Microsomal Ethanol-Oxidizing System (MEOS): The First 30 Years (1968-1998)–A Review

Oxidation of ethanol via alcohol dehydrogenase (ADH) explains various metabolic effects of ethanol but does not account for the tolerance and a number of associated disorders that develop in the alcoholic. These were elucidated by the discovery of the microsomal metabolism of ethanol. The physiologic role of this system comprises gluconeogenesis from ketones, fatty acid metabolism, and detoxification of xenobiotics, including ethanol. After chronic ethanol consumption, the activity of the microsomal ethanol-oxidizing system (MEOS) increases, with an associated rise in cytochromes P-450, especially CYP2E1. This induction is associated with proliferation of the endoplasmic reticulum, both in experimental animals and in humans. The role of MEOS in vivo and its increase after chronic ethanol consumption was shown most conclusively in alcohol dehydrogenase-negative deer mice. Enhanced ethanol oxidation is associated with cross-induction of the metabolism of other drugs, resulting in drug tolerance. Furthermore, there is increased conversion of known hepatotoxic agents (such as CCl₄) to toxic metabolites, which may explain the enhanced susceptibility of alcoholics to the adverse effects of industrial solvents. CYP2E1 also has a high capacity to activate some commonly used drugs, such as acetaminophen, to their toxic metabolites, and to promote carcinogenesis (e.g., from dimethylnitrosamine). Moreover, catabolism of retinol is accelerated and there also is induction of microsomal enzymes involved in lipoprotein production, resulting in hyperlipemia. Contrasting with the chronic effects of ethanol consumption, acute ethanol intake inhibits the metabolism of other drugs through competition for the at least partially shared microsomal pathway. In addition, metabolism by CYP2E1 results in a significant free radical release and acetaldehyde production which, in turn, diminish reduced glutathione (GSH) and other defense systems against oxidative stress. Acetaldehyde also forms adducts with proteins, thereby altering the functions of mitochondria and of repair enzymes. Increases of CYP2E1 and its mRNA prevail in the perivenular zone, the area of maximal liver damage. CYP1A2 and CYP3A4, two other perivenular P-450s, can also sustain the metabolism of ethanol, thereby contributing to MEOS activity and possibly liver injury. By contrast, CYP2E1 inhibitors oppose alcohol-induced liver damage, but heretofore available compounds were too toxic for clinical use. Recently, however, polyenylphosphatidylcholine (PPC), an innocuous mixture of polyunsaturated lecithins extracted from soybeans, was discovered to decrease CYP2E1 activity. PPC (and its active component dilino-leoylphosphatidylcholine) also oppose hepatic oxidative stress and fibrosis. PPC is now being tested clinically for the prevention and treatment of liver disease in the alcoholic.     

Metabolic aspects of alcoholic liver damage: 1984/1985 update. 2: Microsomal enzyme induction and hypermetabolism

In the second part of this review, the effect of ethanol on hepatic microsomal enzymes is primarily discussed. Since ethanol is metabolized via a cytochrome P-450 dependent biotransformation system (MEOS) in hepatic microsomes, the microsomal enzyme induction in the smooth endoplasmic reticulum has to be considered as an adaptive response. This enzyme induction results in an accelerated metabolism of ethanol. However, subsequently, the negative consequences of such a microsomal enzyme induction are predominant. Acetaldehyde production increases and oxygen consumption is enhanced leading to pericentral (perivenular) hypoxia. In addition, microsomal enzyme induction results in an enhanced metabolism of drugs, xenobiotics and hepatotoxins and thus to an increased production of toxic intermediates. Also procarcinogens are activated to a higher degree in microsomes following chronic ethanol consumption. Subsequently, an enhanced microsomal metabolism of vitamin A may explain the low serum concentrations of this vitamin in the alcoholic and may lead to toxic metabolites of retinol. The quantitative role of an enhanced reoxidation of NADH responsible for an increased oxidation of alcohol following chronic ethanol ingestion has still to be determined. However, according to recent investigations, a thyroid hormone induced hypermetabolism seems unlikely.     

Aetiology and pathogenesis of alcoholic liver disease

Until the 1960s, liver disease of the alcoholic patient was attributed exclusively to dietary deficiencies. Since then, however, our understanding of the impact of alcoholism on nutritional status has undergone a progressive evolution. Alcohol, because of its high energy content, was at first perceived to act exclusively as ‘empty calories’ displacing other nutrients in the diet, and causing primary malnutrition through decreased intake of essential nutrients. With improvement in the overall nutrition of the population, the role of primary malnutrition waned and secondary malnutrition was emphasized as a result of a better understanding of maldigestion and malabsorption caused by chronic alcohol consumption and various diseases associated with chronic alcoholism. At the same time, the concept of the direct toxicity of alcohol came to the forefront as an explanation for the widespread cellular injury. Some of the hepatotoxicity was found to result from the metabolic disturbances associated with the oxidation of ethanol via the liver alcohol dehydrogenase (ADH) pathway and the redox changes produced by the generated NADH, which in turn affects the metabolism of lipids, carbohydrates, proteins and purines. Exaggeration of the redox change by the relative hypoxia which prevails physiologically in the perivenular zone contributes to the exacerbation of the ethanol-induced lesions in zone 3. In addition to ADH, ethanol can be oxidized by liver microsomes: studies over the last twenty years have culminated in the molecular elucidation of the ethanol-inducible cytochrome P450IIE1 (CYP2E1) which contributes not only to ethanol metabolism and tolerance, but also to the selective hepatic perivenular toxicity of various xenobiotics. Their activation by CYP2E1 now provides an understanding for the increased susceptibility of the heavy drinker to the toxicity of industrial solvents, anaesthetic agents, commonly prescribed drugs, ‘over the counter’ analgesics, chemical carcinogens and even nutritional factors such as vitamin A. Ethanol causes not only vitamin A depletion but it also enhances its hepatotoxicity. Furthermore, induction of the microsomal pathway contributes to increased acetaldehyde generation, with formation of protein adducts, resulting in antibody production, enzyme inactivation and decreased DNA repair; it is also associated with a striking impairment of the capacity of the liver to utilize oxygen. Moreover, acetaldehyde promotes glutathione depletion, free-radical mediated toxicity and lipid peroxidation. In addition, acetaldehyde affects hepatic collagen synthesis: both in vivo and in vitro (in cultured myofibroblasts and lipocytes), ethanol and its metabolite acetaldehyde were found to increase collagen accumulation and mRNA levels for collagen. This new understanding of the pathogenesis of alcoholic liver disease may eventually improve therapy with drugs and nutrients.     

Inhibition of cytochrome P-450-dependent mixed function oxidation by ethanol

The mechanism of inhibition of cytochrome P-450-dependent mixed function oxidation by ethanol was studied. Ethanol competitively inhibited the binding of hexobarbital to liver microsomes, and increased the low spin signal of cytochrome P-450 in the electron spin resonance spectra. Therefore, ethanol decreased the substrates bound to ferric cytochrome P-450 in the first step of mixed function oxidation. The second step of mixed function oxidation is the reduction of ferric cytochrome P-450-substrate complex by NADPH-cytochrome P-450 reductase. The NADPH-dependent reduction of liver microsomal cytochrome P-450 was biphasic and composed of two first-order reactions. Ethanol decreased the rate constants of the fast and slow phases of microsomal cytochrome P-450 reduction. Thus, it is concluded that the inhibition of drug oxidation by ethanol may be due to the displacement of substrates from cytochrome P-450 and to the inhibition of reduction of cytochrome P-450 by NADPH-cytochrome P-450 reductase.     

Interaction of Ethanol with Drug Toxicity

The interaction of ethanol and drugs is complex. Ethanol interferes with the primary pharmacologic action of many drugs particularly other neuropharmacologically active agents. Ethanol is mainly a central nervous system depressant: sedatives such as the barbiturates tend to enhance these effects and stimulants tend to reduce them. The present review focuses not on this direct interaction but rather on indirect interaction relating to the metabolism of drugs. In that respect, acute and chronic effects of ethanol are usually opposite. Although there are some exceptions, generally, an acute close of ethanol inhibits the metabolism of other drugs, most commonly through competition for an at least partially shared microsomal detoxification pathway, whereas chronic consumption enhances drug metabolism. often because of the induction of liver microsomal enzymes. Although the microsomal process usually results in detoxification of drugs, on occasion the opposite occurs, namely activation of the substrate to a more potent, hepatotoxic metabolite. The interaction of ethanol and drugs is further complicated by the fact that the "induction" of the microsomes after chronic ethanol consumption may be offset, at least in part, by the development of liver damage. In addition to some unique interactions characteristic of some specific drugs, the therapy of the alcoholic must take into account those general factors that determine the degree of interaction, such as blood level of alcohol present, duration of alcohol abuse, degree of liver disease, as well as pretreatment with other drugs and the state of nutrition which may affect the liver's capacity for detoxification.     

Reversible bile acid changes in bile duct obstruction and its potential for hepatocellular injury.

The etiology of hepatocellular dysfunction resulting from chronic biliary obstruction is not clearly understood. Alterations in bile acid metabolism due to changes in microsomal cytochrome P-450 enzyme activities may have a fundamental role in cholestatic liver injury. This study examines the very early changes in both biliary bile acids and hepatic microsomal cytochrome P-450 content after bile duct obstruction in the rat and the effects of the restoration of bile flow after 3 days of biliary obstruction. We found that early induction of cytochrome P-450 may be a fundamental step in the generation of cholestatic liver injury mediated by hepatotoxic bile acids. The rapid reversal of bile acid changes with reconstituted bile flow indicate that the liver is able to quickly recover when obstruction is relieved. Characterization of this fundamental process may ultimately provide a means of modulation of cholestatic hepatotoxicity.     

Ethanol-Induced Enhancement of Cocaine Bioactivation and Irreversible Protein Binding: Evidence Against a Role of Cytochrome P-450IIE1

Chronic ethanol consumption potentiates cocaine-induced liver injury in rodents. Since cocaine has to be bioactivated by a cytochrome P-450-dependent N-oxidative pathway to exert its hepatotoxic effects, we studied the role of the ethanol-inducible P-450IIE1 for cocaine metabolism. Male Sprague-Dawley rats were pretreated with either a liquid diet containing ethanol (30% of calories) for 4 weeks or injected with pyrazole (200 mg/kg/day, ip, for 3 days). Both agents induced microsomal p-nitrophenol hydroxylation which is a probe for the catalytic activity of P-450IIE1. However, only ethanol, but not pyrazole, increased both microsomal cocaine N-demethylase activity (by 47%) and the extent of irreversible binding of [3H]-cocaine to microsomal proteins (by 100%), which was taken as a quantitative endpoint for the formation of a reactive metabolite. Cocaine N-demethylation and irreversible protein binding of cocaine were not inhibited by P-450IIE1 isozyme-selective substrates, nor was the rate of cocaine metabolism and binding decreased by functionally active polyclonal anti-rat P-450IIE1 antibodies. Furthermore, pyrazole pretreatment sensitized cultured hepatocytes to the glutathione-dependent cytotoxic effects of nontoxic concentrations of cocaine. These results indicate that (a) cocaine is not a major substrate for the ethanol-inducible P-450IIE1, (b) the enhancing effects of ethanol on cocaine bioactivation may be due to induction of other P-450 isoforms, and (c) induction of P-450IIE1 may potentiate cocaine-induced hepatocellular toxicity in vitro independently of cocaine metabolism, e.g., by P-450IIE1-dependent oxidative stress.     

The Effects of Chronic Ethanol Ingestion on Ethanol Binding to Hepatic Cytochrome P-450 and on Certain Hepatic and Renal Parameters in the "Long Sleep" and "Short Sleep" Mouse

Male mice selected for genetic differences in ethanol-induced sleep time, thereby designated long sleep (LS) and short sleep (SS), were treated with the Lieber-DeCarli liquid diet for 25 days. This chronic ethanol treatment produced an increase in liver/body weight and kidney/body weight in SS mice only. In addition, chronic ethanol treatment produced significant increases in both LS and SS treated mice in in vivo ethanol elimination, hepatic cytochromes P-450 and B_s, NADPH cytochrome c reductase and hepatic and renal 7-ethoxycoumarin O-de-ethylase activity. Geno-typic differences were observed in the magnitude of response of microsomal ethanol oxidation per mg of microsomal protein (SS > LS). Further, control LS and SS mice possessed substantially different activity of renal 7-ethoxycoumarin O-de-ethylase. Both lines exhibited similar induced renal 7-ethoxycoumarin O-de-ethylase activity after chronic ethanol ingestion. Ethanol binding spectra produced when ethanol was added to hepatic microsomes were examined using double reciprocal plots. Chronic ethanol ingestion produced genotypically related (LS > SS) increases in the absorbance change maximum per mg of microsomal protein. No significant changes in the spectral dissociation constant or absorbance change maximum per nM cytochrome P-450 were observed following ethanol treatment.     

Differential inhibition of individual human liver cytochromes P-450 by cimetidine

Cimetidine binds to cytochrome P-450 and inhibits hepatic metabolism of various drugs in humans. However, cytochrome P-450 is a family of enzymes rather than a single protein, and effects of cimetidine on individual human liver cytochromes P-450 have not been previously characterized. Metabolism of selected substrates and cimetidine-binding assays have been performed using human liver microsomes, purified human liver cytochromes P-450, and cytochrome P-450 complementary DNA-expressed yeast proteins to probe interaction of cimetidine with these individual enzymes. Cimetidine (3.0 mmol/L) in incubations reduced bufuralol hydroxylase activity by 80% and strongly inhibited microsomal nifedipine oxidation (23% +/- 13% of control activity). The same concentration of cimetidine produced intermediate inhibition of cytochrome enzymes responsible for ethoxyresorufin deethylation and aniline hydroxylation (77% +/- 6% and 68% +/- 17% of activity in control microsomal incubations, respectively), but little effect on tolbutamide hydroxylation was observed. Concordantly, the calculated binding constant for the binding of cimetidine to a purified cytochrome P-450 with high tolbutamide hydroxylase activity was 4.4 mmol/L, whereas the calculated binding concentration constant for a purified cytochrome P-450-metabolizing nifedipine was 0.7 mmol/L. These studies show a high variability in the effect of cimetidine on drug metabolism by individual human liver cytochromes P-450. In vitro studies using human liver microsomes and genetically engineered human cytochromes P-450 can be very useful in exploring important clinical questions of hepatic drug metabolism.     

Effect of omeprazole on ethanol oxidation and aniline hydroxylation in rat hepatic microsomes

Omeprazole, a substituted benzimidazole, is a potent gastric acid antisecretory drug, which inhibits the hepatic oxidative drug metabolism in vitro and in vivo. The effect of omeprazole on the microsomal ethanol oxidizing system (MEOS) and, since ethanol-induced cytochrome P-450 reveals a high activity for aniline hydroxylation, on aniline hydroxylase (AH) has been investigated in rat liver microsomes. Omeprazole inhibits microsomal AH activity significantly in a dose dependent manner, while this was not the case for MEOS activity. These data give indirect evidence that the microsomal metabolism of both ethanol and aniline is mediated by different isoenzymes of cytochrome P-450 and that omeprazole exhibits a different affinity to both compounds. Therefore, it must be emphasized that drug interactions with omeprazole have to be tested experimentally in each individual case, since it is impossible to predict such interactions solely on the knowledge of the drug's metabolic pathway.     

Inhibition of rat methylbenzylnitrosamine metabolism by dietary zinc and zinc in vitro.

Methylbenzylnitrosamine is an esophageal-specific carcinogen in the rat, and the incidence of methylbenzylnitrosamine-induced esophageal carcinoma is increased by dietary zinc deficiency. Methylbenzylnitrosamine requires activation by cytochrome P-450 to be mutagenic; the present study examined the role of dietary zinc deficiency and the in vitro addition of zinc on the cytochrome P-450-dependent microsomal metabolism of methylbenzylnitrosamine. Dietary zinc deficiency significantly increased the cytochrome P-450-dependent esophageal and hepatic microsomal metabolism of methylbenzylnitrosamine. These changes occurred without alteration in the specific content of total microsomal cytochrome P-450 of the esophagus or liver. The addition of zinc in vitro, at concentrations found in normal tissues, irreversibly inhibited the esophageal and hepatic cytochrome P-450-dependent microsomal metabolism of methylbenzylnitrosamine. These results suggest that physiological levels of zinc may be an endogenous inhibitor of methylbenzylnitrosamine metabolism. Dietary zinc deficiency appears to reduce this inhibition of cytochrome P-450 activity, resulting in an increase in carcinogen activation.     

Effect of murine schistosomiasis on hepatic cytochrome P-450 and microsomal protein

Experimental hepatic schistosomiasis was produced in CBA mice using a local strain of S. mansoni. A comparative study of the hepatic concentrations of cytochrome P-450 and microsomal protein in the control and infected animals was carried out. S. mansoni infection significantly (P less than 0.05) reduced the liver cytochrome P-450 and microsomal protein. This suggests impairment of drug metabolism in the liver of infected animals. The study calls attention to the possible clinical and pharmacokinetic implications of the late severe S. mansoni infection of the liver in man.     

Cobalt Induction of Hepatic Heme Oxygenase; with Evidence That Cytochrome P-450 Is Not Essential for This Enzyme Activity

Treatment of rats in vivo with cobalt chloride stimulated heme oxidation by hepatic microsomes to levels up to 800% above controls. This treatment also caused increases in liver weight and in total microsomal protein; in contrast, marked decreases were produced in microsomal oxidation of ethylmorphine (80%), and in cytochrome P-450 (60-70%) and heme (30-50%) contents. Cobalt chloride treatment did not affect heme oxidation by the spleen heme oxygenase system.The rate of heme oxidation by hepatic microsomal enzymes and the microsomal content of cytochrome P-450 were found to be unrelated. This conclusion was reached from studies in which microsomal heme oxygenase activity from cobalt-treated animals could be increased by 900% above control levels in the same microsomal preparation in which cytochrome P-450 content was decreased to spectrally unmeasurable amounts after incubation with 4 M urea. The same treatment eliminated ehtylmorphine demethylation and decreased microsomal NADPH-cytochrome c reductase (EC 1.6.2.4) activity by 75%.It is concluded that (i) the hepatic microsomal enzyme system that oxidizes heme compounds is not the same as that which metabolizes drugs, (ii) cytochrome P-450 is not essential for the oxidation of heme by liver cells, (iii) there is no direct relationship between the rate of heme oxidation and the level of NADPH-cytochrome c reductase activity, and (iv) the oxidation of heme is protein-dependent and the active proteins are inducible, but are different from those involved in drug metabolism.     

The Effects of Chronic Ethanol Consumption on Carcinogen Metabolism and on O6-Methylguanine Transferase-Mediated Repair of Alkylated DNA

This article presents a review and update of recent experiments conducted in collaboration with Dr. C. S. Lieber on mechanisms underlying the increased cancer risk associated with alcohol abuse. Ethanol has been found to be a potent inducer of microsomal enzymes involved in carcinogen metabolism in a variety of rat tissues including liver, esophagus, lungs, and intestines. In some of these tissues, ethanol's inductive effect on microsomal cytochrome P-450 enzyme activity may result in enhanced levels of electrophilic metabolites of procarcinogens which are not readily detoxified. In addition, chronic ethanol feeding has been found to depress the activity of O⁶-methylguanine transferase, an enzyme involved in the repair of carcinogen-induced DNA alkylation. The effects of ethanol on carcinogen metabolism and on DNA repair would be expected to enhance the initiation phase of chemically induced cancers.     

Effects of Acute Ethanol Doses or Dietary Phenobarbitol with Carbon Tetrachloride Exposure on Vitamin A Status of Rats

Hepatotoxins such as ethanol and CCl₄ are known to adversely affect vitamin A metabolism, although the effects of acute exposure to these agents have received less evaluation. The purpose of this study was to determine the effects on vitamin A status after a series of acute ethanol doses or a series of CCl₄ inhalation challenges with concurrent phenobarbital exposure in the diet of rats. The depressed hepatic vitamin A seen after one ethanol dose was not sustained after repeated dosings. However, the significantly increased urine and liver radiolabeled vitamin A recovery after three acute ethanol exposure periods suggests adaptive physiologic and metabolic changes after the initial dose. The results of repeated CCl₄/Phenobarbital dosings on vitamin A status paralleled, for the most part, the ethanol results. Thus, the initial acute exposure of hepatotoxic agents causes metabolic changes that are not fully sustained as the animal adapts to these challenges.     

Effect of ethanol on microsomal metabolism of dimethylnitrosamine

Summary Rat liver microsomes were prepared from male and female controls and from animals pretreated for 3 weeks with ethanol, and incubated with dimethylnitrosamine (DMN) and an NADPH-regenerating system. The formation of formaldehyde and nitrite as well as the alkylation of microsomal protein were found to be greatly enhanced, especially in the low DMN concentration range, as a result of long-term ethanol induction. In contrast, ethanol or tetrahydrofuran, when incubated simultaneously with DMN, inhibited microsomal metabolism of the carcinogen.

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Abbreviations DMN dimethylnitrosamine - DEN diethylnitrosamine - cyt. P-450 cytochrome P-450     

Hepatic microsomal protein and cytochrome P-450 in BALB/c mice infected with Leishmania

The effects of infection of mice with Leishmania major on liver microsomal protein and cytochrome P-450 were examined. The levels of hepatic microsomal protein and cytochrome P-450 were monitored at 6, 7, 9 and 12 weeks post-infection. The results indicated that the amount of hepatic microsomal protein and cytochrome P-450 were unchanged throughout the course of infection with L. major, despite the high degree of parasite proliferation in Kupffer cells and marked reduction in phagocytosis. The current results clearly indicate that Leishmania-induced macrophage suppression has no inhibitory effect on hepatic microsomal protein and cytochrome P-450.     

Antibodies to liver/kidney microsome1 in chronic active hepatitis recognize specific forms of hepatic cytochrome P-450

Anti-liver/kidney microsome1-positive sera from children with chronic active hepatitis were studied in an effort to identify the microsomal antigens selected during induction and progression of this autoimmune disease. Immunoblot analysis of sodium dodecyl sulfate gel-resolved microsomal proteins from human and rat liver using anti-liver/kidney microsome1-positive sera revealed a single polypeptide of 48 kilodaltons (human microsomes) or 50 kilodaltons (rat microsomes). Levels of the 50-kilodalton rat microsomal polypeptide were suppressed in vivo by several drugs known to modulate expression of individual forms (enzymes) of hepatic cytochrome P-450, with the largest decrease effected by phenobarbital. Dot blot analysis using a panel of 10 electrophoretically homogeneous rat liver cytochrome P-450 forms under nondenaturing conditions established that the two methylcholanthrene-inducible forms, P-450 BNF-B and P-450 ISF-G (P-450 gene subfamily IA), are selectively recognized by the anti-liver/kidney microsome1 antibodies. These findings demonstrate that sera associated with autoimmune (anti-liver/kidney microsome1) chronic active hepatitis are specifically reactive with select rat hepatic P-450 forms and suggest that these autoantibodies may be principally directed against one or more constitutive forms of the corresponding human liver cytochromes.     

Acetaminophen Hepatotoxicity: Potentiation by Isoniazid

Potentiation of acetaminophen hepatotoxicity has previously been associated with a history of alcohol abuse. Presented here is the case of a 21-yr-old Philippino female with rapidly deteriorating hepatic functions. She had been on isoniazid, 300 mg daily, as prophylaxis against tuberculosis due to a positive tuberculin skin test. She took 3.25 g of acetaminophen for abdominal cramping and subsequently had rapid deterioration of liver function manifested by prolongation of the prothrombin time, elevated ammonia, marked elevation of transaminases, and hyperbilirubinemia. Over the course of 1 wk, these values essentially normalized and she was discharged. Isoniazid induces the cytochrome P-450 system, resulting in increased metabolism of acetaminophen, formation of toxic metabolites, depletion of glutathione stores, and subsequent hepatocellular injury. Patients on isoniazid should use caution when taking acetaminophen since the potentially hepatotoxic effects may be amplified due to induction of the cytochrome P-450 system.     

Lipid peroxidation and antioxidant defense systems in rat liver after chronic ethanol feeding

The effects of chronic ethanol feeding on hepatic lipid peroxidation, ascorbic acid, glutathione and vitamin E levels were investigated in rats fed low or adequate amounts of dietary vitamin E. Hepatic lipid peroxidation was significantly increased after chronic ethanol feeding in rats receiving a low-vitamin E diet, indicating that dietary vitamin E is an important determinant of hepatic lipid peroxidation induced by chronic ethanol feeding. No significant change was observed in hepatic non-heme iron content, but hepatic content of ascorbic acid and glutathione was increased by ethanol feeding. Both low dietary vitamin E and ethanol feeding significantly reduced hepatic alpha-tocopherol content, and the lowest hepatic alpha-tocopherol was found in rats receiving a combination of low vitamin E and ethanol. Plasma alpha-tocopherol was elevated after ethanol feeding, probably because of the associated hyperlipemia. Both the ratio of plasma alpha-tocopherol/plasma lipid and the red blood cell alpha-tocopherol were reduced by ethanol feeding. Furthermore, ethanol feeding caused a marked increase of hepatic alpha-tocopheryl quinone, a metabolite of alpha-tocopherol by free radical reactions. Ethanol feeding caused little changes of alpha-tocopherol and alpha-tocopheryl quinone content in mitochondria, whereas a striking increase in alpha-tocopheryl quinone was observed in microsomes. These data suggest that ethanol feeding causes a marked alteration of vitamin E metabolism in the liver and that the combination of ethanol with a low-vitamin E intake results in a decrease of hepatic alpha-tocopherol content which renders the liver more susceptible to free radical attack.