4-Methylpyrazole inhibits fatty acyl coenzyme synthetase and diminishes catalase-dependent alcohol metabolism: has the contribution of alcohol dehydrogenase to alcohol metabolism been previously overestimated?

Alcohol dehydrogenase (ADH)-deficient deer mice were used as an animal model to investigate the effect of 4-methylpyrazole on alcohol metabolism. After intraperitoneal dosing of these mutant mice with 4-methylpyrazole, rates of ethanol and methanol metabolism in vivo were decreased significantly, by 41% and 35%, respectively. In perfused liver, rates of ethanol metabolism were also decreased up to 61% by 100 microM 4-methylpyrazole. Further, when livers were perfused with methanol, a selective substrate for catalase, rates of methanol metabolism were decreased by 64% by 4-methylpyrazole. It was further determined that 4-methylpyrazole administration caused negligible changes in total hepatic catalase activity and in rates of oxidation of ethanol by isolated microsomes; rather, it acts on catalase-dependent alcohol metabolism by limiting the supply of H2O2. In this study, 4-methylpyrazole inhibited fatty acyl CoA synthetase competitively in liver homogenates. Fatty acyl CoA synthetase is a key enzyme involved in the supply of substrate for peroxisomal oxidation of alcohols via catalase-H2O2. When palmitate was studied, rates of formaldehyde production from methanol were reduced competitively by 4-methylpyrazole; however, when the product palmitoyl CoA was used, the addition of 4-methylpyrazole did not alter activity. 4-Methylpyrazole also inhibited fatty acyl CoA synthetase activity measured directly from CoA disappearance. These data indicate that fatty acyl CoA synthetase is inhibited by 4-methylpyrazole, thus reducing the availability of H2O2 for catalase-dependent alcohol metabolism. Inhibition of methanol metabolism in deer mice expressing ADH indicates that this phenomenon also occurs in species with ADH. Taken together, these data support the hypothesis that the contribution of ADH to alcohol metabolism may have been previously overestimated.