Separation and partial characterization of guinea-pig caseins

Abstract

Guinea-pig caseins A, B and C were purified free of each other by a combination of ion-exchange chromatography and gel filtration. Determination of the amino acid composition showed all 3 caseins to contain a high proportion of proline and glutamic acid, but no cysteine. This apart, the amino acid composition of the 3 caseins was markedly different, though calculated divergence values suggest that some homology may exist between caseins A and B. MW estimates based on amino acid composition were in good agreement with those based on sodium dodecyl sulfate/polyacrylamide-gel electrophoresis. N-Terminal analysis showed lysine, methionine and lysine to be the N-terminal residues of caseins A, B and C, respectively. Two-dimensional separation of tryptic digests revealed a distinctive pattern for each casein. All caseins were shown to be phosphoproteins. The casein C preparation also contained significant amounts of sialic acid, neutral and amino sugars. Each casein apparently represents a separate gene product, and the low MW proteins are not the result of a post-translational cleavage of the largest. All were distinctly different from the whey protein .alpha.-lactalbumin.