Characterization of the proteolytic fragments of bovine colostral IgG1.

Bovine colostral IgG1 was subjected to both papain and pepsin hydrolysis. Papain digestion appeared to be optimal at pH 7.4 in the presence of 0.01 M cysteine. The molecule was split at the COOH-terminal side of the interchain disulfide bond(s), and in addition to Fab fragments, two Fc fragments, designated Fc(I) and Fc(II), were obtained. Both Fc fragments had an identical NH2-terminal sequence, but differed in m.w. by about 10,000, with Fc(II) being the smaller one. Differences were also observed in their circular dichroism (CD) spectra and in their susceptibility to carboxypeptidase hydrolysis. These results suggested that the distinguishing characteristics of the two Fc fragments resided in the COOH-terminal parts of the molecules. Pepsin hydrolysis yielded the expected F(ab')2 and pFc' fragments. This hydrolysis was found to be dependent upon substrate concentration leading to aggregate formation at IgG1 concentrations below 3%.