Inhibition of mitochondrial respiration by analogues of the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium: Structural requirements for accumulation-dependent enhanced inhibitory potency on intact mitochondria☆

Analogues of 1-methyl-4-phenylpyridinium (MPP+), the neurotoxic metabolite of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, were evaluated for inhibition of respiration in intact mitochondria (Mw) and in electron transport particles (ETP). MPP+ exhibits relatively weak inhibitory activity in ETP, but potent inhibition in Mw occurs on account of its energy-dependent accumulation inside mitochondria. The permeant anion tetraphenylborate potentiates the inhibition in both Mw and ETP. Replacement of the 4-phenyl ring of MPP+ by a variety of aromatic and nonaromatic rings, and of the N-methylpyridinium group by other cationic aromatic heterocycles, preserves the inhibitory patterns seen for MPP+. The general observation of enhanced inhibitory potency in Mw for all these permanently charged cations is consistent with our contention that energy-dependent accumulation inside mitochondria represents a passive Nernstian concentration in response to the transmembrane electrochemical gradient. Nonetheless, the magnitude of the inhibitory potentiation seen in Mw relative to ETP varies widely with structure. In particular, less lipophilic analogues, especially those bearing a localized, rather than resonance-stabilized, permanent positive charge, exhibit similar inhibitory activity to MPP+ in ETP, but the inhibition in Mw is not comparably enhanced. For these same analogues, the inhibitory activity in ETP is only weakly potentiated by tetraphenylborate. Since succinate was found to completely reverse the respiratory inhibition in Mw induced by all types of MPP+ analogues investigated, a common site 1 inhibition appears to be involved; thus the different inhibitory patterns observed must be due to structural factors governing membrane transport and distribution properties.