Amino Acid Composition of Chromatographically Separated β-Casein A3

Polyacrylamide gel electrophoresis or starch gel electrophoresis at pH 3 demonstrates that the β-casein type A, as classified at pH 9, actually consists of three species, A1, A2, and A3. Chromatography on cellulose phosphate has been used to separate an A3/B β-casein into its constituents. Amino acid analyses and end group analyses of β-caseins A3 were compared with those of the other genetic variants, A1 and A2β-caseins. The three variants, A1, A2, and A3, have 6, 5, and 4 histidine residues per molecule of ∼24,000 Daltons. The analytical data indicate a substitution of proline for histidine.