5β Reductive pathway of progesterone metabolism in the amphibian ovarian cytosol

The metabolism of ³H-progesterone by the 105,000 x g supernatant (cytosol) fraction of Rana pipiens follicles containing fully grown oocytes was studied using thin layer chromatography. At 18°C, progesterone was rapidly metabolized to products that co migrated with and appeared in the sequence: pregnanedione, pregnanolone and pregnanediol. The pregnanedione fraction appeared to consist primarily of the 5β stereoisomer. At 4°C, 50% of the progesterone was metabolized by the cytosol in 1 to 2 hr compared to 15 to 20 min at 24°C. As a 3 keto-Δ⁴ steroid target tissue, the R. pipiens ovary appears to be unique in that both the 5β and 5α reduced isomers are nearly as effective as the parent compound (progesterone) in inducing a physiological response (germinal vesicle breakdown). Pregnanolone and pregnanediol were relatively ineffective in inducing germinal vesicle breakdown. Thus, the stepwise reduction of pregnanedione may terminate a steroid specific action in the ovarian follicle.