17α,20β,21‐Trihydroxy‐4‐pregnen‐3‐one: the probable maturation‐inducing steroid of the Lusitanian toadfish
17,20β,21‐Trihydroxy‐4‐pregnen‐3‐one (17,20β,21‐P) was identified as the major metabolite of incubations of Lusitanian toadfish Halobatrachus didactylus ovarian follicles with [3H]‐17hydroxyprogesterone. The potency of several steroids in inducing germinal vesicle breakdown of follicle‐enclosed oocytes of Lusitanian toadfish was systematically examined by using an in vitro germinal vesicle breakdown (GVBD) bioassay. 17,20β‐Dihydroxy‐4‐pregnen‐3‐one (17,20β‐P) and 17,20β,21‐P, two confirmed maturation‐inducing steroids (MIS) in teleosts, were the most potent in inducing GVBD with ED50s ranging between 9 and 271 nM. Structure‐activity relationships followed similar patterns to what has been observed in similar bioassays, i.e. a vital requirement for 17‐ and 20β‐hydroxyl groups in C21 steroids and a reduction in activity of 14 and 5–6%, respectively, for 5‐pregnene and 5β‐pregnanes compared to 4‐pregnenes. Corticosteroids, testosterone and 17β‐oestradiol were ineffective. Folliculated oocytes stimulated by pituitary homogenate produced 17,20β,21‐P from endogenous substrates in amounts one order of magnitude higher than 17,20β‐P. These results strongly support the hypothesis that 17,20β,21‐P is the likely MIS in this species.
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Document Type: Research Article
Affiliations: Centro de Ciências do Mar, Universidade do Algarve, Campus de Gambelas, 8000-810 Faro, Portugal
Publication date: 2002-03-01