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Tryptic Degradation of the C_H1 and V_L Regions of IgD and IgE¹

From the Department of Immunopathology, Scripps Clinic and Research Foundation, La Jolla, California 92037

Abstract

Fab fragments of all human Ig classes were digested with trypsin and analyzed by SDS-PAGE for degradation products. Fab fragments of all four IgG subclasses, IgM and IgA₁ were resistant whereas IgD and IgE Fab fragments were degraded into smaller fragments. IgE Fab fragments had already become degraded during the papain digestion used to produce them. The degradations of IgD and IgE Fab fragments were independent of and light chain types. After prolonged tryptic digestion of a IgD Fab fragment, three major fragments remained, which were isolated and characterized by partial amino acid sequence analysis. The fragments consisted of 1) -chains, 2) the constant region of the -chains minus the first three amino acids, and 3) the variable region of the -chain including the first four amino acids of the C₁ region. In contrast to the Fab fragments, IgD Fc fragments and isolated -chains were not degraded into small fragments. The data indicate that the first constant regions of IgD and IgE, but not IgG, IgM, and IgA, are highly susceptible to enzymatic degradation and that association of the light chains with - and -chains renders the V_L regions susceptible to enzymatic attack. Since it is unlikely that the C_H1 regions of - and -chains change the tertiary structure of the V_L regions, it is suggested that monomeric light chains are released after digestion of the C_H1 regions and that the V_L region of these monomers is degraded.

Footnotes

¹ This is publication No. 1643 from the Department of Immunopathology, Scripps Clinic and Research Foundation, La Jolla, California. This work was supported by United States Public Health Service Grant AI-10734 and Biomedical Research Support program Grant RRO-5514.

² Research Fellow of the Swiss National Science Foundation. Present address: Department of Pathology, University of Geneva, CH-1211 Geneva 4, Switzerland.