Functionally Active Thrombomodulin Is Present in Human Platelets

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 M_r 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 K_d (0.4 nM) for thrombin and K_m (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.