Cofactor dependence of factor Xa incorporation into the prothrombinase complex

Exosite binding drives substrate affinity for the activation of coagulation factor X by the intrinsic Xase complex

Factor X activation by the intrinsic Xase complex, composed of factor IXa bound to factor VIIIa on membranes, is essential for the amplified blood coagulation response. The biological significance of this step is evident from bleeding arising from deficiencies in factors VIIIa or IXa in hemophilia. Here, we assess the mechanism(s) that enforce the distinctive specificity of intrinsic Xase for its biological substrate. Active-site function of IXa was assessed with a tripeptidyl substrate (PF-3688). The reversible S1 site binder, 4-aminobenzamidine (pAB), acted as a classical competitive inhibitor of PF-3688 cleavage by Xase. In contrast, pAB acted as a noncompetitive inhibitor of factor X activation. This disconnect between peptidyl substrate and protein substrate cleavage indicates a major role for interactions between factor X and extended sites on Xase in determining substrate affinity. Accordingly, an uncleavable factor X variant, not predicted to engage the active site of IXa within Xase, acted as a classical competitive inhibitor of factor X activation. Fluorescence studies confirmed the binding of factor X to Xase assembled with IXa with a covalently blocked active site. Our findings suggest that the recognition of factor X by the intrinsic Xase complex occurs through a multistep "dock-and-lock" pathway in which the initial interaction between factor X and intrinsic Xase occurs at exosites distant from the active site, followed by active-site docking and bond cleavage.

Phosphatidylserine and FVa regulate FXa structure

Human coagulation FXa (Factor Xa) plays a key role in blood coagulation by activating prothrombin to thrombin on 'stimulated' platelet membranes in the presence of its cofactor FVa (Factor Va). PS (phosphatidylserine) exposure on activated platelet membranes promotes prothrombin activation by FXa by allosterically regulating FXa. To identify the structural basis of this allosteric regulation, we used FRET to monitor changes in FXa length in response to (i) soluble short-chain PS [C6PS (dicaproylphosphatidylserine)], (ii) PS membranes, and (iii) FVa in the presence of C6PS and membranes. We incorporated a FRET pair with donor (fluorescein) at the active site and acceptor (Alexa Fluor® 555) at the FXa N-terminus near the membrane. The results demonstrated that FXa structure changes upon binding of C6PS to two sites: a regulatory site at the N-terminus [identified previously as involving the Gla (γ-carboxyglutamic acid) and EGFN (N-terminus of epidermal growth factor) domains] and a presumptive protein-recognition site in the catalytic domain. Binding of C6PS to the regulatory site increased the interprobe distance by ~3 Å (1 Å=0.1 nm), whereas saturation of both sites increased the distance by a further ~6.4 Å. FXa binding to a membrane produced a smaller increase in length (~1.4 Å), indicating that FXa has a somewhat different structure on a membrane from when bound to C6PS in solution. However, when both FVa2 (a FVa glycoform) and either C6PS- or PS-containing membranes were bound to FXa, the overall change in length was comparable (~5.6-5.8 Å), indicating that C6PS- and PS-containing membranes in conjunction with FVa2 have comparable regulatory effects on FXa. We conclude that the similar functional regulation of FXa by C6PS or membranes in conjunction with FVa2 correlates with similar structural regulation. The results demonstrate the usefulness of FRET in analysing structure-function relationships in FXa and in the FXa·FVa2 complex.

Prothrombin activation on the activated platelet surface optimizes expression of procoagulant activity

Effective hemostasis relies on the timely formation of α-thrombin via prothrombinase, a Ca(2+)-dependent complex of factors Va and Xa assembled on the activated platelet surface, which cleaves prothrombin at Arg271 and Arg320. Whereas initial cleavage at Arg271 generates the inactive intermediate prethrombin-2, initial cleavage at Arg320 generates the enzymatically active intermediate meizothrombin. To determine which of these intermediates is formed when prothrombin is processed on the activated platelet surface, the cleavage of prothrombin, and prothrombin mutants lacking either one of the cleavage sites, was monitored on the surface of either thrombin- or collagen-activated platelets. Regardless of the agonist used, prothrombin was initially cleaved at Arg271 generating prethrombin-2, with α-thrombin formation quickly after via cleavage at Arg320. The pathway used was independent of the source of factor Va (plasma- or platelet-derived) and was unaffected by soluble components of the platelet releasate. When both cleavage sites are presented within the same substrate molecule, Arg271 effectively competes against Arg320 (with an apparent IC(50) = 0.3μM), such that more than 90% to 95% of the initial cleavage occurs at Arg271. We hypothesize that use of the prethrombin-2 pathway serves to optimize the procoagulant activity expressed by activated platelets, by limiting the anticoagulant functions of the alternate intermediate, meizothrombin.

Taking the Thrombin “Fork”

The proverb that probably best exemplifies my career in research is attributable to Yogi Berra (http://www.yogiberra.com/), ie, “when you come to a fork in the road … take it.” My career is a consequence of chance interactions with great mentors and talented students and the opportunities provided by a succession of ground-breaking improvements in technology.

Factor XA binding to phosphatidylserine-containing membranes produces an inactive membrane-bound dimer

Factor Xa (FXa) has a prominent role in amplifying both inflammation and the coagulation cascade. In the coagulation cascade, its main role is catalyzing the proteolytic activation of prothrombin to thrombin. Efficient proteolysis is well known to require phosphatidylserine (PS)-containing membranes that are provided by platelets in vivo. However, soluble, short-chain PS also triggers efficient proteolytic activity and formation of an inactive FXa dimer in solution. In this work, we ask whether PS-containing membranes also trigger formation of an inactive FXa dimer. We determined the proteolytic activity of human FXa toward human Pre2 as a substrate both at fixed membrane concentration (increasing FXa concentration) and at fixed FXa concentration (increasing membrane concentration). Neither of these experiments showed the expected behavior of an increase in activity as FXa bound to membranes, but instead suggested the existence of a membrane-bound inactive form of FXa. We found also that the fluorescence of fluorescein attached to FXa's active site serine was depolarized in a FXa concentration-dependent fashion in the presence of membranes. The fluorescence lifetime of FXa labeled in its active sites with a dansyl fluorophore showed a similar concentration dependence. We explained all these observations in terms of a quantitative model that takes into account dimerization of FXa after binding to a membrane, which yielded estimates of the FXa dimerization constant on a membrane as well as the kinetic constants of the dimer, showing that the dimer is effectively inactive.

Enhanced fibrinolysis by proteolysed coagulation factor Xa

We previously showed that coagulation factor Xa (FXa) enhances activation of the fibrinolysis zymogen plasminogen to plasmin by tissue plasminogen activator (tPA). Implying that proteolytic modulation occurs in situ, intact FXa (FXaalpha) must be sequentially cleaved by plasmin or autoproteolysis, producing FXabeta and Xa33/13, which acquire necessary plasminogen binding sites. The implicit function of Xa33/13 in plasmin generation has not been demonstrated, nor has FXaalpha/beta or Xa33/13 been studied in clot lysis experiments. We now report that purified Xa33/13 increases tPA-dependent plasmin generation by at least 10-fold. Western blots confirmed that in situ conversion of FXaalpha/beta to Xa33/13 correlated to enhanced plasmin generation. Chemical modification of the FXaalpha active site resulted in the proteolytic generation of a product distinct from Xa33/13 and inhibited the enhancement of plasminogen activation. Identical modification of Xa33/13 had no effect on tPA cofactor function. Due to its overwhelming concentration in the clot, fibrin is the accepted tPA cofactor. Nevertheless, at the functional level of tPA that circulates in plasma, FXaalpha/beta or Xa33/13 greatly reduced purified fibrin lysis times by as much as 7-fold. This effect was attenuated at high levels of tPA, suggesting a role when intrinsic plasmin generation is relatively low. FXaalpha/beta or Xa33/13 did not alter the apparent size of fibrin degradation products, but accelerated the initial cleavage of fibrin to fragment X, which is known to optimize the tPA cofactor activity of fibrin. Thus, coagulation FXaalpha undergoes proteolytic modulation to enhance fibrinolysis, possibly by priming the tPA cofactor function of fibrin.

Membrane-dependent interaction of factor Xa and prothrombin with factor Va in the prothrombinase complex

Because all three protein components of prothrombinase, factors (f) Xa and Va and prothrombin, bind to negatively charged membrane phospholipids, the exact role of the membrane in the prothrombinase reaction has not been fully understood. In this study, we prepared deletion derivatives of fXa and prothrombin in which both the Gla and first EGF-like domains of the protease (E2-fXa) as well as the Gla and both kringle domains of the substrate (prethrombin-2) had been deleted. The fVa-mediated catalytic activity of E2-fXa toward prethrombin-2 was analyzed in both the absence and presence of phospholipids composed of 80% phosphatidylcholine (PC) and 20% phosphatidylserine (PS). PCPS markedly accelerated the initial rate of prethrombin-2 activation by E2-fXa, with the cofactor exhibiting saturation only in the presence of phospholipids (apparent K(d) of approximately 60 nM). Competitive kinetic studies in the presence of the two exosite-1-specific ligands Tyr(63)-sulfated hirudin(54-65) and TM456 suggested that while both peptides are highly effective inhibitors of the fVa-mediated activation of prethrombin-2 by E2-fXa in the absence of PCPS, they are ineffective competitors in the presence of phospholipids. Since neither E2-fXa nor prethrombin-2 can interact with membranes, these results suggest that interaction of fVa with PCPS improves the affinity of the activation complex for proexosite-1 of the substrate. Direct binding studies employing OG(488)-EGR-labeled fXa and E2-fXa revealed that the interaction of the Gla domain of fXa with PCPS also induces conformational changes in the protease to facilitate its high-affinity interaction with fVa.

Factor Va alters the conformation of the Na+-binding loop of factor Xa in the prothrombinase complex

Structural and mutagenesis data have indicated that the 220-loop of thrombin is stabilized by a salt-bridge between Glu-217 and Lys-224, thereby facilitating the octahedral coordination of Na (+) with contributions from two carbonyl O atoms of Arg-221a and Lys-224. All three residues are also conserved in fXa and the X-ray crystal structure of fXa indicates that both Glu-217 and Lys-224 are within hydrogen-bonding distance from one another. To investigate the role of these three residues in the catalytic function of fXa and their contribution to interaction with Na (+), we substituted them with Ala and characterized their properties in both amidolytic and proteolytic activity assays. The results indicate that the affinity of all three mutants for interaction with Na (+) has been impaired. The mutant with the greatest loss of affinity for Na (+) (E217A or E217Q) also exhibited a dramatic impairment ( approximately 3-4 orders of magnitude) in its activity toward both synthetic and natural substrates. Interestingly, factor Va (fVa) restored most of the catalytic defect with prothrombin, but not with the synthetic substrate. Both Glu-217 mutants exhibited a near normal affinity for fVa in the prothrombinase assay, but a markedly lower affinity for the cofactor in a direct-binding assay. These results suggest that, similar to thrombin, an ionic interaction between Glu-217 and Lys-224 stabilizes the 220-loop of fXa for binding Na (+). They further support the hypothesis that the Na (+) and fVa-binding sites of fXa are energetically linked and that a cofactor function for fVa in the prothrombinase complex involves inducing a conformational change in the 220-loop of fXa that appears to stabilize this loop in the Na (+)-bound active conformation.

FRET studies with factor X mutants provide insight into the topography of the membrane-bound factor X/Xa

FRET (fluorescence resonance energy transfer) studies have shown that the vitamin K-dependent coagulation proteases bind to membrane surfaces perpendicularly, positioning their active sites above the membrane surfaces. To investigate whether EGF (epidermal growth factor) domains of these proteases play a spacer function in this model of the membrane interaction, we used FRET to measure the distance between the donor fluorescein dye in the active sites of Fl-FPR (fluorescein-D-Phe-Pro-Arg-chloromethane)-inhibited fXa (activated Factor Xa) and its N-terminal EGF deletion mutant (fXa-desEGF1), and the acceptor OR (octadecylrhodamine) dye incorporated into phospholipid vesicles composed of 80% phosphatidylcholine and 20% phosphatidylserine. The average distance of closest approach (L) between fluorescein in the active site and OR at the vesicle surface was determined to be 56+/-1 A (1 A=0.1 nm) and 63+/-1 A for fXa-desEGF1 compared with 72+/-2 A and 75+/-1 A for fXa, in the absence and presence of fVa (activated Factor V) respectively, assuming kappa2=2/3. In comparison, an L value of 95+/-6 A was obtained for a S195C mutant of fXa in the absence of fVa in which fluorescein was attached directly to Cys(195) of fXa. These results suggest that (i) EGF1 plays a spacer function in holding the active site of fXa above the membrane surface, (ii) the average distance between fluorescein attached to Fl-FPR in the active site of fXa and OR at the vesicle surface may not reflect the actual distance of the active-site residue relative to the membrane surface, and (iii) fVa alters the orientation and/or the height of residue 195 above the membrane surface.

Role of the alpha-helix 163-170 in factor Xa catalytic activity

Factor Xa (FXa) is a key protease of the coagulation pathway whose activity is known to be in part modulated by binding to factor Va (FVa) and sodium ions. Previous investigations have established that solvent-exposed, charged residues of the FXa alpha-helix 163-170 (h163-170), Arg(165) and Lys(169), participate in its binding to FVa. In the present study we aimed to investigate the role of the other residues of h163-170 in the catalytic functions of the enzyme. FX derivatives were constructed in which point mutations were made or parts of h163-170 were substituted with the corresponding region of either FVIIa or FIXa. Purified FXa derivatives were compared with wild-type FXa. Kinetic studies in the absence of FVa revealed that, compared with wild-type FXa, key functional parameters (catalytic activity toward prothrombin and tripeptidyl substrates and non-enzymatic interaction of a probe with the S1 site) were diminished by mutations in the NH(2)-terminal portion of h163-170. The defective amidolytic activity of these FXa derivatives appears to result from their impaired interaction with Na(+) because using a higher Na(+) concentration partially restored normal catalytic parameters. Furthermore, kinetic measurements with tripeptidyl substrates or prothrombin indicated that assembly of these FXa derivatives with an excess of FVa in the prothrombinase complex improves their low catalytic efficiency. These data indicate that residues in the NH(2)-terminal portion of the FVa-binding h163-170 are energetically linked to the S1 site and Na(+)-binding site of the protease and that residues Val(163) and Ser(167) play a key role in this interaction.

Further evidence for two functional forms of prothrombinase each specific for either of the two prothrombin activation cleavages

Previous work showed that prothrombin derivatives cleavable only at Arg-320 (rMZ) or Arg-271 (rP2) are partial, rather than competitive, inhibitors of prothrombin activation by prothrombinase. A "ping-pong"-like model, which posits two equilibrating forms of prothrombinase, explained the inhibition pattern. The present studies were undertaken to further investigate this putative mechanism. Two models were developed, one allowing for one form of the enzyme and the other allowing for two forms. Both models also allowed channeling and ratcheting. The models were fit to full time courses of prothrombin, meizothrombin, prethrombin-2, and the B-chain. In the absence of ratcheting and channeling, neither model fits the data. In their presence, however, both models fit very well, and thus they could not be distinguished. Therefore, inhibition of rMZ activation by rP2 was studied. Inhibition was partial and the two-form model fit the data with randomly distributed residuals, whereas the one-form model did not. Initial rates of fluorescein-labeled prothrombin cleavage in the presence of various prothrombin derivatives reported by Brufatto and Nesheim (Brufatto, N., and Nesheim, M. E. (2003) J. Biol. Chem. 278, 6755-6764) were also analyzed using the two models. The two-form model fit the partial inhibition data well, whereas the one-form model did not. In addition, prothrombin at varying concentrations was activated, and subsequently, the initial rates were plotted with respect to the initial prothrombin concentration. When compared with the expected initial rates as determined by the simulation of the models, the two-form model fit the observed rates better than the one-form model. The results obtained here further support the existence of two functional forms of prothrombinase.

Identification of factor Xa residues critical for interaction with protein Z-dependent protease inhibitor: both active site and exosite interactions are required for inhibition

Protein Z-dependent protease inhibitor (ZPI) is a plasma serpin, which can rapidly inactivate factor Xa (fXa) in the presence of protein Z (PZ), negatively charged phospholipids, and Ca2+. To investigate the mechanism by which ZPI inactivates fXa, we expressed the serpin in mammalian cells and characterized its reactivity with both wild-type and selected mutants of fXa that 1) contained substitutions in the autolysis loop and the heparin binding exosite, 2) lacked the first EGF-like domain (fXa-des-EGF-1), or 3) contained the Gla domain of protein C (fXa/PC-Gla). Inhibition studies in both the presence and absence of PZ revealed that Arg-143, Lys-147, and Arg-154 of the autolysis loop and Lys-96, Lys-169, and Lys-236 of the heparin binding exosite are required for recognition of ZPI, with Arg-143 being essential for the interaction. Similar studies with fXa-des-EGF-1 and fXa/PC-Gla suggested that protein-protein interaction with either the Gla or the EGF-1 domain may not play a dominant role in the PZ-dependent recognition of fXa by the serpin on phospholipid vesicles. Further studies showed that an inactive Ser-195 to Ala mutant of fXa effectively competes with wild-type fXa for binding to the non-serpin inhibitors tissue factor pathway inhibitor and recombinant tick anticoagulant peptide, but does not compete for binding to ZPI. This suggests that the catalytic residue of fXa is required for interaction with ZPI.

Antithrombin Reduces Compression-Induced Spinal Cord Injury in Rats

Antithrombin (AT), a natural anticoagulant, has been shown to exert anti-inflammatory activity by promoting the endothelial production of prostaglandin I2 (PGI2), thereby reducing tissue injury. To examine whether AT prevents post-traumatic spinal cord injury (SCI), a pathologic condition in which activated neutrophils are critically involved, we tested the effect of AT on SCI induced by compression trauma in rats. Intravenous administration of AT, either before or after the induction of SCI, significantly reduced SCI-related motor disturbances in these animals. AT also significantly inhibited both intramedullary hemorrhage and the decrease in the number of motor neurons following SCI, and inhibited the accumulation of neutrophils in the damaged segment of the spinal cord by inhibiting the increase in transcription of tumor necrosis factor-alpha (TNF-alpha). AT significantly enhanced the increase in the tissue level of 6-keto-PGF1alpha, a stable metabolite of PGI2, at the injured segment of the cord. These therapeutic effects of AT may not depend on its anticoagulant effect. AT did not show any effects in animals pretreated with indomethacin, a potent inhibitor of prostaglandin synthesis, and iloprost, a stable PGI2 analog, produced effects similar to those of AT. Furthermore, intravenously administered AT accumulated selectively at the injured segment of the spinal cord, where thrombin generation might be increased. These findings suggest that AT may reduce the effects of compression trauma-induced SCI by inhibiting neutrophil activation as a consequence of the AT-mediated inhibition of TNF-alpha production. Such therapeutic effects of AT might be mediated by its promoting the endothelial release of PGI2. These findings strongly suggest AT as a potential agent for treating SCI in the clinical setting.

The critical role of the 185-189-loop in the factor Xa interaction with Na+ and factor Va in the prothrombinase complex

The S1 site (Asp(189)) of factor Xa (fXa) is located on a loop (residues 185-189) that contains three solvent-exposed charged residues (Asp(185), Lys(186), and Glu(188)) below the active-site pocket of the protease. To investigate the role of these residues in the catalytic function of fXa, we expressed three mutants of the protease in which the charges of these residues were neutralized by their substitutions with Ala (D185A, K186A, and E188A). Kinetic studies revealed that E188A has a normal catalytic activity toward small synthetic and natural substrates and inhibitors of fXa; however, the same activities were slightly ( approximately 2-fold) and dramatically ( approximately 20-50-fold) impaired for the D185A and K186A mutants, respectively. Further studies revealed that the affinity of D185A and K186A for interaction with Na(+) has also been altered, with a modest impairment ( approximately 2-fold) for the former and a dramatic impairment for the latter mutant. Both prothrombinase and direct binding studies indicated that K186A also has an approximately 6-fold impaired affinity for factor Va. Interestingly, a saturating concentration of factor Va restored the catalytic defect of K186A in reactions with prothrombin and the recombinant tick anticoagulant peptide that is known to interact with the Na(+) loop of fXa, but not with other substrates. These results suggest that factor Va interacts with 185-189-loop for fXa, which is energetically linked to the Na(+)-binding site of the protease.

Role of the N-terminal Epidermal Growth Factor-like Domain of Factor X/Xa

The functional importance of the N-terminal epidermal growth factor-like domain (EGF-N) of factor X/Xa (FX/Xa) was investigated by constructing an FX mutant in which the exon coding for EGF-N was deleted from FX cDNA. Following expression and purification to homogeneity, the mutant was characterized with respect to its ability to function as a zymogen for either the factor VIIa-tissue factor complex or the factor IXa-factor VIIIa complex and then to function as an enzyme in the prothrombinase complex to catalyze the conversion of prothrombin to thrombin. It was discovered that EGF-N is essential for the recognition and efficient activation of FX by both activators in the presence of the cofactors. On the other hand, the FXa mutant interacted with factor Va with a normal apparent dissociation constant and activated prothrombin with approximately 3-fold lower catalytic efficiency in the prothrombinase complex. Surprisingly, the mutant activated prothrombin with approximately 12-fold better catalytic efficiency than wild-type FXa in the absence of factor Va. The mutant was inactive in both prothrombin time and activated partial thromboplastin time assays; however, it exhibited a similar specific activity in a one-stage FXa clotting assay. These results suggest that EGF-N of FX is required for the cofactor-dependent zymogen activation by both physiological activators, but it plays no apparent role in FXa recognition of the cofactor in the prothrombinase complex.

Proexosite-1-dependent Recognition and Activation of Prothrombin by Taipan Venom

An activator complex from the venom of Oxyuranus scutellatus scutellatus (taipan venom) is known to rapidly activate prothrombin to thrombin. To determine whether, similar to prothrombinase, taipan venom utilizes proexosite-1 on prothrombin for a productive complex assembly, the activation of proexosite-1 mutants of prethrombin-1 by the partially purified venom was studied. It was discovered that basic residues of this site (Arg(35), Lys(36), Arg(67), Lys(70), Arg(73), Arg(75), and Arg(77)) are also crucial for recognition and rapid activation of the substrate by taipan venom. This was evidenced by the observation that the K(m) and k(cat) values for the activation of the charge reversal mutants of prethrombin-1 (in particular K36E, R67E, and K70E) were markedly impaired. Competitive kinetic studies with the Tyr(63)-sulfated hirudin(54-65) peptide revealed that although the peptide inhibits the activation of the wild type zymogen by taipan venom with a K(D) of approximately 2 microm, it is ineffective in inhibiting the activation of mutant zymogens (K(D) > 4-30 microm). Interestingly, an approximately 50-kDa activator, isolated from the taipan venom complex, catalyzed the activation of prothrombin in a factor Va-dependent manner and exhibited identical activation kinetics toward the substrate in the presence of the hirudin peptide. These results suggest that, similar to prothrombinase, proexosite-1 is a cofactor-dependent recognition site for taipan venom.

Challenges for providing effective hemostasis in surgery and trauma

Vascular injury, whether surgical or traumatic, triggers a complex series of regulatory events. The understanding of these events, their interdependence, and their effect on hemostasis and thrombosis, is slowly being unraveled. The current understanding of these processes is reviewed in this paper. The application of this knowledge to the operating theatre has been slow and is severely limited by the lack of effective tools to monitor the coagulopathic status of individual patients. Hence, the initial treatment of patients with severe hemorrhage relies on improving the patient's physiological status and on basic surgical techniques. Should these efforts fail, then a number of topical hemostatic agents, selective inhibitors of fibrinolysis, and procoagulant molecules, such as recombinant factor VIIa, may be utilized. However, many of these agents have not yet been tested in clinical trials and studies are urgently needed to determine efficacy, safety, optimal dosage and time of administration.

Proexosite-1 on prothrombin is a factor Va-dependent recognition site for the prothrombinase complex

Although the contribution of basic residues of exosite-1 to the catalytic function of thrombin has been studied extensively, their role in the specificity of prothrombin recognition by factor Xa in the prothrombinase complex (factor Xa, factor Va, phosphatidylcholine/phosphatidylserine vesicles, and Ca2+) has not been examined. In this study, we prepared several mutants of prethrombin-1 (prothrombin lacking Gla and Kringle-1 domains) in which basic residues of this site (Arg35, Lys36, Arg67, Lys70, Arg73, Arg75, and Arg77 in chymotrypsinogen numbering) were individually substituted with a Glu. Following expression in mammalian cells and purification to homogeneity, these mutants were characterized with respect to their ability to function as zymogens for both factor Xa and the prothrombinase complex. Factor Xa by itself exhibited similar catalytic activity toward both the wild type and mutant substrates; however, its activity in the prothrombinase complex toward most of mutants was severely impaired. Further kinetic studies in the presence of Tyr63-sulfated hirudin-(54-65) peptide suggested that although the peptide inhibits the prothrombinase activation of the wild type zymogen with a KD of 0.5-0.7 microm, it is ineffective in inhibiting the activation of mutant zymogens (KD = 2-30 microm). These results suggest that basic residues of proexosite-1 on prothrombin are factor Va-dependent recognition sites for factor Xa in the prothrombinase complex.

Antithrombin reduces ischemia/reperfusion-induced renal injury in rats by inhibiting leukocyte activation through promotion of prostacyclin production

Antithrombin (AT) supplementation in patients with severe sepsis has been shown to improve organ failures in which activated leukocytes are critically involved. However, the precise mechanism(s) for the therapeutic effects of AT is not well understood. We examined in rats whether AT reduces ischemia/reperfusion (I/R)-induced renal injury by inhibiting leukocyte activation. AT markedly reduced the I/R-induced renal dysfunction and histologic changes, whereas neither dansyl glutamylglycylarginyl chloromethyl ketone-treated factor Xa (DEGR-F.Xa), a selective inhibitor of thrombin generation, nor Trp49-modified AT, which lacks affinity for heparin, had any effect. Renal tissue levels of 6-keto-PGF(1 alpha), a stable metabolite of prostacyclin (PGI(2)), increased after renal I/R. AT enhanced the I/R-induced increases in renal tissue levels of 6-keto-PGF(1 alpha), whereas neither DEGR-F.Xa nor Trp49-modified AT had any effect. AT significantly inhibited I/R-induced decrease in renal tissue blood flow and the increase in the vascular permeability. Ischemia/reperfusion-induced increases in renal tissue levels of tumor necrosis factor-alpha, cytokine-induced neutrophil chemoattractant, and myeloperoxidase were significantly inhibited in animals given AT. Pretreatment of animals with indomethacin reversed the effects induced by AT. Iloprost, an analog of PGI(2), produced effects similar to those induced by AT. These observations strongly suggest that AT reduces the I/R-induced renal injury by inhibiting leukocyte activation. The therapeutic effects of AT might be mainly mediated by PGI(2) released from endothelial cells through interaction of AT with cell surface glycosaminoglycans.

Gla domain-mutated human protein C exhibiting enhanced anticoagulant activity and increased phospholipid binding

Protein C is a member of the vitamin K- dependent protein family. Proteins in this family have similar gamma-carboxyglutamic acid (Gla)-rich domains, but their affinities for negatively charged phospholipid membranes vary more than 1000-fold. We have shown that it is possible to enhance anticoagulant activity and membrane affinity of protein C by selective mutagenesis of the Gla domain. In this study, 3 new mutants, Q10G11N12 (QGN), S23E32D33Y44 (SEDY), and Q10G11N12S23E32D33Y44 (QGNSEDY), were created. In plasma-based coagulation assays, the activated form of QGNSEDY (QGNSEDY-APC) demonstrated approximately 20-fold higher anticoagulant activity than wild-type activated protein C (WT APC), while QGN-APC and SEDY-APC did not. Both normal activated factor V (FVa) and FVa Leiden (Arg506Gln) were degraded much more efficiently by QGNSEDY-APC than by WT APC in the presence as well as in the absence of protein S. Binding of protein C variants to negatively charged phospholipid membranes was investigated using light scattering and the BIAcore technique. QGNSEDY demonstrated 3- to 7-fold enhanced binding as compared with WT protein C, suggesting the membrane affinity to be influenced by several residues located at different parts of the Gla domain. The anticoagulant activity as well as phospholipid binding ability was only enhanced when multiple regions of the Gla domain were modified. The results provide insights into the molecular mechanisms that are involved in determining the binding affinity of the interaction between Gla domains and phospholipid membranes. The unique properties of QGNSEDY-APC suggest this APC variant possibly to have greater therapeutic potential than WT APC.

Nematode anticoagulant protein c2 reveals a site on factor Xa that is important for macromolecular substrate binding to human prothrombinase

The binding of recombinant nematode anticoagulant protein c2 (NAPc2) to either factor X or Xa is a requisite step in the pathway for the potent inhibition of VIIa tissue factor. We have used NAPc2 as a tight binding probe of human Xa to investigate protein substrate recognition by the human prothrombinase complex. NAPc2 binds with high affinity (K(d) approximately 1 nm) to both X and Xa in a way that does not require or occlude the active site of the enzyme. In contrast, NAPc2 is a tight binding, competitive inhibitor of protein substrate cleavage by human Xa incorporated into prothrombinase with saturating concentrations of membranes and Va. By fluorescence binding studies we show that NAPc2 does not interfere with the assembly of human prothrombinase. These are properties expected of an inhibitor that blocks protein substrate recognition by targeting extended macromolecular recognition sites (exosites) on the enzyme complex. A weaker interaction (K(d) = 260-500 nm) observed between NAPc2 and bovine X was restored to a high affinity one in a recombinant chimeric bovine X derivative containing 25 residues from the COOH terminus of the proteinase domain of human X. This region implicated in binding NAPc2 is spatially adjacent to a site previously identified as a potential exosite. Despite the weaker interaction with bovine Xa, NAPc2 was a tight binding competitive inhibitor of protein substrate cleavage by bovine prothrombinase as well. Extended enzymic surfaces elucidated with exosite-directed probes, such as NAPc2, may define a unique region of factor Xa that is modulated following its assembly into prothrombinase and in turn determines the binding specificity of the enzyme complex for its protein substrate.

Interaction of bovine coagulation factor X and its glutamic-acid-containing fragments with phospholipid membranes. A surface plasmon resonance study

The interaction of blood coagulation factor X and its Gla-containing fragments with negatively charged phospholipid membranes composed of 25 mol% phosphatidylserine (PtdSer) and 75 mol% phosphatidylcholine (PtdCho) was studied by surface plasmon resonance. The binding to 100 mol% PtdCho membranes was negligible. The calcium dependence in the membrane binding was evaluated for intact bovine factor X (factor X) and the fragment containing the Gla-domain and the N-terminal EGF (epidermal growth factor)-like domain, Gla-EGFN, from factor X. Both proteins show the same calcium dependence in the membrane binding. Calcium binding is cooperative and half-maximum binding was observed at 1.5 mm and 1.4 mm, with the best fit to the experimental data with three cooperatively bound calcium ions for both the intact protein and the fragment. The dissociation constant (Kd) for binding to membranes containing 25 mol% PtdSer decreased from 4.6 microm for the isolated Gla-domain to 1 microm for the fragments Gla-EGFN and Gla-EGFNC (the Gla-domain and both EGF-like domains) fragments and to 40 nm for the entire protein as zymogen, activated enzyme or in the active-site inhibited form. Analysis of the kinetics of adsorption and desorption confirmed the equilibrium binding data.

Factor Va increases the affinity of factor Xa for prothrombin: a binding study using a novel photoactivable thiol-specific fluorescent probe

The multiprotein complex of factor Xa, factor Va, and prothrombin efficiently generates the blood-clotting agent, thrombin. Here, the formation of the factor Xa*prothrombin complex and the effects of factor Va on this complex were examined using a photoactivable thiol-specific fluorescent probe (LWB), which was synthesized and incorporated into the active site of factor Xa. The use of fluorescent LWB illustrated that factor Xa has an increased affinity for prothrombin in the presence of factor Va. Further exposure of these components to UV light resulted in a specific photocrosslinking of LWB-factor Xa to prothrombin, suggesting a physical association between these proteins. These data demonstrate that LWB can successfully function both as a spectroscopic probe and as a photocrosslinking reagent for studying protein-protein interactions.

Activated protein C reduces the ischemia/reperfusion-induced spinal cord injury in rats by inhibiting neutrophil activation

OBJECTIVE

To examine whether activated protein C (APC) reduces spinal cord injury in rats by inhibiting neutrophil activation after the transient ischemia.

SUMMARY BACKGROUND DATA

Ischemic spinal cord injury is an important pathologic mechanism leading to the paraplegia observed after surgery to repair aortic aneurysms. Activated neutrophils play a pivotal role in the development of ischemia/reperfusion-induced tissue injury. Recently, the authors have reported that APC, a physiologic anticoagulant, prevents lipopolysaccharide-induced pulmonary vascular injury by inhibiting neutrophil activation. These observations strongly suggest that APC reduces ischemia/reperfusion-induced spinal cord injury by inhibiting neutrophil activation.

METHODS

In rats, spinal cord ischemia was induced by using a balloon catheter placed into the aorta. After the transient ischemia, survival and motor function were evaluated, and histologic examination of the spinal cord was performed by using both hematoxylin-and-eosin staining and 2,3,5, -triphenyltetrazolium chloride (TTC) staining 24 hours after the ischemia. Tissue levels of myeloperoxidase and cytokines, including tumor necrosis factor-alpha (TNF-alpha) and rat interleukin-8, were measured in six experimental groups: sham-operated, control, APC (100 microg/kg, intravenous), dansyl glutamyl-glycyl-arginyl chloromethyl ketone-treated activated factor X (DEGR-F.Xa), a selective inhibitor of thrombin generation (1 mg/kg, intravenous), nitrogen mustard-induced leukocytopenia, and diisopropyl fluorophosphate-treated APC (DIP-APC), active site-blocked APC (100 microg/kg, intravenous). APC, DEGR-F.Xa, and DIP-APC were administered intravenously 30 minutes before aortic occlusion. Control and leukocytopenic rats received saline instead of other drugs.

RESULTS

Pretreatment with APC significantly reduced motor disturbances compared with those in control animals. In contrast, neither DEGR-F.Xa nor DIP-APC had any effect. Microinfarctions, evidenced by the absence of TTC staining and histologic change, were markedly reduced in animals given APC. The increases in the tissue levels of TNF-alpha, rat interleukin-8, and myeloperoxidase in the ischemic part of the spinal cord were significantly reduced in animals that received APC. These levels were not reduced in rats given DEGR-F.Xa or DIP-APC. Leukocytopenia produced effects similar to those of APC.

CONCLUSIONS

APC reduced the ischemia/reperfusion-induced spinal cord injury by inhibiting neutrophil activation. The therapeutic mechanisms of APC might depend on its inhibitory effect on the production of TNF-alpha, which is a potent activator of neutrophils. Although the anticoagulant effects of APC might not be related to its ability to inhibit TNF-alpha production, its serine protease activity appears to be essential in the therapeutic mechanism. APC appears to have potential as a therapeutic agent for prevention of spinal cord injury in patients undergoing aortic aneurysm repair.

Activated protein C reduces ischemia/reperfusion-induced renal injury in rats by inhibiting leukocyte activation

We examined whether activated protein C (APC) reduces ischemia/reperfusion (I/R)–induced renal injury by inhibiting leukocyte activation. In a rat model, intravenous administration of APC markedly reduced I/R-induced renal dysfunction and histological changes, whereas intravenous administration of dansyl glutamylglycylarginyl chloromethyl ketone–treated factor Xa (DEGR-FXa; active-site–blocked factor Xa), heparin or diisopropyl fluorophosphate–treated APC (DIP-APC; inactive derivative of ARC) had no effect. Furthermore, APC significantly inhibited the I/R-induced decrease in renal tissue blood flow and the increase in the vascular permeability, whereas neither DEGR-FXa, heparin, nor DIP-APC produced such effects. Renal I/R-induced increases in plasma levels of fibrin degradation products were significantly inhibited by APC, DEGR-FXa, and heparin. These observations suggest that APC reduces I/R-induced renal injury independently of its anticoagulant effects but in a manner dependent on its serine protease activity. Renal levels of tumor necrosis factor- (TNF-), rat interleukin-8, and myeloperoxidase were significantly increased after renal I/R. These increases were significantly inhibited by APC but not by DEGR-FXa, heparin, or DIP-APC. Leukocytopenia produced effects similar to those of APC. These findings strongly suggest that APC protects against I/R-induced renal injury not by inhibiting coagulation abnormalities but by inhibiting activation of leukocytes that play an important role in I/R-induced renal injury. Inhibition of leukocyte activation by APC could be explained by the inhibitory activity of TNF-.

Activated protein C reduces ischemia/reperfusion-induced renal injury in rats by inhibiting leukocyte activation

We examined whether activated protein C (APC) reduces ischemia/reperfusion (I/R)–induced renal injury by inhibiting leukocyte activation. In a rat model, intravenous administration of APC markedly reduced I/R-induced renal dysfunction and histological changes, whereas intravenous administration of dansyl glutamylglycylarginyl chloromethyl ketone–treated factor Xa (DEGR-FXa; active-site–blocked factor Xa), heparin or diisopropyl fluorophosphate–treated APC (DIP-APC; inactive derivative of ARC) had no effect. Furthermore, APC significantly inhibited the I/R-induced decrease in renal tissue blood flow and the increase in the vascular permeability, whereas neither DEGR-FXa, heparin, nor DIP-APC produced such effects. Renal I/R-induced increases in plasma levels of fibrin degradation products were significantly inhibited by APC, DEGR-FXa, and heparin. These observations suggest that APC reduces I/R-induced renal injury independently of its anticoagulant effects but in a manner dependent on its serine protease activity. Renal levels of tumor necrosis factor- (TNF-), rat interleukin-8, and myeloperoxidase were significantly increased after renal I/R. These increases were significantly inhibited by APC but not by DEGR-FXa, heparin, or DIP-APC. Leukocytopenia produced effects similar to those of APC. These findings strongly suggest that APC protects against I/R-induced renal injury not by inhibiting coagulation abnormalities but by inhibiting activation of leukocytes that play an important role in I/R-induced renal injury. Inhibition of leukocyte activation by APC could be explained by the inhibitory activity of TNF-.

Role of leukocytes in spinal cord injury in rats

Spinal cord injury (SCI) is a serious condition that produces life-long disabilities. Only limited therapeutic measures are currently available for its treatment. This review describes the role of leukocytes in pathologic mechanisms of trauma-induced SCI in rats, which contributes to new understanding of the pathologic process involved in SCI and could lead to the development of new therapeutic strategies by which leukocyte activation can be regulated. SCI induced by trauma is a consequence of an initial physical insult that is followed by a progressive injury process which involves various pathochemical events that lead to tissue destruction. Therapeutic intervention in SCI should therefore be directed at reducing or alleviating this secondary process. Although the mechanisms are not fully understood, progressive vascular events, especially activated neutrophil-induced endothelial cell damage, have been shown to be implicated. We have found that some therapeutic agents, which inhibit leukocyte activation directly or indirectly, alleviate the motor disturbances observed in a rat model of SCI. Methylprednisolone (MPS) and GM1 ganglioside, which are the only two pharmacological agents currently clinically available for treatment of acute SCI, do not inhibit neutrophil activation in this rat model. Taken together, these observations raise a possibility that pharmacological agents that inhibit leukocyte activation used in conjunction with MPS or GM1 may have a synergistic effect in the clinical treatment of traumatic SCI in humans.

Antithrombin Reduces Ischemia/Reperfusion Injury of Rat Liver by Increasing the Hepatic Level of Prostacyclin

We investigated whether antithrombin (AT) can reduce ischemia/reperfusion (I/R)-induced injury of rat liver by promoting prostacyclin release from endothelial cells. Although intravenous administration of AT (250 U/kg) markedly reduced hepatic injury, neither dansyl-Glu-Gly-Arg-chloromethyl ketone-treated factor Xa (DEGR-Xa), a selective inhibitor of thrombin generation, nor Trp49-modified AT, which lacks affinity for heparin, had any effect. Hepatic levels of 6-keto-PGF1, a stable prostacyclin (PGI2) metabolite, were increased significantly after I/R of the rat liver. AT significantly increased the hepatic level of 6-keto-PGF1, whereas neither DEGR-Xa nor Trp49-modified AT increased it. Hepatic tissue blood flow was markedly reduced after I/R. Although AT significantly increased the hepatic tissue blood flow after I/R, neither DEGR-Xa nor Trp49-modified AT increased the blood flow. Hepatic levels of cytokine-induced neutrophil chemoattractant (CINC) and myeloperoxidase (MPO) were significantly increased after hepatic I/R. The levels of these two indicators were reduced by AT but were unaffected by either DEGR-Xa or Trp49-modified AT. Pretreatment of animals with indomethacin (IM) completely inhibited the protective effects of AT on the I/R-induced hepatic damage and the leukocyte activation as well as the AT-induced increase in hepatic 6-keto-PGF1 levels after I/R. Iloprost, a stable analog of PGI2, exhibited effects similar to those of AT and also significantly inhibited the exacerbation of liver injury, the decrease in hepatic tissue blood flow, and the increases in hepatic CINC and MPO levels seen in rats subjected to I/R but pretreated with IM. These findings suggest that AT may prevent I/R-induced hepatic injury by increasing the hepatic levels of PGI2 through the interaction of AT with cell-surface glycosaminoglycans, thus increasing hepatic tissue blood flow and inhibiting leukocyte activation in animals subjected to I/R.

Antithrombin Reduces Ischemia/Reperfusion Injury of Rat Liver by Increasing the Hepatic Level of Prostacyclin

We investigated whether antithrombin (AT) can reduce ischemia/reperfusion (I/R)-induced injury of rat liver by promoting prostacyclin release from endothelial cells. Although intravenous administration of AT (250 U/kg) markedly reduced hepatic injury, neither dansyl-Glu-Gly-Arg-chloromethyl ketone-treated factor Xa (DEGR-Xa), a selective inhibitor of thrombin generation, nor Trp49-modified AT, which lacks affinity for heparin, had any effect. Hepatic levels of 6-keto-PGF1, a stable prostacyclin (PGI2) metabolite, were increased significantly after I/R of the rat liver. AT significantly increased the hepatic level of 6-keto-PGF1, whereas neither DEGR-Xa nor Trp49-modified AT increased it. Hepatic tissue blood flow was markedly reduced after I/R. Although AT significantly increased the hepatic tissue blood flow after I/R, neither DEGR-Xa nor Trp49-modified AT increased the blood flow. Hepatic levels of cytokine-induced neutrophil chemoattractant (CINC) and myeloperoxidase (MPO) were significantly increased after hepatic I/R. The levels of these two indicators were reduced by AT but were unaffected by either DEGR-Xa or Trp49-modified AT. Pretreatment of animals with indomethacin (IM) completely inhibited the protective effects of AT on the I/R-induced hepatic damage and the leukocyte activation as well as the AT-induced increase in hepatic 6-keto-PGF1 levels after I/R. Iloprost, a stable analog of PGI2, exhibited effects similar to those of AT and also significantly inhibited the exacerbation of liver injury, the decrease in hepatic tissue blood flow, and the increases in hepatic CINC and MPO levels seen in rats subjected to I/R but pretreated with IM. These findings suggest that AT may prevent I/R-induced hepatic injury by increasing the hepatic levels of PGI2 through the interaction of AT with cell-surface glycosaminoglycans, thus increasing hepatic tissue blood flow and inhibiting leukocyte activation in animals subjected to I/R.

Spinal cord injury in the rat

Only limited therapeutic measures are currently available for the treatment of spinal cord injury. This review describes the pathologic mechanisms of trauma-induced spinal cord injury in rats, which will contribute to new understanding of the pathologic process leading to spinal cord injury and to further development of new therapeutic strategies. Spinal cord injury induced by trauma is a consequence of an initial physical insult and a subsequent progressive injury process that involves various pathochemical events leading to tissue destruction; the latter process should therefore be a target of pharmacological treatment. Recently, activated neutrophils have been shown to be implicated in the latter process of the spinal cord injury in rats. Activated neutrophils damage the endothelial cells by releasing inflammatory mediators such as neutrophil elastase and oxygen free radicals. Adhesion of activated neutrophils to the endothelial cell could also play a role in endothelial cell injury. This endothelial cell injury could in turn induce microcirculatory disturbances leading to spinal cord ischemia. We have found that some therapeutic agents that inhibit neutrophil activation alleviate the motor disturbances observed in the rat model of spinal cord injury. Methylprednisolone (MPS) and GM1 ganglioside, which are the only two pharmacological agents currently clinically available for treatment of acute spinal cord injury, do not inhibit neutrophil activation in this rat model. Taken together, these observations raise a possibility that other pharmacological agents that inhibit neutrophil activation used in conjunction with MPS or GM1 ganglioside may have a synergistic effect in the treatment of traumatic spinal cord injury in humans.

Regions remote from the site of cleavage determine macromolecular substrate recognition by the prothrombinase complex

The proteolytic formation of thrombin is catalyzed by the prothrombinase complex of blood coagulation. The kinetics of prethrombin 2 cleavage was studied to delineate macromolecular substrate structures necessary for recognition at the exosite(s) of prothrombinase. The product, alpha-thrombin, was a linear competitive inhibitor of prethrombin 2 activation without significantly inhibiting peptidyl substrate cleavage by prothrombinase. Prethrombin 2 and alpha-thrombin compete for binding to the exosite without restricting access to the active site of factor Xa within prothrombinase. Inhibition by alpha-thrombin was not altered by saturating concentrations of low molecular weight heparin. Furthermore, proteolytic removal of the fibrinogen recognition site in alpha-thrombin only had a modest effect on its inhibitory properties. Both alpha-thrombin and prethrombin 2 were cleaved with chymotrypsin at Trp148 and separated into component domains. The C-terminal-derived zeta2 fragment retained the ability to selectively inhibit macromolecular substrate cleavage by prothrombinase, while the zeta1 fragment was without effect. As the zeta2 fragment lacks the fibrinogen recognition site, the P1-P3 residues or the intact cleavage site, specific recognition of the macromolecular substrate by the exosite in prothrombinase is achieved through substrate regions, distinct from the fibrinogen recognition or heparin-binding sites, and spatially removed from structures surrounding the scissile bond.

Protein S alters the active site location of activated protein C above the membrane surface. A fluorescence resonance energy transfer study of topography

The location of the active site of membrane-bound activated protein C (APC) relative to the phospholipid surface was determined both in the presence and absence of its cofactor, protein S, using fluorescence resonance energy transfer (FRET). APC was chemically modified to create the FRET donor species, Fl-FPR-APC, with a fluorescein dye (Fl) covalently attached to the active site via a D-Phe-Pro-Arg (FPR) tether and located in the active site near S4. FRET was observed when Fl-FPR-APC was titrated in the presence of Ca2+ ions with phosphatidylcholine/phosphatidylserine (4:1) vesicles containing the FRET acceptor, octadecylrhodamine (OR). Assuming a random orientation of transition dipoles (kappa2 = 2/3), the average distance of closest approach between the fluorescein in the active site of the membrane-bound APC and the OR at the membrane surface is 94 A. The same calcium-dependent distance was obtained for both small and large unilamellar vesicles and for vesicles that contained phosphatidylethanolamine. The active site of membrane-bound APC is therefore located far above the phospholipid surface. Upon addition of protein S, the efficiency of Fl-FPR-APC to OR energy transfer increased due to a protein S-dependent rotational and/or translational movement of the APC protease domain relative to the surface. If this movement were solely translational, then the average height of the fluorescein in the membrane-bound APC.protein S complex would be 84 A above the surface. The extent of Fl-FPR-APC to OR energy transfer was unaltered by the addition of thrombin-inactivated protein S. The protein S effect was also specific for APC, since the addition of protein S to similarly-labeled derivatives of factor Xa, factor IXa, or factor VIIa did not alter the locations of their active sites. This direct measurement demonstrates that the binding of the protein S cofactor to its cognate enzyme elicits a relocation of the active site of APC relative to the membrane surface and thereby provides a structural explanation for the recently observed protein S-dependent change in the site of factor Va cleavage by APC.

Effect of human urinary thrombomodulin on endotoxin-induced intravascular coagulation and pulmonary vascular injury in rats

Adult respiratory distress syndrome (ARDS) and disseminated intravascular coagulation (DIC) are serious complications of sepsis. Thrombomodulin, an important endothelial anticoagulant, binds thrombin to generate activated protein C (APC). To determine whether thrombomodulin purified from human urine (urinary thrombomodulin, UTM) is useful for the treatment of DIC and ARDS in sepsis, we examined the effect of UTM on endotoxin (ET)-induced coagulation abnormalities and pulmonary vascular injury in rats. Intravenous administration of UTM prevented the ET-induced pulmonary accumulation of leukocytes and the increase in pulmonary vascular permeability, as well as ET-induced histological changes such as leukocyte infiltration and pulmonary interstitial edema. On the other hand, dansyl-Glu-Gly-Arg-chloromethyl ketone-treated factor Xa (DEGR-Xa), a selective inhibitor of thrombin generation, did not prevent these effects of ET. UTM did not prevent ET-induced pulmonary accumulation of leukocytes and pulmonary vascular injury in rats pretreated with DEGR-Xa. Our findings suggest that UTM attenuates ET-induced coagulation abnormalities and pulmonary vascular injury. Furthermore, the latter effect may be dependent on the capacity of UTM to activate protein C.

Prothrombinase components can accelerate tissue plasminogen activator-catalyzed plasminogen activation

The enzymatic and cofactor subunits of human prothrombinase, factor Xa (FXa) and factor Va (FVa), respectively, were evaluated as modulators of Glu- and Lys-plasminogen (Pg) activation by tissue plasminogen activator (tPA). The data revealed that both FXa and FVa could accelerate tPA activity by as much as 60-fold for Lys-Pg and > 150-fold for Glu-Pg. This function of FVa depended on pretreatment with plasmin (Pn), whereas the FXa fibrinolytic cofactor activity was endogenous. In the native state, FVa was observed to inhibit the acceleration of Pn generation by FXa. These effects were dependent on Ca2+ and procoagulant phospholipid. Interactions between plasminogen and prothrombinase components were quantified. The apparent Kd for binding to FXa was 35 nM. Strikingly, the affinity between FVa and Pg was increased by approximately 2 orders of magnitude when the FVa was Pn-pretreated (Kd = 0.1 microM). These data cumulatively suggest a mechanism by which Pn production is coordinated with coagulation and localized to sites where procoagulant phospholipid is exposed on a cell surface.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Effect of gamma carboxylation on prothrombinase inhibitory activity of catalytically inactive factor XA

Recombinant catalytically inactive factor Xa (factor rXai) is capable of assembly into inactive prothrombinase complexes, thus serving as a competitive inhibitor (Ki = 0.3nM) of active factor Xa. In order to study the role of gamma carboxylation in prothrombinase complex assembly, we have prepared differentially gamma carboxylated factor rXai and have measured the activities of these proteins in prothrombinase complex inhibition and in extension of plasma clotting. A factor rXai preparation containing 8 out of a possible maximum of 11 g carboxyglutamic acid (GLA) residues was found to be as active as chemically inactivated plasma factor Xa which was fully gamma carboxylated. Loss of a single additional g carboxyglutamic acid in the recombinant protein, however lead to a marked loss in activity. Factor rXai preparation with 8 GLA residues is also detected by a monoclonal antibody specific for a GLA dependent epitope. Thus assembly of the factor Va/Xa complex on phospholipid membranes does not require the presence of all of the g carboxyglutamic acid residues present in the plasma protein.

Biochemistry of factor X

Factor X circulates as a serine protease which is converted to the active form at the point of convergence of the intrinsic and extrinsic coagulation pathways. Subsequently, the enzymatic species, factor Xa, is involved in macromolecular complex formation with its cofactor factor Va, a phospholipid surface and calcium to convert prothrombin into thrombin. The gene encoding factor X shares a number of structural and organisational features in common with the other vitamin K-dependent coagulation proteins, suggesting that they have evolved from a common ancestral gene. Each of the exons encoding these proteins can be considered as a module coding for a homologous domain in each protein. These structural domains in factor X are responsible for specific functional properties including gamma-carboxylase recognition, calcium binding, phospholipid surface interaction, as well as cofactor and substrate binding. Studies of recombinant proteins and proteolytic fragments continue to provide significant insight into structure-function relationships of the protein modules within factor X.

Studies of factor Xa/phospholipid-induced intravascular coagulation in rabbits. Effects of immunodepletion of tissue factor pathway inhibitor

In earlier studies from this laboratory evidence was obtained for a physiological function of tissue factor pathway inhibitor (TFPI) as a regulator of hemostasis capable of preventing thrombotic complications that might otherwise result from exposure of blood to trace amounts of tissue factor (TF). However, it was not possible to conclude that the protective effect of TFPI stemmed solely from inhibition of factor VIIa/TF catalytic activity, since TFPI neutralizes stoichiometric amounts of factor Xa in forming an inhibited factor Xa/TFPI/factor VIIa/TF complex. Therefore, we examined the effects of immunodepletion of TFPI on the extent of coagulation initiated in rabbits by exposure to factor Xa and phospholipid in the absence of TF. In one experimental approach, factor Xa was generated endogenously with the factor X-activating fraction of Russell's viper venom (0.33 microgram/kg) in rabbits receiving an infusion of phosphatidylcholine/phosphatidylserine (PCPS) vesicles, 1 mg/kg over 2 hours. In a second approach, rabbits were injected with a complex of factor Xa (0.75 microgram/kg) and PCPS (12.5 micrograms/kg). In contrast with the observed sensitization of TFPI-depleted rabbits to TF-induced coagulation, TFPI-depleted rabbits were not sensitized to coagulation initiated by factor Xa and phospholipid in the absence of TF. These data support the conclusion that the physiological function of TFPI in regulating TF-dependent coagulation stems primarily from its ability to inhibit factor VIIa/TF catalytic activity.