Deuterium isotope effects for the oxidation of 1-methyl-3-phenyl-3-pyrrolinyl analogues by monoamine oxidase B

The parkinsonian inducing agent, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), is a cyclic tertiary allylamine exhibiting good monoamine oxidase B (MAO-B) substrate properties. MAO-B catalyzes the ring α-carbon 2-electron bioactivation of MPTP to yield the 1-methyl-4-phenyl-2,3-dihydropyridinium species (MPDP⁺). The corresponding 5-membered ring MPTP analogue, 1-methyl-3-phenyl-3-pyrroline, also undergoes MAO-B-catalyzed oxidation to give the 2-electron oxidation product, 1-methyl-3-phenylpyrrole. Here we report the kinetic deuterium isotope effects on V_max and V_max/K_m for the steady-state oxidation of 1-methyl-3-phenyl-3-pyrroline and 1-methyl-3-(4-fluorophenyl)-3-pyrroline by baboon liver MAO-B, using the corresponding pyrroline-2,2,4,5,5-d₅ analogues as the deuterated substrates. The apparent isotope effects for the two substrates were 4.29 and 3.98 on V_max, while the isotope effects on V_max/K_m were found to be 5.71 and 3.37, respectively. The values reported for the oxidation of MPTP by bovine liver MAO-B with MPTP-6,6-d₂, as deuterated substrate, are ^D(V_max) = 3.55; ^D(V_max/K_m) = 8.01. We conclude that the mechanism of the MAO-B-catalyzed oxidation of pyrrolinyl substrates is similar to that of the tetrahydropyridinyl substrates and that a carbon-hydrogen bond cleavage step is, at least partially, rate determining.