Evidence for arginine residues in the immunoglobulin-binding sites of human Clq

The immune complex binding activity of human Clq was lost following treatment of the protein with the arginine-selective reagents cyclohexane 1,2-dione and phenylglyoxal. Both inactivations followed pseudo-first-order kinetics. The affinity of Clq for immune complexes was reduced 7-fold following cyclohexane-1,2-dione treatment, and could be substantially restored by treatment of the modified protein with hydroxylamine. Heat-aggregated IgG protected Clq against inactivation by both reagents. Incorporation of 25 molecules of [7-¹⁴C]phenylglyoxal per Clq molecule completely inactivated the protein. These data are consistent with the presence of arginyl residues in the immunoglobulin recognition sites of human Clq.