Mhn. Hoefnagel et Jt. Wiskich, ALTERNATIVE OXIDASE ACTIVITY AND THE UBIQUINONE REDOX LEVEL IN SOYBEAN COTYLEDON AND ARUM SPADIX MITOCHONDRIA DURING NADH AND SUCCINATE OXIDATION, Plant physiology, 110(4), 1996, pp. 1329-1335
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).