Steroidogenesis in the Ovarian Follicle of Medaka (Oryzias latipes, a daily spawner) during Oocyte Maturation: (oocyte maturation/maturation-inducing hormone/17 α,20β-dihydroxy-4-pregnen-3-one/ medaka/ teleost).

The metabolism of C-labeled steroid precursors by cell-free homogenates of medaka (Oryzias latipes, a daily spawner) ovarian follicles at 12 different developmental stages was examined using thin layer chromatography (TLC). The radioactive metabolites produced were identified and tested for their ability to induce germinal vesicle breakdown (GVBD) in oocytes in an in vitro homologous bioassay. When homogenates of follicles isolated during oocyte maturation were incubated with C-labeled 17α-hydroxyprogesterone, 13 metabolites were detected in TLC. Among these metabolites, one metabolite exhibited very high maturation inducing activity by the in vitro bioassay. This metabolite was identified as 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP) by its chromatographic mobility in TLC and recrystallization to constant specific activity. 17α,20β-DP production was high in follicles collected between 10 and 6 hr before spawning. A much less biologically active metabolite, 17α,20β-dihydroxy-5β-pregnane-3-one appeared in follicles immediately after the formation of 17α,20β-DP. A similar pattern of steroidogenesis was observed when the follicles were incubated with C-labeled pregnenolone and progesterone. The timely synthesis of 17α,20β-DP in medaka at the onset of oocyte maturation, together with the demonstration that this progestogen is the most potent inducer of oocyte maturation in vitro, provides further evidence that 17α,20β-DP is the naturally occurring maturation-inducing hormone in the medaka. The results also suggest that the conversion of 17α,20β-DP to its 5β-reduced metabolite may be an inactivation process.