STEROIDOGENESIS IN THE OVARIAN FOLLICLE OF MEDAKA (ORYZIAS-LATIPES, ADAILY SPAWNER) DURING OOCYTE MATURATION

The metabolism of C-14-labeled steroid precursors by cell-free homogen ates of medaka (Oryzias latipes, a daily spawner) ovarian follicles at 12 different developmental stages was examined using thin layer chrom atography (TLC). The radioactive metabolites produced were identified and tested for their ability to induce germinal vesicle breakdown (GVB D) in oocytes in an in vitro homologous bioassay. When homogenates of follicles isolated during oocyte maturation were incubated with C-14-l abeled 17 alpha-hydroxyprogesterone roxyprogesterone, 13 metabolites w ere detected in TLC. Among these metabolites, one metabolite exhibited very high maturation inducing activity by the in vitro bioassay. This metabolite was identified as 17 alpha,20 beta-dihydroxy-4-pregnen-3-o ne (17 alpha,20 beta-DP) by its chromatographic mobility in TLC and re crystallization to constant specific activity. 17 alpha,20 beta-DP pro duction was high in follicles collected between 10 and 6 hr before spa wning. A much less biologically active metabolite, 17 alpha,20 beta-di hydroxy-5 beta-pregnane-3-one appeared in follicles immediately after the formation of 17 alpha,20 beta-DP. A similar pattern of steroidogen esis was observed when the follicles were incubated with C-14-labeled pregnenolone and progesterone. The timely synthesis of 17 alpha,20 bet a-DP in medaka at the onset of oocyte maturation, together with the de monstration that this progestogen is the most potent inducer of oocyte maturation in vitro, provides further evidence that 17 alpha,20 beta- DP is the naturally occurring maturation-inducing hormone in the medak a. The results also suggest that the conversion of 17 alpha,20 beta-DP to its SP-reduced metabolite may be an inactivation process.