Functionally Active Thrombomodulin Is Present in Human Platelets1

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Abstract
We found functionally active thrombomodulin in human platelets (60±18 molecules per platelet). Protein C appeared not to be activated by thrombin with gel-filtered platelets. However, the activation of protein C by thrombin was accelerated by thrombin-stimulated and washed platelets. This cofactor activity of the platelets was neutralized by the anti-lung thrombomodulin-F(ab′)2 From the Triton X-extract of platelets, thrombomodulin was partially purified by diisopropylphosphoryl-thrombin-agarose affinity chromatography. The Mr of the predominant platelet thrombomodulin was 78,000 before and 109,000 after reduction on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, values identical to those of placental thrombomodulin. The specific activity of the cofactor activity, apparent Kd (0.4 nM) for thrombin and Km (0.67 μM) for protein C of platelet throm bomodulin were also identical to those of placenta thrombomodulin. Thrombomodulin may play a role in activation of protein C on the surface of platelets.

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