APC-cofactor activities of plasma-derived FV:R506 and FV:Q506. Human FV was purified from fresh plasma obtained from an individual with homozygosity for the Arg-506 to Gln mutation (FV:Q506) and from an individual with normal FV (FV:R506). The ability of these FV molecules to work as cofactors to APC in the presence of protein S was studied using a FVIIIa degradation assay. 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 12 nmol/L). The FVIIIa activity measured in the presence of APC and protein S was set to be 100%. (▴), normal FV; (▵), FV:Q506.