Physicochemical and immunochemical studies on bovine IgA and glycoprotein-a.

1. Bovine secretory IgA (SIgA) from colostrum (mol. wt. about 410,000) is composed of four alpha-chains (mol. wt. 61,000), four light chains (mol. wt. 23,000) and one molecule of glycoprotein-a (mol. wt. 70,000-86,000). The alpha-chains are antigenically and physicochemically distinct from the heavy chains of IgM, IgG1 and IgG2 while the light chains are identical to those occurring on other bovine immunoglobulins. Glycoprotein-a and bovine free secretory component are identical and the former name should be abolished. Much of this protein is covalently bonded to IgA. 2. The gel filtration behavior of serum IgA suggests it is a dimer. 3. The elution behavior of IgA and SIgA from ion-exchange columns and the solubility characteristic of SIgA in the presence of Zn2+ are similar to those of human and rabbit IgA. 4. The disc electrophoretic behavior of IgA and SIgA are distinct from IgM, dimeric IgG1, 7-S IgG and glycoprotein-a. Dimeric IgG1 (s20,w = 9.5) is abundant in colostrum and is similar in size to SIgA. 5. Bovine IgA shows physicochemical and immunochemical heterogeneity when studied by gel filtration, disc electrophoresis, immunoelectrophoresis and ultracentrifugational analyses. Lacrimal and nasal SIgA possess antigenic determinants absent on colostral SIgA.