Alterations of specific forms of cytochrome P-450 in rat liver during acute carbon tetrachloride intoxication

Non-toxic Level of Acetaminophen Potentiates Carbon Tetrachloride-Induced Hepatotoxicity in Mice

A wide range of medications are routinely used to maintain and improve human health. Hence, it is essential that we understand and predict adverse effects caused by the combined use of multiple medications. In the present study, we investigated whether the combination of carbon tetrachloride (CCl₄) and acetaminophen (APAP) had a detrimental effect on the liver. Mice injected with APAP (100 mg/kg) showed no significant changes in hepatic injury markers (alanine aminotransferase and aspartate aminotransferase), histopathological findings, pro-inflammatory cytokine levels, or hepatic oxidative stress. In contrast, a single injection of CCl₄ (15 mg/kg) led to a significant increase in hepatic injury, in addition to an increase in pro-inflammatory cytokine levels and oxidative stress. Co-administration of APAP and CCl₄ resulted in exacerbation of these hepatic injuries. Our results suggest that a non-toxic dose of APAP has the potential to increase CCl₄-induced liver damage and oxidative stress.

Vitamin D3-induced hypercalcemia increases carbon tetrachloride-induced hepatotoxicity through elevated oxidative stress in mice

The aim of this study was to determine whether calcium potentiates acute carbon tetrachloride (CCl₄) -induced toxicity. Elevated calcium levels were induced in mice by pre-treatment with cholecalciferol (vitamin D3; V.D3), a compound that has previously been shown to induce hypercalcemia in human and animal models. As seen previously, mice injected with CCl₄ exhibited increased plasma levels of alanine aminotransferase, aspartate aminotransferase, and creatinine; transient body weight loss; and increased lipid peroxidation along with decreased total antioxidant power, glutathione, ATP, and NADPH. Pre-treatment of these animals with V.D3 caused further elevation of the values of these liver functional markers without altering kidney functional markers; continued weight loss; a lower lethal threshold dose of CCl₄; and enhanced effects on lipid peroxidation and total antioxidant power. In contrast, exposure to V.D3 alone had no effect on plasma markers of liver or kidney damage or on total antioxidant power or lipid peroxidation. The potentiating effect of V.D3 was positively correlated with elevation of hepatic calcium levels. Furthermore, direct injection of CaCl₂ also enhanced CCl₄-induced hepatic injury. Since CaCl₂ induced hypercalcemia transiently (within 3 h of injection), our results suggest that calcium enhances the CCl₄-induced hepatotoxicity at an early stage via potentiation of oxidative stress.

Structure-acute toxicity relationship of aromatic hydrocarbons in mice

The structure-acute toxicity relationship of aromatic hydrocarbons was examined in mice. In all test compounds, the acute toxicity was determined under 2 conditions: control LD50 (LD50-cont) and carbon tetrachloride (CCl4)-pretreated LD50 (LD50-CCl4). The CCl4-pretreatment was done in order to evaluate the toxic potency of compound itself without the influence of metabolism. Both log (1/LD50-cont) and log (1/LD50-CCl4) were functions of the log P, n-octanol/water partition coefficient, i.e., log (1/LD50-cont) = 0.080 log P - 1.532 and log (1/LD50-CCl4) = -0.040(logP)2 + 0.157 log P - 1.373. Both equations were statistically significant (P < 0.01). The ratio of LD50-cont/LD50-CCl4 indicated that metabolic activation is more evident in hydrophobic compounds than in hydrophilic compounds. The results suggest that hydrophobicity of the aromatic hydrocarbons plays an important role in determining their acute toxicity.

Oxygen and xenobiotic reductase activities of cytochrome P450

The oxygen reductase and xenobiotic reductase activities of cytochrome P450 (P450) are reviewed. During the oxygen reductase activity of P450, molecular oxygen is reduced to superoxide anion radicals (O2-.) most likely by autooxidation of a P450 ferric-dioxyanion complex. The formation of reactive oxygen species (O2-., hydrogen peroxide, and, notably, hydroxyl free radicals) presents a potential toxication pathway, particularly when effective means of detoxication are lacking. Under anaerobic conditions, P450 may also be involved in the reduction of xenobiotics. During the xenobiotic reductase activity of P450, xenobiotics are reduced by the ferrous xenobiotic complex. After xenobiotic reduction by P450, xenobiotic free radicals are formed that are often capable of reacting directly with tissue macromolecules. Unfortunately, the compounds that are reductively activated by P450 have little structural similarity. The precise molecular mechanism underlying the xenobiotic reductase activity of P450 is, therefore, not yet fully understood. Moreover, description of the molecular mechanisms of xenobiotic and oxygen reduction reactions by P450 is limited by the lack of knowledge of the three-dimensional (3D) structure of the mammalian P450 proteins.

Adrenal activation of carbon tetrachloride: role of microsomal P450 isozymes

Previous investigations demonstrated that carbon tetrachloride (CCl4) was activated by adrenal microsomes, resulting in various functional changes and ultimately in necrosis of the zona reticularis of the gland. Experiments were done to identify the adrenal P450 isozyme(s) involved in the bioactivation of CCl4. Incubation of microsomes from the zona reticularis (ZR) of the guinea pig adrenal cortex with CCl4 plus NADPH caused initiation of lipid peroxidation, covalent binding of CCl4-derived radioactivity to protein, and degradation of cytochrome(s) P450. Preincubation of the microsomal preparations with inhibitory antibodies to P450(17 alpha) or P450C21 decreased the corresponding enzyme activities (17 alpha-hydroxylation and 21-hydroxylation), but did not affect the activation of CCl4. 1-Aminobenzotriazole (ABT), a suicide inhibitor of some P450 isozymes, decreased the enzyme activities catalysed by an adrenal 52,000 Da (52 kDa) isozyme, but had no effect on the function of P450(17 alpha) or P450C21. However, ABT completely inhibited the CCl4-induced LP and covalent binding in adrenal microsomes. The results indicate that adrenal CCl4 activation is catalysed by the 52 kDa P450 isozyme and not by the steroid hydroxylases. Localization of the 52 kDa isozyme to the ZR probably accounts for the selective necrosis of this region of the gland by CCl4.

Response of mouse liver coumarin 7-hydroxylase activity to hepatotoxins: dependence on strain and agent and comparison to other monooxygenases

Acute effects of a single intraperitoneal dose of allyl alcohol (AA, 64 mg/kg), dimethylnitrosamine (DMNA, 30 mg/kg), hexachlorobutadiene (HCBD, 50 mg/kg), carbon tetrachloride (CCl4, 24 mg/kg), cocaine (60 mg/kg) and pyrazole (300 mg/kg) on the hepatic histology and monooxygenases in DBA/2 and C57Bl/6 strains of mice were investigated. All substances caused histologically verified injury to the liver, which varied in appearance and severity depending on the compound and the mouse strain. Responses of P450-catalyzed reactions were highly dependent on the toxin and varied between different monooxygenase (MO) reactions and two mouse strains. In DBA/2 strain, coumarin 7-hydroxylase (COH) activity was increased from 3- to 5-fold by pyrazole, cocaine, HCBD and CCl4. With respect to P450 content and other MO activities, no changes or even decreases were generally observed. Some exceptions to this rule were found: HCBD significantly increased T15 alpha OH, PROD and EROD activities in C57Bl/6 mice, whereas cocaine caused a significant stimulation of T15 alpha OH and PROD in DBA/2 mice, It is concluded that i) different hepatoxins cause different types of liver injury and responses of the monooxygenase complex ("hepatotoxinspecific finger prints"), ii) although DBA/2 and C57Bl/6 mice responded rather similarly to hepatotoxins, also with respect to P450 content and most MO activities, they displayed a profound difference in the behaviour of COH activity, and iii) within the P450 superfamily, the regulation of COH activity seems to be rather unique, also when compared to its structurally close enzyme, testosterone 15 alpha-hydroxylase.

Induction of hepatic drug metabolizing enzymes and interaction with carbon tetrachloride in rats after a single oral exposure to atrazine

A single oral dose (430 mg/kg) of atrazine, a widely employed s-triazine herbicide, was administered to young male rats. There was a significant increase of the in vivo elimination of hexobarbital and a significant induction of the activity of 7-pentoxyresorufin-O-dealkylase, while cytochrome P-450 content and other mixed function oxidase activities remained unaltered. The administration of carbon tetrachloride (CCl4) to atrazine pretreated rats did not substantially augment the impairment of drug metabolizing enzymes brought about by CCl4 alone. Results suggest that atrazine behaves like a relatively weak inducer of phenobarbital-inducible families of cytochrome P-450.

Effect of phenobarbital treatment on carbon tetrachloride-mediated cytochrome P-450 loss and diene conjugate formation

The effect of phenobarbital treatment on the linkage between carbon tetrachloride-mediated cytochrome P-450 loss and lipid peroxidation in rat liver microsomes was studied. Male Sprague-Dawley rats, pretreated with 3 daily i.p. doses of phenobarbital (50 mg/kg) or saline, were orally dosed with carbon tetrachloride (0.01-2.5 ml/kg), with liver microsomes prepared at 7.5-180 min after carbon tetrachloride treatment. In vivo cytochrome P-450 loss displayed pseudo-first-order kinetics, and the initial rates of diene conjugate formation were saturable with dose. Phenobarbital pretreatment decreased the in vivo t0.5,max from 27.0 to 15.6 min, and increased the Kd,app from 0.78 to 1.30 ml/kg for carbon tetrachloride mediated cytochrome P-450 loss. Phenobarbital had no effect on the in vivo Vmax (1.03 to 1.04 delta OD232 nm/min/mg phospholipid) for carbon tetrachloride mediated diene conjugate formation, but decreased the Km,app from 0.22 to 0.10 ml/kg. These results are consistent with destruction of cytochrome P-450 heme resulting from a metabolite which does not leave the site of generation, and with phenobarbital pretreatment enhancing the initiation of lipid peroxidation.

Structure-toxicity relationship of ethylene glycol ethers

The ultimate purpose of the present study was to evaluate correlations between acute in vivo and in vitro toxicity and log P (P is n-octanol-water partition coefficient). The in vitro toxicity to cloned cells (neuroblastoma N18TG-2 and glioma C6) in culture (ED50) and the in vivo toxicity to mice (LD50) of ethylene glycol ethers were studied in terms of the structure-activity relationship. The test ethers showed a wide range of ED50 values in both cells. LD50 was determined under two conditions: LD50-cont. was estimated in mice pretreated with olive oil and LD50-CCl4 in CCl4-pretreated mice. Multiple regression analyses revealed a significant correlation between log 1/LD50 and log P as follows: log (1/LD50-cont.) = -0.120 (log P)2+0.487log P-1.182, and log (1/LD50-CCl4) = -0.128 (log P)2+0.566log P-1.157. There was no significant correlation either between ED50 and LD50 or between ED50 for N18TG-2 and ED50 for C6. The results suggest that metabolic activation might not occur during acute toxicity from the ethers, and that hydrophobicity, expressed as log P, plays an important role in acute toxicity.

Degradation of cytochrome P450 2E1: selective loss after labilization of the enzyme

Mechanism-based inactivation of cytochrome P450 can result in the chemical modification of the heme, the protein, or both as a result of covalent binding of modified heme to the protein. In the present study we took advantage of different modes of inactivation of P450 2E1 by CCl4, 1-aminobenzotriazole, or 3-amino-1,2,4-triazole to investigate parameters which target P450 2E1 for proteolysis from the microsomal membrane. Treatment of mice with CCl4 at the point of maximal induction of P450 2E1 after a single oral dose of acetone resulted in the complete loss of P450 2E1-dependent p-nitrophenol hydroxylation and a 75% loss of immunochemically detectable protein within 1 h of administration. Treatment with 1-aminobenzotriazole at the point of maximal induction caused a complete loss of P450 2E1-dependent p-nitrophenol hydroxylation but only a 12% loss of immunochemically detectable protein 1 h after administration. Treatment of mice with 3-amino-1,2,4-triazole caused a rapid loss of both catalytic activity and microsomal p-nitrophenol hydroxylase activity. However, unlike CCl4 treatment, the activity and enzyme level rebounded 5 and 9 h after treatment. The P450 2E1 ligand, 4-methylpyrazole, administered at the point of maximal induction maintained the acetone-induced catalytic and immunochemical level of P450 2E1. These results suggest that differentially modified forms of P450 2E1 show a characteristic susceptibility to degradation. While there are many potential pathways for protein degradation, the loss of P450 2E1 was associated with increased formation of high molecular weight microsomal ubiquitin conjugates. The formation of ubiquitin-conjugated microsomal protein which correlates with P450 2E1 loss suggests that ubiquitination may represent a proteolytic signal for the rapid and selective proteolysis of certain labilized conformations of P450 2E1 from the endoplasmic reticulum.

Amplification of CCl4 toxicity by chlordecone: destruction of rat hepatic microsomal cytochrome P-450 subpopulation

Previous work has established marked amplification of CCl4 hepatotoxicity by prior exposure to chlordecone (CD). Since CCl4 is toxic by virtue of its bioactivation by the hepatomicrosomal cytochrome P-450 (cyt P-450) system, which is in turn destroyed, our first interest was to determine if cyt P-450 isozymes were selectively destroyed in this interaction. CoCl2 also decreased hepatic P-450 contents, so our other interest was to observe whether CoCl2 selectively decreased or spared CCl4 metabolizing P-450 enzymes. Solubilized hepatic microsomes from variously treated rats were used. The treatment protocol was dietary CD (10 ppm, for 15 d), and CCl4 (100 microliters/kg, ip). The treatments were CD alone, CCl4 alone, CD + CCl4 and with or without CoCl2 (60 mg/kg/d, sc for 2 d) treatment on d 13 and 14 of the dietary protocol. The control group received normal diet and corn oil vehicle. The key mixed-function oxidase (MFO) parameters measured were microsomal protein, cyt P-450 content, and aminopyrine demethylase (APD). Decrease of P-450 levels ranged from 2.2-fold (CD + CCl4) to 1.3-fold (CD + CoCl2). APD activity decreased by 48 and 26.6% in CD + CCl4 and CD + CoCl2 treatments, respectively. Using an anion-exchange high-performance liquid chromatography (HPLC) column, solubilized microsomal hemoproteins were resolved into five peaks. The P-450 content associated with each peak was determined. In CD rats there was slight increase in peak heights, whereas peak heights in CCl4 and control treatments were similar. CoCl2 decreased all peaks, the decrease of peak I being maximal. In CD + CCl4 treatment, absence of peaks II and III was noted. Microsomal proteins stained for heme showed decreased staining intensity of hemo-protein bands, particularly band 4 (MW 52,000), which was absent in CD + CCl4 interaction. These findings suggest that (1) CoCl2 does not selectively decrease or spare any P-450 isozymes and (2) CD + CCl4 interaction does destroy specific P-450 isozymes.

The effect of tetrahydrofuran on biological systems: does a hepatotoxic potential exist?

Tetrahydrofuran (THF) is a widely used solvent in industry and research. THF is a weak toxin, with approximate acute LD50s in the range of 2 to 3 g/kg, 8 to 20 mg/L, and 800 mg/kg following oral, inhalation, and i.v. routes, respectively. The two primary signs of intoxication are narcosis and hepatocellular dysfunction, both occurring at doses of approximately one-half of the lethal dose. Little is known concerning the pharmacokinetics of THF. No evidence exists for genotoxicity due to THF. Ongoing carcinogenicity bioassay tests sponsored by the National Toxicology Program have not yet been completed. Two significant interactions of THF with cellular components have been studied; first, THF inhibits a number of enzymatic reactions at concentrations ranging from 10 to 100 mM. Most notably, THF is an inhibitor of a number of cytochrome P450 (P450) dependent mixed function oxidase activities, with a particular affinity for the alcohol-induced isozyme (P450IIE1). Second, THF has been noted to enhance the toxic action of a number of compounds (i.e. solvent effect), in particular stimulating the more rapid absorption of reactive metabolites. A third factor to be considered is THF's ability to form peroxides upon standing. Little is known concerning the toxicity of THF peroxides. Therefore, while there is little evidence to suggest that THF would be a direct (Type I) hepatotoxin at relatively low doses, its ability to inhibit drug-metabolizing reactions (perhaps more so in alcohol users), and enhance the absorbance of reactive metabolites, are relatively unexplored avenues for THF to contribute to a hepatotoxic response. As a great deal of the human hepatotoxicity (Type II) is "idiosyncratic" and cannot be readily reproduced in experimental animals, this potential route of hepatotoxicity should be considered in future evaluations of human exposure to THF.

Interaction of carbon tetrachloride with beta-naphthoflavone-mediated cytochrome P450 induction in winter flounder (Pseudopleuronectes americanus)

The interaction between beta-naphthoflavone induction (BNF: 100 mg/kg) and carbon tetrachloride (CCl4; 1 ml/kg) hepatotoxicity was examined in the flounder. Treatment groups composed of control, BNF, CCl4, and BNF/CCl4 were compared in terms of cytochrome P450 isozyme content (LM4b; LM2), catalytic activity, isozyme distribution. SGOT-SGPT levels, and pathology. CCl4 administration resulted in significant reductions in both the constitutive P450 (LM2) and the BNF-inducible isozyme (LM4b) as well as elevations in SGPT and SGOT levels. The decline in LM4b isozyme content was reflected by stoichiometric decreases in ethoxyresorufin-O-deethylase activities. BNF/CCl4 coadministration was protective in part against CCl4 hepatotoxicity. Immunohistochemistry indicated that LM4b was diffusely distributed throughout the liver. These interactions have demonstrated a multiple P450 isozyme involvement, the protective nature of BNF against CCl4 hepatotoxicity in the flounder, the ability to maintain an inductive response in face of CCl4 coadministration, and the diffuse distributional pattern of LM4b in the flounder liver.

Phenobarbital pretreatment alters the localization of CCl4-induced changes in rat liver microsomal fatty acids

Phenobarbital treatment induces an isozyme(s) of liver microsomal cytochrome P450 susceptible to CCl4 and enhances the latter's lethality. We have now studied phenobarbital's effect on the specificity of phosphatidyl fatty acid changes in rat liver microsomes. Male Sprague-Dawley rats were pretreated with three daily ip doses of phenobarbital (50 mg/kg) or saline and then orally dosed with CCl4 (2.5 ml/kg). Liver microsomes were prepared 7.5 to 180 min after CCl4 treatment, the lipid fraction was extracted, diene conjugate content was determined, and phospholipids were separated by HPLC for fatty acid content determination. Protein, phospholipid, and phosphatidyl fatty acid residue loss occurred early (7.5 to 30 min) and in some cases later (60 to 180 min) in both pretreated groups, suggesting that two phases of CCl4-mediated injury occurred. Phenobarbital pretreatment accelerated the CCl4-induced formation of diene conjugates in the microsomal lipids. In studies on the separated phospholipids, phenobarbital alone altered microsomal fatty acid content, primarily decreasing arachidonic acid in favor of linoleate, particularly in phosphatidylserine. During the early phase of CCl4 injury, phenobarbital pretreatment shifted the major loss of arachidonic acid from phosphatidylserine to phosphatidylethanolamine. During the later phase, arachidonic acid loss was still prominent, but the most extensive CCl4-induced changes in fatty acids occurred in the neutral lipid fraction, regardless of pretreatment. These changes included loss of neutral lipid linoleic and docosahexanoic acids associated with an increase in palmitic acid. These data demonstrate that phenobarbital pretreatment is associated with a shift in the predominant phospholipid locus from phosphatidylserine to phosphatidylethanolamine for the early CCl4-induced fatty acid changes in rat liver microsomes.

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.

Carbon tetrachloride hepatotoxicity in endotoxin tolerant and polymyxin B-treated rats

Gut-derived endotoxin has been implicated in the hepatotoxic effects of CCl4. The present study has investigated whether two procedures known to block LPS effects would alter the action of CCl4 to decrease hepatic cytochrome P-450 and microsomal drug-metabolizing activity. Administration of polymyxin B or induction of LPS tolerance were shown to attenuate the effect of CCl4 administration to increase SGOT and SGPT levels, signs of hepatic damage. Polymyxin B administration but not LPS tolerance caused a slight decrease in cytochrome P-450. In pretreated animals given CCl4, only those which had received polymyxin B showed a reduced effect of CCl4 to alter cytochrome P-450 level and activity. However, the apparent protective effect was of the same magnitude as the loss of cytochrome P-450 caused by polymyxin B itself. These results suggest that the ability of polymyxin B to ablate the CCl4 loss of P-450 might be due to a reduced metabolic activation of CCl4 by P-450 and not due to any anti-LPS activity. The results suggest that gut-derived LPS does not participate in the effect of CCl4 decreasing cytochrome P-450-mediated reactions. However, participation of LPS in other hepatotoxic effects of CCl4 is not excluded.

Fatty acids in liver microsomal lipids of rats exposed to hypoxia, tetrachloromethane, or both

Arachidonic and docosahexaenoic acid in hepatic microsomal lipids from male Sprague-Dawley rats are greatly lowered when the animals have been exposed to tetrachloromethane; at the same time, palmitic, oleic and linoleic acid are significantly increased. Hypoxia alone causes similar derangements, but to a lesser extent. These are largely corrected 18 h after exposure; the effects induced by tetrachloromethane are persistent. The increases in 16:0, 18:1 and 18:2 suggest that in both cases microsomal enzymes involved in fatty acid metabolism are inhibited, either reversibly or irreversibly. Reduction of oxygen partial pressure during tetrachloromethane exposure has little effect upon hepatotoxicity as judged by hepatic enzymes in serum; only the onset of their release into the bloodstream is earlier.

Impaired androgen 16 alpha-hydroxylation in hepatic microsomes from carbon tetrachloride-cirrhotic male rats

Hepatic cirrhosis produced by repeated inhalation of carbon tetrachloride is associated with reduced levels of microsomal cytochrome P450. In this study the C19-steroids androstenedione and testosterone were used as specific probes of the functional activity of several forms of cytochrome P450 in microsomal fractions from control and cirrhotic rat liver. The principal finding, that androstenedione 16 alpha-hydroxylation and testosterone 2 alpha-, 16 alpha-, and 17 alpha-hydroxylation were reduced to 14%-38% of control activity, strongly suggests that levels of the male sexually differentiated cytochrome P450 (P(450)16 alpha) are decreased in hepatic cirrhosis. The activity of other cytochrome P450-mediated C19-steroid hydroxylases, with the exception of androstenedione 6 beta-hydroxylase, appeared essentially unaltered in microsomes from cirrhotic rats. Cirrhosis induced by carbon tetrachloride was also associated with greatly decreased activity of the microsomal cytochrome P450-independent 17 beta-oxidoreductase, an enzyme that catalyzes the conversion of androstenedione to testosterone. Consequently, and in view of the impaired activity of cytochrome P450-mediated testosterone 17 alpha-hydroxylation, the capacity of cirrhotic microsomes to catalyze the interconversion of androstenedione and testosterone was much lower than that of control microsomes. The present data confirm and extend earlier observations that selective impairment of drug oxidation pathways occurs in hepatic cirrhosis. These changes are unrelated to the acute toxicity produced by carbon tetrachloride exposure. The available evidence supports the assertion that specific forms of cytochrome P450 are subject to altered regulation in cirrhosis.

Binding of reactive metabolites of CCl4 to specific microsomal proteins

The covalent binding of [14C]carbon tetrachloride to microsomal proteins in rat liver microsomes under anaerobic conditions was investigated by SDS-polyacrylamide slab gel electrophoresis and fluorography. Most of the labeled proteins were observed in the molecular weight range of 52-61 kDa, indicating that cytochrome P-450 forms (EC 1.14.14.1) were labeled. Protein bands at the position of the NADPH-cytochrome P-450 reductase (78 kDa) (EC 1.6.2.4) and NADH-cytochrome b5 reductase (33 kDa) (EC 1.6.2.2) also showed radioactivity. The fluorographic pattern of the protein labeling was cytochrome P-450-dependent, as was demonstrated by CO and metyrapone inhibition as well as by pretreatment of rats with inducing drugs such as 3-methylcholanthrene, benzo(a)pyrene, phenobarbitone and Aroclor 1254. Immuno-precipitation with a purified anti-P-450 immunoglobulin against cytochrome P-450 PB-B (52 kDa) of rat liver indicated that this protein contained about 10-20% of the total bound radioactivity in an average ratio of 0.8 mol [14C]CCl4-metabolite/mol cytochrome P-450 PB-B.

Tetrachloromethane metabolism in vivo under normoxia and hypoxia. Biochemical and histopathological effects relative to alkane exhalation

About 250 mumol/kg tetrachloromethane is metabolized by male Sprague-Dawley rats receiving a dose of 500 mumol/kg by inhalation. Determination of enzyme activities and histopathological assessment show that various parameters reflecting cellular injury do not differ for the same amount of tetrachloromethane metabolized under different oxygen partial pressures. On the other hand, the amounts of ethane and pentane exhaled under hypoxic conditions are greater than under normoxia. The previously reported differences in the toxicity of tetrachloromethane upon exposure under normoxia or hypoxia seem to be mainly due to the different amounts of tetrachloromethane metabolized under both conditions.

Lipid peroxidation and molecular damage to polyunsaturated fatty acids in rat liver. Recognition of two classes of hydroperoxides formed under conditions in vivo

The diene conjugates formed during the autoxidation of microsomal lipid extracts, and in endoplasmic reticulum in vivo after exposing rats to CCl4 have been examined by second derivative absorption spectrophotometry. Within a few minutes after administering CCl4 to a rat there is a pronounced signal in microsomal lipid extracts that is ascribed to the cis-trans diene conjugates of microsomal polyunsaturated fatty acids. Somewhat later a second signal becomes evident that is ascribed to trans-trans isomers. The appearance of the trans-trans isomer is very strongly suppressed by prior administration of vitamin E to the rat. It is concluded that the relative contents of cis-trans and trans-trans dienes in lipid extracts of tissue reflect the tissue contents of hydrogen donors as already established for model experiments with polyunsaturated fatty acids in vitro.

Drug metabolizing activity in rats with chronic liver injury induced by carbon tetrachloride: relationship with the content of hydroxyproline in the liver

The drug metabolizing activity of rat liver during long-term administration of carbon tetrachloride (CCl4), and its relationship with the content of hydroxyproline (Hyp) in the liver were examined. The contents of cytochrome P-450 (P-450) and cytochrome b5 (b5) and the metabolization of aniline, aminopyrine, 7-ethoxycoumarin (7-EC) and benzo(a)pyrene (B(a)P) in the microsomal fraction were examined five days after the final administration of CCl4. The contents of P-450 and b5 and the activity to metabolize the four substrates were gradually reduced as the Hyp content in the liver increased. However, aminopyrine N-demethylation and B(a)P hydroxylation, particularly the latter, was more reduced than aniline hydroxylation and 7-EC O-deethylation in the early stage of hepatic fibrosis. Such differences may be due mainly to the different P-450 subtypes affected by CCl4.

Drug metabolism in cirrhosis. Selective changes in cytochrome P-450 isozymes in the choline-deficient rat model

The effect of a choline-deficient diet on microsomal cytochrome P-450 and mixed-function oxidase (MFO) activity was investigated in relation to the development of nutritional cirrhosis. In rats that received the choline-deficient diet for 28 weeks cirrhosis was evident macroscopically and histologically; control rats that received an identical diet supplemented with choline had normal livers. Microsomal cytochrome P-450 and cytochrome b5 were reduced in cirrhotic liver to 50% of control levels. Three MFO activities (ethylmorphine N-demethylase, aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase) were also reduced to 40-70% of control levels. However, the turnover number for the O-deethylation of 7-ethoxycoumarin was not reduced in cirrhotic liver. This finding suggested that certain drug oxidations may be selectively depressed in nutritional cirrhosis. To examine the possibility that selective changes in MFO activity may reflect the suppression of certain cytochrome P-450 isozymes, partially purified fractions of the cytochrome were prepared after solubilisation and hydrophobic affinity chromatography (on n-octylamino-Sepharose 4B) of cirrhotic and control liver microsomes. Analysis of these fractions by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and laser densitometry indicated that a protein band of apparent minimum molecular weight 50.5 kD was primarily affected in cirrhotic rat liver microsomes. Levels of two other bands (apparent minimum molecular weight 48 and 52.5 kD) appeared essentially unaltered. Additional electrophoretic studies, conducted under non-reduced conditions, indicated the haemoprotein nature of protein bands in the 48-55 kD region. These data strongly suggest that cirrhosis produced in rats by a choline-deficient diet is associated with selective decreases in oxidative drug metabolism and individual cytochrome P-450 isozymes.

NADPH-dependent and -independent loss of cytochrome P-450 in control and phenobarbital-induced rat hepatic microsomes incubated with carbon tetrachloride

Carbon tetrachloride-mediated loss of cytochrome P-450 has been compared in hepatic microsomes from untreated and phenobarbital-treated rats. At concentrations of carbon tetrachloride greater than 2.5 mM, a direct effect (i.e., NADPH- independent) on cytochrome P-450 was observed. This apparently arose from the "solvent" properties of carbon tetrachloride as this effect could be duplicated with the physically similar alkyl halide 1,2-dibromo-3-chloropropane. NADPH-dependent loss of cytochrome P-450 occurred at lower concentrations with maximal response occurring at 2.5-5.0 mM. Residual cytochrome P-450 at these concentrations was similar in untreated and phenobarbital-treated microsomes. Mixed-function oxidase activities in phenobarbital-treated microsomes were reduced to levels below those of uninduced controls. The 52-kDa polypeptide(s) in untreated microsomes and that specifically induced in phenobarbital-treated microsomes were susceptible to NADPH-dependent carbon tetrachloride incubation. These data suggest that the susceptibility of specific forms of cytochrome P-450 to carbon tetrachloride can be duplicated in in vitro incubation. Furthermore, data on the direct action of carbon tetrachloride suggest that this route of damage must be taken into consideration when concentrations of carbon tetrachloride of 2.5 mM or greater are used in in vitro incubation systems.

Changes in cytochrome P450 molecular species in rat liver in chloroform intoxication

The effect of CHCl3 on the composition of hepatic microsomal cytochrome P450 species was compared with that of CCl4 in rats pretreated with phenobarbital (PB) or 3-methylcholanthrene (3MC). The administration of CHCl3 hardly affected cytochrome P450 content in non-treated rat liver, but caused a similar degree of depletion in the content as observed after CCl4 administration in PB-pretreated rats. In the pretreatment with 3MC, the administration of CHCl3 brought about a marked decrease in the content to 24% of control after 12 hr, while CCl4 reduced the content only to one-half of control. It was demonstrated by SDS-polyacrylamide gel electrophoresis and Whatman DE-52 anion-exchange chromatography that 3MC-induced P450 species decreased with CHCl3, while it was affected little by CCl4 treatment. The activity of benzo[a]pyrene hydroxylase was altered together with the change in the content of cytochrome P450 species. The administration of CHCl3 to PB-pretreated rats caused the depletion in PB-induced P450. These findings indicate that cytochrome P450 species induced with 3MC as well as PB are highly susceptible to CHCl3 intoxication, whereas the administration of CCl4 depletes the PB-induced species without affecting the 3MC-induced species.

Inhalation pharmacokinetics of carbon tetrachloride in rats based on arterial blood:inhaled air concentration ratios

To estimate the rate of CCl4 metabolism in vivo by using an inhalation pharmacokinetic approach based on arterial blood:air concentration ratios, the blood CCl4 concentrations (Cart) at the end of 5-hr exposure to varying concentrations of CCl4 in inhaled air (Cinh) were determined in male, naive rats and in rats pretreated with po administration of 100 or 200 microliters CCl4/100 g body weight 24 hr before exposure. Hepatic cytochrome P-450 content during and at the end of exposure was also determined. The biphasic nature of the Cart-Cinh curve for naive rats, with a transition at Cinh of about 100 ppm, indicated that CCl4 metabolism is perfusion-limited below 100 ppm and is saturated above 100 ppm. In 100 microliters CCl4-pretreated rats, Cinh at the transition point decreased from 100 to 50 ppm; this percentage decrease was consistent with the decreased cytochrome P-450 content induced by administration of 100 microliters CCl4. In 200 microliters CCl4-pretreated rats, where CCl4 metabolizing enzyme activity was completely inhibited, the Cart-Cinh curve gave a single line with a shallower slope than that of the steeper part of the curve for naive rats, reflecting a loss of cytochrome P-450 content during exposure. The curves of calculated uptake rate showed continued uptake in completely inhibited rats, representing the contribution of fat loading only. The rate of metabolism was approximated by the uptake rate for naive rats minus that for 200 microliters CCl4-pretreated rats, and decreased gradually with increasing Cinh over the range of saturable metabolism. From this rate curve, Vmax and Km for naive rats were 2.7 mg/kg/hr and of the order of 0.3 mg/liter, respectively. The gradual decrease in the rate of metabolism could be interpreted in terms of the rapid loss of cytochrome P-450 content. The Vmax for 100 microliters CCl4-pretreated rats decreased by about 57%, which was in good agreement with the decrease of cytochrome P-450 content. These experiments suggest the usefulness and validity of this approach for studying metabolism of a volatile compound.

Structure-toxicity relationship of monoketones

The structure-toxicity relationship of monoketones was examined in mice. In all test compounds, the acute oral toxicity (LD50) was determined under two conditions; control LD50 (LD50-cont.) and carbon tetrachloride (CCl4)-pretreated LD50 (LD50CCl4). LD50-cont. was larger than LD50-CCl4 in all test compounds, the difference being greater in compounds with longer carbon chains than in those with shorter chains. Both LD50-cont. and LD50-CCl4 were parabolic functions of the partition coefficient, log P, i.e., log (1/LD50-cont.) = -0.120(log P)2 + 0.417log P - 1.734 and log(1/LD50-CCl4) = -0.103(log P)2 + 0.462log P - 1.660, and both equations were statistically significant (P less than 0.01). The log Po values were 1.74 for the former and 2.24 for the latter.

Effect of pretreatment with cimetidine or phenobarbital on lipoperoxidation in carbon tetrachloride- and trichloroethylene-dosed rats

Lipid peroxidation (LP) in vivo as reflected by the exhalation of ethane and n-pentane and by thiobarbituric acid reactive substances (TBARS) in liver microsomes was studied in rats injected with carbon tetrachloride (CCl4) and trichloroethylene (TCE), each at 2 dose levels. Interactions between these chlorinated solvents and cimetidine (CM), an inhibitor of cytochrome P-450-dependent monooxygenases, or phenobarbital (PB) the well known inducer of microsomal enzyme activities were also assessed. A non-hepatotoxic dose of CCl4 did not cause a significant increase in ethane production except in PB-induced rats but did enhance n-pentane elimination, whereas an hepatotoxic dose increased the emission of both hydrocarbons. No interaction between CM and CCl4 could be shown but, as expected, PB potentiated the effect of CCl4. TCE administration led to a moderate dose-independent elevation of n-pentane production but did not affect that of ethane and the effect of TCE was smaller in PB-induced than in CM- or non-pretreated rats. There was no difference in microsomal TBARS content in rats injected with the chlorinated hydrocarbons. The use of butylated hydroxytoluene (BHT) and ethylene diaminetetraacetic acid (EDTA) revealed that direct measurement of TBARS gave inadequate results due to substantial chemical LP in vitro during the whole procedure. With the "ethane-pentane test" it was established that: CM cannot prevent CCl4-induced LP; and TCE hepatotoxicity does not involve increased LP of membrane lipids.

Carbon tetrachloride and 2-isopropyl-4-pentenamide-induced inactivation of cytochrome P-450 leads to heme-derived protein adducts

When CCl4 was incubated with rat liver microsomes from phenobarbital-treated rats in an aerobic or anaerobic atmosphere, over 69% of the heme moiety of cytochrome P-450 was destroyed. At least 45% of the degraded heme under both reaction conditions was accounted for as heme-derived products irreversibly bound to microsomal proteins. Furthermore, 33% of the irreversibly bound products were bound specifically to a 54-kDa form of cytochrome P-450. A structurally different compound, 2-isopropyl-4-pentenamide, also destroyed the heme moiety of cytochrome P-450 and produced heme-derived adducts of microsomal proteins that accounted for 28% of the destroyed heme. These results represent a novel mechanism for the destruction of cytochromes P-450 by xenobiotics.

Cytotoxicity of halogenated alkanes in primary cultures of rat hepatocytes from normal, partial hepatectomized, and preneoplastic/neoplastic liver

Six halogenated hydrocarbons, chloroform, 1,2-dibromoethane (1,2-DBE), 1,1-dichloroethane (1,1-DCE), 1,2-dichloroethane (1,2-DCE), 1,1,1-trichloroethane (1,1,1-TCE), and 1,1,2-trichloroethane (1,1,2-TCE), were evaluated for their cytotoxicity in primary cultures of rat hepatocytes isolated from normal, partially hepatectomized, and preneoplastic/neoplastic rat livers. Preneoplastic/neoplastic lesions of phenotypically altered foci and hepatocyte nodules were induced by either (1) initiation by diethylnitrosamine (DENA) followed by 2 weeks of 0.02% 2-acetylaminofluorene (2-AAF) in the diet and a single gavage dose of carbon tetrachloride 1 week after the start of the 2-AAF diet or (2) initiation by DENA followed by promotion with 500 ppm sodium phenobarbital in the drinking water for 24 weeks. The hepatocytes containing preneoplastic/neoplastic cells isolated from animals treated with either protocol, compared to hepatocytes isolated from normal liver, were resistant to the cytotoxicity of aflatoxin B1 (AFB1). None of the six halogenated alkanes exhibited any difference in their cytotoxicity toward hepatocytes isolated from normal liver or from liver containing preneoplastic/neoplastic lesions induced by either procedure. Hepatocytes isolated from partially hepatectomized animals were resistant to the cytotoxicity of AFB1 and chloroform but not to the cytotoxicity of 1,2-DBE or 1,2-DCE. The ranking of relative cytotoxicity in hepatocytes from untreated rats was 1,2-DBE much greater than 1,2-DCE greater than 1,1,2-TCE greater than 1,1,1-TCE greater than chloroform greater than 1,1-DCE. Treatment with SKF-525A protected the hepatocytes from the cytotoxicity of AFB1 while increasing the cytotoxicity of all six halogenated alkanes. Treatment with diethyl maleate increased the cytotoxicity of AFB1 and all six halogenated alkanes. These observations suggest that preneoplastic/neoplastic rat hepatocytes are not resistant to the cytotoxicity of the six halogenated alkanes because their toxicity might be mediated by a cytochrome P-450 species which is not inhibited by SKF-525A and is not decreased in preneoplastic/neoplastic lesions.

Immunofluorescent determination of the lobular distribution of a constitutive form of hepatic microsomal cytochrome P-450

We have used an immunohistochemical approach to study the lobular distribution of constitutive liver microsomal cytochrome P-450. Cytochrome P-450 isolated (10.3 nmoles per mg protein) from hepatic microsomes from untreated, mature male Sprague-Dawley rats was used to produce antisera in rabbits. The IgG fraction produced single precipitin lines of identity with liver microsomes after double immunodiffusion, precipitated 80% of the total microsomal cytochrome P-450 and inhibited three cytochrome P-450-dependent enzyme activities. By these criteria, the IgG appeared to be specific for a constitutive form (or immunochemically related family) of liver microsomal cytochrome P-450. The pattern of fluorescence after indirect immunofluorescent labeling of liver sections depended on the route of tissue preparation and the concentration of primary antibody. In frozen sections, the labeling was uniform throughout the lobule, whereas in "antigen-depleted" paraffin-embedded sections it was heaviest in the centrilobular and midzonal regions. Increasing the concentration of primary antibody to 500 micrograms per ml inhibited the centrilobular labeling in frozen sections in a concentration-dependent manner. When specific isozymes of cytochrome P-450 were induced with phenobarbital or 3-methylcholanthrene, the constitutive cytochrome P-450 was localized predominantly in the periportal region. Decreases in cytochrome P-450 in rats treated with carbon tetrachloride or 1,2-dibromo-3-chloropropane were associated with antigen loss only in necrotic cells. Regional differences in the loss of antigen in paraffin sections and the inhibition of fluorescence in frozen sections establish that the lobular distribution of constitutive hepatic microsomal cytochrome P-450 is qualitatively heterogeneous and may be altered during hepatocellular responses to chemical treatment.

Effect of ampicillin on hepatic microsomal mixed-function oxidase system in male mice

The effect of ampicillin [(D)-alpha-aminobenzyl penicillin] administration on the hepatic mixed-function oxidase (MFO) system was studied in male mice. Ampicillin (100 mg/kg, i.p., 3 days) decreased the levels of cytochrome P-450, aminopyrine N-demethylase, acetanilide hydroxylase and cytochrome c-reductase activity significantly. In carbon tetrachloride (CCl4)-pretreated mice, ampicillin increased acetanilide hydroxylation compared with CCl4 treatment alone; however, all other parameters of the MFO system remained unchanged. Ampicillin exhibited type II binding with microsomes (trough at 388 nm, peak at 430 nm). Thus ampicillin acts as an inhibitor of the MFO system.

Metabolism of [14C]carbon tetrachloride to exhaled, excreted and bound metabolites. Dose-response, time-course and pharmacokinetics

Fasted male rats were given six doses of 14CCl4 ranging from non-hepatotoxic (0.1 mmole/kg) to severely hepatotoxic (26 mmoles/kg). Time-course and pharmacokinetics of CCl4, 14CO2 and CHCl3 elimination by exhalation were monitored by measuring amounts recovered in breath during discrete 15-min intervals for 8-12 hr. Amounts of 14C-labeled metabolite recovered bound to liver macromolecules at 24 hr and excreted in urine or feces for 24 hr were also determined. Comparison pharmacokinetic studies were done with 14CHCl3 and Na(2)14CO3. After all doses of 14CCl4, the major metabolite was CO2, twenty to thirty times less metabolite was recovered bound to liver macromolecules, and intermediate amounts of metabolite were excreted in urine and feces. CHCl3 was the least abundant metabolite at low CCl4 doses, but the second most abundant at high doses. Stronger associations were found between the magnitude of liver injury at 24 hr (quantitated as serum glutamate-pyruvate transaminase activity) and the extent or rate of CCl4 metabolism by pathways leading to CO2 and CHCl3 than by pathways leading to 14C-metabolites bound in liver or excreted in urine. Time-course and pharmacokinetic data indicated that a major pathway of CCl4 metabolism leading to CO2 became impaired within 2 hr after administration of hepatotoxic doses of CCl4.

Carbon tetrachloride metabolism in vivo and exhalation of volatile alkanes: dependence upon oxygen partial pressure

Metabolism of carbon tetrachloride in rats at atmospheric and reduced oxygen pressure has been determined indirectly by its disappearance from the inhaled air; it is inversely related to oxygen concentration and increases with decreasing partial pressure, as expected for reductive dehalogenation; oxygen partial pressure has been reduced to about a third of normobaric conditions. Concurrently exhalation of ethane and pentane as indication of lipid peroxidation has been monitored, showing a drastic increase when the oxygen partial pressure is reduced in the presence of carbon tetrachloride. Time course and duration of these processes indicate that the total metabolism of carbon tetrachloride is limited by the concomitant destruction of cytochrome P-450; also, oxidative destruction of polyunsaturated fatty acids apparently does not proceed beyond the end of metabolic activation of carbon tetrachloride. The molar ratios of the amount of metabolized carbon tetrachloride to the amounts of exhaled hydrocarbons lead to the same conclusion, namely that "lipid peroxidation" in this case does not proceed as an autocatalytic, self-propagating chain reaction.

Relationships between the pharmacokinetics of carbon tetrachloride conversion to carbon dioxide and chloroform and liver injury

Rate and extent of CCl4 metabolism by pathways leading to CO2 and CHCl3 were evaluated by measuring the amounts of these metabolites exhaled during discrete intervals following six different doses of CCl4. Pulmonary pharmacokinetics of 14CO2 and CHCl3 exhalation after CCl4 administration were compared with those after Na214CO3 and 14CHCl3 administration. Exhalation of 14CO2 metabolite declined more rapidly than expected after hepatotoxic doses of CCl4. This decline could be due to injury associated changes in the metabolism of CCl4.

Isopropanol enhancement of cytochrome P-450-dependent monooxygenase activities and its effects on carbon tetrachloride intoxication

Acute or chronic treatment of rats with isopropanol caused a significant increase in hepatic cytochrome P-450 content and a two- to threefold increase in aniline hydroxylase and 7-ethoxycoumarin O-deethylase activities, but no significant change in ethylmorphine N-demethylase or benzo(a)pyrene hydroxylase activity. In rats treated with isopropanol and challenged with CCl4, liver toxicity of CCl4 was characteristically potentiated, as assessed by elevation of serum glutamic-pyruvic transaminase (SGPT) levels. Isopropanol pretreatment also potentiated CCl4-induced damage to the hepatic monooxygenase system. In addition to a decrease in cytochrome P-450, rats treated with isopropanol and challenged with CCl4 showed a nonspecific decrease not only in aniline hydroxylase and 7-ethoxycoumarin O-deethylase activities, but also in ethylmorphine N-demethylase, benzo(a)pyrene hydroxylase, and NADPH-cytochrome c reductase activities. These results were confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized microsomes. The electrophoretic results showed that isopropanol pretreatment markedly potentiated the CCl4-caused destruction of cytochrome P-450 hemeproteins. The data strongly suggest that isopropanol increases one or more forms of cytochrome P-450 which selectively enhance the metabolism of CCl4 to an active metabolite. This active metabolite then causes a nonselective damage to the microsomal mixed-function oxidase system.

Cytochrome P-450 and halothane metabolism. Decrease in rat liver microsomal P-450 in vitro

Anaerobic in vitro incubation of microsomes from phenobarbital(PB)-induced rats with halothane results in an irreversible decrease of measurable cytochrome P-450. There is a parallel decrease in heme content under the same incubation conditions. However, microsomes from 3-methylcholanthrene(3-MC)-induced or untreated animals do not show a reduction in cytochrome P-450 content. Aerobic incubation with halothane results in a decrease of cytochrome P-450 which can be completely reversed by dialysis or the addition of potassium ferricyanide. These latter treatments only partially restore the cytochrome P-450 levels following anaerobic incubations. The decrease in cytochrome caused by halothane is not associated with measureable heme N-alkyl adduct formation; lipid peroxidation does not play a role as indicated by the lack of effect of 1 mM EDTA or a decrease in glucose-6-phosphatase activity. Halothane metabolites are bound irreversibly to microsomal protein as determined by gel electrophoresis only when the oxygen concentration is very low. The mechanism of cytochrome P-450 decrease is consistent with the formation of a reactive metabolite which binds to the protein portion and also destroys heme.

Time course of the carbon tetrachloride-induced decrease in mitochondrial aldehyde dehydrogenase activity

Hepatic microsomal enzymes like cytochrome P-450 and glucose 6-phosphatase are inhibited after exposure to CCl4 in vivo. Since comparatively less is known about the effects of CCl4 on nonmicrosomal enzymes, we investigated the rapidity by which CCl4 inhibits the low Km mitochondrial aldehyde dehydrogenase (ALDH) isozyme, an enzyme known to be inhibited 24 hr after CCl4 treatment. The activity of this ALDH isozyme was significantly lowered 6 and 12 hr after a single 1 ml/kg intragastric dose of CCl4. The mitochondrial low Km ALDH specific activities exhibited a similar pattern of destruction/inhibition to the documented target enzyme microsomal cytochrome P-450 in that lowest values were observed 6 hr after CCl4. These values were 44 and 37% of control for cytochrome P-450 content and the low Km ALDH activity, respectively. Alcohol dehydrogenase activity, expressed as activity per gram liver, was depressed 12 hr after CCl4 dosing. Finally, the activity of the low Km cytosolic ALDH, the isozyme that metabolizes malondialdehyde at low concentrations, was not affected by CCl4 treatment. The CCl4-induced decline in the activity of the matrix ALDH isozyme occurs earlier than previously reported mitochondrial damage. The study of sensitive enzymes like the low Km ALDH may provide valuable information by which it may be possible to determine the relationship of the truly rapid biochemical effects of CCl4 such as microsomal lipid peroxidation with later effects on nonmicrosomal components.

Effect of chronic ethanol administration on bromobenzene liver toxicity in the rat

Female Sprague-Dawley rats were pair-fed a nutritionally adequate liquid diet containing either ethanol or isocaloric carbohydrate for 3 weeks. In vitro studies showed that chronic ethanol pretreatment preferentially increased the liver microsomal biotransformation of bromobenzene to p-bromophenol (via the toxic 3,4-epoxide) rather than to o-bromophenol (via the nontoxic 2,3-epoxide) and could thus potentiate bromobenzene hepatotoxicity. Bromobenzene administration (500 mg/kg body weight, ip), after an overnight fast, was associated in ethanol-pretreated rats with greater and accelerated liver glutathione depletion and subsequent decrease in liver cytochrome P-450 content than in controls. As assessed histologically and by determination of the rise in activities of serum enzyme markers of liver necrosis, chronic ethanol pretreatment, however, mainly resulted in earlier onset and resolution of bromobenzene-induced liver necrosis, with only a mild increase in the maximal severity of liver lesions. These results suggest that the twofold increase in liver microsomal bromobenzene 3,4-epoxidation by ethanol, being much less than that seen after phenobarbital pretreatment in our animal model and in that of others, is apparently not sufficient to markedly affect the severity of bromobenzene-induced liver toxicity.