Abstract
Amino groups of κ-casein were succinylated at pH 8 to various degrees ranging 4.8 to 93.2 percent. Those modified κ-caseins were used for αs-casein stabilizing test in 0.02 M calcium chloride. The ability of κ-casein to stabilize αs-casein did nct change up to 8.8 percent succinylation, but decreased gradually as the modification reaction proceeded further until 32.3 percent succinylation at which the αs-casein stabilizing ability of κ-casein completely disappeared. The initial succinylation of amino groups occurred on the specific residues rather than in a random fashion among all the free residues. The κ-casein components which have their isoelectric points at 5.8 and 6.1 in 6 M urea were, in the early stage of succinylation at pH 8, modified more rapidly than the rest which have their isoelectric points at more acidic side. It indicates that the, κ-casein components with their pIs around pH 6 are located in the external hydrophilic region of κ-casein complex, and that they probably play important roles in the interaction with αs-casein which brings about the stabilization of αs-casein in the presence of calcium chloride. As previously reported, κ-casein lost its ability to stabilize αs-casein when the isoelectric point moved to pH 5.0-5.2, that of αs-casein, in 6 M urea as a result of succinylation.
