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)

N-acetyl transferase 1: two polymorphisms in coding sequence identified in colorectal cancer patients

“…The NAT1 genotype NATI*4/NAT1*15 was less frequent than expected, if random distribution is assumed, in individuals with a slow NAT2 genotype in both the control and the cancer groups. This may represent genetic linkage between NATI and NAT2, which are closely related and have overlapping substrate specificity (Turesky et al, 1991). This linkage may confound studies of NAT1 and NAT2 polymorphism with disease susceptibility.…”

“…Polymorphisms of NAT2 that result in slow acetylation of metabolites have been associated with bladder cancer (Risch et al, 1995) and, conversely, fast acetylators may be more common in colorectal cancer and some types of breast cancer (Lang et al, 1986;Agundez et al, 1995). However these associations are weak and are not supported by all studies (Bell et al, 1995a;Hubbard et al, 1997).A potentially confounding factor in these assessments is the contribution of NATI, a closely related enzyme that shares some substrate specificity with NAT2 (Hein et al, 1992), which is expressed at higher levels than NAT2 in colonic epithelial cells (Turesky et al, 1991). Recent studies have shown that NATI is also polymorphic and 15 variants have been detected in animal and human gene sequences Weber and Vatsis, 1993;Grant et al, 1997).…”

“…A potentially confounding factor in these assessments is the contribution of NATI, a closely related enzyme that shares some substrate specificity with NAT2 (Hein et al, 1992), which is expressed at higher levels than NAT2 in colonic epithelial cells (Turesky et al, 1991). Recent studies have shown that NATI is also polymorphic and 15 variants have been detected in animal and human gene sequences Weber and Vatsis, 1993;Grant et al, 1997).…”

U.S. dietary exposures to heterocyclic amines*

“…Another predominant HA found in cooked meats, 2 -amino-3,8-dimethylimidazo [ 4,5-f] quinoxaline (MeIQx), causes tumors at multiple sites in rats and mice Bogen, 1994 ). HAs are metabolically activated by P450 and N -acetyltranserase enzymes, and activated forms are observed to bind covalently to DNA at sites including tissues (including prostate ) in which HAs induce cancer in rats (Thorgeirsson, 1984;Thorgeirsson et al, 1983;Rosenkranz and Mermeistein, 1985;Sato et al, 1986;Kato and Yamazoe, 1987;Snyderwine and Battula, 1989;McManus et al, 1990;Turesky et al, 1991;Davis et al, 1993;Kaderlik et al, 1994a,b;Takahashi et al, 1998;Schut and Snyderwine, 1999 ).…”

NAT2 Gene Polymorphism in Bladder Cancer: A Study from North India

“…A recent review describes the nucleotide and aminoacid changes associated with various alleles and deduced phenotype from genotype. It also summarized results of molecular epidemiologic studies assessing the association of NAT1 and NAT2 genotypes with cancer risk of bladder, colon, breast, lung, head and neck and prostate (12,13).…”