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We have modified our radiometric assay for ethylmorphine N-demethylase to examine the metabolism of codeine. We find that the current assay gives excellent separation of metabolites with a zero time activity of 0.08%. The N- and O-demethylations are linear for up to 30 min with up to 1 mg/ml of microsomal protein. The pH profiles show slightly different maxima (N-demethylation, pH 8.0; O-demethylation, pH 7.8). The kinetic parameters for N-demethylation were markedly higher (Vmax = 5.6 nmol/min/mg protein; KM = 714 microM) than those for O-demethylase (Vmax = 0.75 nmol/min/mg protein; KM = 149 microM). These data suggest that the HCHO results primarily from the N-demethylase. Further, the differences in the kinetic parameters and the pH profile suggest that these two activities are catalyzed by different enzymatic systems.


P H Duquette, F J Peterson, D L Crankshaw, N J Lindemann, J L Holtzman. Studies of the metabolic N- and O-demethylation of [6-3H]codeine. Drug metabolism and disposition: the biological fate of chemicals. 1983 Sep-Oct;11(5):477-80

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PMID: 6138234

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