Use of a radioimmunoassay which detects C21 steroids with a 17, 20beta-dihydroxyl configuration to identify and measure steroids involved in final oocyte maturation in female plaice (Pleuronectes platessa).

A radioimmunoassay (RIA) has been developed to detect a range of C21 (pregnane) steroids with a 17,20beta-dihydroxyl (17,20beta) configuration. In conjunction with reverse-phase high-performance liquid chromatography (HPLC), it identifies and quantifies the metabolites of 17,20beta-dihydroxy-4-pregnen-3-one, the putative "maturation-inducing steroid" in female plaice Pleuronectes platessa. Total levels of 17,20beta metabolites which can be extracted from plasma or urine with diethyl ether (i.e., free steroids) are very low (<3 ng/ml). However, total levels of 17,20beta metabolites which can be released by solvolysis (i.e., sulphated steroids) are very high (up to 1 microg/ml in plasma and 10 microg/ml in urine). On HPLC, these sulphated metabolites have been identified (in order of abundance in plasma) as: 5beta-pregnane-3alpha,17,20beta-triol, 5beta-pregnane-3beta,17,20beta-triol, 17, 20beta-dihydroxy-4-pregnen-3-one, and 17, 20beta-dihydroxy-5beta-pregnan-3-one. These steroids are absent from plasmas of fish which have not yet begun final oocyte maturation. The results support the hypothesis that 17, 20beta-dihydroxy-4-pregnen-3-one is the maturation-inducing steroid in plaice but that it is rapidly metabolised to render it inactive. The results also show that the '17,20beta'-RIA, in combination with an overnight acid solvolysis procedure, is a useful procedure for monitoring the effects of exogenous factors (such as gonadotrophin injections) on final oocyte maturation in female plaice.