Characterization of human kappa-casein purified by FPLC.

Because previous purification procedures for human kappa-casein may have caused the loss of some carbohydrate, relatively gentle methods were used. The protein was isolated by a four-step procedure which included isoelectric precipitation of whole casein, gel chromatography on Sephadex G-200 in the presence of SDS, removal of the SDS with Extracti-Gel D, and FPLC chromatography on Mono Q with buffers containing 6 M urea. The purified protein was nearly identical in amino acid composition to that found earlier by amino acid analysis and peptide sequencing and a molar extinction coefficient of 11.2 +/- 0.1 was determined on the basis of amino acid analysis with a norleucine internal standard. Hydrolysis, acylation, and methylsilylation of the carbohydrate, followed by gas chromatographic analysis on a fused silica column, yielded approximately 5% fucose, 17% galactose, 18% N-acetylglucosamine, 8% N-acetylgalactosamine and 7% sialic acid, totaling almost 55% by weight. The percentages from two different donors were almost the same. About 1 mole phosphorus per mole of kappa-casein was also detected. Using low-speed sedimentation equilibrium methods, a molecular weight of only 33,400 was obtained for human kappa-casein, suggesting carbohydrate lability. Human beta-casein with four phosphoryls was stabilized against precipitation by 10 mM Ca+2 ions at a level greater than 95% when the molar ratio of kappa/beta exceeded 0.15.