THE REACTION OF THE PLANT MITOCHONDRIAL CYANIDE-RESISTANT ALTERNATIVE OXIDASE WITH OXYGEN

The dependence of electron flux through the cyanide-resistant respiratory pathway on the redox poise of the ubiquinone pool and oxygen concentration was studied in purified mitochondria isolated from green and etiolated soybean (Glycine max L. Merr. cv. Ransom) cotyledons at different 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 alternative oxidase with respect to oxygen was found to vary between values 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 developed by Siedow and Moore (1993; Biochim. Biophys. Acta 1142, 165-174) which predicts that the K-m for oxygen should decrease as the quinone pool becomes more reduced. A modified kinetic model is developed that incorporates an additional reaction step involving activation of the four-electron reduced oxidase into the earlier kinetic model. The modified model successfully simulates the dependence of the alternative oxidase activity on both ubiquinone pool redox poise and oxygen concentration.