A novel steroid inhibitor for a rat 20alpha-hydroxysteroid dehydrogenase isozyme

20alpha-Hydroxysteroid dehydrogenase (20alpha-HSD, E.C.1.1.1.149) in rat luteal tissue, which catalyzes conversion of progesterone to a biologically inactive steroid, 20alpha-hydroxypregn-4-ene-3-one (20alpha-OHP), suppresses progesterone secretion into the circulation. An increase in 20alpha-HSD activity in luteal tissue in rats is a prerequisite for functional corpus luteum regression. This study was undertaken to find a steroid inhibitor for ovarian cytosolic 20alpha-HSD activity among derivatives based on progesterone structure. A derivative designated as STZ26 (D-homo-16-oxa-4-androstene-3,16alpha-dione) was found to inhibit potently 20alpha-HSD activity in cultured luteal cells. Ovarian 20alpha-HSD activity consists of two isoforms (HSD1 and HSD2). Kinetic analyses of STZ26 for HSD1 and HSD2 showed that the compound suppressed only HSD1 activity by competitive inhibition. Pseudopregnant rats were treated with STZ26 from 13 to 19 days after cervical stimulation. Either an elevation of peripheral 20alpha-OHP levels or a concomitant depletion of peripheral progesterone levels at the end of pseudopregnancy was considerably inhibited in treated animals, although not completely. The results showed that STZ26 is a biologically active inhibitor for HSD1 activity in the luteal tissue and suggested that the depletion of progesterone levels toward the end of pseudopregnancy is not solely due to the elevation of HSD1 activity.