Two immunochemically and physicochemically distinct secretory components from rat exocrine secretion.

Two proteins were isolated from rat milk by using ion-exchange and gel filtration chromatography and shown to be secretory components by the criteria of: a) occurrence as free proteins in exocrine secretions, as well as bound to exocrine IgA while absent from serum and serum IgA, b) immunohistochemical localization in the epithelial cells of the gut, c) dissociability from and in vitro binding to 11 S SIgA, d) cross-reactivity with antisera to mouse and bovine secretory components, and e) significant peptide homology with bovine secretory component. The secretory components were immunochemically and physicochemically distinct as shown by: a) the lack of cross-reactivity between their specific antisera and differential reactivity with anti-bovine FSC and anti-mouse SC, b) different electrophoretic mobilities, c) DEAE elution profiles, d) carbohydrate contents, e) molecular weights, and f) apparent polypeptide chain constitution. Another interesting difference in the two is that only one of the components dissociates spontaneously from SIgA. Possible functions of two secretory components are discussed.