The Ca²⁺-dependence of PC activation by the thrombin-soluble TM complex containing or lacking chondroitin sulfate. Human PC (1 μmol/L) was activated by thrombin (1 nmol/L) in the presence of recombinant soluble TM (200 nmol/L) lacking chondroitin sulfate in the absence (○) or presence (•) of approximately 0.8 μmol/L heparin in TBS buffer containing 1 mg/mL BSA, 0.1% PEG 8000, and indicated concentrations of Ca²⁺. After 15 minutes of activation at room temperature, the thrombin activity was inhibited by antithrombin and the amount of PC generated was determined in an amidolytic activity assay described under the Materials and Methods. PC activation by the thrombin-TM complex containing chondroitin sulfate (□) under the same experimental conditions is also shown.