ALTERNATIVE OXIDASE ACTIVITY AND THE UBIQUINONE REDOX LEVEL IN SOYBEAN COTYLEDON AND ARUM SPADIX MITOCHONDRIA DURING NADH AND SUCCINATE OXIDATION

In Arum and soybean (Glycine max L.) mitochondria, the dependence of t he alternative oxidase activity on the redox level of ubiquinone, with NADH and succinate as substrates, was studied, using a voltametric pr ocedure to measure the ubiquinone redox poise in the mitochondrial mem brane. The results showed that when the enzyme was activated by pyruva te the relationship between the alternative oxidase rate and the redox state of the ubiquinone pool was the same for both NADH and succinate oxidations. In the absence of pyruvate the alternative oxidase had an apparent lower affinity for ubiquinol. This was more marked with NADH than with succinate and was possibly due to pyruvate production durin g succinate oxidation or to an activation of the alternative oxidase b y succinate itself. In Arum spadix (unlike soybean cotyledon) mitochon dria, succinate oxidation via the alternative oxidase maintained the u biquinone pool in a partially reduced state (60%), whereas NADH oxidat ion kept it almost completely reduced. Previous data comparing mitocho ndria from thermogenic and nonthermogenic tissues have not examined th e full range of ubiquinone redox levels in both tissues, leading to th e suggestion that the activity of alternative oxidase for Arum was dif ferent from nonthermogenic tissues. When the complete range of redox s tates of ubiquinone is used and the oxidase is fully activated, the al ternative oxidase from thermogenic tissue (Arum) behaves similarly to that of nonthermogenic tissue (soybean).