Aromatic amines are well known as occupational carcinogens and are found in cooked foods, tobacco smoke, synthetic fuels, and agricultural chemicals. For the primary arylamines, metabolic N-oxidation by hepatic cytochromes-P-450 is generally regarded as an initial activation step leading to carcinogenesis. The metabolic activation of 4-aminobiphenyl, 2-naphthylamine, and several heterocyclic amines has been shown recently to be catalyzed by rat cytochrome P-450ISF-G and by its human ortholog, cytochrome P-45OPA. We now report that human hepatic microsomal caffeine 3-demethylation, the initial major step in caffeine biotransformation in humans, is selectively catalyzed by cytochrome P-450PA. Caffeine 3-demethylation was highly correlated with 4-aminobiphenyl N-oxidation (r = 0.99; P < 0.0005) in hepatic microsomal preparations obtained from 22 human organ donors, and both activities were similarly decreased by the selective inhibitor, 7,8-benzoflavone. The rates of microsomal caffeine 3-demethylation, 4-aminobiphenyl Noxidation, and phenacetin O-deethylation were also significantly correlated with each other and with the levels of immunoreactive human cytochrome P 450PA. Moreover, a rabbit polyclonal antibody raised to human cytochrome P-450PA was shown to inhibit strongly all three of these activities and to inhibit the N-oxidation of the carcinogen 2-naphthylamine and the heterocyclic amines, 2-amino-6-methyldipyrido-[1,2-a:3',2'-d]imidazole and 2-amino-3-methylimidazo[4,5-fquinoline. Human liver cytochrome P-45OPA was also shown to catalyze caffeine 3-demethylation, 4-aminobiphenyl N-oxidation, and phenacetin 0-deethylation. Thus, estimation of caffeine 3-demethylation activity in humans may be useful in the characterization of arylamine N-oxidation phenotypes and in the assessment of whether or not the hepatic levels of cytochrome P-450PA, as affected by environmental or genetic factors, contribute to interindividual differences in susceptibility to arylamine-induced cancers.The carcinogenicity of arylamines has been well established in both humans and experimental animals (1). Humans are frequently exposed to arylamines such as 4-aminobiphenyl (ABP), 2-naphthylamine (2-NA), and o-toluidine in mainstream and sidestream cigarette smoke (2) and to mutagenic and carcinogenic heterocyclic arylamines in cooked foods (3). Arylamines are also found in coal-and shale-derived oils (4) and in agricultural chemicals (5), and they are used in a variety of industrial processes (1,6,7).Metabolic N-oxidation of primary arylamines, catalyzed by hepatic cytochromes P-450 (P-450s), is a critical initial activation step leading to carcinogenesis (reviewed in refs. 8 and 9). ¶ Studies with purified rat and rabbit P-450s have shown high specificity for the N-oxidation of 2-acetylaminofluorene, 2-NA, ABP, and several heterocyclic amines to their proximate carcinogenic and/or mutagenic forms by the P450IA2 gene products in various species (8)(9)(10)(11)(12)(13)(14)(15). In humans, the orthologous P-450 (16, 17), termed P-45...

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)

Tamoxifen has been a mainstay of adjuvant therapy for breast cancer for many years. We sought to determine if genetic variability in the tamoxifen metabolic pathway influenced overall survival in breast cancer patients treated with tamoxifen. We examined functional polymorphisms in CYP2D6, the P450 catalyzing the formation of active tamoxifen metabolites, and UGT2B15, a Phase II enzyme facilitating the elimination of active metabolite in a retrospective study of breast cancer patients. We also examined whether the combination of variant alleles in SULT1A1 and UGT2B15 had more of an impact on overall survival in tamoxifen-treated patients than when the genes were examined separately. We conducted a retrospective study using archived paraffin blocks for DNA extraction and data from pathology reports and hospital tumor registry data for information on clinical characteristics, treatment, and outcomes (162 patients receiving tamoxifen and 175 who did not). Genotypes for CYP2D6 and UGT2B15 were obtained and Cox proportional hazards modeling was performed. After adjusting for age, race, stage of disease at diagnosis, and hormone receptor status, we found no significant association between CYP2D6 genotype and overall survival in either group of breast cancer patients. Tamoxifen-treated patients with UGT2B15 high activity genotypes had increased risk of recurrence and poorer survival. When UGT2B15 and SULT1A1 'at-risk' alleles were combined, women with two variant alleles had significantly greater risk of recurrence and poorer survival than those with common alleles. These studies indicate that genetic variation in Phase II conjugating enzymes can influence the efficacy of tamoxifen therapy for breast cancer.

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