an intrinsic difference in their sensitivity but by a differential effect of membrane incorporation on their cumene hydroperoxide sensitivity.

Mitochondrial oxidation of p-phenylenediamine derivatives in vitro: structure-activity relationships and correlation with myotoxic activity in vivo

A number of p-phenylenediamine derivatives are known to cause necrosis of skeletal and/or cardiac muscle when administered to experimental animals. Compounds of this type are oxidized to semiquinonedi-imines or quinonedi-imines by mitochondria in vitro, establishing alternative pathways for electron transport in the respiratory chain with concomitant decreases in respiratory control and ADP:O ratios. Muscle mitochondria were found to be particularly effective in promoting p-phenylenediamine oxidation in vitro and the magnitude of the mitochondrial effects of the various compounds tested correlated well with their ability to cause muscle necrosis in vivo. It is suggested that mitochondrial oxidation may be involved in the initiation of the myotoxic effects of these compounds and account for their target-site specificity.

Rat pulmonary lipoxygenase: dioxygenase activity and role in xenobiotic metabolism

1. Dioxygenase activity and the ability of pregnant rat lung lipoxygenase to oxidize xenobiotics were examined in vitro under a variety of experimental conditions. 2. More than 90% of the dioxygenase activity towards linoleic acid in the lung homogenate was found to be associated with the cytosolic fraction. The cytosolic enzyme exhibited pH optima at 6.5 and 9.5, the activity being two-fold greater at pH 9.5. To observe maximal dioxygenase activity (about 0.7 mumol of 13-hydroperoxylinoleic acid formed/min per mg protein) at pH 9.5, the presence of 6.0 mM linoleic acid was required. 3. Benzidine oxidation occurred at maximal rate of pH 6.5 when the reaction medium contained 1.0 mM benzidine and 13.5 mM linoleic acid. All eight xenobiotics tested were oxidized at significant rates by the lung cytosolic lipoxygenase. 4. Both dioxygenase activity and benzidine oxidation were inhibited by the inhibitors of lipoxygenase, viz. nordihydroguaiaretic acid, BHT, caffeic acid, esculetin, and gossypol, in a concentration-dependent manner. 5. The results suggest that oxidation of xenobiotics by lipoxygenase may be an important pathway of metabolism in the mammalian lung.

Effect of alpha-tocopherol on hepatic mixed function oxidases in hepatic ischemia/reperfusion

This study was done to determine the relationship between microsomal lipid peroxidation during hepatic ischemia/reperfusion and alteration in cytochrome P-450-dependent drug metabolism. Rats were pretreated with alpha-tocopherol to inhibit lipid peroxidation or with vehicle (soybean oil) and then subjected to 60 min no-flow hepatic ischemia in vivo. Control animals were time-matched sham-ischemic animals. After 1, 5 or 24 hr of reperfusion, liver microsomes were isolated and cytochrome P-450 and mixed function oxidases were studied. In vehicle-treated ischemic rats, serum ALT levels peaked at 5 hr (5,242 +/- 682 U/L) and were significantly reduced by alpha-tocopherol pretreatment (1,854 +/- 229 U/L, p less than 0.01). Similarly, microsomal lipid peroxidation was elevated in the vehicle-treated ischemic group, but this elevation was prevented by alpha-tocopherol pretreatment. Microsomal cytochrome P-450 content and aminopyrine-N-demethylase activity were both decreased in vehicle-treated ischemic rats to 60% and 70% of sham-ischemic control levels, respectively. Although alpha-tocopherol restored cytochrome P-450 content to the level of sham-ischemic control rats, aminopyrine-N-demethylase activity remained at 76% of control with alpha-tocopherol treatment (p less than 0.01 compared with sham-ischemic control). In contrast to what was seen with cytochrome P-450 and aminopyrine-N-demethylase, aniline p-hydroxylase activity was elevated in the vehicle-treated ischemic rats compared with sham-ischemic control rats. These increases were prevented by alpha-tocopherol pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical modification of lysine residues in cytochrome P450LM2 (P450IIB4): influence on heme liganding of arylamines

Treatment of cytochrome P450LM2 with fluorescein isothiocyanate to introduce up to two equivalents of fluorophore per polypeptide chain resulted in the selective derivatization of lysine residues. CD-spectral measurements revealed the overall conformation as well as the immediate heme environment of the hemoprotein to remain unaffected by attachment of the label. Modification caused decreased affinity of p-phenylenediamine and other 4-substituted anilines for the heme site, whereas there was a rise in the extent of substrate interaction. Experiments with pigment containing acetylated lysines gave analogous results, suggesting that the observed phenomenon was due to charge neutralization. There was linear correlation between the Hammett sigma P values and both the optical dissociation constants for arylamine binding to intact enzyme and the dipole moments of the anilines, indicating that basicity along with electronic factors controlled heme liganding; lipophilicity appeared to be of minor importance. Introduction of fluorescein isothiocyanate into the oxygenase was found to influence the bond-making process through modulating basicity of the nitrogenous compounds, but perturbation of optimal spacial orientation of the amine nitrogen toward the heme iron also might have been operative. The lysines studied seem to represent metabolically inactive elements of the substrate channel located on the cytosolic surface of the aggregates, as evidenced by steady-state fluorescence measurements. A hydrophilic segment in the cytochrome P450LM2 molecule that would accommodate the critical residues is discussed.

Biochemical effects of combined gases of nitrogen dioxide and ozone. IV. Changes of lipid peroxidation and antioxidative protective systems in rat lungs upon life span exposure

Lipid peroxide production, antioxidant contents and activities of antioxidative protective enzymes were examined in lungs of rats exposed to clean air (control group), 0.05 ppm O3, 0.05 ppm O3 + 0.04 ppm NO2 and 0.05 ppm O3 + 0.4 ppm NO2 for 22 months. The results were compared with our previous data in rats exposed to 0.04 ppm NO2, 0.4 ppm NO2 and 4 ppm NO2 for their life span (Sagai et al., Toxicol. Appl. Pharmacol., 73, (1984) 444-456). TBA values used as an index of lipid peroxidation in the lungs were increased maximally at 9 months, but were decreased below control values in animals exposed for 18 and 22 months. Nonprotein sulfhydryl (NPSH) contents were increased maximally at 9 months, and after 18 and 22 months were decreased significantly below control values. Vitamin E (VE) contents showed a similar trend. On the other hand, enzyme activities of glucose-6-phosphate dehydrogenase (G6PD), 6-phosphogluconate dehydrogenase (6PGD), glutathione reductase (GR), glutathione peroxidase measured by using cumene hydroperoxide (cum.OOH) substrate (GPx-cum.OOH), glutathione peroxidase measured by using H2O2 as a substrate (GPx-H2O2), glutathione S-transferase (GSH-Tase) and superoxide dismutase (SOD) did not show any significant changes during this experiment. The results show that lipid peroxidation in lungs was increased synergistically by a combination of NO2 and O3 at ambient levels, and that the time of maximum lipid peroxide production was shorter than with NO2 alone. The protective ability against lipid peroxides was higher with increased lipid peroxide levels, but the inducibility was not maintained through a life span exposure to the combined gases. Additionally, two small adenomas were observed in 2 out of 18 rats in the 0.05 ppm O3 + 0.04 ppm NO2 group and a large adenoma was observed in 1 out of 18 animals in the 0.05 ppm + 0.4 ppm NO2 group exposed for 22 months.

Formation and reactions of the Wurster's blue radical cation during the reaction of N,N,N',N'-tetramethyl-p-phenylenediamine with oxyhemoglobin

The aromatic amine N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) reacted directly with oxyhemoglobin in a catalytic reaction resulting in formation of ferrihemoglobin. The second order rate constant of the reaction was found to be 5.5 M-1.s-1. The stable Wurster's blue radical cation produced ferrihemoglobin at rates greater 10(3) M-1.s-1, i.e. more than two orders of magnitude faster than the parent amine. In contrast to the reactions of aminophenols with hemoglobin, free hydrogen peroxide was formed which additionally contributed to ferrihemoglobin formation. Since ferrihemoglobin formation proceeded by two orders of magnitude faster than autoxidation of TMPD, oxyhemoglobin itself acted as an oxidase/peroxidase resulting in electron abstraction from the amino alone pair electrons.

Reactions of the Wurster's blue radical cation with thiols, and some properties of the reaction products

Formation of 1-electron oxidation products of aromatic amines in biological systems have been ascertained. The mechanisms of the toxic actions of the aminyl radicals and their corresponding detoxication reactions are much less established. During the studies of reactions of GSH with the N,N,N',N'-tetramethyl-p-phenylenediamine radical cation (TMPD) (Wurster's blue) two pathways were detected: (1) a slow second order reaction (k = 5 M-1.s-1) which gave the parent amine and (ultimately) GSSG, and (2) a fast, complex reaction which yielded 2-(glutathione-S-yl)-N,N,N',N'-tetramethyl-p-phenylenediamine (2-GS-TMPD). From kinetic reasons, this reaction was suggested to be composed of a rapid disproportionation reaction followed by a reductive 1,4-Michael-addition. This reaction pathway prevailed at GSH concentrations below 1 mM. At higher GSH concentrations formation of the thioether was suppressed. This hypothesis was confirmed when the reaction of the highly labile N,N,N',N'-tetramethyl-p-quinonediiminium dication (TMQDI++) with GSH was followed: In this case, thioether formation outweighed clearly reductive mechanisms, the latter yielding ultimately the amine and GSSG. Similar to N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), 2-GS-TMPD was also capable of producing ferrihemoglobin in a catalytic reaction. Its rate, however, was only 3% that observed with the parent amine. During this reaction the thioether was apparently oxidized to the corresponding quinonediiminium dication, which gave the corresponding quinonemonoimine on acidification.

Interaction of benzanthrone with cytochrome P450: altered patterns of hepatic xenobiotic metabolism in rats

Benzanthrone, an anthraquinone dye intermediate, is commonly used for the synthesis of a number of polycyclic vat and disperse dyes. Our prior studies have shown that benzanthrone can be metabolized by rat hepatic microsomal cytochrome P450 (P450) (Biochem. Int., 18, 1989, 1237). In this study, the interaction of benzanthrone with rat hepatic microsomal P-450 and its effect on xenobiotic metabolism have been investigated. Parenteral administration of benzanthrone (40 mg/kg body weight) for 3, 7, or 21 days caused no change in the relative body weight or organ weight of rats. The levels of P450 were found to be reduced (33%-50%) in all the benzanthrone-exposed animals at all the time periods. In vitro addition of benzanthrone caused a spectral change with oxidized P450 and concentration-dependent reduction in the carbon monoxide spectrum of dithionite-reduced P450. The addition of benzanthrone to hepatic microsomes prepared from phenobarbital-treated rats resulted in spectral changes characterized by an absorbance maximum at 397 nm indicative of type I binding. In vitro addition of benzanthrone showed a concentration-dependent inhibition of hepatic aminopyrine N-demethylase (APD) and ethoxyresorufin-O-deethylase (ERD) activities with respective I50 values of 9.5 x 10(-4) and 8.0 x 10(-5) M. However, the inhibition of aryl hydrocarbon hydroxylase (AHH) even at the highest concentration of benzanthrone (10(-2) M), was of the order of only 29%. In vivo administration of benzanthrone also led to the inhibition of APD, AHH, and ERD activities at all treatment times although the magnitude of inhibition was of a lower order.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydroperoxide-dependent epoxidation of unsaturated fatty acids in the broad bean (Vicia faba L.)

Incubation of linoleic acid with the 105,000g particle fraction of the homogenate of the broad bean (Vicia faba L.) led to the formation of the following products: 13(S)-hydroxy-9(Z),11(E)-octadecadienoic acid, 9,10-epoxy-12(Z)-octadecenoic acid (9(R),10(S)/9(S)/10(R), 80/20), 12,13-epoxy-9(Z)-octadecenoic acid (12(S),13(R)/12(R)/13(S), 64/36), and 9,10-epoxy-13(S)-hydroxy-11(E)-octadecenoic acid (9(S),10(R)/9(R),10(S), 91/9). Oleic acid incubated with the enzyme preparation in the presence of 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid or cumene hydroperoxide was converted into 9,10-epoxyoctadecanoic acid (9(R),10(S)/9(S),10(R), 79/21). Two enzyme activities were involved in the formation of the products, an omega 6-lipoxygenase and a hydroperoxide-dependent epoxygenase. The lipoxygenase, but not the epoxygenase, was inhibited by low concentrations of 5,8,11,14-eicosatetraynoic acid and nordihydroguaiaretic acid. In contrast, the epoxygenase, but not the lipoxygenase, was readily inactivated in the presence of 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid. Studies with 18O2-labeled 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid showed that the epoxide oxygens of 9,10-epoxyoctadecanoic acid and of 9,10-epoxy-13(S)-hydroxy-11(E)-octadecenoic acid were derived from hydroperoxide and not from molecular oxygen.

Partial purification and characterization of a peroxidase activity from human placenta

Peroxidases can metabolize a variety of xenobiotics to reactive intermediates capable of binding to protein or DNA. The potential role of these enzymes in fetotoxicity has not been explored. In this study, the presence of peroxidase activity was observed in human term and pre-term placenta. Human term placental peroxidase activity (HTPP) was partially purified by concanavalin A affinity chromatography from CaCl2 extracts of the particulate fraction. HTPP appears to be a membrane-bound glycoprotein. Arachidonic acid-dependent oxidation of guaiacol was not observed, suggesting that the peroxidase activity was not due to prostaglandin synthase. Moreover, HTPP preparations were devoid of catalase and spectrally dissimilar from human haemoglobin, cytochrome P-450, eosinophil peroxidase and myloperoxidase, suggesting an endogenous origin. An Mr of approx. 119,000 was determined for HTPP by gel filtration. Cathodic slab-PAGE of cetyltrialkylammonium bromide-solubilized HTPP yielded two peroxidase-staining bands.

Enhancement of hydroperoxide-dependent lipid peroxidation in rat liver microsomes by ascorbic acid

Simultaneous addition of ascorbic acid and organic hydroperoxides to rat liver microsomes resulted in enhanced lipid peroxidation (approximately threefold) relative to incubation of organic hydroperoxides with microsomes alone. No lipid peroxidation was evident in incubations of ascorbate alone with microsomes. The stimulatory effect of ascorbate on linoleic acid hydroperoxide (LAHP)-dependent peroxidation was evident at all times whereas stimulation of cumene hydroperoxide (CHP)-dependent peroxidation occurred after a lag phase of up to 20 min. EDTA did not inhibit CHP-dependent lipid peroxidation but completely abolished ascorbate enhancement of lipid peroxidation. Likewise, EDTA did not significantly inhibit peroxidation by LAHP but dramatically reduced ascorbate enhancement of lipid peroxidation. The results reveal a synergistic prooxidant effect of ascorbic acid on hydroperoxide-dependent lipid peroxidation. The inhibitory effect of EDTA on enhanced peroxidation suggests a possible role for endogenous metals mobilized by hydroperoxide-dependent oxidations of microsomal components.

Electron spin resonance studies on the degradation of hydroperoxides by rat liver cytosol

Incubation of tBuOOH (in the concentration range 200 microM to 20 mM) with rat liver post-microsomal supernatant in the presence of the spin trap DMPO gives three radical species, which can be observed by electron spin resonance spectroscopy. The first of these is the ascorbyl radical (which decreases in concentration with time), the other two are identified as spin adducts of alkoxyl and carbon-centred radicals; these latter species increase in concentration with time. Addition of NADH, but not NADPH, led to an increase in concentration of the alkoxyl and carbon-centred radical adducts and a decrease in the concentration of the ascorbyl radical. Results obtained in the presence of iron chelators and other ligands suggest that the generating system is an NADH-dependent enzyme that reduces tBuOOH by one-electron to give initially the tBuO. radical. Results from experiments carried out on dialysed cytosol samples lend support to this conclusion.

The action of hydrogen peroxide on the formation of thiobarbituric acid-reactive material from microsomes, liposomes or from DNA damaged by bleomycin or phenanthroline. Artefacts in the thiobarbituric acid test

Incubation of rat-liver microsomes, previously azide-treated to inhibit catalase, with H2O2 caused a loss of cytochrome P-450 but not of cytochrome b5. This loss of P-450 was not prevented by scavengers of hydroxyl radical, chain-breaking antioxidants or metal ion-chelating agents. Application of the thiobarbituric acid (TBA) assay to the reaction mixture suggested that H2O2 induces lipid peroxidation, but this was found to be due largely or completely to an effect of H2O2 on the TBA assay. By contrast, addition of ascorbic acid and Fe(III) to the microsomes led to lipid peroxidation and P-450 degradation: both processes were inhibited by chelating agents and chain-breaking antioxidants, but not by hydroxyl radical scavengers. H2O2 inhibited ascorbate/Fe(III)-induced microsomal lipid peroxidation, but part of this effect was dues to an action of H2O2 in the TBA test itself. H2O2 also decreased the colour measured after carrying out the TBA test upon authentic malondialdehyde, tetraethoxypropane, a DNA-Cu2+/o-phenanthroline system in the presence of a reducing agent, ox-brain phospholipid liposomes in the presence of Fe(III) and ascorbate, or a bleomycin-ion iron/DNA/ascorbate system. Caution must be used in interpreting the results of TBA tests upon systems containing H2O2.

Effects of acute and sub-chronic administration of iron nitrilotriacetate in the rat

Parenteral administration of iron nitrilotriacetate (FeNTA) to rats resulted in marked loss in body weight, and increases in liver/and kidney/body weight ratios. Fatalities, due to renal failure, depended on dosage and age of the animals, and were greater (70%) after a single large dose (12 mg iron) than after repeated smaller doses (30%). FeNTA administered subchronically gave rise to an increase in ethane exhalation, and to decreased liver glutathione peroxidase activity, and decreased cytochrome P-450 concentration and benzphetamine N-demethylase activity. It also resulted in severe renal tubular necrosis, with deposition of iron in the tubular cells and loss of brush border alkaline phosphatase activity, resulting in a dose-dependent diuresis, with increased urinary excretion of glucose, iron and lipid peroxidation products, and decreased urine creatinine concentration. NTA alone had none of these effects but slightly decreased the hepatic concentration of iron.

Cumene hydroperoxide-dependent oxidation of NNN'N'-tetramethyl-p-phenylenediamine and 7-ethoxycoumarin by cytochrome P-450. Comparison between the haemoproteins from liver and olfactory tissue

The interaction of cytochromes P-450 of the liver and olfactory epithelium of male hamsters with cumene hydroperoxide (CHP) has been characterized with regard to the ability of CHP to (1) support 7-ethoxycoumarin-O-de-ethylase (ECOD) activity, (2) support the oxidation of NNN'N'-tetramethyl-p-phenylenediamme (peroxidase activity) and (3) cause inactivation of cytochrome P-450. In the liver, CHP was found to support both ECOD and peroxidase activities while causing only minimal inactivation of cytochrome P-450. In contrast, in the olfactory epithelium CHP was virtually unable to support ECOD activity, peroxidase activity was 4-fold greater than in the liver, and extensive inactivation of cytochrome P-450 occurred. The reasons for these differences have been investigated with particular reference to the mode of cytochrome P-450-catalysed decomposition of CHP, that is, via homolytic or heterolytic cleavage of the hydroperoxide dioxygen bond. In both tissues, cumenol (2-phenylpropan-2-ol) was the major product of CHP decomposition detected. The radical scavenger nitrosobenzene inhibited cumenol formation by 84% in the olfactory epithelium, but by only 38% in the liver. This may indicate that dioxygen-bond scission occurs predominantly homolytically in the nasal tissue, whereas there is a balance between homolysis and heterolysis in the liver. It is suggested that the inability of CHP to support ECOD activity in the olfactory epithelium and the extensive inactivation of cytochrome P-450 that it causes both stem from decomposition of the hydroperoxide occurring homolytically rather than heterolytically in this tissue.

The effects of cadmium, mercury and lead in vitro on hepatic microsomal mixed function oxidase and lipid peroxidation

In this study, the authors investigated the effect of cadmium, mercury and lead in vitro on hepatic microsomal mixed function oxidase and lipid peroxidation. The results showed that these metals could inhibit the activity of hepatic microsomal aniline hydroxylase, decrease the concentration of hepatic microsomal cytochrome P-450 and increase the concentration of the inactive form of hemoprotein, P-420. Besides, they could enhance hepatic microsomal lipid peroxidation. There were marked concentration-dependent responses in these microsomal reactions. Thus, it may be suggested that cadmium, mercury and lead are capable of impairing hepatic microsomal mixed function oxidase in vitro by stimulating membrane lipid peroxidation.

Detection of peroxyl and alkoxyl radicals produced by reaction of hydroperoxides with rat liver microsomal fractions

E.s.r. spin trapping using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used to detect peroxyl, alkoxyl and carbon-centred radicals produced by reaction of t-butyl hydroperoxide (tBuOOH) with rat liver microsomal fraction. The similarity of the hyperfine coupling constants of the peroxyl and alkoxyl radical adducts to those obtained previously with isolated enzymes suggests that these species are the tBuOO. and tBuO. adducts. The effects of metal-ion chelators, heat denaturation, enzyme inhibitors and reducing equivalents demonstrate that these species arise from reaction of tBuOOH with a haem enzyme such as cytochrome P-450 or cytochrome b5. In the absence of NADPH or NADH the previously undetected peroxyl radical adduct is the major species observed. In the presence of these reducing equivalents the alkoxyl and carbon-centred radical adducts predominate, which is in accord with product studies on similar systems. These results demonstrate that both reductive and oxidative decomposition of tBuOOH can occur in rat liver microsomal fraction with the reductive pathway favoured in the presence of NADH or NADPH.

Studies on the mechanism of enhancement of butylated hydroxytoluene-induced mouse lung toxicity by butylated hydroxyanisole

The studies described in this report were designed to probe possible mechanisms whereby butylated hydroxyanisole (BHA) is able to enhance butylated hydroxytoluene (BHT)-induced mouse lung toxicity. In experiments with mouse lung slices, BHA enhanced the covalent binding of BHT to protein, indicating that the interaction between BHA and BHT takes place in the lung. Subcutaneous administration of either BHA (250 mg/kg) or diethyl maleate (DEM, 1 ml/kg) to male CD-1 mice produced a similar enhancement of BHT-induced lung toxicity. In contrast to DEM, the administration of BHA (250 or 1500 mg/kg) did not decrease mouse lung glutathione levels, suggesting that the effect of BHA is not due to the depletion of glutathione levels. We previously observed that in the presence of model peroxidases a unique interaction occurs between BHA and BHT, resulting in the increased metabolic activation of BHT. Upon the addition of hydrogen peroxide or various hydroperoxides to mouse lung microsomes, BHA significantly increased the covalent binding of BHT to protein. BHA also stimulated the rate of formation of hydrogen peroxide by 4.7-fold in mouse lung microsomes. Likewise, hydrogen peroxide resulting from the NADPH cytochrome P-450 (c) reductase-catalyzed redox cycling of tert-butylhydroquinone, a microsomal metabolite of BHA, supported the peroxidase-dependent BHA-enhanced formation of BHT-quinone methide. These results suggest that BHA could facilitate the activation of BHT in the lung as a result of both the increased formation of hydrogen peroxide and the subsequent peroxidase-dependent formation of BHT-quinone methide from the direct interaction of BHA with BHT.

Hydroperoxidase activity of lipoxygenase: hydrogen peroxide-dependent oxidation of xenobiotics

Since H2O2 is one of the major biologically available peroxides, its ability to support hydroperoxidase activity of highly purified soybean lipoxygenase was examined by monitoring co-oxidation of selected xenobiotics. All of the eight chemicals tested were found to be oxidized in the presence of H2O2. Tetramethylbenzidine oxidation was completely inhibited by the classical lipoxygenase inhibitor nordihydroguaiaretic acid. The reaction was enzymatic in nature and exhibited a acidic pH optimum. The data clearly indicate, for the first time, that H2O2 can efficiently replace fatty acid hydroperoxide in a xenobiotic oxidation reaction medicated by the hydroperoxidase activity of lipoxygenase.

Olfactory cytochrome P-450. Studies with suicide substrates of the haemoprotein

1. The olfactory epithelium of male hamsters has been found to be extremely active in the cumene hydroperoxide-supported oxidation of tetramethylphenylenediamine, and this peroxidase activity has been shown to be cytochrome P-450-dependent. 2. The interaction of a series of suicide substrates of cytochrome P-450 with the hepatic and olfactory mono-oxygenase systems has been assessed by determination of peroxidase, 7-ethoxycoumarin O-de-ethylase (ECOD) and 7-ethoxyresorufin O-de-ethylase (EROD) activities after treatment in vivo with these compounds. Chloramphenicol, OOS-trimethylphosphorothiolate and two dihydropyridines [DDC (3,5-diethoxycarbonyl-1,4-dihydrocollidine) and 4-ethyl DDC (3,5-diethoxycarbonyl-4-ethyl-1,4-dihydro-2,6-dimethylpyridine)] all caused similar percentage inhibitions of hepatic and olfactory activities, but the absolute amounts of enzymic activity lost were considerably greater in the latter tissue. In contrast, halothane had little effect upon hepatic cytochrome P-450-dependent reactions, whereas it severely inhibited those of the olfactory epithelium. 3. The time course of loss and recovery of hepatic and olfactory peroxidase, ECOD and EROD activities after a single dose of 4-ethyl DDC was studied. The rates of loss of activity observed were very similar, irrespective of tissue or reaction examined. In the olfactory epithelium, all three activities recovered concurrently and at a rate similar to that of the hepatic peroxidase activity. In contrast, the hepatic de-ethylation of 7-ethoxycoumarin and 7-ethoxy-resorufin recovered significantly more rapidly. 4. It is suggested that this behaviour is due to 4-ethyl DDC acting not only as a suicidal inhibitor but also as an inducer of certain forms of cytochrome P-450 in the liver; in the olfactory epithelium, however, inactivation, but not induction, occurs. Classical inducing agents were reported to have no effect upon olfactory cytochrome P-450, and in the present study neither phenobarbitone nor beta-naphthoflavone treatment had any effect upon olfactory cytochrome P-450-dependent reactions, although it induced those of the liver.

Bioactivation of catechol in rat and human bone marrow cells

o-Benzoquinone-glutathione (GSH) conjugate formation and covalent binding of [14C]catechol to protein were utilized as probes of bioactivation of catechol in both rat and human white bone marrow cell systems. Conjugate formation and binding occurred in the absence of exogenous hydrogen peroxide, but were markedly stimulated by its addition. Protein-binding and conjugate formation using rat cells in the presence of exogenous peroxide were increased by the presence of phenol whereas GSH and hydroquinone inhibited binding. Similarly, protein-binding in the absence of exogenous peroxide was inhibited by GSH and exacerbated by phenol. Prostaglandin synthase, the peroxidatic function of which may also utilize hydrogen peroxide as a substrate, appeared on the basis of experiments using arachidonic acid to play only a minor role in bioactivation of catechol in rat bone marrow cells. These results show that peroxide-dependent bioactivation of catechol occurs in rat and human bone marrow cells and that hydroquinone and GSH inhibit whereas phenol stimulates bioactivation.

Effect of oxygen concentration on microsomal oxidation of ethanol and generation of oxygen radicals

The iron-catalysed production of hydroxyl radicals, by rat liver microsomes (microsomal fractions), assessed by the oxidation of substrate scavengers and ethanol, displayed a biphasic response to the concentration of O2 (varied from 3 to 70%), reaching a maximal value with 20% O2. The decreased rates of hydroxyl-radical generation at lower O2 concentrations correlates with lower rates of production of H2O2, the precursor of hydroxyl radical, whereas the decreased rates at elevated O2 concentrations correlate with lower rates (relative to 20% O2) of activity of NADPH-cytochrome P-450 reductase, which reduces iron and is responsible for redox cycling of iron by the microsomes. The oxidation of aniline or aminopyrine and the cytochrome P-450/oxygen-radical-independent oxidation of ethanol also displayed a biphasic response to the concentration of O2, reaching a maximum at 20% O2, which correlates with the dithionite-reducible CO-binding spectra of cytochrome P-450. Microsomal lipid peroxidation increased as the concentration of O2 was raised from 3 to 7 to 20% O2, and then began to level off. This different pattern of malondialdehyde generation compared with hydroxyl-radical production probably reflects the lack of a role for hydroxyl radical in microsomal lipid peroxidation. These results point to the complex role for O2 in microsomal generation of oxygen radicals, which is due in part to the critical necessity for maintaining the redox state of autoxidizable components of the reaction system.

Protective effect of beta-naphthoflavone against NO2 toxicity in mice with genetically inducible lung cytochrome P450

The effects of the cytochrome P450 inducer beta-naphthoflavone (BNF) on NO2 toxicity were studied in two strains of mice. In one strain (C57B1/6J), cytochrome P450 could be induced by the aromatic hydrocarbon, while in the other strain (DBA/2J) cytochrome P450 was not inducible by this compound. Mice were treated with BNF before and during 4 days of exposure to 20 ppm NO2. The body growth of NO2-exposed mice improved only in BNF-treated C57B1/6J mice. In this strain, BNF reduced both pulmonary edema (as measured by wet and dry lung weights or as assessed by histological studies) and lung peroxidation (as measured by malondialdehyde). This protective effect of BNF on NO2 toxicity in C57B1/6J mice was associated with an increase in the components of the cytochrome P450 system (cytochrome P450 and cytochrome b5), whereas the activities of pulmonary antioxidant enzymes (superoxide dismutase, glutathione peroxidase, and glutathione reductase) were not significantly increased. These data suggest that the induction of the cytochrome P450 system may be important in promoting NO2 tolerance in those strains of mice in which the cytochrome P450 system is genetically inducible.