Purification and characterization of C4‐binding protein from human serum

Activation of the classical pathway of complement proceeds through the transient formation of a C3 convertase, a bimolecular complex due to an interaction of fragments C4b and C2a from components C4 and C2 [ 1,2]. This C3 convertase is highly labile and retains only shortly its capacity for p roteolyzing C3. In [3,4] control factors were described which could destabilize the C3 convertase. A protein has been purified from human serum, which is able to bind to native C4b and termed C4-binding protein (C4bp); this protein is also able to inhibit the formation of the C3 convertase and to accelerate the decay of its catalytic activity [5,6]. This protein is probably similar to a macromolecular cofactor described in [7] and detailed in [8,9]. C4bp has been purified from mouse, guinea pig and human plasma [5,10,11]. Most of the techniques described are complex and time-consuming. This paper describes a simplified method for the purification of C4bp from human plasma, to give the major characteristics of the purified protein and to demonstrate one of its interactions with native C4b.