Purification and Partial Characterization of a Unique Group of Phosphoproteins from Rat Milk Whey

Abstract

A group of 4 electrophoretically distinct but related proteins were isolated from rat milk whey. These have been classified as P.1-P.4 in order of their decreasing electrophoretic mobility in an alkaline buffer system. All 4 members of this group of proteins are immunologically identical and apparently constitute a single protein which is differentially phosphorylated in the ratio of 3:2:1:0. Sodium dodecyl sulfate gel electrophoresis yielded constant estimates of MW of approximately 20,700. Enzymatic dephosphorylation of purified P.1 generated the other 3 members of the group, demonstrating the commonality of the peptide component. This group of proteins in total makes a significant contribution to the total whey proteins being approximately 5 mg/ml in whole rat milk in the approximate proportion of 1:1.5:2:1.5. The P.1, P.2 and P.3 were isolated in sufficient quantities to permit further characterization. The partial amino acid analyses of each of the 3 forms were similar.