CONTRIBUTION OF ACIDIC ADH(CLASS III) TO ALCHOL METABOLISM

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(1) A fraction of acidic alcohol dehydrogenase (Class III ADH) activity of the mouse liver corresponded to 10-42% that of basic ADH (Class I) known as a key enzyme in alcohol metabolism in vivo, when ethanol was used as a substrate at concentrations of blood level (15-100 mM).(2) The activity of acidic ADH was enhanced with the increase in hydrophobicity of the reaction solution. This activation was due to a marked increase of the catalytic efficiency (Kcat/Km), based on a marked decrease of the Km for ethanol. On the other had, the activity of basic ADH was strongly depressed with the increase in the solution hydrophobicity.(3) Acidic ADH was found to localize mainly in the sinusoidal endothelial cells of the mouse liver, whereas basic ADH was shown to distribute within the cytoplasmic matrix of hepatocytes. Staining the liver tissues with a hydrophobic probe (Nile red) showed that the cytoplasm was hydrophobic, and moreover, the intracelluar hydrophobicity was higher in the sinusoida … More l cells than in the parenchymal cells. These results suggest that intracellular acidic ADH activity was highted, whereas the basic ADH activity was lowered by the effects of intracellular hydrophobicity, compared with their activities measured by the conventional in vitro method.(4) Class III ADH was found in the liver of a so-called "ADH^-" deermouse that possessed a capacity of alcohol metabolism more than 50% that of a "ADH^+" strain. A class of ADH which "ADH^-" strain was genetically deficient in was identified as class I.(5) The contributions of basic ADH and catalase to the total alcohol metabolism in the mouse were estimated to be about 50% and 20%, respectively, by additional administration of specific inhibitors (capronamide for basic ADH and aminotriazole for catalase) after ethanol administration at a dose of 3.0 g/kg (i. p.). The total metabolism was almost completely depressed by adminstration of 4-methylpyrazole, which inhibited not only class I ADH, but also class III ADH and catalase pathway. Taken together with the results from in vitro studies, the alcohol metabolism accounting for about 30% of the total one, which was independent to class I ADH and catalase, was suggested to be due to acidic ADH.Thus, it was concluded that acidic ADH (Class III) may play an important role in alcohol metabolism in vivo and the contributions of the two major ADHs (Class I and III) to the metabolism were reversely regulated in response to the intracellular hydrophobicity. Less