Induction of a sustained fibrinolytic response by BRL 26921 in vitro.

The role of thrombus-binding in the fibrinolytic response to the acylated streptokinase.plasminogen activator complex, BRL 26921, has been examined using human plasma clots, radiolabelled with 125I-fibrin, in vitro. When clots were briefly exposed to BRL 26921, washed and returned to homologous plasma, lysis continued for up to 3 hours and attained approximately 25% of that lysis achieved by incubating with BRL 26921 for 5 hours. This continuing lysis was potentiated by return of exposed clots to alpha 2-antiplasmin-depleted plasma, or buffer and is attributed to an initial uptake of BRL 26921 rather than the binding of exogenous plasmin that was observed for streptokinase and high concentrations of urokinase. The sustained lysis is not explained by transfer of loosely-associated surface material or by dissociation of agent from the clot with reuptake from a dilute systemic pool. The response can be attributed, at least in part, to specific fibrin binding, mediated by kringles 1-4, for a low-molecular weight plasminogen (Val442) variant was less active.