Calcium ion modulation of meizothrombin autolysis at Arg55-Asp56 and catalytic activity

Comparative sequence analysis of vitamin K-dependent coagulation factors

BACKGROUND

Prothrombin, protein C, and factors VII, IX, and X are vitamin K (VK)-dependent coagulation proteins that play an important role in the initiation, amplification, and subsequent attenuation of the coagulation response. Blood coagulation evolved in the common vertebrate ancestor as a specialization of the complement system and immune response, which in turn bear close evolutionary ties with developmental enzyme cascades. There is currently no comprehensive analysis of the evolutionary changes experienced by these coagulation proteins during the radiation of vertebrates and little is known about conservation of residues that are important for zymogen activation and catalysis.

OBJECTIVES

To characterize the conservation level of functionally important residues among VK-dependent coagulation proteins from different vertebrate lineages.

METHODS

The conservation level of residues important for zymogen activation and catalysis was analyzed in >1600 primary sequences of VK-dependent proteins.

RESULTS

Functionally important residues are most conserved in prothrombin and least conserved in protein C. Some of the most profound functional modifications in protein C occurred in the ancestor of bony fish when the basic residue in the activation site was replaced by an aromatic residue. Furthermore, during the radiation of placental mammals from marsupials, protein C acquired a cysteine-rich insert that introduced an additional disulfide in the EGF1 domain and evolved a proprotein convertase cleavage site in the activation peptide linker that also became significantly elongated.

CONCLUSIONS

Sequence variabilities at functionally important residues may lead to interspecies differences in the zymogen activation and catalytic properties of orthologous VK-dependent proteins.

Enhancing the anticoagulant profile of meizothrombin

Meizothrombin is an active intermediate generated during the proteolytic activation of prothrombin to thrombin in the penultimate step of the coagulation cascade. Structurally, meizothrombin differs from thrombin because it retains the auxiliary Gla domain and two kringles. Functionally, meizothrombin shares with thrombin the ability to cleave procoagulant (fibrinogen), prothrombotic (PAR1) and anticoagulant (protein C) substrates, although its specificity toward fibrinogen and PAR1 is less pronounced. In this study we report information on the structural architecture of meizothrombin resolved by SAXS and single molecule FRET as an elongated arrangement of its individual domains. In addition, we show the properties of a meizothrombin construct analogous to the anticoagulant thrombin mutant W215A/E217A currently in Phase I for the treatment of thrombotic complications and stroke. The findings reveal new structural and functional aspects of meizothrombin that advance our understanding of a key intermediate of the prothrombin activation pathway.

High throughput protease profiling comprehensively defines active site specificity for thrombin and ADAMTS13

We have combined random 6 amino acid substrate phage display with high throughput sequencing to comprehensively define the active site specificity of the serine protease thrombin and the metalloprotease ADAMTS13. The substrate motif for thrombin was determined by >6,700 cleaved peptides, and was highly concordant with previous studies. In contrast, ADAMTS13 cleaved only 96 peptides (out of >10⁷ sequences), with no apparent consensus motif. However, when the hexapeptide library was substituted into the P3-P3′ interval of VWF73, an exosite-engaging substrate of ADAMTS13, 1670 unique peptides were cleaved. ADAMTS13 exhibited a general preference for aliphatic amino acids throughout the P3-P3′ interval, except at P2 where Arg was tolerated. The cleaved peptides assembled into a motif dominated by P3 Leu, and bulky aliphatic residues at P1 and P1′. Overall, the P3-P2′ amino acid sequence of von Willebrand Factor appears optimally evolved for ADAMTS13 recognition. These data confirm the critical role of exosite engagement for substrates to gain access to the active site of ADAMTS13, and define the substrate recognition motif for ADAMTS13. Combining substrate phage display with high throughput sequencing is a powerful approach for comprehensively defining the active site specificity of proteases.

Effect of zymogen domains and active site occupation on activation of prothrombin by von Willebrand factor-binding protein

Prothrombin is conformationally activated by von Willebrand factor-binding protein (vWbp) from Staphylococcus aureus through insertion of the NH(2)-terminal residues of vWbp into the prothrombin catalytic domain. The rate of prothrombin activation by vWbp(1-263) is controlled by a hysteretic kinetic mechanism initiated by substrate binding. The present study evaluates activation of prothrombin by full-length vWbp(1-474) through activity progress curve analysis. Additional interactions from the COOH-terminal half of vWbp(1-474) strengthened the initial binding of vWbp to prothrombin, resulting in higher activity and an ∼100-fold enhancement in affinity. The affinities of vWbp(1-263) or vWbp(1-474) were compared by equilibrium binding to the prothrombin derivatives prethrombin 1, prethrombin 2, thrombin, meizothrombin, and meizothrombin(des-fragment 1) and their corresponding active site-blocked analogs. Loss of fragment 1 in prethrombin 1 enhanced affinity for both vWbp(1-263) and vWbp(1-474), with a 30-45% increase in Gibbs free energy, implicating a regulatory role for fragment 1 in the activation mechanism. Active site labeling of all prothrombin derivatives with D-Phe-Pro-Arg-chloromethyl ketone, analogous to irreversible binding of a substrate, decreased their K(D) values for vWbp into the subnanomolar range, reflecting the dependence of the activating conformational change on substrate binding. The results suggest a role for prothrombin domains in the pathophysiological activation of prothrombin by vWbp, and may reveal a function for autocatalysis of the vWbp·prothrombin complexes during initiation of blood coagulation.

Evaluation of the efficacy and safety of rivaroxaban using a computer model for blood coagulation

Rivaroxaban is an oral, direct Factor Xa inhibitor approved in the European Union and several other countries for the prevention of venous thromboembolism in adult patients undergoing elective hip or knee replacement surgery and is in advanced clinical development for the treatment of thromboembolic disorders. Its mechanism of action is antithrombin independent and differs from that of other anticoagulants, such as warfarin (a vitamin K antagonist), enoxaparin (an indirect thrombin/Factor Xa inhibitor) and dabigatran (a direct thrombin inhibitor). A blood coagulation computer model has been developed, based on several published models and preclinical and clinical data. Unlike previous models, the current model takes into account both the intrinsic and extrinsic pathways of the coagulation cascade, and possesses some unique features, including a blood flow component and a portfolio of drug action mechanisms. This study aimed to use the model to compare the mechanism of action of rivaroxaban with that of warfarin, and to evaluate the efficacy and safety of different rivaroxaban doses with other anticoagulants included in the model. Rather than reproducing known standard clinical measurements, such as the prothrombin time and activated partial thromboplastin time clotting tests, the anticoagulant benchmarking was based on a simulation of physiologically plausible clotting scenarios. Compared with warfarin, rivaroxaban showed a favourable sensitivity for tissue factor concentration inducing clotting, and a steep concentration–effect relationship, rapidly flattening towards higher inhibitor concentrations, both suggesting a broad therapeutic window. The predicted dosing window is highly accordant with the final dose recommendation based upon extensive clinical studies.

Optimization of fed-batch parameters and harvest time of CHO cell cultures for a glycosylated product with multiple mechanisms of inactivation

Optimization of fed-batch feeding parameters was explored for a system with multiple mechanisms of product inactivation. In particular, two separate mechanisms of inactivation were identified for the recombinant tissue-type activator (r-tPA) protein. Dynamic inactivation models were written to describe particular r-tPA glycoform inactivation in the presence and absence of free-glucose. A glucose-independent inactivation mechanism was identified, and inactivation rate constants were found dependent upon the presence of glycosylation of r-tPA at N184. Inactivation rate constants of the glucose-dependent mechanism were not affected by glycosylation at N184. Fed-batch optimization was performed for r-tPA production by CHO cell culture in a stirred-tank reactor with glucose, glutamine and asparagine feed. Feeding profiles in which culture supernatant concentrations of free-glucose and amino acids (combined glutamine and asparagine) were used as control variables, were evaluated for a wide variety of set points. Simulation results for a controlled feeding strategy yielded an optimum at set points of 1.51 g L(-1) glucose and 1.18 g L(-1) of amino acids. Optimization was also performed in absence of metabolite control using fixed feed-flow rates initiate during the exponential growth phase. Fixed feed-flow results displayed a family of optimum solutions along a mass flow rate ratio of 3.15 of glucose to amino acids. Comparison of the two feeding strategies showed a slight advantage of rapid feeding at a fixed flow rate as opposed to metabolite control for a product with multiple mechanisms of inactivation.

A control switch for prothrombinase: characterization of a hirudin-like pentapeptide from the COOH terminus of factor Va heavy chain that regulates the rate and pathway for prothrombin activation

Membrane-bound factor Xa alone catalyzes prothrombin activation following initial cleavage at Arg(271) and prethrombin 2 formation (pre2 pathway). Factor Va directs prothrombin activation by factor Xa through the meizothrombin pathway, characterized by initial cleavage at Arg(320) (meizo pathway). We have shown previously that a pentapeptide encompassing amino acid sequence 695-699 from the COOH terminus of the heavy chain of factor Va (Asp-Tyr-Asp-Tyr-Gln, DYDYQ) inhibits prothrombin activation by prothrombinase in a competitive manner with respect to substrate. To understand the mechanism of inhibition of thrombin formation by DYDYQ, we have studied prothrombin activation by gel electrophoresis. Titration of plasma-derived prothrombin activation by prothrombinase, with increasing concentrations of peptide, resulted in complete inhibition of the meizo pathway. However, thrombin formation still occurred through the pre2 pathway. These data demonstrate that the peptide preferentially inhibits initial cleavage of prothrombin by prothrombinase at Arg(320). These findings were corroborated by studying the activation of recombinant mutant prothrombin molecules rMZ-II (R155A/R284A/R271A) and rP2-II (R155A/R284A/R320A) which can be only cleaved at Arg(320) and Arg(271), respectively. Cleavage of rMZ-II by prothrombinase was completely inhibited by low concentrations of DYDYQ, whereas high concentrations of pentapeptide were required to inhibit cleavage of rP2-II. The pentapeptide also interfered with prothrombin cleavage by membrane-bound factor Xa alone in the absence of factor Va increasing the rate for cleavage at Arg(271) of plasma-derived prothrombin or rP2-II. Our data demonstrate that pentapeptide DYDYQ has opposing effects on membrane-bound factor Xa for prothrombin cleavage, depending on the incorporation of factor Va in prothrombinase.

The Structural Integrity of Anion Binding Exosite I of Thrombin Is Required and Sufficient for Timely Cleavage and Activation of Factor V and Factor VIII

Alpha-thrombin has two separate electropositive binding exosites (anion binding exosite I, ABE-I and anion binding exosite II, ABE-II) that are involved in substrate tethering necessary for efficient catalysis. Alpha-thrombin catalyzes the activation of factor V and factor VIII following discrete proteolytic cleavages. Requirement for both anion binding exosites of the enzyme has been suggested for the activation of both procofactors by alpha-thrombin. We have used plasma-derived alpha-thrombin, beta-thrombin (a thrombin molecule that has only ABE-II available), and a recombinant prothrombin molecule rMZ-II (R155A/R284A/R271A) that can only be cleaved at Arg(320) (resulting in an enzymatically active molecule that has only ABE-I exposed, rMZ-IIa) to ascertain the role of each exosite for procofactor activation. We have also employed a synthetic sulfated pentapeptide (DY(SO(3)(-))DY(SO(3)(-))Q, designated D5Q1,2) as an exosite-directed inhibitor of thrombin. The clotting time obtained with beta-thrombin was increased by approximately 8-fold, whereas rMZ-IIa was 4-fold less efficient in promoting clotting than alpha-thrombin under similar experimental conditions. Alpha-thrombin readily activated factor V following cleavages at Arg(709), Arg(1018), and Arg(1545) and factor VIII following proteolysis at Arg(372), Arg(740), and Arg(1689). Cleavage of both procofactors by alpha-thrombin was significantly inhibited by D5Q1,2. In contrast, beta-thrombin was unable to cleave factor V at Arg(1545) and factor VIII at both Arg(372) and Arg(1689). The former is required for light chain formation and expression of optimum factor Va cofactor activity, whereas the latter two cleavages are a prerequisite for expression of factor VIIIa cofactor activity. Beta-thrombin was found to cleave factor V at Arg(709) and factor VIII at Arg(740), albeit less efficiently than alpha-thrombin. The sulfated pentapeptide inhibited moderately both cleavages by beta-thrombin. Under similar experimental conditions, membrane-bound rMZ-IIa cleaved and activated both procofactor molecules. Activation of the two procofactors by membrane-bound rMZ-IIa was severely impaired by D5Q1,2. Overall the data demonstrate that ABE-I alone of alpha-thrombin can account for the interaction of both procofactors with alpha-thrombin resulting in their timely and efficient activation. Because formation of meizothrombin precedes that of alpha-thrombin, our findings also imply that meizothrombin may be the physiological activator of both procofactors in vivo in the presence of a procoagulant membrane surface during the early stages of coagulation.

Practical applications of snake venom toxins in haemostasis

Snake venom toxins affecting haemostasis have facilitated extensively the routine assays of haemostatic parameters in the coagulation laboratory. Snake venom thrombin-like enzymes (SVTLE) are used for fibrinogen/fibrinogen breakdown product assay and for the detection of fibrinogen dysfunction. SVTLE are not inhibited by heparin and can thus can be used for assaying antithrombin III and other haemostatic variables in heparin-containing samples. Snake venoms are a rich source of prothrombin activators and these are utilised in prothrombin assays, for studying dysprothrombinaemias and for preparing meizothrombin and non-enzymic forms of prothrombin. Russell's viper (Daboia russelli) venom (RVV) contains toxins which have been used to assay blood clotting factors V, VII, X, platelet factor 3 and, importantly, lupus anticoagulants (LA). Other prothrombin activators (from the taipan, Australian brown snake and saw-scaled viper) have now been used to assay LA. Protein C and activated protein C resistance can be measured by means of RVV and Protac, a fast acting inhibitor from Southern copperhead snake venom and von Willebrand factor can be studied with botrocetin from Bothrops jararaca venom. The disintegrins, a large family of Arg-Gly-Asp (RGD)-containing snake venom proteins, show potential for studying platelet glycoprotein receptors, notably, GPIIb/IIIa and Ib. Snake venom toxins affecting haemostasis are also used in the therapeutic setting: Ancrod (from the Malayan pit viper, Calloselasma rhodostoma), in particular, has been used as an anticoagulant to achieve 'therapeutic defibrination'. Other snake venom proteins show promise in the treatment of a range of haemostatic disorders.

Role of prothrombin fragment 1 in the pathway of regulatory exosite I formation during conversion of human prothrombin to thrombin

Prothrombin (Pro) activation by factor Xa generates the thrombin catalytic site and exosites I and II. The role of fragment 1 (F1) in the pathway of exosite I expression during Pro activation was characterized in equilibrium binding studies using hirudin(54-65) labeled with 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoate ([NBD]Hir(54-65)(SO3-)) or 5-(carboxy)fluorescein ([5F]Hir(54-65)(SO3-)). [NBD]Hir(54-65)(SO3-) distinguished exosite I environments on Pro, prethrombin 1 (Pre 1), and prethrombin 2 (Pre 2) but bound with the same affinities as [5F]Hir(54-65)(SO3-). Conversion of Pro to Pre 1 caused a 7-fold increase in affinity for the peptides. Conversely, fragment 1.2 (F1.2) decreased the affinity of Pre 2 for [5F]Hir(54-65)(SO3-) by 3-fold. This was correlated with a 16-fold increased affinity of F1.2 for Pre 2 in comparison to thrombin, demonstrating an enhancing effect of F1 on F1.2 binding. The active intermediate, meizothrombin, demonstrated a 50- to 220-fold increase in exosite affinity. Free thrombin and thrombin.F1.2 complex bound [5F]Hir(54-65)(SO3-) with indistinguishable affinity, indicating that the effect of F1 on peptide binding was eliminated upon expression of catalytic activity and exosite I. The results demonstrate a new zymogen-specific role for F1 in modulating the affinity of ligands for exosite I. This may reflect a direct interaction between the F1 and Pre 2 domains in Pro that is lost upon folding of the zymogen activation domain. The effect of F1 on (pro)exosite I and the role of (pro)exosite I in factor Va-dependent substrate recognition suggest that the Pro activation pathway may be regulated by (pro)exosite I interactions with factor Va.

Diagnostic uses of snake venom

Snake venom toxins are invaluable for the assay of coagulation factors and for the study of haemostasis generally. Thrombin-like enzymes (SVTLE) are used for fibrinogen and fibrinogen breakdown product assays as well as detecting dysfibrinogenaemias. Since SVTLE are not inhibited by heparin, they can be used for assaying antithrombin III in samples containing heparin. Snake venom prothrombin activators are utilised in prothrombin assays, whilst Russell's viper venom (RVV) can be used to assay clotting factors V, VII, X and lupus anticoagulants (LA). Activators from the taipan, Australian brown snake and saw-scaled viper have also been used to assay LA. Protein C (PC) and activated PC (APC) resistance can be measured by means of RVV, Protac (from Southern copperhead snake venom) and STA-Staclot (from Crotalus viridis helleri) whilst von Willebrand factor can be studied with Botrocetin (Bothrops jararaca). Finally, snake venom C-type lectins and metalloproteinase disintegrins are being used to study platelet glycoprotein receptors and show great potential for use in the routine coagulation laboratory.

Fluorescence properties and functional roles of tryptophan residues 60d, 96, 148, 207, and 215 of thrombin

Conservative Trp-to-Phe mutations were individually created in human thrombin at positions 60d, 96, 148, 207, and 215. Fluorescence intensities for these residues varied by a factor of 6. Residues 60d, 96, 148, and 215 transferred energy to the thrombin inhibitor 5-dimethylaminonaphthalene-1-sulfonylarginine-N-(3-ethyl-1,5- pentanediyl)amide efficiently, but residue 207 did not. Intensities correlated inversely with exposure to solvent, and measured and theoretical energy transfer efficiencies agreed well. Function was measured with respect to fibrinogen clotting, platelet and factor V activation, inhibition by antithrombin, and the thrombomodulin-dependent activation of protein C and thrombin-activable fibrinolysis inhibitor (TAFI). All activities of W96F and W207F ranged from 74 to 154% of the wild-type activity. This was also true for W148F, except for inhibition by antithrombin, where it showed 60% activity. W60dF was deficient by 30, 57, and 43% with fibrinogen clotting, platelet activation, and factor V cleavage (Arg(1006)), respectively. W215F was deficient by 90, 55, and 56% with fibrinogen clotting, platelet activation, and factor V cleavage (Arg(1536)). With protein C and TAFI, W96F, W148F, and W207F were normal. W60dF, however, was 76 and 23% of normal levels with protein C and TAFI, respectively. In contrast, W215F was 25 and 124% of normal levels in these reactions. Thus, many activities of thrombin are retained upon substitution of Trp with Phe at positions 96, 148, and 207. Trp(60d), however, appears to be very important for TAFI activation, and Trp(215) appears to very important for clotting and protein C activation.

A dimeric form of prothrombin on membrane surfaces

Blood coagulation requires the conversion of zymogens to active enzymes. These reactions are facilitated by Ca2+-dependent protein binding to membrane surfaces containing anionic phospholipids. Here it is shown that only in the presence of both Ca2+ and phospholipid vesicles composed of phosphatidylcholine and phosphatidylserine can a prothrombin dimer be chemically cross-linked. A cross-linker containing evenly spaced reactive groups was prepared by activating the carboxy groups of a ten-residue glutamic acid peptide and allowed to react with physiological concentrations of prothrombin. When Ca2+ and anionic phospholipids were both present during exposure to the cross-linker, it was found that more than 50% of the prothrombin was trapped as a chemically defined dimer with reaction times of the order of 1 min. The dimer yield remained high even when reactions were performed at high phospholipid-to-protein ratios at protein concentrations an order of magnitude less than physiological. Amino acid sequencing of a CNBr peptide produced from the purified dimer localized the cross-link to residues Lys341 and Lys427 of prothrombin. The specificity and high yield under mild conditions of the cross-linking suggest that dimeric membrane bound prothrombin might be a physiologically relevant substrate for the formation of thrombin. Prothrombinase converts this modified protein to an enzyme that catalyses the hydrolysis of a thrombin chromogenic substrate as efficiently as thrombin and is inhibited by a thrombin active-site directed inhibitor, but is a thrombin dimer. The thrombin dimer has impaired activity compared with thrombin with respect to physiological functions requiring binding to exosite I. A model based on the known structure of thrombin is presented that can account for the prothrombin dimer and the properties of the dimeric thrombin formed from it.

New insights into the regulation of the blood clotting cascade derived from the X-ray crystal structure of bovine meizothrombin des F1 in complex with PPACK

BACKGROUND

The conversion of prothrombin to thrombin by factor Xa is the penultimate step in the blood clotting cascade. In vivo, where the conversion occurs primarily on activated platelets in association with factor Va and Ca2+ ions, meizothrombin is the major intermediate of the two step reaction. Meizothrombin rapidly loses the fragment 1 domain (F1) by autolysis to become meizothrombin des F1 (mzTBN-F1). The physiological properties of mzTBN-F1 differ dramatically from those of thrombin due to the presence of prothrombin fragment 2 (F2), which remains covalently attached to the activated thrombin domain in mzTBN-F1.

RESULTS

The crystal structure of mzTBN-F1 has been determined at 3.1 A resolution by molecular replacement, using only the thrombin domain, and refined to R and Rfree values of 0.205 and 0.242, respectively. The protease active site was inhibited with D-Phe-Pro-Arg-chloromethylketone (PPACK) to reduce autolysis. The mobile linker chain connecting the so-called kringle and thrombin domains and the first two N-acetylglucosamine residues attached to the latter were seen in electron-density maps improved with the program SQUASH. Previously these regions had only been modeled.

CONCLUSIONS

The F2 kringle domain in mzTBN-F1 is bound to the electropositive heparin-binding site on thrombin in an orientation that is systematically shifted and has significantly more interdomain contacts compared to a noncovalent complex of free F2 and free thrombin. F2 in mzTBN-F1 forms novel hydrogen bonds to the carbohydrate chain of thrombin and perhaps stabilizes a unique, rigid conformation of the gamma-autolysis loop through non-local effects. The F2 linker chain, which does not interfere with the active site or fibrinogen-recognition site, is arranged so that the two sites cleaved by factor Xa are separated by 36 A. The two mzTBN-F1 molecules in the asymmetric unit share a tight 'dimer' contact in which the active site of one molecule is partially blocked by the F2 kringle domain of its partner. This interaction suggests a new model for prothrombin organization.

Inhibition of meizothrombin and meizothrombin(desF1) by heparin cofactor II

Meizothrombin and meizothrombin(desF1) are intermediates formed during the conversion of prothrombin to thrombin by factor Xa, factor Va, phospholipids, and Ca2+ (prothrombinase). These intermediates are active toward synthetic peptide substrates but have limited ability to interact with platelets or macromolecular substrates such as fibrinogen. Meizothrombin and meizothrombin(desF1) activate protein C, however, and may exert primarily an anticoagulant effect. In this study, we investigated the inhibition of meizothrombin and meizothrombin(desF1) by two glycosaminoglycan-dependent protease inhibitors, heparin cofactor II (HCII) and antithrombin (AT). Purified recombinant meizothrombin and meizothrombin(desF1) were inhibited by HCII in the presence of dermatan sulfate with maximal second-order rate constants of 8 x 10(6) M-1.min-1 and 1.8 x 10(7) M-1.min-1, respectively, but were inhibited less than one-tenth as fast by AT in the presence of heparin. Similarly, the products of the prothrombinase reaction were inhibited in situ more effectively by HCII than by AT. When HCII and dermatan sulfate were present continuously during the prothrombinase reaction, meizothrombin was trapped as a sodium dodecyl sulfate-stable complex with HCII and no amidolytic activity could be detected with a thrombin substrate. Our findings indicate that HCII is an effective inhibitor of meizothrombin and meizothrombin(desF1) and, therefore, might regulate the anticoagulant activity of these proteases.

Functional characterization of recombinant human meizothrombin and Meizothrombin(desF1). Thrombomodulin-dependent activation of protein C and thrombin-activatable fibrinolysis inhibitor (TAFI), platelet aggregation, antithrombin-III inhibition

Recombinant human prothrombin (rII) and two mutant forms (R155A, R271A,R284A (rMZ) and R271A,R284A (rMZdesF1)) were expressed in mammalian cells. Following activation and purification, recombinant thrombin (rIIa) and stable analogues of meizothrombin (rMZa) and meizothrombin(desF1) (rMZdesF1a) were obtained. Studies of the activation of protein C in the presence of recombinant soluble thrombomodulin (TM) show TM-dependent stimulation of protein C activation by all three enzymes and, in the presence of phosphatidylserine/phosphatidylcholine phospholipid vesicles, rMZa is 6-fold more potent than rIIa. In the presence of TM, rMZa was also shown to be an effective activator of TAFI (thrombin-activatable fibrinolysis inhibitor) (Bajzar, L., Manuel, R., and Nesheim, M. E. (1995) J. Biol. Chem. 270, 14477-14484). All three enzymes were capable of inducing platelet aggregation, but 60-fold higher concentrations of rMZa and rMZdesF1a were required to achieve the effects obtained with rIIa. Second order rate constants (M-1.min-1) for inhibition by antithrombin III (AT-III) were 2.44 x 10(5) (rIIa), 6.10 x 10(4) (rMZa), and 1.05 x 10(5) (rMZdesF1a). The inhibition of rMZa and rMZdesF1a by AT-III is not affected by heparin. All three enzymes bound similarly to hirudin. The results of this and previous studies imply that full-length meizothrombin has marginal procoagulant properties compared to thrombin. However, meizothrombin has potent anticoagulant properties, expressed through TM-dependent activation of protein C, and can contribute to down-regulation of fibrinolysis through the TM-dependent activation of TAFI.