Troglitazone is a competitive inhibitor of 3β-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 pregnenolone secretion, implicating inhibition of the 3β-hydroxysteroid dehydrogenase enzyme. This study determined whether troglitazone is a direct inhibitor of 3β-hydroxysteroid dehydrogenase activity. STUDY DESIGN: Homogenates of porcine granulosa cells underwent classic enzyme kinetic analysis through Lineweaver-Burke and Dixon plotting. Human ovarian homogenates were also assayed for the effects of troglitazone on 3β-hydroxysteroid dehydrogenase enzyme activity. Enzyme kinetics data were analyzed by the HyperKinetics software program. Analysis of variance was used to determine statistical significance for human ovarian homogenate experiments. RESULTS: In porcine granulosa cells Lineweaver-Burke analysis found that troglitazone inhibition of 3β-hydroxysteroid dehydrogenase enzyme activity was competitive in nature, with 5 μg/mL troglitazone increasing the apparent Michaelis constant from 1.3 to 4.3 βmol/L (no change in maximum velocity). Dixon plot analysis demonstrated that the inhibition constant for troglitazone of 3β-hydroxysteroid dehydrogenase is approximately 6.5 μg/mL, which is in the same order of magnitude as its therapeutic concentration in blood. Troglitazone also significantly decreased the activity of 3β-hydroxysteroid dehydrogenase in homogenates of human ovarian tissue. CONCLUSION: We conclude that troglitazone can inhibit steroidogenesis in the ovary by direct competitive inhibition of 3β-hydroxysteroid dehydrogenase.