Cleavage of prothrombin by prothrombinase assembled on PC/PS vesicles. Human prothrombin was radioactively labeled with ¹²⁵I and assessed for its ability to be cleaved either by human prothrombinase (5nM factor Xa and 0.5nM factor Va) (A) or by factor Xa alone (B) assembled on PC/PS vesicles as detailed in “Prothrombin activation time courses.” Aliquots of the reaction mixtures were quenched at timed intervals and subjected to SDS-PAGE. The gels were analyzed by staining with Coomassie blue (A, left panel) to visualize unlabeled protein, and by phosphorimaging (A, right panel, B) to visualize ¹²⁵I-prothrombin and fragments generated by prothrombinase. Time points are indicated at the top of each panel. Molecular weight markers are indicated on the left, and the locations of prothrombin (II), the fragment 1.2.A chain of meizothrombin (F1.2.A), fragment 1.2 (F1.2), prethrombin-2 (P2), and the B chain, are indicated on the right.