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A further study of 17 beta-hydroxysteroid oxidoreductase in the human testis: mechanism of in vitro activation.

The mechanism and specificity of product activation of 17 beta-hydroxysteroid oxidoreductase for interconversion of androstenedione and testosterone were investigated using cell-free homogenates of human testes as an enzyme preparation. The reactions of oxidation and reduction catalyzed by the enzyme followed the Michaelis-Menten formula Addition of testosterone to the incubation medium resulted in increased Vmax and decreased the Km of reductase function of the enzyme. Androstenedione added to the incubation medium increased both Vmax and Km of the enzyme for the oxidase function. The apparent Km of the reductase function for [3H] androstenedione as a substrate in the presence of a fixed concentration of [14C] testosterone was found identical with the apparent Kd for the androstenedione as an effector (activator) for the oxidase function of the enzyme calculated from the simultaneous increase of [14C] androstnedione produced from the fixed concentration of [14C] testosterone. Similarly, the apparent Km of the oxidase function for [3H] testosterone as a substrate in the presence of a fixed concentration of [14C] androstenedione was identical to the apparent Kd for the testosterone as an effector (activator) for the reductase function of the enzyme calculated from the simultaneous increase of [14C] testosterone produced from the fixed concentration of [14C] androstenedione. None of the free 17-oxo and 17 beta-hydroxy steroids examined showed substrate activation or product inhibition of the enzyme except for androstenedione which showed product inhibition at high concentration. The present findings in combination with our previous data suggest the presence of two sites on the 17 beta-hydroxysteroid oxidoreductase of the human testis, one specific for androstenedione and the other specific for testosterone, each serving as an active site for the substrate and also as an activation site for the other active site.