Interaction of tPA-S⁴⁸¹A with fibrin and plasminogen. (A) The specific binding of catalytically inactive human tPA-S⁴⁸¹A and WT-tPA (Bound tPA) to fibrin was measured.²⁴  tPA-S⁴⁸¹A and WT-tPA bind to fibrin with comparable affinity. Data represent results from 1 of 3 independent experiments. (B) tPA-S⁴⁸¹A inhibits the binding of ¹²⁵I-WT-tPA (5 nM) to fibrin.²⁴  Data represent results from 1 of 3 independent experiments. (C) Binding of tPA-S⁴⁸¹A to plasminogen. tPA-S⁴⁸¹A was incubated with plasminogen (Plasgen; 100 nM each) and fibrin in the absence (lane 2) or presence (lane 3) of 1 µM uPA and in the absence of fibrin (lane 5). Lane 4 shows the positive control (80 nM human tPA-S⁴⁸¹A). Complexes were immunoprecipitated with anti-plasminogen antibody and detected with anti-tPA antibody.²⁶  tPA-S⁴⁸¹A binds to plasminogen, forming a stable complex (lane 2) in the presence, but not in the absence, of fibrin, as shown in lane 5, where tPA-S⁴⁸¹A and plasminogen were incubated alone. Binding of tPA-S⁴⁸¹A is inhibited by uPA (lane 3). Lane 1 shows the molecular weight markers. Data represent results from 1 of 3 independent experiments. mtPA, mutant tPA; plasgen, plasminogen.