Reactive half-cystine peptides of the secretory component of human exocrine immunoglobulin A

On the basis of previous work the two forms of human secretory component, namely that which is covalently bound as a part of the exocrine immunoglobulin A molecule and the free form, are probably different states of the same protein. From autoradiographs of trypic peptide maps of bound and free secretory components which were radioactively alkylated after partial reduction, it was concluded that the same half-cystines in each are sensitive to reduction. in the present work the easily reduced half-cystines of the bound and free secretory components have been studies in more detail. In each form there are two such half-cystines. In the case of bound secretory component they provide the linkage to the remainder of the exocrine immunoglobulin A molecule. Peptides from the sensitive half-cystines were isolated from tryptic-peptic digests of free secretory component and sequenced. By diagonal electrophoresis these two peptides were shown to be joined in an intrachain disulfide bridge. Therefore, it is proposed that the exocrine immunoglobulin A molecule becomes fully assembled when a single, reactive intrachain disulfide bridge in free secretory component rearranges to yeild two interchain bridges with dimeric serum-type immunoglobulin A. This process is thought to occur within the epithelial lining cells of mucous membranes.