Abstract
Reactions between Ca and casein were investigated at Ca concentrations ranging from zero to ten mM. αs-Casein began to precipitate at Ca 3 mM and was completely stabilized by κ-casein. κ-Casein tended to aggregate randomly in the presence of 5 mM Ca. S20, w values were 14.4 and 1.8 for κ-casein and αs-casein, respectively, in the absence of Ca. Caseins were found in gel filtration to be brought into polymerization at such low Ca concentrations that no aggregated particles were visible. αs-κ Complex did not grow as fast as αs-casein did in the presence of Ca. The amounts of Ca bound to caseins were determined at pH 7 and 8 in the presence of 1 to 10 mM Ca using the followings; equilibrium dialysis, gel filtration and centrifugation. The minimum amount of Ca to be necessary for the initiation of αs-casein precipitation was about 12 moles per mole casein. Weak adsorption of Ca, due to some structural factor, was indicated since the amounts of bound Ca obtained by centrifugation were remarkably lower than those determined by the other methods. The states of bound Ca were studied by gel filtration of 45Ca-caseinates with an eluant containing no Ca. More Ca was bound to the αs-κ complex in the early stage of the reaction than to the same quantity of αs-casein only. But the amount of Ca bound to the complex did not vary with reaction time while that bound to αs-casein increased greatly accompanied by the progress of polymerization. The binding intensity of Ca in κ-casein was so weak that only a small amount of Ca was eluted together with κ-casein in the gel filtration of Ca-κ-caseinate.
