DIFFERENTIAL STEROIDOGENIC ENZYME-ACTIVITIES IN DIFFERENT POPULATIONS OF RAT LEYDIG-CELLS

Previous reports have described the presence of 2 distinct populations of Leydig cells (I and II) in the mature rat testis which differ in hCG[human chorionic gonadotropin]-stimulated testosterone production. The present study examines differences in steroidogenic enzyme activity between the different Leydig cell populations. Population I may be further separated into 2 subpopulations (IA and IB) by centrifugation of dispersed testicular cells in a discontinuous Metrizamide gradient. The luteinizing hormone receptor number, as measured by radiolabeled hCG binding, was the same in all 3 populations (.apprx. 100 f[femto]mol/106 Leydig cells). hCG-stimulated testosterone production was highest in population II (253 ng/106 Leydig cells), while testosterone production in population IB (183 ng/106 Leydig cells) was markedly greater than that in population IA (51 ng/106 Leydig cells). The activities of the key steroidogenic enzymes, .delta.5-3.beta.-hydroxysteroid dehydrogenase-isomerase, 17-hydroxylase, C17-C20 lyase, and 17-ketosteroid reductase, were measured in the 3 populations using 3H-labeled substrates. .delta.5-3.beta.-Hydroxysteroid dehydrogenase-isomerase activity did not differ between populations IA and IB, but both were 2-fold higher than that in population II. 17-Hydroxylase activity did not differ between populations IB and II, but was significantly lower in IA. Differences in C17-C20 lyase activity among the 3 populations paralleled differences in hCG-stimulated testosterone production. Population II showed a 3.6-fold higher activity, and population IB showed a 2.4-fold higher activity than that in population IA. 17-Ketosteroid reductase activity did not differ markedly among the 3 populations. Differences in maximal testosterone production among the 3 Leydig cell populations reflect, at least in part, differences in the activity of C17-C20 lyase.