The apoprotein of cytochrome P450 3A2 does not selectively direct alkylation to one of four pyrrole rings of N-vinylprotoporphyrin IX

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A variety of porphyrinogenic xenobiotics cause mechanism-based inactivation of a number of rat hepatic cytochrome P450 isozymes. As a result, a mixture of four N-alkylprotoporphyin IX (N-alkylPP) regioisomers, with ferrochelatase-inhibitory properties, are formed. This study was designed to determine whether all four regioisomers are produced on the active site of a single P450 isozyme, or whether the apoprotein of individual P450 isozymes selectively directs the alkylation to one of four pyrrole nitrogen atoms of N-alkylPP. Administration of 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) to untreated and dexamethasone-pretreated rats results in the formation of four regioisomers of N-vinylprotoporphyrin IX (N-vinylPP). Following coadministration of TTMS with troleandomycin, N-vinylPP formation is reduced to 40 and 43% of control in untreated and dexamethasone-pretreated male rats. Since troleandomycin is a selective inhibitor of rat hepatic P450 3A, the difference in N-vinylPP regioisomer formation between control (TTMS alone) and TTMS plus troleandomycin coadministered rats represents the difference due to P450 3A. This allowed us to calculate that all four regioisomers of N-vinylPP are formed by P450 3A isozymes in both untreated and dexamethasone-pretreated male rats. Since untreated male rats express P450 3A2, but not P450 3A1, we conclude that the apoprotein of rat hepatic P450 3A2 does not selectively direct the alkylation to one of four pyrrole nitrogens of N-vinylPP.