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Isolation and Partial Characterization of Rat Casein Proteins¹

Department of Dairy Science, University of Illinois, Urbana 61801

ABSTRACT

Casein was isolated from rat milk by high speed centrifugation. Polyacrylamide disc gel electrophoresis of the whole casein yielded three major protein zones designated C.l, C.2, and C.3 in order of their decreasing electrophoretic mobility in the alkaline system. Zone 3 subsequently contained two possibly related bands, C.3.1 and C.3.2. The presence of phosphate in all four zones was indicated by staining and conformed by phosphorus-32 labeling studies. A glycoprotein character was indicated by all zones.

Separation of the constituents of rat casein by diethylaminoethyl-cellulose ion exchange chromatography yielded the same four major protein entities. Three milk-specific phosphoproteins unique to rat whey eluted from such columns in the same general region as the casein constituents but appear to be otherwise unrelated to the four major components of micellar casein. Gel electrophoresis in sodium dodecyl sulfate systems yielded apparent molecular weight estimates of approximately 24,000 for C.1, 38,000 for C.2, and 28,000 for C.3.1 and C.3.2.

¹ This work was supported under Contract US NO1-CB-23856 of the National Cancer Institute, National Institutes of Health, and Hatch Projects 35-320 and 35-351 of the Illinois Agricultural Experiment Station.