Fibrinogen interaction with platelet receptors.

In summary: Incubation of platelets with ADP or proteolytic enzymes (chymotrypsin or pronase) results in an exposure of two classes of specific binding sites on platelet surface: low and high affinity fibrinogen receptors. Fibrinogen interaction with these receptors results in platelet aggregation. High affinity fibrinogen receptors are not exposed on thrombasthenic platelets stimulated by ADP but are rendered available on chymotrypsin-treated thrombasthenic platelets; low affinity receptors cannot be exposed by ADP or chymotrypsin on these platelets. Availability of high affinity fibrinogen receptors on thrombasthenic platelets may depend on the residual glycoprotein IIIa. Fibrinogen receptors appear to be associated with glycoproteins IIb, IIIa and a 66,000 Mr platelet membrane component that is exposed during proteolysis of platelet membranes. Some of the platelet-binding sites on the fibrinogen molecule appear to be associated with the COOH-terminal portion of the gamma chain (gamma 374-411). Additional binding sites may also be located in the COOH-terminal portion of the A alpha chain. The conformation of the fibrinogen molecule may be important in its interaction with platelets. Platelet aggregation may result from bridging platelets by fibrinogen molecule in the presence of bivalent cations. In conclusion, platelet interaction with fibrinogen is a complex process involving different binding sites of the fibrinogen molecule. Our own data and review of literature suggest that platelet-interaction with fibrinogen is of major significance in hemostasis.