Assessing the APC cofactor activity in an FVIIIa degradation assay. Preformed tenase complexes containing phospholipid-bound FIXa and FVIIIa were added to increasing amounts of FV (final concentration, 0.5-1.3 nM) mixed with APC (final concentration, 5 nM) and protein S (final concentration, 5 nM), and the FVIIIa degradation was followed. FV-WT, ; FV-Ile359Thr, ▪; FV-Arg506Gln, ▴. Each data point represents the mean of 3 independent experiments performed in duplicate. Error bars represent ± SD.