THE REACTION OF THE PLANT MITOCHONDRIAL CYANIDE-RESISTANT ALTERNATIVEOXIDASE WITH OXYGEN

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.