ORGANIC-ACID ACTIVATION OF THE ALTERNATIVE OXIDASE OF PLANT-MITOCHONDRIA

Alternative oxidase activity (oxygen uptake in the presence of KCN, an timycin or myxothiazol) in mitochondria isolated from the roots of soy bean seedlings was very slow, even with succinate as substrate. This a ctivity was stimulated substantially (100-400%) by the addition of pyr uvate, with half maximal stimulation occurring at 0.1 mM pyruvate. Mit ochondria from soybean shoots displayed high alternative oxidase activ ity with succinate and malate as substrates but lower activity with ex ogenous NADH; addition of pyruvate stimulated the activity with NADH u p to that seen with succinate. This stimulation of cyanide-insensitive NADH oxidation was seen also with mitochondria from other species. Hy droxypyruvate and oxoglutarate could substitute for pyruvate, although higher concentrations were required to achieve maximum stimulation. P yruvate stimulation of cyanide-insensitive oxygen uptake was observed with exogenous quinols as substrates, with sub-mitochondrial particles , and in the presence of the pyruvate transport inhibitor, cyanohydrox ycinnamic acid, but was not observed with detergent-solubilised mitoch ondria. It is suggested that pyruvate acts allosterically on the alter native oxidase to stimulate its activity. The implications of these fi ndings for respiration in vivo are discussed.