Bioactivation of MPTP: reactive metabolites and possible biochemical sequelae.
Expression of the selective nigrostriatal neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine [MPTP] requires its bioactivation by MAO B which leads to the formation of potentially reactive metabolites including the 2-electron oxidation product, 1-methyl-4-phenyl-2,3-dihydropyridinium species [MPDP+] and the 4-electron oxidation product, the 1-methyl-4-phenyl pyridinium species [MPP+]. The latter metabolite accumulates in brain striatal tissues, is a substrate for dopaminergic active uptake systems and is an inhibitor of mitochondrial NADH dehydrogenase, a respiratory chain enzyme located in the inner mitochondrial membrane. In intact mitochondria this inhibition of respiration may be facilitated by active uptake of MPP+, a process dependent on the membrane electrical gradient. In considering possible mechanisms involved in the biochemical effects of MPP+, its redox cycling potential appears to be much lower than its chemical congener paraquat, based on attempted radical formation by chemical or enzymic reduction. Theoretically, a carbon-centered radical intermediate could be formed by 1-electron reduction of MPP+, or by 1-electron oxidation of 1-methyl-4-phenyl-1,2-dihydropyridine, the free base form of MPDP+. The 1-electron reduction of such a radical could form 1-methyl-4-phenyl-1,4-dihydropyridine [DHP]. Synthetic DHP is neurotoxic in C57B mice, and its administration leads to the formation of MPP+ in the brain, presumably through rapid auto-oxidation. The hydrolysis of DHP would yield 3-phenylglutaraldehyde and methylamine. Recent studies demonstrating the formation of methylamine in brain mitochondrial preparations containing MPTP support our suggestion that DHP may be a brain metabolite of MPTP.
D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D015655:1-Methyl-4-phenylpyridinium; D000818:Animals; D001692:Biological Transport; D001711:Biotransformation; D003342:Corpus Striatum / Q000187:drug effects / Q000378:metabolism; D004298:Dopamine / Q000502:physiology; D005609:Free Radicals; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D008995:Monoamine Oxidase / Q000378:metabolism; D010084:Oxidation-Reduction; D011725:Pyridines / Q000378:metabolism* / Q000633:toxicity; D011726:Pyridinium Compounds / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D013378:Substantia Nigra / Q000187:drug effects / Q000378:metabolism
D005609:Free Radicals; D011725:Pyridines; D011726:Pyridinium Compounds; D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D008995:Monoamine Oxidase; D015655:1-Methyl-4-phenylpyridinium; D004298:Dopamine