APC-catalyzed inactivation of plasma-derived factor Va bound to thrombin-activated platelets. Platelets (1 × 109/mL) in the presence of RGDS peptide (1 mmol/L) from a normal individual were treated with 2 NIH U/mL (20 nmol/L) of α-thrombin for 5 minutes to both activate the platelet and release and activate platelet-derived factor Va. Hirudin (30 nmol/L) was added to inhibit thrombin. APC (0.25 nmol/L) was then added to initiate the reaction and residual cofactor activity was monitored as described in Fig 1. (▪) The inactivation of platelet-derived factor Va on thrombin-activated platelets. After 2.5 hours, purified normal plasma-derived factor Va (3.0 nmol/L) was added (arrowhead) to the activated platelet/platelet factor Va/APC mixture (•). After the addition of plasma-derived factor Va, samples of the reaction mixture were immediately assayed for cofactor activity. Values are expressed as the initial rate (in micromoles per liter of IIa generation per minute) of prothrombinase activity, which is directly proportional to the amount of functional cofactor. No additional APC was added to the reaction mixture.
