[3H]Ro 16–6491, a Selective Probe for Affinity Labelling of Monoamine Oxidase Type B in Human Brain and Platelet Membranes

: [3H]Ro 16–6491 [N-(2-aminoethyl)-p-chloroben-zamide HCl], a reversible “mechanism-based” inhibitor of monoamine oxidase (MAO) type B, binds selectively and with high affinity to the active site of MAO-B in brain and platelet membranes. Under normal conditions, the binding of [3H]Ro 16–6491 is fully reversible. However, [3H]Ro 16–6491 could be irreversibly bound (covalently) to membranes by the addition of the reducing agent NaBH3CN to the sample and adjusting to pH 4.5 with acetic acid. No irreversible labelling occurred in the absence of NaBH3CN and at neutral pH. The presence of the irreversible MAO-B inhibitor /-deprenyl completely abolished the irreversible labelling of the membranes by [3H]Ro 16–6491. The selective inactivation of MAO-B, e.g., by /-deprenyl prevented the covalent incorporation of [3H]Ro 16–6491 whereas selective inhibition of the MAO-A by clorgyline was without effect. The covalent linkage to membranes of unlabelled Ro 16–6491 and Ro 19–6327 (a selective and reversible MAO-B inhibitor closely related to Ro 16–6491) after the addition of NaBH3CN at pH 4.5 irreversibly inactivated MAO-B activity whereas MAO-A activity was unaffected. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of labelled membranes showed that [3H]Ro 16–6491 was incorporated into a single polypeptide with a molecular mass identical to the one labelled by [3H]pargyline (58 kilodaltons). Our results indicate that the polypeptide that is covalently labelled by [3H]Ro 16–6491 corresponds to one of the two MAO-B subunits. Therefore, [3H]Ro 16–6491 represents a selective probe for affinity labelling of MAO-B and for the investigation of the structural composition of the active site of the enzyme. Whether the reduction with NaBH3CN at pH 4.5 of the [3H]Ro 16–6491-MAO-B complex results in the formation of a stable adduct with the amino acid chain of the MAO-B or with its prosthetic group, FAD, remains to be elucidated.