Cleavage of F.IX by HNE or F.XIa in the presence of pAB. F.IX (4.0 μmol/L) was incubated at 37°C in the presence of pAB (4.0 μmol/L) and after approximately 100 seconds, F.XIa (500 nmol/L) or HNE (400 nmol/L) was added and the incubations continued. (A) shows the fluorescence intensity values recorded for F.IX treated with F.XIa (•) or HNE (○) with excitation and emission wavelengths of 336 and 376 nm, respectively, and a cutoff filter of 350 nm in the emission beam using a Perkin Elmer LS50B Luminescence Spectrometer. (B) (F.XIa-treated F.IX) and (C) (HNE-treated F.IX) show reducing SDS-PAGE analysis of aliquots (4 μg protein each) removed from the reactions in (A) after 0, 0.5, 2, 5, 10, 20, 40, and 60 minutes of incubation with protease (lanes 1 through 8, respectively, in each case) using 5% to 20% linear polyacrylamide gradient gels after staining with Coomassie Blue. The positions of molecular weight markers (in kilodaltons) are indicated at the left of each panel.