Chromatographic Separation of the Proteins of Mouse Milk

A variety of chromatographic methods have been evaluated in order to develop a system for the quantitative fractionation of the caseins of mouse milk. Dissolving freeze-dried acid-precipitated protein in 8 M-urea in the presence of 0.5% n-octyl-beta-D-glucopyranoside and 2-mercaptoethanol, or diluting skimmed milk in the same buffer containing EDTA, gave good resolution of the component proteins by FPLC on Mono S cation-exchange resin. Five individual protein components were separated from acid-precipitated protein fraction together with a small peak containing a mixture of the high molecular weight serum components. In skimmed milk, those peaks due to the whey proteins were much larger, but did not interfere with the separation of the caseins. Amino acid analysis of the acid-precipitated proteins showed them to be whey acid protein, alpha-, beta- and kappa-caseins and another unidentified, but apparently phosphorylated, protein. Apparent molecular weights of the latter proteins, as determined by gel electrophoresis, were 44.2, 26.3, 30.9 and 25.1 kDa. Relative levels of the identified caseins were somewhat variable between individual mice with the mean ratio of alpha: beta: kappa being approximately 14: 12: 1.