Rat casein: isolation and characterization of two major fractions

1. In rat milk, casein exists as particles of 77 nm mean diameter, similar in appearance to the casein micelles in the milk of other species. 2. The heterogeneity of rat casein was investigated by polyacrylamide gel electrophoresis and by chromatography on DEAE-cellulose, both in the presence of 8 M-urea. 3. The chromatography yielded two fractions, D2 and D4. 4. In chemical composition, D2 resembles the kappa-caseins of other species, while D4 resembles the alpha 5-caseins of other species. 5. D2 is soluble in 80 mM-CaCl2 at 37 degrees C, but D4 is insoluble under these conditions. 6. In 80 mM-CaCl2 at 37 degrees C, D2 prevents the precipitation of D4 by the formation of particles similar to those found in rat milk.