Roles of Two Major Alcohol Dehydrogenases, ADH1 (Class I) and ADH3 (Class III), in the Adaptive Enhancement of Alcohol Metabolism Induced by Chronic Alcohol Consumption in Mice.

AIMS: It is still unclear which enzymes contribute to the adaptive enhancement of alcohol metabolism by chronic alcohol consumption (CAC). ADH1 (Class I) has the lowest Km for ethanol and the highest sensitivity for 4-methylpyrazole (4MP) among ADH isozymes, while ADH3 (Class III) has the highest Km and the lowest sensitivity. We investigated how these two major ADHs relate to the adaptive enhancement of alcohol metabolism. METHODS: Male mice with different ADH genotypes (WT, Adh1-/- and Adh3-/-) were subjected to CAC experiment using a 10% ethanol solution for 1 month. Alcohol elimination rate (AER) was measured after ethanol injection at a 4.0 g/kg dose. 4MP-sensitive and -insensitive AERs were measured by the simultaneous administration of 4MP at a dose of 0.5 mmol/kg in order to estimate ADH1 and non-ADH1 pathways. RESULTS: AER was enhanced by CAC in all ADH genotypes, especially more than twofold in Adh1-/- mice, with increasing ADH1 and/or ADH3 liver contents, but not CYP2E1 content. 4MP-sensitive AER was also increased by CAC in WT and Adh3-/- strains, which was greater in Adh3-/- than in WT mice. The sensitive AER was increased even in Adh1-/- mice probably due to the increase in ADH3, which is semi-sensitive for 4MP. 4MP-insensitive AER was also increased in WT and Adh1-/- by CAC, but not in Adh3-/- mice. CONCLUSION: ADH1 contributes to the enhancement of alcohol metabolism by CAC, particularly in the absence of ADH3. ADH3 also contributes to the enhancement as a non-ADH1 pathway, especially in the absence of ADH1.