Activation of aminoimidazole carcinogens by nitrosation: Mutagenicity and nucleotide adducts

Abstract

2-Amino-3-methylimidazo[4,5- f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline (MeIQx) are heterocyclic amines (HCAs) derived from high temperature cooking of meat and thought to cause colon cancer in humans. Reactive nitrogen oxygen species, which are mediators of the inflammatory response, can convert these amines to the corresponding N-nitrosamines, N-NO-IQ and N-NO-MeIQx. This study was designed to evaluate whether these N-nitrosamines are genotoxic and could be responsible, in part, for the high incidence of colon cancer in individuals with colitis. Such an association would counsel reduced intake of well-done red meat by colitis patients. Mutagenicity was evaluated by reversion of a lacZ frameshift allele in three different E. coli strains. Strains DJ701 and DJ702 express recombinant ( S. typhimurium) aromatic amine N-acetyltransferase (NAT); DJ702 also expresses recombinant human cytochrome P450 1A2 and NADPH-P450 reductase; and DJ2002 served as an N-acetyltransferase-negative control. In strain DJ701, N-NO-IQ and N-NO-MeIQx elicited dose-dependent mutagenicity, which was not further increased in DJ702. Neither nitrosamine was mutagenic in strain DJ2002. While both N-nitrosamines are stable for >4 h (pH 7.4, 37 °C), they react with DNA or 2′-deoxyguanosine 3′-monophosphate at lower pH (5.5) to form adducts. HOCl, a component of the inflammatory response, increased adduct formation, as measured by 32P-postlabeling. Following treatment with nuclease P1 and separation by two-dimensional thin-layer chromatography and then HPLC, N-NO-IQ and N-NO-MeIQx were shown to form the same adducts as those formed by N-OH-MeIQx or N-OH-IQ, namely N-(deoxyguanosin-8-yl) adducts. In summary, these N-nitrosamines are genotoxic and might be alternatives to their hydroxylamine analogues as activated intermediates leading to initiation of colon cancer in individuals with colitis.