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Isolation and Characterization of Caseins from Rat Milk¹

Department of Biochemistry, College of Biological Sciences, University of Minnesota, St. Paul 55108

ABSTRACT

Whole rat casein, precipitated from milk at pH 4.0, was resolved into three major fractions on diethylaminoethyl-cellulose at pH 7 in the presence of urea and mercaptoethanol. Each fraction, upon further analysis, appeared to consist of at least two closely related components. The minimum molecular weights of these major casein fractions, by sodium dodecyl sulfate-gel electrophoresis, were approximately 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 nitrogen-terminal sequence analysis. The 25,500 dalton component exhibits homology with ß-casein from other species.

¹ Paper No. 10,774, Scientific Journal Series, Minnesota Agricultural Experiment Station.

² Guest scientist, September 1976 to September 1977. Address: Netherlands Institute for Dairy Research, Ede, Netherlands.