Identification of in vitro cytochrome P450 modulators to detect induction by prototype inducers in the mallard duckling (Anas platyrhynchos)

Seven modulators of mammalian monooxygenase activity were screened for thei r ability to selectively stimulate or inhibit in vitro monooxygenase activi ties of hepatic microsomes from mallard ducklings treated with phenobarbita l, beta-naphthoflavone, 3,3',4,4',5-pentachlorobiphenyl or vehicle. Microso mes were assayed fluorometrically for four monooxygenases: benzyloxy-, etho xy-, methoxy-, and pentoxyresorufin-O-dealkylase, in combination with each of the seven modulators. Four combinations: alpha-naphthoflavone and 2-meth ylbenzimidazole with benzyloxyresorufin, and Proadifen with methoxy- and et hoxyresorufin, respectively, were evaluated further. beta-Naphthoflavone-tr eated groups were clearly distinguished from the corn oil vehicle control g roup by all of the assays and by the effects of the modulators in three of the four assay/modulator combinations. Enzyme activities of the phenobarbit al and saline groups were statistically similar (P greater than or equal to 0.05) when assayed without modulator added, but each assay/modulator combi nation distinguished between these groups. The PCB-treated group was distin guished from the corn oil vehicle control group only for BROD activity, wit h or without the presence of modulator. Graphing of per cent. modulation of BROD activity Versus initial BROD activity provided the dearest distinctio n between all of the study groups. Identification of these selective in vit ro modulators may improve detection and measurement of low level cytochrome P450 induction in avian species. Also, both the monooxygenase activities i nduced and the impacts of the modulators indicated differences between mamm alian and avian cytochromes P450. (C) 1999 Elsevier Science Inc. All rights reserved.