Isolation and partial characterization of rate casein proteins.

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.1, 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 cluted 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.