Preparation and characterization of rabbit IgG fractions

Rabbit IgG has been purified and further fractionated by deae-cellulose column chromatography to yield three fractions, Fr-I,-II and -III-1 that span a broad range in the heterogeneous molecular population of IgG. Immunodiffusion established that these fractions are free from contamination by IgA, IgM, transferrin, and hemopexin. The three fractions were assayed for neutral hexose and sialic acid; Fr-III-1 and Fr-I yielded the highest and lowest values for these components, resepctively. The order of elution from DEAE-cellulose and the electrophoretic mobilities of the fractions correlate with their content of sialic acid. Digestion of Fr-I with papain is much more rapid and complete than that of Fr-III-1. Treatment with neuraminidase to remove sialic acid from each fraction does not affect the comparative susceptibility to digestion by papain. The fractions can also be distinguished by the relative amounts of H-L half molecules and H chains formed under identical conditions of reduction with cysteine and by differences in their ability to bind with IgG receptors on the fetal rabbit yolk sac membrane. The results are discussed in the context of the structures of IgG in the heterogeneous population of molecules and the possible relation to the oligosaccharide moieties of the IgG molecules in that population.