INHIBITION OF COMPLEX-I BY HYDROPHOBIC ANALOGS OF N-METHYL-4-PHENYLPYRIDINIUM (MPP(-SELECTIVE ELECTRODE TO MEASURE THEIR ACCUMULATION BY MITOCHONDRIA AND ELECTRON-TRANSPORT PARTICLES()) AND THE USE OF AN ION)

N-Methyl-4-phenylpyridinium (MPP(+)), the neurotoxic metabolite of N-m ethyl-4-phenyl-1,2,3,6-tetrahydropyridine, kills dopaminergic neurons after its accumulation in mitochondria where it inhibits Complex I of the respiratory chain. MPP(+) inhibits respiration by binding to both a hydrophobic and a hydrophilic site on Complex I and this inhibition is increased by the lipophilic tetraphenylboron anion (TPB-) which fac ilitates movement of MPP(+) through membranes and its penetration to t he hydrophobic binding site on Complex I. To investigate the inhibitio n of respiration by MPP(+)-like compounds, we have measured simultaneo usly NADH-linked mitochondrial respiration and the uptake and accumula tion of the N-benzyl-4-styrylpyridinium and N-ethyl-4-styrylpyridinium cations in mitochondria using ion-selective electrodes. The data prov ide direct evidence that TPB- increases the inhibition not by increasi ng matrix concentration but by facilitating access to the inhibitory s ites on Complex I. We have also compared the rates of uptake of MPP(+) analogues of varied lipophilicity by the inner membrane and the devel opment of inhibition of NADH oxidation, using an inverted mitochondria l inner membrane preparation and appropriate ion-selective electrodes. These experiments demonstrated that the amount of MPPC analogue bound to the inner membrane greatly exceeded the quantity required for comp lete inhibition of NADH oxidation. Moreover, binding to the membrane o ccurred much more rapidly than the development of inhibition with all MPP(+) analogues tested. This suggests that the attainment of a correc t orientation of these compounds within the membrane and the binding s ite may be a rate-limiting step in the development of inhibition.