1. The reduction of mitochondrial NAD(P) by 2-oxoglutarate was monitored as a measure of 2-oxoglutarate dehydrogenase activity in its intramitochondrial locale. In the absence of ADP, steady-state reduction of NAD(P) by 0.5 mM-2-oxoglutarate in the presence of 0.5 mM-L-malate was markedly increased by extramitochondrial Ca2+, with 50% activation at pCa 6.58, when the Na+ concentration was 10 mM, the Pi concentration ws 5 mM and the added Mg2+ concentration was 1 mM. Omission of Pi resulted in 50% activation at pCa 6.77; omission of Mg2+ resulted in 50% activation at pCA greater than or equal to 7.3. 2. The activation of 2-oxoglutarate dehydrogenase could be reversed on addition of an excess of EGTA. The rate of inactivation was dependent on the concentration of Na+, with K0.5 2.5 mM, which is consistent with the rate of withdrawal of Ca2+ from the mitochondria being the limiting factor. 3. The steady-state reduction of cytochrome c by 2-oxoglutarate (0.5 mM) also showed a marked dependence on pCa in the absence of ADP; in the presence of an excess of ADP, no such effect of Ca2+ was detectable. 4. Mitochondria from the hearts of senescent rats showed an undiminished rate of dehydrogenase activation by Ca2+ but a rate of inactivation by excess EGTA that was diminished by 40%. Direct studies of Ca2+ egress with Arsenazo III confirmed a decrement in rate with old age. 5. Studies of 2-oxoglutarate dehydrogenase activity as a function of the mitochondrial context of Ca2+, as measured by atomic-absorption spectrophotometry, showed half-maximal activation at a mitochondrial content of 1.0 nmol of Ca2+/mg of protein, and saturation at 3 nmol/mg. 6. These findings support the model advanced by Denton, Richards & Chin [(1978) Biochem. J. 176, 899-906], of a control of the tricarboxylate cycle by intramitochondrial Ca2+, and demonstrate the range of mitochondrial Ca2+ content over which this may occur. In addition, they raise the possibility of a disturbance of this control mechanism in old age.

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