Characterization of Glutathione Conjugates Derived from Reactive Metabolites of Bakuchiol

Bakuchiol belongs to a family of monoterpene phenols occurring in plant Psoralea corylifolia L., a traditional herbal medicine. Bakuchiol has also demonstrated multiple pharmacologic activities. However, metabolism of bakuchiol had never been investigated. The major objective of the present study was to study the metabolic pathways of bakuchiol in order to identify potential reactive metabolites. A total of five glutathione (GSH) conjugates (M1-M5) were detected in rat/human liver microsomes containing NADPH, GSH, and bakuchiol. M1 and M2 resulted from GSH conjugated on the phenol ring. M3, M4, and M5 were derived from GSH adducted on the side chain. The results displayed that bakuchiol can be bioactivated by oxidation of the phenol moiety to the corresponding ortho-quinone and by epoxidation of the aliphatic side chain to epoxide metabolites. No bakuchiol-derived GSH conjugates were detected in urine of rats given bakuchiol, but six corresponding cysteinylglycine (Cys-Gly) conjugates and mercapturic acids were observed instead. A 2'-iodoxybenzoic acid-mediated oxidation reaction of bakuchiol in the presence of GSH produced M1 and M2, and m-chloroperoxybenzoicacid-mediated oxidation of bakuchiol trapped with GSH generated M3 and M4. The four synthetic metabolites were detected in microsomal incubations. In addition, recombinant P450 enzyme incubations showed that CYP 1A2 was the predominant P450 responsible for the metabolism of bakuchiol. In summary, our results demonstrated that bakuchiol can be bioactivated to quinone and epoxide metabolites. These findings facilitate the understanding of the mechanisms of bakuchiol-induced cytotoxicity.