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Keywords:

Alanine, Binding Sites, Catalysis, Cell Line, Enzyme Activation, Fibrin, Fibrinolysis, Glutamates, Glutamic Acid, Humans, Kinetics, Lysine, Mutagenesis, Site-Directed, Plasminogen, Plasminogen Activator Inhibitor 1, Recombinant Proteins, Serine, Solutions, Urinary Plasminogen Activator, Science & Technology, Life Sciences & Biomedicine, Biochemistry & Molecular Biology, DOSE SYNERGISTIC COMBINATIONS, ACUTE MYOCARDIAL-INFARCTION, SITE-SPECIFIC MUTAGENESIS, HUMAN-PLASMA, PRO-UROKINASE, MONOCLONAL-ANTIBODY, CLOT LYSIS, CORONARY THROMBOLYSIS, 2-CHAIN UROKINASE, SARCOMA-CELLS, Urokinase-Type Plasminogen Activator, 03 Chemical Sciences, 06 Biological Sciences, 11 Medical and Health Sciences, 31 Biological sciences, 32 Biomedical and clinical sciences, 34 Chemical sciences

Abstract:

The activation of plasminogen at the surface of fibrin by single-chain urokinase-type plasminogen activator (scu-PA) was investigated using recombinant forms of a plasmin-resistant mutant of scu-PA, rscu-PA-Glu158, and an inactive catalytic site mutant of human plasminogen, rPg-Ala741. Conversion of cleavable 125I-labeled single-chain proteins to their two-chain forms, was quantitated by radioisotope counting of protein bands on reduced SDS-polyacrylamide gels. The efficiency of the activation (moles of plasmin generated per mol of plasminogen activator) of native Glu-plasminogen bound to degraded fibrin was comparable for scu-PA and its two-chain form (tcu-PA) and approximately 4-fold lower for rscu-PA-Glu158. The corresponding values with rPg-Ala741 were 4-fold or 9-fold lower for scu-PA or rscu-PA-Glu158, as compared to tcu-PA. In contrast, in solution in the absence of fibrin, the efficiency of scu-PA for activation of rPg-Ala741 was 100-fold lower than that of tcu-PA. Initial activation rates of rPg-Ala741 (32.7 fmol/well containing 50 microliters of solution) with 4 nM tcu-PA were comparable in solution and bound to degraded fibrin (v(o) = 1.01 and 1.16 fmol/min, respectively). In contrast, with 4 nM scu-PA the corresponding values when rPg-Ala741 was bound to degraded fibrin were 20-fold higher as compared to the soluble phase (v(o) = 0.23 and 0.012 fmol/min, respectively). Comparable results were obtained when using Glu- or Lys-forms of rPg-Ala741. Furthermore, in the presence of normal human plasma, activation of Glu-plasminogen bound to degraded fibrin was found to be about 2.5-fold more efficient with scu-PA than with tcu-PA. These findings indicate that the fibrin specificity of scu-PA does not require its conversion to tcu-PA, nor conversion of Glu- to Lys-plasminogen, but appears to be due to the additional binding of plasminogen to partially digested fibrin; scu-PA may thus represent a physiological functional form of u-PA in plasma.