Effects of pyrazole and 3-amino-1,2,4-triazole on methanol and ethanol metabolism by the rat

Abstract

The metabolism of methanol- 14C and ethanol-1- 14C in rats was evaluated from the rates of 14CO 2 production. 3-Amino,1,2,4-triazole, a known catalase inhibitor, decreased by 10 and 35 per cent the rates of oxidation of ethanol and methanol, whereas pyrazole, an alcohol dehydrogenase inhibitor, decreased the rates 85 and 50 per cent respectively. However, the simultaneous use of both inhibitors gave the same effects produced by pyrazole alone. Thus the relative contributions in vivo to alcohol metabolism of rat liver alcohol dehydrogenase and catalase-mediated peroxidation, cannot be estimated only in this way. Rat liver alcohol dehydrogenase was purified 14·7 times. At pH 7·0 and 30°, the K m for methanol was 340 mM and for ethanol 0·26 mM. The V max/ e was 2·36 nM for methanol and 22·3 nM for ethanol (NADH × U −1 × 1 −1 × sec −1). 3-Amino-1,2,4-triazole inhibited the purified enzyme with a K i of 55 mM for methanol and 33 mM for ethanol. The K i of pyrazole was 2·3 mM for methanol and 2·2 mM for ethanol. The amount of alcohol dehydrogenase present in rat liver, with the found kinetic constants, can account for the ethanol oxidation in vivo, but fails to account, as methanol dehydrogenase, for the observed pyrazole-sensitive methanol oxidation. A mechanism for the complete oxidation of methanol to CO 2 and water through the concerted action of catalase and alcohol dehydrogenase is suggested. 3-Amino-1,2,4-triazole in a dose of 1 g/kg decreases more than 90 per cent of the catalatic activity of catalase, but under certain conditions in vitro, only about 50 per cent of the peroxidative activity of catalase towards methanol and ethanol. Consequently, the degree of catalase-mediated peroxidation should not be controlled or estimated from the residual catalatic activity when using catalase inhibitors. Pyrazole, at a dose of 0·3 g/kg, does not affect catalase activity 1 hr after administration, but decreases it more than 90 per cent after 24 hr. This effect is completely prevented in the presence of alcohol.