Academic literature on the topic 'Thrombin-Inhibitory Activity'

Thrombin, a key enzyme of the serine protease superfamily, plays an integral role in the blood coagulation cascade and thrombotic diseases. In view of this, it is worthwhile to establish a method to screen thrombin inhibitors (such as natural flavonoid-type inhibitors) as well as investigate their structure activity relationships. Virtual screening using molecular docking technique was used to screen 103 flavonoids. Out of this number, 42 target compounds were selected, and their inhibitory effects on thrombin assayed by chromogenic substrate method. The results indicated that the carbon-carbon double bond group at the C2, C3 sites and the carbonyl group at the C4 sites of flavones were essential for thrombin inhibition, whereas the methoxy and O-glycosyl groups reduced thrombin inhibition. Noteworthy, introduction of OH groups at different positions on flavonoids either decreased or increased anti-thrombin potential. Myricetin exhibited the highest inhibitory potential against thrombin with an IC50 value of 56 μM. Purposively, the established molecular docking virtual screening method is not limited to exploring flavonoid structure activity relationships to anti-thrombin activity but also usefully discovering other natural active constituents.

Spumigins are marine natural products derived from cyanobacteria Nodularia spumigena, which mimics the structure of the d-Phe-Pro-Arg sequence and is crucial for binding to the active site of serine proteases thrombin and factor Xa. Biological evaluation of spumigins showed that spumigins with a (2S,4S)-4-methylproline central core represent potential lead compounds for the development of a new structural type of direct thrombin inhibitors. Herein, we represent synthesis and thrombin inhibitory activity of a focused library of spumigins analogues with indoline ring or l-proline as a central core. Novel compounds show additional insight into the structure and biological effects of spumigins. The most active analogue was found to be a derivative containing l-proline central core with low micromolar thrombin inhibitory activity.

Objective: The management of snake bite envenomation is a global challenge affecting millions of people. Immunotherapy is still regarded as the treatment of choice; however, their subsequent adverse effects restrict their potential use for therapy against snake venom poisoning. In recent years, more attention has been given to the exploration of indigenous medicinal plants for their outstanding benefits for the treatment of several diseases and disorders, including snake venom poisoning. Hesperetin is a naturally occurring compound derived from a flavanone glycoside, hesperidin and is obtained from various citrus fruits. It is known to possess significant inhibitory activity against snake venom serine proteases. The aim of our present study was to investigate the significant inhibitory action of hesperetin on thrombin-like serine protease from sharp-nosed pit viper (Deinagkistrodon acutus) snake venom.Methods: We have employed molecular docking analysis by implementing the state-of-the-art docking program to validate the hypothesis of the prospective inhibitory properties of hesperetin on thrombin-like serine proteases of snake venom. AutoDock 4.2, InterProSurf, MGLTools, and MTiAutoDock were utilized for the molecular docking analysis of thrombin-like serine protease obtained from the snake venom of sharp-nosed pit viper with the natural compound hesperetin.Results: The results generated from in silico based approach reveals the significant inhibitory role of hesperetin against thrombin-like snake venom proteases, which might lead to the drug designing of the inhibitors of snake venom serine proteases.Conclusion: The implementation of molecular docking analysis by the employment of state-of-the-art docking program supports the potential of inhibitory activity of naturally obtained hesperetin compound on thrombin-like serine proteases of snake venom. The generated in silico results suggests that the novel structure hesperetin - flavanone might act as a potent inhibitor of thrombin-like snake venom proteases, and unlocks the possibilities for designing drugs of the inhibitors of snake venom serine proteases. Moreover, the investigation of the novel compound obtained from natural sources for their inhibitory activity against snake venom serine proteases would lead to the discovery of newer inhibitory compound from a highly uninvestigated research arena.

SummaryThrombomodulin (TM) is a cofactor for the thrombin-catalyzed activation of anticoagulant protein C. However, we have no evidence that thrombomodulin actually activates protein C during blood coagulation processing, nor do we know whether this activated protein C acts as an anticoagulant. We studied the inhibitory action of recombinant human soluble TM (rhs-TM) on thrombin generation in whole plasma. Human plasma was activated with small amounts of tissue factor using phospholipid vesicles in place of activated platelets. Thrombin generation was observed. The addition of only 2 nM of rhs-TM prevented rapid generation of thrombin and reduced the total amount of thrombin generated. In order to study the influence of the protein C activation pathway on this inhibitory action of rhs-TM, protein C-depleted plasma was used. rhs-TM had little inhibitory effect on protein C-depleted plasma. However, the addition of protein C caused a delay in thrombin generation and a reduction of the maximum thrombin concentration. We concluded that the anticoagulant activity of rhs-TM was amplified by the protein C activation pathway.

Abstract Novel quercetin derivatives were prepared to change its physicochemical properties and effects on activity of proteolytic enzymes. For them preparation, the selective protection procedures some of the quercetin hydroxyl groups and acylation of the others with acylchlorides were used. The ability of these compounds to inhibit the activity of serine proteases e.g. trypsin, thrombin, urokinase and elastase was studied. In micromolar range, tested derivatives were the most potent inhibitors of thrombin. There was estimated better inhibition of thrombin for prenylated, acetylated, feruloyl and caffeoyl quercetin esters. Slight inhibitory effect of all quercetin derivatives on elastase was found. Among tested derivatives only diquercetin displayed better inhibiton. Trypsin and urokinase were inhibited by quercetin at comparable level. Slight improvement in inhibitory effect of trypsin and urokinase was seen for chloronaphtoquinone quercetin that revealed enhanced inhibiton of thrombin, too. However, no influence on elastase activity was determined for this compound. Obtained results indicate that certain modifications of quercetin structure could improve its biological properties.

Abstract Abstract 4177 Introduction National and international organizations recognize that low molecular weight heparins (LMWHs) are distinct entities and that they should not be used interchangeably in clinical practice. Physicochemical properties, such as molecular weight anti-Xa/anti-IIa ratios, are used by health authorities to establish LMWHs differentiation. LMWHs are multitargeted drugs in which small differences in physicochemical properties may lead to significantly different inhibition of blood coagulation. In the present in vitro study we have investigated if thrombin generation assay, reflecting the overall coagulation process, is a suitable tool for the LMWH variability evalution. Materials and methods LMWHs (bemiparin, enoxaparin, nadroparin, dalteparin and tinzaparin) and unfractionated heparin (UFH) were added in vitro in normal citrated platelet rich plasma from 10 healthy individuals, at clinically relevant concentrations (ranging from 0.2 to 1 anti-Xa IU/ml). Thrombin generation was studied with Thrombogram-Thrombinoscope® assay using diluted thromboplastin (Innovin®, 1/1000 final dilution, Siemens France). The concentrations doubling the lag-time, the time to Peak, the IC50 for Peak, the endogenous thrombin potential (ETP) and the mean rate index of propagation phase (MRI) were determined for each compound. Results LMWHs compared on their anti-Xa activity basis showed variable inhibitory effect on thrombin generation. At equivalent anti-Xa concentrations tinzaparin was significantly more potent than the other LMWHs, being almost similar to UFH profile. Enoxaparin, nadroparin and dalteparin showed a similar inhibitory activity. Bemiparin had the lesser pronounced inhibitory effect on thrombin generation. The impact of anti-Xa and anti-IIa activities on each phase of thrombin generation process is different. Thrombogram chronometric parameters are mainly influenced by the anti-IIa activity. Similarly, ETP inhibition depends more on anti-IIa rather than anti-Xa activity. The MRI of the propagation phase is more sensitive to the anti-Xa activity of LMWHs. Conclusion Thrombin generation assessment in platelet rich plasma allows to evaluate the anticoagulant fingerprint of LMWHs and to differentiate them on global and functional criteria. Each LMWH has a particular inhibitory impact on each phase of thrombin generation process. These functional criteria need to be standardized and probably required for a better characterization of LMWHs heterogeneity by health authorities. They could be used also to evaluate bioequivalence of generic and original LMWHs. Disclosures: No relevant conflicts of interest to declare.

SummaryAn amidrazonophenylalanine derivative, LB30057, inhibits the catalytic activity of thrombin potently by interaction with the active site of thrombin, and has high water solubility. In the present study, we evaluated the effect of LB30057 on the biological activities of thrombin at various tissues, and determined whether thrombin inhibition by LB30057 could reduce the incidence of occlusive thrombosis in an in vivo animal model. Treatment with LB30057 to human plasma prolonged clotting times in a concentration-dependent manner. LB30057 suppressed significantly thrombin-induced phosphatidylserine (PS) exposure in platelets, suggesting that LB30057 could inhibit blood coagulation accelerated by PS exposure. In human platelets, soluble thrombin- and clot-induced platelet aggregation was inhibited by LB30057 potently. Consistent with this finding, LB30057 showed concentration-dependent inhibitory effects on serotonin secretion and P-selectin expression induced by thrombin in platelets. In the blood vessel isolated from the guinea pig, treatment with LB30057 resulted in a concentration-dependent inhibition of thrombin-induced vascular contraction. In vivo study revealed that LB30057 following oral administration significantly increased the time to occlusion and improved carotid arterial patency using rat carotid artery thrombosis model. All these results suggest that LB30057 is a potent inhibitor of biological activities of thrombin at various target tissues and, therefore, might be developed as an anti-thrombotic agent for treatment and prevention of thrombotic diseases.

SummaryWe compare the relative activities of surface-bound and fluid-phase thrombin and their inhibition by heparin and Intimatan, a novel heparin cofactor II (HCII) agonist. In vitro, we compared the observed amidolytic activities of fluid-phase and surface-bound thrombin with the expected activities based upon 125I-specific activity. In vivo, we compared the inhibitory effects of heparin and Intimatan on thrombin activity bound to injured vessel walls. In vitro, the correlations between observed and expected activities of fluid-phase and surface-bound thrombin, were: r = 0.9974, p < 0.001; and r = 0.9678, p < 0.001; respectively. In vivo, injured vessel wall surface-bound thrombin activity persisted for > 24 h. This activity was not inhibited by heparin, but was inhibited by Intimatan, p < 0.001.We conclude that surface-bound thrombin is as active as fluid-phase thrombin and remains protected from inhibition by heparin, thereby contributing to vessel wall thrombogenicity following injury. In contrast, surface-bound thrombin is inhibited by Intimatan, thereby effectively decreasing vessel wall thrombogenicity following injury in vivo.

Blood coagulation is a process that occurs normally in an animal during bleeding, to prevent the loss of blood from the body and is assisted by two key players -blood platelets and coagulation factors particularly thrombin. When this process becomes extensive, it leads to thrombosis, a condition which can lead to severities such as cardiac failure, brain hemorrhage,renal failure and deep vein thrombosis. Anti-thrombotic drugs which are administered to combat thrombosis can be classified into three categories - anticoagulants, antiplatelet drugs (also known as blood thinners) and fibrinolytic drugs. The anticoagulant and antiplatelet drugs constitute the anti-thrombotic drugs which ensure the prevention of thrombosis whereas fibrinolytic drugs are administered after the thrombotic plaque is formed in order to dissolve the clot. The present day human society has been plagued by cardiovascular disease (also known as coronary heart disease) due to sedentary lifestyle and unhealthy food habits. The major fear in case of this disease is the blockage of the coronary artery by theatherosclerotic plaque, which then results in decreased blood supply to the cardiac tissue. When the plaque completely blocks the artery,owing to the blood pressure, the atherosclerotic plaque ruptures resulting in the formation of a thrombus (blood clot), which cuts off the blood supply to the cardiac tissue and can finally lead to a cardiac arrest. Therefore, anti-thrombotic drugs play a pivotal role in combating heart disease. Amongst these, the administration of anticoagulant and antiplatelet drugs is highly crucial to prevent the formation of the thrombus in patients suffering from heart disease. There have been many drugs in the market at present but they can cause side effects such as unwanted bleeding, allergy and non-specific reactions. Also, the current need is to ensure that the anti-thrombotic drugs administered do not hamper haemostasis, although they prevent blood coagulation. Hence, there is a constant need to discover newer anti-thrombotic drugs from unexplored microbial sources. This thesis describes the identification of compounds from endophytic fungi that inhibit thrombin and platelet aggregation. We have screened forty eight endophytic fungi isolated from the plants Datura metel and Cassia fistulafor thrombin inhibitory activity. Two isolates from D. metel -Dm 3.3, Dm 14.2 and three isolates from C. fistula - C2, C3, C4 tested positive for thrombin inhibitory activity. When each of these five endophytic fungal isolates was further tested for their potential to inhibit thrombin and platelet aggregation separately; Dm 3.3, C2 and C3 showed potent anti-thrombotic properties. The present thesis work describes the identification of anti-thrombotic metabolites of two of these three endophytic fungal isolates namely - Dm 3.3 and C2. Based on ITS sequence analysis, Dm 3.3 was identified taxonomically to be Colletotrichum gloeosporioides. The hexane extract of endophytic fungal isolate,C. gloeosporioides showed higher inhibition of purified thrombin (Sigma thrombin) and plasma thrombin as well as the potential to inhibit platelet aggregation induced by ADP, when compared to the ethyl acetate extract of the fungus. When the hexane extract of C. gloeosporioides was analysed by GC-MS, two major metabolite peaks were identified by the NIST library data -retinoic acid and 6β-hydroxytestosterone. There have been several reports on the presence of retinal in fungi such as Mucormucedo and have suggested its role as an intermediate in the biosynthesis of the morphogen, trisporic acid (a retinoid derivative) with a vital role in the sexual reproductive phase of fungi in inducing the formation of zygophores (gametes) from the mycelium. There have also been reports which speculate that the homologs of the enzyme RALDH (retinaldehyde dehydrogenase) in fungi could possibly convert retinal to retinoic acid. The presence of steroids in fungi has been extensively reported to play a key role to induce meiosis in the antheridials and oogonium of the fungus -Saprolegniaferax. Therefore, both retinoid compounds and steroids play a crucial role in the sporulation of fungi, thus justifying the presence of the two metabolites, retinoic acid and 6β-hydroxytestosterone, in C. gloeosporioides. Since, we had observed two major metabolite peaks in the hexane extract of C. gloeosporioides which corresponded to retinoic acid and 6β-hydroxytestosterone, we investigated the anti-thrombotic properties of both the standards - retinoic acid and 6β-hydroxytestosterone. We observed that retinoic acid, but not 6β-hydroxytestosterone, showed potent inhibition of both thrombin and platelet aggregation.The three forms of vitamin A namely - retinoic acid, retinal and retinol, showed potent inhibition of thrombin and platelet aggregation.Retinoic acid showed the maximum inhibition of thrombin (IC50values: Sigma thrombin – 67µg/ml; plasma thrombin – 49µg/ml), while retinol showed the maximum inhibition of platelet aggregation (97% inhibition at 120µg/ml). Therefore, we can infer that retinoic acid to be the active anti-thrombotic secondary metabolite present in C. gloeosporioides. In parallel to the work on the anti-thrombotic properties of the D. metel endophytic fungus –C. gloeosporioides (Dm 3.3), we have also investigated the thrombin and platelet aggregation inhibitory metabolites of C2, the endophytic fungus isolated from the plant Cassia fistula. The molecular taxonomic method of ITS PCR enabled us to identify the endophytic fungus C2 as Penicillium sclerotiorum. The ethyl acetate extract of the mycelium of Penicillium sclerotiorumwhen grown in potato dextrose broth showed the highest thrombin inhibitory activity. Hence, this extract was subjected to a bio-assay guided column chromatographic fractionation using silica gel column. Three fraction pools were obtained, each of which was found to possess a unique anti-thrombotic active principle (AP). Active principle 1 (AP1) was purified from fraction pool I by preparative TLC, while active principle 2 (AP2) was purified from fraction pool II by column chromatography (sephadex LH20 column followed by silica gel column) and preparative TLC. The compound AP1 showed inhibition of both Sigma thrombin (IC50 value – 58µg/ml) and plasma thrombin (IC50 value – 89µg/ml) as well as the inhibition of platelet aggregation (70% inhibition at 120µg/ml).AP2 showed inhibition of Sigma thrombin (IC50 value – 104µg/ml) and plasma thrombin (IC50 value – 107µg/ml) as well as the inhibition of platelet aggregation (41% inhibition at 120µg/ml). Amongst these metabolites, AP1 showed higher anti-thrombotic potential as compared to AP2. Thus, AP1 and AP2 are the two active metabolites purified from P. sclerotiorum which show potent anti-thrombotic properties. The structure elucidation of the active principles – AP1 and AP2 purified from the Cassia fistula endophytic fungus, Penicillium sclerotiorum, was performed using state-of-the art analytical techniques such as IR spectroscopy, UV spectroscopy, LC-MS, NMR spectroscopy and micro-analysis. The mass of AP1 was analysed by mass spectrometry to be 415 m/z and the structure characterization of this metabolite is awaited. The structure of AP2 was elucidated to be 5-(2,4-dinitrophenyl)hydrazono-2-bornanol, which has a mass of 390 m/z (as analysed by mass spectrometry) and belongs to the class of monoterpenoid derivatives called camphenes. Thus, the thesis work describes the identification of anti-thrombotic compounds from two endophytic fungi – Colletotrichum gloeosporioides and Penicillium sclerotiorum. Since these compounds have a microbial origin, there is a potential advantage to scale up their production by exploiting the tools of media optimization and environmental stress application. Structure of the anti-thrombotic secondary metabolites which were identified in this study

Two monoclonal antibodies (mAb's) against antithrombin III (ATIII) were characterized with respect to their ability to interfere with ATIII activity. AT III activity was measured by its ability to inhibit the amidolitic activity of thrombin on the substrate BCP-100. Incubation of 150 ng of ATIII with 28pg mAb A36R2 prior to addition of 50 ng thrombin totally abolishes the inhibitory effect of ATIII on thrombin. Incubation of 200ng of ATIII with 10 μg of mAb B26R4 prior to addition of 75 ng thrombin raises the inhibitory effects of ATIII from 37% to 100%. We examined the effect of these mAb's on binding of antithrombin III-thrombin (ATIII-Th) complexes to bovine corneal endothelial cells. 120 pg/ml mAb's are reacted with 2 μg/ml ATIII-Th complexes prior to their addition to the cells. mAb A36R2 completely blocks ATIII-Th complexes binding. In contrast, mAb B26R4 enhances binding up to 250% of the control binding.We conclude that mAb A36R2 prevents binding of thrombin to ATIII by recognizing an epitope on ATIII close to thrombin binding site or that its binding to ATIII induces a conformational change in the thrombin binding site thus it no longer recognizes thrombin. mAb B26R4 has a heparin-like effect on ATIII: Its binding to ATIII induces conformational changes which improve thrombin binding to ATIII. There is a correlation between inhibition and enhancement of thrombin binding to ATIII and of ATIII-Th complexes binding to cells by the two mAb's. These mAb's may provide a new tool to control the activity of ATIII and to identify the cellular binding site on the ATIII-Th complex.This research was supported by a grant from the National Council for Research and Development, Israel and G.S.F. München, Germany.

Binding of fibrinogen by thrombin was measured by inhibition of amidolysis of S2238 and found to be 12 μM. Upon digestion of fibrinogen with plasmin (0.16ugs/mg fibrinogen) for 4 hours at 37°C, thrombin binding activity remained in the supernatants upon heat treatment. The thrombin binding activity in the dialyzed supernatant reached a maximum after two hours coinciding with maximal release of B 1-42 and 45-39 KDa chain fragments. Measured immunologically, levels of fragment E at this time were 45% of the maximum generated after 4 hours digestion. FPA levels in the dialyzed supernatant (measured by RIA and HPLC) after thrombin treatment, were zero and did not increase until 1½ hours after the beginning of digestion, reaching a maximum at 4 hours. The thrombin binding activity generated was stable to further plasmin action. Upon gel chromatography of 2 and 4 hour supernatants, thrombin binding activity coincided closely with fragment E, measured immunologically. Further purification showed the fragment to have Ki for thrombin amidolytic activity of 0.5μM. The fragment also inhibited the thrombin clotting time of plasma but did not affect fibrin monomer polymerization.The fragment was susceptible to very slow inactivation by thrombin but not arvin (though it did inhibit arvin amidolytic activity). A thrombin binding (thrombin inhibitory) fragment is therefore generated during the early stages of f ibrinogenolysis and may be the result of protection by 45 and 39 KDa A α carboxy terminus fragments since E fragments generated in later stages (in the presence of 29 and 25 KDa fragments) do not have this property. These findings may give interesting new insight into thrombin/fibrinogen interaction.

MD-805 is a potent thrombin-inhibitor having the structure of tri-pods; Arg skeletone, N-terminal side and C-terminal side. MD-805 showed weaker inhibitory activity to other enzymes than thrombin. In this report, to gather more detailed informations about the structural features of serine enzymes concerning the specificity, we experimentally examined the structure-activity relationship (SAR) of a number of arginine derivatives including MD-805 and theoretically generated a MD-805-trypsin complex model using the results of X-ray crystallography of MD-805 and BPTI-trypsin complex by calculation in principle to minimize van der Waals contacts, and thus we discussed to interpret SAR from the molecular level. SAR of C-terminal side of arginine derivatives was obtained with the inhibitory activity to trypsin, plasmin, and glandular kallikrein and compared with the previous results of thrombin, the followings being indicated: (1) The hydrophobic binding pocket (HBP), which was reported by us to be at least partly similar in stereogeometry between trypsin and thrombin, had the depth corresponded to the length of ethylpiperidine, (2) concerning the site (termed the P site) next to HBP, there were large differences in stereogeometry between trypsin and thrombin; the P site of trypsin could accept propyl and phenyl group attached to 4-position of piperidine, while that of thrombin was unable to accept them and (3) the P sites of plasmin and glandular kallikrein resembled that of trypsin in being able to accept phenyl group. MD-805-trypsin complex model supported the reasonable understanding that the stereogeometrical similarity in HBP between thrombin and trypsin was attributable to the high homology in amino acid sequences in Ser-195 loop and that the dissimilarity in the P sites between thrombin and the others was attributable to 9 amino acids insertion found only in thrombin (Loop B). Furthermore, many dansylarginine derivatives showed very strong inhibition for pseudocholinesterase, however, SAR for C-terminal side of these derivatives revealed the similarity and dissimilarity in HBP and the P site between pseudocholinesterase and the proteases described above.

The dependence of the anticoagulant properties of heparin upon charge density may reflect structural factors that are important in anti-thrombin effect. We have previously demonstrated that in the absence of antithrombin III (AT III) unfractionated heparin inhibits the catalytic effect of thrombin upon platelet activation. In the present study we evaluated the thrombin-binding properties of heparin fractions obtained by ion-exchange chromatography on DEAE-Sephacei. We found that these fractions were able to bind to thrombin with an affinity that increased with their charge density. This was shown by their inhibitory effect in the absence of AT III on thrombin-catalyzed platelet factor Va formation and by the ability of active site blocked thrombin to prevent the heparin-dependent inactivation of thrombin by AT III. However, their increase in charge density and thus affinity for thrombin was found to go along with an increase in AT III-binding sites, as measured by the heparin-dependent increase of the intrinsic fluorescence of AT III. Moreover all heparin fractions showed the same specific antithrombin activity when the molar concentration of AT III-binding heparin was taken into account. We also investigated the thrombin-binding properties of two heparin fractions obtained by affinity chromatography on AT III-Sepharose. The AT III low affinity fraction was practically devoid of any inhibitory effect on the rate of the thrombin-catalyzed factor Va formation, indicating a low, if any, affinity for thrombin. In contrast the AT III-independent inhibition of thrombin was completely recovered from the AT III high affinity fraction. In addition, we also established that when the heparin fraction from the DEAE-Sephacel column, with the lowest charge density and very low in AT III binding material, was modified by the incorporation of sulfate groups so as to achieve a higher charge density, it obtained a higher affinity for thrombin but this modification caused the loss of half the AT III binding sites. In conclusion, it is apparent that fractionation of crude heparin on a DEAE-Sephacel column or on an AT III-Sepharose column does not result exclusively in a separation of either the thrombin-binding or the AT III-binding heparin fractions.

It is now widely recognized that the activation of phospholipase C by specific agonists leads to the formation of two second messengers: (1) inositol trisphosphate, which releases Ca2+ from the endoplasmic reticulum to the cytosol and (2) 1,2- diacylglycerol, which stimulates protein kinase C. In the past few years, GTP-binding proteins have been associated with the regulation of phospholipase C. However, the identity of the GTP-binding protein involved and the type of association with phospholipase C is not yet known. It is now recognized that there are two types of phospholipase C enzymes: (a) a soluble enzyme that has been characterized in several tissues and does not preferentially hydrolyze polyphospholinositides and (b) membrane-bound enzymes that are coupled to the receptors, specifically hydrolyzing polyphosphoinositides and activated by membrane guanine nucleotide-binding proteins. Recent reports have tried to assess the involvement of GTP-binding proteins in the agonist-induced stimulation of phospholipase C, and various related aspects have been reported. These are concerned with: (a) detection of various GTP-binding proteins in platelets, (b) the effects of known inhibitors of GTP-binding proteins such as GDPgS or pertussis toxin on the agonist-induced stimulation of phospholipase C, (c) the direct effects of stimulators of GTP-binding proteins such as GTP, GTP-analogs and fluoride on phospholipase C activity, (d) the possible association of GTP-binding proteins to cytosolic phospholipase C that would then lead to degradation of the membrane-bound inositides and (e) cytosolic phospholipase C response to the activation of cell surface receptors. The emerging information has had contradictory conclusions. (1) Pretreatment of saponin-permeabilized platelets with pertussis toxin has been shown to enhance and to inhibit the thrombin-induced activation of phospholipase C. Therefore, it is not clear if a G protein that is affected by pertussis toxin in a manner similar to Gi or Go plays a central role in activation of phospholipase C. (2) Studies on the effect of GDPβ;S are also conflicting indicating that there may be GTP-independent and/or -dependent pathways for the activation of phosphoinositide hydrolysis. (3) A cytosolic phospholipase C is activated by GTP, and it has been advanced that this activity might trigger the hydrolysis of membrane-bound inositides. A cytosolic GTP-binding protein might be involved in this action, and it is speculated that an α-subunit might be released to the cytoplasm by a receptor-coupled mechanism to activate phospholipase C. However, no direct evidence exists to support this conclusion. Moreover, the exact contribution of phospholipase C from the membranes or the cytosol to inositide hydrolysis in response to cellular agonists and the relationship of those activites to membrane-bound or soluble GTP-binding proteins are unknown. Our results indicate that the stimulation of phospholipase C in platelets by GDPβS and thrombin are affected differently by GDPβS. GDPgSinhibits the formation of inositol phosphates produced by GTPγS but not that induced by thrombin. Thrombin, therefore, can directly stimulate phospholipase C without the involvement of a “stimulatory” GTP-binding protein, such as Gs, for the agonist stimulation of adenylate cyclase. However, an “inhibitory” GTP-binding protein might have some influence on thrombin-stimulated phospholipase C, since in the presence of GDPγS thrombin produces a more profound stimulation of phospholipase C.This “inhibitory” GTP-binding protein might be ADP-ribosylated by pertussis toxin because pertussis toxin can also enhance thrombin action on phospholipase C activity. Therefore, phospholipase C that responds to thrombin could be different from the one that responds to GTPγS. Cytosolic phospholipase C can be activated by GTP or GTP analogs, and the one that responds to thrombin should be coupled to the receptors present in the plasma membrane. The initial action of thrombin is to directly activate the plasma membrane-bound phospholipase C and the mechanism of this activation is probably related to the proteolytic action of thrombin or the activation of platelet proteases by thrombin. In agreement with this, trypsin can also directly activate platelet phospholipase C and, subsequently, GTPyS produces further activation of phospholipase C. If these two mechanisms are operative in platelets, the inhibition of cytosolic phospholipase C by GDPβS would allow a larger fraction of inositides for degradation of the thrombin-stimulated phospholipase C, as our results show.

Activation of platelets by agonists acting via cell surface-located receptors apparently involves as an early event in transmembrane signalling an interaction of the agonist-occupied receptor with a guanine nucleotide-binding regulatory protein (G-protein). The activated G-protein, then, transduces the information to the effector molecule, being responsible for the changes in intracellular second messengers. At least two changes in intracellular signal molecules are often found to be associated with platelet activation by agonists, i.e., increases in inositol trisphosphate and diacylglycerol levels caused by activation of a polyphosphoinositide-specific phospholipase C and decrease in cyclic AMP concentration caused by inhibition of adenylate cyclase.Both actions of platelet-activating agents apparently involve G-proteins as transducing elements. Generally, the function of a G-protein in signal transduction can be measured either by its ability to regulate the activity of the effector molecule (phospholipase C or adenylate cyclase) or the binding affinity of an agonist to its specific receptor or by the abitlity of the G-protein to bind and hydrolyze GTP or one of its analogs in response to agonist-activated receptors. Some platelet-activating agonists (e.g. thrombin) can cause both adenylate cyclase inhibition and phospholipase C activation, whereas others induce either inhibition of adenylate cyclase (e.g. α2-adrenoceptor agonists) or activation of phospholipase C (e.g. stable endoperoxide analogs) . It is not yet known whether the simultaneous activation of two signal transduction systems is due to activation of two separate G-proteins by one receptor, to two distinct receptors activating each a distinct G-protein or to activation of two effector molecules by one G-protein.For some of the G-proteins, rather specific compounds are available causing inactivation of their function. In comparison to Gs, the stimulatory G-protein of the adenylate cyclase system, the adenylate cyclase inhibitory Gi-protein is rather specifically inactivated by ADP-ribosylation of its a-subunit by pertussis toxin, “unfortunately” not acting in intact platelets, and by SH-group reactive agents such as N-ethylmaleimide and diamide, apparently also affecting the Giα-subunit. Both of these treatments completely block α2-adrenoceptor-induced GTPase stimulation and adenylate cyclase inhibition and also thrombin-induced inhibition of adenylate cyclase. In order to know whether the G-protein coupling receptors to phospholipase C is similar to or different from the Gi-protein, high affinity GTPase stimulation by agents known to activate phospholipase C was evaluated in platelet membranes. The data obtained indicated that GTPase stimulation by agents causing both adenylate cyclase inhibition and phospholipase C activation is reduced, but only partially, by the above mentioned Gi-inactivating agents, while stimulation of GTPase by agents stimulating only phospholipase C is not affected by these treatments. These data suggested that the G-protein regulating phospholipase C activity in platelet membranes is different from the Gi-protein and may also not be a substrate for pertussis toxin. Measuring thrombin stimulation of inositol phosphate and diacylglycerol formation in saponin-permeabilized platelets, apparently contradictory data were reported after pertussis toxin treatment, being without effect or causing even an increase in thrombin stimulation of inositol phosphate formation (Lapetina: BBA 884, 219, 1986) or being inhibitory to thrombin stimulation of diacylglycerol formation (Brass et al.: JBC 261, 16838, 1986). These data indicate that the nature of the phospholipase C-related G-protein(s) is not yet defined and that their elucidation requires more specific tools as well as purification and reconstitution experiments. Preliminary data suggest that some antibiotics may serve as useful tools to characterize the phospho-lipase-related G-proteins. The possible role of G-protein phosphorylation by intracellular signal molecule-activated protein kinases in attenuation of signal transduction in platelets will be discussed.

A chemically synthesized heparin pentasaccharide (PS) (Instir tut Choay, Paris, France) has been reported to exhibit an antithrombotic action in a rabbit stasis induced thrombosis model, in an IV dose range of 25-200 ug/kg (0.5 to 5 ug/ml circulating concentrations) . Ex vivo plasma analysis from treated animals revealed expected angi-factor Xa activity byboth amidolytic and coagulant (Heptest®) methods. Nodirect inhibitory effect against factor IIa by amidolytic or coagulant methods was observed. Global anticoagulant activities were not found by PT and APTT methods; however, a hypocoagulable thrombelastographic pattern was demonstrated for native whole blood. Platelet activation remained unaffected at the antithrombotic dosages of PS. An attempt was made to more specifically elucidate the anti-factor Xa mediated antithrombotic mechanism of action of PS. The effect of this agent was studied in several thrombin generation assays in human and rabbit plasmas supplemented in vitro with a 0-5 ug/ml concentrations of PS. The following systems were used: 1) activated prothrombin complex (FEIBA®) , thromboplastin-Ca+2/synthetic substrate; 2) prothrombin complex concentrate (Konyne), thrombo- plastin-Ca+2/synthetic substrate; 3) cephalin-ellagic acid-Ca+2/ synthetic substrate (modified Fischer method); 4)cephalin-Ca / synthetic substrate (modified Ofosu method); 5) FEIBA /FPA re- dioimmunoassay; 6)whole blood prothrombin consumption/clot; and 7) cephalin-Ca+2/clot (Hicks-Pitney method). All assays produced a concentration-dependent effect to a 35-50% inhibition of generated thrombin with 4-5 ug/ml PS concentrations. Assay #4 also revealed that although PS inhibited the generation of thrombin, the thrombin that was generated was inhibitedby natural plasma proteins at normal kinetic rates. In ex vivo studies similar concentration-dependent inhibition of thrombin generation was observed using assays #3 and #4. These results indicate that the inhibition of induced venous stasis thrombosis is associated with the inhibition of thrombin generation byPS without a direct inhibition of pre-formed thrombin. Furthermore, these results indicate that thrombotic events may be controlled at pre-thrombin coagulation stages by agents with sole anti-factor Xa activity.

We have previously proposed that the steps in coagulation most sensitive to inhibition by heparin are the thrombin-dependent activation of factor V and factor VIII. This observation was based on the demonstration that therapeutic concentrations of heparin or 1μM of the thrombin specific inhibitor, phe-pro-arg CH2Cl (PPACK) completely inhibited the activation of prothrombin when contact-activated plasma (CAP) was recalcified for up to 1 min. Under similar conditions, heparin and PPACK only partially inhibited the activation of factor X. Moreover, the addition of thrombin (lOnM) to CAP 1 min before that of heparin or PPACK reversed their inhibitory effects. We now provide further support for our hypothesis by showing that when the activity of thrombin is suppressed by heparin or PPACK, efficient activation of radiolabelled prothrombin occurs only when the factor Xa then present activates factor V and factor VIII. We compared the effects of HEP of PPACK on the following four systems for initiating the activation of prothrombin: (1) CAP; (2) CAP + lOnM thrombin; (3) CAP + InM Xa and (4) unactivated plasma + InM Xa + InM Va + coagulant phospholipids. In each system, the enzymes were added 1 min before the heparin or PPACK. In the absence of heparin or PPACK, all four systems generated the same amount of thrombin activity in 45s. Complete inhibition of prothrombin activation by heparin and PPACK was observed only in system 1 which did not contain exogenous thrombin or factor Xa. No inhibition by heparin or PPACK was observed when thrombin or factor Xa was added to CAP in systems (2) and (3). Only partial inhibition was observed in system (4) which contained exogenous prothrombi-nase complex. Factor Xa thus provides an effective by-pass mechanism for the activation of factor VIII and factor V in plasma containing therapeutic concentrations of heparin. Our data provide further evidence that the heparin-antithrombin III system is not effective in inactivating factor Xa. These results support the hypothesis that in unactivated normal plasma, the primary anticoagulant effect of heparin is the inhibition of the thrombin-dependent activation of factor V and factor VIII.

Blood coagulation Factor XIIla (FXIIIa) is a thrombin activated transglutaminase (TG) that is involved in the final step of fibrin stabilization. FXIIIa inhibits fibrinolysis by crosslinking α-2-plasmin inhibitor (α-2-PI) to fibrin. A thrombin-independent TG has been identified in vascular cells and tissues -from human, rabbit, rat, porcine and bovine sources. The vascular TG had several properties similar to the well characterized guinea pig liver TG. Both enzymes had similar molecular weights (80-90 kDa) and similar chromatographic and electrophoretic properties. Both enzymes preferentially crosslinked α-chains of fibrinogen and their TG activities were independent of thrombin treatment. Finally, both enzymes reacted with polyclonal and monoclonal antibodies to guinea pig liver TG. However, the TG from cultured adult bovine aortic endothelial (ABAE) cells exhibited a novel Ca++/Mg++ dependence for enzymatic activity which was distinct from purified liver TG. TG from confluent ABAE cells and rabbit vascular smooth muscle cells had between 4-7 fold higher TG activity compared to rapidly dividing (nonconfluent) cells -from the same passage. The difference in activity was not due to enhanced degradation of TG catalyzed isopeptide bonds by nonconfluent cells Upon examination by immunoblots using anti-TG antibodies, the TG antigen in nonconfluent cells appeared extensively degraded. Furthermore, guanosine-5'-triphosphate (GTP) was nearly 3-fold more inhibitory to TG from confluent cells compared to nonconfluent cells. Proteases, GTP and divalent cation levels may be modulating intracellular TG activity. The TG antigen detected by imm-unohistochemical techniques was predominantly associated with endothelial and smooth muscle cells of arteries, veins, venules and capillaries. TG antigen also codistributed with fibronectin antigen along the hepatic sinusoids. The ABAE cell TG crosslinked α-2-PI to fibrinogen. The modified fibrinogen was 40-fold more resistant to plasminolysis compared to unmodified fibrinogen. In conclusion, the presence of a thrombin-independent TG in blood vessels may provide an alternate pathway to inhibit fibrinolysis and promote clot stabilization.

A novel anti-platelet agent, 4-cyano-5,5-bis(4-methoxy-phenyl)-4-pentenoic acid(E-5510), has been shown to inhibit platelet aggregation and secretion induced by thrombin as well as by other inducers such as ADP, collagen and PAF. Although E-5510 can act as a cyclooxygenase inhibitor,inhibition of cyclooxygenase may not be the primary mode of action since this compound effectively inhibits thrombin-induced platelet activation. In this paper, effects^of E-5510 on arachidonic acid (AA) metabolism, intracellular Ca++ and cAMP in human platelets are examined.(1) Effect on AA metabolism. Platelets prelabeled with [14 C]-AA were stimulated with thrombin. E-5510 inhibited not only thromboxane A2 and HHT generation but also 12-HETE generation in a dose-dependent fashion. The total AA released was also reduced by E-5510. An almost 50% reduction was obtained by 10 uM of this compound. On the other hand, a cyclooxygenase inhibitor such as U-53059 increased 12-HETE generation in a dose-dependent fashion. In addition to the inhibition of AA metabolism, E-5510 exerted inhibitory effects on phosphatidic acidgeneration, which suggests the possible inhibition of phospholipase C activity by this compound.(2) Effect on intracellular Ca++ and protein phosphorylation. Intracellular Ca++ mobilization was examined using Fura-2 loaded human platelet suspension. The increase in intracellular Ca induced by thrombin was inhibited by E-5510 and the increase in phosphorylation of 40 K protein was also suppressed by this compound after the stimulation of human platelets by thrombin.(3) Effect on cAMP. Platelets were incubated with 10-100 uM of E-5510 and the cAMP content in human platelets was measured. E-5510 increased the cAMP content in a dose-dependent fashion. In platelet homogenate, E-5510 inhibited phosphodiesterase activity with an IC50 of 10 uM.These results suggest that E-5510 may inhibit platelet aggregation and secretion through the multiple modes of action, such as inhibition of phospholipase C, phosphodiesterase and cyclooxygenase, in the process of platelet activation.