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

钙和无机磷酸盐的存在超过了它们的溶解度，并与酪蛋白相互作用。磷酸钙酪蛋白胶束与酪蛋白相结合，称为胶体磷酸钙（CCP）。虽然CCP通过磷酸基团交联酪蛋白，但交联的机制尚未得到充分的阐明。在本研究中获得了以下结果。从全人酪蛋白的人工酪蛋白胶束中分离得到的ccp交联酪蛋白中含有少量的β -酪蛋白3-P成分，而含有大量的4-P和5-P成分。ccp交联酪蛋白是在人β -酪蛋白的3-P和4-P组分与kappa-酪蛋白的人工酪蛋白胶束中形成的。相反，1-P和2-P对偶产物的人工酪蛋白胶束未形成交联。综上所述，CCP至少需要3个磷酸基团进行酪蛋白交联。将钙、磷酸盐和柠檬酸盐加入到酪蛋白酸钠溶液中形成ccp交联酪蛋白。这表明酪蛋白的亚胶束结构和有序结构对于CCP交联酪蛋白是不必要的。当α <s1>-酪蛋白的氨基被乙酰化、柠檬酸化或琥珀化时，修饰的α <s1>-酪蛋白比天然的α <s1>-酪蛋白更难被CCP交联。蛋清核黄素结合蛋白也通过其磷酸基团被CCP交联。

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.