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In vivo and in vitro formation of morphinone from morphine in rat
S Yamano1, A Takahashi, T Todaka
1Faculty of Pharmaceutical Sciences, Fukuoka University, Japan.
Abstract:
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.
Insights
Morphine is metabolized to morphinone, a toxic substance, in rats and other mammals. This metabolic pathway, involving liver enzymes, was confirmed through in vivo and in vitro studies.
Area of Science:
- Pharmacology
- Toxicology
- Biochemistry
Background:
- Morphine is a widely used analgesic.
- Understanding its metabolic pathways is crucial for safety and efficacy.
- Morphinone is a known toxic metabolite of morphine.
Purpose of the Study:
- To identify and quantify morphinone and its adducts in vivo.
- To investigate the enzymatic formation of morphinone from morphine across various mammalian species.
- To characterize the enzyme responsible for morphinone formation in rats.
Main Methods:
- High-performance liquid chromatography (HPLC) for metabolite identification and quantification in rat bile.
- Incubation of liver supernatants from various species with morphine and cofactors (NAD+, NADP+) to assess morphinone production.
- Fractionation of liver homogenates (microsomal, cytosolic) to determine enzyme localization.
- Characterization of rat liver microsomal enzyme activity.
Main Results:
- Morphinone and its glutathione adduct (MO-GSH) were identified in rat bile.
- Liver enzymes from rat, guinea pig, rabbit, mouse, hamster, and bovine produced morphinone from morphine, with NAD+ being a more efficient cofactor.
- Enzyme activity localization varied, with rats showing predominantly microsomal activity, while guinea pigs, hamsters, and bovines had mainly cytosolic activity.
- Enzyme kinetics, pH dependency, and inhibitor susceptibility were characterized in male and female rat microsomes.
Conclusions:
- The metabolism of morphine to morphinone in rats was confirmed via in vivo and in vitro experiments.
- The pathway of morphine metabolism to morphinone appears to be common across several mammalian species.
- Further research into this pathway may elucidate morphine's toxicological profile and inform clinical use.