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Human IgM molecules were treated with Na(2)SO(3) 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.

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