Study of tissue-type plasminogen activator binding sites on fibrin using distinct fragments of fibrinogen

It is well established that tissue-type plasminogen activator (t-PA) binds to the D region of fibrin(ogen) and that two distinct CNBr fragments of fibrinogen (FCB), FCB-2 and FCB-5, comprising parts of this region, stimulate plasminogen activation by t-PA. In the present work, ligand-binding studies were performed to characterize the interactions between t-PA and the corresponding fibrin regions using a well defined model of a fibrin surface and both FCB-2 and FCB-5 in liquid and solid phase. Binding isotherms showed a characteristic Langmuir adsorption saturation profile. The dissociation constants determined for the binding of t-PA to immobilized FCB-2 (Kd = 0.70 +/- 0.10 nM) and FCB-5 (Kd = 0.47 +/- 0.08 nM) were of the same order of magnitude as the Kd for fibrin binding (Kd = 1 +/- 0.2 nM). The specificity of the binding was demonstrated by the ability of soluble FCB-2 and FCB-5 to inhibit t-PA binding to solid-phase fibrin (Ki = 3.3 microM and 6.4 microM, respectively). The binding of t-PA to fibrin and to immobilized FCB-2 was partially inhibited by the lysine analogue 6-aminohexanoic acid (Ki = 123 +/- 47 microM and 364 microM, respectively) but was not modified by carboxypeptidase B, thus indicating involvement of internal lysine residues. Removal of lysine residues by treatment with, successively, plasmin and carboxypeptidase B, produced only a partial inhibition of t-PA binding, thus confirming the existence of both a lysine-dependent and a lysine-independent mechanism of binding of t-PA to both fibrin and FCB-2. In contrast, the binding of t-PA to FCB-5 was not significantly affected by 6-aminohexanoic acid. Altogether, these data indicate that the mechanism of binding of t-PA to fibrin involves mainly a lysine-independent interaction with the D region which is contributed by sequences present in FCB-5 and FCB-2; contribution to binding by a lysine-dependent interaction was detected only in FCB-2 and is probably of minor relevance as suggested by the limited effect of 6-aminohexanoic acid.

The mutation in fibrinogen Bicêtre II (gamma Asn308-->Lys) does not affect the binding of t-PA and plasminogen to fibrin

The aim of this study was to investigate the interactions of t-PA and plasminogen with fibrin derived from an abnormal fibrinogen detected in a 40-year-old male patient who had had an episode of thrombophlebitis with pulmonary embolism. An abnormal fibrinogen was diagnosed on the basis of prolonged thrombin and reptilase times also detected in two other family members. Fibrinogen purified from plasma, in the presence of protease inhibitors, by glycine precipitations, gel filtration and affinity chromatography, was devoid of plasminogen, fibronectin, and vWf. SDS-PAGE analysis according to Laemmli under reducing conditions, showed an abnormal gamma chain (approximately 50% of the total) migrating in a more anodic position (M(r) 48 kDa). By PCR amplification and DNA sequencing, the abnormality was identified as an Asn308-->Lys mutation of the gamma chain. Since such a mutation constitutes a new plasmin cleavage site as first reported for fibrinogen Kyoto I, it may modify interactions of plasminogen and t-PA with carboxy-terminal lysine residues. Ligand-binding studies were therefore performed using intact and plasmin-degraded fibrin surfaces obtained from the abnormal fibrinogen. The plasminogen and t-PA binding isotherms obtained with the abnormal fibrinogen were similar to the control. Moreover, the stimulation by fibrin of plasminogen activation by t-PA was not different from the control. These results suggest (i) that the lysine 308 residue may not be exposed to plasmin cleavage in fibrin, and (ii) that the thrombotic accident of the propositus cannot be explained by an abnormality of the plasminogen/t-PA binding to fibrin.

Sodium dodecyl sulfate-induced dissociation of complexes between human tissue plasminogen activator and its specific inhibitor

The stability of complexes between serine proteinases and their inhibitors after sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis has been claimed to indicate covalent bond formation. In this work we have investigated the effects of SDS on the stability of complexes between single-chain or two-chain tissue plasminogen activator (t-PA) and its inhibitor (PAI-1). Complexes formed by incubation of t-PA with PAI-1 for 15 min at 22 degrees C were further incubated with various amounts of SDS before being subjected to SDS-polyacrylamide gel electrophoresis. The molecular species in the gels were identified both by zymography or by autoradiography after immunoblotting with antibodies directed against either t-PA or PAI-1. It was demonstrated that the interaction of SDS with t-PA.PAI-1 complexes before electrophoresis resulted in a transition from the complexed state to the free forms of t-PA and PAI-1 in a time- and dose-dependent manner. The first-order dissociation rate constant in the presence of 35 mM SDS at 22 degrees C had a koff value of 1.4 x 10(-2) min-1, which corresponds to a half-life of 49.5 min. The t-PA released from the complexes was fibrinolytically active, whereas the released PAI-1 inhibited activator-dependent fibrinolysis. In a similar fashion, the well characterized nonacylated pair alpha 1-proteinase inhibitor-elastase was dissociated by SDS treatment, confirming the validity of our experimental approach to demonstrate the reversibility of t-PA.PAI-1 complexes. These results demonstrate that SDS-polyacrylamide gel electrophoresis traps the molecular species in the state in which the proteins existed prior to the analysis, and they suggest that under the conditions used, the interaction of t-PA with PAI-1 results in the formation of nonacylated reversible complexes. This phenomenon may be relevant to the pathophysiology of fibrinolysis and to the general mechanism of serine proteinase-inhibitor complex formation.

Lipoprotein(a) impairs generation of plasmin by fibrin-bound tissue-type plasminogen activator. In vitro studies in a plasma milieu

Apoprotein(a), (apo[a]), the specific antigen of lipoprotein(a) (Lp[a]), consists of structural domains (a serine protease unit, kringles 4 and 5) with marked homology to those of the corresponding domains in plasminogen. In this study, we have investigated the impact of this unique structural mimicry on the binding and activation of plasminogen by fibrin-bound tissue-type plasminogen activator at the plasma-fibrin interface. We found that the total amount of plasmin generated on the surface of fibrin was decreased in the presence of high concentrations of Lp(a): 197 +/- 65 fmol in plasmas with greater than 60 mg/dl Lp(a) versus 287 +/- 112 fmol in control plasmas. A similar effect was also apparent in the corresponding euglobulin fractions (554 +/- 169 fmol versus 754 +/- 310 fmol), the latter lacking the plasminogen-binding proteins alpha 2-antiplasmin and histidine-rich glycoprotein, but containing Lp(a). The difference between plasma samples was significant (p less than 0.05) as calculated from the percent decrease in plasmin generated from plasmas with high levels of Lp(a) relative to that generated in the paired controls with low Lp(a) levels. The involvement of Lp(a) was verified in a reconstituted system consisting of normal human plasma supplemented with 100 mg/dl of either purified Lp(a) or low density lipoprotein. Lp(a) produced a decrease of 30% in the generation of plasmin (180 fmol versus 255 fmol in plasma, and 485 fmol versus 705 fmol in the euglobulin fraction). Moreover, using a radiolabeled sheep antibody against human apo(a), we were able to demonstrate the binding of 40 fmol Lp(a) to fibrin during ongoing plasminogen activation. These results indicate that Lp(a) impairs the binding of plasminogen to fibrin and thereby decreases the generation of plasmin by occupying C-terminal lysine residues unveiled on the fibrin surface by plasmin degradation as recently reported (Circulation 1990;82[suppl III]:III-92). In consequence, impairment of fibrinolysis and accumulation of Lp(a) at sites of vascular injury may occur, factors that may be important in the development of atherosclerosis and associated thrombosis.

Functional identification of t-PA in crude and purified systems

In the present study the activation of glu-plasminogen by fibrin-bound t-PA was determined by integrating spectrophotometric and computer analysis of the reaction. Our aim was to determine the parameters of the activation and to characterize the functional activity of t-PA. Our results indicate that the t-PA present in purified or crude sources such as plasma or culture supernatants, can be specifically identified using the methodology described in this report.