Functional activity of protein S determined with use of protein C activated by venom activator.

A new screening procedure, an easy and specific assay for determining functional Protein S activity, has been developed, with use of Protein C activated by venom activator (Protac). Purified Protein C (100% amidolytic activity) was activated by venom activator (6 units/mL). To a mixture of 100 microL of Protein S-deficient plasma, 20 microL of sample plasma, 100 microL of cephalin (Actin), and 20 microL of activated Protein C we added 100 microL of 25 mmol/L CaCl2 solution and measured the clotting time with a KC 10 coagulometer. The functional Protein S activity correlated well with the concentrations of Protein S antigen measured by enzyme immunoassay and the Laurell rocket technique (r = 0.810 and 0.850, respectively) in normal subjects, patients with myocardial infarction undergoing warfarin therapy, and patients with liver cirrhosis.