2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD) INDUCES HEPATIC CYTOCHROMEP450-DEPENDENT ARACHIDONIC-ACID EPOXYGENATION IN DIVERSE AVIAN ORDERS- REGIOISOMER SELECTIVITY AND IMMUNOCHEMICAL COMPARISON OF THE TCDD-INDUCED P450S TO CYP1A4 AND 1A5

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) markedly induced cytochrome P450 (CYP)-dependent arachidonic acid metabolism in liver microsomes from hatchlings of four avian species belonging to four different orde rs: chick, pigeon, cormorant, and great blue heron, increasing formati on of arachidonic acid epoxides (EETs), monohydroxyeicosatetraenoic ac ids (HETEs), omega-1, and omega-2 OH arachidonic acid products by five fold or more. Microsomes from TCDD-induced hatchling chicks had the hi ghest activity and the least restricted EET regioselectivity, omega-OH arachidonic acid, the principal constitutive metabolite in chick and pigeon liver microsomes and a major product for cormorant and great bl ue heron was not induced by TCDD. Constitutive EET formation in avian liver microsomes was very low except in cormorant microsomes where 8,9 -EET was generated almost exclusively. Western blots of liver microsom es using polyclonal antisera to chick embryo-derived CYP1A4 and 1A5 re cognized two TCDD-induced bands in each of the species. The chick band s had the same molecular weights as CYP1A4 and 1A5 (55 and 55.5 kDa, r espectively) but those of the other species differed. Immunopurified a ntiserum monospecific for CYP1A5 recognized a band in microsomes from all of the avian species, and monospecific antiserum for CYP1A4 recogn ized a band in microsomes from chick, pigeon, and great blue heron. An tiCYP1A4 and 1A5 IgG immunoinhibited TCDD-induced mixed function oxida se activity completely in chick and chick embryo microsomes and only p artially in the other avian microsomes, The results demonstrate that ( 1) TCDD causes much greater induction of CYP-dependent arachidonic aci d metabolism, and of arachidonic acid epoxygenation in particular, in avian than in mammalian species; (2) TCDD induces two CYP1A-related en zymes in birds as in mammals; (3) CYP1A enzymes in the birds other tha n chicks are not identical to CYP1A4 and 1A5 but share some enzymatic and immunochemical characteristics with them; (4) constitutive omega-O H arachidonic acid in all of the avian species and 8,9-EET in cormoran t are formed by CYP enzymes unrelated to CYP1A; and (5) two distinct c haracteristics of avian CYP1A enzymes are the acquisition by avian CYP 1A4-related P450 of unique epitope(s) and by CYP1A5-related P450 of un usual catalytic effectiveness for arachidonic acid epoxygenation. (C) 1998 Academic Press.