The effect of benzylation of tryptophan residues of immunoglobulin G (IgG) on the interaction with macrophage receptors was studied by measuring the competition between free IgG and immune complexes (sheep red cells covered with IgG antibodies). 2. The modification of approximately three tryptophan side chains greatly decreased the capacity of IgG for binding to the phagocytic cell. Under conditions in which normal IgG inhibited the binding of immune complexes to macrophages by 49 +/- 5% and the urea-treated IgG control inhibited it by 40 +/- 11%, IgG modified by 2-hydroxy-5-nitrobenzyl bromide in the presence of urea inhibited the binding by 7.5 +/- 5%. 3. These results indicate that tryptophan residues are located at or near the site of IgG that binds to the macrophage receptor and that they are important for the interaction.
