Metabolism of carcinogenic heterocyclic and aromatic amines by recombinant human cytochrome P450 enzymes

Abstract

The N-hydroxylation of carcinogenic arylamines represents an initial step in their metabolic activation. Animal studies have shown that this reaction is catalyzed by the cytochrome P450 (P450) enzymes P450 1A1 and P450 1A2. In this study, utilizing enzymes expressed in Escherichia coli (and purified) or in human B-lymphoblastoid cells, the catalytic activities of recombinant human P450 1A1, P450 1A2, and P450 3A4 for N-hydroxylation of several carcinogenic arylamines were determined. P450 1A2 from both expression systems catalyzed the N-hydroxylation of 4-aminobiphenyl and the heterocyclic amines, 2-amino-3-methylimidazo[4,5-f/quinoline (IQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Rates were similar, with values of 1.1-7.8 nmol/min/nmol P450. In contrast, P450 1A1 catalyzed N-hydroxylation of only PhIP, and no activity was observed with P450 3A4. Further kinetic analysis with purified P450 1A2 showed similar Km and Vmax values for N-hydroxylation of the arylamines. Furafylline and fluvoxamine, inhibitors of P450 1A2 activity in human liver microsomes, were found to be inhibitory of the recombinant P450 1A2 N-hydroxylation activity. Results from this study are supportive of a major role for human P450 1A2 in the metabolic activation of arylamines.

Regulation and function of family 1 and family 2 UDP-glucuronosyltransferase genes (UGT1A, UGT2B) in human oesophagus

“…PhIP undergoes N-hydroxylation by cytochromes P4501A1 and P4501A2 at the two-amino position [42] to generate the mutagenic metabolite [43]. Since P4501A1 is not expressed abundantly in liver [44] and P4501A2 is limited to expression in hepatic tissue, it has been proposed [45] that N-hydroxy-PhIP is transported from the liver in the circulation to target tissues where it forms DNA adducts.…”

Inter-Individual Variation in the Metabolic Activation of Heterocyclic Amines and Susceptibility to Prostate Cancer

“…The formation of 4-hydroxy PhIP was qualitatively confirmed by the standard samples and the formation of 4-hydroxy PhiP was consistent as reported by Hammons et. al, (23), the formation of 4-hydroxy PhIP by human CYP1A1 was shown in Fig. 8.…”

“…The positive correlation between meat consumption and cancer incidence such as those of the stomach (5), prostate (6), breast (7,8), lung (9) in human populations (10) along with carcinogenicity to rats and mice (11)(12)(13) of these compounds suggest that they may be an important factor in diet-related human cancers. The critical step in HCA bioactivation is catalyzed by the CYP mediated A/-hydroxylation, forming its major metabolite, A/-hydroxy HCA (14)(15)(16)(17). The presence of CYP1A2, CYP1A1 and CYP1B1 mRNA in the human prostate has been reported (18).…”

“…Chemical toxicity and carcinogenicity in extrahepatic tissues are frequently results from in situ metabolic activation mediated by the target organ and the toxicity of a given compound is linked to its metabolism in the target tissue (19)(20)(21). Prostate tissues obtained from rats fed with HCAs, were histopathologically evaluated and found to contain prostate carcinomas (22,23). Risk assessment of potential human carcinogens based primarily on data obtained from experimental animals.…”

HPLC determination of caffeine and paraxanthine in urine: An assay for cytochrome P450 1A2 activity

“…Pharmaceutical substrates for P450 1A2 include acetaminophen, estradiol, theophylline, warfarin, imipramine, olanzapine, and others. P450 1A2 is also responsible for the electrophilic activation of aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons [5,6]. Heterocyclic amines, produced during the cooking of red meat for instance, are metabolized by P450 1A2 to electrophilic metabolites that have been shown to cause DNA mutations and cancer [7,8].…”