Microsomal ethanol oxidizing system (MEOS): current status of its characterization and its role

A novel metabolic disorder in the degradation pathway of endogenous methanol due to a mutation in the gene of alcohol dehydrogenase

BACKGROUND

A small amount of methanol is produced endogenously in the human body but it is efficiently metabolized by alcohol dehydrogenase (ADH) and other enzymes, and the products eliminated without harm. In this study, we present a new entity of inborn error of methanol metabolism due to a mutation in the ADH1C gene coding for the γ subunit that is part of several ADH isoenzymes.

RESULTS

This disorder was discovered in an 11.58-year-old boy. During one 9-month hospital admission, he had periods of 1-4 days during which he was comatose, and between these periods he was sometimes verbose and euphoric, and had ataxia, dysarthria. Following hemodialysis treatments, he became conscious and appeared healthy. Organ evaluations and his laboratory tests were normal. Toxicological evaluation of his blood showed a high methanol level [12.2 mg/dL (3.8 mmol/L), normal range up to 3.5 mg/dL (1.09 mmol/L) while the formaldehyde level was undetectable. The finding of liver function tests that were within normal limits, coupled with a normal eye examination and size of the liver, elevated blood methanol levels and an undetectable formaldehyde level, suggested ADH insufficiency. Adding zinc to the drug regimen 15 mg/daily dramatically reduced the patient's methanol level and alleviated the abnormal symptoms. When zinc supplementation was discontinued, the patient relapsed into a coma and hemodialysis was once again required. A homozygous mutation in ADH1C gene located at exon 3 was found, and both parents were heterozygous for this mutation.

CONCLUSION

Accumulation of methanol due to mutation in ADH1C gene may result in drunkenness and ataxia, and leads to coma. This condition can be successfully treated with zinc supplementation as the cofactor of ADH.

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.

Alcoholic steatohepatitis (ASH) and alcoholic hepatitis (AH): cascade of events, clinical aspects, and pharmacotherapy options

INTRODUCTION

Clinicians caring for patients with alcoholic hepatitis (AH) are often confronted with the question of the best pharmacotherapy to be used.

AREAS COVERED

This article covers metabolic aspects of alcohol as the basis of understanding pharmacotherapy and to facilitate choosing the drug therapeutic options for patients with severe AH.

EXPERT OPINION

Alcoholic steatohepatitis (ASH) and alcoholic hepatitis (AH) as terms are often used interchangeably in scientific literature but a stringent differentiation is recommended for proper clarity. As opposed to ASH, the clinical course of AH is often severe and requires an effective drug treatment strategy, in addition to absolute alcohol abstinence and nutritional support. Drug options include corticosteroids as a first choice and pentoxifylline, an inhibitor of phosphodiesterase, as a second line therapy, especially in patients with contraindications for a corticosteroid therapy such as infections or sepsis. At seven days under corticosteroids, treatment should be terminated in non-responders, and patients must then be evaluated for liver transplantation. Pentoxifylline is not effective as a rescue therapy for these patients. Other treatments such as infliximab, propylthiouracil, N-acetylcysteine, silymarin, colchicine, insulin and glucagon, oxandrolone, testosterone, and polyunsaturated lecithin are not effective in severe AH. For liver transplantation, few patients will be eligible.

Liver Injury by Carbon Tetrachloride Intoxication in 16 Patients Treated with Forced Ventilation to Accelerate Toxin Removal via the Lungs: A Clinical Report

Carbon tetrachloride (CCl₄) is an efficient but highly toxic solvent, used in households and commercially in the industry under regulatory surveillance to ensure safety at the working place and to protect the workers’ health. However, acute unintentional or intentional intoxications by CCl₄ may rarely occur and are potentially life-threatening. In this review article, therapy options are discussed that are based on a literature review of traditional poisoning cases and the clinical experience with 16 patients with acute poisoning by CCl₄. Among various therapy options, the CO₂-induced hyperventilation therapy will be considered in detail as the most promising approach. This special therapy was developed because only around 1% of the intoxicating CCl₄ is responsible for the liver injury after conversion to toxic radicals via microsomal cytochrome P450 2E1 whereas 99% of the solvent will leave the body unchanged by exhalation. Therefore, to enhance CCl₄ elimination through the lungs, CO₂ is added to the inspiration air at a flow rate of 2⁻3 L min−1 in order to achieve hyperventilation with a respiratory volume of 25⁻30 L min−1. Under this therapy, the clinical course was favorable in 15/16 patients, corresponding to 93.8%. In essence, patients with acute CCl₄ intoxication should be treated by forced ventilation.

Effects of long-term ethanol consumption on jejunal lipase and disaccharidase activities in male and female rats

AIM: To study the effect of long-term ethanol consumption on jejunal lipase and disaccharidase (sucrase, maltase, and lactase) activities in rats and its gender difference.

METHODS: Age-matched male and female Wistar rats were fed control or ethanol-containing liquid diets for 12 wk following the Lieber-DeCarli model. According to both the plasma aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, 40 rats were divided into four groups as follows: male control group (MC), male ethanol group (ME), female control group (FC), and female ethanol group (FE).

RESULTS: After ethanol feeding for 12 wk, the results revealed that plasma AST and ALT activities of group ME were significantly increased by 58% and 92%, respectively, than those of group MC (P<0.05). Similarly, plasma AST and ALT activities of group FE were also significantly increased by 61% and 188%, respectively, than those of group FC (P<0.05). Fat accumulation was observed in both ethanol-treated groups, while fatty changes were more severe in group FE than those in group ME. The induction of hepatic microsomal cytochrome P450 2E1 (CYP2E1) was obviously seen in group ME and group FE, but was not detected in group MC and group FC. Jejunal lipase activity of group ME was significantly increased by 1.25-fold than that of group MC (P<0.05). In contrast to, sucrase, maltase, and lactase activities of group ME were significantly decreased by 63%, 62% and 67%, respectively, than those of group MC (P<0.05). Similarly, activities of these three enzymes of group FE were also significantly decreased by 43%, 46% and 52%, respectively, than those of group FC (P<0.05). There were no significant epithelial changes of the duodenal mucosa in any group.

CONCLUSION: Long-term ethanol consumption significantly can increase jejunal lipase and decrease jejunal disaccharidase activities in both male and female rats.

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.

Metabolism of alcohol

Most tissues of the body contain enzymes capable of ethanol oxidation or nonoxidative metabolism, but significant activity occurs only in the liver and, to a lesser extent, in the stomach. Hence, medical consequences are predominant in these organs. In the liver, ethanol oxidation generates an excess of reducing equivalents, primarily as NADH, causing hepatotoxicity. An additional system, containing cytochromes P-450 inducible by chronic alcohol feeding, was demonstrated in liver microsomes and found to be a major cause of hepatotoxicity.

Alcoholic fatty liver: its pathogenesis and mechanism of progression to inflammation and fibrosis

Liver disease in the alcoholic is due not only to malnutrition but also to ethanol's hepatotoxicity linked to its metabolism by means of the alcohol dehydrogenase and cytochrome P450 2E1 (CYP2E1) pathways and the resulting production of toxic acetaldehyde. In addition, alcohol dehydrogenase-mediated ethanol metabolism generates the reduced form of nicotinamide adenine dinucleotide (NADH), which promotes steatosis by stimulating the synthesis of fatty acids and opposing their oxidation. Steatosis is also promoted by excess dietary lipids and can be attenuated by their replacement with medium-chain triglycerides. Through reduction of pyruvate, elevated NADH also increases lactate, which stimulates collagen synthesis in myofibroblasts. Furthermore, CYP2E1 activity is inducible by its substrates, not only ethanol but also fatty acids. Their excess and metabolism by means of this pathway generate release of free radicals, which cause oxidative stress, with peroxidation of lipids and membrane damage, including altered enzyme activities. Products of lipid peroxidation such as 4-hydroxynonenal stimulate collagen generation and fibrosis, which are further increased through diminished feedback inhibition of collagen synthesis because acetaldehyde forms adducts with the carboxyl-terminal propeptide of procollagen in hepatic stellate cells. Acetaldehyde is also toxic to the mitochondria, and it aggravates their oxidative stress by binding to reduced glutathione and promoting its leakage. Oxidative stress and associated cellular injury promote inflammation, which is aggravated by increased production of the proinflammatory cytokine tumor necrosis factor-alpha in the Kupffer cells. These are activated by induction of their CYP2E1 as well as by endotoxin. The endotoxin-stimulated tumor necrosis factor-alpha release is decreased by dilinoleoylphosphatidylcholine, the active phosphatidylcholine (PC) species of polyenylphosphatidylcholine (PPC). Moreover, defense mechanisms provided by peroxisome proliferator-activated receptor alpha and omega fatty acid oxidation are readily overwhelmed, particularly in female rats and also in women who have low hepatic induction of fatty acid-binding protein (L-FABPc). Accordingly, the intracellular concentration of free fatty acids may become high enough to injure membranes, thereby contributing to necrosis, inflammation, and progression to fibrosis and cirrhosis. Eventually, hepatic S-adenosylmethionine and PCs become depleted in the alcoholic, with impairment of their multiple cellular functions, which can be restored by PC replenishment. Thus, prevention and therapy opposing the development of steatosis and its progression to more severe injury can be achieved by a multifactorial approach: control of alcohol consumption, avoidance of obesity and of excess dietary long-chain fatty acids, or their replacement with medium-chain fatty acids, and replenishment of S-adenosylmethionine and PCs by using PPC. Progress in the understanding of the pathogenesis of alcoholic fatty liver and its progression to inflammation and fibrosis has resulted in prospects for their better prevention and treatment.

Ontogeny of ethanol elimination and ethanol-induced hypothermia

Ontogeny of ethanol elimination rates and ethanol-induced hypothermia were examined as possible mechanisms contributing to the marked reduction in ethanol sensitivity early in life (Little et al., 1996; Silveri & Spear, 1998) and the notable gender difference in ethanol sleep-time seen in adult animals (Silveri & Spear, 1998). Elimination rates and brain/blood ethanol levels were determined following doses of 1.5 or 4.5 g/kg ethanol in male and female Sprague-Dawley rats at postnatal days (P)16, 26, 36, or 56. Animals were sacrificed at 40, 80, or 160 min post-injection, with ethanol elimination rates estimated from the slope of the regression of blood and brain alcohol levels across the three sampling periods. P16 animals exhibited the slowest rate of ethanol metabolism, while no gender effects were evident at any age. Observed ontogenetic increases in ethanol hypothermia were not systematically related to the ontogeny of ethanol metabolism. Factors other than ontogenetic changes in ethanol metabolism, hypothermia, or the distribution of ethanol between brain and blood must underlie the relative insensitivity to ethanol often reported in young and adolescent organisms, a fruitful area for future studies given the frequent use and misuse of alcohol by human adolescents.

The effect of hypermetabolism induced by burn trauma on the ethanol-oxidizing capacity of the liver

OBJECTIVE

To study the rate of elimination of ethanol after a major burn trauma.

DESIGN

Prospective, controlled study.

SETTING

National burns unit in a Swedish university hospital.

PATIENTS AND SUBJECTS

Eight consecutive patients suffering from 18%-72% total burned surface area and nine healthy male control subjects.

INTERVENTIONS

The patients received ethanol, 0.35-0.60 g/kg body weight intravenously, during 1 hr. This was repeated daily during the first week postburn. The control subjects received the same amount of ethanol once.

MEASUREMENTS AND MAIN RESULTS

Blood samples were drawn at 20- to 30-min intervals during 5 hrs after the start of the infusion. Serum ethanol was determined by headspace gas chromatography. The rate of elimination of ethanol was calculated from the concentration time profile. In the control subjects, the median elimination rate was 0.074 g/kg/hr (range, 0.059-0.083 g/kg/hr). In the patients, it was already 0.138 g/kg/hr (range, 0.111-0.201 g/kg/hr) on the first day; this increased even further over the following 6 days, reaching 0.183 g/kg/hr (range, 0.150-0.218 g/kg/hr) on the seventh day.

CONCLUSIONS

Ethanol elimination is augmented postburn. A more effective reoxidation of reduced nicotinamide adenine dinucleotide seems the most likely explanation for the increased rate of ethanol elimination in these hypermetabolic trauma patients. This finding suggests that the oxidative capacity of the liver may be assessed by studying the rate of ethanol elimination in burn victims.

Ethanol metabolism, toxicity and genetic polymorphism

The relationships between the individual (and racial) differences in alcohol metabolism and toxicity, and the genetic polymorphism of alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), and cytochrome P-4502E1(CYPIIE1) were reviewed. In recent studies involving DNA analysis, it was found that a deficiency of the ALDH2 isozyme (ALDH2*2) was responsible for the flushing symptoms as well as other vasomotor symptoms caused by a higher acetaldehyde level after alcohol consumption. Deficiency of ALDH2 activity has been found prevalently only among people of Mongoloid origin, and the deficiency of ALDH2 prevents them from developing alcohol dependence due to the unpleasant physical effects of the flushing symptom. It was reported that Mongoloids such as Japanese and Chinese people carry the enzymatically active (ALDH2*1) subunit and/or the inactive (ALDH2*2) one, and that a low proportion of ALDH2 deficiency (ALDH2*2 allele frequency) was found in alcoholics compared with healthy controls. It was also reported that polymorphism of ALDH2 and/or CYP2E1 may be associated with the susceptibility to alcohol-induced liver injury. Concerning blood ethanol elimination kinetics, it was reported that the c2 gene of CYP2E1 and the ALDH2*1 gene may have greater effects on ethanol and acetaldehyde elimination than the other genotypes, when the blood ethanol level is below 20 m M.

Metabolism of ethanol and some associated adverse effects on the liver and the stomach

Current knowledge of alcohol oxidation and its effects on hepatic metabolism and its toxicity are summarized. This includes an evaluation of the relationship of the level of consumption to its interaction with nutrients (especially retinoids, carotenoids, and folate) and the development of various stages of liver disease. Ethanol metabolism in the stomach and its link to pathology and Helicobacter pylori is reviewed. Promising therapeutic approaches evolving from newly gained insight in the pathogenesis of medical complications of alcoholism are outlined. At present, the established approach for the prevention and treatment of alcoholism are outlined. At present, the established approach for the prevention and treatment of alcoholic liver injury is to control alcohol abuse, with the judicial application of selective antioxidant therapy, instituted at early stages, prior to the social or medical disintegration of the patient, and associated with antiinflammatory agents at the acute phase of alcoholic hepatitis. In addition, effective antifibrotic therapy may soon become available.

Ethanol metabolism, cirrhosis and alcoholism

Alcohol-induced tissue damage results from associated nutritional deficiencies as well as some direct toxic effects, which have now been linked to the metabolism of ethanol. The main pathway involves liver alcohol dehydrogenase which catalyzes the oxidation of ethanol to acetaldehyde, with a shift to a more reduced state, and results in metabolic disturbances, such as hyperlactacidemia, acidosis, hyperglycemia, hyperuricemia and fatty liver. More severe toxic manifestations are produced by an accessory pathway, the microsomal ethanol oxidizing system involving an ethanol-inducible cytochrome P450 (2E1). After chronic ethanol consumption, there is a 4- to 10-fold induction of 2E1, associated not only with increased acetaldehyde generation but also with production of oxygen radicals that promote lipid peroxidation. Most importantly, 2E1 activates many xenobiotics to toxic metabolites. These include solvents commonly used in industry, anaesthetic agents, medications such as isoniazid, over the counter analgesics (acetaminophen), illicit drugs (cocaine), chemical carcinogens, and even vitamin A and its precursor beta-carotene. Furthermore, enhanced microsomal degradation of retinoids (together with increased hepatic mobilization) promotes their depletion and associated pathology. Induction of 2E1 also yields increased acetaldehyde generation, with formation of protein adducts, resulting in antibody production, enzyme inactivation, decreased DNA repair, impaired utilization of oxygen, glutathione depletion, free radical-mediated toxicity, lipid peroxidation, and increased collagen synthesis. New therapies include adenosyl-L-methionine which, in baboons, replenishes glutathione, and attenuates mitochondrial lesions. In addition, polyenylphosphatidylcholine (PPC) fully prevents ethanol-induced septal fibrosis and cirrhosis, opposes ethanol-induced hepatic phospholipid depletion, decreased phosphatidylethanolamine methyltransferase activity and activation of hepatic lipocytes, whereas its dilinoleoyl species increases collagenase activity. Current clinical trials with PPC are targeted on susceptible populations, namely heavy drinkers at precirrhotic stages.

Effect of aniline on ethanol oxidation and carbohydrate metabolism in isolated hepatocytes

The addition of aniline to isolated hepatocytes derived from fasted rats and incubated with ethanol, caused a 30-60% decrease in the rate of ethanol oxidation. The degree of inhibition was dependent on aniline concentration, 5 mM causing near-maximal inhibition. Aniline reduced the activity of alcohol dehydrogenase in a noncompetitive manner, but had no effect on aldehyde dehydrogenase activity nor on reducing-equivalent transfer between the cytoplasm and mitochondria. The inhibition of alcohol dehydrogenase by aniline was associated with a decrease in the inhibitory effects of ethanol on glycolysis. Aniline, added to hepatocytes in the presence or absence of ethanol, inhibited gluconeogenesis from lactate and pyruvate, but not from sorbitol or fructose.

Kinetic interactions between 4-methylpyrazole and ethanol in healthy humans

4-Methylpyrazole (4-MP), a potent inhibitor of alcohol dehydrogenase activity, is a candidate to replace ethanol as the antidote for methanol and ethylene glycol intoxications, because it has a longer duration of action and apparently fewer adverse effects. To study a probable mutual inhibitory effect between ethanol and 4-MP on their elimination, two studies were performed in healthy human volunteers using double-blind crossover designs. In study A1 4-MP in the presumed therapeutic dose range of 10 to 20 mg/kg caused a 40% reduction in the rate of elimination of ethanol in 12 subjects given 0.5 to 0.7 g/kg of ethanol. These data suggest that such doses of 4-MP inhibit alcohol dehydrogenase activity in humans in vivo and would be effective at blocking methanol or ethylene glycol metabolism. In study B, ethanol (0.6 g/kg followed by 0.2 g/kg twice) significantly decreased the rate of elimination of 4-MP (5 mg/kg, given intravenously to four subjects). These moderate doses of ethanol also inhibited the rate of urinary excretion of 4-carboxypyrazole, the primary metabolite of 4-MP in humans. Data suggest that ethanol inhibits 4-MP metabolism, thereby increasing the duration of therapeutic blood levels of 4-MP in the body. This mutual interaction may have clinical implications, because most self-poisoned patients have also ingested ethanol. Theoretically, methanol and ethylene glycol might also show such interactions with 4-MP.

Role of oxidative stress and antioxidant therapy in alcoholic and nonalcoholic liver diseases

The main pathway for the hepatic oxidation of ethanol to acetaldehyde proceeds via ADH and is associated with the reduction of NAD to NADH; the latter produces a striking redox change with various associated metabolic disorders. NADH also inhibits xanthine dehydrogenase activity, resulting in a shift of purine oxidation to xanthine oxidase, thereby promoting the generation of oxygen-free radical species. NADH also supports microsomal oxidations, including that of ethanol, in part via transhydrogenation to NADPH. In addition to the classic alcohol dehydrogenase pathway, ethanol can also be reduced by an accessory but inducible microsomal ethanoloxidizing system. This induction is associated with proliferation of the endoplasmic reticulum, both in experimental animals and in humans, and is accompanied by increased oxidation of NADPH with resulting H2O2 generation. There is also a concomitant 4- to 10-fold induction of cytochrome P4502E1 (2E1) both in rats and in humans, with hepatic perivenular preponderance. This 2E1 induction contributes to the well-known lipid peroxidation associated with alcoholic liver injury, as demonstrated by increased rates of superoxide radical production and lipid peroxidation correlating with the amount of 2E1 in liver microsomal preparations and the inhibition of lipid peroxidation in liver microsomes by antibodies against 2E1 in control and ethanol-fed rats. Indeed, 2E1 is rather "leaky" and its operation results in a significant release of free radicals. In addition, induction of this microsomal system results in enhanced acetaldehyde production, which in turn impairs defense systems against oxidative stress. For instance, it decreases GSH by various mechanisms, including binding to cysteine or by provoking its leakage out of the mitochondria and of the cell. Hepatic GSH depletion after chronic alcohol consumption was shown both in experimental animals and in humans. Alcohol-induced increased GSH turnover was demonstrated indirectly by a rise in alpha-amino-n-butyric acid in rats and baboons and in volunteers given alcohol. The ultimate precursor of cysteine (one of the three amino acids of GSH) is methionine. Methionine, however, must be first activated to S-adenosylmethionine by an enzyme which is depressed by alcoholic liver disease. This block can be bypassed by SAMe administration which restores hepatic SAMe levels and attenuates parameters of ethanol-induced liver injury significantly such as the increase in circulating transaminases, mitochondrial lesions, and leakage of mitochondrial enzymes (e.g., glutamic dehydrogenase) into the bloodstream. SAMe also contributes to the methylation of phosphatidylethanolamine to phosphatidylcholine. The methyltransferase involved is strikingly depressed by alcohol consumption, but this can be corrected, and hepatic phosphatidylcholine levels restored, by the administration of a mixture of polyunsaturated phospholipids (polyenylphosphatidylcholine). In addition, PPC provided total protection against alcohol-induced septal fibrosis and cirrhosis in the baboon and it abolished an associated twofold rise in hepatic F2-isoprostanes, a product of lipid peroxidation. A similar effect was observed in rats given CCl4. Thus, PPC prevented CCl4- and alcohol-induced lipid peroxidation in rats and baboons, respectively, while it attenuated the associated liver injury. Similar studies are ongoing in humans.

Role of superoxide anion radicals in ethanol metabolism by blood monocyte-derived human macrophages

The effects of a number of additives on the rate of conversion of ethanol (1 mg/ml; 21.7 mM) to acetate by monolayers of blood monocyte-derived human macrophages were investigated. The additives studied were superoxide dismutase (SOD; 1,500 U/ml), catalase (1,500 U/ml), tetrahydrofurane (20 mM), and PMA (20 nM), either singly or in various combinations. SOD, catalase, SOD plus catalase, tetrahydrofurane, and tetrahydrofurane plus SOD inhibited ethanol oxidation by 49.2, 12.1, 52.9, 60.4, and 66.8%, respectively. PMA caused a 4.0-8.3-fold increase in the rate of ethanol metabolism and this increase was completely suppressed in the presence of SOD. The data indicate that a substantial proportion of the ethanol metabolism by both unstimulated and PMA-stimulated blood monocyte-derived macrophages was dependent on the generation of superoxide anion radicals.

Microsomal induction, alcohol, and lipoprotein metabolism: is there a three-way relationship?

The role of ethanol as a microsomal enzyme-inducing agent and as a modulator of lipid metabolism is reviewed. In an attempt to ascertain the mechanisms underlying the latter effects we examined the changes in hepatic triglyceride lipase (HTGL), hepatic high density lipoprotein (HDL) binding, and apolipoprotein secretion mediated by ethanol in a variety of experimental situations. Chronic administration of ethanol to rats decreased the ability of the liver to secrete HTGL, but primary liver cultures prepared from both ethanol and sucrose-fed rats secreted more HTGL when acutely exposed to ethanol over a 3-day period than when grown in a control medium. Hep G2 cells when grown in ethanol-containing medium for 14-28 days manifested increased HDL-binding capacity; apolipoprotein-A1 secretion was increased by ethanol but apolipoprotein B secretion was not affected. These findings suggest that increased plasma HDL concentrations which follow chronic ethanol ingestion may be due, at least in part, to increased hepatic secretion and reduced intravascular conversion of the lipoprotein despite enhanced reuptake by the liver; they are not consistent with an ethanol-mediated alteration in very low density lipoprotein secretion by the liver.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 15/2. Metabolism and metabolic effects of ethanol, including interaction with drugs, carcinogens and nutrition

Different pathways of alcohol metabolism, the alcohol dehydrogenase pathway, the microsomal ethanol-oxidizing system and the catalase pathway are discussed. Alcohol consumption leads to accelerated ethanol metabolism by different mechanisms including an increased microsomal function. Microsomal induction leads to interactions of ethanol with drugs, hepatotoxic agents, steroids, vitamins and to an increased activation of mutagens/carcinogens. A number of ethanol-related complications may be explained by the production of its first metabolite, acetaldehyde, such as alterations of mitochondria, increased lipid peroxidation and microtubular alterations with its adverse effects on various cellular activities, including disturbances of cell division. Nutritional factors in alcoholics such as malnutrition are discussed especially with respect to its possible relation to cancer.

Neuroglial and neuroblastoma cell lines are capable of metabolizing ethanol via an alcohol-dehydrogenase-independent pathway

Two neuroglial cell lines (U-251 MG and C6) had a substantial capacity to convert ethanol to acetate in vitro largely by an alcohol dehydrogenase (ADH)-independent mechanism and three neuroblastoma cell lines (IMR-32, NB41A3, and Neuro-2a) had a lesser but significant ethanol-metabolizing capacity which was also either partly or largely ADH-independent. The ADH-independent pathway of ethanol metabolism by neural cells appeared to be dependent on one or more isoenzymes of cytochrome P-450. The data emphasize the possibility that the neurotoxicity of ethanol may be related to a relatively high ethanol-metabolizing capability of neural tissue and particularly of neuroglial cells.

A study of three groups of urban men from the general population with different alcohol habits and drug use and their serum levels of liver-related enzymes and haematological variables

A sample of 200 men from the general population was investigated concerning alcohol consumption in relation to laboratory findings. The relation between symptoms of alcoholism (subjective relative loss of control over drinking, blackouts and morning drinks) and the alcohol consumption was also studied. The subjects were divided into three groups: (1) a group with low alcohol consumption without symptoms of alcoholism, (II) an intermediate group with low, moderate or high alcohol consumption and one or more alcohol symptoms and (III) a heavy-drinking group with two or more symptoms. The heavy-drinking group had significantly higher serum bilirubin, aspartate amino-transferase (ASAT), creatine kinase and lactate dehydrogenase values than the other two groups. Gamma-glutamyl transpeptidase (GGT) showed no relation to alcohol consumption. The use of liver-metabolized drugs was investigated. Ten of the 53 heavy drinkers were taking such drugs, because of illness, and the other 43 were not. The heavy drinkers taking drugs showed pathological laboratory values throughout, in contrast to the subjects of the other subgroups. Serum GGT was high in the drug-using groups but was not significantly elevated in the groups taking only alcohol and no drugs.

Assessment of the role of non-ADH ethanol oxidation in vivo and in hepatocytes from deermice

Deermice genetically lacking alcohol dehydrogenase (ADH-) were used to quantitate the effect of 4-methylpyrazole (4-MP) on non-ADH pathways in hepatocytes and in vivo. Although primarily an inhibitor of ADH, 4-methylpyrazole was also found to inhibit competitively the activity of the microsomal ethanol-oxidizing system (MEOS) in deermouse liver microsomes. The degree of 4-MP inhibition in ADH- deermice then served to correct for the effect of 4-MP on non-ADH pathways in deermice having ADH (ADH+). In ADH+ hepatocytes, the percent contributions of non-ADH pathways were calculated to be 28% at 10 mM and 52% at 50 mM ethanol. When a similar correction was applied to in vivo ethanol clearance rates in ADH+ deermice, non-ADH pathways were found to contribute 42% below 10 mM and 63% at 40-70 mM blood ethanol. The catalase inhibitor 3-amino-1,2,4-triazole, while reducing catalase-mediated peroxidation of ethanol by 83-94%, had only a slight effect on blood ethanol clearance at ethanol concentrations below 10 mM, and no effect at all at 40-70 mM ethanol. These results indicate that non-ADH pathways (primarily MEOS) play a significant role in ethanol oxidation in vivo and in hepatocytes in vitro.

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.

Contribution of non-ADH pathways to ethanol oxidation in hepatocytes from fed and hyperthyroid rats. Effect of fructose and xylitol

The metabolism of (1R)[1-3H]ethanol, [2-3H]lactate or [2-3H]xylitol was studied in hepatocytes from fed or T3-treated rats in the presence or absence of fructose or xylitol. The yields of tritium in ethanol, lactate, water, glycerol and glucose were determined. A simple model, describing the metabolic fate of tritium from these substrates is presented. The model allows estimation of the ethanol oxidation rate by the non-alcohol dehydrogenase pathways from the relative yield of tritium in water and glucose. The calculations are based on a comparison of the fate of the 1-proR-hydrogen of ethanol and the hydrogen bound to carbon 2 of lactate (or xylitol) under identical condition. In our calculations we have taken into account that the reactions catalyzed by lactate dehydrogenase and alcohol dehydrogenase are reversible and that lactate or ethanol labelled during the metabolism of the other tritiated substrates will contribute to the tritium found in water. The contribution of non-ADH pathways to ethanol oxidation varied from 10 to 50% and was correlated to changes in the lactate/pyruvate ratio from 80 to 500. In T3-treated rats the activity of non-ADH pathways were greater than in fed rats for the same lactate/pyruvate ratio.

Stereochemical studies on the cytochrome P-450 and hydroxyl radical dependent pathways of 2-butanol oxidation by microsomes from chow-fed, phenobarbital-treated, and ethanol-treated rats

Microsomes have the potential to oxidize alcohols by two pathways, one dependent on hydroxyl radicals (.OH) and the other dependent on cytochrome P-450 in which .OH does not seem to be involved. The possibility that these two pathways may display differences in stereospecificity was evaluated by comparing the oxidation of (+)-2-butanol, (-)-2-butanol, and racemic 2-butanol. Microsomes oxidized 2-butanol to 2-butanone by a reaction which was partially sensitive to carbon monoxide and to competitive .OH scavengers. Desferrioxamine, which completely blocks the production of .OH by microsomes, inhibited the oxidation of ethanol by about 60%, while the oxidation of 2-butanol and 1-butanol was decreased by only 30%. Vmax values for the oxidation of ethanol, 1-butanol, and 2-butanol were 17.7, 6.2, and 23.8 nmol min-1 (mg of protein)-1, respectively, in the absence of desferrioxamine and 5.9, 4.7, and 13.6 nmol min-1 (mg of protein)-1, respectively, in the presence of desferrioxamine. 2-Butanol appears to be a particularly good alcohol substrate for the cytochrome P-450 dependent pathway of alcohol oxidation. Chronic ethanol consumption, which induces the microsomal alcohol oxidizing system, resulted in a 3-fold increase in the rate of 2-butanol oxidation. Most of this increment reflected an increased rate of metabolism by the cytochrome P-450 pathway. A type 2 binding spectrum was observed for the interaction of 2-butanol with microsomes from ethanol-fed rats, but not with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Acute effects of ethanol in the control of protein synthesis in isolated rat liver cells

The acute effect of ethanol on hepatic protein synthesis is a rather controversial issue. In view of the conflicting reports on this subject, the effect of ethanol on protein labeling from L-[3H]valine in isolated liver cells was studied under a variety of experimental conditions. When tracer doses of the isotope were utilized, ethanol consistently decreased the rate of protein labeling, regardless of the metabolic conditions of the cells. This inhibition was not prevented by doses of 4-methylpyrazole large enough to abolish all the characteristic metabolic effects of ethanol, and it was not related to perturbations on the rates of L-valine transport and/or proteolysis. When ethanol was tested in the presence of saturating doses of L-[3H]valine no effect on protein labeling was observed. These observations suggest that the ethanol effect in decreasing protein labelling from tracer doses of the radioactive precursor does not reflect variations in the rate of protein synthesis but reflects changes in the specific activity of the precursor. These changes probably are secondary to variations in the dimensions of the amino acid pool utilized for protein synthesis. Even though it showed a lack of effect when tested alone, in the presence of saturating doses of the radioactive precursor ethanol inhibited the stimulatory effects on protein synthesis mediated by glucose and several gluconeogenic substrates. This effect of ethanol was not prevented by inhibitors of alcohol dehydrogenase, indicating that a shift of the NAD system to a more reduced state is not the mediator of its action. It is suggested that ethanol probably acted by changing the steady-state levels of some common effector(s) generated from the metabolism of all these fuels or else by preventing the inactivation of a translational repressor.

Hepatic thyroid hormone levels following chronic alcohol consumption: direct experimental evidence in rats against the existence of a hyperthyroid hepatic state

To study the effect of chronic alcohol consumption on hepatic levels of thyroid hormones, female Sprague-Dawley rats (n = 24) were pair-fed nutritionally adequate liquid diets containing either ethanol (36% of total calories) or isocaloric carbohydrates for 21 days. Compared to controls, chronic alcohol consumption failed to result in a significant change of hepatic thyroid hormone levels (thyroxine: 14.7 +/- 1.81 ng per gm of liver wet weight vs. 15.0 +/- 1.59; triiodothyronine: 2.60 +/- 0.16 ng per gm of liver wet weight vs. 2.66 +/- 0.18). Similar results were obtained when the hepatic levels of thyroid hormones were expressed per total liver, per gram of liver protein or per 100 gm of body weight. Moreover, prolonged alcohol ingestion led to a significant reduction of serum total thyroxine by 31.6% (p less than 0.001), free thyroxine by 38.9% (p less than 0.02), total triiodothyronine by 40.2% (p less than 0.001) and free triiodothyronine by 56.1% (p less than 0.001) when compared to their pair-fed controls, whereas thyroid-stimulating hormone levels remained virtually unchanged. These data, therefore, clearly show that chronic alcohol consumption is incapable of creating a hyperthyroid hepatic state in rats, and limit the rationale for antithyroid treatment in patients with alcoholic liver disease.

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.

Uninterrupted prolonged ethanol oxidation as a main pathogenetic factor of alcoholic liver damage: evidence from a new liquid diet animal model

Marked fatty infiltration and degenerative or mild inflammatory changes including eosinophilic cytoplasmic degeneration in centrilobular cells and focal inflammatory changes with cell necrosis were observed in livers of rats maintained for 12 weeks on a nutritionally adequate and balanced liquid ethanol diet. The animals continuously oxidized ethanol due to the supplementation of the diet with a low dose of 4-methylpyrazole (4-MP, an alcohol dehydrogenase inhibitor), that decreased ethanol elimination by about 20%. In other, equicalorically pair-fed groups of rats receiving (a) a similar ethanol-containing liquid diet without 4-MP or (b) a diet with 4-MP and 20% less ethanol, only a few minor changes were seen. The liver histology of rats pair-fed a control diet with a 4 times higher doses of 4-MP was completely normal. The results indicate that the prolonged imbalance of hepatic metabolism due to the uninterrupted oxidation of ethanol is a crucial factor in the development of alcoholic liver injury.

Evidence for two ethanol oxidizing pathways in reconstituted mixed-function oxidase systems

The oxidation of ethanol and typical hydroxyl radical scavengers by NADPH-cytochrome P-450 reductase and cytochrome P-450 purified from phenobarbital-treated rats were studied. Ethanol and the scavengers could be oxidized by the reductase system itself. This system was inhibited by superoxide dismutase, competing hydroxyl radical scavengers and desferrioxamine, but stimulated by either EDTA or iron. These results suggest that an iron-catalyzed Haber-Weiss reaction might be involved in the mechanism by which the reductase mediates the oxidation of typical hydroxyl radical scavengers and ethanol. The addition of cytochrome P-450 had no effect on the oxidation of the scavengers, whereas the oxidation of ethanol was enhanced two-to three-fold over the reductase-dependent rate. The oxidation of ethanol was dependent on both the amount of reductase and P-450. There was no effect of competing scavengers, superoxide dismutase or desferrioxamine on the increased rate of ethanol oxidation produced upon addition of cytochrome P-450. Organic hydroperoxides supported the oxidation of ethanol, but not that of the scavengers when added directly to cytochrome P-450. These results suggest that two independent pathways are operative in supporting NADPH-dependent microsomal oxidation of ethanol. One pathway involves hydroxyl radicals which can be generated by the reductase, whereas the other pathway requires the combined presence of both the reductase and cytochrome P-450, and appears to be independent of oxygen radicals.

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.

Role of catalase and hydroxyl radicals in the oxidation of methanol by rat liver microsomes

In view of the presence of adventitious catalase in isolated microsomes, and the requirement for H2O2, it has been suggested that NADPH-dependent oxidation of methanol by rat liver microsomes was mediated exclusively by the peroxidatic activity of catalase. However, H2O2 may also serve as a precursor of the hydroxyl radical, and methanol reacts with hydroxyl radicals to produce formaldehyde. Inhibition of H2O2 production should therefore decrease methanol oxidation by either a hydroxyl radical-dependent pathway or a catalase-dependent pathway. To attempt to clarify some of the controversies concerning the roles of H2O2 and catalase in the microsomal pathway of oxidation of short chain alcohols, studies were carried out to determine the nature of the pathway responsible for methanol oxidation by isolated microsomes. In the absence of the catalase inhibitor azide, methanol may be oxidized primarily by the peroxidatic activity of catalase since there was little effect on methanol oxidation by competing hydroxyl radical scavengers. Azide, which inhibited catalase activity greater than 95%, inhibited NADPH-dependent oxidation of methanol by 30-50%. The azide-insensitive (catalase-independent) pathway of methanol oxidation was inhibited by scavengers of hydroxyl radicals. The inhibition of the scavengers reflected the rate constant for interaction with hydroxyl radicals and was greater at lower concentrations of methanol than at higher concentrations, suggesting competition between the scavengers and methanol. The addition of H2O2 stimulated the oxidation of methanol in the presence of azide; H2O2 may serve as a precursor of the hydroxyl radical. Iron-EDTA, which is known to increase hydroxyl radical production, stimulated the oxidation of methanol in the absence and presence of azide. The stimulation by iron-EDTA was blocked by the competing hydroxyl radical scavengers even in the absence of azide, suggesting that the added iron-EDTA favorably with microsomal catalase for H2O2 to produce hydroxyl radicals (or a species with the oxidizing power of the hydroxyl radical). These results suggest that in microsomes, depending on the absence or presence of azide, methanol may be oxidized by two primary pathways, one involving the peroxidatic activity of catalase, and the other in which hydroxyl radicals, generated from microsomal electron transfer, play a role. In view of the crucial role played by H2O2 in both pathways, inhibition of H2O2 formation should not be interpreted solely as evidence for a role for catalase in the microsomal oxidation of alcohols.

Ether inhibition of ethanol metabolism in isolated rat liver parenchymal cells

The effect of diethyl ether on ethanol metabolism was studied in isolated rat hepatocytes and ether was found to inhibit ethanol oxidation in a dose-dependent manner. At ethanol concentrations of approximately 30 mM, diethyl ether inhibited ethanol oxidation by approximately 58%, 40%, and 20% at ether concentrations of 30, 20, and 10 mM, respectively. This inhibition was also seen at a low ethanol concentration (5.4 mM) and in pyruvate (5 mM)-stimulated hepatocytes which exhibited increased rates of ethanol metabolism closer to in vivo rates. Accumulation of acetaldehyde from ethanol in cyanamide (400 micron M)-treated hepatocyte suspensions was also reduced by approximately 16% by 30 mM ether. It was concluded that inhibition of ethanol metabolism by diethyl ether might be of practical importance in studies of ethanol metabolism in ether anesthetized animals.