The Use of Lithium Chloride to Study Human Milk Micelles

Various methods have been used to study the dissociation of milk micelles in attempts to determine their structure and the interactions that stabilize them. These include the addition of urea, cooling to alter hydrophobic bonding, the addition of EDTA to sequester calcium, and changes in pH to alter molecular charge. For this study, the mild chaotropic agent LiCl was added to human milk micelles, and measurements were made on the relative percentages of the six different phosphorylation levels of β-casein (CN) at various LiCl concentrations for different lengths of time and at different temperatures. Added LiCl had little effect at 37°C but caused maximal dissociation, mainly of the β-CN species with higher phosphorylation levels, at 23°C and 4°C between 1 and 2 M concentration. Comparison was made with 2-M additions of NaCl, MgCl2, and KCl at 4°C, with LiCl showing the only appreciable change. The results suggest that Li+ may displace Ca+2 in protein-Ca+2-protein or protein-colloidal calcium phosphate-protein salt bridges and that the nonphosphorylated form of human β-CN may change its conformation and mode of interaction upon phosphorylation. Lithium chloride may be useful to study the dissociation of the different CN in bovine milk micelles.