Effect of KCl-medium on succinate oxidation and hydrogen peroxide generation in mitochondria of sugar beet taproot

The effect of substitution of KCl for sucrose in the reaction medium on succinate oxidation and hydrogen peroxide generation was investigated in the mitochondria isolated from stored taproots of sugar beet (Beta vulgaris L.). In a sucrose-containing medium, oxidation of succinate was inhibited by oxaloacetate; this inhibition was especially pronounced upon a decrease in substrate concentration and eliminated in the presence of glutamate, which removed oxaloacetate in the course of transamination. Irrespective of succinate concentration, substitution of KCl for sucrose in the medium considerably enhanced suppression of succinate oxidation apparently as a result of slow activation of succinate dehydrogenase (SDH) by its substrate. In this case, mitochondria showed the symptoms of uncoupling, lower values of membrane potential (Delta I), respiratory control (RC), and ADP/O induced by electrophoretic transport of potassium via K+ channel of mitochondria. KCl-dependent suppression of succinate oxidation by taproot mitochondria was accompanied by a considerable inhibition of H2O2 production as compared with the sucrose-containing medium. These results indicate that in the presence of potassium ions, Delta I dissipates, suppression of succinate oxidation by oxaloacetate increases, and succinate-dependent generation of ROS in sugar beet mitochondria is inhibited. A possible physiological role of oxaloacetate-restricted SDH activity in the suppression of respiration of storage organs protecting mitochondria from oxidative stress is discussed.