| This study examined the ability of rainbow trout embryos (Oncorhynchus mykiss) to metabolize various steroids including pregnenolone (P 5), progesterone (P4), testosterone (T), estradiol (E 2), and estrone (E1), some of which are present in the oocytes of teleost fishes. In vitro incubations, followed by separation of steroids by high performance liquid chromatography (HPLC), were used throughout the study. Gas chromatography-mass spectroscopy (GC-MS) was used to identify some steroid metabolites, and steroid hormone concentration was measured by radioimmunoassay (RIA). The study demonstrated the presence of P5 in oocytes and embryos, and the ability of newly-hatched to yolk-absorbance stage embryos to convert P5 to a single metabolite, 7α-hydroxypregnenolone (7αOHP5). Using specific P450 inhibitors and P4501A1 immuno-neutralization, evidence was provided to show that P4501A1 is involved in 7αOHP 5 production. Several metabolites were derived from P4 metabolism, with 5α-pregnan-3β,7α-diol-20-one (5αPDO), identified by GC-MS, being the major end-product. A 5α-reduced P4, which was negatively correlated with the production of 5αPDO, was also tentatively characterized. The metabolites produced from radiolabelled T and E2 suggested the presence of 17β-hydroxysteroid dehydrogenase. These findings show that rainbow trout embryos metabolize various yolk steroids, and I hypothesized that the end-products are then excreted. The effect of o,p′ DDT and o,p′ DDE at 0.01 or 1.0 mg.l−1 on the in vitro steroid biosynthesis by ovarian follicles, and steroid metabolism by the embryos of rainbow trout was also investigated. Embryos were exposed in ovo and also co-incubated with the toxicants to test the hypothesis that direct toxicant-induced alteration of steroidogenic and catabolic enzymes contribute to a suppression of plasma steroids. The results indicated that steroidogenic enzyme activity in the ovarian follicle and embryonic tissue were not directly affected by o,p ′ DDT and o,p′ DDE at exposure levels of 0.01 and 1.0 mg.l−1, thus suggesting that the observed changes in plasma steroid levels following DDT and DDE exposure reported in the literatures cannot be explained on the basis of toxicant effects on steroid hormone processing, at least at the sub-lethal concentrations of o,p ′ DDT and o,p′ DDE used in the present studies. |
