Heat-induced Complex Formation between κ-Casein and α-Lactalbumin

Abstract

It is considered that heat treatment affects the properties of milk such as heat-stability and chymosin clottability. The heat-induced interaction between κ-casein and α-lactalbumin was investigated under various conditions (buffer, pH, ionic strength, temperature). The results obtained, using Sephacryl S-300 gel filtration and sodium dodecylsulfate-polyacrylamide gel electrophoresis, indicated that a complex of the two proteins was formed in 35 Mm phosphate buffer, pH 7.6, containing 0.4_M NaCl on heat treatment at 90°C for 30 min, while no complex was formed in 10 mM imidazole-HC1 buffer, pH 7.1, containing 70 mM KC1 with the same heat treatment. A κ-casein-α-lactalbumin complex was formed at high pH by heat treatment, and was dissociated in the presence of 2-mercaptoethanol. Under complex forming conditions, a change in the higher-order structure of α-lactalbumin was observed by ultraviolet absorption and fluorescence experiments. Though the participation of specific amino acid residues in the complex formation could not be clarified by the chemical modification procedure, the re-arrangement of the disulfide cross-links did take place in the reaction. An optical study on the complex formation did not indicate the involvement of a hydrophobic bond.