Redox state of quinone affects sensitivity of Acanthamoeba castellanii mitochondrial uncoupling protein to purine nucleotides
Résumé
We studied free fatty acid-induced uncoupling activity in A. castellanii mitochondria in nonphosphorylating state. Either succinate or external NADH was used as a respiratory substrate to determine the proton conductance curves and the respiratory rate versus the quinone reduction level relationships. Our determinations of the membranous quinone reduction level in nonphosphorylating mitochondria show that activation of UCP activity leads to a purine nucleotide-sensitive decrease in quinone redox state. The gradual decrease in the rate of quinone-reducing pathways (titration of dehydrogenases activity) progressively leads to a full inhibitory effect of GDP on linoleic-acid induced proton conductance. This inhibition cannot be attributed to changes in the membrane potential. Indeed, the lack of GDP inhibitory effect observed when decrease in respiratory rate is accompanied by an increase in the quinone reduction level (titration of quinol-oxidizing pathway) proves that the inhibition by nucleotides can be revealed only for a low quinone redox state. It must be underlined that in A. castellanii nonphosphorylating mitochondria, the transition of the inhibitory effect of GDP on linoleic acid-induced UCP-mediated uncoupling is observed for the same range of quinone reduction level (between 50% and 40%) as that observed previously for phosphorylating conditions. This observation drawn from the two different metabolic states of mitochondria indicates that quinone could affect UCP activity through sensitivity to purine nucleotides.
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