The effects of 16α-cyanopregnenolone, 3-methylcholanthrene (3-MC), Clophen A 50 (Cl 50) and phenobarbital on the total amount of cytochrome P-450 and the metabolism of 4-androstene-3,17-dione in liver microsomes from rainbow trout were studied. Only 3-MC and CI 50 caused marked changes in the cytochrome P-450 levels and the cytochrome P-450-dependent steroid hydroxylase activities. A varying response to 3-MC and Cl 50 was seen in fish of different ages and sex. Different responses of 6β-hydroxylase towards α-naphtoflavone, SKF 525-A and metyrapone in vitro were seen in 3-MC- and Cl 50-treated fish when compared to control fish. It is suggested that endocrine factors may be involved in the regulation of cytochrome P-450-mediated metabolism in fish and the presence of multiple forms of cytochrome P-450 in trout liver is indicated.
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