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 mitochondria by a high reduction level of the ubiquino ne pool in the presence of pyruvate. In mitochondria from both thermog enic (Arum italicum spadices) and nonthermogenic (Glycine max cotyledo ns) tissues AOX is 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 transient h igh reduction level of the Q-pool, but is when pyruvate is added befor e the transient reduction. Pyruvate also protects the enzyme against i nhibition during catalytic turnover. Although this activation is not a ccompanied by a reduction of the covalent disulfide bond, the same act ivation can be achieved with dithiothreitol (DTT). It is suggested tha t a part of the activation by DTT is not the result of reducing the co valent disulfide bond, and the relation between these types of activat ion is discussed. The importance of this activation for the in vivo re gulation and its relation to previously reported activators is discuss ed. A mechanism is proposed in which it is suggested that AOX is inact ivated by its product (oxidized ubiquinone) during catalysis and that this inhibition is prevented in the presence of pyruvate. The inhibiti on 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 restorati on of activity is not related to the redox process involved in reducin g the covalent disulfide-bond. (C) 1998 Academic Press.