Enzymatic conversion of morphine to pseudomorphine

The interaction of morphine with an enzymatic oxidation-reduction system was investigated. It has been reported that morphine is converted to a highly water-soluble metabolite by incubation with horseradish peroxidase (HRP), albumin and H 2O 2 (Misra and Mitchell, Experientia 27, 1442 (1971)). After incubating [ 14C]morphine with HRP and H 2O 2, we detected two radioactive compounds with t.l.c. One compound corresponded to morphine. Using spectral analysis, the other, 14C-labeled compound, was identified as pseudomorphine, a dimerized oxidation product of two molecules of morphine. Using compounds structurally related to morphine, it was determined that: (1) a free phenolic hydroxyl group in position 3 was necessary for the enzymatic oxidation of morphine to pseudomorphine; (2) a carbonyl group in position 6 of the morphinan ring prevents the formation of the dimer; and (3) substitution of other functional groups on the morphine molecule did not affect the peroxidase-catalyzed dimerization. By optimizing reaction conditions, measurable rates of peroxidase-catalyzed pseudomorphine formation were observed at morphine concentrations as low as 5 × 10 −7 M. The nature of peroxidase enzymes and the phenolic character of morphine suggest that the formation of pseudomorphine proceeds through a morphine-free radical intermediate.