Mechanism of the enhanced intrinsic activity of single-chain urokinase-type plasminogen activator during ongoing fibrinolysis

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.

Molecular assembly of plasminogen and tissue-type plasminogen activator on an evolving fibrin surface

A well characterized model of an intact and a degraded surface of fibrin that represents the states of fibrin during the initiation and the progression of fibrinolysis was used to quantitatively characterize the molecular interplay between tissue-type plasminogen activator (t-PA), plasminogen and fibrin. The molecular assembly of t-PA and plasminogen on these surfaces was investigated using combinations of proteins that preclude complications due to side reactions caused by generated plasmin: native plasminogen with di-isopropylphosphofluoridate-inactivated t-PA, and a recombinant human plasminogen with the active-site Ser741 mutagenized to Ala which renders the catalytic site inactive. Under these conditions, neither the affinity nor the maximal number of binding sites for plasminogen were modified by the presence of t-PA, indicating that binding sites for plasminogen pre-exist in intact fibrin and are not dependent on the presence of t-PA. In contrast, when plasminogen activation is allowed, increasing binding of plasminogen to the progressively degraded fibrin surface is directly correlated (r = 0.98) to the appearance of the fibrin E-fragment as shown using a monoclonal antibody (FDP-14) that has its epitope in the E domain of fibrin. t-PA was shown to bind with a high affinity to both the intact (Kd = 3.3 +/- 0.6 nM) and the degraded surface of fibrin (Kd = 1.2 +/- 0.4 nM). Binding of t-PA to carboxy-terminal lysine residues of degraded fibrin was shown to be efficiently competed by physiological concentrations of plasminogen (2 microM), indicating that the affinity of t-PA for these residues was lower than that of plasminogen (Kd = 0.66 +/- 0.22 microM) and unrelated to the high affinity of t-PA for specific binding sites on intact fibrin. These data confirm and establish that the generation of carboxy-terminal lysine residues on fibrin during ongoing fibrinolysis, and the binding of plasminogen to these sites, is an important pathway in the acceleration of clot dissolution.

Apolipoprotein(a) and plasminogen interactions with fibrin: a study with recombinant apolipoprotein(a) and isolated plasminogen fragments

Lipoprotein(a) [Lp(a)], but not low-density lipoprotein (LDL), was previously shown to impair the generation of fibrin-bound plasmin [Rouy et al. (1991) Arterioscler. Thromb. 11, 629-638] by a mechanism involving binding of Lp(a) to fibrin. It was therefore suggested that the binding was mediated by apolipoprotein(a) [apo(a)], a glycoprotein absent from LDL which has a high degree of homology with plasminogen, the precursor of the fibrinolytic enzyme plasmin. Here we have evaluated this hypothesis by performing comparative fibrin binding studies using a recombinant form of apo(a) containing 17 copies of the apo(a) domain resembling kringle 4 of plasminogen, native Lp(a), and Glu-plasminogen (Glu1-Asn791). Attempts were also made to identify the kringle domains involved in such interactions using isolated elastase-derived plasminogen fragments. The binding experiments were performed using a well-characterized model of an intact and of a plasmin-digested fibrin surface as described by Fleury and Anglés-Cano [(1991) Biochemistry 30, 7630-7638]. Binding of r-apo(a) to the fibrin surfaces was of high affinity (Kd = 26 +/- 8.4 nM for intact fibrin and 7.7 +/- 4.6 nM for plasmin-degraded fibrin) and obeyed the Langmuir equation for adsorption at interfaces. The binding to both surfaces was inhibited by the lysine analogue AMCHA and was completely abolished upon treatment of the degraded surface with carboxypeptidase B, indicating that r-apo(a) binds to both the intrachain lysines of intact fibrin and the carboxy-terminal lysines of degraded fibrin. As expected from these results, both r-apo(a) and native Lp(a) inhibited the binding of Glu-plasminogen to the fibrin surfaces.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the binding of plasminogen to fibrin surfaces: the role of carboxy-terminal lysines

In the present study we have quantitatively characterized the interaction of purified human Glu- and Lys-plasminogen with intact and degraded fibrin by ligand-binding experiments using a radioisotopic dilution method and antibodies against human plasminogen. A fibrinogen monolayer was covalently linked to a solid support with polyglutaraldehyde and was treated with thrombin or with thrombin and then plasmin to respectively obtain intact and degraded fibrin surfaces. Under these conditions, a well-defined surface of fibrin is obtained (410 +/- 4 fmol/cm2) and, except for a 39-kDa fragment, most of the fibrin degradation products remain bound to the support. New binding sites for plasminogen were detected on the degraded surface of fibrin. These sites were identified as carboxy-terminal lysine residues both by inhibition of the binding by the lysine analogue 6-aminohexanoic acid and by carboxy-terminal end-group digestion with carboxypeptidase B. The binding curves exhibited a characteristic Langmuir adsorption isotherm saturation profile. The data were therefore analyzed accordingly, assuming a single-site binding model to simplify the analysis. Equilibrium dissociation constants (Kd) and the maximum number of binding sites (Bmax) were derived from linearized expression of the Langmuir isotherm equation. The Kd for the binding of Glu-plasminogen to intact fibrin was 0.99 +/- 0.17 microM and for degraded fibrin was 0.66 +/- 0.22 microM. The Kd for the binding of Lys-plasminogen to intact fibrin was 0.41 +/- 0.22 microM and for degraded fibrin was 0.51 +/- 0.12 microM.(ABSTRACT TRUNCATED AT 250 WORDS)