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Fractionation and Some Properties of -Casein Variants

Netherlands Institute for Dairy Research, Ede, The Netherlands

ABSTRACT

The whole -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 _s1-casein against Ca²⁺-ions were measured. As far as the fractions of one variant are considered, only differences in sialic acid content were found. These observations are consistent with the model of the -casein molecule, developed by MacKinlay and Wake. Amino acid analysis, carried out with the main fractions, revealed that an aspartic acid residue in -casein A (0.60) has been replaced by an alanine residue in -casein B (0.52).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 -casein A.