Product-inhibition of testicular steroidogenic enzymes in the marmoset Saguinus oedipus

Abstract

It has been reported that the marmoset testis converts pregnenolone to testosterone through the delta-4 pathway, with progesterone and 17-hydroxyprogesterone the major intermediate metabolites. The following studies were to determine the inhibition of specifie testicular steroidogenic enzymes by the products of these enzymes, and to verify our initial delta-4 results with the marmoset testis. Marmoset testicular fragments (50 mg) were incubated with 10 μCi (200nmol) of the following tritium-labelled substrates: pregnenolone, progesterone, 17-hydroxyprogesterone, androstenedione. Radiolabelled pregnenolone was incubated in the presence of unlabelled (200 nmol) progesterone or unlabelled 17-hydroxypregnenolone. Radiolabelled progesterone was incubated with 200nmol of unlabelled 17-hydroxyprogesterone, while radiolabelled 17-hydroxyprogesterone was incubated with 200nmol of unlabelled androstenedione. Radiolabelled androstenedione was incubated with 200 nmol of unlabelled testosterone. All radiolabelied incubates were compared to appropriate controls receiving no unlabelled product. Unlabelled progesterone decreased the metabolism of labelled pregnenolone by 19.6% compared to that of control incubates, while unlabelled 17-hydroxypregnenolone had no detectable effect upon the metabolism of pregnenolone. 17-Hydroxypregnenolone and dehydroepiandrosterone were not identified in any of the pregnenolone incubates. Unlabelled 17-hydroxyprogesterone substantially decreased the metabolism of progesterone, resulting in a 26.6% decline in the metabolism of progesterone compared to controls. Similarly, unlabelled androstenedione decreased the metabolism of 17-hydroxyprogesterone by 20.3% compared to controls receiving no unlabelled androstenedione. Unlabelled testosterone had no effect upon the metabolism of androstenedione by marmoset testicular fragments. These results suggest that the products of selected marmoset testicular steroidogenic enzymes can competitively inhibit the metabolism of their respective substrates, and that pregnenolone is converted to testosterone through the delta-4 pathway in the marmoset testis.