Coumarin metabolism by rat esophageal microsomes and cytochrome P450 2A3

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Abstract

The rat esophagus is strikingly sensitive to tumor induction by nitrosamines, and it has been hypothesized that this tissue contains cytochrome P450 enzymes (P450s) which catalyze the metabolic activation of these carcinogens. The metabolic capacity of the esophagus is not well characterized. In the study described here, the products of 14C-coumarin metabolism by rat esophageal microsomes were identified and quantified. Metabolite characterization was by LC/MS/MS and GC/MS and comparison to standards, quantification was by radioflow HPLC. The coumarin metabolites formed by rat esophageal microsomes were compared to those formed by P450 2A3. The major metabolites formed by esophageal microsomes were 8-Hydroxycoumarin, o-Hydroxyphenylacetaldehyde (o-HPA), and o-hydroxyphenylacetic acid (o-HPAA). A smaller amount of 5-hydroxycoumarin, about one-third the 8-hydroxycoumarin, was also formed. o-HPA and o-HPAA are products of coumarin 3,4-epoxidation. The relative rates of coumarin 8-Hydroxylation and 3,4-epoxidation were similar. Coumarin 8-hydroxylation has not previously been reported as a major pathway in any tissue, and no P450s have yet been reported to catalyze this reaction. P450 2A3 catalyzed both the 7-hydroxylation and 3,4-epoxidation of coumarin. P450 2A3 was previously characterized as a coumarin 7-hydroxylase, however, in this study, we report that it catalyzes the formation of o-HPA more efficiently. The Km and Vmax were 1.3 ± 0.35 μM and 0.65 ± 0.06 nmol/min/nmol P450 for coumarin 7-hydroxylation and 1.4 ± 0.58 μM and 3.1 ± 0.46 nmol/min/nmol P450 for o-HPA formation.

Original languageEnglish (US)
Pages (from-to)1386-1392
Number of pages7
JournalChemical Research in Toxicology
Volume14
Issue number10
DOIs
StatePublished - Oct 31 2001

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Microsomes
Metabolism
Cytochrome P-450 Enzyme System
Rats
Hydroxylation
Epoxidation
Metabolites
Esophagus
Tissue
Nitrosamines
Acids
Carcinogens
coumarin
Tumors
Chemical activation
High Pressure Liquid Chromatography
2-hydroxyphenylacetaldehyde
Neoplasms

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Coumarin metabolism by rat esophageal microsomes and cytochrome P450 2A3. / von Weymarn, Linda B; Murphy, Sharon E.

In: Chemical Research in Toxicology, Vol. 14, No. 10, 31.10.2001, p. 1386-1392.

Research output: Contribution to journalArticle

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abstract = "The rat esophagus is strikingly sensitive to tumor induction by nitrosamines, and it has been hypothesized that this tissue contains cytochrome P450 enzymes (P450s) which catalyze the metabolic activation of these carcinogens. The metabolic capacity of the esophagus is not well characterized. In the study described here, the products of 14C-coumarin metabolism by rat esophageal microsomes were identified and quantified. Metabolite characterization was by LC/MS/MS and GC/MS and comparison to standards, quantification was by radioflow HPLC. The coumarin metabolites formed by rat esophageal microsomes were compared to those formed by P450 2A3. The major metabolites formed by esophageal microsomes were 8-Hydroxycoumarin, o-Hydroxyphenylacetaldehyde (o-HPA), and o-hydroxyphenylacetic acid (o-HPAA). A smaller amount of 5-hydroxycoumarin, about one-third the 8-hydroxycoumarin, was also formed. o-HPA and o-HPAA are products of coumarin 3,4-epoxidation. The relative rates of coumarin 8-Hydroxylation and 3,4-epoxidation were similar. Coumarin 8-hydroxylation has not previously been reported as a major pathway in any tissue, and no P450s have yet been reported to catalyze this reaction. P450 2A3 catalyzed both the 7-hydroxylation and 3,4-epoxidation of coumarin. P450 2A3 was previously characterized as a coumarin 7-hydroxylase, however, in this study, we report that it catalyzes the formation of o-HPA more efficiently. The Km and Vmax were 1.3 ± 0.35 μM and 0.65 ± 0.06 nmol/min/nmol P450 for coumarin 7-hydroxylation and 1.4 ± 0.58 μM and 3.1 ± 0.46 nmol/min/nmol P450 for o-HPA formation.",
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