Investigations of the structural function of the J chain in human immunoglobulin M

Human IgM molecules were treated with Na₂SO₃ or mercaptoethylamine in concentrations ranging from 2 to 14mm or 2 to 22mm respectively. The dissociation of IgM to IgM_s varied from 0% to 100%. At the intermediate concentrations of either reagent the amount of freed J chains was less than expected. In an attempt to find an explanation for this, IgM was partially dissociated to IgM_s with mercaptoethylamine. The IgM_s isolated by gel filtration was divided according to the ascending and descending portions of the elution curve. These portions were treated with 24mm-mercaptoethylamine and analysed for the presence of J chains. Only the ascending portion contained free J chains. Thus, after mild reduction where not all the IgM molecules are dissociated to IgM_s, some J chains remain covalently attached to some IgM_s molecules although most of the J chains are freed. It was concluded that the J chain could serve as a ‘hitch’ for IgM_s molecules forming intact IgM.