Isolation and properties of human kappa-casein.

Human kappa-casein was isolated from human whole casein by gel filtration with Sephadex G-200 and hydroxylapatite chromatography in the presence of sodium dodecyl sulfate (SDS). The kappa-casein was calcium-insensitive and did stabilize human beta-casein and bovine alpha s1-casein against precipitation by calcium ions. Formation of micelles from human beta- and kappa-caseins, and calcium ions was confirmed by electron microscopic observation. On SDS-polyacrylamide gel electrophoresis (SDS-PAGE), a single band was obtained. The formation of para-kappa-caseins by chymosin was confirmed by SDS-PAGE. Two para-kappa-caseins with apparent molecular weights of 13,000 and 11,000 appeared. The molecular weight of intact human kappa-casein was estimated to be approximately 33,000. The human kappa-casein contained about 40% carbohydrate (15% galactose, 3% fucose, 15% hexosamines, and 5% sialic acid) and 0.10% (1 mol/mol) phosphorus. Its amino acid composition was similar to that of bovine kappa-casein except for serine, glutamic acid, and lysine contents.