Activation of factor XI by thrombin in the presence of activated platelets.

Recombinant factor XI (60 nM) and α-thrombin (1 nM) were incubated at 37°C in the presence of activated platelets (0.5 × 10⁸/mL), ZnCl₂ (25 μM), CaCl₂ (2 mM), and high-molecular-weight kininogen (50 nM), as described in “Materials and methods.” At various time points, aliquots were tested for factor XIa generation using the chromogenic substrate S-2366. Wild-type recombinant factor XI (○), factor XI–R226 (▪), and factor XI–N248 (▵) are shown. Data are mean values (± SEM) for 3 separate experiments, each performed in triplicate.