Abstract
The activation of the classical pathway of complement may occur when C1 is bound to the Fc portion of immunoglobulin G (IgG) in immune complexes. In order to study the properties of C1 binding and activation we have prepared covalently cross-linked complexes of isolated Fc molecules. The Fc of human IgG was isolated from either limited papain digests or trypsin digests by gel filtration and ion exchange chromatography. Using a C1 binding assay we found that monomeric Fc (papain) did not bind C1 even at concentrations as high as 1 mg/ml. Monomeric Fc (trypsin) did bind C1 weakly. The binding properties of polymeric Fc complexes produced by covalent cross-linking with dimethyl suberimidate in polyethylene glycol were remarkably different from those observed with monomeric Fc. Less than 0.2 µg/ml of Fc complexes found more than 50% of the C1 in the assay.
