The effect of APC cleavage site mutations in recombinant FV on the expression of APC-cofactor activity. Increasing concentrations of wtFV, FV:Q306, FV:Q506, FV:Q306/Q506, and FV:Q679 were added to mixtures of APC and protein S and the stimulation of the APC-mediated FVIIIa degradation was studied. In the assay, the final concentrations of the proteins were APC (5.0 nmol/L), protein S (4.4 nmol/L) and FV (0 to 11 nmol/L). A, (▵), FV:Q306/Q506; (⧫), FV:Q506; (○), FV:Q306, and (▪), wt FV. B, (▿), wtFV; (▾), FV:Q679. The FVIIIa activity measured in the presence of APC and protein S alone was set to be 100%. Each data point in A represent the mean of three experiments, whereas the data in B represent one of two experiments with similar results.