Interactions of calcium with the pyridine nucleotides

Association constants were determined for the 1:1 interactions of calcium with NAD⁺, NADH, NADP⁺, and NADPH in aqueous systems (pH 7, 25 °C) by use of a calcium-sensitive electrode. The order of binding of calcium to these pyridine nucleotides appears to be NAD⁺ < NADH < NADP⁺ < NADPH with association constants of 0.2 × 10², 0.3 × 10², 0.9 × 10², and 2 × 10², respectively. Calorimetric experiments revealed that all of these interactions are endothermic with enthalpy changes of 1, 2, 2, and 3 kcal/mol, respectively.