Characterizations of mouse hepatic microsomal monooxygenase catalyzing 11beta-hydroxylation of osaterone acetate.

Osaterone acetate (17alpha-acetoxy-6-chloro-2-oxa-4,6-pregnadiene-3,20-dione, OA) is a new steroidal antiandrogen. There is a marked species difference in the metabolism of OA in that 11beta-hydroxylated metabolites are found in the plasma, feces, and urine of mice after oral administration of OA, but there is very little metabolism in rats and humans. OA reduces the adrenal gland weight in mice, but not in rats, and this effect in mice might be explained by the species difference in 11beta-hydroxylation activity. The objectives of this study were to elucidate the enzyme(s) involved in this particular oxidation and to explain the species difference observed. Mouse hepatic microsomes oxidize OA to 11beta-OH OA, and this oxidation requires NADPH as a cofactor. The use of various competitive and allosteric inhibitors of cytochrome P450 and flavin-containing monooxygenase (i.e. CO, N-octylamine, and methimazole) showed that the oxidation of OA was catalyzed by cytochrome P450. In microsomes from mice pretreated with phenobarbital (a CYP2B-selective inducer), 3-methylcholanthrene (a CYP1A-selective inducer), pregnenolone-16alpha-carbonitrile (a CYP3A-selective inducer), and EtOH (a CYP2E-selective inducer), an increase in the rates of oxidation was seen only in microsomes from EtOH-treated animals. However, metyrapone, a selective inhibitor for enzymes of the cytochrome P45011B and P4502B family, inhibited mouse hepatic microsomal 11beta-hydroxylation by < 30%. The results obtained showed that the production of 11beta-OH OA may be catalyzed by a novel cytochrome P450 in mouse liver.