GlcCer enhances factor Va inactivation by APC:protein S.

Purified factor Va was incubated with APC:protein S and varying lipids to allow factor Va inactivation for times indicated, and then residual factor Va activity was determined using clotting assays as described in “Patients, materials, and methods.” (A) Time course for 1.5 nM factor Va inactivation by final concentrations of 6 nM APC and 18 nM protein S in the presence of 5 mM CaCl₂ and GlcCer (●) or PC vesicles (21.5 μM) (○), or buffer (×) at 37°C. Controls without APC:protein S included GlcCer (▵) or PC vesicles (*). The factor Va activity observed at 0 time was defined as 100%. (B) For studies using multicomponent vesicles, various concentrations of PC (○), PC/GlcCer (●), PC/PS (▵), and PC/PS/GlcCer (▴) vesicles were incubated with final concentrations of 1 nM APC, 18 nM protein S, and 1.5 nM factor Va for 3 minutes at 37°C, and then residual factor Va was determined. The factor Va activity observed with no lipid vesicles added was defined as 100%.