Oxidation of the alcohol dehydrogenase inhibitor pyrazole to 4-hydroxypyrazole by microsomes. Effect of cytochrome P-450 inducing agents.

Pyrazole and its analogues are widely used to inhibit alcohol dehydrogenase and to block the metabolism of ethanol. Experiments were conducted to demonstrate that pyrazole is oxidized to 4-hydroxypyrazole by isolated rat liver microsomes. A HPLC procedure employing UV and electrochemical detection was developed for the separation and quantitation of 4-hydroxypyrazole. Pyrazole metabolism was NADPH-dependent, sensitive to inhibition by carbon monoxide, and was depressed in the presence of other substrates such as aniline or ethanol. Prior treatment of rats with either pyrazole or 4-methylpyrazole resulted in an increase in pyrazole oxidation to 4-hydroxypyrazole by microsomes. Increases were observed when rates were expressed either per mg of protein or per nmol of P-450. Microsomes from pyrazole- and 4-methylpyrazole-treated rats had Km values for pyrazole of about 0.29 and 0.14 mM, respectively, and Vmax values of about 0.5 and 0.7 nmol of 4-hydroxypyrazole per min per mg of protein, respectively. Chronic consumption of ethanol for 24 days resulted in an increase in pyrazole oxidation (per mg of protein and per nmol of P-450) as compared to pair-fed controls. By contrast, phenobarbital treatment lowered the rate of production of 4-hydroxypyrazole. Treatment with 3-methylcholanthrene resulted in an increase in pyrazole oxidation when rates were expressed per mg of protein, but not per nmol of P-450. These results show that pyrazole is oxidized to 4-hydroxypyrazole by microsomes in a P-450-dependent manner and that this metabolism can be increased by certain inducers, e.g. pyrazole, 4-methylpyrazole, and chronic ethanol treatment.