Stoichiometric Inhibition of Various 3β-Hydroxysteroid Dehydrogenases by a Substrate Analogue

A synthetic steroid (2α-cyano-4, 4, 17α-trimethyl-androst-5-en-17β-ol-3-one) behaves as a stoichiometric inhibitor of 3β-hydroxysteroid dehydrogenase in a purified extract of testosterone-induced Pseudomonas testosteroni, and in homogenates of bovine and rat adrenal tissues. This inhibitor titrates the activity of the bacterial 3(17-)β-hydroxysteroid dehydrogenase with three 3β-and two 17β-hydroxysteroidal substrates, and that of the mammalian 3β-hydroxysteroid dehydrogenases with the 3β-hydroxysteroidal substrates. Human adrenal tissue, hyperplastic due to a selectively deficient activity of 3β-hydroxysteroid dehydrogenase, does not bind the inhibitor, while suitable control tissue does. The inhibitor-binding capacity of the purified bacterial preparation is of the same order of magnitude as the amount of inhibitor required to activate the 3β-hydroxysteroid dehydrogenase in this preparation. These results suggest that the prolonged effects of this steroidal cyano-ketone in vivo result, at least in part, from its extraordinarily tight binding very near to the active center of 3βhydroxysteroid dehydrogenase.