Identification of 17,20β,21-trihydroxy-4-pregnen-3-one as an oocyte maturation-inducing steroid in black porgy, Acanthopagrus schlegeli

The identity of the maturation-inducing steroid (MIS) in black porgy, Acanthopagrus schlegeli, a marine protandrous teleost, is unknown. Previous studies demonstrated that two teleost MISs, the progestins 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) and 17,20β-dihydroxy-4-pregnen-3-one (DHP) can induce maturation of black porgy oocytes in vitro. The purpose of the present study was to identify the major progestin produced during oocyte maturation (OM) in black porgy and investigate whether its secretion increases during this process. Females were injected twice with a LHRH analog to induce OM. Ovarian follicles undergoing OM were incubated in vitro with tritiated [3H]pregnenolone precursor and the tritiated products were extracted, purified, and identified by HPLC, TLC, acetylation, and recrystallization. Significant amounts of tritiated products were biosynthesized from [3H]pregnenolone that co-migrated with 20β-S but not with DHP on HPLC and TLC. Similar TLC profiles were obtained with the tritiated products isolated from the HPLC/TLC 20β-S fraction and standard 20β-S after the acetylation reaction. The identity of the tritiated products as 20β-S was confirmed by recrystallization. 20β-S had a slightly higher potency than DHP in the inducing in vitro final oocyte maturation. Plasma 20β-S concentrations increased significantly during the oocyte maturation after injection with a LHRH analog. The present data suggest that 20β-S is the MIS in black porgy.