Reversible Heat-Induced Dissociation of the Alpha-Casein Complex

Sephadex gel filtration and ultracentrifugation at temperatures up to 85 C demonstrafed a reversible heat-induced dissociation of the alpha-casein complex into a.~-easein and K-casein, which occurred between 45 and 54 C. Presumably, the dissociation is the result of a transition from predominantly hydrophilie interactions (hydrogen bonds) between a,~-and K-casein at low temperature to hydrophobie interactions involving only ~-casein at elevated temperature.Although the alpha-casein fraction has long been known to be composed of a~,-and K-casein it behaves as a homogenous protein in free boundary electrophoresis and ultracentrifu~'ation.The eomplex is formed spontaneously when the constituent proteins are mixed at pH 7 in the weip.'htratio of 4a~,:1 ~ (15).During this oroeess, a mutual stabilization takes place and the a~,-casein is rendered stable towards moderate coneentration of calcium, and the Kcasein is prevented from self-ao'gregation (]5).These factors are unique and important properties of the complex, and for this reason it is of interest to explore the effect of various processing treatments, such as heat.on the interactions responsible for the stability of the complex.Previous findings by light-scatterino' (7) have suggested that the complex dissociates at elevated temperature and that the disaggreaation is followed by aggregation.Reeent result~ obtained by hot ultraeentrifugation (S) have shown that the protein fraction underooes a rever.*ibledissociation at 85 C.The purpose of this study has been to investigate further the sequence of the heat changes in the alpha-casein. Experimental ProcedureAlpha-casein was prepared by the method of Hipp et ah (6) and Warner (13).a~,-Casein