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Purification and Partial Characterization of a Unique Group of Phosphoproteins from Rat Milk Whey¹

Department of Dairy Science, University of Illinois, Urbana 61801

ABSTRACT

A group of four electrophoretically distinct but related proteins has been isolated from rat milk whey. These have been classified as P.l to P.4 in order of their decreasing electrophoretic mobility in an alkaline buffer system. All four 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 molecular weight of approximately 20,700. Enzymatic dephosphorylation of purified P.l generated the other three 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.l, P.2, and P. 3 were isolated in sufficient quantities to permit further characterization. The partial amino acid analyses of each of the three forms were similar. They had common extinction coefficients, E¹₁ % cm A28C of 4.9 and A₂₈₀/A₂₉₀ ratios of about 1.36

¹ This investigation was supported under Project US NOl-CB-23856 of the National Cancer Institute, National Institutes of Health, and Hatch Projects 35-320 and 35-351 of the Illinois Agriculture Experiment Station.