Effect of Bound Phosphate on the Calcium-binding Ability and Calcium-dependent Precipitability of Human β-Casein

Abstract

The calcium-binding ability and calcium-dependent precipitability of human β-casein were studied and compared regarding components with different phosphorus contents. The calcium-binding ability and phosphorus content showed a linear correlation except for the component carrying no phosphate (0-P component). Calcium-dependent precipitability was also related to the phosphorus content, the components with more than 3 phosphates coagulating rapidly in the presence of 20 mM calcium, while the other components were quite stable even in this solution. However, solutions of the 0-P and 1-P components were turbid in the absence of calcium, because of strong hydrophobic interactions. When the 1-P and 5-P components were mixed in the presence of calcium, the 5-P component was protected from calcium-dependent precipitation. The 2-P component also showed a little stabilizing activity, but the 0-P component did not. These results suggest that the formation of human casein micelles may be quite complicated.