Titration of human brain type-B monoamine oxidase

The interaction of the substrate-selective irreversible inhibitor J-508 [N-methyl-N-propargyl-(1-indanyl)-ammonium hydrochloride] with the B form of human brain monoamine oxidase has been investigated, and the conditions necessary for this inhibitor to "titrate" the concentration of this enzyme form determined. It was found that the concentration of monoamine oxidase-B determined in this way was the same when either benzylamine or beta-phenethylamine was used to assay for activity, which would indicate that this enzyme form is not heterogeneous. Furthermore, the variation in activity from sample to sample was found to be due to a variation in the concentration of available monoamine oxidase-B active centers, raher than due to a variation in the molecular turnover numbers of this enzyme form towards its amine substrates.