Kinetic properties of steroid 19-hydroxylase and estrogen synthetase from porcine ovary microsomes.

Abstract

Abstract Androstenedione and 19-hydroxyandrostenedione are mutually competitive inhibitors in the steroid aromatization reaction catalyzed by a small-particle fraction from sow ovary. Apparent Michaelis constants ( K M ) of 1.0μM and 1.7μM, and inhibition constants ( K I ) of 0.4μM and 2.0μM, were calculated for androstenedione and 19-hydroxyandrostenedione respectively, using data from competitive inhibition experiments. The K M for androstenedione calculated from simple single substrate experiments was 0.8μM. Aromatization of both androstenedione and 19-hydroxyandrostenedione, as well as 19-hydroxylation of androstenedione, was inhibited by carbon monoxide. The aromatization reactions had Warburg constants of 0.9 for androstenedione and 2.4 for 19-hydroxyandrostenedione, and 19-hydroxylation of androstenedione had a Warburg constant of 1.4. The results are consistent with the presence of a single steroid binding site on a single enzyme complex, estrogen synthetase, which catalyzes all three of these reactions.