NAD(+)-dependent retinol dehydrogenase in liver microsomes.
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Alcohol Oxidoreductases↗
Biomedical subjects
C S Lieber
Publications and source records attributed to C S Lieber.
Mechanism of ethanol induced hepatic injury.
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) which 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. Induction also results in energy wastage and 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 synthesis, thereby promoting fibrosis.
Acetaldehyde↗
Interaction of ethanol with drugs, hepatotoxic agents, carcinogens and vitamins.
Ethanol has been shown to have a multitude of acute and chronic interactions with xenobiotic agents, many of which can now be explained on the basis of the existence of a newly recognized microsomal ethanol oxidizing system (MEOS) involving a specific cytochrome P-450 (P450IIE1). Although such a system was proposed already two decades ago, its role was viewed with skepticism: until recently, it was commonly believed that the primary pathway for hepatic ethanol metabolism is due almost exclusively to the activity of cytosolic alcohol dehydrogenase, with a minor contribution from peroxisomal catalase. It is now recognized, however, that liver microsomes (through MEOS) participate in ethanol metabolism. The existence of this system and its inducibility contribute to the metabolic tolerance to ethanol in the alcoholic. Cross induction of other microsomal enzymes also explains the tolerance to many commonly used drugs. Most importantly, the alcohol-inducible form (P450IIE1) has a unique capacity to activate xenobiotic agents to toxic metabolites, thereby explaining the unusual susceptibility of the alcoholic to the adverse effects of other drugs, hepatotoxic agents, carcinogens and even vitamins.
Alcohol Dehydrogenase↗
Effects of arachidonic acid on hepatic lipids in ethanol-fed rats.
The effects of arachidonic acid supplementation on rats fed ethanol employing an ad libitum schedule have been reported to be different from those observed when rats are fed in more limiting, matched fashion. To reexamine this issue, rats were fed unrestricted amounts of a diet in which 36% of the energy was provided by either ethanol or isocaloric amounts of carbohydrate. In half the animals, 7% of fat consisted of arachidonic acid. Despite earlier reports to the contrary, arachidonic acid had no effect on weight gain and did not attenuate the ethanol-induced fatty liver. Arachidonate supplementation tended to increase hepatic total lipids and triacylglycerols, and to potentiate the ethanol-induced elevation of cholesterol esters. Our present results are consistent with those previously reported using pair-feeding techniques in which dietary intakes are somewhat limited. Thus, regardless of the feeding technique employed, relative arachidonic deficiency cannot be involved to explain the lipid accumulation observed after chronic ethanol consumption.
Animals↗
Ethanol oxidation by deermice mitochondria under physiologic conditions.
Mitochondria obtained from alcohol dehydrogenase-positive or - negative deermice do not oxidize significant amounts of ethanol at pH 7.4. A slight activity, equivalent to less than 0.3% of the elimination rate in alcohol dehydrogenase-negative deermice was observed at pH 10; it was strongly inhibited by cyanide and thiourea, and was not dependent on exogenous NAD. Whereas ethanol oxidation by the cytosol of alcohol dehydrogenase-positive deermice was time-dependent, that of mitochondria from alcohol dehydrogenase-negative deermice was not. These findings indicate that deermice mitochondria do not oxidize ethanol at physiological pH, and that the mitochondrial system is not likely to play a significant physiologic role.
Alcohol Dehydrogenase↗
Interaction of ethanol with enflurane metabolism and toxicity: role of P450IIE1.
Administration of enflurane (EF), a widely-used anesthetic agent, sometimes results in occult liver injury. As hepatic cytochromes P450 oxidize EF to a reactive intermediate, we assessed whether one such microsomal enzyme, ethanol-inducible P450IIE1, plays an obligatory role in EF metabolic activation and hepatotoxicity. Liver microsomes from rats fed ethanol (36% of total calories for 14 days) oxidized 1 mM EF (measured by its defluorination) at rates nearly 10-fold greater than those from control rats, reflecting the markedly enhanced content of immunoreactive microsomal P450IIE1 in the former animals. P450IIE1 involvement in hepatic EF oxidation was further suggested by the pronounced inhibition of microsomal defluorination noted with P450IIE1 antibodies and with ethanol, a specific substrate for this enzyme. EF administration to rats treated chronically with ethanol caused significant elevations in plasma levels of aspartate and alanine aminotransferases and glutamate dehydrogenase, indicative of hepatic injury, whereas concurrent treatment of naive rats with EF and ethanol failed to produce the same effect. Our results imply that ethanol-inducible P450IIE1 is the primary catalyst of hepatic EF bioactivation and that the increased bioactivation occurring in vivo secondary to chronic ethanol consumption is attendant with an increased incidence of EF hepatotoxicity.
Animals↗
Human gastric alcohol dehydrogenase: its inhibition by H2-receptor antagonists, and its effect on the bioavailability of ethanol.
Two types of alcohol dehydrogenase isoenzymes (differing in their affinity for ethanol, sensitivity to 4-methylpyrazole, and electrophoretic migration) have been identified in the human stomach. At the high ethanol concentrations prevailing in the gastric lumen during alcohol consumption, the sum of their activities could account for significant oxidation of ethanol. In vitro, these activities were inhibited by cimetidine and ranitidine, but not by famotidine. In vivo, therapeutic doses of cimetidine (but not of famotidine) increased blood ethanol levels when ethanol was given orally, but not when it was given intravenously, indicating a significant contribution of the gastric ADH to the bioavailability and thereby the potential toxicity of ethanol.
Alcohol Dehydrogenase↗
Alcoholism: a disease of internal medicine.
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Alcoholism↗
The role of SAMe in the treatment of liver disease.
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Humans↗
Interaction of alcohol with other drugs and nutrients. Implication for the therapy of alcoholic liver disease.
Two decades of research in ethanol metabolism have culminated in the molecular elucidation of an ethanol-inducible cytochrome P450 (P450IIE1) which is not only involved with ethanol metabolism and ethanol tolerance, but also with the activation of a number of xenobiotics. The unique ability of P450IIE1 to activate xenobiotic agents now appears to be responsible for the increased susceptibility of the heavy drinker to hepatotoxic industrial solvents, commonly used drugs, over-the-counter medications and chemical carcinogens. It also explains some of the interaction of ethanol with nutritional factors, such as hepatic vitamin A: enhanced microsomal degradation of retinoids (together with hepatic mobilisation) promotes depletion. Treatment, however, is complicated by the fact that ethanol also enhances the toxicity of excess vitamin A. All pathways of ethanol metabolism result in the production of acetaldehyde, the toxicity of which has been reviewed (Lieber 1982). New aspects discussed here include the formation of acetaldehyde-protein adducts and an associated immune response that may play a pathogenic role. Also discussed are the implications of ethanol-induced alterations in microtubules, mitochondria and plasma membranes, as they relate, in part, to accompanying acetaldehyde-induced toxicity, to the production of free radicals or to lipid peroxidation-mediated injury associated with glutathione depletion. There is also depletion of S-adenosyl-L-methionine (SAMe). Administration of synthetic SAMe results in a partial correction of the SAMe depletion and a consequent restoration of glutathione levels. Other beneficial effects of SAMe include a significant attenuation of the increase in plasma aspartate transaminase and glutamate dehydrogenase activities. Mitochondrial damage, including giant forms, documented by light and electron microscopy, is also attenuated by SAMe. Thus, the new understanding of the pathophysiology of alcohol-induced liver damage has led to more successful therapy with drugs and nutritional factors.
Animals↗
Human immunodeficiency virus and hepatitis B virus infections in alcoholics.
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Acquired Immunodeficiency Syndrome↗
Role of peroxisomal fatty acid beta-oxidation in ethanol metabolism.
The contribution of peroxisomal fatty acid beta-oxidation to ethanol metabolism was examined in deermice hepatocytes. Addition of 1 mM oleate to hepatocytes isolated from fasted alcohol dehydrogenase (ADH)-positive deermice in the presence of 4-methylpyrazole or to hepatocytes from fasted or fed ADH-negative deermice produced only a slight and statistically not significant increase in ethanol oxidation. Lactate (10 mM), which is not a peroxisomal substrate, showed a greater effect on ethanol oxidation. There was also a lack of oleate effect on the oxidation of ethanol by hepatocytes of ADH-positive deermice. Furthermore, in ADH-negative deermice, the catalase inhibitor azide (0.1 mM) did not inhibit the increase in ethanol oxidation by oleate and lactate. The rate of oleate oxidation by hepatocytes from fasted ADH-negative deermice was much lower than that of ethanol. These results indicate that in deermice hepatocytes, peroxisomal fatty acid oxidation does not play major role in ethanol metabolism.
Alcohol Dehydrogenase↗
The microsomal ethanol oxidizing system mediates metabolic tolerance to ethanol in deermice lacking alcohol dehydrogenase.
Metabolic tolerance to ethanol has been attributed to enhanced mitochondrial reoxidation of reducing equivalents produced in the alcohol dehydrogenase (ADH) pathway or to non-ADH mechanisms. To resolve this issue, deermice lacking low Km hepatic ADH were fed for 2 weeks a liquid diet containing ethanol or isocaloric carbohydrate and hepatocytes were isolated. Ethanol (50 mM) oxidation increased (9.8 vs 4.5 nmol/min/10(6) cells in controls). To differentiate which of two non-ADH pathways (the microsomal ethanol oxidizing system (MEOS) or catalase) was responsible for the induction, four approaches were used. First, MEOS was assayed in hepatic microsomes and found to be increased (24.4 vs 6.8 nmol/min/mg protein in controls). Second, hepatocyte ethanol metabolism was measured after addition of the catalase inhibitor azide (0.1 mM) and found to be unchanged. By contrast, the competitive MEOS inhibitor, 1-butanol, depressed metabolism in a concentration-dependent manner. A third approach relied on measurement of isotope effects known to be different for MEOS and catalase. From the isotope effect values, MEOS was calculated to contribute 85% or more of total ethanol oxidation by cells from both ethanol-fed and control animals. A fourth approach involved in vivo pretreatment with pyrazole (300 mg/kg/day for 2 days), which reduced peroxidation by catalase to 13% of control values in liver homogenates while inducing MEOS activity to 152% of controls. Hepatocytes from pyrazole-treated deermice showed a 47% increase in ethanol metabolism, paralleling the MEOS induction and contrasting with the catalase suppression. These results indicate that since metabolic tolerance occurs in the absence of ADH, it is not necessarily ADH mediated, and further, that MEOS rather than catalase accounts for basal ethanol metabolism and its increase after chronic ethanol treatment.
1-Butanol↗
Metabolism of retinol and retinoic acid by human liver cytochrome P450IIC8.
Liver microsomes obtained from nine subjects were found to metabolize retinol to polar metabolites, including 4-hydroxyretinol. In a reconstituted monooxygenase system containing human liver P450IIC8, retinol was converted to 4-hydroxyretinol and other polar metabolites, with a Km of 0.071 mM and a Vmax of 1.73 nmol/min/nmol P450. Neither P450IIC9 nor P450IIE1, two other purified human P450s, displayed significant retinol hydroxylase activity. Immunoblots performed with a monospecific antibody directed against human P450IIC8 revealed that appreciable amounts of this enzyme were present in human liver microsomes. The same antibody significantly inhibited retinol metabolism in liver microsomes and in the system reconstituted with P450IIC8. The system reconstituted with P450IIC8 also converted retinoic acid to polar metabolites. Thus, this study shows, for the first time, metabolism of two physiologic substrates by a human liver cytochrome P450 related to a group of "constitutive" rodent P450s believed to participate in the metabolism of endogenous compounds. Through its involvement in vitamin A metabolism, P450IIC8 may participate in maintaining the balance between those vitamin A concentrations that promote cellular integrity (and oppose the development of cancer) and those concentrations that cause cellular toxicity.
Adult↗
Enhanced stimulatory effect of high density lipoproteins (HDL) and other agonists on vascular prostacyclin production in rats fed alcohol-containing diets.
Chronic administration of liquid diets containing 36% of energy as ethanol to rats increased the serum level of high density lipoproteins (HDL) by 40% and potentiated 2 to 3-fold the stimulatory effect of these lipoproteins on the production of prostacyclin by aortic rings, as compared to pair-fed controls given isocaloric carbohydrate instead of ethanol. Cross-incubations between aortic rings and HDL from either alcohol-fed or pair-fed control rats revealed two factors operating in opposite directions. On the one hand, the predominant mechanism for the potentiation was increased reactivity of the vessel to these lipoproteins. This increased reactivity was also apparent with other agonists of prostacyclin formation. On the other hand, for equal amounts of either cholesterol or protein, the HDL from alcohol-fed rats were less stimulatory than those from controls. This was associated with a smaller content of arachidonate in the HDL of alcohol-fed rats. These ethanol effects on prostacyclin, a potent vasodilator and platelet antiaggregator, may contribute to the decreased incidence of ischemic heart disease observed in moderate alcohol drinkers.
Alcohol Drinking↗
Characteristics of butanol metabolism in alcohol dehydrogenase-deficient deermice.
Deermice lacking the low-Km alcohol dehydrogenase eliminated butan-1-ol, a substrate for microsomal oxidation but not for catalase, at 117 mumol/min per kg body wt. Microsomal fractions and hepatocytes metabolized butan-1-ol also (Vmax. = 6.7 nmol/min per nmol of cytochrome P-450, Km = 0.85 mM; Vmax. = 5.3 nmol/min per 10(6) cells, Km = 0.71 mM respectively). These results are consistent with alcohol oxidation by the microsomal system in these deermice.
Alcohol Dehydrogenase↗
Increased hepatic retinal dehydrogenase activity after phenobarbital and ethanol administration.
Cytosolic retinal dehydrogenase (EC 1.2.1.36, retinal: NAD+ oxidoreductase) activity was measured by assessing the conversion of retinal to retinoic acid by HPLC. In vitro, acetaldehyde, chloral hydrate and disulfiram were found to be inhibitors, whereas 95% of the activity remained in the presence of cyanide or in the absence of oxygen. In rats, retinal dehydrogenase activity prevailed over that of retinal oxidase. By contrast, in deermice, 80% of retinal oxidation was due to the oxidase rather than the retinal dehydrogenase activity in a normal strain (ADH+) as well as in one lacking alcohol dehydrogenase (ADH-). In ADH- deermice, retinal oxidase activity was greater than in ADH+ animals. In vivo, in the rat, chronic ethanol administration resulted in a significant increase of the dehydrogenase activity in the liver, but not in other tissues. After phenobarbital administration, hepatic retinal dehydrogenase activity was increased 8-fold, but no extrahepatic induction was observed. Conversely, feeding rats with a diet devoid of the precursor for the substrate (retinal) by replacing retinyl acetate with an equivalent amount of retinoic acid resulted in decreased retinal dehydrogenase activity. Under conditions in which retinal dehydrogenase activity is rate-limiting for the metabolism of retinal to retinoic acid, its induction after phenobarbital or ethanol administration may contribute to hepatic vitamin A depletion.
Aldehyde Oxidoreductases↗