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.