The effect of HNE on the cofactor activity of Factor V and Va. Purified human F.V (A), and F.Va (B), (600 nmol/L) were incubated at 37°C with purified HNE (2 nmol/L in A; 60 nmol/L in B) in HBS, pH 7.4, 5 mmol/L CaCl₂ and 3.0 μmol/L DAPA (•) or with 50 μmol/L PCPS (▪). Control incubation mixtures for both F.V (A) and F.Va (B) in the absence of HNE with (□) or without (○) the addition of PCPS vesicles are shown. At the times indicated, aliquots were withdrawn and HNE activity was quenched with 10.0 μmol/L AAPV-CMK in HBS, pH 7.4 containing 0.1% BSA and 500 μmol/L PCPS. F.Va cofactor activity was then determined using 1.4 μmol/L prothrombin and 5.0 nmol/L F.Xa in the prothrombinase assay (see Materials and Methods). The results shown are expressed as initial rates of mol thrombin (hIIa) generated per second per mole hF.Va.