Effect of KCl Medium on Malate Oxidation in Mitochondria of Sugar Beet Taproot

Isolated mitochondria were obtained from growing and stored sugar beet (Beta vulgaris L.) taproots. These preparations were used to monitor the mitochondrial matrix volume and malate oxidation after the replacement of sucrose with KCl in the reaction medium. The transfer of mitochondria from sucrose-containing isolation medium to the isoosmotic KCl solution initiated spontaneous or energy-dependent (in the presence of respiratory substrate) swelling whose kinetic parameters (the initial rate and amplitude) were virtually independent of the plant age. At the same time, effects of KCl-induced swelling on oxidative and phosphorylating activities of mitochondria were age-dependent. In mitochondria from growing taproots, K⁺ ions stimulated nonphosphorylating malate oxidation, thereby decreasing the respiratory control ratio and the ADP/O coefficient. The incubation of mitochondria from stored taproots in KCl solution induced a short-term activation and subsequent progressive inhibition of malate oxidation but did not inhibit the oxidation of exogenous NADH. The inhibition of malate oxidation was not released by adding ADP or uncouplers and was enhanced in the presence of valinomycin. The swelling of mitochondria in KCl solutions did not impair the integrity of mitochondrial membranes and did not preclude stimulation of malate oxidation by exogenous NAD. It is supposed that the KCl-induced inhibition of respiration is related to a large increase in the matrix volume and a drastic decrease in the concentration of a coenzyme NAD. Previous studies with isolated mitochondria from stored taproots showed that the mitochondrial NAD level was a rate-limiting factor of malate oxidation assayed in the sucrose-containing media. A possible role of K⁺-transporting mechanisms in regulation of mitochondrial matrix volume and metabolic activity of plant mitochondria is discussed.