Reexamination of the polymeric distributions of kappa-casein isolated from bovine milk.

kappa-Casein the stabilizing protein of the colloidal milk protein complex was purified from bovine skim milk by the method of McKenzie and Wake (Biochim, Biophys. Acta. 47, 240, 1961). The preparations were examined by sodium dodecyl sulfate gel electrophoresis in the presence and absence of a reducing agent. In the presence of a reducing agent, the kappa-casein migrates as a single low molecular weight band. However, in the absence of a reducing agent, a characteristic pattern of aggregates of varying molecular weight was observed with components ranging from monomer to octamer in integer steps. Densitometry of the Coomassie blue stained gels showed an almost equal distribution of components in each band; carbohydrate staining showed preferential location of sugar residues in lower molecular weight components. Treatment with chymosin (rennin) caused a downward shift in apparent molecular weight for each band with no change in the relative intensity of the Coomassie blue stained bands. Similar gel patterns were observed in whole caseins and partially purified kappa-caseins, indicating that this size distribution is a natural disulfide-linked reporter for the distribution of kappa-casein in casein colloids (micelles).