Competition between Arg271 and Arg320 within the same prothrombin molecule for cleavage by prothrombinase assembled on the activated platelet membrane. (A) To titrate the Arg271 cleavage site within the same molecule as the Arg320 cleavage site, prothrombinase assays were performed using mixtures of prothrombin (containing both Arg271 and Arg320 as potential cleavage sites) and rMZ (containing only Arg320). (B) Reactions were performed, and data were analyzed as described in Figure 5, to determine the rate of generation of fragment 1.2.A as an indication of initial cleavage at Arg320, and of fragment 1.2 as an indication of initial cleavage at Arg271. Data are represented as the concentration of Arg271 versus the percentage of initial cleavage at Arg271.