Binding of Calcium to Serum Albumin I. Stoichiometry and Intrinsic Association Constant at Physiological pH, Ionic Strength, and Temperature

Ultrafiltration and spectrophotometric methods have shown that calcium is bound reversibly to serum albumin by a single class of 12±1 identical, independent binding sites with an association constant = 90-100 litres/mole at 37 °C in unbuffered solutions with pH 7.4 and ionic strength 0.15–0.16. Tea. albumin, and calcium ion concentration were varied over 11-, 6-, and 88-fold ranges. Albumins prepared by different methods had the same affinity. An unrecognized high citrate content of purified albumin is shown to be a plausible cause of previous disparate binding results.