Plasminogen Activation by Tissue Plasminogen Activator in the Presence of Stimulating CNBr Fragment FCB-2

Plasminogen activation by tissue-type plasminogen activator (t-PA) is stimulated by fibrin. In a purified system maximal fibrin-enhanced plasmin formation occurs with a delay after an initial phase of slow plasmin formation (lag phase). In the present study purified stimulating CNBr-fragment FCB-2 of fibrinogen was used, and kinetics of plasminogen activation by t-PA were analyzed with respect to the lag phase. At constant FCB-2 concentration the duration of the lag phase decreased with increasing Concentrations of tPA and plasminogen. During this period the rate of plasmin formation/min increased linearly with time with a slope dependent on the initial concentrations of FCB-2, plasminogen, and t-PA. Plasmin pretreatment of FCB-2 resulted in a doseand time-dependent shortening of the lag phase, and at plasmin concentrations 21 nM and preincubation times 23 min maximal plasmin formation occurred without a lag phase. Kinetics during the phase of maximal and constant plasmin formation were not influenced by plasmin pretreatment of FCB-2. We therefore conclude that maximal tPA-dependent plasmin formation in a system stimulated by purified FCB-2 requires plasmin modification of FCB-2.