Activation of the Plant Alternative Oxidase by High Reduction Levels of the Q-Pool and Pyruvate

This report describes the activation of the alternative oxidase (AOX) of higher plant mitochondriaby a high reduction level of the ubiquinone poolin the presence of pyruvate. In mitochondria from both thermogenic (Arum italicumspadices) and nonthermogenic (Glycine maxcotyledons) tissues AOXis activated when the Q-pool becomes highly reduced in the presence of pyruvate. Pyruvate is essential for this activation. The enzyme is not activated when pyruvate is added after a transienthigh reduction level of the Q-pool, but is when pyruvate is added before the transient reduction. Pyruvate also protects the enzyme against inhibitionduring catalytic turnover. Although this activation is not accompanied by a reduction of the covalent disulfide bond, the same activation can be achieved with dithiothreitol (DTT). It is suggested that a partof the activation by DTT is not the result of reducing the covalent disulfide bond, and the relation between these types of activation is discussed.The importance of this activation for thein vivoregulation and its relation to previously reportedactivators is discussed. A mechanism is proposedin which it is suggested that AOX is inactivated by its product (oxidized ubiquinone) during catalysis and that this inhibition is prevented in the presence of pyruvate. The inhibition can be reversed by a reductive process, achieved by high levels of reduction of the Q-pool or by DTT, but not by pyruvate. This restoration of activity is not related to the redox process involved in reducing the covalent disulfide bond.