title: Uptake and accumulation of 1-methyl-4-phenylpyridinium by rat liver mitochondria measured using an ion-selective electrode.
abstract: The compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes selective destruction of nigrostriatal dopaminergic neurons in primates, giving rise to a condition resembling Parkinson's disease. The toxicity of MPTP is believed to be due to its metabolite 1-methyl-4-phenylpyridinium (MPP+). MPP+ is an inhibitor of mitochondrial respiration at the NADH-ubiquinone oxidoreductase site and this, together with its selective transport into dopaminergic nerve terminals, accounts for its neurotoxicity. In this paper an electrode selective for MPP+ was developed and used to measure the rate of uptake and the steady-state accumulation of MPP+ in rat liver mitochondria. The initial rates of MPP+ uptake were not saturable, confirming previous work that the transport of MPP+ is not carrier-mediated. The membrane potential of mitochondria respiring on succinate was decreased by MPP+ and the steady-state accumulation ratio of MPP+ did not come to equilibrium with the mitochondrial transmembrane potential gradient (delta psi). The effect of the cation exchanger tetraphenylboron (5 microM) was to increase the initial rate of MPP+ uptake by about 20-fold and the steady-state accumulation by about 2-fold. This suggests that there may be a mechanism of efflux of MPP+ from mitochondria which allows MPP+ to cycle across the membrane and thus decrease delta psi. These data indicate that MPP+ interacts with mitochondria independently of its inhibition of NADH-ubiquinone oxidoreductase, and these alternative interactions may be of relevance for its mechanism of neurotoxicity.
authors.affiliation: Department of Biochemistry, Trinity College, Dublin, Ireland.
authors.forename: G P
authors.initials: GP
authors.lastname: Davey
authors.forename: K F
authors.initials: KF
authors.lastname: Tipton
authors.forename: M P
authors.initials: MP
authors.lastname: Murphy
mesh_terms: D015655:1-Methyl-4-phenylpyridinium / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002451:Cell Compartmentation / Q000187:drug effects; D004566:Electrodes; D005260:Female; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D008930:Mitochondria, Liver / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013775:Tetraphenylborate / Q000494:pharmacology
chemical_list: D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D013775:Tetraphenylborate; D015655:1-Methyl-4-phenylpyridinium