Inactivation of monoamine oxidase by 3,3-dimethyl analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 1-methyl-4-phenyl-2,3-dihydropyridinium iron. Dramatic effect of β-mercaptoethanol on substrate turnover and enzyme inactivity

It was previously shown (Sayre, L.M., Arora, P.K., Feke S.C., and Urbach, F.L. (1986) J. Am. Chem. Soc. 108, 2464-2466) that 1,3,3-trimethyl-4-phenyl-2,3-dihydropyridinium salt (the 3,3-dimethyl analogue of 1-methyl-4-phenyl-2,3-dihydropyridinium ion or MPDP⁺) is a good model for MPDP⁺ on the basis of its redox potential and was used to show that MPDP⁺ is unlikely to possess reactivity characteristics which could contribute to the neurotoxicity observed with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). 3,3-Dimethyl-MPTP and 3,3-dimethyl-MPDP⁺ are now shown to interact with monoamine oxidase similar to MPTP and MPDP⁺, but only in the presence of β-mercaptoethanol (β-ME). In the absence of β-ME, mixed competitive-noncompetitive inhibition kinetics are observed for 3,3-dimethyl MPTP and 3,3-dimethyl-MPDP⁺, whereas competitive inhibition kinetics are exhibited by MPTP. In the presence of β-ME, however, 3,3-dimethyl-MPTP also is a competitive inhibitor. 3,3-Dimethyl-MPTP and 3,3-dimethyl-MPDP⁺ also are time-dependent inactivators of monoamine oxidase, having identical kinetic constants, as is the case with MPTP and MPDP⁺. In the presence of β-ME, but not glutathione, the rate of inactivation increases dramatically. When [β-ME] and [3,3-dimethyl-MPTP] or [3,3-dimethyl-MPDP⁺] are varied, there is an optimal concentration of 1.0 mM for all three at which maximal inactivation rates are obtained. Another dramatic effect of the β-ME is to lower the partition ratio for inactivation from > 50 to about one. This suggests that the effect of the β-ME toward inactivation may be to induce a conformational change in the enzyme, which reorients an active site nucleophile for attack on the activated species. Support for involvement of an active site nucleophile is the finding that inactivation does not lead to a flavin adduct. Three possible mechanisms for inactivation of monoamine oxidase by MPTP and MPDP⁺ are suggested.