Effect of WT protein S, D95A, D95N, D78A, and Q79A variants on thrombin generation. Thrombin generation was measured in protein S–deficient plasma supplemented with 9nM APC, 100nM antibodies against TFPI, and 0 to 120nM WT protein S (A), protein S D95A (B), protein S D95N (C), or 90nM purified WT (dashed line) or purified protein S D95A (dotted line; D). Protein S concentrations are positioned adjacent to the peaks to which they refer. The cofactor activity of 60nM WT protein S and protein S variants D95A, D78A, and Q79A was compared at 9nM APC (E). Typical experiments are shown (n = 3). Whereas the cofactor activity of WT protein S is highly dependent on the APC concentration used (Figure 1B), that of protein S D95A is not, explaining the difference in fold activity between WT protein S and protein S D95A in Figures 2 and 3. Dose-response data from titrations with WT protein S, protein S D95A, and protein S D95N in the presence of 9nM APC are shown in panel F (data are expressed as mean ± SD of 2 independent experiments performed in duplicate). Inset in panel B shows recognition of WT protein S and protein S D95A in media by polyclonal antibodies and a monoclonal antibody recognizing only γ-carboxylated Gla domains. Inset in panel D shows the SeeBlue-prestained marker, plasma-purified protein S from Enzyme Research Laboratories Ltd (lane 1), purified recombinant WT protein S (lane 2), and purified protein S D95A (lane 3) visualized with silver staining.