Binding of fibrinogen to platelet integrin alpha IIb beta 3 in solution as monitored by tracer sedimentation equilibrium.

Fibrinogen showed essentially no binding (KD > 1 mM) to platelet alpha IIb beta 3 integrin in solution in the presence of Triton or octylglucoside above critical micellar concentrations. Under these conditions the integrin was an alpha beta monomer. After removal of the detergent from the Triton containing buffer (25 mM Tris/HCl;, 150 mM NaCl, 1 mM CaCl2, 1 mM MgCl2, pH 7.4) the integrin formed aggregates with hexamers as the most prominent species, as demonstrated by analytical ultracentrifugation and electron microscopy. Tracer sedimentation equilibrium experiments indicate that fibrinogen binds to the integrin aggregates, but with a surprisingly large KD (at least 3 microM). This value is 10- to 100-fold higher than values determined by solid phase assays or with integrins reconstituted onto lipid bilayers.