Monoamine oxidase B(MAO-B) is the major catalyst for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) oxidation in human brain and other tissues.

A new in vitro radiometric method has been developed for the direct assay of the oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydrophyridine (MPTP) to its main neurotoxic metabolite 1-methyl-4-phenylpyridinium. This assay has been used to show that the rate of oxidation of MPTP parallels that of phenylethylamine in a range of human and rodent tissues, providing strong evidence that this reaction is predominantly catalysed by monoamine oxidase B (MAO-B). In human brain the reaction was inhibited by selective doses of the MAO-B inhibitor (-)-deprenyl. When dopamine was added to the incubation mixture, products of MPTP oxidation appeared to form a complex with it.