Caseins are cross-linked through their ester phosphate groups by colloidal calcium phosphate

Abstract

Artificial casei micelles were prepared by adding 30 mM calcium, 22 mM phosphate and 10 mM citrate to sodium caseinate solutions, and the content of the casein aggregates cross-linked by colloidal calcium phosphate was determined by high-performance gel chromatography on a TSK-GEL G4000SW column in the presence of 6 M urea. The content of the casein aggregates cross-linked by colloidal calcium phosphate in artificial whole casein micelles was 48 of total casein, and their relative casein composition determined by high-performance ion-exchange chromatography was 53.1% for α s1-casein, 15.8% for α s2-casein, 31.1% for β-casein and 0% for κ-casein. The order of cross-linking by colloidal calcium phosphate agreed with that of the ester phosphate content of casein constituents. The content of the casein aggregates cross-linked by colloidal calcium phosphate was higher in α-κ-casein micelles than in β-κ-casein micelles. κ- and γ-caseins and dephosphorylated α s1-casein were not cross-linked by colloidal calcium phosphate. Although κ-casein was not cross-linked, chemically phosphorylated κ-casein, of which the average phosphate content was 8.5 per molecule, was cross-linked. It is concluded that caseins are cross-linked through their ester phosphate groups of colloidal calcium phosphate.