Platelet release protein which inhibits plasminogen activators.

An inhibitor of plasminogen activator has been identified in human platelets by the technique of sodium dodecyl sulphate polyacrylamide gel electrophoresis and zymography. The inhibitor has a molecular weight of about 40 000 and is distinct from known plasma protease inhibitors. It is associated almost exclusively with platelets, with only trace amounts in platelet free plasma. The inhibitor is released during platelet aggregation or in vitro coagulation. This inhibitor inhibits both tissue type plasminogen activator and urokinase but has no effect on plasmin. It forms a 1:1 complex with tissue type plasminogen activator, which retains activity detectable under the analytical conditions used. A similar complex with urokinase either forms less readily or retains less activity.