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 IgMs 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 IgMs with mercaptoethylamine. The IgMs 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 IgMs, some J chains remain covalently attached to some IgMs molecules although most of the J chains are freed. It was concluded that the J chain could serve as a ‘hitch’ for IgMs molecules forming intact IgM.

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