The Effects of Histidine Residue Modification on the Immune Precipitating Ability of Rabbit IgG

Treatment of anti-ovalbumin rabbit IgG with diethylpyrocarbonate (DEPC) at concentrations up to 100 μM led to a progressive decrease in the rates of formation of insoluble immune complexes, without affecting the final extent of immune complex formation. DEPC concentrations approximately 10-fold higher were needed to give comparable decreases in the rates of immune complex formation by F(ab′)2. Treatment of DEPC-treated IgG with hydroxylamine led to substantial restoration of the rates of formation of insoluble immune complexes. Carbethoxylation of two histidine residues per IgG molecule had little effect on rates of formation of insoluble immune complexes, but these rates were markedly decreased in samples of IgG with four to five histidines per molecule modified. There were parallel decreases in the protein A-binding activity and in the rates of formation of insoluble immune complexes in IgG treated with increasing concentrations of DEPC. The presence of complement protein C1q restored the rates of formation of insoluble immune complexes of DEPC-treated IgG.