Effects of colloidal calcium phosphate content and free calcium ion concentration in the milk serum on the dissociation of bovine casein micelles

Abstract

Summary The strength of binding of the individual caseins and the nature of the bonding within bovine casein micelles were examined through dissociation of the micelles by dialysis of skim milk either against phosphate-free buffers containing 3 or 6 mm-CaCl 2 , or against buffers that were nearly saturated with respect to micellar calcium phosphate, but which had a free Ca 2+ concentration in the range 0·4–5·9 mm. Dissociation was followed by ultracentrifuging the dialysed milks and determining the partition of the total and the individual caseins between the pellet and serum. During dialysis against the phosphate-free buffers both colloidal Ca and P i in the milks decreased and about 30 % of the P i could be removed without significant casein dissociation. With further loss of P i , however, increasing dissociation occurred and the proportions of the individual caseins retained in the casein pellet were in the order α s2 - > α s1 - > β- ≈ κ-casein. Dialysis against the calcium phosphate buffers resulted in no loss of colloidal P i but colloidal Ca increased with the free Ca 2+ concentration of the buffer. Little change in the casein partition occurred in the presence of more than 1 mm free Ca 2+ , but serum casein increased markedly at lower levels, and the strength of binding of the individual caseins in the pelleted casein was in the order α s2 -> α s1 - > β- > κ-casein. In both types of buffer, dissociation is considered to occur through the breaking of linkages between the caseins and inorganic constituents. Analysis of the amino acids in a calcium phosphate-rich material obtained after exhaustive proteolytic digestion of casein micelles suggests that these linkages involve the phosphate centres of the caseins.