Deuterium isotope effect measurements on the interactions of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine with monoamine oxidase B.
Kinetic deuterium isotope effects for the noncompetitive, intermolecular monoamine oxidase B-catalyzed oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the corresponding 1-methyl-4-phenyl-2,3-dihydropyridinium species MPDP+ were found to be 3.55 on Vmax and 8.01 on Vmax/Km with MPTP-6,6-d2 as the deuterated substrate. Similar values were obtained with MPTP-2,2,6-d4 and MPTP-CD3-2,2,6,6-d4. The deuterium isotope effect for the electrochemical oxidation of 1 mM MPTP-2,2,6,6-d4 was only 1.35. These results indicate that the monoamine oxidase B-catalyzed oxidation of this substrate may not proceed via a reaction pathway involving alpha-carbon deprotonation of an aminium radical intermediate. Isotope effect measurements also established that the rate of inactivation of monoamine oxidase B by MPTP is unaffected by replacement of the C-6 methylene protons with deuterons, but is retarded by replacement of the C-2 methylene protons (DKi = 1.9). The mechanism-based inactivation of monoamine oxidase B by MPTP, therefore, is likely to mediated by a species derived from the enzyme-generated 2,3-dihydropyridinium oxidation product.
Ottoboni
S
S
Department of Pharmaceutical Chemistry, University of California, San Francisco 94143.
Caldera
P
P
Trevor
A
A
Castagnoli
N
N
D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D000818:Animals; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D015198:Designer Drugs; D003903:Deuterium; D007553:Isotope Labeling; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008930:Mitochondria, Liver / Q000201:enzymology; D008995:Monoamine Oxidase / Q000378:metabolism*; D009498:Neurotoxins / Q000378:metabolism*; D011485:Protein Binding; D011725:Pyridines / Q000378:metabolism*
D015198:Designer Drugs; D009498:Neurotoxins; D011725:Pyridines; D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D003903:Deuterium; D008995:Monoamine Oxidase