Characterization of k-Casein Obtained by Fractionation with Trichloroacetic Acid in a Concentrated Urea Solution

Abstract

K-casein-rich preparations were isolated from both isoelectric whole casein and crude a-casein by fractiona-tion with trichloroacetic acid (12%) in urea solution (6.6M). Preparations fractionated from isoelectric whole casein at 4[degree]C. and at room temperatures (20-24[degree]C.) contained 77% and 55% of K-casein, respectively. The latter preparations were non-stabilizing to os-casein. The preparation fractionated from crude a-casein contained 92% K-casein. The residual proteins consisted principally of B-casein. The residual proteins consisted principally of B-casein and a small amount of X -casein. Sedimentation coefficients for K-casein in the 92% preparation were S20 = 12.9 (polymer) in phosphate buffer at pH 7.0, /2 = 0.1 and S c=o = 1.4 (monomer) in phosphate KOH buffer at pH 12.2, /2 = 0.19. 20 A weight average molecular weight of 24,000 was calculated for the monomeric species by Archibald''s approach-to-equilibrium method. An isoelectric point at pH 4.1 was determined by free-boundary electro-phoresis. This preparation stabilized 90% of 0s-casein in a 1:10, K-/as -casein mixture in the presence of 0.02[image]_CaCl2- Following a treatment with rennin, 25.7% of the protein nitrogen was recovered as a soluble protein fraction.