Morphine is a potent analgesic and is used worldwide in the clinical management of severe acute and chronic pain. It is known, however, that the repeated administration of morphine results in tolerance and physical dependence. It has been shown that the treatment of the rat and mouse with high doses of morphine decreases the hepatic glutathione (GSH) content and increases the levels of serum transaminase activity.1-3) These adverse reactions of morphine have long been thought to be due, at least in part, to the generation of reactive metabolite(s) that can bind to GSH and tissue macromolecules. We found morphine 6-dehydrogenase, which catalyzes the conversion of morphine to morphinone in the guinea pig livers and purified the enzyme.4) The product with the enzyme was identified as the 2-mercaptoethanol (ME) adduct of morphinone (MO-ME) which was formed by nonenzymatic binding of morphinone to ME in the incubation medium via a Michael addition, indicating that morphinone is a reactive electrophile. This finding led us to the identification of the morphinone-GSH adduct (MO-GSH) in the bile of guinea pigs given morphine. 5) Thus the metabolic pathway of morphine to morphinone and subsequently to the MO-GSH was first demonstrated in the guinea pig (Fig. 1). The latter reaction was considered to occur mainly through a nonenzymatic binding of morphinone to GSH and, in part, in a reaction catalyzed by the GSH S-transferase.6) In vivo studies in the guinea pig 6) and rat 7) showed that most morphinone formed from morphine is commonly excreted as MO-GSH into the bile and that the sum of morphinone and MO-GSH accounted for about 10% in the guinea pig and 8% in the rat of the administered dose. These observations indicate that the pathway of morphine to morphinone catalyzed by morphine 6-dehydrogenase is one of the main routes in morphine metabolism, at least in guinea pigs and rats.In the mouse, morphinone is nine-fold more toxic than morphine 2) and is a potent antagonist of morphine. 8) This metabolite was also found to block naloxone binding irreversibly in the mitochondrial-synaptosomal fractions of the mouse brain, 8) to bind covalently to tissue macromolecules through sulfhydryl groups in the mouse, 9) and to cause hepatotoxicity in the rat.10,11) These observations suggest that morphinone formed from morphine may affect the analgesic action of morphine and has a potential role in morphine-induced toxicity including the development of tolerance, although most morphinone would be trapped by intracellular GSH to form MO-GSH and excreted readily into the bile. It is therefore of toxicologic and pharmacologic interest to assess the metabolic pathway of morphine to morphinone in animals and humans.We previously reported that the livers of rabbit, mouse, hamster, and bovine as well as guinea pig and rat have the ability to produce morphinone from morphine. 7) However, the in vivo and in vitro formation of morphinone and MO- Morphinone, identified in the bile of guinea pigs and rats given morphine, is a reactive electrophi...

1. Morphinone, a toxic metabolite, and its glutathione adduct (MO-GSH) were identified in the bile of rat after subcutaneous injection of morphine (25 mg/kg) by hplc procedures. The amounts of morphinone and MO-GSH excreted in the 12-h bile were 0.8 +/- 0.3 and 8.4 +/- 4.3% respectively. 2. The 9000 g supernatants of rat, guinea pig, rabbit, mouse, hamster and bovine livers produced morphinone from morphine in the presence of either NAD+ or NADP+, NAD+ was a more efficient cofactor than NADP+ except in the guinea pig which equally utilized both cofactors. With NAD+ as cofactor, the amounts of morphinone formed in rat and guinea pig were 5.70 and 5.82 mumol/g liver/30 min respectively and were three-to-four times those in other species. 3. The enzyme activity responsible for formation of morphinone from morphine in the rat was almost exclusively distributed in the microsomal fraction, whereas guinea pig, hamster and bovine expressed the enzyme activity mainly in the cytosolic fraction. Rabbit and mouse gave higher activity in the cytosolic and microsomal fractions respectively, but other fractions of both species contained considerable activity. 4. The enzyme activities in male and female rat microsomes were characterized with respect to developmental pattern, kinetic parameters, pH dependency and susceptibility to inhibitors. 5. In conclusion the metabolism of morphine to morphinone in rat was confirmed by in vivo and in vitro experiments. It is also suggested that this pathway is a common route in morphine metabolism in several mammalian species.

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