Da. Day et Jt. Wiskich, REGULATION OF ALTERNATIVE OXIDASE ACTIVITY IN HIGHER-PLANTS, Journal of bioenergetics and biomembranes, 27(4), 1995, pp. 379-385
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.