Metabolic activation of carcinogenic heterocyclic aromatic amines by human liver and colon

Abstract

The metabolic activation of the food-borne rodent carcinogens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) was compared with that of the known human carcinogen 4-aminobiphenyl (ABP), using human liver microsomes, human and rat liver cytosols, and human colon cytosol. All of these aromatic amines were readily activated by N-hydroxylation with human liver microsomes (2.3-5.3 nmol/min/mg protein), with PhIP and ABP exhibiting the highest rates of cytochrome P450IA2-dependent N-oxidation, followed by MeIQx, IQ and Glu-P-1. In contrast, while ABP and 2-aminofluorene were readily N-acetylated (1.7-2.3 nmol/min/mg protein) by the polymorphic human liver cytosolic N-acetyltransferase, none of the heterocyclic amines were detectable as substrates (less than 0.05 nmol/min/mg protein). Likewise, only low activity was observed (0.11 nmol/min/mg protein) for the N-acetylation of p-aminobenzoic acid, a selective substrate for the human monomorphic liver N-acetyltransferase. The radiolabeled N-hydroxy (N-OH) arylamine metabolites were synthesized and their reactivity with DNA was examined. Each derivative bound covalently with DNA at neutral pH (7.0), with highest levels of binding observed for N-OH-IQ and N-OH-PhIP. Incubation at acidic pH (5.0) resulted in increased levels of DNA binding, suggesting formation of reactive arylnitrenium ion intermediates. These N-OH arylamines were further activated to DNA-bound products by human hepatic O-acetyltransferase. Acetyl coenzyme A (AcCoA)-dependent, cytosol-catalyzed DNA binding was greatest for N-OH-ABP and N-OH-Glu-P-1, followed by N-OH-PhIP, N-OH-MeIQx and N-OH-IQ; and both rapid and slow acetylator phenotypes were apparent. Rat liver cytosol also catalyzed AcCoA-dependent DNA binding of the N-OH arylamines; and substrate specificities were comparable to human liver, except that N-OH-MeIQx and N-OH-PhIP gave relatively higher and lower activities respectively. Human colon cytosols likewise displayed AcCoA-dependent DNA binding activity for the N-OH substrates. Metabolic activity was generally lower than that found with the rapid acetylator liver cytosols; however, substrate specificity was variable and phenotypic differences in colon O-acetyltransferase activity could not be readily discerned. This may be due, at least in part, to the varied contribution of the monomorphic acetyltransferase, which would be expected to participate in the enzymatic acetylation of some of these N-OH arylamines.(ABSTRACT TRUNCATED AT 400 WORDS)