The main components of the murine plasma fibrinolytic system, including fibrinogen, plasminogen, α₂‐antiplasmin, tissue‐type plasminogen activator and plasminogen activator inhibitor‐1, were purified to homogeneity and their interactions were quantitated and compared with those of the human counterparts.

Initial activation rates of murine and human plasminogen by autologous tissue‐type plasminogen activator were comparable (catalytic efficiencies, k₂/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 (k₂/K_m=0.01 mM⁻¹ s⁻¹). Plasminogen activation by tissue‐type plasminogen activator was stimulated 100‐ and 160‐fold in autologous murine and human systems, respectively, with saturating concentrations of 0.45 and 0.32 μM, respectively, of CNBr‐digested fibrinogen. Nearly quantitative binding (85–90%) of tissue‐type plasminogen activator to fibrin was observed both in autologous and heterologous systems.

Murine and human plasmin were very rapidly inhibited by autologous and heterologous α₂‐antiplasmin (second‐order inhibition rate constants, k_1,app, of 2.1–2.3×10⁷ M⁻¹ s⁻¹) and murine and human tissue‐type plasminogen activator were very rapidly inhibited by autologous or heterologous plasminogen activator inhibitor‐1 (k_1,app of 1.8–4.9×10⁷M⁻¹ s⁻¹). Two‐chain murine tissue‐type plasminogen activator (added at a concentration of 1 μg/ml) was inhibited in normal or plasminogen activator inhibitor‐1‐deficient murine plasma with half‐lives of 6.5 min and 4.2 min, respectively, as compared to 80 min for human tissue‐type plasminogen activator, suggesting that murine plasma contains proteinase inhibitors other than plasminogen activator inhibitor‐1 which efficiently inhibit autologous tissue‐type plasminogen activator.

Clot lysis experiments in autologous plasma revealed that the murine plasma fibrinolytic system is more resistant to activation than the human system (20–30% clot lysis in 2 h with 100 nM tissue‐type plasminogen activator 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 mechanisms appear to be involved in this relative resistance observed in the murine system, including resistance of murine plasminogen to quantitative activation and short plasma half‐life of murine tissue‐type plasminogen activator.

Thus, although these quantitative interactions between 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‐type plasminogen activator than human plasma clots.