The CH3 domain of pig immunoglobulin G. A study of structural heterogeneity and enzymic fragmentation.

The CH3 domain of pig immunoglobulin G has been isolated as the pFc' fragment by peptic hydrolysis at pH 4.5. By ion-exchange chromatography in a dissociating medium pH 3.0 the heterogeneous pFc' fragment could be resolved into three variants differing in electric charge. The variants were very similar in amino acid composition but differed mainly in the histidine content. The peptides obtained from the pFc' fragment by tryptic hydrolysis were separated by ion-exchange chromatography and characterized by amino acid composition. It appeared that the CH3 domain is split by tryptic hydrolysis to four large peptides of 20-30 amino acid residues in length and to a number of small peptides. The large peptides represent segments of the polypeptide chain suitable for study of the location of the binding site for the Fc receptor.