Mechanisms of citrate oxidation by percoll-purified mitochondria from potato tuber.

The mechanisms and accurate control of citrate oxidation by Percoll-purified potato (Solanum tuberosum) tuber mitochondria were characterized in various metabolic conditions by recording time course evolution of the citric acid cycle related intermediates and O(2) consumption. Intact potato tuber mitochondria showed good rates of citrate oxidation, provided that nonlimiting amounts of NAD(+) and thiamine pyrophosphate were present in the matrix space. Addition of ATP increased initial oxidation rates, by activation of the energy-dependent net citrate uptake, and stimulated succinate and malate formation. When the intramitochondrial NADH to NAD(+) ratio was high, alpha-ketoglutarate only was excreted from the matrix space. After addition of ADP, aspartate, or oxaloacetate, which decreased the NADH to NAD(+) ratio, flux rates through the Krebs cycle dehydrogenases were strongly increased and alpha-ketoglutarate, succinate, and malate accumulated up to steady-state concentrations in the reaction medium. It was concluded that NADH to NAD(+) ratio could be the primary signal for coordination of fluxes through electron transport chain or malate dehydrogenase and NAD(+)-linked Krebs cycle dehydrogenases. In addition, these results clearly showed that the tricarboxylic acid cycle could serve as an important source of carbon skeletons for extra-mitochondrial synthetic processes, according to supply and demand of metabolites.