Structural studies on bovine immunoglobulin M

Abstract

Heavy (μ) and light polypeptide chains were prepared from normal bovine IgM and were found to have molecular weights of 61 800 and 22 800. Heavy chain from a human pathological IgM had a molecular weight of 65 200. Bovine μ-chain contained 12% carbohydrate but the light chain contained less than 0.5%. The human μ-chain was found to have 15% carbohydrate. The molecular weights of the carbohydrate free heavy chains from bovine and human IgM are therefore 54 400 and 55 400, respectively. Amino acid analysis and fingerprinting of the bovine chains showed that a light chain is linked to a heavy chain by a single disulphide bridge and that a heavy chain is linked to other heavy chains by 3 disulphide bridges. One of the inter-heavy chain bridges is near to the C-terminus of the chain and another forms the inter-subunit bridge. Fingerprinting and amino analysis also showed the presence of at least four carbohydrate sites on the μ-chain, three of which are near to the inter-heavy chain disulphide bridges. The carbohydrate site near to the disulphide bridge at the C-terminal end of the chain appears to have a variable carbohydrate moiety. The similarity between bovine and human IgM as regards the arrangement of inter-chain disulphide bridges and carbohydrate sites suggests that these structural features might be essential characteristics of the IgM molecule.