Interactions Between the Cytochrome Pathway and the Alternative Oxidase in Isolated Acanthamoeba castellanii Mitochondria

The steady-state activity of the two quinol-oxidizing pathways of Acanthamoeba castellanii mitochondria, the phosphorylating cytochrome pathway (i.e. the benzohydroxamate(BHAM)-resistant respiration in state 3) and the alternative oxidase (i.e. the KCN-resistant respiration), is shown to be fixed by ubiquinone (Q) pool redox state independently of the reducing substrate (succinate or exogenous reduced nicotinamide adenine dinucleotide (NADH)), indicating that the active Q pool is homogenous. For both pathways, activity increases with the Q reduction level (up to 80%). However, the cytochrome pathway respiration partially inhibited (about 50%) by myxothiazol decreases when the Q reduction level increases above 80%. The decrease can be explained by the Q cycle mechanism of complex III. It is also shown that BHAM has an influence on the relationship between the rate of ADP phosphorylation and the Q reduction level when alternative oxidase is active, and that KCN has an influence on the relationship between the alternative oxidase activity and the Q reduction level. These unexpected effects of BHAM and KCN observed at a given Q reduction level are likely due to functional connections between the two pathways activities or to protein–protein interaction.