Troglitazone is a competitive inhibitor of 3 beta-hydroxysteroid dehydrogenase enzyme in the ovary

OBJECTIVE: Troglitazone is a potent inhibitor of progesterone release from porcine granulosa cells. This is associated with a marked increase in pregn enolone secretion, implicating inhibition of the 3 beta -hydroxysteroid deh ydrogenase enzyme. This study determined whether troglitazone is a direct i nhibitor of 3 beta -hydroxysteroid dehydrogenase activity. STUDY DESIGN: Homogenates of porcine granulosa cells underwent,classic enzy me kinetic analysis through Lineweaver-Burke and Dixon plotting. Human ovar ian homogenates were also assayed for the effects of troglitazone on 3 beta -hydroxysteroid dehydrogenase enzyme activity Enzyme kinetics data were an alyzed by the HyperKinetics software program. Analysis of variance was used to determine statistical significance for human ovarian homogenate experim ents. RESULTS: In porcine granulosa cells Lineweaver-Burke analysis found that tr oglitazone inhibition of 3 beta -hydroxysteroid dehydrogenase enzyme activi ty was competitive in nature, with 5 mug/mL troglitazone increasing the app arent Michaelis constant from 1.3 to 4.3 mu mol/L (no change in maximum vel ocity). Dixon plot analysis demonstrated that the inhibition constant for t roglitazone of 3 beta -hydroxysteroid dehydrogenase is approximately 6.5 mu g/ml, which is in the same order of magnitude as its therapeutic concentrat ion in blood. Troglitazone also significantly decreased the activity of 3 b eta -hydroxysteroid dehydrogenase in homogenates of human ovarian tissue. CONCLUSION: We conclude that troglitazone can inhibit steroidogenesis in th e ovary by direct competitive inhibition of 3 beta -hydroxysteroid dehydrog enase.