Human secretory component. NH2-terminal amino acid sequences and peptide maps of the form occurring in exocrine immunoglobulin A and the free form.

Secretory component is found either covalently linked to exocrine immunoglobulin A or in a free form unassociated with other protein. Previous work has demonstrated considerable similarities between the two forms in terms of size, composition, and antigenic determinants. The comparison has now been extended to the primary structure of bound and free secretory components obtained from human colostrum. Their tryptic peptide maps are similar; the amino acid sequences of the first 14 residues, as determined by an automated sequenator, are identical; and they show no structural homologies with any other proteins. The data support the concept that secretory component is a unique protein species, a fraction of which forms a complex specifically with immunoglobulin A. been definitely established; nor is it known if there are any specific regulatory mechanisms which influence the synthesis of the diverse components. Combination of SC with IgA is specific since SC is not normally found associated with any other protein in secretions or serum (3, 4). However, not all the SC in secretions occurs as a part of secretory IgA; a portion exists in a free state uncoupled to other protein. These two forms of SC, referred to as bound secretory component (BSC) and free secretory component (FSC) have been isolated and compared in both human (5, 6) and rabbit (7), and were found to be similar in terms of size (71,000 daltons for human SC), composition, and antigenic determinants. More detailed comparison of the two forms of SC requires information on their primary structure. Toward this aim we now report results of peptide mapping studies and amino acid sequence determinations of their NH2-terminal regions.