Fractionation and Some Properties of κ-Casein Variants

The whole K-caseins A and B are fractionated by column chromatography on DEAE cellulose with buffer solutions containing 2-mercapto-ethanol. Of the various obtained fractions, the contents of sialic acid, phosphorus, and cystine were determined. Moreover, their sensitivity to rennin and ability to stabilize a~-casein against Ca~+-ions were measured. As far as the fractions of one variant are considered, only differences in sialie acid content were found. These observations are consistent with the model of the K-casein molecule, developed by MacKinlay and Wake. Amino acid analysis, carried out with the main fractions, revealed that an aspartic acid residue in K-casein A (0.60) has been replaced by an alanine residue in K-casein B (0.52). Curiously enough, it has been found further that the peptide chain of ~-casein B contains two more amino acids than that of K-casein A.