Metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline in human hepatocytes

2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid is a major detoxication pathway catalyzed by cytochrome P450 1A2

S. Langouët, D. H. Welti, N. Kerriguy, L. B. Fay, T. Huynh-Ba, J. Markovic, F. P. Guengerich, A. Guillouzo, R. J. Turesky

Research output: Contribution to journalArticle

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Abstract

Metabolic pathways of the mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) remain incompletely characterized in humans. In this study, the metabolism of MeIQx was investigated in primary human hepatocytes. Six metabolites were characterized by UV and mass spectroscopy. Novel metabolites were additionally characterized by 1H NMR spectroscopy. The carcinogenic metabolite, 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline, which is formed by cytochrome P450 1A2 (P450 1A2), was found to be transformed into the N2-glucuronide conjugate, N2-(β-1-glucosiduronyl)-2-(hydroxyamino)-3,8-dimethylimidazo [4,5-f]quinoxaline. The phase II conjugates N2-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid and N2-(β-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline, as well as the 7-oxo derivatives of MeIQx and N-desmethyl-MeIQx, 2-amino-3,8-dimethyl-6-hydro-7H-imidazo-[4,5-f]quinoxalin-7-one (7-oxo-MeIQx), and 2-amino-6-hydro-8-methyl-7H-imidazo[4,5-f]quinoxalin-7-one (N-desmethyl-7-oxe-MeIQx), thought to be formed exclusively by the intestinal flora, were also identified. A novel metabolite was characterized as 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH), and it was the predominant metabolite formed in hepatocytes exposed to MeIQx at levels approaching human exposure. IQx-8-COOH formation is catalyzed by P450 1A2. This metabolite is a detoxication product and does not induce umuC gene expression in Salmonella typhimurium strain NM2009. IQx-8-COOH is also the principal oxidation product of MeIQx excreted in human urine [Turesky, R., et al. (1998) Chem. Res. Toxicol. 11, 217-225]. Thus, P450 1A2 is involved in both the metabolic activation and detoxication of this procarcinogen in humans. Analogous metabolism experiments were conducted with hepatocytes of untreated rats and rats pretreated with the P450 inducer 3-methycholanthrene. Unlike human hepatocytes, the rat cell preparations did not produce IQx-8-COOH but catalyzed the formation of 2-amino-3,8-dimethyl-5-hydroxyimidazo[4,5-f]-quinoxaline as a major P450-mediated detoxication product. In conclusion, our results provide evidence of a novel MeIQx metabolism pathway in humans through P450 1A2-mediated C8-oxidation of MeIQx to form IQx-8-COOH. This biotransformation pathway has not been detected in experimental animal species. Considerable interspecies differences exist in the metabolism of MeIQx by P450s, which may affect the biological activity of this mutagen and must be considered when assessing human health risk.

Original languageEnglish (US)
Pages (from-to)211-221
Number of pages11
JournalChemical research in toxicology
Volume14
Issue number2
DOIs
StatePublished - Jan 1 2001

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2-amino-3,8-dimethylimidazo(4,5-f)quinoxaline
Cytochrome P-450 CYP1A2
Quinoxalines
Metabolites
Carboxylic Acids
Metabolism
Hepatocytes
Rats
Mutagens
Oxidation
Salmonella
Health risks
Glucuronides
Bioactivity
Gene expression
Nuclear magnetic resonance spectroscopy
2-amino-3-methylimidazo(4,5-f)quinoxaline
Animals
Salmonella typhimurium
Biotransformation

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Metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline in human hepatocytes : 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid is a major detoxication pathway catalyzed by cytochrome P450 1A2. / Langouët, S.; Welti, D. H.; Kerriguy, N.; Fay, L. B.; Huynh-Ba, T.; Markovic, J.; Guengerich, F. P.; Guillouzo, A.; Turesky, R. J.

In: Chemical research in toxicology, Vol. 14, No. 2, 01.01.2001, p. 211-221.

Research output: Contribution to journalArticle

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title = "Metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline in human hepatocytes: 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid is a major detoxication pathway catalyzed by cytochrome P450 1A2",
abstract = "Metabolic pathways of the mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) remain incompletely characterized in humans. In this study, the metabolism of MeIQx was investigated in primary human hepatocytes. Six metabolites were characterized by UV and mass spectroscopy. Novel metabolites were additionally characterized by 1H NMR spectroscopy. The carcinogenic metabolite, 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline, which is formed by cytochrome P450 1A2 (P450 1A2), was found to be transformed into the N2-glucuronide conjugate, N2-(β-1-glucosiduronyl)-2-(hydroxyamino)-3,8-dimethylimidazo [4,5-f]quinoxaline. The phase II conjugates N2-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid and N2-(β-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline, as well as the 7-oxo derivatives of MeIQx and N-desmethyl-MeIQx, 2-amino-3,8-dimethyl-6-hydro-7H-imidazo-[4,5-f]quinoxalin-7-one (7-oxo-MeIQx), and 2-amino-6-hydro-8-methyl-7H-imidazo[4,5-f]quinoxalin-7-one (N-desmethyl-7-oxe-MeIQx), thought to be formed exclusively by the intestinal flora, were also identified. A novel metabolite was characterized as 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH), and it was the predominant metabolite formed in hepatocytes exposed to MeIQx at levels approaching human exposure. IQx-8-COOH formation is catalyzed by P450 1A2. This metabolite is a detoxication product and does not induce umuC gene expression in Salmonella typhimurium strain NM2009. IQx-8-COOH is also the principal oxidation product of MeIQx excreted in human urine [Turesky, R., et al. (1998) Chem. Res. Toxicol. 11, 217-225]. Thus, P450 1A2 is involved in both the metabolic activation and detoxication of this procarcinogen in humans. Analogous metabolism experiments were conducted with hepatocytes of untreated rats and rats pretreated with the P450 inducer 3-methycholanthrene. Unlike human hepatocytes, the rat cell preparations did not produce IQx-8-COOH but catalyzed the formation of 2-amino-3,8-dimethyl-5-hydroxyimidazo[4,5-f]-quinoxaline as a major P450-mediated detoxication product. In conclusion, our results provide evidence of a novel MeIQx metabolism pathway in humans through P450 1A2-mediated C8-oxidation of MeIQx to form IQx-8-COOH. This biotransformation pathway has not been detected in experimental animal species. Considerable interspecies differences exist in the metabolism of MeIQx by P450s, which may affect the biological activity of this mutagen and must be considered when assessing human health risk.",
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T1 - Metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline in human hepatocytes

T2 - 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid is a major detoxication pathway catalyzed by cytochrome P450 1A2

AU - Langouët, S.

AU - Welti, D. H.

AU - Kerriguy, N.

AU - Fay, L. B.

AU - Huynh-Ba, T.

AU - Markovic, J.

AU - Guengerich, F. P.

AU - Guillouzo, A.

AU - Turesky, R. J.

PY - 2001/1/1

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N2 - Metabolic pathways of the mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) remain incompletely characterized in humans. In this study, the metabolism of MeIQx was investigated in primary human hepatocytes. Six metabolites were characterized by UV and mass spectroscopy. Novel metabolites were additionally characterized by 1H NMR spectroscopy. The carcinogenic metabolite, 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline, which is formed by cytochrome P450 1A2 (P450 1A2), was found to be transformed into the N2-glucuronide conjugate, N2-(β-1-glucosiduronyl)-2-(hydroxyamino)-3,8-dimethylimidazo [4,5-f]quinoxaline. The phase II conjugates N2-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid and N2-(β-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline, as well as the 7-oxo derivatives of MeIQx and N-desmethyl-MeIQx, 2-amino-3,8-dimethyl-6-hydro-7H-imidazo-[4,5-f]quinoxalin-7-one (7-oxo-MeIQx), and 2-amino-6-hydro-8-methyl-7H-imidazo[4,5-f]quinoxalin-7-one (N-desmethyl-7-oxe-MeIQx), thought to be formed exclusively by the intestinal flora, were also identified. A novel metabolite was characterized as 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH), and it was the predominant metabolite formed in hepatocytes exposed to MeIQx at levels approaching human exposure. IQx-8-COOH formation is catalyzed by P450 1A2. This metabolite is a detoxication product and does not induce umuC gene expression in Salmonella typhimurium strain NM2009. IQx-8-COOH is also the principal oxidation product of MeIQx excreted in human urine [Turesky, R., et al. (1998) Chem. Res. Toxicol. 11, 217-225]. Thus, P450 1A2 is involved in both the metabolic activation and detoxication of this procarcinogen in humans. Analogous metabolism experiments were conducted with hepatocytes of untreated rats and rats pretreated with the P450 inducer 3-methycholanthrene. Unlike human hepatocytes, the rat cell preparations did not produce IQx-8-COOH but catalyzed the formation of 2-amino-3,8-dimethyl-5-hydroxyimidazo[4,5-f]-quinoxaline as a major P450-mediated detoxication product. In conclusion, our results provide evidence of a novel MeIQx metabolism pathway in humans through P450 1A2-mediated C8-oxidation of MeIQx to form IQx-8-COOH. This biotransformation pathway has not been detected in experimental animal species. Considerable interspecies differences exist in the metabolism of MeIQx by P450s, which may affect the biological activity of this mutagen and must be considered when assessing human health risk.

AB - Metabolic pathways of the mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) remain incompletely characterized in humans. In this study, the metabolism of MeIQx was investigated in primary human hepatocytes. Six metabolites were characterized by UV and mass spectroscopy. Novel metabolites were additionally characterized by 1H NMR spectroscopy. The carcinogenic metabolite, 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline, which is formed by cytochrome P450 1A2 (P450 1A2), was found to be transformed into the N2-glucuronide conjugate, N2-(β-1-glucosiduronyl)-2-(hydroxyamino)-3,8-dimethylimidazo [4,5-f]quinoxaline. The phase II conjugates N2-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid and N2-(β-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline, as well as the 7-oxo derivatives of MeIQx and N-desmethyl-MeIQx, 2-amino-3,8-dimethyl-6-hydro-7H-imidazo-[4,5-f]quinoxalin-7-one (7-oxo-MeIQx), and 2-amino-6-hydro-8-methyl-7H-imidazo[4,5-f]quinoxalin-7-one (N-desmethyl-7-oxe-MeIQx), thought to be formed exclusively by the intestinal flora, were also identified. A novel metabolite was characterized as 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH), and it was the predominant metabolite formed in hepatocytes exposed to MeIQx at levels approaching human exposure. IQx-8-COOH formation is catalyzed by P450 1A2. This metabolite is a detoxication product and does not induce umuC gene expression in Salmonella typhimurium strain NM2009. IQx-8-COOH is also the principal oxidation product of MeIQx excreted in human urine [Turesky, R., et al. (1998) Chem. Res. Toxicol. 11, 217-225]. Thus, P450 1A2 is involved in both the metabolic activation and detoxication of this procarcinogen in humans. Analogous metabolism experiments were conducted with hepatocytes of untreated rats and rats pretreated with the P450 inducer 3-methycholanthrene. Unlike human hepatocytes, the rat cell preparations did not produce IQx-8-COOH but catalyzed the formation of 2-amino-3,8-dimethyl-5-hydroxyimidazo[4,5-f]-quinoxaline as a major P450-mediated detoxication product. In conclusion, our results provide evidence of a novel MeIQx metabolism pathway in humans through P450 1A2-mediated C8-oxidation of MeIQx to form IQx-8-COOH. This biotransformation pathway has not been detected in experimental animal species. Considerable interspecies differences exist in the metabolism of MeIQx by P450s, which may affect the biological activity of this mutagen and must be considered when assessing human health risk.

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