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Bleeding, the serious complication of thrombolytic therapy with tissue type plasminogen activator (t-PA), is a result from lysis of fibrin in hemostatic plugs and from the systemic lytic state caused by unopposed plasmin. It suggests that structural changes resulting from incorporation of fragment X into clots promote their lysis and the attenuation of thrombolytic therapy-induced fragment X formation might reduce the risk of bleeding.

Heparin binds fibrin and, by bridging thrombin onto fibrin, promotes the formation of a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin. Because thrombin binds γA/γ′-fibrin, a variant with an extended γ-chain, with higher affinity than the bulk γA/γA-fibrin, γA/γ′-fibrin affords bound thrombin more protection from inhibition by antithrombin–heparin. We examined the effect of Zn+ on heparin–thrombin–fibrin complex formation because Zn+ modulates heparin–protein interactions. Zn+ increased the affinity of heparin for γA/γA- and γA/γ′-fibrin by 4.3- and 3.7-fold, respectively, but had no effect on the affinity of thrombin for either form of fibrin. In contrast, in the presence of heparin, Zn+ increased the affinity of thrombin for γA/γA-fibrin 4-fold (from a Kd value of 0.8 to 0.2 μM) and slowed the rate of thrombin dissociation from γA/γA-fibrin clots. These findings suggest that Zn+ enhances the formation of ternary heparin–thrombin–fibrin complexes with γA/γA-fibrin but does not influence the already high affinity interaction of thrombin with γA/γ′-fibrin. Consistent with this concept, in the presence of Zn+, γA/γA-fibrin protected thrombin from inhibition by antithrombin–heparin to a similar extent as γA/γ′-fibrin. Therefore, by enhancing the binding of heparin to fibrin, physiological concentrations of Zn+ render fibrin-bound thrombin more protected from inhibition by antithrombin. Because fibrin-bound thrombin can trigger thrombus expansion, these findings help to explain why recurrent thrombosis can occur despite heparin treatment. Heparin binds fibrin and, by bridging thrombin onto fibrin, promotes the formation of a ternary heparin–thrombin–fibrin complex that protects thrombin from inhibition by antithrombin. Because thrombin binds γA/γ′-fibrin, a variant with an extended γ-chain, with higher affinity than the bulk γA/γA-fibrin, γA/γ′-fibrin affords bound thrombin more protection from inhibition by antithrombin–heparin. We examined the effect of Zn+ on heparin–thrombin–fibrin complex formation because Zn+ modulates heparin–protein interactions. Zn+ increased the affinity of heparin for γA/γA- and γA/γ′-fibrin by 4.3- and 3.7-fold, respectively, but had no effect on the affinity of thrombin for either form of fibrin. In contrast, in the presence of heparin, Zn+ increased the affinity of thrombin for γA/γA-fibrin 4-fold (from a Kd value of 0.8 to 0.2 μM) and slowed the rate of thrombin dissociation from γA/γA-fibrin clots. These findings suggest that Zn+ enhances the formation of ternary heparin–thrombin–fibrin complexes with γA/γA-fibrin but does not influence the already high affinity interaction of thrombin with γA/γ′-fibrin. Consistent with this concept, in the presence of Zn+, γA/γA-fibrin protected thrombin from inhibition by antithrombin–heparin to a similar extent as γA/γ′-fibrin. Therefore, by enhancing the binding of heparin to fibrin, physiological concentrations of Zn+ render fibrin-bound thrombin more protected from inhibition by antithrombin. Because fibrin-bound thrombin can trigger thrombus expansion, these findings help to explain why recurrent thrombosis can occur despite heparin treatment. [ABSTRACT FROM AUTHOR]