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...

DNA cleavage by quinones contained in diesel exhaust particles (DEP) was examined in a cell-free system using supercoiled FX174 DNA as the target DNA. In the presence of Cu(II) and NADPH, 9,10-phenanthrenequinone (PQ) caused the transformation of the supercoiled FX174 DNA into open circular and then linear forms in a concentration-dependent manner. This DNA damage by PQ was decreased by catalase, a superoxide anion scavenger and a Cu(I)-specific chelator, but not by superoxide dismutase and a hydroxyl radical scavenger, suggesting that the ultimate reactive product responsible for the DNA scission may be Cu(I)-OOH generated from hydrogen peroxide and Cu(I) rather than hydroxyl radicals. In addition, 1,2-naphthoquinone (1,2-NQ) damaged DNA more severely than PQ, while 1,4-NQ and 9,10-anthraquinone (AQ) did not induce significant DNA damage. When a purified aldo-keto reductase (AKR) 1C isozyme, which catalyzes the two-electron reduction of PQ, was included in the reaction mixture, the PQ-induced DNA damage became more extensive. Addition of the AKR1C isozyme also increased the 1,2-NQ-induced DNA damage and conferred the ability to cause DNA damage on 1,4-NQ, but had no effect on AQ. The severity of the DNA damage induced by DEP quinones was solely related to both NADPH consumption and reactive oxygen species (ROS) gen-