γA/γ′ fibrinogen inhibits thrombin-induced platelet aggregation

The minor gamma A/gamma' fibrinogen isoform contains a high affinity binding site for thrombin exosite II that is lacking in the major gamma A/gamma A fibrinogen isoform. We therefore investigated the biological consequences of the gamma' chain binding to thrombin. Thrombin-induced platelet aggregation was inhibited by gamma A/gamma' fibrinogen. Carboxyl terminal peptide fragment gamma'410-427 from the gamma' chain was also inhibitory, with an IC50 of similar to 200 mu M in whole plasma. Deletion of the pepticle from either the amino or carboxyl end significantly decreased inhibition. In contrast to thrombin-induced platelet aggregation, aggregation induced by epinephrine, ADP, arachidonic acid, or SFLLRN pepticle showed little inhibition by the gamma' peptide. The inhibition of thrombin-induced platelet aggregation was not due to direct inhibition of the thrombin active site, since cleavage of a small peptidyl substrate was 91% of normal even in the presence of 1 mM gamma'410-427. The gamma'410-427 peptide blocked platelet adhesion to immobilized thrombin under both static and flow conditions, blocked soluble thrombin binding to platelet GP1b alpha, and inhibited PAR1 cleavage by thrombin. These results suggest that the gamma' chain of fibrinogen inhibits thrombin-induced platelet aggregation by binding to thrombin exosite II. Thrombin that is bound to the gamma' chain is thereby prevented from activating platelets, while retaining its amidolytic activity.