Rabbit antibodies against pneumococcus capsular polysaccharides of Types II, III, VI, VII, and XII were isolated from type-specific antisera by precipitation with homologous antigen and hydrolyzed with either subtilisin, chymotrypsin or trypsin. Purified antibodies were also hydrolyzed with papain; the two polypeptides with active antibody sites and the crystallizable polypeptide were separated, studied in the ultracentrifuge and hydrolyzed with either subtilisin or chymotrypsin. The resulting peptides were separated on filter paper by electrophoresis and chromatography.
1. The two polypeptides with antigen-combining activity, fractions I and II, each constituted about 25 per cent of the original antibody molecule; the crystallizable polypeptide did not combine with antigen, fraction III, and represented about 35 per cent of the original antibody molecule. About 10 to 15 per cent of the original antibody molecule was hydrolyzed by papain into about 35 peptides.
2. The peptide patterns obtained for hydrolyzates of fraction I were almost identical with those obtained for fraction II obtained from the same antibody and were quite different for those obtained for fraction III.
3. Many of the peptide spots in the patterns obtained with whole antibody hydrolysates contained at least two peptides derived from different parts of the antibody molecule.
4. Differences were observed in the peptide patterns for different antibodies that suggested the existence of differences in both primary and tertiary structures among these antibodies.
