Identification of a nonproteolytically activated C3 with CR 1-binding properties in sera from subjects with primary biliary cirrhosis.

Complement factor C3 associated with immune complex-like material was precipitated by 2.7% (w/v) polyethylene glycol (PEG) in sera from patients with primary biliary cirrhosis. Radioimmunoassay analysis demonstrated that this C3 expressed antigens which are specific for bound and denatured C3 but absent in native C3. Gelfiltration of the PEG precipitate revealed a C3 peak which corresponded to a molecular weight of more than 2 X 10(7) daltons but sucrose-gradient centrifugation of the same material showed that the C3 had a sedimentation coefficient of 4-7 S. SDS-PAGE followed by immunoblotting presented intact C3 alpha and C3 beta chains, and immune electrophoresis showed that the PEG-precipitated C3 was slower than native C3. The biological properties of the PEG-precipitable C3 were demonstrated by the inhibitory effect on the binding of immune complexes containing C3b to complement receptor 1. These data are not compatible with coprecipitation of native C3, nor with immune complex-bound C3b.