title: The mechanism of plasminogen activation and fibrin dissolution by single chain urokinase-type plasminogen activator in a plasma milieu in vitro.
abstract: The relative contribution of several mechanisms to plasminogen activation and fibrin dissolution by urokinase-type plasminogen activator (u-PA) in vitro was quantitated. The activation of plasminogen by recombinant single chain u-PA (rscu-PA), by its two chain derivative (rtcu-PA) and by a plasmin-resistant mutant, rscu-PA-Glu158, obeys Michaelis-Menten kinetics with catalytic efficiencies of 0.00064, 0.046, and 0.00005 L/mumol.s for native plasminogen (Glu-plasminogen) and of 0.0061, 1.21, and 0.0004 L/mumol.s for partially degraded plasminogen (Lys-plasminogen). In a purified system consisting of a fibrin clot submerged in a plasminogen solution, the equi-effective doses (50% lysis in one hour) for rscu-PA, rtcu-PA, and rscu-PA-Glu158 were 16, 6.5, and 32,000 ng/mL for Glu-plasminogen and two- to fourfold lower for Lys-plasminogen. In a plasma milieu, 50% lysis in two hours was obtained for a plasma clot with 2.1 micrograms/mL rscu-PA, 0.5 micrograms/mL rtcu-PA, and greater than 200 micrograms/mL rscu-PA-Glu158 and for a purified fibrin clot with 1.3 micrograms/mL rscu-PA and 0.27 microgram/mL rtcu-PA. After predigestion of a purified fibrin clot with plasmin, the apparent potency of rscu-PA and rtcu-PA increased by 40% and 20%, respectively. In conclusion, rscu-PA has an intrinsic plasminogen activating potential that is only about 1% of that of rtcu-PA and that is 13 times higher than that of rscu-PA-Glu158. Conformational transition of Glu-plasminogen to Lys-plasminogen enhances its sensitivity to activation by all u-PA moieties ten- to 20-fold. Predigestion of fibrin clots with associated increased binding of plasminogen results in a minor apparent increase of the fibrinolytic potency of rscu-PA and rtcu-PA. The relative fibrinolytic potency of rtcu-PA is two to three orders of magnitude higher than that of rscu-PA-Glu158 but only two- to five-fold higher than that of rscu-PA, both in purified systems and in a plasma milieu. These results indicate that conversion of rscu-PA to rtcu-PA constitutes the primary mechanism of fibrin dissolution.
authors.affiliation: Center for Thrombosis and Vascular Research, University of Leuven, Belgium.
authors.forename: H R
authors.initials: HR
authors.lastname: Lijnen
authors.forename: B
authors.initials: B
authors.lastname: Van Hoef
authors.forename: F
authors.initials: F
authors.lastname: De Cock
authors.forename: D
authors.initials: D
authors.lastname: Collen
mesh_terms: D001777:Blood Coagulation; D004789:Enzyme Activation; D005337:Fibrin / Q000378:metabolism* / Q000502:physiology; D005342:Fibrinolysis; D006801:Humans; D007700:Kinetics; D010949:Plasma / Q000502:physiology*; D010958:Plasminogen / Q000097:blood* / Q000378:metabolism; D011994:Recombinant Proteins / Q000097:blood / Q000494:pharmacology; D014568:Urokinase-Type Plasminogen Activator / Q000097:blood / Q000494:pharmacology*; D000979:alpha-2-Antiplasmin / Q000097:blood
chemical_list: D011994:Recombinant Proteins; D000979:alpha-2-Antiplasmin; D005337:Fibrin; D010958:Plasminogen; D014568:Urokinase-Type Plasminogen Activator