THE LEAST NUMBER OF PHOSPHATE GROUPS FOR CROSS-LINKING OF CASEIN BY COLLOIDAL CALCIUM-PHOSPHATE

Artificial casein micelles were formed with whole human casein at 20 mM calcium, 17 mM phosphate, and 10 mM citrate. The casein micelles disaggregated by 6 M urea were separated by high performance gel chromatography on a TSK-GEL G4000SW column into crosslinked and monomeric fractions. When the crosslinked casein fraction was analyzed by high performance ion-exchange chromatography on a TSK-GEL DEAE-5PW column, a small peak, representing the 3-P component of human beta-casein, and distinct peaks of the 4-P and 5-P components were found. Artificial casein micelles were formed from mixtures of each purified component of human beta-casein and bovine kappa-casein, disaggregated by urea, and separated on a TSK-GEL G4000SW column. The casein aggregates crosslinked by colloidal calcium phosphate were formed in artificial casein micelles of the 3-P and 4-P components. In contrast, no crosslinkage was formed in artificial micelles of the 1-P and 2-P components. The results indicate that at least three phosphate groups are needed for crosslinking of casein by colloidal calcium phosphate.