Is secretory component disulfide-bonded to both monomer subunits in human secretory IgA dimer?

Selective hepatobiliary transport of human polymeric IgA in mice

Both subclasses of human polymeric IgA (pIgA) were selectively transported from the serum into the bile of mice relative to human IgG or IgM. Removal of human pIgA from serum corresponded to the clearance kinetics shown for murine pIgA. The biliary pIgA was intact as determined by sucrose density gradient ultracentrifugation. This hepatic uptake was specific for the IgA isotype and occurred independently of receptors in the liver specific for glycoproteins that terminate with galactose or mannose moieties. Desialylation of human pIgA resulted in its rapid clearance from serum and subsequent deposition in the liver in a manner similar to most other desialylated serum glycoproteins. The desialylated pIgA present in bile was also an intact molecule; thus the asialoglycoprotein receptor may represent an additional mechanism for the transport of serum pIgA into bile.

Complex formation between secretory component and human immunoglobulins related to their content of J chain

The J-chain content of 3 IgM and 24 IgA preparations was quantitated by an immunochemical technique after reduction with 20mM dithiothreitol. The amounts released ranged from undetectable (less than 0.1 mg) to 7.3 mg per 100 mg of Ig. Most of the J-chain-deficient proteins were monomeric, but four polymeric IgA preparations were found to contain only 0.2-0.8 mg of J chain per 100 mg. The SC-binding capacity of these polymers, expressed as percentage of the amount added (2.5 mu g SC/100 mug Ig), was 6%-12% compared with 69%-82% for IgA and IgM polymers that contained more than 4.0 mg of J chain per 100 mg. Some monomeric IgA preparations showed a slight SC-binding capacity, which was explained by the presence of contaminating J-chain-positive polymers. Bound J chain therefore seems to be a structural prerequisite for a specific noncovalent complexing of Ig polymers with SC.

Binding of secretory component to dimers of immunoglobulin A in vitro. Mechanism of the covalent bond formation

A complex between secretory component and an immunoglobulin A (IgA) myeloma dimer has been studied in vitro as a model to elucidate the mechanism of the formation of disulfide bonds during assembly in vivo of secretory immunoglobin A. A small amount of free thiol groups, totally about 0.4 groups per mole of protein, were shown to be present on both the heavy and light chains of the IgA dimer, but not on its J-chain, while no such groups could be demonstrated on free secretory component. The SH-groups on IgA most likely exist as a result of incomplete oxidation of some intra-or interchain disulfide bonds of the molecule, analogous to what has been suggested for IgG. Several types of evidence indicated that the disulfide bonds between secretory component and IgA are formed after the noncovalent association of the two proteins by a sulfhydryl group-disulfide bond exchange reaction, in which the small amount of free sulfhydryl groups on the IgA dimer initiate the reaction by reducing a reactive disulfide bond on secretory component. This exchange reaction, which thus proceeds by the mechanism of so-called disulfide interchange reactions, requires certain conformational features of one or both of the proteins and leads to the formation of presumably two new interchain disulfide bonds between secretory component and IgA. The reaction does not progress to completion, however, but ends in an equilibrium so that a small proportion of the secretory component molecules always are unattached by disulfide bonds.

Blocking effect of J chain and J-chain antibody on the binding of secretory component to human IgA and IgM

Isolated released J chain showed only a small affinity for free secretory component (SC), as indicated by a marginal but reproducible blocking effect on the binding of SC to Ig polymers. The SC-binding site was completely blocked by J-chain antibody in those Ig polymers where the bound J chains were accessible to the antibody. Along with the established masking effect of SC on the antigenicity of J chains present in secretory IgA, these results are compatible with the idea that the conformation of Ig-associated J chains contributes to the SC-binding site of Ig polymers.