IS COMPLEX-II INVOLVED IN THE INHIBITION OF MITOCHONDRIAL RESPIRATIONBY N-METHYL-4-PHENYLPYRIDINIUM CATION (MMP-METHYL-BETA-CARBOLINES() AND N)

It has been reported that N-methyl-beta-carbolinium analogues of the n eurotoxic N-methyl-4-phenylpyridinium cation (MPP+) inhibit NADH-linke d mitochondrial oxidations, as well as mitochondrial respiration on su ccinate nearly to the same extent [Fields, Albores, Neafsey and Collin s (1992) Arch. Biochem. Biophys. 294, 539-544]. Those authors further claimed that MPP+ itself also blocks respiration through succinate deh ydrogenase, in addition to its well-known effect on NADH dehydrogenase (Complex I), and concluded that both effects may contribute to the de velopment of Parkinsonian symptoms. Since N-methyl-beta-carboliniums a re thought to be endogenous metabolites, these findings, if verified, would have important implications on the etiology of idiopathic Parkin sonism. We have re-examined these observations, using mitochondria aft er full activation of succinate dehydrogenase, as well as submitochond rial particles, in which complexities due to membrane transport are no t present. We report the following observations. (1) N-Methyl-beta-car boliniums inhibit mitochondrial respiration on NAD+-linked substrates in a time-dependent manner, and the inhibition is potentiated by the p resence of tetraphenylboron anion (TPB-), as expected for positively c harged compounds. (2) Unlike MPP+ itself, however, these compounds are uncouplers at higher concentrations, so that the effects seen in Stat e 3 cannot be assigned exclusively to inhibition of NADH oxidation. (3 ) The effects on succinate oxidation in mitochondria, in which the ful l activity of the enzyme is expressed, are 1-1.5 orders of magnitude l ower than on respiration via Complex I and are thus unlikely to contri bute significantly to the neurotoxicity. (4) The effect of MPP+ on mit ochondrial respiration via succinate dehydrogenase is trivial, in acco rd with previous reports from several laboratories, but contradicting the findings of Fields et al. (cited above). (5) In submitochondrial p articles the inhibition of NADH oxidation (via the complete respirator y chain) has been confirmed, but it differs markedly from the action o f MPP+ in two respects. First, the enhancement by TPB- is very small; secondly, the inhibition of NADH oxidation measured using ubiquinone ( Q) analogues is far lower, suggesting that Complex I is not the only t arget. (6) In submitochondrial particles the inhibition of succinate o xidation by either O2 or Q analogues is incomplete, trivial or absent. (7) We thus conclude that we find no basis for assigning any potentia l biological effect of N-methyl-beta-carboliniums to the blockade of s uccinate oxidation.