A Study of the Association of Human Secretory Component with IgA and IgM Proteins

Abstract

Human secretory component (SC) was isolated from colostral whey, and the binding of ¹²⁵I-SC to purified IgA and IgM monoclonal proteins was studied using two methods to separate free from immunoglobulin-bound ¹²⁵I-SC: a) gel filtration on Sephadex G-200, and b) precipitation of ¹²⁵I-SC-Ig complexes with anti-Ig antibody. Both IgA dimeric proteins and IgM pentamers bound ¹²⁵I-SC with approximately one SC-binding site per mole of polymer and similar affinity. Assuming a reversible equilibrium, an apparent association constant ≃ 10⁸ M^-1 was calculated to govern the binding of ¹²⁵I-SC to immunoglobulin polymers. The assignment of a single association constant may be an oversimplication, particularly for the case of IgA polymers, since evidence was obtained that disulfide bonds were formed in the ¹²⁵I-SC-IgA complex. Despite the complexity of the reaction, binding of ¹²⁵I-SC to both IgA and IgM polymers could be analyzed by standard methods of saturation analysis, and both were shown to have a similar affinity for ¹²⁵I-SC. No differences were noted in the affinity of ¹²⁵I-SC binding to the IgA1 and IgA2 subclasses. Binding of monomeric IgA and IgM proteins could not be measured and was at least 100-fold lower than that found for IgA and IgM polymers. Complexes of ¹²⁵I-SC with IgA dimers were presumed to involve covalent bond formation, since these complexes did not dissociate in guanidine-HCl. One IgA2 trimer did not form a covalent bond since it was completely dissociated in guanidine. In contrast, ¹²⁵I-SC-IgM complexes were dissociated in denaturing solvent, indicating that such complexes were held together primarily by non-covalent bonds. Experiments with (Fc)₅ µ isolated by high temperature tryptic digestion of IgM showed that binding of ¹²⁵I-SC was to the Fc region of IgM proteins. It was suggested that the binding of SC with similar affinity to both IgA and IgM polymers may be important in the biologic function of both these immunoglobulin classes.