The reaction of the plant mitochondrial cyanide-resistant alternative oxidase with oxygen

Abstract

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 μM. These are generally higher than values of the K m for oxygen of the alternative oxidase reported previously (0.5 to 2.0 μM). 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.