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Structure of Human Immunoglobulin M

II. Isolation of a High Molecular Qeight Fc Fragment of IgM Composed of Several Fc Subunits^1,2,

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

Abstract

A 10.6 S fragment with an approximate molecular weight of 320,000, which represented the Fc portion of the human IgM was isolated by digesting the IgM in the absence of cysteine with activated papain. Comparison of antigenic properties, amino acid composition and carbohydrate content of this fragment with those of the Fc fragment obtained from the subunits of IgM previously, indicated that this 10.6 S fragment was composed of the Fc portions of IgM subunits. The 10.6 S fragment dissociated into 3.2 S Fc units by mild reduction followed by alkylation. Thus it was evidenced that the intersubunit disulfide bonds of IgM are located on the Fc portions of the subunits.

Footnotes

This investigation was supported in part by grants from Japan Ministry of Education and Waksman Research Foundation in Japan.

² Abbreviations were used for immunoglobulins and their fragments as follows. IgM, IgG, Fab and Fc were used according to the definition which appeared in "Nomenclature for Human Immunoglobulins" (Bull. W. H. O., 30: 447, 1964). Fab-(IgMs) and Fc(IgMs) were used for the Fab-like and Fc-like fragments obtained by papain digestion of the 7 S subunits of IgM. In the previous paper (7), Fab (IgMs) was called Ms pap Ia or Ms pap Ib and Fc(IgMs) was called Ms pap III.