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 x 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 microM. 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.