Investigations of mechanisms of reactive metabolite formation from (R)-(+)-pulegone

Abstract

1. (R)-(+)-Pulegone is a monoterpene that is oxidized by cytochromes P-450 to reactive metabolites that initiate events in the pathogenesis of hepatotoxicity in mice, rats and humans.

2. Selective labelling of (R)-(+)-pulegone with deuterium revealed that menthofuran was a proximate hepatotoxic metabolite formed by oxidation of the allylic methyl groups of pulegone. Incubations of pulegone with mouse liver microsomes in an atmosphere of ¹⁸O₂ resulted in the formation of menthofuran that contained only oxygen-18 in the furan moiety. These results are consistent with oxidation of pulegone to an allylic alcohol that reacts intramolecularly with the ketone moiety to form a hemiketal that subsequently dehydrates to generate menthofuran.

3. Studies on the metabolism of menthofuran revealed that it is oxidized by cytochromes P-450 to an electrophilic γ-ketoenal that reacts with nucleophilic groups on proteins to form covalent adducts. In addition, diastereomeric mintlactones are formed. Investigations with H₂¹⁸O and ¹⁸O₂ are indicative of a furan epoxide intermediate, or a precursor, in the formation of the γ-ketoenal and mintlactones.