Ethanol Oxidation by a Component of Liver Microsomes Rich in Cytochrome P-450

Cytochrome P450 2E1 and its roles in disease

Cytochrome P450 (P450) 2E1 is the major P450 enzyme involved in ethanol metabolism. That role is shared with two other enzymes that oxidize ethanol, alcohol dehydrogenase and catalase. P450 2E1 is also involved in the bioactivation of a number of low molecular weight cancer suspects, as validated in vivo in mouse models where cancers could be attenuated by deletion of Cyp2e1. P450 2E1 does not have a role in global production of reactive oxygen species but localized roles are possible, e.g. in mitochondria. The structures, conformations, and catalytic mechanisms of P450 2E1 have some unusual features among P450s. The concentration of hepatic P450 varies ≥10-fold among humans, possibly in part due to single nucleotide variants. The level of P450 2E1 may have relevance in the rates of oxidation of drugs, particularly acetaminophen and anesthetics.

Microsomal Ethanol-Oxidizing System: Success Over 50 Years and an Encouraging Future

Fifty years ago, in 1968, the pioneering scientists Charles S. Lieber and Leonore M. DeCarli discovered the capacity for liver microsomes to oxidize ethanol (EtOH) and named it the microsomal ethanol-oxidizing system (MEOS), which revolutionized clinical and experimental alcohol research. The last 50 years of MEOS are now reviewed and highlighted. Since its discovery and as outlined in a plethora of studies, significant insight was gained regarding the fascinating nature of MEOS: (i) MEOS is distinct from alcohol dehydrogenase and catalase, representing a multienzyme complex with cytochrome P450 (CYP) and its preferred isoenzyme CYP 2E1, NADPH-cytochrome P450 reductase, and phospholipids; (ii) it plays a significant role in alcohol metabolism at high alcohol concentrations and after induction due to prolonged alcohol use; (iii) hydroxyl radicals and superoxide radicals promote microsomal EtOH oxidation, assisted by phospholipid peroxides; (iv) new aspects focus on microsomal oxidative stress through generation of reactive oxygen species (ROS), with intermediates such as hydroxyethyl radical, ethoxy radical, acetyl radical, singlet radical, hydroxyl radical, alkoxyl radical, and peroxyl radical; (v) triggered by CYP 2E1, ROS are involved in the initiation and perpetuation of alcoholic liver injury, consequently shifting the previous nutrition-based concept to a clear molecular-based disease; (vi) intestinal CYP 2E1 induction and ROS are involved in endotoxemia, leaky gut, and intestinal microbiome modifications, together with hepatic CYP 2E1 and liver injury; (vii) circulating blood CYP 2E1 exosomes may be of diagnostic value; (viii) circadian rhythms provide high MEOS activities associated with significant alcohol metabolism and potential toxicity risks as a largely neglected topic; and (ix) a variety of genetic animal models are useful and have been applied elucidating mechanistic aspects of MEOS. In essence, MEOS along with its CYP 2E1 component currently explains several mechanistic steps leading to alcoholic liver injury and has a promising future in alcohol research.

Alcoholic Liver Disease: Alcohol Metabolism, Cascade of Molecular Mechanisms, Cellular Targets, and Clinical Aspects

Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

Metabolism of ethanol and associated hepatotoxicity

Over the last three decades, direct hepatotoxic effects of ethanol were established, some of which were linked to redox changes produced by NADH generated via the alcohol dehydrogenase (ADH) pathway and shown to affect the metabolism of lipids, carbohydrates, proteins, and purines. It was also determined that ethanol can be oxidized by a microsomal ethanol oxidizing system (MEOS) involving a specific cytochrome P-450; this newly discovered ethanol-inducible cytochrome P-450 (P-450 IIEi) contributes to ethanol metabolism, tolerance, energy wastage (with associated weight loss), and the selective hepatic perivenular toxicity of various xenobiotics. Their activation by P-450IIEi now provides an understanding of the increased susceptibility of the heavy drinker to the toxicity of industrial solvents, anaesthetic agents, commonly prescribed drugs, over-the-counter analgesics, and chemical carcinogens. P-450 induction also explains depletion (and toxicity) of nutritional factors such as vitamin A. As a consequence, treatment with vitamin A and other nutritional factors is beneficial, but must take into account a narrowed therapeutic window in alcoholics who have increased needs for nutrients and also display an enhanced susceptibility to some of their adverse effects. Acetaldehyde (the metabolite produced from ethanol by either ADH or MEOS) impairs hepatic oxygen utilization and forms protein adducts, resulting in antibody production, enzyme inactivation, and decreased DNA repair. It also stimulates collagen production by the vitamin A storing cells (lipocytes) and myofibroblasts, and causes glutathione depletion. Supplementation with S-adenosyl-L-methionine partly corrects the depletion and associated mitochondrial injury, whereas administration of polyunsaturated lecithin opposes the fibrosis. Thus, at the cellular level, the classic dichotomy between the nutritional and toxic effects of ethanol has now been bridged. The understanding of how the ensuing injury eventually results in irreversible scarring or cirrhosis may provide us with improved modalities for treatment and prevention.

Ethanol-induced oxidative stress: basic knowledge

After a general introduction, the main pathways of ethanol metabolism (alcohol dehydrogenase, catalase, coupling of catalase with NADPH oxidase and microsomal ethanol-oxidizing system) are shortly reviewed. The cytochrome P(450) isoform (CYP2E1) specifically involved in ethanol oxidation is discussed. The acetaldehyde metabolism and the shift of the NAD/NADH ratio in the cellular environment (reductive stress) are stressed. The toxic effects of acetaldehyde are mentioned. The ethanol-induced oxidative stress: the increased MDA formation by incubated liver preparations, the absorption of conjugated dienes in mitochondrial and microsomal lipids and the decrease in the most unsaturated fatty acids in liver cell membranes are discussed. The formation of carbon-centered (1-hydroxyethyl) and oxygen-centered (hydroxyl) radicals during the metabolism of ethanol is considered: the generation of hydroxyethyl radicals, which occurs likely during the process of univalent reduction of dioxygen, is highlighted and is carried out by ferric cytochrome P(450) oxy-complex (P(450)-Fe(3+)O(2) (.-)) formed during the reduction of heme-oxygen. The ethanol-induced lipid peroxidation has been evaluated, and it has been shown that plasma F(2)-isoprostanes are increased in ethanol toxicity.

The discovery of the microsomal ethanol oxidizing system and its physiologic and pathologic role

Oxidation of ethanol via alcohol dehydrogenase (ADH) explains various metabolic effects of ethanol but does not account for the tolerance. This fact, as well as the discovery of the proliferation of the smooth endoplasmic reticulum (SER) after chronic alcohol consumption, suggested the existence of an additional pathway which was then described by Lieber and DeCarli, namely the microsomal ethanol oxidizing system (MEOS), involving cytochrome P450. The existence of this system was initially challenged but the effect of ethanol on liver microsomes was confirmed by Remmer and his group. After chronic ethanol consumption, the activity of the MEOS increases, with an associated rise in cytochrome P450, especially CYP2E1, most conclusively shown in alcohol dehydrogenase negative deer mice. There is also cross-induction of the metabolism of other drugs, resulting in drug tolerance. Furthermore, the conversion of hepatotoxic agents to toxic metabolites increases, which explains the enhanced susceptibility of alcoholics to the adverse effects of various xenobiotics, including industrial solvents. CYP2E1 also activates some commonly used drugs (such as acetaminophen) to their toxic metabolites, and promotes carcinogenesis. In addition, catabolism of retinol is accelerated resulting in its depletion. Contrasting with the stimulating effects of chronic consumption, acute ethanol intake inhibits the metabolism of other drugs. Moreover, metabolism by CYP2E1 results in a significant release of free radicals which, in turn, diminishes reduced glutathione (GSH) and other defense systems against oxidative stress which plays a major pathogenic role in alcoholic liver disease. CYP1A2 and CYP3A4, two other perivenular P450s, also sustain the metabolism of ethanol, thereby contributing to MEOS activity and possibly liver injury. CYP2E1 has also a physiologic role which comprises gluconeogenesis from ketones, oxidation of fatty acids, and detoxification of xenobiotics other than ethanol. Excess of these physiological substrates (such as seen in obesity and diabetes) also leads to CYP2E1 induction and nonalcoholic fatty liver disease (NAFLD), which includes nonalcoholic fatty liver and nonalcoholic steatohepatitis (NASH), with pathological lesions similar to those observed in alcoholic steatohepatitis. Increases of CYP2E1 and its mRNA prevail in the perivenular zone, the area of maximal liver damage. CYP2E1 up-regulation was also demonstrated in obese patients as well as in rat models of obesity and NASH. Furthermore, NASH is increasingly recognized as a precursor to more severe liver disease, sometimes evolving into "cryptogenic" cirrhosis. The prevalence of NAFLD averages 20% and that of NASH 2% to 3% in the general population, making these conditions the most common liver diseases in the United States. Considering the pathogenic role that up-regulation of CYP2E1 also plays in alcoholic liver disease (vide supra), it is apparent that a major therapeutic challenge is now to find a way to control this toxic process. CYP2E1 inhibitors oppose alcohol-induced liver damage, but heretofore available compounds are too toxic for clinical use. Recently, however, polyenylphosphatidylcholine (PPC), an innocuous mixture of polyunsaturated phosphatidylcholines extracted from soybeans (and its active component dilinoleoylphosphatidylcholine), were discovered to decrease CYP2E1 activity. PPC also opposes hepatic oxidative stress and fibrosis. It is now being tested clinically.

Gender differences in ethanol oxidation and cytochrome P4502E1 content and functions in hepatic microsomes from alcohol-preferring and non-preferring rats

1. We have studied the hepatic microsomal metabolism of ethanol (MEOS), CYP2E1 expression and catalytic activity, and the response to phenobarbital (PB) induction or CCl4 challenge in rats of either sex genetically selected for their preference (P) or aversion (NP) for ethanol. 2. In P versus NP females, the amount of both total cytochrome P450 and P450 binding to metyrapone was lower, whereas the activities of MEOS, aniline 4-hydroxylase (4-AOH), and 4-nitrophenol hydroxylase (PNP-OH) as well as the level of immunodetectable CYP2E1 content were consistently higher. By contrast, no substantial differences were observed between P and NP males. 3. Despite an apparent down-regulation of CYP2E1 expression occurring in all rats as a result of PB induction, P females maintained higher 2E1 levels and showed enhanced MEOS, 4-AOH and PNP-OH activities with respect to NP females. No such changes were detected in the male counterparts. 4. No sex-related differences in CCl4-mediated inhibition of monooxygenase or MEOS activities were evident between P and NP animals. 5. These results indicate that, in females only, the behavioural trait of ethanol preference is apparently associated not only with higher constitutive levels of CYP2E1 and rate of microsomal metabolism of ethanol but also with altered susceptibility to PB induction.

Hepatic, metabolic and toxic effects of ethanol: 1991 update

Until two decades ago, dietary deficiencies were considered to be the only reason for alcoholics to develop liver disease. As the overall nutrition of the population improved, more emphasis was placed on secondary malnutrition and direct hepatotoxic effects of ethanol were established. Ethanol is hepatotoxic through redox changes produced by the NADH generated in its oxidation via the alcohol dehydrogenase pathway, which in turn affects the metabolism of lipids, carbohydrates, proteins, and purines. Ethanol is also oxidized in liver microsomes by an ethanol-inducible cytochrome P-450 (P-450IIE1) that contributes to ethanol metabolism and tolerance, and activates xenobiotics to toxic radicals thereby explaining increased vulnerability of the heavy drinker to industrial solvents, anesthetic agents, commonly prescribed drugs, over-the-counter analgesics, chemical carcinogens, and even nutritional factors such as vitamin A. In addition, ethanol depresses hepatic levels of vitamin A, even when administered with diets containing large amounts of the vitamin, reflecting, in part, accelerated microsomal degradation through newly discovered microsomal pathways of retinol metabolism, inducible by either ethanol or drug administration. The hepatic depletion of vitamin A is strikingly exacerbated when ethanol and other drugs were given together, mimicking a common clinical occurrence. Microsomal induction also results in increased production of acetaldehyde. Acetaldehyde, in turn, causes injury through the formation of protein adducts, resulting in antibody production, enzyme inactivation, decreased DNA repair, and alterations in microtubules, plasma membranes and mitochondria with a striking impairment of oxygen utilization. Acetaldehyde also causes glutathione depletion and lipid peroxidation, and stimulates hepatic collagen production by the vitamin A storing cells (lipocytes) and myofibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired hepatic elimination of paranitrophenol and its metabolites in the rat following chronic ethanol pretreatment

Chronic ethanol feeding has been shown to enhance hepatic microsomal drug oxidation in humans and in laboratory animals. However, the effects of chronic ethanol administration on drug conjugation are less conclusive. We have studied the effects of chronic ethanol feeding on (a) the conjugation and elimination of p-nitrophenol (PNP) by the isolated perfused rat liver, (b) the formation of PNP glucuronide by hepatic microsomal PNP-glucuronyltransferase in vitro and (c) the hepatic content of UDP-glucuronic acid (UDPGA). PNP elimination from the isolated perfused rat liver was best described as a combination of parallel saturable and first-order processes. Ethanol pretreatment did not influence the former but resulted in a 48% reduction in the rate of elimination by the latter. This was associated with a significant reduction in recovery of PNP-glucuronide from bile, but no change in concentrations of PNP glucuronide or sulfate in perfusate. Michaelis constants and Vmax for PNP-glucuronyltransferase in native and solubilized microsomes and UDPGA concentrations in liver were not influenced by ethanol pretreatment. These results suggest that chronic ethanol treatment reduces PNP elimination in the intact liver primarily via a reduction in the biliary excretion of PNP glucuronide without altering glucuronidation per se.

Ethanol oxidation and toxicity: role of alcohol P-450 oxygenase

The isolation and characterization of ethanol-inducible rabbit liver microsomal cytochrome P-450, termed P-450 3a or P-450ALC, has provided definitive evidence for the role of this enzyme in alcohol oxidation. From findings on the distribution, substrate specificity, and mechanism of action of P-450ALC we have suggested "alcohol P-450 oxygenase" as a more biochemically accurate name than "microsomal ethanol-oxidizing system." The present review is concerned with studies in this and other laboratories on activities and inducers associated with this versatile enzyme. Numerous xenobiotics, including alcohols and ketones, nitrosamines, aromatic compounds, and halogenated alkanes, alkenes, and ethers, are known to undergo increased microsomal metabolism after chronic exposure of various species to ethanol. Diverse compounds and treatments may induce P-450ALC, including the administration of ten or more chemically different compounds, fasting, or the diabetic state. Whether a common mechanism of induction is involved is unknown at this time. As direct evidence that P-450ALC catalyzes numerous metabolic reactions, the purified rabbit enzyme has been used in a reconstituted system to demonstrate various metabolic transformations, including the oxidation of various alcohols, acetone, acetol, p-nitrophenol, and aniline, the dealkylation of substituted nitrosamines, the reductive dechlorination of carbon tetrachloride, carbon tetrachloride-induced lipid peroxidation, and acetaminophen activation to form the glutathione conjugate.

Biochemical basis of alcoholism: statements and hypotheses of present research

Experimental results and theoretical considerations on the biology of alcoholism are devoted to the following topics: genetically determined differences in metabolic tolerance; participation of the alternative alcohol metabolizing systems in chronic alcohol intake; genetically determined differences in functional tolerance of the CNS to the hypnotic effect of alcohol; cross tolerance between alcohol and centrally active drugs; dissociation of tolerance and cross tolerance from physical dependence; permanent effect of uncontrolled drinking behavior induced by alkaloid metabolites in the CNS; genetically determined alterations in the function of opiate receptors; and genetic predisposition to addiction due to innate endorphin deficiency. For the purpose of introducing the most important research teams and their main work, statements from selected publications of individual groups have been classified as to subject matter and summarized. Although the number for summary-quotations had to be restricted, the criterion for selection was the relevance to the etiology of alcoholism rather than consequences of alcohol drinking.

Ethanol metabolism in vivo by the microsomal ethanol-oxidizing system in deermice lacking alcohol dehydrogenase (ADH)

To assess the importance of non-ADH ethanol metabolism, ADH-negative and ADH-positive deermice were fed liquid diets containing ethanol or isocaloric carbohydrate for 2-4 weeks. Blood ethanol disappearance rate increased significantly after chronic ethanol feeding in both strains. Although at low ethanol concentrations (between 5 and 10 mM) there was no significant difference between ethanol-fed and pair-fed control animals, at high ethanol concentrations (between 40 and 70 mM) blood ethanol elimination rates were increased significantly after chronic ethanol feeding in both ADH-positive and ADH-negative animals. There was no significant effect of the catalase inhibitor 3-amino-1,2,4-triazole on the ethanol elimination/rates in both strains. Whereas catalase and ADH activities were not altered after chronic ethanol treatment, the activity of the microsomal ethanol-oxidizing system (MEOS) was enhanced three to four times in both strains, and microsomal cytochrome P-450 content was also increased significantly. When MEOS activity was expressed per cytochrome P-450 content, it was higher in ADH-negative than in ADH-positive animals, and it increased after ethanol administration. When microsomal proteins were separated by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, ethanol-fed animals had a distinct band which reflected the increase in microsomal cytochrome P-450 content and seemed to reflect a unique form of cytochrome P-450 induced by ethanol. Thus, despite the absence of the ADH pathway, a large amount of ethanol was metabolized by MEOS in ADH-negative deermice; this was associated with increased blood ethanol elimination rates, enhanced MEOS activity, and quantitative and qualitative changes of cytochrome P-450.

Metabolism and metabolic effects of alcohol

The author provides an excellent overview of the three major pathways for the metabolism of ethanol. Many of the toxic effects of ethanol can be attributed to two specific products, hydrogen and acetaldehyde, and these effects are explored in detail.

Increased microsomal oxidation of hydroxyl radical scavenging agents and ethanol after chronic consumption of ethanol

The oxidation of ethanol by rat liver microsomes is increased after chronic ethanol consumption. Previous experiments indicated that hydroxyl radicals play a role in the mechanism whereby microsomes oxidize ethanol. Experiments were therefore carried out to evaluate the role of these radicals in ethanol oxidation by microsomes from ethanol-fed rats, and to determine whether the increase in ethanol oxidation by these induced microsomes correlates with an increase in the generation of hydroxyl radicals. Rat liver microsomes from ethanol-fed rats catalyzed the oxidation of two typical hydroxyl radical scavenging agents, dimethylsulfoxide and 2-keto-4-thiomethylbutyric acid, at rates which were two- to threefold greater than rates found with control microsomes. This increased rate of oxidation of hydroxyl radical scavengers was similar to the increased rate of microsomal oxidation of ethanol. Azide, which inhibits contaminating catalase in microsomes, increased the oxidation of dimethyl sulfoxide and 2-keto-4-thiomethylbutyric acid by both microsomal preparations. This suggests that H2O2 may serve as the microsomal precursor of the hydroxyl radical. Cross competition for oxidation between ethanol and the hydroxyl radical scavenging agents was observed. Moreover, the oxidation of ethanol, dimethyl sulfoxide, or 2-keto-4-thiomethylbutyric acid was inhibited by other compounds which interact with hydroxyl radicals, e.g., benzoate, and the free-radical, spin-trapping agent, 5,5-dimethyl-1-pyrroline-N-oxide. These results suggest that the increase in the rate of ethanol oxidation found with microsomes from ethanol-fed rats may be due, at least in part, to an increase in the rate of production of hydroxyl radicals by these induced microsomes. Increased production of oxyradicals may possibly result in oxidative damage to the liver cell as a result of ethanol consumption.

Alcohol dehydrogenase (ADH) independent ethanol metabolism in deermice lacking ADH

To assess the importance of non-ADH ethanol metabolism, ADH-negative (ADH-) and ADH-positive (ADH+) deermice were fed for 2-4 weeks liquid diets containing ethanol or isocaloric carbohydrate. They consumed progressively increasing amounts of ethanol. Blood ethanol clearance (BEC) increased significantly in both strains. It remained almost unchanged at low ethanol concentrations (5-10 mM), but at high levels (40-70 mM) BEC was strikingly increased with significant differences between ethanol-fed and control animals. Kinetics were consistent with the activity of a non-ADH high Km system such as the microsomal ethanol-oxidizing system (MEOS). Naive ADH- had a more active MEOS and more abundant SER than naive ADH+. After ethanol feeding, MEOS was increased 3-4 times in both strains. There was striking proliferation of SER and cytochrome P-450 was enhanced significantly. Expressed per P-450, MEOS activity was higher in ADH- than ADH+. Thus despite absence of ADH, ADH- deermice can consume large amounts of ethanol: this is associated with increased BEC, SER proliferation, enhanced MEOS activity and quantitative and qualitative changes of cytochrome P-450.

Microsomal ethanol oxidizing system (MEOS): interaction with ethanol, drugs and carcinogens

Several studies in our unit showed that in men, baboons, rats and deermice, blood ethanol clearance is significantly accelerated at ethanol concentrations higher than the levels needed to effectively saturate the low Km forms of ADH present in animals, thereby incriminating a high Km non-ADH system such as microsomal ethanol oxidizing system (MEOS). Furthermore, kinetics of blood ethanol clearance were consistent with the Km of MEOS. After chronic ethanol consumption, there was an increase in rates of ethanol elimination and in the activity of MEOS. There was an associated rise in microsomal cytochrome P-450, including a form (different from that of a non-ADH pathway of ethanol metabolism and its increase after chronic ethanol consumption was most conclusively shown in ADH-negative deermice. Microsomal induction was also associated with enhanced metabolism of other drugs, resulting in metabolic drug tolerance. In addition, there was increased activation of known hepatotoxic agents (such as CCl4 and acetaminophen) which may explain the enhanced susceptibility of alcoholics to the toxicity of solvents and commonly used drugs. There was enhanced activation of procarcinogens, sometimes at concentrations much lower than those required for other microsomal inducers. Moreover, catabolism of retinoic acid was accelerated possibly contributing to hepatic vitamin A depletion. In conclusion, after chronic ethanol consumption, enhanced MEOS activity and concomitant cytochrome P-450 changes may contribute to accelerated ethanol and drug metabolism and associated activation of hepatotoxic agents and carcinogens.

The role of cytochrome P-450 in the hydroperoxide-catalyzed oxidation of alcohols by rat-liver microsomes

The organic hydroperoxide cumene hydroperoxide is capable of oxidizing ethanol to acetaldehyde in the presence of either catalase, purified cytochrome P-450 or rat liver microsomes. Other hemoproteins like horseradish peroxidase, cytochrome c or hemoglobin were ineffective. In addition to ethanol, higher alcohols like 1-propanol, 1-butanol and 1-pentanol are also oxidized to their corresponding aldehydes to a lesser extent. Other organic hydroxyperoxides will replace cumene hydroperoxide in oxidizing ethanol but less effectively. The cumene-hydroperoxide-dependent ethanol oxidation in microsomes was inhibited partially by cytochrome P-450 inhibitors but was unaffected by catalase inhibitors. Phenobarbital pretreatment of rats increased the specific activity of the cumene-hydroperoxide-dependent ethanol oxidation per mg of microsomes about seven-fold. The evidence suggests that cytochrome P-450 rather than catalase is the enzyme responsible for hydroperoxide-dependent ethanol oxidation. However, when H2O2 is used in place of cumene hydroperoxide, the microsomal ethanol oxidation closely resembles the catalase system.

Metabolism of alcohol at high concentrations: role and biochemical nature of the hepatic microsomal ethanol oxidizing system

At intermediate and higher alcohol concentrations, ethanol metabolism proceeds via alcohol dehydrogenase (ADH) and the microsomal ethanol oxidizing system (MEOS), whereas catalase plays no significant role. Following prolonged ethanol consumption, an enhancement of both MEOS activity as well as the rates of ethanol metabolism occurs; the latter persisted despite inhibition of ADH by pyrazole and catalase by sodium axide, suggesting the involvement of MEOS in the adaptive increase. MEOS exhibits characteristics similar to those of other microsomal drug metabolizing enzymes and can be differentiated and isolated from both ADH and catalase activities. Reconstitution of MEOS activity was achieved with partially purified cytochrome P-450 and NADPH-cytochrome c reductase in the presence of synthetic phospholipid.

Effect of chronic alcohol consumption on ethanol and acetaldehyde metabolism

Hepatic metabolism of ethanol to acetaldehyde by the alcohol dehydrogenase (ADH) pathway is associated with the generation of reducing equivalents as NADH. Conversely, reducing equivalents are consumed when ethanol oxidation is catalyzed by the NADPH dependent microsomal ethanol oxidizing system (MEOS). Since the major fraction of ethanol metabolism proceeds via ADH and since the oxidation of acetaldehyde also generates NADH, an excess of reducing equivalents is produced. This explains a variety of effects following acute ethanol administration, including hyperlactacidemia, hyperuricemia, enhanced lipogenesis and depressed lipid oxidation. To the extent that ethanol is oxidized by the alternate MEOS pathway, it slows the metabolism of other microsomal substrates. Following chronic ethanol consumption, adaptive microsomal changes prevail, which include enhanced ethanol and drug metabolism, and increased lipoprotein production. Eventually, injury develops with alterations of the rough endoplasmic reticulum and structural and functional abnormalities of the mitochondria.

Microsomal ethanol oxidation: activity in vitro and in vivo

Studies by several investigators have confirmed that the microsomal fraction of mammalian liver oxidizes ethanol to acetaldehyde in a reaction that requires NADPH and oxygen. Efforts to identify the enzymes involved have produced conflicting opinions of the reaction mechanism, however. Initially, the microsomal mixed function oxidase system was assumed to be capable of oxidizing ethanol in a mechanism that did not involve either alcohol dehydrogenase or catalase. Later evidence suggested that the oxidative enzyme was, in fact, catalase, a contaminant of microsomal preparations and that the mixed function oxidase system merely furnished hydrogen peroxide to the reaction. Much current research supports the latter interpretation. Other workers provide evidence that favors a system in which catalase does not participate. Attempts to define the reaction process have involved studies with catalase inhibitors, kinetic studies of the different reaction systems, and physical separation of catalase from the microsomal components. Questions of the mechanism of microsomal ethanol oxidation may prove to be purely academic, however. Efforts to prove that the system has significant in vivo activity generally have not been successful.