Structure of Human Immunoglobulin M

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Abstract
Summary: Pepsin fragmentation of a purified IgM obtained from a patient with macroglobulinemia was studied. Three fragments were obtained. These fragments were all related structurally to the Fab-(IgMs) fragment obtained by papain digestion of the subunit of IgM. The first, 5.6 S fragment (F(ab′)2(IgM)), was converted quantitatively to 4.2 S fragments (Fab′(IgM)) by mild reduction and alkylation. No change in antigenicity was observed by this treatment. Thus the 5.6 S fragment seemed to be a dimer in which two monomeric 4.2 S fragments were linked by a disulfide bond. The second, 3.3 S fragment (Fab pep(IgM)), was similar to the Fab(IgM) obtained by papain digestion of the IgM or its subunit. It lacked a part of the µ chain portion (Fd′µ) of F(ab′)2(IgM) on which the particular disulfide bond linking the two Fab′ fragments seemed to be located. It was noted that the part of Fd′µ mentioned carried some antigenic determinants of the µ chain. The third, 2.4 S fragment (Fab piece(IgM)), was a part of the Fab pep(IgM) consisting of a part of κ chain and a part of µ chain. It seemed to lack a part of the Fab pep(IgM) on which the disulfide bond(s) linking a κ chain and a µ chain portions are located. Time study of the digestion showed that pepsin first cleaved off the Fc portion of the IgM and yielded the 5.6 S dimeric fragment, F(ab′)2. Then the F(ab′)2 was degraded to the 3.3 S monomer, Fab pep, and further to the 2.4 S fragment, Fab piece.

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