In a system of washed human platelets, Ca2+and purified human factors X anc II, a sufficient amount of thrombin is generated in about 10 minutes to aggregate the platelets. This thrombin is formed through the activation of FX by the platelets. In a system with either FX or FII present, no aggregation occurs. In addition no aggregation is observed when hirudin, a specific thrombin inhibitor, or when soybean trypsin inhibitor, which inhibits factor Xa, are added to the mixture. The formation of factor Xa can be monitored indirectly through the generation of thrombin, in the presence of an excess of prothrombin, using a thrombin sensitive chromogenic substrate. When washed platelets are incubated with FX alone for 10 minutes, no aggregation occurs and after the addition of prothrombin aggregation starts within 6 minutes. These findings confirm that washed platelets possess a factor X activating property. The generation of FXa proceeds in the absence of added Ca2+, whereas in the presence of Ca2+factor Xa activity reaches a maximum in 3 minutes, whereafter the activity progressively decreases. This may be due to the binding of Xa to the platelets in the presence of calcium ions.
A soybean trypsin inhibitor. Crystallization and x-ray crystallographic study.
