M. Ribascarbo et al., THE REACTION OF THE PLANT MITOCHONDRIAL CYANIDE-RESISTANT ALTERNATIVEOXIDASE WITH OXYGEN, Biochimica et biophysica acta. Bioenergetics, 1188(3), 1994, pp. 205-212
The dependence of electron flux through the cyanide-resistant respirat
ory pathway on the redox poise of the ubiquinone pool and oxygen conce
ntration was studied in purified mitochondria isolated from green and
etiolated soybean (Glycine max L. Merr. cv. Ransom) cotyledons at diff
erent ages (4 and 10 days after planting), soybean roots and mung bean
(Vigna radiata L.R. Vilcz) hypocotyls. In soybean, the K-m of the alt
ernative oxidase with respect to oxygen was found to vary between valu
es of 10 and 20 mu M. These are generally higher than values of the K-
m for oxygen of the alternative oxidase reported previously (0.5 to 2.
0 mu M). In addition, the value of the K-m for oxygen varied with the
redox poise of the ubiquinone pool, measured voltametrically; the more
reduced the quinone pool, the larger the observed K-m. These results
are at variance with the behavior expected of the kinetic model develo
ped by Siedow and Moore (1993; Biochim. Biophys. Acta 1142, 165-174) w
hich predicts that the K-m for oxygen should decrease as the quinone p
ool becomes more reduced. A modified kinetic model is developed that i
ncorporates an additional reaction step involving activation of the fo
ur-electron reduced oxidase into the earlier kinetic model. The modifi
ed model successfully simulates the dependence of the alternative oxid
ase activity on both ubiquinone pool redox poise and oxygen concentrat
ion.