Differential effect of platelets on plasminogen activation by tissue plasminogen activator, urokinase, and streptokinase.

In this report, we have examined the effects of platelets on plasminogen activation by different activators. Platelets enhance activation of plasminogen by both 1- and 2-chain tissue plasminogen activator (t-PA). The primary effect of platelets is to lower the Km with a corresponding 5-8-fold increase in the kcat/Km. The effect is saturable with respect to the platelet concentration. Platelets enhance activation of both glu- and lys-plasminogen by t-PA. Platelets have no effect on plasminogen activation by streptokinase, and high and low molecular weight urokinase. Thus, there are marked differences in the effects of platelets on plasminogen activation depending on the plasminogen activator. These differences are likely to reflect differences in the interaction between platelets and the plasminogen activators.