BINDING OF COMPLEMENT PROTEINS C1Q AND C4BP TO SERUM AMYLOID-P COMPONENT (SAP) IN SOLID CONTRA LIQUID-PHASE

Serum amyloid P component (SAP), a member of the conserved pentraxin f amily of plasma proteins, binds calcium dependently to its ligands. Th e authors investigated SAPs interaction with the complement proteins C 4b binding protein (C4bp) and C1q by ELISA, immunoelectrophoresis and electron microscopy. Binding of these proteins to SAP was demonstrated when SAP was immobilized using F(ab')(2) anti-SAP, but not when SAP r eacted with these proteins in liquid phase; thus the binding to human SAP was markedly phase state dependent. Presaturation of solid phase S AP with heparin. which binds SAP with high affinity, did not interfere with the subsequent binding of C4bp or C1q to SAP. In contrast, colla gen I and IV showed partial competition with the binding of C1q to SAP . Using fresh serum, immobilized native SAP bound C4bp whereas binding of C1q/C1 could not be demonstrated. Altogether the results indicate that firm binding of C1q and C4bp to SAP requires that SAP is presente d on a solid phase, that C1q and C4bp react with sites distinct from t he heparin binding site, and that C1q and collagen I share binding sit es on SAP. Immobilized native SAP, aggregated SAP and SAP-heparan-sulp hate complexes induced no detectable complement activation.