The controlling effect of carbohydrate in human, rabbit and bovine immunoglobulin G on proteolysis by papin.

About two-thirds of the hexose of human and rabbit immunoglobulin G (IgG) was located in the Fc fragment and one-third in the ;hinge' region of the gamma (heavy) polypeptide chain at the junction of the Fab and Fc fragments. In contrast, bovine IgG contained more hexose in the ;hinge' region than in the Fc fragment. The initial cleavage of susceptible IgG molecules into Fab and Fc fragments by papain under the conditions given by Porter (1959) had reached completion after digestion for 2hr., though bovine IgG was digested somewhat more slowly than human or rabbit IgG. The release of ;hinge' peptides from human and rabbit IgG had also reached completion by 2hr., but was slower from bovine IgG and continued for several hours longer. Since bovine IgG molecules contained on the average a greater amount of hexose in the ;hinge' region, carbohydrate on this part of the gamma-chain may influence not only the initial rate of enzymic hydrolysis into Fab and Fc fragments, but also, and to a greater extent, the rate of further limited hydrolysis of the N-terminal regions of the Fc fragment. The presence of carbohydrate in the ;hinge' region does not appear to account for the resistance of some IgG molecules to papain digestion and of some Fc fragments to N-terminal degradation.