CHARACTERIZATION OF THE MURINE PLASMA FIBRINOLYTIC SYSTEM

The main components of the murine plasma fibrinolytic system, includin g fibrinogen, plasminogen, alpha(2)-antiplasmin, tissue-type plasminog en activator and plasminogen activator inhibitor-1, were purified to h omogeneity and their interactions were quantitated and compared with t hose of the human counterparts. Initial activation rates of murine and human plasminogen by autologous tissue-type plasminogen activator wer e comparable (catalytic efficiencies, k(2)/K-m, of 0.4 and 0.6 mM(-1) s(-1), respectively), but murine plasminogen appeared to be resistant to activation by human tissue-type plasminogen activator (k(2)/K-m = 0 .01 mM(-1) s(-1)). Plasminogen activation by tissue-type plasminogen a ctivator was stimulated 100- and 160-fold in autologous murine and hum an systems, respectively, with saturating concentrations of 0.45 and 0 .32 mu M, respectively, of CNBr-digested fibrinogen. Nearly quantitati ve binding (85-90%) of tissue-type plasminogen activator to fibrin was observed both in autologous and heterologous systems. Murine and huma n plasmin were very rapidly inhibited by autologous and heterologous a ,antiplasmin (second-order inhibition rate constants, k(1,app,) of 2.1 -2.3 X 10(7) M(-1) s(-1)) and murine and human tissue-type plasminogen activator were very rapidly inhibited by autologous or heterologous p lasminogen activator inhibitor-1 (k(1,app) of 1.8-4.9 x 10(7) M(-1) s( -1)). Two-chain murine tissue-type plasminogen activator (added at a c oncentration of 1 mu g/ml) was inhibited in normal or plasminogen acti vator inhibitor-1-deficient murine plasma with half-lives of 6.5 min a nd 4.2 min, respectively, as compared to 80 min for human tissue-type plasminogen activator, suggesting that murine plasma contains proteina se inhibitors other than plasminogen activator inhibitor-1 which effic iently inhibit autologous tissue-type plasminogen activator. Clot lysi s experiments in autologous plasma revealed that the murine plasma fib rinolytic system is more resistant to activation than the human system (20-30% clot lysis in 2 h with 100 nM tissue-type plasminogen activat or in the murine system, as compared to 50% clot lysis in 2 h with 3.5 nM tissue-type plasminogen activator in the human system). Several me chanisms appear to be involved in this relative resistance observed in the murine system, including resistance of murine plasminogen to quan titative activation and short plasma half-life of murine tissue-type p lasminogen activator. Thus, although these quantitative interactions b etween purified components of the murine fibrinolytic system appear to be comparable to those between the human counterparts, murine plasma clots are > 30-fold more resistant to lysis with autologous tissue-typ e plasminogen activator than human plasma clots.