Inactivation of membrane-bound human factor VIII by the APC/protein S complex.

Recombinant factor VIII (60 nM) was incubated with APC (5 nM) and protein S (100 nM) in the presence of PCPS vesicles (10 μM). At selected time intervals aliquots of the mixture were assayed for factor VIII activity as described27 using a 2-stage clotting assay (filled circles). Filled triangles represent the progress of the reaction in the presence of factor V (4 nM), and the filled diamonds represents the progress of the reaction in the presence of 8 nM inhibitory immunoglobulin fraction eluted from the factor V-Sepharose column. The filled inverted triangles represent the progress of the reaction in the presence of a commercially available preparation of nonimmune human IgG (8 nM). Incubation of recombinant factor VIII alone at 37°C did not result in any significant loss of cofactor function (filled squares). The error bars represent the average value found in 3 independent experiments.