_k-Casein components having various carbohydrate contents were prepared by diethyl-aminoethyl-cellulose chromatography and the interactions of each _k-casein component both with α_s1, -casein and with β-casein were examined by Sepharose 4B gel chromatography, ultra-centrifugal experiments and viscosity measurements. Each _k-casein component could form complex with α_s1- and β-casein in the absence and presence of CaCl₂. Molecular weight of complexes of unfractionated _k-casein both with α_s1, -casein and with β-casein were about 70×10⁴ at 37°C in the absence of CaCl₂, while those of complexes of each _k-casein component with α_s1- and β-casein were about 50×10⁴. Stokes radii of complexes increased with increasing calcium ion. While sedimentation coefficient at 37°C of complex with β-casein had almost the same value, those of complexes with α_s1-casein decreased with increase of carbohydrate content of _k-casein components. Intrinsic viscosity of complex of _k-casein component having much carbohydrate was almost the same among tested temperatures. It is suggested that heterogeneity of _k-casein is necessary to form large complex and that the carbohydrate moiety of _k-casein contributes the stability of casein complex.

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