In Vitro inhibition by ketoconazole of human testicular steroid oxidoreductases

An oral antimycotic agent, ketoconazole has been demonstrated to be an inhibitor of cytochrome P-450-dependent monooxygenases. To investigate its effect on steroid oxido-reductases, in vitro studies were carried out using subcellular fractions of human testes. Ketoconazole competitively inhibited activities of 3β-hydroxy-5-ene-steroid oxidoreductase/ isomerase and NADH-linked 20α-hydroxysteroid oxidoreductase for steroid substrate and the K_i values were 2.9 and 0.9 μM, respectively. In contrast, ketoconazole inhibited neither 17β-hydroxysteroid oxidoreductase nor NADPH-linked 20α-hydroxysteroid oxido-reductase, indicating that the two 20α-hydroxysteroid oxidoreductases are distinct. Further, ketoconazole inhibited non-competitively the above enzyme activities for the corresponding cofactors of NAD and NADH. From the binding mode of ketoconazole to cytochrome P-450 and the present findings, it appears likely that the agent binds to a site which is different from that of steroids or pyridine nucleotides.