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Structure of Human Immunoglobulin M

III. Pepsin Fragmentation of IgM^1,2,

From the Third Department of Internal Medicine, Osaka University School of Medicine, Osaka, Japan

Abstract

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 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 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.

Footnotes

This study was partly supported by grants from the Ministry of Education and from Wacksman Research Foundation in Japan.

² The following abbreviations are used for the immunoglobulins and their subunits. IgM, IgG, Fab and Fc are used according to the definition appeared in "Nomenclature for human immunoglobulins" (Bull. W. H. O., 30: 447. 1964). Fab(IgMs) is used for the Fab-like fragment and Fc(IgMs) for the Fc-like fragment, both of which were obtained by papain digestion of the subunit of IgM. The Fab(IgMs) was called Ms pap Ia or Ms pap Ib and the Fc(IgMs) was called Ms pap III in the previous paper (7).