Reconstituted micelle formation using reduced, carboxymethylated bovine κ-casein and human β-casein

In milk, κ-casein, a mixture of disulfide-bonded polymers, stabilizes and regulates the size of the unique colloidal complex of protein, Ca ²⁺ and inorganic phosphate (P_i) termed the casein (CN) micelle. However, reduced, carboxymethylated bovine κ-CN (RCM-κ) forms fibrils at 37°C and its micelle-forming ability is in question. Here, the doubly- and quadruply-phosphorylated human β-CN forms and 1:1 (wt:wt) mixtures were combined with RCM-κ at different β/κ weight ratios. Turbidity (OD_{400 nm}) and a lack of precipitation up to 37°C were used as an index of micelle formation. Studies were with 0, 5 and 10 mM Ca²⁺ and 4 and 8 mM P_i. The RCM-κ does form concentration-dependent micelles. Also, β-CN phosphorylation level influences micelle formation. Complexes were low-temperature reversible and RCM-κ fibrils were seen. There appears to be equilibrium between fibrillar and soluble forms since the solution still stabilized after fibril removal. The RCM-κ stabilized better than native bovine κ-CN.