Reversibility of fibrinogen fragment D₁ binding to human platelets: Comparison with native fibrinogen

To gain further insight into the mechanism responsible for rendering fibrinogen bound to stimulated platelets irreversible to dissociation by EDTA or excess unlabeled fibrinogen, the present study compared the reversibility of platelet interactions with fibrinogen and its plasmic degradation product, fragment D₁. Like fibrinogen binding, the binding of fragment D₁ became progressively less sensitive to dissociation by EDTA, PGE₁, or excess unlabeled fibrinogen. Thus in the presence of EDTA, 70 ± 19% and 55 ± 24% (mean ± S.D., n=9) of bound fragment D₁ failed to dissociate from platelets 60 min after stimulation with 0.15 U/ml thrombin or the combination of 5μM ADP and 5μM epinephrine, respectively, compared to 75 ± 8% and 52 ± 17% of platelet-bound, intact fibrinogen. In contrast, platelet stimulation with chymotrypsin or Zn⁺² failed to support the development of irreversible fragment D₁ or fibrinogen binding. Only 8 ± 6% and 9 ± 3% of bound fragment D₁ remained associated with chymotrypsin- or Zn⁺² -treated platelets, respectively, compared to 7 ± 11% and 15 ± 6% (mean ± S.D., n=3) of platelet-associated fibrinogen. These observations suggest that irreversible fragment D₁ and fibrinogen binding to platelets occurs by a similar mechanism that requires neither fibrinogen alpha chain 95-97 or 572-574 RGD sequences nor multivalent ligand-receptor interactions.