The Formation of Casein Micelles Reconstituted with Ca⁺² and Added Inorganic Phosphate is Influenced by the Non-phosphorylated Form of Human β-Casein

Abstract

The β-casein (CN) human milk fraction is comprised of a single protein phosphorylated at levels from 0 to 5. Component interactions are dependent on the phosphorylation level. Here, 3 mg/ml of β-CN-0P, β-CN-2P, β-CN-4P, a 2P/4P 1:1 (wt:wt) mixture, or a mixture of all six forms in the ratio in human milk, were mixed with bovine κ-CN at a κ/β molar ratio of 0.33. Measurements were with 0, 5 and 10 mM Ca⁺² and 4 and 8 mM added inorganic phosphate (P_i). The turbidity (OD_400 nm) and a lack of precipitation as T increased from 4 to 37°C was an index of micelle formation. The results indicate: (1) while micelles will form with Ca⁺² alone, added P_i has a significant enhancing effect on micelle formation; (2) the patterns of micelle formation as a function of T are influenced by the β-CN-0P and β-CN-1P forms of β-CN to an unexpected extent.