Suspension of the calcium-sensitive human β-caseins by human κ-casein

The β-casein (CN) fraction of human milk exists as a single protein entity phosphorylated at various levels from zero to five (β-CN-0P to β-CN-5P). Since the β-CN fraction is precipitated by the calcium ions in milk, a stabilizing protein is needed to form a suspension of casein micelles for ready ingestion by the infant. That stabilization is known to be carried out by κ-CN but it is also thought possible that the 0P and 1P β-CN moieties may play a role. To examine the effects of different phosphorylation levels, 10 mM CaCl₂ was added to each of the purified proteins phosphorylated (P) at different levels. Without κ-CN, precipitation of the different β-CN forms varied from 78 to 99%. Human κ-CN was then added to each to give κ/β molar ratios varying from 0.01 to 0.25. Some stabilization was observed even at the lowest ratio and more than 90% of the protein was suspended in all cases at the highest ratio. Interaction of low levels of κ-CN with the different forms of β-CN to create a suspension was somewhat dependent on the phosphorylation level and the possibility of forming intra- or intermolecular Ca⁺² bridges or cross-links. Similar ratios of the β-CN-4P to either the 0P or 1P form and of the 2P to the 1P form showed that neither the 0P nor 1P form had any stabilizing ability. In fact, coprecipitation occurred so that with either the 4P or 2P forms present, higher percentages of the 0P and 1P forms precipitated.