Inactivation of factor VIIIa by human APC and protein S.

(A) Factor VIIIa activity was determined for reactions containing 200 nmol/L factor VIIIa alone (open squares), factor VIIIa plus 40 nmol/L APC (open circles) and factor VIIIa plus APC plus 250 nmol/L protein S (closed circles). (B) Residual A1 subunit was determined for the above reactions run in the absence (open circles) and presence (closed circles) of protein S. Curves were derived from scans of the Western blots obtained using the 58.12 anti-A1 subunit antibody. These data are shown in the inset, where lanes 1-7 and 8-14 represent time points at 0, 1, 3, 5, 10, 15, and 20 minutes after addition of APC and APC plus protein S, respectively. (C) Residual A2 subunit was determined for the above reactions run in the absence (open circles) and presence (closed circles) of protein S. Curves were derived from scans of the Western blots obtained using the R8B12 anti-A2 subunit antibody, which is shown in the inset. Lane designations are as in B. Low levels of the C-terminal derived fragment (A2c; approximately 10% total A2) were identified in the control (time = 0) lanes before addition of APC. The origin of this material is not known.