Effects of modification of ε-amino groups on the interaction of κ- and [formula omitted

Abstract

1. 1.|The reaction of κ-casein B and α s1 - casein C with formaldehyde suggested that their free amino groups might be essential for their native functions. Other amino group-modifying reagents were then employed since the specificity of formaldehyde is doubtful. 2. 2.|The capacity of κ-casein B for stabilizing α s1 - casein C in the presence of Ca 2+ is abruptly abolished after five of its nine positively charged lysine residues per molecule (monomer mol. wt., 19 000) are converted to uncharged homocitrulline residues by carbamylation. A marked decrease in sedimentation coefficient at 1% protein concentration coincided with the loss of protective colloid function of carbamylated κ-casein B. These findings indicate that changes in conformation and/or aggregation of κ-casein B occur due to the increase in net negative charge that accompanied carbamylation of the fifth lysine residue per molecule. 3. 3.|In other experiments the lysine residues of κ- and α s - caseins were converted to homoarginine, ε-N,N- dimethyllysine and ε-N- isopropyllysine residues. Such modification retains the charges of the lysine residues while sterically hindering their ε-amino groups. Since the native properties of the κ-casein B and α s1 - casein C were conserved, it seems doubtful that a specific ε-amino group is critically involved in the interaction of these proteins.