Impact of oxidative stress on Acanthamoeba castellanii mitochondrial bioenergetics depends on cell growth stage

Addition of a moderate (1.4 mM) concentration of H 2 O 2 to protozoon Acanthamoeba castellanii cell cultures at different growth phases caused a different response to oxidative stress. H 2 O 2 treatment of exponentially growing cells significantly delayed their growth; however, in mitochondria isolated from these cells, no damage to their bioenergetic function was observed. In contrast, addition of H 2 O 2 to A. castellanii cells approaching the stationary phase did not influence their growth and viability while seriously affecting mitochondrial bioenergetic function. Although mitochondrial integrity was maintained, oxidative damage was revealed in the reduction of cytochrome pathway activity, uncoupling protein activity, and the efficiency of oxidative phosphorylation as well as the membrane potential and the endogenous ubiquinone reduction level of the resting state. An increase in the alternative oxidase protein level and activity as well as an increase in the membranous ubiquinone content were observed in mitochondria isolated from late H 2 O 2 -treated cells. For the first time, the regulation of ubiquinone content in the inner mitochondrial membrane is shown to play a role in the response to oxidative stress. A physiological role for the higher activity of the alternative oxidase in response to oxidative stress in unicellular organisms, such as amoeba A. castellanii , is discussed.