Studies of the Human Testis. VIII. Product Activation of 17β-Hydroxysteroid Oxidoreductase for Testosterone

Abstract

Cell-free homogenates prepared from human testis tissue were incubated with either 1.3 × 10⁻⁵m[4-¹⁴C]testosteroneand 2× 10⁻⁴m NADP or 1.3 × 10⁻⁵ [4-¹⁴C]androstenedione and 2 × 10⁻⁴m NADPH. Addition of non-radioactive androstenedione and testosterone to the incubation medium increased the formation of [¹⁴C]androstenedione from [4-¹⁴C]testosterone and [¹⁴C]testosterone from [4-¹⁴C]androstenedione, respectively, while addition of product nucleotide NADPH or NADP, respectively, decreased the conversions. The addition of androstenedione to the incubation medium changed the apparent optimal pH of 17β-hydroxysteroid oxidoreductase for testosterone from 8.6 to 8.0. It appears likely that in a cell-free system human testicular 17β-hydroxysteroid oxidoreductase not solubilized and still attached to membrane is activated by the product of the reaction catalyzed by the enzyme.