Role of oc2-antiplasmin

The mechanism of activation of human Glu-plasminogen by fibrin-bound tissue-type plasminogen activator (t-PA) in a plasma environment or in a reconstituted system was characterized. A heterogeneous system was used, allowing the setting of experimental conditions as close as possible to the physiological fibrin/plasma interphase, and permitting the separate analysis of the products present in each of the phases as a function of time. The generation of plasmin was monitored both by spectrophotometric analysis and by radioisotopic analysis with a plasmin-selective chromogenic substrate and radiolabelled Glu-plasminogen respectively. Plasmin(ogen)-derived products were identified by SDS/PAGE followed by autoradiography and/or immunoblotting. When the activation was performed in a plasma environment, the products identified on the fibrin surface were Glu-plasmin (90 %) and Glu-plasminogen (10 %), whereas in the soluble phase only complexes between Glu-plasmin and its fast-acting inhibitor were detected. Identical results were obtained with a reconstituted system comprising solid-phase fibrin, t-PA, Glu-plasminogen and a2-antiplasmin. In contrast, when a2-antiplasmin was omitted from the solution, Lys-plasmin was progressively generated on to the fibrin surface (30 %) and released to the soluble phase. In the presence of a2-antiplasmin or in plasma, the amount of active plasmin generated on the fibrin surface was lower than in the absence of the inhibitor: in a representative experiment the initial velocity of plasmin generation was 2.8 x 10-3, 2.0 x 10-3 and 1.8 x 10-3 (AA405/min) for 200 nM-plasminogen, 200 nM-plasminogen plus 100 nM-a2-antiplasmin and native plasma respectively. Our results indicate that in plasma or in a reconstituted purified system containing plasminogen and cx2-antiplasmin at a ratio similar to that found in plasma (1) the activation pathway of native Glu-plasminogen proceeds directly to the formation of Glu-plasmin, (2) Lys-plasminogen is not an intermediate of the reaction and therefore (3) Lys-plasmin is not the final active product. However, in the absence of the inhibitor, Lys-plasmin and probably Lys-plasminogen, which is more readily activated to plasmin than is Gluplasminogen, are generated as well.