REGULATION OF ALTERNATIVE OXIDASE ACTIVITY IN HIGHER-PLANTS

Plant mitochondria contain two terminal oxidases: cytochrome oxidase a nd the cyanide-insensitive alternative oxidase. Electron pardoning bet ween the two pathways is regulated by the redox poise of the ubiquinon e pool and the activation state of the alternative oxidase. The altern ative oxidase appears to exist as a dimer which is active in the reduc ed, noncovalently linked form and inactive when in the oxidized, coval ently linked form. Reduction of the oxidase in isolated tobacco mitoch ondria occurs upon oxidation of isocitrate or malate and may be mediat ed by matrix NAD(P)H. The activity of the reduced oxidase is governed by certain other organic acids, notably pyruvate, which appear to inte ract directly with the enzyme. Pyruvate alters the interaction between the alternative oxidase and ubiquinol so that the oxidase becomes act ive at much lower levels of ubiquinol and competes with the cytochrome pathway for electrons. These requirements for activation of the alter native oxidase constitute a sophisticated feed-forward control mechani sm which determines the extent to which electrons are directed away fr om the energy-conserving cytochrome pathway to the non-energy conservi ng alternative oxidase. Such a mechanism fits well with the proposed r ole of the alternative oxidase as a protective enzyme which prevents o ver-reduction of the cytochrome chain and fermentation of accumulated pyruvate.