ISOLATION OF N-VINYLPROTOPORPHYRIN-IX AFTER HEPATIC CYTOCHROME-P450 INACTIVATION BY 3-[(ARYLTHIO)ETHYL]SYDNONE IN CHICK-EMBRYOS PRETREATED WITH PHENOBARBITAL, GLUTETHIMIDE, DEXAMETHASONE, AND BETA-NAPHTHOFLAVONE - DIFFERENTIAL INHIBITION OF FERROCHELATASE BY N-VINYLPROTOPORPHYRIN REGIOISOMERS

Several xenobiotics cause hepatic porphyrin accumulation through mecha nism-based inactivation of cytochrome P450 (P450) and heme alkylation. Loss of iron from the alkylated heme results in formation of an N-alk ylporphyrin, which is a potent inhibitor of ferrochelatase. N-Vinylpro toporphyrin IX (N-vinylPP) was identified in chick embryo liver after in ovo administration of 3-[(arylthio)ethyl]sydnone (TTMS). Pretreatme nt of chick embryos with beta-naphthoflavone, which causes a 90-fold i ncrease in P450 1A levels, did not increase the formation of N-vinylPP after TTMS administration, showing that the heme moiety of P450 1A do es not contribute to the formation of N-vinylPP. Increased amounts of N-vinylPP were isolated from dexamethasone-, phenobarbital-, and glute thimide-pretreated chick embryos, and it is possible that P450 2H and/ or a P450 3A-like isozyme contributes to the formation of N-vinylPP. T he ring B-substituted (N-B) regioisomer of N-vinylPP constituted the l owest percentage of the total regioisomers (9-13%) in untreated and dr ug-induced chick embryos, thus supporting the concept that ring B of h eme is occluded by a protein residue in the P450 active site. Previous ly, the finding that the N-B regioisomer of N-ethylprotoporphyrin IX h ad one fifth the potency of the ring A-substituted (N-A) regioisomer a s a ferrochelatase inhibitor led to a proposal that an A-C ring tilt w as important in N-alkylprotoporphyrins for ferrochelatase inhibition. The finding in the present study that the N-A and N-B regioisomers of N-vinylPP are equipotent does not support the above proposal. The ring C-substituted (N-C) and ring D-substitutedd (N-D) regioisomers of N-v inylPP had low potency.