1. 1. Amino acid analysis of carboxymethylated and aminoethylated polypeptide chains from a human 19-S immunoglobulin M has shown that 1 cysteine residue per light chain and 4 cysteine residues per heavy chain are most probably involved in the inter-chain disulphide bridges. This has been substantiated by the isolation and amino acid analysis of a single-labelled peptide from the [14C] carboxymethylated light chain and 4 labelled peptides from the [14C]carboxymethylated heavy chain. Each of these peptides contains 1 residue of carboxymethylcysteine. Analogous peptides have also been isolated from oxidized IgM and contain 1 residue of cysteic acid in each case. 2. 2. The results show that 1 disulphide bridge links a light chain to a heavy chain and that each heavy chain is most probably linked to other heavy chains by 3 disulphide bridges. 3. 3. The peptides arising from the 3 inter-heavy chain bridges are also glycopeptides, and a fourth glycopeptide has been isolated. This indicates that the heavy chain has at least 4 major carbohydrate sites. 4. 4. Treatment of the human 19-S immunoglobulin M and its reduced and alkylated subunits, with trypsin, produces 3.6-S Fabu, 6.3-S F(ab′)2u and 4.5-S Fab′u fragments. Immunochemical and peptide analysis has shown that the Fabu fragment consists of a light (κ) polypeptide chain linked by a single disulphide bridge to a piece (Fdu) of the heavy (μ) polypeptide chain. The F(ab′)2u fragment consists of 2 Fab′u fragments linked by a single inter-heavy chain disulphide bridge. Each Fab′u fragment contains a κ-chain linked by a single disulphide bridge to a piece (Fd′u) of the μ-chain that is slightly larger than the Fdu. 5. 5. Treatment of reduced and alkylated subunits with papain in the presence of cysteine produces Fabu and Fcu fragments. Immunochemical and peptide analysis of the Fcu fragment has shown that it consists of a part of the μ-chain which is distinct from that in the Fab′ fragment. Two inter-heavy chain disulphide bridges have been found in the Fcu, one of which forms the linkage between subunits. 6. 6. The Fdu part of the Fabu fragment contains a major site of carbohydrate and the Fd′u part of the F(ab′)2u fragment contains an additional carbohydrate site situated near the inter-heavy chain disulphide bridge. The Fcu contains 2 further carbohydrate sites also situated near the inter-heavy chain disulphide bridges. © 1969.
