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Human exposure to heterocyclic aromatic amines such as MeIQx (2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline) may be monitored by measuring the levels of the heterocyclic aromatic amine in urine. In order to investigate the contribution of N-oxidation to the metabolism of MeIQx in vivo, we developed a biomonitoring procedure for the analysis and quantification of the N2-glucuronide conjugate of 2-hydroxyamino-3,8-dimethylimidazo[4,5-f]quinoxaline in human urine. Subjects (n = 66) in the dietary study ingested a uniform diet of cooked meat containing known amounts of MeIQx, and urine was collected after consumption of the test meal. A method based on solid-phase extraction and immunoaffinity separation was used to isolate N2-(beta-1-glucosiduronyl)-2-hydroxyamino-3,8-dimethylimidazo++ +[4,5-f]quinoxaline and its stable isotope-labeled internal standard from urine. The isolated conjugate was converted to the deaminated product 2-hydroxy-3,8-dimethylimidazo[4,5-f]quinoxaline by treatment with acetic acid under moderate heating. 2-Hydroxy-3,8-dimethylimidazo[4,5-f]quinoxaline and the [2H3]methyl analog were derivatized to form the corresponding 3,5-bis(trifluoromethyl)benzyl ether derivatives and quantified by capillary gas chromatography-negative ion chemical ionization mass spectrometry employing selected ion monitoring procedures. The amounts of N2-(beta-1-glucosiduronyl)-2-hydroxyamino-3,8-dimethylimidazo++ +[4,5-f]quinoxaline recovered in urine collected 0-12 h after the test meal accounted for 2.2-17.1% of the ingested dose, with a median value of 9.5%. The variability in the proportion of the dose excreted among the subjects may be reflective of several factors, including interindividual variation in the enzymic activity of CYP1A2 and/or conjugation reactions of the N-hydroxylamine metabolite with N-glucuronosyltransferase(s).

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are heterocyclic amines formed during the cooking of meat and fish. Both are genotoxic in a number of test systems and are carcinogenic in rats and mice. Human exposure to these compounds via dietary sources has been estimated to be under 1 microg/kg body wt. per day, although most laboratory animal studies have been conducted at doses in excess of 10 mg/kg body wt. per day. We are using accelerator mass spectrometry (AMS), a tool for measuring isotopes with attomole sensitivity, to study the dosimetry of protein and DNA adduct formation by low doses of MeIQx and PhIP in rodents and comparing the adduct levels to those formed in humans. The results of these studies show: 1, protein and DNA adduct levels in rodents are dose-dependent; 2, adduct levels in human tissues and blood are generally greater than in rodents administered equivalent doses; and 3, metabolite profiles differ substantially between humans and rodents for both MeIQx and PhIP, with more N-hydroxylation (bioactivation) and less ring oxidation (detoxification) in humans. These data suggest that rodent models do not accurately represent the human response to heterocyclic amine exposure.

The catalytic efficiences of cytochrome P450 (P450)-mediated N-oxidation of the 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by recombinant human P450 1A2 were 10-19-fold higher than rat P450 1A2, while methoxyresorufin O-demethylation activity was comparable for both P450s. Similar findings were observed with rat and human liver microsomal samples. Interspecies differences in P450 enzyme expression and catalytic activities may be significant and must be considered when assessing human health risk.

The mutagenic activity of 9 brands of commercial meat extracts were compared using Ames tester strain TA 98 with hepatic S-9 activation prior to concentration by solvent extraction. In a preincubation test system where 20 mg of intact meat extract preparations were tested per plate, about half of the brands were non-mutagenic whereas the other half increased the spontaneous revertant number by a factor of 2.5-3 (MF). The mutagenic principals could be extracted into methylene chloride under alkaline conditions and the mutagenic activity of these basic organic extracts ranged in potency from a mutation factor of 7-40 (280-1600 revertants) per 2 g equivalents of intact meat extracts in the standard Ames test. A quantitative correlation was obtained between the two trials (intact and extracted samples). The commercial meat extract with the greatest activity contained approximately 70 ng/g of total amino-imidazoquinoline and amino-imidazoquinoxaline compounds while the weakest brand of meat extract contained less than 10 ng/g of these heterocyclic aromatic amines. The human intake of these heterocyclic amines from meat extract in bouillon soups was estimated at maximally 100 ng/day and is approximately 10(3) times less than the estimated total daily intake of all heterocyclic amines which are known to be formed in protein-rich heated foods.