Purification and characterization of four components of rat caseins

Rat casein components (C1-, C3A-, C3B- and C4-casein) were extensively purified from rat milk, and the properties of these proteins were compared with those of other caseins including rat C2-casein. C1-casein was precipitated by a low concentration of CaCl₂ (1.5 mM). Both C3A- and C3B-casein were less sensitive to Ca²⁺ than were C1- and C2-casein, and the presence of 20 mM CaCl₂ was required at 37°C for their precipitation. C4-casein was absolutely insensitive to Ca²⁺. This protein exhibited the ability to stabilize all of the other rat casein components against Ca²⁺-dependent precipitation. In addition, C4-casein contained sialic acid, galactose and $\text{N-}\text{acetylgalactosamine}$. Therefore, C4-casein appears to be a bovine ϰ-casein-like protein.