Comparison of the interaction of pyrazole and its metabolite 4-hydroxypyrazole with rat liver microsomes

Pyrazole is known to interact with and to induce cytochrome P-450 IIE1. Since pyrazole is oxidized by rat liver microsomes to 4-hydroxypyrazole, and several of the actions of pyrazole have been ascribed to its metabolite, experiments were conducted to evaluate the interactions of 4-hydroxypyrazole with microsomes, and to compare these to pyrazole itself. Rats were infected with doses of 4-hydroxypyrazole ranging from 2 to 100 mg/kg body weight/day for 2 days. A slight increase of total cytochrome P-450 was observed at low doses, folowed by a decrease at higher concentrations. NADPH-cytochrome P-450 reductase activity was not affected. The oxidation of aniline or dimethylnitrosamine was increased about 50% by the 4-hydroxypyrazole treatment; however, this extent of increase was much less than that produced by pyrazole treatment. In vitro, 4-hydroxypyrazole produced a type II binding spectrum with microsomes, with a peak at about 425 nm and a trough at about 395 nm. The affinity for 4-hydroxypyrazole was increased from a value of about 0.60 mM in control microsomes to a value of about 0.40 mM in microsomes from pyrazole-treated rats. These values are 2-fold greater than those observed with pyrazole as the ligand. 4-Hydroxypyrazole inhibited the microsomal oxidation of ethanol; kinetics of inhibition were mixed. The apparent K(l) for 4-hydroxypyrazole inhibition of ethanol oxidation by microsomes was about 4 mM, which is about an order of magnitude greater than that for pyrazole. The in vivo and in vitro interactions of 4-hydroxypyrazole with microsomes appear to be similar to those described for pyrazole; however, these interactions are considerably less effective than those of the parent drug, pyrazole. Thus, although some actions of pyrazole may be due to the metabolite 4-hydroxypyrazole, it appears that the induction of P-450 IIE1 and the in vitro interactions of pyrazole with microsomes is not likely to be mediated by prior metabolism of pyrazole to 4-hydroxypyrazole.