Studies on the complement-binding site of rabbit immunoglobulin G—I. Modification of tryptophan residues and their role in anticomplementary activity of rabbit IgG

Abstract

The effect of tryptophan modification of rabbit IgG and its fragment Fc on anticomplementary activity was studied using a tryptophan-specific reagent 2-hydroxy-5-nitrobenzyl bromide. In order to locate the position of the critical tryptophan residues, modified IgG was subjected to pepsin treatment and a peptide containing modified tryptophan residues was isolated and identified as belonging to a fragment derived from Fc near the hinge region (Pep V). In order to determine if the complement-binding site of IgG could be attributed to one particular peptide from the Fc region, the interaction of normal and tryptophan-modified peptides with complement was investigated. Peptides were tested for their effect on complement fixation either directly, under specified conditions, or in the presence of aggregated IgG. In the latter case, Pep V produced an effect different from that of the other peptides. Tryptophan modification of this peptide altered its behavior implying that at least one tyrptophan residue in Pep V is involved in complement fixation by IgG. Further studies are required to localize more precisely the complement-binding site of IgG.