Studies of the Human Testis. V. Properties of Δ5-3β and 17β-Hydroxysteroid Dehydrogenases in the Biosynthesis of Testosterone from Dehydroepiandrosterone

Abstract

The properties of Δ⁵-β3-hydroxysteroid dehydrogenase and 17β-hydroxysteroid dehydrogenase in the human testis were examined using cell-free homogenates with added cofactors. Michaelis constants of the Δ⁵-β3-hydroxysteroid dehydrogenase enzyme at 37 C and pH 7.4 were 8.2 × 10^–7M for dehydroepiandrosterone and 2.9 × 10^–6M for androstenediol. The optimal pH for both substrates was approximately 8.15. Dehydroepiandrosterone and androstenediol are competitive substrates for the enzyme. When free and conjugated C₁₉ steroids in the order of 10^–6 were added, androstenedione and testosterone inhibited the enzyme activity for dehydroepiandrosterone while the activity for androstenediol was inhibited by addition of dehydroepiandrosterone and its sulfate as well as by androstenedione and testosterone. 17β-Hydroxysteroid dehydrogenase had two apparent Michaelis constants for dehydroepiandrosterone, 3.3 × 10^–6M at low substrate concentrations and 1 × 10^–5M at high substrate concentrations. The enzyme activities for dehydroepiandrosterone and androstenedione were found to be enhanced by addition of the 17β-hydroxysteroids examined and slightly inhibited by addition of dehydroepiandrosterone-sulfate and androstenediol-3-monosulfate. Androstenedione caused an inhibition of the 17β-hydroxysteroid dehydrogenase for dehydroepiandrosterone. The interconversion between androstenedione and testosterone by the enzyme favored testosterone formation. Following simultaneous incubation of ³H-dehydroepiandrosterone and ¹⁴Candrostenediol in equal amounts, initially more testosterone was produced from dehydroepiandrosterone than from androstenediol under the conditions employed, while subsequently with accumulation of androstenediol more testosterone was produced from androstenediol.