N-Hydroxy-MeIQx is the major microsomal oxidation product of the dietary carcinogen MeIQx with human liver

Abstract

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), one of the most abundant of the heterocyclic aromatic amines formed during the cooking of meat, is genotoxic and carcinogenic in rodents. MeIQx requires metabolic activation by P450 before it can exert these effects. Whilst there is indirect evidence that the mutagenic product is N-hydroxy-MeIQx (N-OHMeIQx), we have now identified this unequivocally following incubation of the amine with human hepatic microsomal fraction. A mixture of unlabelled MeIQx, [13C,15N2]MeIQx and [14C]MeIQx was used as substrate and the products analysed by HPLC-thermospray mass spectrometry. Characteristic doublet ions, 3 mass units apart, were found at m/z 214/217 ([M+H]+) from the parent compound, MeIQx and at 230/233 ([M+H]+) from N-OHMeIQx. The presence of a doublet ion at m/z 214/217 with the doublet at 230/233 [M+H+] provided additional evidence that this was N-OHMeIQx, as facile loss of 'O' is characteristic of N-hydroxylamines. Further evidence for the identity of the major metabolite, which accounted for approximately 90% of all microsomal metabolism, was obtained by comparing the mutagenicity of the HPLC eluate using Salmonella typhimurium YG1024, which is particularly sensitive to N-hydroxylamines, and TA98/1,8-DNP6 which is resistant to most N-hydroxylamines. Ninety-five per cent of direct-acting mutagenicity present in the reaction mixture was associated with a single peak, which co-eluted with N-OHMeIQx, as indicated by mass spectrometry. In the presence of a metabolic activation system, only one additional mutagenic peak, corresponding to unchanged MeIQx, could be detected. MeIQx (5 microM) was N-hydroxylated at a rate of 77 +/- 11 pmol/mg/min (mean +/- SEM, n = 4) by human liver microsomes. The specific inhibitor of human CYP1A2, furafylline (5 microM) inhibited the N-hydroxylation of MeIQx by > 90%. These data show that N-OHMeIQx is both the major oxidation product and the major genotoxic product of MeIQx generated by microsomal fractions of human liver and that the reaction is catalysed almost exclusively by CYP1A2.

Food Borne Carcinogens: A Dead End?

“…Among the different CYPs, those of family 1 have been found to participate in the metabolism of HCAs. CYP1A2 shows the highest activity for N-hydroxylation or activation of the HCAs to the ultimate mutagenic specie Glatt, 2006;Rich et al, 1992;Zhao et al, 1994). CYP1A2 activity may vary from person to person and can be activated by lifestyle factors such as smoking or eating habits (Lang et al, 1994).…”

Heterocyclic amines: evaluation of their role in diet associated human cancer

“…olites of MeIQx and PhIP and using liver microsomal fractions showed that humans efficiently N-hydroxylate both amines (Table 2 ) but do not produce the C-oxidation detoxication products [27,28 ]. Animal The metabolism of heterocyclic amines studies using chemical modifiers of enzyme activity and specific antibodies raised against CYP enzymes showed Our experiments with both animals and man indicated that MeIQx and PhIP are extensively metabolised prior that CYP1A sub-family enzymes were involved in the oxidative metabolism of MeIQx and PhIP and that the is the N-hydroxylamine generated by N-oxidation of the exocyclic amino function.…”

Tumorigenesis of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), but not enhancing effects of concomitant high-fat diet, on lung carcinogenesis in female A/J mice

“…It is well known that MeIQx is converted to genotoxic metabolites by liver cyP1A2 (33,34). A previous study showed an increase of about 1.5-2-fold in the expression levels of cyP1A2 mrnA in livers treated with MeIQx (26).…”