Ethanol-oxidizing Activity Research Articles

Variation in gastric alcohol dehydrogenase and the risk of alcohol dependence.

Alcohol dependence is both a medical and socioeconomic problem. The disease is multifactorial, i.e. its development is attributable to gene-gene and gene-environment interactions. Multi-centre studies investigating the genetic background of alcoholism stress the role of genes encoding enzymes of the ethanol decomposition pathway in the human body, particularly alcohol dehydrogenase (ADH), in the development of alcohol dependence. Among five classes of alcohol dehydrogenases, class I and IV isoenzymes have been found to be associated with alcohol dependence. Class IV is of particular interest due to its occurrence in the upper gastrointestinal tract, mainly in the stomach. No activity of the enzyme has been demonstrated in the liver. Single nucleotide polymorphism (SNP) of the gene encoding ADH class IV (ADH7) affects its ethanol-oxidizing activity in the gastric lumen, thereby influencing the first-pass metabolism (FPM) of the substance. The findings published by various research centres have demonstrated that specific SNP changes in the ADH7 gene are of different significance for the risk of alcohol dependence according to the population studied.

Methanol and ethanol oxidase respiratory chains of the methylotrophic acetic acid bacterium, Acetobacter methanolicus.

Acetobacter methanolicus is a unique acetic acid bacterium which has a methanol oxidase respiratory chain, as seen in methylotrophs, in addition to its ethanol oxidase respiratory chain. In this study, the relationship between methanol and ethanol oxidase respiratory chains was investigated. The organism is able to grow by oxidizing several carbon sources, including methanol, glycerol, and glucose. Cells grown on methanol exhibited a high methanol-oxidizing activity and contained large amounts of methanol dehydrogenase and soluble cytochromes c. Cells grown on glycerol showed higher oxygen uptake rate and dehydrogenase activity with ethanol but little methanol-oxidizing activity. Furthermore, two different terminal oxidases, cytochrome c and ubiquinol oxidases, have been shown to be involved in the respiratory chain; cytochrome c oxidase predominates in cells grown on methanol while ubiquinol oxidase predominates in cells grown on glycerol. Both terminal oxidases could be solubilized from the membranes and separated from each other. The cytochrome c oxidase and the ubiquinol oxidase have been shown to be a cytochrome co and a cytochrome bo, respectively. Methanol-oxidizing activity was diminished by several treatments that disrupt the integrity of the cells. The activity of the intact cells was inhibited with NaCl and/or EDTA, which disturbed the interaction between methanol dehydrogenase and cytochrome c. Ethanol-oxidizing activity in the membranes was inhibited with 2-heptyl-4-hydroxyquinoline N-oxide, which inhibited ubiquinol oxidase but not cytochrome c oxidase. Alcohol dehydrogenase has been purified from the membranes of glycerol-grown cells and shown to reduce ubiquinone-10 as well as a short side-chain homologue in detergent solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcohol oxidation by isozyme 3a of liver microsomal cytochrome P-450

Liver microsomes from rabbits treated chronically with ethanol were solubilized and fractionated to yield a new isozyme of cytochrome P-450 in a homogeneous state. This cytochrome, designated as isozyme 3a on the basis of its relative electrophoretic mobility, is distinct from the known isozymes as judged by its spectral properties, minimal molecular weight, amino acid composition, and NH 2- and COOH-terminal amino acid sequences. In addition, peptide mapping by high performance liquid chromatography following trypsinolysis indicates that form 3a is a unique gene product. This cytochrome has unusually high activity in the oxidation of ethanol and other alcohols to aldehydes and in the p-hydroxylation of aniline as compared with the other isozymes of P-450. The ethanol-oxidizing activity of isozyme 3a, which requires the presence of NADPH and NADPH-cytochrome P-450 reductase and is stimulated by the presence of phosphatidylcholine, is not due to contamination by catalase or an NAD +-or NADP +-dependent alcohol dehydrogenase.

Catalytic activity of cytochrome P-450 isozyme 3a isolated from liver microsomes of ethanol-treated rabbits. Oxidation of alcohols.

Cytochrome P-450 isozyme 3a, isolated from hepatic microsomes of rabbits treated chronically with ethanol, was found to have a unique substrate specificity when compared with isozymes 2, 3b, 3c, and 4. Form 3a has unusually high activity in the p-hydroxylation of aniline and in the oxidation of alcohols to aldehydes. These properties are reflected in the increased activities of these substrates in microsomes from ethanol-treated rabbits as compared to microsomes from untreated animals or those administered phenobarbital or 5,6-benzoflavone. The ethanol-oxidizing activity of isozyme 3a, which requires the presence of NADPH and NADPH cytochrome P-450 reductase and is stimulated by the presence of phospholipid, was shown not to be due to contaminating catalase or an NAD- or NADP-dependent alcohol dehydrogenase. Isozyme 3a catalyzes the oxidation of methanol, 1-propanol, and 1-butanol as well as ethanol; the relationships between the apparent Km values for these alcohols and their octanol-water partition coefficients is in accord with the known hydrophobic nature of the P-450 binding site. Whereas typical substrates of isozyme 2 are known to be metabolized with the stoichiometry predicted of a monooxygenase reaction, with isozyme 3a the sum of acetaldehyde formed from ethanol and of hydrogen peroxide generated is inadequate to account for the NADPH and oxygen consumed. Free hydroxyl radicals appear to mediate the slow oxidation of ethanol in the presence of the reductase alone but not the faster rate catalyzed by P-450 isozyme 3a. The results obtained, however, do not rule out the involvement of hydroxyl radical equivalent generated and bound at the active site of the cytochrome.

Effect of dietary ethanol and cholesterol on metabolic functions of hepatic mitochondria and microsomes from the monkey, Macaca nemestrina.

Monkeys (Macaca nemestrina) were divided into four groups, and each group was fed a particular diet. The variables in the diets were as follows: diet A, 0.3 mg cholesterol/kcal nutrient; diet B, 1.0 mg cholesterol/kcal nutrient; diet C, 0.3 mg cholesterol/kcal nutrient, ethanol (36% of calories); diet D, 1.0 mg cholesterol/kcal nutrient, ethanol (36% of calories). Monkeys on the diets containing ethanol developed fatty liver. Mitochondria from ethanol-fed animals demonstrated significant decreases in uncoupler-stimulated, state 3, and state 4 succinate oxidation activity; respiratory control ratio; and ATP content. Liver microsomes isolated from the ethanol-fed groups demonstrated increased ethanol oxidizing activity with either NADPH or H2O2 as cosubstrate. Aniline hydroxylase and aminopyrine-N-demethylase activities were also elevated in ethanol-fed animals. The alterations in these functional properties were related primarily to ethanol in the diets. Cholesterol, while being less of a perturbant than ethanol, did elicit a significant decrease in cytochrome oxidase activity of mitochondria and a small but statistically significant increase in microsomal-associated ethanol oxidation activity. It appeared to potentiate the effect of ethanol in lowering mitochondrial respiratory control and ATP concentrations.

Induction of hepatic microsomal ethanol-oxidizing system activity following chronic ethanol consumption: Respective roles of ethanol and carbohydrates

Induction of hepatic microsomal ethanol-oxidizing system activity following chronic ethanol consumption: Respective roles of ethanol and carbohydrates

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

A cytochrome P-450-rich fraction free of alcohol dehydrogenase activity and containing only small amounts of catalase was prepared from rat liver microsomes and shown to catalyze the NADPH-dependent oxidation of ethanol. The recoveries of cytochrome P-450 and the activity of the NADPH-dependent ethanol oxidation were 15.6% and 14.0%, respectively. The addition of separated cytochrome c reductase to the cytochrome P-450 fraction resulted in an increase in the ethanol-oxidizing activity, while the addition of microsomal lipid had a mild inhibitory effect on the activity catalyzed by the combined cytochrome P-450 and cytochrome c reductase fractions. Substitution of the NADPH-generating system by a system generating H2O2 also resulted in oxidation of ethanol by the cytochrome P-450-rich fraction, although at a slightly slower rate. The separated catalase fraction oxidized ethanol only in the presence of a system generating H2O2. It is concluded that ethanol oxidation can be catalyzed by a microsomal component rich in cytochrome P-450 and separate from contaminating alcohol dehydrogenase and catalase.

Contribution of “substrate shuttles” in the transport of extramitochondrial reducing equivalents by hepatic mitochondria from chronic alcohol-fed mice

Effects of chronic alcohol treatment have been investigated on the rates of extramitochondrial NADH utilization by hepatic mitochondria in the presence or absence of “malate-aspartate shuttle,” oxidation of ethanol, α-glycerophosphate, and the activity of succinic dehydrogenase, along with the changes in the intrahepatic distribution of aspartate aminotransferase. The rates of blood alcohol clearance, hepatic alcohol dehydrogenase activity, and NADPH-dependent microsomal ethanol oxidation were also studied after different time intervals of alcohol withdrawal from chronically alcohol-fed animals. Hepatic mitochondria from chronically ethanol-fed mice (ethanol withheld 20 hr before sacrifice) utilized extramitochondrial NADH at rates 25–40% higher than the corresponding pair-fed controls. Addition of malateaspartate shuttle components to mitochondria from control and ethanol-fed groups resulted in 70 and 90% stimulation of NADH utilization, respectively. Mitochondria from both groups showed respiratory control upon ADP addition (state 3). Preincubation with amino-oxyacetate or hydrazine, which inhibit aspartate aminotransferase activity, prevented the stimulatory effect of malate-aspartate shuttle on NADH utilization. Mitochondria from livers of chronic ethanol-fed mice in the presence of reconstituted malate-aspartate shuttle showed 30–40% higher utilization of ethanol than the corresponding pair-fed control animals. The rate of mitochondrial α-glycerophosphate utilization by alcohol-fed animals was significantly higher than the control group. Succinic dehydrogenase activity measured as an index of mitochondrial permeability in the absence of Ca 2+ showed 85% higher activity in alcoholtreated group than the control animals. Chronic ethanol feeding for 4 weeks resulted in an increase in the activity of hepatic aspartate aminotransferase in the cytoplasmic fraction and a corresponding decrease in the mitochondrial fraction. Alcohol withdrawal from chronic alcohol-fed animals resulted in a decrease in the blood alcohol clearance rate after 10 days. Furthermore, a lack of correlation was observed between the rates of blood alcohol clearance and the activity of hepatic alcohol dehydrogenase on one hand, and between the rates of blood alcohol clearance and the microsomal ethanol-oxidizing activity on the other.

Effect of ethanol administration on hepatic ethanol and drug-metabolizing enzymes and on rates of ethanol degradation.

Abstract Ethanol administration was recently shown to enhance the activities of microsomal enzyme systems which metabolize ethanol and other drugs. The present study was designed to establish the time sequence and the mechanism that may account for the increases in enzyme levels, and to determine whether the increases in microsomal ethanol-oxidizing activity are accompanied by comparable changes in the rates of ethanol degradation in vivo. Male rats were fed ethanol for periods of 24, 48, and 72 hours, 7, and 14 days. Whereas liver weights and the cytosol enzymes alcohol dehydrogenase and lactate dehydrogenase were not changed, significant increases were obtained as early as 24 hours after ethanol administration for microsomal ethanol-oxidizing activity and at 48 hours for aniline hydroxylase and cytochrome P-450. Microsomal protein concentration was increased only at 14 days. Protein turnover studies demonstrated no changes in the rate of incorporation of 1- 14 C-l-leucine into microsomal protein, but significant decreases in the rate of degradation of 1- 14 C-l-leucine from microsomal protein were found at 14 days. The rates of ethanol disappearance from the blood were increased; however, the calculated mean increase of the microsomal ethanol-oxidizing activity accounted for only 13.8 per cent of the mean increase in the rate of in vivo ethanol disappearance at 7 days. The findings suggest that the mechanism of induction by ethanol is different from that of other known inducers, and that in vivo factors beside the absolute increase in microsomal enzyme activity must account for the observed changes in the rates of ethanol degradation.

Hepatic Microsomal Ethanol-oxidizing System: IN VITRO CHARACTERISTICS AND ADAPTIVE PROPERTIES IN VIVO

A hepatic microsomal ethanol-oxidizing system is described both in men and rats. It is distinguished from alcohol dehydrogenase by its subcellular localization (cytosol for alcohol dehydrogenase, microsomes for this system), its pH optimum (physiological pH versus pH 10 to 11 for alcohol dehydrogenase), and its cofactor requirements (NADPH versus NAD+ for alcohol dehydrogenase). It also requires oxygen and is inhibited by CO, properties commonly found among microsomal drug-detoxifying enzymes. That catalase is probably not involved was revealed by the partial or complete failure of cyanide, pyrazole, azide, or 3-amino-1,2,4-triazole to inhibit the NADPH-dependent microsomal ethanol-oxidizing system under conditions which diminished catalase activity. Moreover, a combination of administration in vivo of pyrazole and addition in vitro of azide virtually blocked catalase activity and abolished 95% of a H2O2-dependent microsomal ethanol oxidation, whereas two-thirds of the activity of the NADPH-dependent ethanol oxidation persisted. Ethanol feeding resulted in a striking rise of hepatic NADPH-dependent microsomal ethanol-oxidizing activity, whereas under the same conditions, activities of alcohol dehydrogenase in the cytosol and of microsomal as well as of total hepatic catalase did not increase. Furthermore, blood ethanol clearance was accelerated, which suggests that microsomal ethanol oxidation may play a role in vivo. Pyrazole, which inhibits alcohol dehydrogenase strongly (affecting also other hepatic functions, including microsomal enzymes) markedly reduced but did not block ethanol metabolism in vivo or in liver slices. Even after pyrazole, ethanol clearance rates remained significantly higher in ethanol-pretreated rats. The existence of a microsomal ethanol-oxidizing system, especially its capacity to increase in activity adaptively after ethanol feeding, may explain various effects of ethanol, including proliferation of hepatic smooth endoplasmic reticulum, induction of other hepatic microsomal drug-detoxifying enzymes, and the metabolic tolerance to ethanol which develops in alcoholics.