Abstract

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.