Biotransformation of coumarin by rodent and human cytochromes P-450: Metabolic basis of tissue-selective toxicity in olfactory mucosa of rats and mice

Coumarin was previously found to cause tissue-selective toxicity in the olf actory mucosa (OM) of rats and mice, with rats being the more sensitive spe cies. The aim of this study was to explore the role of target tissue biotra nsformation in OM-selective toxicity and the metabolic basis of the species differences in coumarin toxicity. At least six coumarin metabolites were d etected in OM microsomal reactions, with o-hydroxyphenyl-acetaldehyde (o-HP A) being the most abundant. Formation of o-HPA was inhibited by reduced glu tathione, confirming its origin from a reactive intermediate. There were si gnificant differences in the rates and metabolite profiles of coumarin meta bolism in the livers of Wistar rats and C57BL/6 mice. The rates of metaboli c activation of coumarin, as indicated by the formation of o-HPA, were comp arable in OM microsomes of the two species but about 25- and 3-fold higher in OM than in liver microsomes of rats and mice, respectively. Thus, target tissue activation seems to play an important role in the tissue-selective toxicity, whereas differences in the rates of hepatic metabolism may be res ponsible for the species difference in olfactory toxicity. Purified, hetero logously expressed mouse CYP2A5 and CYP2G1 produced 7-hydroxycoumarin and o -HPA as the predominant products, respectively. Kinetic analysis and immuno inhibition studies indicated that the OM-specific CYP2G1 plays the major ro le in metabolic activation of coumarin. Furthermore, of 13 human cytochrome P-450s (P-450s) examined, five (CYP1A1, CYP1A2, CYP2B6, CYP2E1, and CYP3A4 ) were active in the metabolic activation of coumarin, suggesting a potenti al risk of coumarin toxicity in humans.