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

Abstract

κ-casein-rich preparations were isolated from both isoelectric whole casein and crude α-casein by fractionation with trichloroacetic acid (12%) in urea solution (6.6 M). Preparations fractionated from isoelectric whole casein at 4 C and at room temperatures (20–24 C) contained ≅77% and 55% of κ-casein, respectively. The latter preparations were nonstabilizing to αs-casein. The preparation fractionated from crude α-casein contained ≅92% κ-casein. The residual proteins consisted principally of β-casein and a small amount of λ-casein. Sedimentation coefficients for κ-casein in the 92% preparation were S20 = 12.9 (polymer) in phosphate buffer at pH 7.0, Γ/2 = 0.1 and S20c=o = 1.4 (monomer) in phosphate :KOH buffer at pH 12.2, Γ/2 = 0.19. 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 electrophoresis. This preparation stabilized 90% of αs-casein in a 1:10, κ-/αs-casein mixture in the presence of 0.02 m CaCl2. Following a treatment with rennin, 25.7% of the protein nitrogen was recovered as a soluble protein fraction. © 1962, American Dairy Science Association. All rights reserved.