Isolation and Characterization of Caseins from Rat Milk

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Abstract
Whole rat casein, precipitated from milk at pH 4.0, was resolved into 3 major fractions on diethylaminoethylcellulose at pH 7 in the presence of urea and mercaptoethanol. Each fraction, upon further analysis, appeared to consist of at least 2 closely related components. The minimum MW of these major casein fractions, by sodium dodecyl sulfate-gel electrophoresis, were .apprx. 25,500, 21,500 and 20,000. Under the usual dissociating conditions of sodium dodecyl sulfate-gel electrophoresis, the 20,000 dalton fraction occurs as a dimer of 40,000 daltons which slowly dissociates into the monomer. The 21,500 dalton fraction is sensitive to proteolytic attack at pH 6.6 by the milk-clotting enzyme chymosin (EC 3.4.23.4). The various casein components were characterized further by starch gel electrophoresis, phosphorus and sialic acid content, amino acid analysis, carbon-terminal group determination, and N-terminal sequence analysis. The 25,500 dalton component exhibits homology with .beta.-casein from other species.

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