Studies on circulating immune complexes of the liver disease. 3. Conditions for Clq binding assay and low molecular weight Clq reactants.

Assay conditions for Clq binding activity (ClqBA) of aggregated human gamma globulin and pathologic sera were studied. The Clq component of complement was isolated by precipitation method and effectively radioiodinated using lactoperoxidase without loss of binding activity. The assay method was reproducible and the binding activity of normal sera was 5.0 +/- 2.4% (mean +/- S.D.). Amount of 125I-Clq, in the range of 50--1000 ng per assay tube, did not influence the binding activity. Competitive inhibition of intrinsic Clq was very limited. Sample sera for the determination can be stored at --20 degrees C and freezing-throwing of the sera did not increase ClqBA. In normal sera, the existence of solubilizer of Clq-reactant-complex or inhibitor of Clq-reactant binding working at 37 degrees C is suggested. For the low molecular weight reactants in pathologic sera reacting with Clq at 4 degrees C, following substances are proposed: 1. abnormal IgG, 2. hapten-IgG complexes or 3. anionic substances.