Cell-binding and internalization of soluble fibrin by platelets.

Centrifuged human platelets bound soluble 125I-labelled fibrin, mediated by a plasma factor. Binding was inhibited by D-phenylalanyl-L-prolyl-L-arginyl- chloromethane (PPACK), which specifically blocks thrombin. As the binding-promoting principle was adsorbed to barium citrate, it was tentatively characterized as prothrombin, suggesting that it might be converted to thrombin at the cell surface. The peptide GRGDSP failed to inhibit binding, thus eliminating the glycoprotein IIb/IIIa complex as a receptor. Most likely, a thrombin - fibrin complex is recognized by a cell receptor, possibly protease-nexin I. In a platelet concentrate, the cells also internalized 125I-labelled fibrin, providing evidence that platelets are involved in the clearance of circulating fibrin - monomer complexes. Engulfment was again inhibited by PPACK or hirudin but not by an antibody against the glycoprotein IIb/IIIa complex.