title: Coumarin metabolism by rat esophageal microsomes and cytochrome P450 2A3.
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 microM and 0.65 +/- 0.06 nmol/min/nmol P450 for coumarin 7-hydroxylation and 1.4 +/- 0.58 microM and 3.1 +/- 0.46 nmol/min/nmol P450 for o-HPA formation.
authors.affiliation: Department of Biochemistry, Molecular Biology & Biophysics, University of Minnesota Cancer Center, Minneapolis, Minnesota 55455, USA.
authors.forename: L B
authors.initials: LB
authors.lastname: von Weymarn
authors.forename: S E
authors.initials: SE
authors.lastname: Murphy
mesh_terms: D000818:Animals; D000925:Anticoagulants / Q000378:metabolism* / Q000493:pharmacokinetics; D001189:Aryl Hydrocarbon Hydroxylases; D003374:Coumarins / Q000378:metabolism* / Q000493:pharmacokinetics; D065637:Cytochrome P-450 CYP2A6; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004947:Esophagus / Q000187:drug effects / Q000502:physiology; D006900:Hydroxylation; D008297:Male; D008861:Microsomes; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D051381:Rats; D011916:Rats, Inbred F344
chemical_list: D000925:Anticoagulants; D003374:Coumarins; D003577:Cytochrome P-450 Enzyme System; C030123:coumarin; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; D065637:Cytochrome P-450 CYP2A6