A study of the association of human secretory component with IgA and IgM proteins
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 congruent to 10-8 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 mu 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.
Available from: Per Brandtzaeg
- "e J chain is mandatory for specific combination of SC with dimeric IgA and pentameric IgM, it is not clear how this polypeptide is involved in the binding process. A direct interaction between SC and J chain is not likely since the purified dimeric polypeptide shows only marginal blocking of the binding of SC to the Ig polymers (Brandtzaeg, 1975b). Weicker & Underdown (1975) initially reported that IgM pentamers and IgA dimers bind SC with similar affinity. Conversely, in our laboratory a much stronger non-covalent interaction was found between pentameric IgM and SC than between dimeric IgA and SC (Brandtzaeg, 1974d), and the apparent equilibrium constant of association (Ka) was estimated to be 2-7-12 5 tim"
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ABSTRACT: At least seven models have been proposed for the epithelial transport of IgA, and each model presents particular features which are not generally appreciated. Much of the confusion in this field has been caused by the many conflicting reports about the cellular origin of the secretory component (SC) and the mode in which it is expressed by secretory epithelial cells. The transport model proposed in 1973-74 on the basis of test-tube experiments and immunohistochemical studies has now gained considerable support from observations made on both normal and neoplastic living epithelial cells According to this model, the J ("joining') chain and SC represent "the lock and key' in the selective external translocation of both dimeric IgA and pentameric IgM through serous-type secretory epithelial cell. Incorporation of J chains into these two Ig isotypes during their production in gland-associated immunocytes induces a configurational fit (binding site) allowing them to combine by specific non-covalent interactions with SC in the plasma membrance of the epithelial cell. After being formed on the basolateral surface of the cell, the SC-IgA and SC-IgM complexes are transported in cytoplasmic vesicles to the gland lumen along with some free SC. Covalent stabilization of human secretory IgA during this process depends on unique possibilities for disulphide-exchange reactions and is not an inherent feature of the transport model.
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ABSTRACT: Secretory component (SC) isolated from human colostrum was radioiodinated with chloramin-T method and it was subjected to
a radioimmunoassay for quantitation of SC. A monospecific rabbit anti-sera against human SC which could react with free SC
as well as immunoglobulin bound SC was used for the assay.
The present method can measure as low as 300ng/ml of SC. The level (mean ±2 S.D.) of serum SC in healthy subjects was determined
to be 31.3 ±7.2Μg/ml. Studies with Sephadex G-200 gel filtration demonstrated that SC in the normal sera existed mostly as
a form of SC-IgA. In contrast, free SC was never detected in them.
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ABSTRACT: Inthe rat,allreceptor-bindableimmunoglobulin A (IgA),and 1-4% of injected asialoglycoprotein(ASG),are transportedfrom blood to bileintact.The major fractionof the ASG isdegraded inhepaticlysosomes.The study described here was designed to elucidate the sortingthatoccurs inhepatocytessubsequent to receptorbindingof ligandsnot sharing the same fate.We show thatconjugationofproteinwith the Boltonand Hunter reagentcan be used as a probe forthe lysosomalpathway, since50% ofthe reagentisreleasedintobile afterlysosomal degradation of internalizedprotein.Radiolabelingby iodinemonochloride was alternatively used to followthe directpathways thatdeliverintactIgAand ASG to bile. Afterintravenousinjectionoflabeledproteins,firstintactASG and IgA,and then radioactive catabolitesfrom degraded protein,were releasedintobile.No proteolyticintermediateswere detected,and the transportof IgAor ASG directlyto bilewas not affectedby the lysosomal proteaseinhibitorleupeptin.These observationsindicatethatdivergenceofthe directbiliary transportpathways from the degradation pathway occurs at a stage preceding deliveryto lysosomes,possiblyatthe cellsurface.Competition studiesshowed thatallthreepathways (includingthe biliarytransportof intactASG) are receptormediated, but even atsupersatu- ratingdoses the uptake and processingof IgA and ASG occur independently.We propose thatIgAand ASG receptorsare notfrequentlyinjuxtapositionon the plasma membrane, but thatASG, afterbindingtoitsreceptor,isoccasionallymissortedintothe biliarytransportpool.
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