Steroidogenic pathways to 17,20β-dihydroxy-4-pregnen-3-one and 17,20β,21-trihydroxy-4-pregnen-3-one in the ovarian follicles of the bambooleaf wrasse Pseudolabrus sieboldi

Intact follicles of the bambooleaf wrasse were incubated with different radioactively labeled steroid precursors during final oocyte maturation (FOM), and the maturation-inducing hormone (MIH) of this species was examined to compare the activities of the steroid metabolites produced on germinal vesicle breakdown (GVBD) in vitro. Of the metabolites obtained by thin layer chromatography, two steroids, 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) and 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S), both had the greatest effect on GVBD in vitro. During FOM, a shift in steroidogenic enzymes, a decrease in C17,20-lyase activity, and an increase in 20β-HSD activity were found. In addition to this shift, continuous high 21-hydroxylase activity throughout all the oocyte developmental stages was observed, and, in all likelihood, this 21-hydroxylase activity enables ovarian follicles to synthesize enough 17,21-P. Consequently, activated 20β-hydroxysteroid dehydrogenase converted 17-P and 17,21-P to 17,20β-P and 20β-S, respectively, during FOM. Thus, the present study not only provides evidence on the physiological role of 17,20β-P and 20β-S as MIH in the bambooleaf wrasse, but also details the enzymatic kinetics of 17,20β-P and 20β-S biosynthesis.