Comparative thermodynamic linkage study of the calcium-induced solubility of bovine and caprine caseins

In this paper, the calcium-induced solubility profiles of isolated caprine alpha s1-casein and both bovine and caprine whole caseins, containing known amounts of alpha s1, have been analyzed and compared using Wyman's theory of thermodynamic linkage in conjunction with nonlinear regression analysis. All of the solubility profiles could be described by an initial salting-out followed by salting-in. These events were quantified by a salting-out constant, k1, and a salting-in constant, k2. These constants are correlated with association constants for salt binding so that n and m, the number of moles of calcium bound to different classes of sites which induce the respective changes in solubility, could also be quantitated. The purified alpha s1-casein from caprine milk differs from its bovine counterparts in that the parameters k1 and n are somewhat smaller, indicating less salt binding at both 24 and 1 degree C. Conversely, for whole caseins, k, was 6 times larger for caprine, which contains significantly less alpha s1-casein, than for bovine whole casein. This suggests stronger Ca2+-casein interactions in caprine whole casein as compared to whole bovine casein. Binding of Ca2+ ions to stronger affinity sites results in greater "salting-out" for caprine whole casein. Studies on temperature and ionic strength dependence of the comparative solubilities confirm this hypothesis