Isolation and characterization of canine free secretory component

Free secretory component (FSC) was isolated for structural analysis from canine colostrum, using a combination of immunoabsorbants, gel filtration and ion exchange chromatography. The yield of FSC from one liter of colostrum was approximately 100 mg. With alkaline urea and sodium dodecyl sulfate (SDS) disc gel electrophoresis, FSC migrated as a single band with a corresponding mol wt (MW) of 77,500 daltons ± 6500. The carbohydrate content of FSC was 19% by weight (6% neutral hexose, 6.4% glucosamine and 6.6% galactosamine). The amino acid composition of FSC was similar to those reported for the human rabbit and cow. The amino acid sequence for the 12 NH₂ terminal residues of FSC was: Lys-Ser-Pro-Ile-Phe-Gly-Pro-Glu-Glu-Val-Asn-Ile. The first 10 residues of this sequence was identical to that of human and bovine FSC. Canine FSC was further subjected to cyanogen bromide cleavage and the products were analyzed by SDS gel electrophoresis. Two bands were observed with estimated MW's of 55,000 daltons ± 5000 and 17,000 ± 1500. Prior reduction of the larger band with 2 mercaptoethanol indicated it was composed of several small fragments joined by disulfide bonds, whereas the smaller bands appeared to be a single fragment. Further analysis of these fragments may help elucidate the attachment site of secretory component to the alpha chain of colostral IgA.