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

Abstract

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 3beta-hydroxysteroid dehydrogenase enzyme. This study determined whether troglitazone is a direct inhibitor of 3beta-hydroxysteroid dehydrogenase activity. 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 3beta-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. In porcine granulosa cells Lineweaver-Burke analysis found that troglitazone inhibition of 3beta-hydroxysteroid dehydrogenase enzyme activity was competitive in nature, with 5 microg/mL troglitazone increasing the apparent Michaelis constant from 1.3 to 4.3 micromol/L (no change in maximum velocity). Dixon plot analysis demonstrated that the inhibition constant for troglitazone of 3beta-hydroxysteroid dehydrogenase is approximately 6.5 microg/mL, which is in the same order of magnitude as its therapeutic concentration in blood. Troglitazone also significantly decreased the activity of 3beta-hydroxysteroid dehydrogenase in homogenates of human ovarian tissue. We conclude that troglitazone can inhibit steroidogenesis in the ovary by direct competitive inhibition of 3beta-hydroxysteroid dehydrogenase.