The Role of the Finger Domain of Tissue-type Plasminogen Activator (t-PA) and Plasminogen Activator Inhibitor 1 (PAI-1) in Initiation and Progression of Thrombolysis

Summary We further investigated the role of the finger (F) and the kringle-2 (K2) domains of tissue-type plasminogen activator (t-PA) in fibrin-stimulated plasminogen activation. To that end, the action of purified (wt) t-PA or of variants lacking F (del.F) or K2 (del.K2) was assessed either in a static, human whole blood clot-lysis system or in whole blood thrombi generated in the “Chandler loop”. In both clot-lysis systems, significant differences were observed for the initiation of thrombolysis with equimolar concentrations of the t-PA variants. A relatively minor “lag phase” occurred in thrombolysis mediated by wt t-PA, whereas a 6.4-fold and 1.6-fold extension is found for del.F and del.K2, respectively. We observed identical lag-times, characteristic for each t-PA variant, in platelet-rich heads and in platelet-poor tails of thrombi. Since plasminogen activator inhibitor 1 (PAI-1) is preferentially retained in the platelet-rich heads, we conclude that the inhibitor does not interfere with the initial stage of thrombolysis but exerts its action in later stages, resulting in a reduction of the rate of clot lysis. A complementation clot-lysis assay was devised to study a potential interplay of del.F and del.K2. Accordingly, clot lysis was determined with combinations of del.F and del.K2 that were inversely varied in relation to equipotent dosage to distinguish between additive, antagonistic or synergistic effects of these variants. The isobole for combinations of del.F and del.K2 shows an independent, additive action of del.F and del.K2 in clot lysis. Under the conditions employed, namely a relatively high concentration of fibrin and Glu-plasminogen and a low concentration of t-PA variant, our data show: i) the crucial role of the F domain and the lack of effect of PAI-1 in initiation of thrombolysis, ii) the lack of importance of the fibrimbinding domains of t-PA and the regulatory role of PAI-1 in advanced stages of thrombolysis.