Distinct testicular 17-ketosteroid reductases, one in interstitial tissue and one in seminiferous tubules. Differential modulation by testosterone and metabolites of testosterone.

The final step in the biosynthesis of testosterone is the reduction of androstenedione, which is catalyzed by the microsomal enzyme 17-ketosteroid reductase. Evidence is presented which suggests that there are two distinct 17-ketosteroid reductases in rat testes, one in interstitial tissue and one in seminiferous tubules. The two enzymes have different pH optima, 5.6 for the one from interstitial tissue and 6.5 for the one from seminiferous tubules. At the optimum pH, a 70-fold difference in Km values was observed, 17 muM for the interstitial tissue enzyme and 0.25 muM for the enzyme from seminiferous tubules. Testosterone and metabolites of testosterone have very different effects of each of these enzyme activities. The interstitial tissue enzyme activity is inhibited by testosterone and several 5alpha-reduced metabolites of testosterone and by estrogens. The most potent inhibitor of the steroids investigated was 5alpha-androstane-3alpha, 17beta-diol, followed by 17beta-estradiol approximately equal to dihydrotestosterone greater than testosterone greater than estrone greater than estriol. 5alpha-Androstane-3alpha, 17beta-diol and 17beta-estradiol were shown to act by competitive inhibition with apparent Ki values of 2.2 and 3.7 muM, respectively. In contrast, it was demonstrated that among the above steroids, only dihydrotestosterone inhibits the 17-ketosteroid reductase activity of seminiferous tubules and this inhibition was only observed at very high concentrations of inhibitor. Testosterone stimulated the 17-ketosteroid reductase activity of seminiferous tubules. 5alpha-Androstane-3alpha, 17beta-diol at low concentrations stimulated the enzyme activity from seminiferous tubules, while it had no effect at high concentrations. The remainder of the steroids tested had no effect on the 17-ketosteroid reductase activity of seminiferous tubules. The difference in response of the two enzyme activities suggests a mechanism for local regulation of testosterone synthesis in each testicular compartment that does not involve directly pituitary gonadotropins.