A rapid assay for protein C activity in whole plasma using a protein C activator from the venom of Agkistrodon contortrix is described. Plasma samples are incubated with venom activator in the presence and absence of anti-protein C antibody for 8 minutes at 37 degrees C in microtiter plate wells, followed by addition of a chromogenic substrate for activated protein C and measurement of the absorbance at 405 nm after 10-20 minutes incubation at 37 degrees C. The difference in absorbance between plasma assayed in the presence and absence of anti-protein C antibody is used to calculate the protein C activity by reference to a dilution curve of normal plasma. With this assay, mean protein C activity in 33 normal subjects was 87%. Thirty-eight patients receiving warfarin had a mean protein C of 34%. Twenty-four patients with disseminated intravascular coagulation had a mean protein C of 24%. The venom activator appears to preferentially activate gamma-carboxylated protein C, indicating specificity for the physiologically active form of the molecule.
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Francis et al. (1987) studied this question.