Interactions of the neurotoxin MPTP and its demethylated derivative (PTP) with monoamine oxidase-B.

The kinetics of the interactions of MPTP and its N-des-methyl-derivative (PTP) have been studied. Both were mechanism-based inhibitors as well as substrates for the enzyme. Analysis of the reaction progress-curves for the formation of the corresponding dihydropyridine derivatives allowed the kinetic parameters for the process and the partition ratio, which corresponds to the number of mol. of product formed per mol. of enzyme inactivated, to be determined for both compounds. The conversion of MPTP to its corresponding pyridinium-ion derivative through the action of MAO-B is known to be essential for its neurotoxicity. PTP has been reported not to be neurotoxic, although it appears to be a relatively good substrate for MAO-B as well as acting as a mechanism-based inhibitor. Studies of the changes in absorbance spectra during the MAO-B catalysed oxidation were consistent with the formation of the corresponding pyridinium-ion derivative (MPP+), which is known to be the effective neurotoxin, as the end-product when MPTP was oxidized. In contrast the oxidation of PTP appeared to stop at the dihydropyridine stage with no significant further oxidation to the corresponding pyridine-derivative.