Bioactivation of morphine in human liver: Isolation and identification of morphinone, a toxic metabolite

Morphinone, identified in the bile of guinea pigs and rats given morphine, is a reactive electrophile and has the ability to bind to glutathione (GSH) and tissue macromolecules, leading to GSH depletion and cell damage. We previously demonstrated that the livers of various animal species are capable of forming morphinone from morphine. In this study, we examined whether the human liver can produce morphinone from morphine. HPLC analysis revealed that the incubation of morphine with the 9000xg supernatant of human liver in the presence of NAD(P) and 2-mercaptoethanol (ME) gave a peak corresponding to the synthetic morphinone-ME adduct (MO-ME), which is readily formed by a nonenzymatic reaction of morphinone with ME. The reaction product was isolated and was unambiguously identified as MO-ME using FAB-MS and NMR analyses in comparison with synthetic MO-ME. The conversion of morphine to morphinone required NAD(P), and NAD was a preferred cofactor over NADP. All the 9000xg supernatants from six human livers could produce morphinone at different rates, ranging from 30 to 120 nmol/g liver/30 min with NAD at pH 7.4. The enzyme activity responsible for the formation of morphinone from morphine was mainly localized in the microsomes. The microsomal enzyme activity was inhibited by steroids, lithocholic acid and indomethacin. Among these compounds, steroids with a 17β-hydroxyl group almost completely depressed morphinone formation. In conclusion, the metabolic pathway of morphine to morphinone, a toxic metabolite, in human was shown for the first time in in vitro experiments.