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A null point titration method has been used to determine the free Ca2+ concentration in the matrix of rat liver and heart mitochondria which depends on addition of A-23187 to ruthenium red-inhibited mitochondria.The matrix free calcium was calculated from a knowledge of the extramitochondrial free calcium at which no net movement of calcium occurs and the ApH which, under these conditions, governs the equilibrium distribution of calcium across the inner mitochondrial membrane.Using this technique it was found that the relationship between total and free calcium was linear in the range studied (2-50 nmol/mg of protein).The activity coefficient for intramitochondrial calcium was 7.0 X for liver and 4.0 X
Coll et al. (Sun,) studied this question.