Metabolism of Testosterone and 5α-Dihydrotestosterone in Vitro by the Seminiferous Tubules of the Mature Rat

One hundred mg seminiferous tubules (ST) from adult rat testes were incubated for 1 hr with testosterone-³H (T) or dihydrotestosterone-³H (DHT). The quantities of the metabolites, androstenedione (A), DHT, 3α-androstandiol (3α-Adiol) and its 3β-epimer (3β-Adiol) were quantified by double isotope dilution.

In incubations with 0.33 nmoles of T/ml medium the percent substrate converted to these metabolites was of the order of 11% for A, 1.5% for DHT and 4% for 3α-Adiol. As the concentration of T in the medium was raised the amount of DHT formed continued to increase while that of 3α- Adiol levelled off. When the concentration of T in the medium exceeded 5.0 nmoles/ml (5.0 ì 10⁻⁶M) the quantities of DHT formed exceeded that of 3α-Adiol. The reversal in ratio in favor of DHT appeared to be due to inhibition of 3α-oxidoreductase activity rather than to saturation of the 3a-oxidoreductase, its inhibition by 3α-Adiol or lack of cofactors: When DHT was used as the substrate up to 50 times more 3α-Adiol was formed than the maximum that could be obtained from T. Substituting NADH generating system for NADPH generating system did not alter 3α-oxidoreductase activity. Addition of 3α-Adiol to the incubation did not influence 3α-reduction of DHT. The inhibitory effect of T on 3a-reduction was confirmed by incubation of ST with 1.66 nmoles/ml DHT in the presence or absence of 15.0 nmoles/ml T. The presence of T reduced by 50% the amount of 3α-Adiol accumulated. Time-sequence studies indicated that DHT formation preceded that of 3α-Adiol. In incubations of ST with DHT, 3a-Adiol was the major metabolite (approx. 70% of the substrate). Small amounts of 3β-Adiol were also identified (approx. 1.5%).

The profile of metabolites formed by 2–4 mg interstitial tissue from T or DHT differed from that formed by ST under identical conditions. There was relatively more A and 3β-Adiol and less DHT and 3α-Adiol formed by IT than by ST. Thus the findings in incubations of ST were unlikely to be due to contaminating IT. Estradiol and estrone were not formed from T in detectable amounts by either the ST or the IT. (Endocrinology95: 1306, 1974)