ABSTRACT
Various fluorescent substrates have been used as specific indicators of induction or activity of different cytochrome P450 isozymes in both fish and mammalian species. In an attempt to identify additional definitive fluorescent substrates for use in fish, we examined a series of 7-alkoxyphenoxazones, 7-alkoxycoumarins and 7-alkoxyquinolines as substrates in O-dealkylation assays with hepatic microsomes from rainbow trout (Oncorhynchus mykiss). Microsomes were prepared after 48 hr of treatment with beta-naphthoflavone (beta-NF), pregnenolone-16 alpha-carbonitrile (PCN), phenobarbital (PB), isosafrole (ISF), or dexamethasone (DEX). Total P450 spectra were obtained, and spectral binding studies were performed. Microsomal O-dealkylation rates were greater after ISF treatment than after beta-NF treatment for 7-methoxy-, 7-ethoxy-, 7-propoxy- and 7-benzyloxyphenoxazones but not for 7-butoxyphenoxazone. DEX treatment resulted in a significant elevation of pentoxyphenoxazone metabolism (about a 144-fold increase) compared with microsomes induced by beta-NF (11-fold) and ISF (37-fold). The rates of dealkylation of the alkoxyphenoxazones by ISF-treated microsomes occurred in the following order: methoxy > ethoxy > propoxy > benzxyloxy > butoxy > pentoxy. When beta-NF-treated microsomes were used, the 7-alkoxyphenoxazones were metabolized as follows: methoxy > ethoxy > propoxy > butoxy > benzyloxy = pentoxy, while the order of metabolism of the 7-alkoxycoumarins was: ethoxy >> butoxy > propoxy = methoxy > benzyloxy > pentoxy. None of the other treatments significantly increased the rate of metabolism of any of the alkoxycoumarins. Treatment with beta-NF did not significantly elevate the rate of metabolism of any of the alkoxyquinolines. DEX treatment produced significant elevations in the rate of metabolism of benzyloxy-, ethoxy-, and butoxy- = pentoxy- = propoxyquinoline, in that order. ISF treatment significantly elevated the rate of metabolism of benzyloxy-, methoxy- and butoxyquinoline, in that order. These results suggest that some of these new fluorescent substrates can be used to characterize induction of rainbow trout hepatic microsomal monooxygenase activity by ISF and DEX, in addition to the commonly used ethoxyphenoxazone and ethoxycoumarin for the characterization of induction by beta-NF or other 3-methylcholanthrene-type P450 inducers. Distinction between ISF-type and beta-NF-type inducers in rainbow trout hepatic microsomes may best be made using 7-methoxycoumarin as a substrate. Distinction between ISF-type and DEX-type inducers and between beta-NF-type and DEX-type inducers may best be made using 7-methoxyphenoxazone as a substrate.(ABSTRACT TRUNCATED AT 400 WORDS)
