IDENTIFICATION OF MULTIPLE STEROID HYDROXYLASES IN DAPHNIA-MAGNA AND THEIR MODULATION BY XENOBIOTICS

Steroid hydroxylase activities were characterized in Daphnia magna and evaluated for potential use as biomarkers of xenobiotic exposure. Mic rosomes prepared from Daphnia magna generated a single NADPH-dependent metabolite of [C-14]testosterone. However, intact daphnids excreted a t least 10 polar metabolites of [ C]testosterone into the test medium. Six of these metabolites were identified as 2alpha-, 16beta-, 6beta-, 6alpha-, 7alpha-, and 15alpha-[C-14]hydroxytestosterone. The unidenti fied metabolites are also presumed to be hydroxylated products of test osterone, based on their relative migrations during TLC. The inefficie nt metabolism of [C-14]testosterone during the in vitro microsomal inc ubations may have been due to the release of P450 inhibitors during mi crosome preparation. Exposure of daphnids to the P450 modulators pheno barbital, beta-naphthoflavone, piperonyl butoxide, and malathion diffe rentially inhibited the steroid hydroxylase activities. Results from t his study indicate that Daphnia magna expresses several P450 enzymes a nd that these enzymes are differentially modulated by xenobiotic expos ure. Steroid hydroxylase activities may serve not only as a biomarker of toxicant exposure, but also as a predictor of toxicant effects invo lving perturbations of steroid hormone homeostasis.