Isolation of C4-binding protein from guinea pig plasma and demonstration of its function as a control protein of the...

A decay-accelerating factor of the classical complement pathway C3 convertase, C4b,2a, has been purified to homogeneity from guinea pig plasma by a 5-step procedure that includes 5% polyethyleneglycol-4000 (PEG-4000) precipitation, Sepharose 6B gel filtration, heparin-Sepharose chromatography, DE-52 anion exchange chromatography, and Sepharose-C4gp affinity chromatography. The protein elicited a monospecific antiserum in a rabbit and was found with the Mancini technique in both normal and C4-deficient guinea pig plasma at a concentration of 60 microgram/ml. The purified protein gave a single stained band of 550,000 m.w. on SDS-PAGE under nonreducing conditions and a single band of 72,000 m.w. with reduction and alkylation. On the basis of its m.w. and subunit structure, ability to bind to a C4 affinity column, and ability to regulate the classical C system by accelerating the decay of the classical C3 convertase this protein represents the guinea pig analog of the human C4-binding protein.