Indexed on: 01 Feb '91Published on: 01 Feb '91Published in: The Protein Journal
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 Å 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 1500g. 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 αs1- (a calcium insoluble casein) and the stabilizing protein κ-casein, eliminating the complications arising from β- and minor casein forms. Two distinct genetic variants αs1-A andB were used. Analysis of αs1-A colloid stability profiles yielded a precipitation (salting-out) constantk1, as well as colloid stability (salting-in) parameterk2. No variations ofk1 ork2 were found with increasing amounts of κ-casein. From the variation of the amount of colloidal casein capable of being stabilized vs. amount of added κ-casein an association constant of 4 L/g could be calculated for the complexation of αs1-A and κ-casein. For the αs1-B and κ-casein micelles, an additional Ca2+-dependent colloidal destabilization parameter,k3, was added to the existingk1 andk2 parameters in order to fully describe this more complex system. Furthermore, the value ofk3 decreased with increasing concentration of κ-casein. These results were analyzed with respect to the specific deletion which occurs in αs1-caseinA in order to determine the sites responsible for these Ca2+-induced quaternary structural effects.