Kinetic analysis of the effects of glycosaminoglycans and lipoproteins on urokinase-mediated plasminogen activation

The effect of fucoidan, heparin and cyanogen bromide-fibrinogen on the activation of human glutamic-plasminogen by tissue plasminogen activator

Earlier studies on the stimulatory effect of fucoidan, heparin, and cyanogen bromide (CNBr)-fibrinogen digest on the in-vitro activation of glutamic type plasminogen by tissue plasminogen activator, which were performed using subphysiologic ionic strengths of buffers, gave inconsistent results because of the variation in the ionic strengths of the buffers used. Studies were therefore conducted on the effect of these cofactors using 0.05 mol/l Tris buffer containing a physiologic concentration of sodium chloride. The double reciprocal plots of the activation of glutamic type plasminogen by tissue plasminogen activator in the presence of fucoidan and 6-aminohexanoic acid (6-AH) or heparin and 6-AH showed a four- to six-fold increase in K(cat), while the K(m) remained unchanged. On the other hand, there was greater than six-fold lowering of K(m) from 0.213 to 0.035 micromol/l in the presence of CNBr-fibrinogen, while K(cat) was only slightly increased. The ratios of the initial rate of plasmin generation in the presence or absence of the cofactors were plotted against the inverse of the volume fraction of glutamic type plasminogen or of tissue plasminogen activator after serial dilution. The results suggested that the enhancements by fucoidan and 6-AH or CNBr-fibrinogen were due to their interactions directed towards glutamic type plasminogen, while for heparin and 6-AH, the interaction was directed towards tissue plasminogen activator. Circular dichroism studies in the near ultraviolet range (250-308 nm) showed that 6-AH enhanced the circular dichroism spectra of glutamic type plasminogen around certain chromophores, while fucoidan and heparin had no effect, suggesting that the enhancement by the cofactors may be related to the favorable conformational changes of glutamic type plasminogen by 6-AH.

Glycosaminoglycan mimetics (RGTA) modulate adult skeletal muscle satellite cell proliferation in vitro

Muscle regeneration occurs through the activation of satellite cells, which are stimulated to proliferate and to fuse into myofibers that will reconstitute the damaged muscle. We have previously reported that a family of new compounds called "regenerating agents" (RGTAs), which are polymers engineered to mimic heparan sulfates, stimulate in vivo tissue repair. One of these agents, RG1192, a dextran derivative substituted by CarboxyMethyl, Benzylamide, and Sulfate (noted CMBS, RGTA type), was shown to improve greatly the regeneration of rat skeletal muscle after severe crushing, denervation, and acute ischemia. In vitro, these compounds mimic the protecting and stabilizing properties of heparin or heparan sulfates toward heparin-binding growth factors (HBGFs). We hypothesized that RGTA could act by increasing the bioavailability of some HBGF involved in myoblast growth and thus asked whether RGTA would alter the ability of satellite cells to proliferate. Its effect was tested on primary cultures of rat satellite cells. The RG1192 stimulated the proliferation of satellite cells in vitro in a dose-dependent manner. It appeared to be as efficient as natural glycosaminoglycans (GAGs; heparan sulfate, dermatan sulfate, or keratan sulfate) in stimulating satellite cell proliferation but was about 100 times more efficient than heparin. RG1192 stimulated satellite cell proliferation by increasing the potency of fibroblast growth factor 2 and scatter factor-hepatocyte growth factor. It also partially restored myoblast proliferation of satellite cells with chlorate-induced hyposulfation. Taken together, our results explain to some extent the improving effect of RGTA with a CMBS structure, such as the RG1192, on muscle regeneration in vivo by providing support for the hypothesis that RGTA may act by increasing the potency of some HBGFs during the proliferation phase of the regenerating muscle.

Regulation of urokinase/urokinase receptor interaction by heparin-like glycosaminoglycans

We show here that the interaction between the urokinase-type plasminogen activator and its receptor, which plays a critical role in cell invasion, is regulated by heparan sulfate present on the cell surface and in the extracellular matrix. Heparan sulfate oligomers showing a composition close to the dimeric repeats of heparin (glucosamine-NSO(3)(6-OSO(3))-iduronic acid(2-OSO(3))) n = 5 and n > 5, where iduronic acid may alternate with glucuronic acid, exhibit affinity for urokinase plasminogen activator and confer specificity on urokinase/urokinase receptor interaction. Cell surface clearance of heparan sulfate reduces the affinity of such interaction with a parallel decrease of specific urokinase binding in the presence of an unaltered expression of receptor. Transfection of human urokinase plasminogen activator receptor in normal Chinese hamster ovary fibroblasts and in Chinese hamster ovary cells defective for the synthesis of sulfated glycosaminoglycans results in specific urokinase/receptor interaction only in nondefective cells. Heparan sulfate/urokinase and receptor/urokinase interactions exhibit similar K(d) values. We concluded that heparan sulfate functions as an adaptor molecule that confers specificity on urokinase/receptor binding.

Importance of GlcUAbeta1-3GalNAc(4S,6S) in chondroitin sulfate E for t-PA- and u-PA-mediated Glu-plasminogen activation

Chondroitin sulfate E (CSE) markedly enhanced plasminogen activation by tissue plasminogen activators (t-PAs) and urinary plasminogen activator (u-PA) in vitro; in the presence of 10 microg/ml of CSE, the potentiation factors of single chain of t-PA, two chain of t-PA and u-PA were 400, 140 and 130, respectively. Though the potentiation activity of CSE gradually decreased when it was depolymerized by chondroitinase ABC, the specific disaccharide from CSE still showed significant activity. Glycosaminoglycan (GAG) from sea cucumber, which possesses a very similar core structure to CSE, but has additional sulfated fucose branches exhibit very weak activity. These results suggested that the minimal structural requirement in CSE to enhance plasminogen activation by plasminogen activators is GlcUAbeta1-3GalNAc(4S,6S) and that additional branching sugars abolish the activity.

Mechanism of enhancement by fucoidan and CNBr-fibrinogen digest of the activation of glu-plasminogen by tissue plasminogen activator

The interactions of fucoidan with human glutamic type plasminogen (Glu-Plg), porcine pancreatic elastase digested plasminogen fractions and two chain tissue plasminogen activator t-PA) were investigated using fucoidan-Sepharose affinity chromatography. The results showed a high degree of affinity between fucoidan-Sepharose and Glu-Plg or PlgK(1-3) but not with PlgK4 or mini-Plg. Fucoidan-Sepharose also showed a high affinity for t-PA, which was largely reversed by 0.002 M 6-aminohexanoic acid (6-AH). The addition of fucoidan and CNBr-fibrinogen digest (CNBr-Fbg) gave the highest enhancement of the in vitro activation of Glu-Plg by t-PA in the presence of 0.002 M 6-AH. The results of affinity chromatography and enhancement studies suggested a template mechanism, since increasing the concentrations of any one of the two cofactors reversed the enhancement. Enzyme kinetic studies, using double reciprocal plots, showed that the addition of fucoidan-6-AH increased Kcat by 7-fold without affecting Km and addition of CNBr-Fbg lowered Km by 5-fold without significantly affecting Kcat while addition of the two cofactors lowered Km by 16-fold without significantly affecting Kcat. The enhancement by fucoidan-6-AH or by CNBr-Fbg of the in vitro activation of Glu-Plg by t-PA was reversed by plasminogen activator inhibitor 1 (PAI-1). Fucoidan-Sepharose affinity chromatography revealed that the binding of PAI-1 with fucoidan may be responsible for the reversal of the enhancement by fucoidan-6-AH.

The potential mechanism for the effect of heparin on tissue plasminogen activator-mediated plasminogen activation

The effects and possible role of heparin on tissue plasminogen activator-mediated plasminogen activation was thoroughly investigated. Direct analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that heparin increased the conversion of plasminogen to plasmin. Experiments by fluorescence quenching suggested that the stimulation of tissue plasminogen activator activity probably was due to a direct binding of heparin to tissue plasminogen activator, causing a conformational change of tissue plasminogen activator and rendering it more accessible to plasminogen interaction. The absence of additive stimulation effects on tissue plasminogen activator-mediated plasminogen activation when both heparin and fibrinogen were present also implied that both compounds interacted with tissue plasminogen activator via the same domain; it appeared to be most likely via the kringle-2 domain in tissue plasminogen activator based on studies using epsilon-aminocaproic acid as an inhibitor. Unlike heparin-induced stimulation of antithrombin-thrombin interaction, the heparin-induced stimulation of tissue plasminogen activator did not seem to follow a template model. Only in the presence of a high plasminogen or a low tissue plasminogen activator concentration, massive stimulation of tissue plasminogen activator activity was observed via a pseudotemplate model. The results suggest that precautions concerning high heparin dose should be given during its conjunctive clinical use with tissue plasminogen activator in thrombolytic therapy to reduce the risk of hemorrhage.

Signal transduction by a protease cascade

The dorsoventral axis of the Drosophila embryo is determined by a spatial cue generated by ovarian somatic cells. This cue is communicated to the embryo through an extracellular serine protease cascade active only on the ventral side of the embryo. Studies of the proteases and somatically expressed proteins involved in this signalling process suggest a working model for how the protease cascade is locally activated hours after the ovarian somatic cells have degenerated.

Sulfated glycosaminoglycans enhance tumor cell invasion in vitro by stimulating plasminogen activation

Metastasizing tumor cells invade host tissues by degrading extracellular matrix constituents. We report here that the highly sulfated glycosaminoglycans, heparin and heparan sulfate, as well as the sulfated polysaccharide, fucoidan, significantly enhanced tumor cell invasion in vitro into fibrin, the basement membrane extract, Matrigel, or through a basement membrane-like extracellular matrix. The enhancement of tumor cell invasion was due to a stimulation of the proteolytic cascade of plasminogen activation since the effect required plasminogen activation and was abolished by inhibitors of urokinase-type plasminogen activator (uPA) or plasmin. Sulfated polysaccharides enhanced five reactions of tumor-cell initiated plasminogen activation in a dose-dependent manner. They amplified plasminogen activation in culture supernatants up to 70-fold by stimulating (i) pro-uPA activation by plasmin and (ii) plasminogen activation by uPA. (iii) In addition, sulfated polysaccharides partially protected plasmin from inactivation by alpha 2-antiplasmin. Sulfated polysaccharides also stimulated tumor-cell associated plasminogen activation, e.g., (iv) cell surface pro-uPA activation by plasmin and (v) plasminogen activation by cell surface uPA. These results suggest that sulfated glycosaminoglycans liberated by tumor-cell mediated extracellular matrix degradation in vivo might amplify pericellular plasminogen activation and locally enhance tumor cell invasion in a positive feedback manner.

Interaction of fucoidan with proteases and inhibitors of coagulation and fibrinolysis

The interactions of fucoidan with glutamic plasminogen (Glu-Plg), two-chain tissue plasminogen activator (t-PA), LMwt-urokinase, thrombin, and antithrombin III (AT-III) were investigated using fucoidan-sepharose affinity chromatography. The results showed 1) a high degree of affinity between fucoidan-sepharose and Glu-Plg; Lmwt-urokinase and thrombin while t-Pa and AT-III did not bind with fucoidan-sepharose. 2) The double reciprocal plot for the LMwt-urokinase activation of Glu-Plg showed that plasminogen activator inhibitor (PAI-1) inhibited this reaction in a noncompetitive manner and that the presence of fucoidan decreased Km for this interaction by 50% and increased Kcat by 30-fold, 3) The double reciprocal plot for the t-PA activation of Glu-Plg showed that PAI-1 inhibited this reaction in a competitive manner and that fucoidan in conjunction with 6-aminohexanoic acid (6-AH) increased Kcat for this interaction by 5-fold without affecting Km. 4) Fucoidan enhanced the interaction of thrombin with both AT-III and heparin cofactor II (HC-II) and it was more effective than unfractionated heparin of LMwt-heparin in enhancing the interaction of HC-II with thrombin.

TSG-6: an IL-1/TNF-inducible protein with anti-inflammatory activity

The pro-inflammatory cytokines IL-1 and TNF-alpha are primary mediators of the acute phase response, the complex reaction of the mammalian organism to infection and injury. Among the genes activated by TNF-alpha and IL-1 in a variety of cells is TNF-stimulated gene 6 (TSG-6). The TSG-6 cDNA encodes a secreted 35 kDa glycoprotein which is abundant in synovial fluids of patients with various forms of arthritis and detectable in serum of patients with different inflammatory or autoimmune disorders. TSG-6 protein consists of two structural domains: a hyaluronan-binding link module, the characteristic domain of the hyaladherin family of proteins, and a C-terminal CUB domain, present in a variety of diverse proteins. TSG-6 forms a stable complex with components of the plasma protein inter-alpha-inhibitor (I[alpha]I), a Kunitz-type serine protease inhibitor. TSG-6 and I(alpha)I synergize to inhibit plasmin, a serine protease involved in the activation of matrix metalloproteinases which are part of the proteolytic cascade associated with inflammation. Recombinant human TSG-6 protein exerts a potent anti-inflammatory effect in a murine model of acute inflammation. Modulation of the proteolytic network associated with inflammatory processes may be a mechanism whereby TSG-6, in cooperation with I(alpha)I, inhibits inflammation. Activation of the TSG-6 gene by pro-inflammatory cytokines, presence of TSG-6 protein in inflammatory lesions and its anti-inflammatory effect suggest a role for TSG-6 in a negative feed-back control of the inflammatory response.

Ligand-induced conformational change of lipoprotein(a)

Lipoprotein(a) undergoes a dramatic, reversible conformational change on binding 6-amino-hexanoic acid (6-AHA), as measured by a decrease in the sedimentation rate, the magnitude of which is directly proportional to apo(a) mass. A similar reversible transition from a compact to an extended form has been shown to occur in plasminogen on occupation of a weak lysine binding site. The magnitude of the change in Lp(a) with large apo(a) is about 2.5 times that seen for plasminogen, however. Regardless of apo(a) size, binding analysis indicated that 1.4-4 molecules of 6-AHA bound per Lp(a) particle; the midpoint of the conformational change occurs at 6-AHA concentrations of 100-200 mM. Since rhesus Lp(a), which lacks both kringle V and the strong lysine binding site on kringle IV 10, also undergoes a similar conformational change, the phenomenon may be attributable to weak sites, possibly located in K-IV 5-8. Compact Lp(a), i.e., native Lp(a), had a frictional ratio (f/f0) of 1.2 that was independent of apo(a) mass, implying constant shape and hydration. For Lp(a) in saturating 6-AHA, f/f0 ranged from 1.5 to over 2.1 for the largest apo(a) with 32 K-IV, indicating a linear relationship between hydrodynamic volume and number of kringles, as expected for an extended conformation. However, only the variable portion of apo(a) represented by the K-IV 2 domains, participates in the conformational change; the invariant K-IV 3-9 domains remain close to the surface. These results suggest that apo(a) is maintained in a compact state through interactions between weak lysine binding sites and multiple lysines on apoB and/or apo(a), and that these interactions can be disrupted by 6-AHA, a lysine analog.

Isolation and characterization of heparan sulfate from crude porcine intestinal mucosal peptidoglycan heparin

A method for the preparation of heparan sulfate from peptidoglycan heparin is described. The objective of this research was to provide a basis for the development and validation of an industrial process to support the preclinical development of heparan sulfate and/or heparan sulfate derivatives. In the preparation of heparan sulfate, heparin was recovered by alcohol fractionation and dermatan sulfate was isolated by selective precipitation. The remaining crude heparan sulfate was fractionated by anion-exchange chromatography into five subfractions. The biological activities of these subfractions were examined by anticoagulant and amidolytic assays. Molecular weight and molecular size were determined using capillary viscometry and polyacrylamide gel electrophoresis. Charge density and degree of sulfation were determined by cellulose acetate electrophoresis and elemental analysis. Oligosaccharide and disaccharide analysis relied on enzymatic depolymerization using heparin lyases followed by polyacrylamide gel and capillary electrophoresis. 1H NMR analysis provided detailed structural information on each subfraction. Crude heparin sulfate and its subfractions showed significant differences in physical, structural and biological properties.

Lipoprotein (a) in the regulation of fibrinolysis

Elevated plasma levels of lipoprotein(a) [LP(a)] are associated with increased an risk of developing atherosclerosis. This increased risk may be due to an Lp(a)-mediated depression of fibrinolytic activity. Lp(a) regulates fibrinolysis by controlling the activity of plasminogen activators. Lp(a) is a low density lipoprotein with an apoprotein(a) subunit which has a high degree of homology with the fibrinolytic zymogen plasminogen. The apoprotein(a) subunit contains up to thirty seven copies of a domain homologous to the plasminogen kringle 4 domain, which enables Lp(a) to bind to fibrin. The subunit also has a zymogen domain, but it is not activated by plasminogen activators. Lp(a) inhibits plasminogen activation by competing with plasminogen for access to plasminogen activators bound to vascular surfaces. Lp(a) also competes with the irreversible inhibitor of plasminogen activators, plasminogen activator inhibitor-1. Therefore increases in Lp(a) concentration may decrease fibrinolytic activity by preventing activation of plasminogen, but Lp(a) may also prolong plasminogen activation by preventing the irreversible inhibition of the activators. At elevated levels of Lp(a) the decreased rate of plasmin generation may not be offset by the prolongation in plasminogen activation, and fibrinolysis will be inhibited.

Effects of heparan sulfate analogue or other sulfated polysaccharides on the activation of plasminogen by t-PA or u-PA

Effects of heparan sulfate analogue (HHS-5) and other sulfated polysaccharides on the fibrinolytic system were investigated. Activation rate of native plasminogen (Glu-plg) by tissue plasminogen activator (t-PA), urinary plasminogen activator (u-PA) and single-chain u-PA (scu-PA) was enhanced in the presence of HHS-5 dose dependently up to 10 micrograms/ml. Activation rate of Glu-plg by sct-PA was more enhanced than that by tct-PA in its presence. HHS-5 enhanced the activation of Glu-plg by t-PA or u-PA compared to the activation of Lys-plg. SDS-PAGE confirmed that the enhancement of the hydrolysis of S-2251 by the mixture of Glu-plg, sct-PA and HHS-5 was due to the facilitation of the conversion of Glu-plg to plasmin. HHS-5 only slightly enhanced the amidolytic activity of sct-PA or tct-PA. The enhancement of the activation of plasminogen by t-PA or u-PA was more significant in the presence of HHS-5 than in the presence of chondroitin sulfate C, dextran sulfate or heparin. It is possible that the enhancement of the activation of Glu-plg by activators in the presence of HHS-5 was due to the conformational change of Glu-plg upon interaction with HHS-5.

Lipoprotein (a) regulates plasmin generation and inhibition

The relationship between lipoprotein (a) (Lp(a)) and atherosclerosis has been appreciated for a number of years. Only in recent years, however, has the structural relationship of Lp(a) to plasminogen resulted in studies of the effect of this lipoprotein on fibrinolysis. Lp(a) inhibits activation of plasminogen by tissue-type (t-PA) and urinary-type (u-PA) plasminogen activators. These inhibitory reactions are surface-dependent. When Lp(a) binds to fibrin, fibrinogen, heparin or cells it blocks activation of plasminogen by t-PA. u-PA-mediated activation of plasminogen is blocked on surfaces including heparin and chondroitin sulfate. Lp(a) also favors inhibition of plasmin by alpha 2-antiplasmin (alpha 2-AP). The ability of Lp(a) to compete with plasmin for fibrin binding displaces plasmin into solution where alpha 2-AP rapidly inhibits this proteinase. These effects are all antifibrinolytic. Lp(a) also exhibits one profibrinolytic effect, since it blocks inhibition of t-PA by plasminogen activator type 1 in the presence of fibrinogen or heparin. Thus, Lp(a) modulates most of the reactions involved in plasmin generation and inhibition. Its overall effect will depend primarily on the concentrations of Lp(a), PAI-1 and t-PA in vivo.

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.

Lipoprotein(a): a kinetic study of its influence on fibrin-dependent plasminogen activation by prourokinase or tissue plasminogen activator

Lipoprotein(a) [Lp(a)] has been postulated to inhibit fibrinolysis due to its structural homology to plasminogen. Indeed, it has been reported that Lp(a) competitively inhibits the promotion by fibrin of tissue plasminogen activator (t-PA)-catalyzed plasminogen activation. However, it has also been reported that this inhibition is uncompetitive. No studies have been published, to our knowledge, of the effect of Lp(a) on prourokinase (pro-UK)-catalyzed plasminogen activation. Plasminogen activation by pro-UK or a plasmin-resistant mutant pro-UK was previously shown to be promoted by fibrin fragment E2, whereas that by t-PA is promoted by fragment D. Therefore, the influence of Lp(a) on the kinetics of these two reactions was examined. When Lp(a) was added (90-600 nM), no change in the rate of plasmin generation by Ala158-pro-UK was observed. Consistent with this, immobilized Lp(a) also failed to bind to fragment E2, whereas it did bind to D dimer. When t-PA-catalyzed plasminogen activation in the presence of D dimer was measured, uncompetitive inhibition by Lp(a) was found, but only at low concentrations of D dimer (< 0.5 microM) or t-PA (0.05 nM). At higher concentrations of D dimer and t-PA, instead of inhibition, Lp(a) induced a 2.4-fold promotion of plasminogen activation. Similarly, Lp(a) enhanced (up to 2.5-fold) plasminogen binding to immobilized fibrin in both buffer and plasma milieus at the physiological concentration of plasminogen (2.0 microM). In conclusion, Lp(a) had no effect on plasminogen activation by pro-UK and induced only limited inhibition of activation by t-PA.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of proteoglycans in cell adhesion, migration and proliferation

Proteoglycans comprise a part of the extracellular matrix that participates in the molecular events that regulate cell adhesion, migration and proliferation. Their structural diversity and tissue distribution suggest a functional versatility not generally encountered for other extracellular matrix components. This versatility is mainly dictated by their molecular interactions and their ability to regulate the activity of key molecules involved in several biological events. This molecular cooperativity either promotes or inhibits cell adhesion, migration and proliferation. A growing number of studies indicate that proteoglycans can play a direct role in these cellular events by functioning either as receptors or as ligands for molecules that are required for these events to occur. Such studies support a role for proteoglycans as important effectors of cellular processes that constitute the basis of development and disease.

Ionic modulation of the effects of heparin on plasminogen activation by tissue plasminogen activator: the effects of ionic strength, divalent cations, and chloride

Ionic strength, divalent cations, and Cl- modulate the ability of the glycosaminoglycan heparin to stimulate the activation of human plasminogen (Pg) by tissue-type Pg activator. Kinetic analysis of Pg activation indicates that heparin is inhibitory, stimulatory, or nonstimulatory as a function of ionic strength. While increasing ionic strength inhibits Pg activation in the absence of heparin, in it presence an activation phase followed by an inhibitory phase is observed. Divalent cations, inhibitors of activation in the absence of heparin, increase the rate of activation in its presence. Kinetic analysis demonstrates that divalent cations augment the heparin stimulatory effect a maximum of 60-fold due to increases in kcat without changes in Km of the reaction. This effect is heparin-specific, since activation is not affected by Ca2+ in the presence of heparan sulfate or de-N-sulfated heparin. Also, Cl- inhibits Pg activation in the presence of heparin by acting as a competitive inhibitor (Kic of 100 mM). Furthermore, inhibition by Cl- reduces the overall magnitude of heparin stimulation of Pg activation. These results suggest that physiologic ions in combination with heparin may be significant effectors of Pg activation in the vascular microenvironment.

Euglobulin clot lysis induced by tissue-type plasminogen activator is reduced in subjects with increased levels of lipoprotein (a)

Several reports have evaluated the in vitro effect of lipoprotein(a) [Lp(a)] levels on the fibrinolytic system, suggesting that high Lp(a) levels may inhibit fibrinolysis by competing for plasminogen binding in different systems. We have studied plasminogen activation induced by tissue-type plasminogen activator (t-PA), as well as other fibrinolytic parameters, in 25 subjects with Lp(a) levels greater than 30 mg/dl and the results were compared with those found in 23 subjects with Lp(a) less than 30 mg/dl. Both groups were similar in age, sex distribution, living habits and lipid pattern. Plasminogen activation, when measured by t-PA-induced euglobulin clot lysis, was significantly decreased in the group with elevated Lp(a) levels (lysis time, 16.7 +/- 3.3 min) compared with the group with low Lp(a) levels (11.8 +/- 2.0 min), although 8 of the 25 subjects with high Lp(a) levels showed plasminogen activation within the range of the control group. A positive significant correlation between Lp(a) levels and t-PA-induced euglobulin clot lysis time was found. No statistical differences were demonstrated between groups for the other fibrinolytic parameters studied. Addition of purified Lp(a) to the euglobulin fraction or to plasma resulted in a decrease in euglobulin clot lysis. The present study shows that t-PA induced plasminogen activation is decreased in individuals with high circulating levels of Lp(a) supporting the hypothesis that Lp(a) may interfere with the physiological functions of plasminogen.