Fibrinogen^AEK forms a polymer following incubation with enterokinase enzyme but not thrombin. Representative turbidity analyses using plasma isolated from WT (closed circles) and Fib^AEK (open symbols) mice following incubation with (A) thrombin or (B) enterokinase enzyme. Similar representative turbidity analyses using purified WT fibrinogen (closed circles) and fibrinogen^AEK (open symbols) following incubation with (C) thrombin or (D) enterokinase enzyme. The purified fibrinogen concentration in each analysis was 0.04 mg/mL. To prevent spurious thrombin activity, reactions with enterokinase enzyme included the addition of the specific thrombin inhibitor lepirudin at 0.025 mg/mL. (E) Scanning electron micrographs of products formed from reaction mixtures of purified WT fibrinogen or fibrinogen^AEK with thrombin or enterokinase. Scale bar, 2.5 μm.