Mechanism of cross-linking of casein by colloidal calcium phosphae in casein micelles

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Calcium and inorganic phosphate are present in excess of their Solubility and interact with casein. Calcium phosphate ity casein micelles, which is associated with casein, is called colloidal calcium phosphate(CCP). Although CCP cross-links caseins through phosphate groups, the mechanism of cross-linking has not yet been elucidated sufficiently. In the present study following results were obtained.A small amount of the 3-P component of human beta-casein and a considerable amount of the 4-P and 5-P components were found in the CCPcross-linked casein separated from artificial casein micelles of whole human casein. CCP-cross-linked casein was formed in artificial casein micelles of tlie 3-P and 4-P components of human beta-casein with kappa-casein. In contrast, no cross-linitage was formed in artificial casein micelles of the 1-P and 2-P coutponeiits. It was concluded that at least three phosphate groups were need fGr cross-linking of casein by CCP.CCP-cross-linked casein taras formed when calcium, phosphate and citrate were added to sodium caseinate solution. This suggested that the submicelle structure and ordered structures of casein were unnecessary for caseins to be cross-linked by CCP.When amino groups of alpha_<s1>-casein were acetylated, citraconilated, orsuccinylated, modified alpha_<s1>-casein was somewhat more hardly cross-linked by CCP than native one.Egg white ribofravin-binding protein is also cross-linked through its phosphate groups by CCP.