Monomer characterization and studies of self-association of the major beta-casein of human milk.

The casein form that has four organic phosphoryl groups, beta-casein (CN)-4P, is the major constituent (approximately 35%) of the beta-CN fraction of human milk and should play an important role in micelle structure and formation. In 3.3 M urea, the monomer is present with a molecular mass of 24,500 Da and a sedimentation coefficient of 1.3 S (Svedberg units, 10(-13)s). In 0.02 M NaCl and 0.01 M imidazole (low salt buffer) at pH 7, the sedimentation coefficient was 1.5 S, which increased to 14 S at 37 degrees C. Laser light scattering in low salt buffer and 9 mg/ml of protein indicated monomers with a radius of about 4 nm at 4 degrees C. The size of the radius increased as temperature increased, and, at 37 degrees C, the radius was about 12 nm. The molecular mass suggested the presence of about 47 monomers per polymer. In 0.25 M NaCl and with 10 nM Ca2+ prior to precipitation, the polymer attained a maximum radius of about 15 nm, which perhaps is the size of the smallest human milk micelles. The low value for reduced viscosity of 8.2 ml/g for the calcium-induced polymer was independent of protein concentration, suggesting a spherical shape and fixed size. Calcium apparently binds strongly to the phosphates; the dissociation constant was 8.1 x 10(-4) M. Other constituents of milk, such as inorganic orthophosphate, may contribute to differences in the manner by which beta-CN, with various phosphorylation levels, participate in micelle formation.