Immunoglobulin A was isolated from canine serum and compared to canine secretory IgA (colostral) to clarify molecular weight and antigenic differences. Serum and secretory IgA differ in molecular weight by approximately 75,000 and have sedimentation coefficients of 10 and 11.7 S, respectively; yet disulfide reduction decreased the sedimentation coefficient of both molecules to approximately 7 S. Gel filtration of unreduced secretory IgA in 6 m guanidine-HCl dissociated 13% of the molecule in a mannrr analogous to the dissociation of rabbit colostral IgA. A portion of this dissociated protein, which was approximately the size of γ chains, possessed antigenic determinants specific for secretory IgA which were localized to the Fcα fragment produced by tryptic digestion. The presence of a secretory piece-like protein on canine secretory IgA seems certain, although it was not identified specifically in polyacrylamide gels. It is proposed that both serum and secretory IgA molecules consist of two covalently linked subunits, each composed of two a and two light chains. While the greater mass of canine secretory IgA may be due in small part to a slightly larger a chain, most of the extra mass can be accounted for by the attachment of an additional but as yet incompletely characterized component which is similar to the secretory piece found in humans and rabbits.
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