Studies on an Inhibitor of Plasminogen Activators in Human Platelets

Summary. Whole platelets obtained by gel filtration of platelet‐rich plasma inhibited urokinase activity. An inhibitor of urokinase and tissue activator was partially purified by chromatography of washed, lyophilized human platelets on DEAE‐cellulose. Inhibition was immediate, proportional to the quantity of platelet extract present and competitive. The inhibitor was labile on storage at 20°C, but stable for at least 3 weeks at −20°C; it was destroyed by heating to 56°C for 30 min. The approximate molecular weight was 45 000 as determined by gel filtration. The platelet extracts were free of molecules antigenically related to α2‐macroglobulin, α1‐antitrypsin, antithrombin III, C1 inactivator, or factor XIII.