Purification of disulphide-linked alpha s2- and kappa-casein from bovine milk

Naturally occurring disulphide-linked alpha s2- and kappa-casein in bovine milk were purified by gel chromatography on a column of Sepharose CL-6B. Four fractions (A-D) were obtained by elution with ammonium acetate-urea buffer. Fractions A and B, identified by SDS gel electrophoresis and amino acid sequence analysis, corresponded to disulphide-linked kappa-casein and alpha s2-casein respectively. Fraction C consisted of a mixture of alpha s1-, alpha s2-, and beta-casein. Separation of fraction C into its components was achieved by reversed-phase HPLC. The stability of the disulphide bridges in alpha s2- and kappa-casein was shown to differ with respect to reducing agents (dithioerythritol and 2-mercaptoethanol).