Competitive binding of thrombospondin, fibronectin, and fibrinogen to adsorbed proteins in multicomponent systems

The proteins thrombospondin, fibronectin, and fibrinogen are distributed among the plasma, the basement membrane of the vascular endothelium, and the storage pool of the unstimulated platelet. Upon platelet stimulation these proteins undergo redistribution and coexist both in solution and in bound states on the platelet and other surfaces. In our earlier studies of complex formation in systems involving these proteins in solution and in the surface-immobilized form we have shown that they interact with each other on the surface but that there is inhibition of binding to the surface when all three proteins are present. The concentration dependence of the interactions in such multicomponent systems was therefore studied. For reference, ovalbumin was included as a noninteracting protein. We have shown (a) that no three-component chemical complex is formed at the surface; (b) that the inhibition of binding to a surface-attached protein by any of the three proteins in solution is competitive; and (c) that proteins form complexes in solution, and that the dissociation constant of these complexes are on the order of 5 × 10^-8 M. The results suggest that thrombospondin secreted by the stimulated platelet will interact mainly with fibronectin and fibrinogen in the plasma with little involvement of the platelet or the extracellular matrix surface.