Fibrinogen Binds to Heparin: The Relationship of the Binding of Other Adhesive Proteins to Heparin

The binding of heparin to fibrinogen was characterized. We have used an assay employing heparin coupled to Sepharose CL-6B to show that 125I-fibrinogen binds to heparin in a time-dependent, divalent ion-independent, saturable, and reversible manner. A total of 1.75 mg of fibrinogen could bind to 1 ml of heparin-Sepharose CL-6B with a Kd of 4.03 × 10−7 M. Binding was completely inhibited by other adhesive proteins including fibronectin, von Willebrand factor (vWF), and vitronectin with IC50s of 7.2-8.8 μM. The binding of 125I-fibrinogen to heparin-Sepharose CL-6B was blocked by a monoclonal antibody (52k-2) and a polyclonal antibody (FK-2). 52k-2 inhibits the binding of vWF, and FK-2 blocks the binding of fibronectin to heparin-Sepharose CL-6B. It is therefore likely that fibrinogen has a heparin-binding site(s) which may have some overlap with those of other adhesive proteins, although the physiological role of the fibrinogen heparin-binding domain is uncertain at present.