The nature of the binding for prothrombinase at the platelet surface as revealed by lipolytic enzymes

Differential Roles for the Coagulation Factors XI and XII in Regulating the Physical Biology of Fibrin

In the contact activation pathway of the coagulation, zymogen factor XII (FXII) is converted to FXIIa, which triggers activation of FXI leading to the activation of FIX and subsequent thrombin generation and fibrin formation. Feedback activation of FXI by thrombin has been shown to promote thrombin generation in a FXII-independent manner and FXIIa can bypass FXI to directly activate FX and prothrombin in the presence of highly negatively charged molecules, such as long-chain polyphosphates (LC polyP). We sought to determine whether activation of FXII or FXI differentially regulate the physical biology of fibrin formation. Fibrin formation was initiated with tissue factor, ellagic acid (EA), or LC polyP in the presence of inhibitors of FXI and FXII. Our data demonstrated that inhibition of FXI decreased the rate of fibrin formation and fiber network density, and increased the fibrin network strength and rate of fibrinolysis when gelation was initiated via the contact activation pathway with EA. FXII inhibition decreased the fibrin formation and fibrin density, and increased the fibrinolysis rate only when fibrin formation was initiated via the contact activation pathway with LC polyP. Overall, we demonstrate that inhibition of FXI and FXII distinctly alter the biophysical properties of fibrin.

Collagen can selectively trigger a platelet secretory phenotype via glycoprotein VI

Platelets are not only central actors of hemostasis and thrombosis but also of other processes including inflammation, angiogenesis, and tissue regeneration. Accumulating evidence indicates that these “non classical” functions of platelets do not necessarily rely on their well-known ability to form thrombi upon activation. This suggests the existence of non-thrombotic alternative states of platelets activation. We investigated this possibility through dose-response analysis of thrombin- and collagen-induced changes in platelet phenotype, with regards to morphological and functional markers of platelet activation including shape change, aggregation, P-selectin and phosphatidylserine surface expression, integrin activation, and release of soluble factors. We show that collagen at low dose (0.25 µg/mL) selectively triggers a platelet secretory phenotype characterized by the release of dense- and alpha granule-derived soluble factors without causing any of the other major platelet changes that usually accompany thrombus formation. Using a blocking antibody to glycoprotein VI (GPVI), we further show that this response is mediated by GPVI. Taken together, our results show that platelet activation goes beyond the mechanisms leading to platelet aggregation and also includes alternative platelet phenotypes that might contribute to their thrombus-independent functions.

Superactivated Platelets

Platelets contribute to hemostasis by forming the platelet plug and then contributing to coagulation by providing a catalytic surface where thrombin generation occurs efficiently. This catalytic activity, known as the platelet procoagulant response, is being recognized as a nuanced response. This review examines platelets’ response to strong stimuli, which results in the formation of a platelet subpopulation (superactivated platelets) with several unique properties, including enhanced procoagulant activity. These platelets contribute uniquely to thrombus architecture and seem to have thrombus regulatory activity. Superactivated platelets’ role in diseases of thrombosis and hemostasis, as either potentiating or mitigating factors, is not currently known, but may be an important pharmacological target.

Effects of plasma dilution on tissue-factor-induced thrombin generation and thromboelastography: partly compensating role of platelets

BACKGROUND

Bleeding upon major surgery or severe trauma is treated by transfusion with crystalloids, colloids, or plasma. This treatment, however, can lead to dilutional coagulopathy and impaired hemostasis. We investigated the suitability of two integrative coagulation tests to measure the hemostatic activity of diluted plasma.

STUDY DESIGN AND METHODS

Plasma from healthy donors was diluted in vitro with saline or colloid (venofundin or gelofusin). Coagulant activity in response to tissue factor was monitored by calibrated automated thrombin (CAT) generation and rotational thromboelastography (TEG), detecting formation of elastic fibrin clots. Plasma from patients receiving fluid infusion during coronary artery bypass grafting (CABG) was analyzed with the same assays.

RESULTS

Optimal activity of CAT and TEG assays required the presence of 10 pmol per L tissue factor and 4 micromol per L phospholipid vesicles or 100 x 10(9) platelets (PLTs) per L. Strikingly, thrombin generation and clot formation became impaired at a higher extent of dilution with PLTs present (< or =40% plasma) than with phospholipid vesicles present (< or =60% plasma). Colloids aggravated the dilution effect on clot formation, but FFP antagonized the dilution effect on thrombin and clot formation. In contrast, fibrinogen and Factor (F)XIII only restored the impaired clot formation. In plasma samples from patients undergoing CABG, CAT and TEG assay variables were altered to an extent corresponding with the volume of fluid infusion.

CONCLUSION

Thrombin generation and clot formation are reduced at a plasma dilution of more than 40 percent. In either process, PLTs can partly compensate for the dilution effect. In vitro dilution with colloids impaired fibrin clot elasticity compared to saline.

The protein kinase C inhibitor RO318220 potentiates thrombin-stimulated platelet-supported prothrombinase activity

Prothrombinase activity was tested on thrombin- and SFLLRN-activated platelets treated with RO318220, a potent inhibitor of protein kinase C. RO318220 completely inhibited platelet dense and alpha-granule secretion at a concentration of 20 microM but had no effect on prothrombinase activity in the presence of excess factor Va (20 nM). This indicates that protein kinase C activity and agonist-initiated secretion are not necessary for the development of a procoagulant surface. Treatment with 75 to 150 microM RO318220 potentiated platelet-supported thrombin generation up to 280% of control platelets with no change in Kd appFXa. Treated with increasing concentrations of RO318220, an increasing proportion of thrombin-stimulated platelets bound annexin V with decreasing binding sites per platelet. A lower mean forward scatter (FSC-H) of platelets treated with RO318220 suggested platelet vesiculation as a result of RO318220 treatment; however, 100 microM calpeptin pretreatment eliminated the decrease in FSC-H without affecting either the increase in platelets positive for annexin V binding, the decrease in binding sites per platelet, or the 3-fold increase in prothrombinase activity. Thus, RO318220 appears to increase prothrombinase activity by increasing platelet responsiveness to thrombin rather than by inducing platelet vesiculation. This suggests that RO318220 inhibits a signaling molecule within a negative regulatory pathway that governs platelet procoagulant surface changes.

Platelets and thrombin generation

This review examines the evidence that platelets play a major role in localizing and controlling the burst of thrombin generation leading to fibrin clot formation. From the first functional description of platelets, it has been recognized that platelets supply factors that support the activation of prothrombin. Studies have demonstrated that on activation, the amount of one specific lipid, phosphatidylserine, is significantly increased on the outer leaflet of platelet membranes. When it was found that phosphatidylserine containing lipid extracts could be substituted for platelets in clotting assays, this suggested the possibility that changes in platelet lipid composition were necessary and sufficient to account for platelet surface thrombin generation. Because a growing body of data suggest that platelet-binding proteins provide much of the specificity for platelet thrombin generation, we review in this report data suggesting that changes in lipid composition are necessary but not sufficient to account for platelet surface regulation of thrombin generation. Also, we review data suggesting that platelets from different individuals differ in their capacity to generate thrombin, whereas platelets from a single subject support thrombin generation in a reproducible manner. Individual differences in platelet thrombin generation might be accounted for by differences in platelet-binding proteins.

Mechanism of factor Va inactivation by plasmin. Loss of A2 and A3 domains from a Ca2+-dependent complex of fragments bound to phospholipid

The coagulation cofactor Va (FVa) is a noncovalent heterodimer consisting of a heavy chain (FVaH) and a light chain (FVaL). Previously, the fibrinolytic effector plasmin (Pn) has been shown to inhibit FVa function. To understand this mechanism, the fragmentation profile of human FVa by Pn and the noncovalent association of the derived fragments were determined in the presence of Ca(2+) using anionic phospholipid (aPL)-coated microtiter wells and large (1 microm) aPL micelles as affinity matrices. Following Pn inactivation of aPL-bound FVa, a total of 16 fragments were observed and their NH(2) termini sequenced. These had apparent molecular weights and starting residues as follows (single letter abbreviation is used): 50(L1766), 48(L1766), 43(Q1828), 40(Q1828), 30(S1546), 12(T1657), and 7(S1546) kDa from FVaL; and 65(A1), 50(A1), 45(A1), 34(S349), 30(L94), 30(M110), and 3 small <5(W457, W457, and K365) kDa from FVaH. Of these, 50(L1766), 48(1766), 43(Q1828), and 40(Q1828) spanning the C1/C2 domains, and 30(L94), but not the similar 30(M110), positioned within the A1 domain remained associated with aPL. These were detected antigenically during Pn- or tissue plasminogen activator-mediated lysis of fibrin clot formed in plasma. Chelation by EDTA dissociated the 30(L94)-kDa fragment, which was observed to associate with intact FVaL upon recalcification, indicating that the Leu-94 to Lys-109 region of the A1 domain plays a critical role in the FVaL and FVaH Ca(2+)-dependent association. By using domain-specific monoclonal antibodies and an assay for thrombin generation, loss of FVa prothrombinase function was coincident with proteolysis at sites in the A2 and A3 domains resulting in their dissociation. Inactivation of FV or FVa by Pn was independent of the thrombophilic R506Q mutation. These results identify the molecular composition of Pn-cleaved FVa that remains bound to membrane as largely A1-C1/C2 in the presence of Ca(2+) and suggest that Pn inhibits FVa by a process involving A2 and A3 domain dissociation.

Impaired redistribution of aminophospholipids with distinctive cell shape change during Ca2+-induced activation of platelets from a patient with Scott syndrome

We have investigated phospholipid redistribution, membrane vesicle shedding, shape change, and granule release following A23187 activation of platelets from a patient with Scott syndrome, characterized by impaired transmembrane migration of phosphatidylserine (PS) accompanied by haemorrhagic complications, and two of her children. Electron spin resonance spectroscopy measurement of phospholipids redistribution showed that the internalization of PS was unaffected by the disorder but, after activation, PS exposure was significantly reduced in platelets from the homozygous-type patient. Vesicle shedding was also reduced in these platelets. However, the slow redistribution of phosphatidylcholine was similar to that observed in normal platelets. When treated with calpeptin, platelets from the homozygous-type patient, unlike normal or heterozygous Scott syndrome platelets, showed a smoothly rounded shape without filopods after activation. Following A23187 activation of normal platelets, filopod formation was consecutive to the re-exposition of aminophospholipids on the outer leaflet of the plasma membrane, and the existence of a floppase (outward aminoPLs translocase) has been suggested. In homozygous Scott syndrome platelets the deficiency in PS re-exposition, the absence of filopod formation, and low vesicle shedding are correlated with each other, and argue in favour of a disruption of the proposed floppase activity.

Activation of the alternative pathway of complement by calcium-loaded erythrocytes resulting from loss of membrane phospholipid asymmetry

The aminophospholipids phosphatidylserine (PS) and phosphatidylethanolamine (PE) are exposed on the outer membrane leaflet of deoxygenated and irreversibly sickled erythrocytes and senescent normal cells. PS exposure on erythrocytes results in the expression of procoagulant activity for the conversion of prothrombin to thrombin. Because liposomes or vesicles composed of aminophospholipids can activate the alternative pathway of complement, the possibility that increased exposure of PS and PE on intact erythrocytes would also make them capable of activating the alternative pathway was examined. Loss of normal membrane phospholipid asymmetry was induced by incubation of erythrocytes with calcium (Ca2+) and the calcium ionophore A23187. PS exposure on 60% of erythrocytes was confirmed by binding of fluorescein isothiocyanate-conjugated annexin V. Expression of procoagulant activity, measured with the Russell's viper venom clotting assay, was significantly increased on the Ca2+/A23187-treated erythrocytes. In addition, the erythrocytes became capable of activating the alternative pathway of complement, as judged by an increase in cell-bound C3b after incubation with serum and a decrease in alternative pathway hemolytic activity of the serum. The effect could be reversed by incubation of the Ca2+/A23187-treated erythrocytes under conditions that induced recovery of normal membrane phospholipid asymmetry. In contrast, tetrathionate-treated erythrocytes showed no increase in binding of annexin V and no procoagulant activity and failed to activate the alternative pathway of complement. These findings demonstrate that loss of phospholipid asymmetry in erythrocytes not only results in expression of procoagulant activity but also renders the cells capable of activating the alternative pathway of complement.

Thrombin Generation by Apoptotic Vascular Smooth Muscle Cells

Thrombin activation requires assembly of a prothrombinase complex of activated coagulation factors on an anionic phospholipid surface, classically provided by activated platelets. We have previously shown that anionic phosphatidylserine is exposed by rat vascular smooth muscle cells (VSMCs) undergoing apoptosis after serum withdrawal. In this study, using a chromogenic assay, we have shown thrombin generation by apoptotic VSMCs expressing c-myc (VSMC-myc) with an area under the thrombin-generation curve (AUC) of 305 ± 17 nmol⋅min/L and a peak thrombin (PT) of 154 ± 9 nmol/L. The thrombin-generating potential of the apoptotic VSMC-myc cells was greater than that of unactivated platelets (P = .003 for AUC; P = .0002 for PT) and similar to calcium-ionophore activated platelets (AUC of 332 ± 15 nmol⋅min/L, P = .3; PT of 172 ± 8 nmol/L, P = .2). Thrombin activation was also seen with apoptotic human VSMCs (AUC of 211 ± 8 nmol⋅min/L; PT of 103 ± 4 nmol/L) and was inhibited by annexin V (P < .0001 for AUC and PT). VSMC-myc cells maintained in serum generated less thrombin than after serum withdrawal (P = .0002 for AUC and PT). VSMCs derived from human coronary atherosclerotic plaques that apoptose even in serum also generated thrombin (AUC of 260 ± 2 nmol⋅min/L; PT of 128 ± 4 nmol/L). We conclude that apoptotic VSMCs possess a significant thrombin-generating capacity secondary to phosphatidylserine exposure. Apoptotic cells within atherosclerotic plaques may allow local thrombin activation, thereby contributing to disease progression.

Phospholipid asymmetry in red blood cells and spectrin-free vesicles during prolonged storage

Erythrocytes and spectrin-free DMPC-induced vesicles released from the cells were incubated for 3 weeks at 6 degrees C under conditions of metabolic ATP-depletion. Phosphatidylserine (PS) asymmetry was monitored during this period by use of the prothrombinase assay. Prothrombinase activities measured at the beginning of the incubation period indicated that approximately 0.06% of PS was located at the outer layer of the red cell membrane, whereas in DMPC-induced vesicles approximately 1.5% the PS was exposed on the outside. After completion of the incubation period PS exposure on the outside of red cells and vesicles was increased by no more than 5-fold. On the other hand, with vesicles prepared with a significantly increased (4-fold) ATP-content to sustain translocase activity, the incubation process resulted in a surprisingly high (20-fold) increase of PS exposure. With vanadate, an inhibitor of the aminophospholipid translocase, included in the incubation medium, the redistribution of PS was even more pronounced. These observations indicate that PS asymmetry in spectrin-free vesicles can not be directly correlated to either ATP content or translocase activity and suggest that besides the aminophospholipid translocase and the membrane skeleton, other mechanisms must be involved in maintaining phospholipid asymmetry.

Membrane phospholipid asymmetry in DMPC-induced human red cell vesicles

Vesicles that do not contain spectrin were released from human erythrocytes by incubation with dimyristoylphosphatidylcholine. The transbilayer orientation of membrane phospholipids was subsequently determined by two independent methods. Incubation with phospholipase A2 revealed that the phospholipid asymmetry observed in red blood cells was essentially preserved in vesicles. By use of the prothrombinase assay a still highly asymmetric distribution of phosphatidylserine could be demonstrated in spite of its slightly increased exposure on the vesicle surface. These results show that membrane phospholipid asymmetry can be maintained in a system that does not contain an intact membrane skeleton or spectrin.

Function of intracellular [Ca2+]i in exocytosis and transbilayer movement in human platelets surface-labeled with the fluorescent probe 1-(4-trimethylammonio)phenyl)-6-phenyl-1,3,5-hexatriene

Ellipsometry indicated that 1-(4-(trimethylammonio)phenyl-6-phenylhexa-1,3,5-triene (TMA-DPH) bound to platelets in a reversible and saturable way. Accordingly, the fluorescence intensity (F) of a suspension of TMA-DPH-labeled platelets was described as a quantity, determined by the amount of TMA-DPH bound to the platelet surface. Most platelet activators elevated F to a degree that correlate well with the secretion of serotonin evoked by these activators. The increase in F levels reflected the increase in outer membrane surface area following exocytosis. However, activators that evoked prolonged (> 2.5 min) and strong (> 600 nM) elevations of cytosolic [Ca2+]i increased F to levels that were much higher than expected from the maximal increase in surface area due to exocytosis. This high increase in F was caused by inward transbilayer movement of TMA-DPH over the plasma membrane and the subsequent labeling of cytosolic membrane sides. The kinetics of exocytosis and changes in cytosolic [Ca2+]i were studied by stopped-flow mixing of platelets with agonist. Thrombin-induced exocytosis had a delay of only 3 s, which was shortened when external CaCl2 or ADP was present. This correlated well with a faster rise in [Ca2+]i in the presence of CaCl2 or ADP, indicating that exocytosis was linked in time to elevation of [Ca2+]i. By itself, ADP was unable to evoke exocytosis and it elicited a [Ca2+]i transient of much shorter duration than thrombin, but with similar maximum. We concluded that both exocytosis and transbilayer movement were associated with elevation of [Ca2+]i: exocytosis required a moderate, relatively prolonged rise and transbilayer movement was accompanied by a stronger rise of even longer duration. Influx of external Ca2+ was essential for transbilayer movement, but not for exocytosis.

The effect of virus infection on the adherence of leukocytes or platelets to endothelial cells

It has been reported that atherosclerotic lesions contain genomic material belonging to members of the herpes family. This suggests that latent viral infection may be one of the atherogenic triggers. In this study we show that early infection of endothelial cell monolayers with Herpes Simplex virus type 1 (HSV-1) or Cytomegalovirus (CMV) results in an increased monocyte (MC) and polymorphonuclear leukocyte (PMN) adherence, but not in an increased platelet adhesion. Further, is demonstrated that MC and PMN respond differently to virus infected endothelial cell monolayers: PMN adhesion to CMV infected cells is approximately 430% of the control adherence, while the MC adherence is increased to 160%. Also, a difference in virus acting is observed: the adherence of MC or PMN to HSV-1 infected endothelial cells is caused by a secreted adherence promoting factor, while the adherence of MC or PMN to CMV infected endothelial cells seems to be a cell-bound phenomenon. In addition, it was demonstrated that the augmentation of MC or PMN adherence to virus infected endothelial cells is sensitive to tunicamycin, suggesting that both virus infections induce the expression of glycoproteins on the endothelial cell membrane, which is responsible for the MC and PMN adhesion. Thus, HSV-1 and CMV infection of endothelium results in an increased adherence of leukocytes which is suggested, irrespective of the precise nature of the mechanism of virus induced atherosclerosis, to be the earliest event associated with endothelium cell damage.

Membrane fluidity of non-activated and activated human blood platelets

The steady-state fluorescence anisotropy of membranes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) or its 4'-trimethylammonio derivative, TMA-DPH, is generally considered a measure for the lipid order and, hence, inversely related to membrane fluidity. We now report that anisotropy values of DPH- and TMA-DPH-labeled human platelets are considerably influenced by experimental conditions like the platelet concentration, which do not affect membrane fluidity. Activation of platelets with thrombin increases, but activation with ionomycin decreases anisotropy values with both labels. Such anisotropy changes are not detected in platelet membranes or platelet lipids, when isolated after activation of the intact platelets. We present evidence that the anisotropy changes of intact platelets are not a consequence of modified lipid composition (e.g., as would be induced by phospholipase A2 activity) but are, at least partially, caused by changed optical properties of the cell suspension. Measurement of membrane fluidity of platelets by fluorescence polarization is severely hindered by a high turbidity of the platelet suspension and also by changes in the turbidity and platelet morphology during the activation process.

Fluoride-dependent calcium-induced platelet procoagulant activity shows that calpain is involved in increased phospholipid transbilayer movement

Treatment of platelets with fluoride (10 mM) was found to result in a transient increase in Ca2+-permeability of the platelet plasma membrane. This phenomenon was used to provide supplementary evidence for the suggestions made earlier (Comfurius et al. (1985) Biochim. Biophys. Acta 815, 143; Verhallen et al. (1987) Biochim. Biophys. Acta 903, 206), that cytoskeletal disrupture by calpain is involved in the process leading to transbilayer movement of phosphatidylserine during expression of platelet procoagulant activity. This was achieved by relating both calpain activity and exposure of phosphatidylserine with platelet procoagulant activity. It was found that only upon addition of extracellular Ca2+ to fluoride-treated platelets, procoagulant activity, expressed as prothrombinase activity, and calpain activity, estimated from protein patterns after gel electrophoresis, were generated. Both Ca2+-inducible prothrombinase activity and calpain activity followed an identical time-course during incubation with fluoride: after a time-lag of about 10 min they sharply increased towards a peak level. Upon further incubation with fluoride, both activities decreased towards a final plateau, still above basal level. The presence of leupeptin during incubation with fluoride was found to inhibit Ca2+-inducible calpain activity and prothrombinase activity in an identical way. Ca2+-inducible exposure of phosphatidylserine, as determined with extracellular phospholipase A2, showed a similar pattern as Ca2+-inducible calpain activity and prothrombinase activity. From the strict parallelism between prothrombinase activity, calpain activity and exposure of phosphatidylserine, it is concluded that calpain plays an important role in the activation-dependent transbilayer movement of phosphatidylserine during expression of platelet procoagulant activity. It is suggested that degradation of the platelet membrane-skeleton by calpain disturbs the structural organization of the lipid bilayer of the platelet plasma membrane leading to enhanced transbilayer movement of phospholipids and appearance of phosphatidylserine at the platelet outer surface.

Correlation between calpain-mediated cytoskeletal degradation and expression of platelet procoagulant activity. A role for the platelet membrane-skeleton in the regulation of membrane lipid asymmetry?

The relationship between platelet calpain-activity and platelet procoagulant-activity was investigated by comparison of the time course of their generation after platelet stimulation by calcium ionophore A23187, or by the combined action of collagen and thrombin, or during exposure of platelets to the local anesthetics dibucaine or tetracaine. In addition, the Ca2+ dose-response curves of both activities in intact platelets, obtained by stimulation with A23187 in the presence of Ca2+/HEDTA-buffers, were compared. Platelet procoagulant activity was determined by assaying for prothrombinase activity in the presence of saturating concentrations of factors Xa, Va, and prothrombin. Platelet calpain activity was monitored by the degradation of its major substrates (filamin, talin, myosin) and the formation of their fragments as judged from protein patterns after gel electrophoresis. Platelet stimulation by A23187 resulted in a fast increase in prothrombinase activity, reaching its maximum level after about 20 seconds. Filamin and talin were completely hydrolysed within 15 s, and myosin was partly degraded between 15 and 30 s after platelet activation. When platelets were activated by collagen plus thrombin, prothrombinase activity was generated with a sigmoid time course, the steepest increase being observed between 1 and 2 min after platelet activation. Proteolysis of filamin and talin occurred between 0.5 and 1.5 min after platelet activation, while degradation of myosin became visible after 2 to 2.5 min. Dibucaine and tetracaine were both found to be potent stimulators of prothrombinase activity, with half-maximal activities obtained at 0.7 and 2.8 mM, respectively. Using suboptimal concentrations of both local anesthetics, it was found that the generation of prothrombinase activity closely paralleled that of calpain activity over a time course of 1 hour. Ca2+ titration of intact platelets using A23187 and Ca2+/HEDTA buffers, revealed half-maximal response at about 15 microM free Ca2+ for both calpain and prothrombinase activity. These findings strongly suggest a causal relationship between generation of a procoagulant platelet surface and calpain-mediated degradation of filamin, talin, and myosin. Since an increased procoagulant activity reflects an increased exposure of phosphatidylserine at the platelet outer surface, the present findings suggest that platelet cytoskeletal proteins are involved in the regulation of membrane lipid asymmetry.

The effect of Ca2+, phospholipid and factor V on the anti-(factor Xa) activity of heparin and its high-affinity oligosaccharides

The influence of Ca2+, phospholipid and Factor V was determined on the rate of inactivation of Factor Xa by antithrombin III, in the absence and in the presence of unfractionated heparin and of three high-affinity heparin oligosaccharides in the Mr range 1500-6000. In the absence of heparin the addition of Ca2+, phospholipid and Factor V caused a 4-fold decrease in rate of inactivation of Factor Xa. As concentrations of unfractionated heparin were increased the protective effect of Ca2+/phospholipid/Factor V was gradually abolished, and at a concentration of 2.4 nM there were no differences in rates of neutralization of Factor Xa in the presence or absence of Ca2+, phospholipid and Factor V. In contrast, heparin decasaccharide (Mr 3000) and pentasaccharide (Mr 1500) fragments were unable to overcome the protective effect of Ca2+/phospholipid/Factor V; in the presence of these components their catalytic efficiencies were 16-fold and 40-fold less respectively than that of unfractionated heparin. A heparin 20-22-saccharide fragment (Mr approx. 6000) gave similar inactivation rates in the presence and in the absence of Ca2+/phospholipid/Factor V. Human and bovine Factor Xa gave similar results. These results indicate that in the presence of Ca2+/phospholipid/Factor V optimum inhibition of Factor Xa requires a saccharide sequence of heparin additional to that involved in binding to antithrombin III. The use of free enzyme for the assessment of anti-(Factor Xa) activity of low-Mr heparin fractions could give misleading results.

Uncoupling of the membrane skeleton from the lipid bilayer. The cause of accelerated phospholipid flip-flop leading to an enhanced procoagulant activity of sickled cells

We have previously reported that the normal membrane phospholipid organization is altered in sickled erythrocytes. More recently, we presented evidence of enhanced transbilayer movement of phosphatidylcholine (PC) in deoxygenated reversibly sickled cells (RSC) and put forward the hypothesis that these abnormalities in phospholipid organization are confined to the characteristic protrusions of these cells. To test this hypothesis, we studied the free spicules released from RSC by repeated sickling and unsickling as well as the remnant despiculated cells. The rate of transbilayer movement of PC in the membrane of deoxygenated remnant despiculated cells was determined by following the fate of 14C-labelled PC, previously introduced into the outer monolayer under fully oxygenated conditions using a PC-specific phospholipid exchange protein from beef liver. The rate of transbilayer movement of PC in the remnant despiculated cells was significantly slower than in deoxygenated native RSC and was not very much different from that in oxygenated native RSC or irreversibly sickled cells. The free spicules had the same lipid composition as the native cells, but were deficient in spectrin. These spicules markedly enhanced the rate of thrombin formation in the presence of purified prothrombinase (Factor Xa, Factor Va, and Ca2+) and prothrombin, indicating the exposure of a significant fraction of phosphatidylserine (PS) in the outer monolayer. This effect was not observed when the spicules in this assay were replaced by normal erythrocytes, deoxygenated native RSC, or a deoxygenated sample of RSC after repetitive sickling/unsickling. The results are interpreted to indicate that the destabilization of the lipid bilayer in sickled cells, expressed by the enhanced flip-flop of PC and the exposure of PS in the outer monolayer, occurs predominantly in those parts of the membrane that are in spicular form.

Inhibition of prothrombinase complex by plasma proteinase inhibitors

The rate of inactivation of human coagulation factor Xa by the plasma proteinase inhibitors antithrombin III and alpha 1-antitrypsin has been studied in the presence of the accessory components which constitute the prothrombinase complex. The rate of inactivation of factor Xa by antithrombin III was found to be decreased in the presence of phospholipid vesicles with high affinity for factor Xa. The second-order rate constant for the reaction fell from 6.21 X 10(4) to 3.40 X 10(4) M-1 min-1 in the presence of 20 microM phospholipid. Purified factor Va had no effect on the rate of inactivation of factor Xa in the absence of phospholipid. In the presence of phospholipid, factor Va increased the protective effect displayed by phospholipid, further reducing the rate constant to 2.20 X 10(4) M-1 min-1. The rate of inactivation of factor Xa by alpha 1-antitrypsin was unaffected under these conditions. Platelet-bound prothrombinase complex was formed by incubation of factor Xa with washed human platelets activated by a mixture of collagen and thrombin. The prothrombinase activity was inhibited by antithrombin III was a second-order rate constant of 0.85 X 10(4) M-1 min-1. This rate was obtained in both the presence and absence of exogenous factor Va. Platelet factor 3 vesicles, isolated from platelet aggregation supernatants, also formed prothrombinase complex in the presence of factor Va, and this was inhibited by antithrombin III at the same rate as the platelet-bound complex. There was no protection of the platelet-bound prothrombinase complex from inhibition by alpha 1-antitrypsin.

Stimulation of prothrombinase activity of platelets and erythrocytes by sub-lytic treatment with phospholipase C from Clostridium welchii

Treatment of platelets or red cells with small amounts of phospholipase C from Clostridium welchii enables both cells, prior to the onset of lysis, to stimulate prothrombin conversion by coagulation factor Xa and Va in the presence of calcium. Phospholipase C treatment of both cells also exposes significant amounts of phosphatidylserine at the outer surface. The level of phosphatidic acid formed from diglycerides produced by phospholipase C action, is similar to that formed in activated platelets upon triggering the phosphatidylinositol cycle. A possible involvement of this cycle to activate platelets to become more procoagulant is discussed.

Decreased [20-3H]phorbol-12,13-dibutyrate binding to phospholipase C pretreated epidermal cells

Incubation of mouse epidermal cells (HEL-37) with phospholipase C (Clostridium perfringens) caused about a 50% decrease in the specific binding of [20-3H]phorbol-12,13-dibutyrate. Phospholipase C caused a decrease in the apparent number of binding sites from 2.86 X 10(5) to 1.21 X 10(5) per cell with little effect on ligand affinity. The decrease was observed with purified phospholipase containing no detectable protease activity, and when cells were fixed with glutaraldehyde. The phorbol-12,13-dibutyrate binding capacity of treated cells was recovered within 4 h incubation in complete medium. The results suggest that not all phorbol ester binding sites are equivalent, with differences arising either from varying phospholipid requirements or from membrane localisation.

Surface-governed molecular regulation of blood coagulation

Among extracellular biological processes the spatial control of blood clotting is a unique phenomenon. Localization in space has very important consequences in both normal and pathological conditions. Under physiological circumstances a clot is formed only in the vicinity of injury, albeit the prerequisites of coagulation are almost completely given in the whole circulation. The local character of blood clotting is secured by the following major conditions: The regulatory signal initiating coagulation-the damaged vascular wall-is itself a surface on which the majority of clotting reactions take place. The first enzyme, factor XII, of the intrinsic coagulation pathway is activated on the collagen fibers exposed in the damaged vascular wall, although the significance of this reaction in respect of the clotting process is ambiguous. On the membrane of platelets adhered to the damaged blood vessel is activated factor XI, too, which is a well-established participant of the intrinsic clotting process. The further consecutive reactions of coagulation are confined to the surface produced by injury, because the enzymes involved contain gamma-carboxyl-glutamyl side chains which are anchored through calcium bridges to the phospholipids of the platelet membrane. The last enzyme of the sequence is thrombin, which is released from the surface. The reactions taking place on the surface form an enzyme cascade, which amplifies the relatively weak triggering signal by several orders of magnitudes. Amplification is ensured not only by the enzyme-substrate relationship of the consecutive reaction partners, but also by spatial confinement, which endows the process with higher efficacy than could be expected on a statistical basis from reactions in solution. It contributes to the efficiency of enzyme cascade that the non-enzymatic regulatory proteins increase the activity of factors IXa and Xa, and thereby the overall process. While the partner of factor IXa, factor VIII, is captured from plasma, factor V, the partner of factor Xa, is derived from the platelets adhered to the damaged surface and orients the binding of factor Xa. The surface localization ensures the protection of the members of clotting system: In the activator complexes found on the surface, the spatial arrangement of clotting factors prevents the inactivation of factors by physiological inhibitors or by proteolytic enzymes and specific antibodies that appear in the circulation in pathological conditions. Platelet factor 4, derived from platelets, binds heparin and thereby markedly decreases the reactivity of antithrombin III, the physiological inhibitor of clotting factors. The above two circumstances are