Inhibition of microsomal oxidation of ethanol by pyrazole and 4-methylpyrazole in vitro. Increased effectiveness after induction by pyrazole and 4-methylpyrazole.

Pyrazole and 4-methylpyrazole, which are inhibitors of alcohol dehydrogenase, were also found to be effective inhibitors of the oxidation of ethanol by liver microsomes (microsomal fractions) in vitro. Ethanol oxidation by microsomes from rats previously treated for 2 or 3 days with either pyrazole or 4-methylpyrazole appeared to be especially sensitive to inhibition in vitro by pyrazole or 4-methylpyrazole. The kinetics of inhibition by pyrazole or 4-methylpyrazole in all microsomal preparations were mixed, as the Km for ethanol was elevated while Vmax was lowered. However, Ki values for pyrazole (about 0.35 mM) and especially 4-methylpyrazole (about 0.03-0.10 mM) were much lower than those found with the saline controls (about 0.7-1.1 mM). In contrast, Ki values for dimethyl sulphoxide as an inhibitor of microsomal ethanol oxidation were similar in all microsomal preparations. Pyrazole and 4-methylpyrazole reacted with microsomes to produce type II spectral changes whose magnitude increased after treatment with either pyrazole or 4-methylpyrazole. Thus the increased inhibitory effectiveness of pyrazole and 4-methylpyrazole appears to be associated with increased interactions with the cytochrome P-450 isoenzyme(s) induced by these compounds. These isoenzymes have properties similar to those of the isoenzyme induced by chronic ethanol treatment. Therefore, caution is needed in the use of pyrazole or 4-methylpyrazole to assess pathways of ethanol metabolism, especially after chronic ethanol treatment, since these agents, besides inhibiting alcohol dehydrogenase, are also effective inhibitors of microsomal ethanol oxidation.