3-(4-[(3-Chlorophenyl)methoxy]phenyl)-5-[(methylamino)methyl]-2- oxazolidinone methanesulphonate (compound MD 780236) is a selective inhibitor of the B-form of monoamine oxidase. Inhibition involves an initial non-covalent interaction between enzyme and inhibitor followed by a time-dependent process resulting in irreversible inhibition. The initial, reversible, phase of inhibition was found to be competitive with respect to phenethylamine and 5-hydroxytryptamine, and a comparison of the Ki values indicated the affinity of the inhibitor for the B-form of the enzyme to be some 7-fold greater than its affinity for the A-form. This selectivity was considerably enhanced by preincubation of the enzyme and inhibitor. Time courses showed that complete inhibition was not achieved under conditions where the inhibitor concentration was over 100-fold greater than that of the enzyme. Assay of the activity of monoamine oxidase by determining the release of hydrogen peroxide fluorometrically showed compound MD 780236 to be a substrate for, as well as an inhibitor of, monoamine oxidase, and kinetic analysis revealed that the rate of product formation was some 530-fold greater than that of the process leading to irreversible inhibition of the B-form of the enzyme.

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