Carbonyl reduction of metyrapone in human liver

Carbonyl reduction was investigated in cytosolic and microsomal fractions of human liver using the ketone metyrapone as a substrate. The cytosolic enzyme has a stronger preference for NADPH over NADH than the microsomal enzyme: the former shows only 14% of the NADPH-supported activity while the latter exhibits 36% activity with NADH. Barbitone and quercitrin, the classic inhibitors of carbonyl reductases, do not affect metyrapone reduction in either fraction. Dicumarol and indomethacin, the specific inhibitors of NAD(P)H : quinone-oxidoreductase and dihydrodiol dehydrogenase, respectively, only slightly decreased metyrapol formation. In contrast, 5α-dihydrotestosterone, the active form of the androgen steroid testosterone, inhibited metyrapone reduction very strongly in the microsomal fractions and is postulated to be the physiological substrate of the enzyme. This resembles the situation in mouse liver [E. Maser and K. J. Netter, Biochem Pharmacol 38: 3049–3054, 1989] wherem icrosomal metyrapone reductase was inhibited by steroids and the purified enzyme was demonstrated to mediate androsterone oxidation. Immunoblot analysis revealed antigenic cross-reaction of antibodies against the 34 kDa metyrapone reductase from mouse liver microsomes with the homologous protein in human liver microsomes pointing to structural homologies between the respective enzymes of the two species. These results—together with previous findings, which have shown that there exist functional as well as structural relationships between microsomal mouse liver metyrapone reductase and 3α-hydroxysteroid dehydrogenase from Pseudomonas testosteroni (E. Maser, U. Oppermann and K. J. Netter, EurJ Pharmacol 183: 1366, 1990]—suggest that metyrapone reduction in human liver microsomes might be catalysed by a microsomal hydroxysteroid dehydrogenase.