Interaction of calcium ion with bovine caseins

In order to clarify the interaction of calcium ion with casein, the volume change associated with the interaction was measured by dilatometric procedures. When CaCl 2 was added to the casein solutions at neutral pH, a volume increase occurred and reached a constant saturated value of about 700 ml per 10 6 g protein with increasing CaCl 2 concentrations for whole-, α s - and β-casein solutions, but there was no volume change for κ-casein solution. On the other hand, the binding of calcium ion to the casein fractions was determined by a gel filtration procedure at pH 6.0 to 9.0. The number of Ca 2+ ions bound to the caseins increased with the CaCl 2 concentration and pH value, and the relative order of binding capacities for the caseins was: α s -casein > whole-casein > β-casein > κ-casein. It was found that the volume changes obtained by the dilatometry were smaller than the calculated volume increases based on the assumption that these are caused by the binding of Ca 2+ ion to the caseins. Therefore it is necessary to introduce another factor which reduces the volume increase due to the Ca 2+ ion binding in order to reasonably explain the measured volume changes. At present it is presumed that there occurs the unfolding of peptide chain of casein molecule on Ca 2+ ion binding, which has been known to decrease the volume of the protein solution.