Portal de Periódicos da CAPES

Human secretory component—VI

1977; Elsevier BV; Volume: 14; Issue: 3 Linguagem: Inglês

10.1016/0019-2791(77)90192-6

1878-237X

Per Brandtzæg,

T-cell and B-cell Immunology

The interaction between native free secretory component (SC) and J chain-containing IgA and IgM polymers was studied with regard to forces involved in Ig-SC complex formation, number of SC-binding sites (n) per polymer, apparent equilibrium constant of association (Ka), and physicochemical properties of the complexes formed. The Ig-SC association followed a dose-response pattern indicating that no more than one SC-binding site of high affinity and limited capacity is present per polymer, since c was determined to be 0.6-0.7 for both dimeric IgA and pentameric IgM. Nevertheless, the non-covalent forces involved in the complex formation were much stronger for IgM than for IgA. Moreover, although Ka for both types of polymers was determined to be in the range 107–108M−1, it appeared to be 2.7–12.5 times higher for IgM than for IgA. Since several uncontrolled variables were involved in the determination of Ka, a competitive binding test was established. It was substantiated that SC has 5–30 times better affinity for IgM than for IgA, depending on the pair of protein preparations compared. This may reflect the ‘bonus effect’ of a higher molar J chain-content in IgM than in IgA, since the configuration of the SC-binding site apparently is determined by the polymer-associated J chains. Oxidation-dependent stabilization and packing of the quaternary structure took place more readily for the IgA-SC than for the IgM-SC complexes, as demonstrated by a relatively smaller specific SC displacement in the former and by a more decreased dissociation and lowered I-determinant reactivity of bound SC after dialysis. These results seemed to be explained by a greater tendency of IgA to become involved in disulphide-exchange reactions with the bound SC. Better affinity of SC for IgM may confer glandular transport advantage on this polymer compared with dimeric IgA, whereas better covalent stabilization of the latter may account for the superior survival of IgA antibodies in exocrine secretions.