The Role of the Alternative Oxidase in Stabilizing the in Vivo Reduction State of the Ubiquinone Pool and the Activation State of the Alternative Oxidase

A possible function for the alternative (nonphosphorylating) pathway is to stabilize the reduction state of the ubiquinone pool (Q_r/Q_t), thereby avoiding an increase in free radical production. If the Q_r/Q_t were stabilized by the alternative pathway, then Q_r/Q_t should be less stable when the alternative pathway is blocked. Q_r/Q_t increased when we exposed roots of Poa annua (L.) to increasing concentrations of KCN (an inhibitor of the cytochrome pathway). However, when salicylhydroxamic acid, an inhibitor of the alternative pathway, was added at the same time, Q_r/Q_t increased significantly more. Therefore, we conclude that the alternative pathway stabilizes Q_r/Q_t. Salicylhydroxamic acid increasingly inhibited respiration with increasing concentrations of KCN. In the experiments described here the alternative oxidase protein was invariably in its reduced (high-activity) state. Therefore, changes in the reduction state of the alternative oxidase cannot account for an increase in activity of the alternative pathway upon titration with KCN. The pyruvate concentration in intact roots increased only after the alternative pathway was blocked or the cytochrome pathway was severely inhibited. The significance of the pyruvate concentration and Q_r/Q_t on the activity of the alternative pathway in intact roots is discussed.