CALCIUM-INDUCED ASSOCIATIONS OF THE CASEINS - THERMODYNAMIC LINKAGE OF CALCIUM-BINDING TO COLLOIDAL STABILITY OF CASEIN MICELLES

The caseins occur in milk as colloidal complexes of protein aggregates, calcium, and inorganic phosphate. As determined by electron microscopy, these particles are spherical and have approximately a 650 angstrom radius (casein micelles). In the absence of calcium, the protein aggregates themselves (submicelles) have been shown to result from mainly hydrophobic interactions. The fractional concentration of stable colloidal casein micelles can be obtained in a calcium caseinate solution by centrifugation at 1500 g. Thus, the amount of stable colloid present with varying Ca2+ concentrations can be determined and then analyzed by application of equations derived from Wyman's Thermodynamic Linkage Theory. Ca2+ -induced colloid stability profiles were obtained experimentally for model micelles consisting of only alpha-s-1-(a calcium insoluble casein) and the stabilizing protein kappa-casein, eliminating the complications arising from beta- and minor casein forms. Two distinct genetic variants alpha-s-1-A and B were used. Analysis of alpha-s-1-A colloid stability profiles yielded a precipitation (salting-out) constant k1, as well as colloid stability (salting-in) parameter k2. No variations of k1 or k2 were found with increasing amounts of kappa-casein. From the variation of the amount of colloidal casein capable of being stabilized vs. amount of added kappa-casein an association constant of 4 L/g could be calculated for the complexation of alpha-s-1-A and kappa-casein. For the alpha-s-1-B and kappa-casein micelles, an additional Ca2+ -dependent colloidal destabilization parameter, k3, was added to the existing k1 and k2 parameters in order to fully describe this more complex system. Furthermore, the value of k3 decreased with increasing concentration of kappa-casein. These results were analyzed with respect to the specific deletion which occurs in alpha-s-1-casein A in order to determine the sites responsible for these Ca2+ -induced quaternary structural effects.