Molecular determinants in the bioactivation of the dopaminergic neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

Abstract

Nineteen analogs of the dopaminergic neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) have been used as probes to study the structural parameters that influence MAO-catalyzed oxidation. In this study, the efficiency of enzyme-catalyzed substrate oxidation was found to be unrelated to parameters such as the ionization potential, dipole moment, net atomic charge at C5 and the dihedral angle between the phenyl ring and the tetrahydropyridine moiety. Conformational analysis revealed that substitution at the C2' position of MPTP yields atropisomers. It is suggested that one of these atropisomers would be either inactive or substantially less active than the other. Therefore, the relative oxidative efficiency and toxicity of these compounds reported earlier may have been significantly underestimated. Based on the conformational analysis and other data, a rudimentary model of the MAO substrate site has been developed which partially explains the substrate specificities of MAO A and MAO B. Each substrate binding site can be divided into two regions, (a) an amine-binding pocket (for the tetrahydropyridine moiety), and (b) a 'bulky substituent' region (for the phenyl group and its substituents). The length of the substrate binding site (measured along the long axis of MPTP) is approximately 8.5 A, and the width of the 'amine-binding' pocket is approximately 2.5 A (from C3 to C5). The 'bulky substituent' region contains a central area for binding the phenyl group of MPTP. This central area is flanked by two hydrophobic pockets, P2' and P3'. In MAO A, the pocket P2'-A is oriented 45-135 degrees relative to the plane of the tetrahydropyridine moiety, with a radius of 3.1 A from C2' of the phenyl ring. The radius of a similar but smaller pocket, P2'-B, in MAO B, is approximately 2.7 A. In MAO B, the pocket P3'-B (radius 2.36 A from C3') is larger than a similar pocket P3'-A (radius 1.70 A from C3') in MAO A. The foregoing characterization suggests that differences in the size and topography of both of the substituent pockets play an important role in determining the substrate specificities of these two isozymes.