Binding characteristics of human serum amyloid P component.

Serum amyloid P component (SAP), a normal human plasma glycoprotein, was found in a solid phase ELISA to have Ca2+-dependent binding for keyhole limpet haemocyanin (KLH), pectic acid, trinitrophenylated (TNP) macromolecules, and plastic surfaces. The binding to TNP-KLH was used to develop a sensitive ELISA. The binding of SAP to the ligands mentioned was inhibited by EDTA, KLH, pectic acid, TNP-conjugated macromolecules (bovine serum albumin, polyacrylhydrazide), and p-nitrophenylarsonic acid. Underivatized and DNP-conjugated macromolecules did not inhibit the SAP binding; arsenilic acid, picric acid, and dinitrophenyl were weak inhibitors. SAP bound to TNP-agarose was eluated by either EDTA or p-nitrophenylarsonic acid. Thus, a unique region of SAP is responsible for the polyspecific binding. We suggest that the polyspecific binding of SAP takes place through a Ca2+ bridge: half of the metal coordination sphere is occupied by SAP, with the other half available to interact with metal ligand.