Search

The thrombolytic therapy is usually associated with bleeding risks due to a systemic fibrinogenolysis. We investigated whether it is possible to block fibrinogenolysis without impairing the lytic activity of fibrin-bound plasmin molecules. The present data show that at increased a2-antiplasmin levels as high as 6-fold the normal concentration fibrinolysis was not impaired, while the haemostatic parameters as aPTT, TT and the fibrinogen concentration remained normal. Essentially similar results were observed with all plasminogen activators (streptokinase, urokinase and a t-PA mutant) assayed. The use of a2-macroglobulin leads to identical observations. With aprotinin, however, a complete inhibition of the lysis was observed. The data presented here show that it is possible with the combined use of a plasminogen activator and a2-antiplasmin to reach at least in vitro a normal fibrinolysis without impairment of the haemostatic parameters.

The pharmacological and pharmacokinetic behaviour of a recombinant tissue-type plasminogen activator (t-PA) mutant with Asn 117 and Asn 184 substituted by Gln (G4) was compared to t-PA wild type expressed in the same cell line. Although the t 1 2 α and β were almost similar for both molecules, the plasma clearance of the partially deglycosylated molecules was 2.3 times slower than that of the native molecule. The relative size of the area under the curve in the β-phase was markedly increased from 27–54.4%. Infusion of less than half of the dose of t-PA wild type (mg/kg) resulted in similar thrombolytic effects. The data presented show that the mutant analysed is more potent than t-PA wild type in vivo. Although the mutant has an increased specific activity and a lowered clearance rate, the thrombolytic data do not demonstrate that these properties are cumulative.

The specific thrombin inhibitors r-hirudin and a synthetic peptide (I) D-FPRP(G)4-NGDFEEIPEEYL were compared in in vitro tests. r-hirudin proved to be the superior compound with respect to inhibition of amidolytic small substrate turnover that is catalysed by soluble and immobilised thrombin as well as to inhibition of fibrinogen activation. In an in vitro clot model significantly higher molar concentrations of peptide I are needed to achieve fibrin bound thrombin inhibition equivalent to that of r-hirudin. Stable complexes consisting of thrombin and hirudin oppose labile complexes containing the synthetic peptide. The latter leads to a regaining of thrombin activity with subsequent additional fibrin accretion. Analyses of the mixtures of thrombin and peptide I display a time dependent release of amino-terminal D-FPR peptide (III) exhibiting, similar to the residual fragment (peptide II), only weak inhibitory activity. Peptide I and the carboxy-terminal fragment induce, within a certain concentration range, an increase in thrombin activity and clot growth.

Experimental gram-negative sepsis was induced in the rat by Klebsiella pneumoniae. Although bacteria are susceptible to the treatment with the antibiotic Tobramycin, DIC could not be prevented. DIC was manifested by a leuko- and thrombocytopenia, decreases in fibrinogen and AT III and an increase of the aPTT. In this model the therapeutic treatment with human AT III was evaluated. To determine the optimal concentration of AT III a prestudy in a LPS induced DIC in the rat was performed. It was shown that a bolus i.v. injection of 500 U/kg improved survival and DIC, and was thus chosen for the Klebsiella sepsis model. The infectious load was adjusted to yield a mortality rate of 90-100% in the untreated Klebsiella group and a reduction to about 40-50% of the mortality rate by Tobramycin. It was found that AT III reduced mortality in the Klebsiella induced sepsis not only when given prophylactically but was effective even when administrated in a late stage of the DIC, i.e. 3 or 5 h post infection.

Haemostasis is a system of finely adjusted interactions between cells, enzymatic reaction cascades and inhibitors. Disturbances of this balance occur in many disorders, especially in inflammatory processes, septicaemia and cancer. In such cases malignant cells and infectious organisms activate the plasmatic enzyme cascades, especially of the coagulation and fibrinolysis cascades. The resulting consumption and proteolytic degradation of the regulatory proteins contribute to hypercoagulability and secondarily to reactive fibrinolysis, and these may then lead to local thromboses and haemorrhages. These pathogenic events culminate in disseminated intravascular coagulation (DIC), frequently with organ failure and death. Factors of both plasmatic systems are also "misused" by malignant cells for the purposes of growth and metastasis. Prominent examples of this misuse are the formation of a protective fibrin shield against the endogenous defence mechanisms and the local degradation of tissues for tumor proliferation as well as for cell permeation and invasion. In the search for a potential therapy a number of protease inhibitors, predominantly of enzymes of coagulation and fibrinolysis, have been tested in vivo with regard to their efficacy. So far, however, it has not been possible to find a new uniform treatment principle to inhibit the growth and/or metastasis of different types of tumor. The haemorrhagic diathesis and thromboses frequently associated with tumors are generally treated by substitution with plasma components, especially concentrates of coagulation factors and inhibitors.

Concentrations of annexins I to VI were quantitatively determined in extracts of placenta and different human cell types. They were detectable in all extracts studied, but lymphocytes/monocytes, endothelial cells and fibroblasts had very high annexin contents. The results indicate cell type specific annexin-repertoires. Annexins are intracellular proteins lacking signal sequences but which are detectable in trace amounts in plasma of healthy humans. The majority of plasma samples drawn from 14 patients suffering from myocardial infarction had elevated annexin III, IV and V concentrations. Shortly after infarction increased annexin levels were detected, reaching maximal values 24 to 48 h later. In the course of the following days annexin concentrations returned towards normal plasma levels.

In the present in vitro study the stabilities of the thrombin-hirudin and the thrombin-antithrombin III complexes were investigated. After incubation of the complexes with free inhibitors the thrombin-antithrombin III levels were determined by ELISA. The thrombin-hirudin complex proved to be stable in the presence of antithrombin III or heparin. However, in the presence of heparin and plasma equivalent concentrations of antithrombin III, the thrombin-hirudin complex dissociated and hirudin was displaced. In contrast, both thrombin-antithrombin III and thrombin-antithrombin III/heparin complexes are very stable even in the presence of a large excess of hirudin.

A binding protein for plasminogen activator inhibitor 1 (PAI-1-BP) was isolated from human plasma by a four-step procedure. 1) The 7 S globulin fraction of plasma was isolated by gel filtration on Sephacryl S-300. 2) Human endothelial cell-type plasminogen activator inhibitor (PAI-1), pretreated with 12 M urea, was added to this fraction (22 micrograms of PAI-1/ml of plasma), and a PAI-1 antigen peak with apparent mass 450 kDa (representing 65% of PAI-1 antigen and 85% of PAI activity) was isolated by gel filtration of this mixture. 3) The PAI-1.PAI-1-BP complex was further purified by immunoadsorption on an immobilized murine monoclonal antibody directed against PAI-1 (MA-7D4) and by elution with 4 M KSCN. 4) The complex was then dissociated by addition of excess human tissue-type plasminogen activator (t-PA), and t-PA and PAI-1 antigen (t-PA.PAI-1 complexes and free t-PA and PAI-1) were removed by immunoadsorption on monoclonal antibodies directed against t-PA (MA-62E8) and against PAI-1 (MA-7D4 and MA-12A4). Sodium dodecyl sulfate-gel electrophoresis of the purified material under nonreducing conditions revealed two bands with apparent mass approximately equal to 150 kDa and two bands with mass 74 and 68 kDa. Reduced sodium dodecyl sulfate-gel electrophoresis displayed two main bands with apparent masses of 73 and 64 kDa. The PAI-1-BP reacts with urea-treated, but not with inactive PAI-1. t-PA dissociates the complex between PAI-1 and PAI-1-BP. PAI-1 in complex with PAI-1-BP is 2-3-fold more stable at 37 degrees C than purified PAI-1, suggesting that PAI-1-BP may stabilize PAI-1 in blood. The concentration of PAI-1-BP in plasma determined by titration with PAI-1 is approximately 130 mg/liter. The isolated PAI-1-BP was shown to be identical to S protein (vitronectin) both by cross-reactivity with monospecific rabbit antisera and by NH2-terminal amino acid sequence analysis. The gel filtration behavior, mobility on sodium dodecyl sulfate-gel electrophoresis, and concentration in plasma suggest that PAI-1-BP is a multimer (presumably a dimer) of S protein accounting for approximately 35% of the S protein in plasma.

It is now generally well accepted that heparin and related substances increase the fibrinolytic activity in vivo . The stimulation of the amidolytic, plasminogenolytic and fibrinogenolytic activity of tissue plasminogen activator and urinary plasminogen activator through heparin was investigated in vitro . A concentration-dependent stimulation of the plasminogenolytic and fibrinogenolytic activity of both urinary and tissue-type plasminogen activators was observed in the presence of heparin. No heparin dependence was observed in the amidolytic assay. Heparin stimulates the plasminogenolytic activity of tissue plasminogen activator in the same manner as fibrin. Both activators form complexes with heparin; the heparin-binding-site seems to be identical or related with the fibrin-binding-site of tissue plasminogen activator. The physiological role of these interactions is discussed.

The clinical relevance of tissue-plasminogen-activator (t-PA) as a potent thrombolytic agent has recently been established. It has however been recognized that t-PA does not fulfill all conditions required for an ideal thrombolytic pharmaceutical agent; for example, its physiological stability and its short half life in vivo necessitate the use of very large clinical doses. We have therefore attempted to develop novel mutant t-PA proteins with improved properties by creating mutants by site-directed mutagenesis in M13 bacteriophage. Seventeen mutants were designed, cloned, and expressed in CHO cells. Modifications were of three types: alterations to glycosylation sites, truncations of the N- or C-termini, and amino acids changes at the cleavage site utilized to generate the two chain form of t-PA. The mutant proteins were analyzed in vitro for specific activity, fibrin dependence of the plasminogen activation, fibrin affinity, and susceptibility to inhibition by PAI.In brief, the results are: 1) some unglycosylated and partially glycosylated molecules obtained by mutagenesis are characterized by several-fold higher specific activity than wild type t-PA; 2) truncation at the C-terminus by three amino acids yields a molecule with increased fibrin specificity; 3) mutations at the cleavage site lead zo a decreased inhibition by PAI; and 4) recombinants of these genes have been constructed and the proteins were shown to possess multiple improved properties. The use of site directed mutagenesis has proved to be a powerful instrument to modulate the biological properties of t-PA.

A binding protein for plasminogen activator inhibitor 1 (PAI-1-BP) was isolated from human plasma by a four-step procedure. 1) The 7 S globulin fraction of plasma was isolated by gel filtration on Sephacryl S-300. 2) Human endothelial cell-type plasminogen activator inhibitor (PAI-1), pretreated with 12 M urea, was added to this fraction (22 micrograms of PAI-1/ml of plasma), and a PAI-1 antigen peak with apparent mass 450 kDa (representing 65% of PAI-1 antigen and 85% of PAI activity) was isolated by gel filtration of this mixture. 3) The PAI-1.PAI-1-BP complex was further purified by immunoadsorption on an immobilized murine monoclonal antibody directed against PAI-1 (MA-7D4) and by elution with 4 M KSCN. 4) The complex was then dissociated by addition of excess human tissue-type plasminogen activator (t-PA), and t-PA and PAI-1 antigen (t-PA.PAI-1 complexes and free t-PA and PAI-1) were removed by immunoadsorption on monoclonal antibodies directed against t-PA (MA-62E8) and against PAI-1 (MA-7D4 and MA-12A4). Sodium dodecyl sulfate-gel electrophoresis of the purified material under nonreducing conditions revealed two bands with apparent mass approximately equal to 150 kDa and two bands with mass 74 and 68 kDa. Reduced sodium dodecyl sulfate-gel electrophoresis displayed two main bands with apparent masses of 73 and 64 kDa. The PAI-1-BP reacts with urea-treated, but not with inactive PAI-1. t-PA dissociates the complex between PAI-1 and PAI-1-BP. PAI-1 in complex with PAI-1-BP is 2-3-fold more stable at 37 degrees C than purified PAI-1, suggesting that PAI-1-BP may stabilize PAI-1 in blood. The concentration of PAI-1-BP in plasma determined by titration with PAI-1 is approximately 130 mg/liter. The isolated PAI-1-BP was shown to be identical to S protein (vitronectin) both by cross-reactivity with monospecific rabbit antisera and by NH2-terminal amino acid sequence analysis. The gel filtration behavior, mobility on sodium dodecyl sulfate-gel electrophoresis, and concentration in plasma suggest that PAI-1-BP is a multimer (presumably a dimer) of S protein accounting for approximately 35% of the S protein in plasma.

In the last decades, tremendous advances have been made in the elucidation of the biochemical mechanism of the fibrinolytic system. The activators of plasminogen have been accurately characterized and the mechanism of the activation reaction studied in great details. Recently four new inhibitors of the plasminogen activators have been identified, their biochemical and physiological properties are under study. Although the various roles of plasmin, only its function in the lysis of fibrin will be considered in this review.