Interchain and intrachain disulfide bridges of a human immunoglobulin M: detection of a unique fragment.

Studies of the amino acid sequences around half-cystine residues in an immunoglobulin M have revealed an unexpectedly high number. At least 14 different sequences were found in the mu chain (V(HIII) subclass). Four of these were involved in interchain disulfide bridges and at least 10 in intrachain bridges. Five came from the kappa chain (kappa(III) subclass, Inv b). Several others, although having a high degree of homology, were not identical with those of either the mu or kappa chains. These results support the concept of an additional fragment in gammaM molecules, although its function and localization remain to be determined.