Structural Studies on Casein Micelles of Human Milk: Dissociation of ß-Casein of Different Phosphorylation Levels Induced by Cooling and Ethylenediaminetetraacetate

¹ Departments of Biochemistry and Pediatrics, Loma Linda University School of Medicine, Loma Linda, CA 92350

Information on the structure of human casein micelles has been obtained from dissociation of ß-casein (CN). Two approaches were used: cooling at 4°C and addition of EDTA. An initial loss of about 80% of the protein optical density occurred upon cooling to 4°C. Dissociation was time dependent, and at 24 h about 10% remained. However, mean size and voluminosity of micelles increased, as indicated by laser light scattering and viscosity measurements. This process was reversible, and 95% of the protein reentered the micelles upon incubation for 3 h at 37°C. Upon cooling, amounts of nonphosphorylated ß-CN increased, and singly phosphorylated ß-CN levels were almost constant relative to the total ß-CN in micelles. Upon addition of EDTA (0 to 5 mM), the forms with three to five phosphates were the major dissociating constituents; EDTA that was added by dialysis produced similar results but at lower concentrations. These data suggest that, in the absence of significant amounts of _s1-CN, nonphosphorylated and singly phosphorylated human ß-CN may form a framework, as proposed for _s1-CN for bovine milk, along with the colloidal calcium phosphate for the development of the final micelle structure by addition of the more highly phosphorylated forms. The results also indicate that human casein micelles have a less rigid structure than those of other species.

Key Words: human milk • casein micelles • micelle dissociation • micelle structure

Submitted on March 25, 1996
Accepted on September 30, 1996

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