Mechanisms of cytochrome P450 1A2-mediated formation of N-hydroxy arylamines and heterocyclic amines and their reaction with guanyl residues

Abstract

Amine oxidation reactions are catalyzed by cytochrome P450 (P450) and peroxidase enzymes; both types of enzymes appear to function via aminium radical intermediates. N-Dealkylation is favored over N-oxygenation for secondary and tertiary amines with both kinds of enzymes, but in the peroxidase-like enzymes N-oxygenation is even less favorable because of apparent restriction of the Fe-O complex in the active site. Among the rat liver P450s many of the carcinogenic primary arylamines and heterocyclic amines are N-oxygenated by P450 1A2 to form the N-hydroxy arylamine derivatives. Studies with human liver P450s also indicate that P450 1A2 plays a major role in such reactions, although some arylamines such as 4,4'-methylene-bis (3-chloroaniline) and dapsone are preferentially N-oxygenated by P450 3A4. Caffeine N3-demethylation has been developed as a useful marker of P450 1A2 levels in humans; the knowledge that P450 1A2 is the major phenacetin O-deethylase also allows insight into previous human interaction studies. 2-Ethynylnaphthalene is a useful mechanism-based inactivator of rat and rabbit P450 1A2 but not human P450 1A2 enzymes; the peptides labeled in the enzymes have been identified, along with the region in rat P450 1A2 that is modified with the photoaffinity label 4-azidobiphenyl. Microcrystals of rabbit P450 1A2 have been obtained as a first course to realizing the three-dimensional structures of these enzymes. Evidence is also presented that the major C8-guanyl DNA adducts resulting from these arylamines and heterocyclic amines in DNA may be formed via rearrangement of an initial N7-guanyl-2-arylamine adduct: reaction of N-acetoxy-2-aminofluorene with C8-methylguanine derivatives led to the formation of stable N7-substituted species, and reaction of N-acetoxy-2-aminofluorene with C8-bromoguanine yielded N-(C8-guanosinyl)-2-aminofluorene in a reaction best rationalized by such a mechanism.