The presence of histidine residues at or near the C1q binding site of rabbit immunoglobulin G.

Treatment of covalently crosslinked rabbit IgG oligomers with diethylpyrocarbonate resulted in the loss of their C1q binding activity. The inactivation was a first-order process with respect to time in the range 0-8 min, and modifier concentration from 0 to 2.39 mM. Hydroxylamine treatment of diethylpyrocarbonate-treated IgG oligomers led to 80% recovery of their C1q binding activity. Diethylpyrocarbonate treatment of IgG oligomers had little effect on their absorbance at 278 nm, but led to an increase in their absorbance at 242 nm. The apparent pKa of the modified residues was 6.91 +/- 0.12. These data are consistent with diethylpyrocarbonate modification of histidine residues leading to loss of C1q binding activity in rabbit IgG oligomers. Modification of four histidine residues per IgG molecule was associated with the loss of C1q binding activity. Thus, there may be two histidine residues at or near the C1q binding sites in the CH2 domains of rabbit IgG.