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 alpha-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-d(5) 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-d(2), 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.