The Presence of Antithrombin Activity in Platelets

The platelet has been implicated as the nidus around which clot formation occurs. We therefore decided to investigate whether any antithrombotic activity was associated with platelets. Thrombin activity was measured using the chromogenic substrate S-2238. Freshly prepared human platelet rich plasma (PRP) and platelet poor plasma (PPP) were found to have similar amounts of antithrombin activity. This antithrombin activity could be easily removed from a platelet pellet by repeated washing. However, sonicating PRP resulted in the appearance of additional antithrombin activity. After 6 minutes of sonication, the antithrombin activity of PRP decreased to 35% of presonication levels, whereas the antithrombin activity of PPP remained 100%. When subjected to Laurell Immunoelectrophoresis using an antibody to antithrombin III (AT-III), a greater amount of antigen was detected in PRP than in PPP. We conclude that platelets contain antithrombin material which is probably AT-III.