Binding Properties of the Human Complement Protein Clq

The interaction between human Clq and immunoglobulins was measured quantitatively by determining the ability of IgG, IgM, and (Fc)5µ to inhibit the binding of 125I-labeled Clq to IgM covalently linked to cyanogen bromide activated Sepharose. The following inhibition constants were determined: Ki for IgM = 6.42 x 10-6 m; Ki for (Fc)5µ = 4.35 x 10-6 m; and Ki for IgG = 1.10 x 10-4 m. The heat aggregation of IgM and IgG increased the ability of these proteins to bind 125I-labeled Clq, but had no significant effect on the binding properties of (Fc)5µ. The binding between the 125I-labeled Clq and the IgM-Sepharose complex was inhibitable with aromatic and alkyl diamino compounds. The most potent inhibitor was 2,5-diaminotoluene.