Proteolysis of factor Va by factor Xa and activated protein C

Activated protein C, protein S, and tissue factor pathway inhibitor cooperate to inhibit thrombin activation

INTRODUCTION

Thrombin, the enzyme which converts fibrinogen into a fibrin clot, is produced by the prothrombinase complex, composed of factor Xa (FXa) and factor Va (FVa). Down-regulation of this process is critical, as excess thrombin can lead to life-threatening thrombotic events. FXa and FVa are inhibited by the anticoagulants tissue factor pathway inhibitor alpha (TFPIα) and activated protein C (APC), respectively, and their common cofactor protein S (PS). However, prothrombinase is resistant to either of these inhibitory systems in isolation.

MATERIALS AND METHODS

We hypothesized that these anticoagulants function best together, and tested this hypothesis using purified proteins and plasma-based systems.

RESULTS

In plasma, TFPIα had greater anticoagulant activity in the presence of APC and PS, maximum PS activity required both TFPIα and APC, and antibodies against TFPI and APC had an additive procoagulant effect, which was mimicked by an antibody against PS alone. In purified protein systems, TFPIα dose-dependently inhibited thrombin activation by prothrombinase, but only in the presence of APC, and this activity was enhanced by PS. Conversely, FXa protected FVa from cleavage by APC, even in the presence of PS, and TFPIα reversed this protection. However, prothrombinase assembled on platelets was still protected from inhibition, even in the presence of TFPIα, APC, and PS.

CONCLUSIONS

We propose a model of prothrombinase inhibition through combined targeting of both FXa and FVa, and that this mechanism enables down-regulation of thrombin activation outside of a platelet clot. Platelets protect prothrombinase from inhibition, however, supporting a procoagulant environment within the clot.

The Procoagulant Snake Venom Serine Protease Potentially Having a Dual, Blood Coagulation Factor V and X-Activating Activity

A procoagulant snake venom serine protease was isolated from the venom of the nose-horned viper (Vipera ammodytes ammodytes). This 34 kDa glycoprotein, termed VaaSP-VX, possesses five kDa N-linked carbohydrates. Amino acid sequencing showed VaaSP-VX to be a chymotrypsin-like serine protease. Structurally, it is highly homologous to VaaSP-6 from the same venom and to nikobin from the venom of Vipera nikolskii, neither of which have known functions. VaaSP-VX does not affect platelets. The specific proteolysis of blood coagulation factors X and V by VaaSP-VX suggests that its blood-coagulation-inducing effect is due to its ability to activate these two blood coagulation factors, which following activation, combine to form the prothrombinase complex. VaaSP-VX may thus represent the first example of a serine protease with such a dual activity, which makes it a highly suitable candidate to replace diluted Russell’s viper venom in lupus anticoagulant testing, thus achieving greater reliability of the analysis. As a blood-coagulation-promoting substance that is resistant to serpin inhibition, VaaSP-VX is also interesting from the therapeutic point of view for treating patients suffering from hemophilia.

Proteolysis of plasma-derived factor V following its endocytosis by megakaryocytes forms the platelet-derived factor V/Va pool

BACKGROUND

Central to appropriate thrombin formation at sites of vascular injury is the concerted assembly of plasma- and/or platelet-derived factor (F) Va and FXa on the activated platelet surface. While the plasma-derived procofactor, FV, must be proteolytically activated by α-thrombin to FVa to function in prothrombinase, the platelet molecule is released from α-granules in a partially activated state, obviating the need for proteolytic activation.

OBJECTIVES

The current study was performed to test the hypothesis that subsequent to its endocytosis by megakaryocytes, plasma-derived FV is proteolytically processed to form the platelet-derived pool.

METHODS & RESULTS

Subsequent to FV endocytosis, a time-dependent increase in FV proteolytic products was observed in megakaryocyte lysates by SDS-PAGE followed by phosphorimaging or western blotting. This cleavage was specific and resulted in the formation of products similar in size to FV/Va present in a platelet lysate as well as to the α-thrombin-activated FVa heavy chain and light chain, and their respective precursors. Other proteolytic products were unique to endocytosed FV. The product/precursor relationships of these fragments were defined using anti-FV heavy and light chain antibodies with defined epitopes. Activity measurements indicated that megakaryocyte-derived FV fragments exhibited substantial FVa cofactor activity that was comparable to platelet-derived FV/Va.

CONCLUSIONS

Taken together, these observations suggest that prior to its packaging in α-granules endocytosed FV undergoes proteolysis by one or more specific megakaryocyte protease(s) to form the partially activated platelet-derived pool.

Modeling of human factor Va inactivation by activated protein C

BACKGROUND

Because understanding of the inventory, connectivity and dynamics of the components characterizing the process of coagulation is relatively mature, it has become an attractive target for physiochemical modeling. Such models can potentially improve the design of therapeutics. The prothrombinase complex (composed of the protease factor (F)Xa and its cofactor FVa) plays a central role in this network as the main producer of thrombin, which catalyses both the activation of platelets and the conversion of fibrinogen to fibrin, the main substances of a clot. A key negative feedback loop that prevents clot propagation beyond the site of injury is the thrombin-dependent generation of activated protein C (APC), an enzyme that inactivates FVa, thus neutralizing the prothrombinase complex. APC inactivation of FVa is complex, involving the production of partially active intermediates and "protection" of FVa from APC by both FXa and prothrombin. An empirically validated mathematical model of this process would be useful in advancing the predictive capacity of comprehensive models of coagulation.

RESULTS

A model of human APC inactivation of prothrombinase was constructed in a stepwise fashion by analyzing time courses of FVa inactivation in empirical reaction systems with increasing number of interacting components and generating corresponding model constructs of each reaction system. Reaction mechanisms, rate constants and equilibrium constants informing these model constructs were initially derived from various research groups reporting on APC inactivation of FVa in isolation, or in the presence of FXa or prothrombin. Model predictions were assessed against empirical data measuring the appearance and disappearance of multiple FVa degradation intermediates as well as prothrombinase activity changes, with plasma proteins derived from multiple preparations. Our work integrates previously published findings and through the cooperative analysis of in vitro experiments and mathematical constructs we are able to produce a final validated model that includes 24 chemical reactions and interactions with 14 unique rate constants which describe the flux in concentrations of 24 species.

CONCLUSION

This study highlights the complexity of the inactivation process and provides a module of equations describing the Protein C pathway that can be integrated into existing comprehensive mathematical models describing tissue factor initiated coagulation.

Coagulation procofactor activation by factor XIa

SUMMARY BACKGROUND

In the extrinsic pathway, the essential procofactors factor (F) V and FVIII are activated to FVa and FVIIIa by thrombin. In the contact pathway and its clinical diagnostic test, the activated partial thromboplastin time (APTT) assay, the sources of procofactor activation are unknown. In the APTT assay, FXII is activated on a negatively charged surface and proceeds to activate FXI, which activates FIX upon the addition of Ca(2+). FIXa feeds thrombin generation through activation of FX. FIXa is an extremely poor catalyst in the absence of its FVIIIa cofactor, which, in the intrinsic FXase complex, increases FXa generation by approximately 10(7). One potential APTT procofactor activator in this setting is FXIa.

OBJECTIVE

To test the hypothesis that FXIa can activate FVIII and FV.

METHODS

Recombinant FVIII and plasma FV were treated with FXIa, and the activities and integrities of each procofactor were measured using commercial clotting assays and sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

Kinetic analyses of FXIa-catalyzed activation and inactivation of FV and FVIII are reported, and the the timing and sites of cleavage are defined.

CONCLUSIONS

FXIa activates both procofactors at plasma protein concentrations, and computational modeling suggests that procofactor activation during the preincubation phase of the APTT assay is critical to the performance of the assay. As the APTT assay is the primary tool for the diagnosis and management of hemophilias A and B, as well as in the determination of FVIII inhibitors, these findings have potential implications in the clinical setting.

Carbohydrate moieties on the procofactor factor V, but not the derived cofactor factor Va, regulate its inactivation by activated protein C

Factor V (FV) is a single-chain plasma protein containing 13-25% carbohydrate by mass. Studies were done to determine if these carbohydrate moieties altered the activated protein C (APC)-catalyzed cleavage and inactivation of both FV and the cofactor which results from its activation by alpha-thrombin, factor Va(IIa) (FVa(IIa)). Treatment of purified FV with N-glycanase and neuraminidase under nonprotein-denaturing conditions removed approximately 20-30% of the carbohydrate from the heavy chain region of the molecule. When glycosidase-treated FV was analyzed in an aPTT (activated partial thromboplastin time)-based APC sensitivity assay, the APC sensitivity ratio (APC-SR) increased from 2.34 to 3.33. In contrast, when glycosidase-treated FV was activated with alpha-thrombin, the addition of the resulting FVa(IIa) to the plasma-based APC sensitivity assay produced no substantial increase in the APC-SR. Additional functional analyses of the APC-catalyzed inactivation of FVa(IIa) in an assay consisting of purified components indicated that both glycosidase-treated and untreated FVa(IIa) expressed identical cofactor activities and were inactivated at identical rates. Analyses of the APC-catalyzed cleavage of glycosidase-treated FV at Arg(306), the initial cleavage site, revealed a 10-fold rate increase when compared to untreated FV. In contrast, and consistent with functional assays, similar analyses of FVa(IIa), derived from those FV species, revealed near-identical rates of APC-catalyzed cleavage at both the Arg(506) and Arg(306)sites. These combined results indicate that N-linked carbohydrate moieties play a substantial role in the APC-catalyzed cleavage and inactivation of FV but not FVa(IIa) at position Arg(306) and that the Arg(306) cleavage sites of FV and FVa(IIa) are distinct substrates for APC.

Coagulation process proceeds on cultured human mesangial cells via expression of factor V

BACKGROUND

In a previous clinicopathological study, we observed mesangial factor V expression accompanied by the intact form of cross-linked fibrin deposition in the active type of IgA nephropathy. The conversion of prothrombin to thrombin by factor Xa is potently accelerated more than 104-fold by the presence of factor V, which is a membrane-bound cofactor. Another membrane-bound cofactor, tissue factor, is known to play an initiating role in the coagulation cascade and to be synthesized in mesangial cells (MCs) by the stimulation of tumor necrosis factor-alpha (TNF-alpha). However, the synthesis of factor V, which plays on the terminating stage of prothrombin activation, has not been reported previously in MCs by in vitro study. Our current study tested the coagulation process via expression of factor V by the stimulation of proinflammatory cytokine, TNF-alpha, in cultured human MCs.

METHODS

To evaluate factor V protein expression, immunoperoxidase staining with densitometric evaluation and Western blot analysis were conducted after stimulation of TNF-alpha. To test factor V activity, stimulated MCs were incubated in combination with factor Xa, prothrombin, fibrinogen and factor XIII, and fibrin production on MCs was assessed after immunoperoxidase staining on the cell surface. In a blocking test using an antibody against factor V, suppression of fibrin production was evaluated to clarify the role of factor V activity. For the evaluation of factor V mRNA expression in cultured human MCs, in situ hybridization and Northern blot analysis were performed.

RESULTS

Factor V protein expression in MCs after TNF-alpha stimulation increased both time- and dose-dependently. As a marker of factor V activity with exogenous factor Xa, fibrin production on TNF-alpha-stimulated MCs was increased in a time-dependent manner and was inhibited by the addition of anti-factor V antibody. Factor V mRNA was identified in MCs by in situ hybridization and showed an increase after stimulation with TNF-alpha on Northern blot analysis.

CONCLUSIONS

Our data suggest that the coagulation process proceeds on MCs as the result of increased expression of endogenous factor V activity on its cell surface in cooperation with exogenous factor Xa.

Soluble phosphatidylserine binds to a single identified site in the C2 domain of human factor Va

Factor V(a) (FV(a)) is a cofactor for the serine protease factor X(a) that activates prothrombin to thrombin in the presence of Ca(2+) and a membrane surface. FV(a) is a heterodimer composed of one heavy chain (A1 and A2 domains) and one light chain (A3, C1, and C2 domains). We use fluorescence, circular dichroism, and equilibrium dialysis to demonstrate that (1) the FV C2 domain expressed in Sf9 cells binds one molecule of C6PS with a k(d) of approximately 2 microM, (2) stabilizing changes occur in the FV C2 domain upon C6PS binding, (3) the C6PS binding site in the FV C2 domain is located near residue Cys(2113), which reacts with DTNB, and (4) binding to a PS-containing membrane is an order of magnitude tighter than that to soluble C6PS. Coupled with a recently published crystal structure of the C2 domain, these results support a model for the mechanism of C2-membrane interaction.

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

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

The role of the membrane in the inactivation of factor va by plasmin. Amino acid region 307-348 of factor V plays a critical role in factor Va cofactor function

The mechanism of inactivation of bovine factor Va by plasmin was studied in the presence and absence of phospholipid vesicles (PCPS vesicles). Following 60-min incubation with plasmin (4 nm) membrane-bound factor Va (400 nm) is completely inactive, whereas in the absence of phospholipid vesicles following a 1-h incubation period, the cofactor retains 90% of its initial cofactor activity. Amino acid sequencing of the fragments deriving from cleavage of factor Va by plasmin demonstrated that while both chains of factor Va are cleaved by plasmin, only cleavage of the heavy chain correlates with inactivation of the cofactor. In the presence of a membrane surface the heavy chain of the bovine cofactor is first cleaved at Arg(348) to generate a fragment of M(r) 47,000 containing the NH(2)-terminal part of the cofactor (amino acid residues 1-348) and a M(r) 42,000 fragment (amino acid residues 349-713). This cleavage is associated with minimal loss in cofactor activity. Complete loss of activity of the membrane-bound cofactor coincides with three cleavages at the COOH-terminal portion of the M(r) 47,000 fragment: Lys(309), Lys(310), and Arg(313). These cleavages result in the release of the COOH terminus of the molecule and the production of a M(r) 40,000 fragment containing the NH(2)-terminal portion of the factor Va molecule. Factor Va was treated with plasmin in the absence of phospholipid vesicles followed by the addition of PCPS vesicles and activated protein C (APC). A rapid inactivation of the cofactor was observed as a result of cleavage of the M(r) 47,000 fragment at Arg(306) by APC and appearance of a M(r) 39,000 fragment. These data suggest a critical role of the amino acid sequence 307-348 of factor Va. A 42-amino acid peptide encompassing the region 307-348 of human factor Va (N42R) was found to be a good inhibitor of factor Va clotting activity with an IC(50) of approximately 1.3 microm. These data suggest that plasmin is a potent inactivator of factor Va and that region 307-348 of the cofactor plays a critical role in cofactor function and may be responsible for the interaction of the cofactor with factor Xa and/or prothrombin.

Mesangial factor V expression colocalized with fibrin deposition in IgA nephropathy

BACKGROUND

Factor V in its active form (Va) plays a key role at the termination of the intrinsic coagulation pathway, serving as a membrane-bound cofactor for the conversion of prothrombin to thrombin by factor Xa. Cross-linked fibrin (XFb) is often observed in mesangial areas in active types of human glomerulonephritis. In this study, to clarify contribution of factor V in intramesangial coagulation, mesangial factor V expression and its relationship to mesangial proliferation and fibrin deposition in IgA nephropathy (IgAN) were investigated.

METHODS

Twenty-two patients with IgAN were studied. XFb was detected in renal biopsy specimens using anti-d-dimer antibody combined with plasmin exposure, and factor V was detected with rabbit antibody against human factor V. Double-labeling immunohistochemistry was used to investigate the relationship of the glomerular distribution of factor V to XFb. The relationship of factor V staining to the activity index or XFb deposition was evaluated. The expression of factor V mRNA was assessed by in situ hybridization in relationship to the antigen staining of alpha-smooth muscle actin (alpha-SMA). The ultrastructural distribution of factor V in glomeruli was studied by immunoelectron microscopy.

RESULTS

XFb and factor V were observed in the mesangium and along capillary loops in seven and nine specimens, respectively. Factor V had intense, frequent expression in the proliferating and necrotizing areas, showing a significant relationship to XFb (P < 0.05). Furthermore, XFb deposition and factor V expression were markedly correlated with disease activity (P = 0.005 and P = 0.008, respectively). By double-labeling experiments, XFb and factor V were often seen colocalized in mesangial areas of the glomeruli, which showed necrotizing lesions and/or intense cellular proliferation. By in situ hybridization, factor V mRNA was detected mainly in the mesangial cells, which were positive for alpha-SMA, and partly in the endothelial cells. By immunoelectron microscopy, factor V presence was confirmed in the mesangium and endothelium.

CONCLUSION

The present findings suggest that factor V is strongly expressed in mesangial cells in active IgAN accompanied with mesangial proliferation and may exert procoagulant activity, leading to intramesangial coagulation.

The action of Lonomia achelous caterpillars venom on human factor V

The bleeding syndrome produced by contact with the Lonomia achelous caterpillars is characterized by a decrease of fibrinogen, factor XIII, plasminogen, and factor V with normal platelets. In this study, we report the effect of crude hemolymph and some semipurified chromatographic fractions on human factor V. Incubation of factor V with crude hemolymph resulted in an increase in procoagulant activity, followed by a subsequent decline in factor V activity. Identical results were obtained with fraction I, whereas with fraction II there was only a decrease in activity reaching its minimum at 30 minutes. fraction III did not modify the activity of factor V. All concentrations of fraction I tested produced an initial rise and subsequent fall in activity. However, at lower relative concentrations of fraction I, more sustained increases in activity were observed. The activator and inactivator activities present in fraction I show differences in temperature and pH stability, susceptibility to different inhibitors, and in SDS/PAGE pattern. The factor V activator is a thermostable protein, with maximum activity at acid pH and is inhibited by o-phenantroline, EDTA, and EGTA, while the factor V inactivator is thermolabile, presents maximum activity at basic pH, precipitates at pH 5.0, and is completely inhibited by iodoacetic acid and TLCK. It is partially blocked by diisopropyl fluorophosphate, phenylmethylsulfonyl fluoride, and p-chloromercuribenzoic acid. These results suggest that the activator should be a metallo-proteinase, while the inactivator is a serine or cysteine proteinase with a serine, histidine, or cysteine residue in the active site.

Partial glycosylation at asparagine-2181 of the second C-type domain of human factor V modulates assembly of the prothrombinase complex

Thrombin-activated factor Va exists as two isoforms, factor Va(1) and factor Va(2), which differ in the size of their light chains and their affinity for biological membranes. The heterogeneity of the light chain remained following incubation of factor Va with N-glycanase. However, we found that the factor V C2 domain, which contains a single potential glycosylation site at Asn-2181, was partially glycosylated when expressed in COS cells. To confirm the structural basis for factor Va(1) and factor Va(2), we mutated Asn-2181 to glutamine (N2181Q) and expressed this mutant using a B domain deletion construct (rHFV des B) in COS cells. Thrombin activation of N2181Q released a light chain with mobility identical to that of factor Va(2) on SDS-PAGE. The functional properties of purified N2181Q were similar to those of factor Va(2) in prothrombinase assays carried out in the presence of limiting concentrations of phosphatidylserine. The binding of human factor Va(1) and factor Va(2) to 75:25 POPC/POPS vesicles was also investigated in equilibrium binding assays using proteins containing a fluorescein-labeled heavy chain. The affinity of human factor Va(2) binding to POPC/POPS vesicles was approximately 3-fold higher than that of factor Va(1). These results indicate that partial glycosylation of factor V at asparagine-2181 is the structural basis of the light chain doublet and that the presence of this oligosaccharide reduces the affinity of factor Va for biological membranes.

A model describing the inactivation of factor Va by APC: bond cleavage, fragment dissociation, and product inhibition

The inactivation of factor Va is a complex process which includes bond cleavage (at three sites) and dissociation of the A2N.A2C peptides, with intermediate activity in each species. Quantitation of the functional consequences of each step in the reaction has allowed for understanding of the presentation of disease in individuals possessing the factor V polymorphism factor VLEIDEN. APC cleavage of membrane-bound bovine factor Va (Arg306, Arg505, Arg662) leads to the dissociation of fragments of the A2 domain, residues 307-713 (A2N.A2C + A2C-peptide), leaving behind the membrane-bound A1.LC species. Evaluation of the dissociation process by light scattering yields invariant mass loss estimates as a function of APC concentration. The rate constant for A2 fragment dissociation varies with [APC], reaching a maximal value of k = 0.028 s-1, the unimolecular rate constant for A2 domain fragment dissociation. The APC binding site resides in the factor Va light chain (LC) (Kd = 7 nM), suggesting that the membrane-bound LC.A1 product would act to sequester APC. This inhibitory interaction (LC.A1.APC) is demonstrated to exist with either purified factor Va LC or the products of factor Va inactivation. Utilizing these experimental data and the reported rates of bond cleavage, binding constants, and product activity values for factor Va partial inactivation products, a model is developed which describes factor Va inactivation and accounts for the defect in factor VLEIDEN. The model accurately predicts the rates of inactivation of factor Va and factor VaLEIDEN, and the effect of product inhibition. Modeled reaction progress diagrams and activity profiles (from either factor Va or factor VaLEIDEN) are coincident with experimentally derived data, providing a mechanistic and kinetic explanation for all steps in the inactivation of normal factor Va and the pathology associated with factor VLEIDEN.

Importance of individual activated protein C cleavage site regions in coagulation factor V for factor Va inactivation and for factor Xa activation

Activated protein C (APC) cleavage of Factor Va (FVa) at residues R506 and R306 correlates with its inactivation. APC resistance and increased thrombotic risk are due to the mutation R506Q in Factor V (FV). To study the effects of individual cleavages in FVa by APC and the importance of regions near the cleavage sites, the following recombinant (r) human FVs were prepared and purified: wild-type, Q306-rFV, Q506-rFV, and Q306Q506-rFV. All had similar time courses for thrombin activation. Q506-rFVa was cleaved by APC at R306 and was moderately resistant to APC in plasma-clotting assays and in prothrombinase assays measuring FVa residual activity, in agreement with studies of purified plasma-derived Q506-FVa. Q306-rFVa was cleaved by APC at R506 and gave a low APC-resistance ratio similar to Q506-rFVa in clotting assays, whereas unactivated Q306-rFV gave a near-normal APC-resistance ratio. When FVa residual activity was measured after long exposure to APC, Q306-rFVa was inactivated by only < or = 40% under conditions where Q506-rFVa was inactivated > 90%, supporting the hypothesis that efficient inactivation of normal FVa by APC requires cleavage at R306. In addition, the heavy chain of Q306-rFVa was cleaved at R506 much more rapidly than activity was lost, suggesting that FVa cleaved at only R506 is partially active. Under the same conditions, Q306Q506-rFVa lost no activity and was not cleaved by APC. Therefore, cleavage at either R506 or R306 appears essential for significant inactivation of FVa by APC. Modest loss of activity, probably due to cleavage at R679, was observed for the single site rFVa mutants, as evidenced by a second phase of inactivation. Q306Q506-rFVa had a low activity-to-antigen ratio of 0.50-0.77, possibly due to abnormal Factor Xa (FXa) binding. Furthermore, Q306Q506-rFV was very resistant to cleavage and activation by FXa. Q306Q506-rFV appeared to bind FXa and inhibit FXa's ability to activate normal FV. Thus, APC may downregulate FV/Va partly by impairing FXa-binding sites upon cleavage at R306 and R506. This study shows that R306 is the most important cleavage site for normal efficient inactivation of FVa by APC and supports other studies suggesting that regions near R306 and R506 provide FXa-binding sites and that FVa cleaved at only R506 retains partial activity.

Protein C activation and factor Va inactivation on human umbilical vein endothelial cells

The inactivation of factor Va was examined on primary cultures of human umbilical vein endothelial cells (HUVECs), either after addition of activated protein C (APC) or after addition of alpha-thrombin and protein C (PC) zymogen. Factor Va proteolysis was visualized by Western blot analysis using a monoclonal antibody (alpha HVaHC No. 17) to the factor Va heavy chain (HC), and cofactor activity was followed both in a clotting assay using factor V-deficient plasma and by quantitation of prothrombinase function. APC generation was monitored using the substrate 6-(D-VPR)amino-1-naphthalenebutylsulfonamide (D-VPR-ANSNHC4H9), which permits quantitation of APC at 10 pmol/L. Addition of APC (5 nmol/L) to an adherent HUVEC monolayer (3.5 x 10(5) cells per well) resulted in a 75% inactivation of factor Va (20 nmol/L) within 10 minutes, with complete loss of cofactor activity within 2 hours. Measurements of the rate of cleavage at Arg506 and Arg306 in the presence and absence of the HUVEC monolayer indicated that the APC-dependent cleavage of the factor Va HC at Arg506 was accelerated in the presence of HUVECs, while cleavage at Arg306 was dependent on the presence of the HUVEC surface. Factor Va inactivation proceeded with initial cleavage of the factor Va HC at Arg506, generating an M(r) 75,000 species. Further proteolysis at Arg306 generated an M(r) 30,000 product. When protein C (0.5 mumol/L), alpha-thrombin (1 nmol/L), and factor Va (20 nmol/L) were added to HUVECs an APC generation rate of 1.56 +/- 0.11 x 10(-14) mol/min per cell was observed. With APC generated in situ, cleavage at Arg506 on the HUVEC surface is followed by cleavage at Arg306, generating M(r) 75,000 and M(r) 30,000 fragments, respectively. In addition, the appearance of two novel products derived from the factor Va HC are observed when thrombin is present on the HUVEC surface: the HC is processed through limited thrombin proteolysis to generate an M(r) 97,000 fragment, which is further processed by APC to generate an M(r) 43,000 fragment. NH2-terminal sequence analysis of the M(r) 97,000 fragment revealed that the thrombin cleavage occurs in the COOH-terminus of the intact factor Va HC since both the intact HC as well as the M(r) 97,000 fragment have the same sequence. Our data demonstrate that the inactivation of factor Va on the HUVEC surface, initiated either by APC addition or PC activation, follows a mechanism whereby cleavage is observed first at Arg506 followed by a second cleavage at Arg306. The latter cleavage is dependent on the availability of the HUVEC surface. This mechanism of inactivation of factor Va is similar to that observed on synthetic phospholipid vesicles.

The effect of Arg306-->Ala and Arg506-->Gln substitutions in the inactivation of recombinant human factor Va by activated protein C and protein S

Factor Va (fVa) is inactivated by activated protein C (APC) by cleavage of the heavy chain at Arg306, Arg506, and Arg679. Site-directed mutagenesis of human factor V cDNA was used to substitute Arg306-->Ala (rfVa306A) and Arg506-->Gln (rfVa506Q). Both the single and double mutants (rfVa306A/506Q) were constructed. The activation of these procofactors by alpha-thrombin and their inactivation by APC were assessed in coagulation assays using factor V-deficient plasma. All recombinant and wild-type proteins had similar initial cofactor activity and identical activation products (a factor Va molecule composed of light and heavy chains). Inactivation of factor Va purified from human plasma (fVaPLASMA) in HBS Ca2+ +0.5% BSA or in conditioned media by APC in the presence of phospholipid vesicles resulted in identical inactivation profiles and displayed identical cleavage patterns. Recombinant wild-type factor Va (rfVaWT) was inactivated by APC in the presence of phospholipid vesicles at an overall rate slower than fVaPLASMA. The rfVa306A and rfVa506Q mutants were each inactivated at rates slower than rfVaWT and fVaPLASMA. Following a 90-min incubation with APC, rfVa306A and rfVa506Q retain approximately 30-40% of the initial cofactor activity. The double mutant, rfVa306A/506Q, was completely resistant to cleavage and inactivation by APC retaining 100% of the initial cofactor activity following a 90-min incubation in the presence of APC. Recombinant fVaWT, rfVa306A, rfVa506Q, and rfVa306A/506Q were also used to evaluate the effect of protein S on the individual cleavage sites of the cofactor by APC. The initial rates of rfVaWT and rfVa306A inactivation in the presence of protein S were unchanged, indicating cleavage at Arg506 is not affected by protein S. The initial rate of rfVa506Q inactivation was increased, suggesting protein S slightly accelerates the cleavage at Arg306. Overall, the data demonstrate high specificity with respect to cleavage sites for APC on factor Va and demonstrate that cleavages of the cofactor at both Arg306 and Arg506 are required for efficient factor Va inactivation.

A Factor V Genetic Component Differing From Factor V R506Q Contributes to the Activated Protein C Resistance Phenotype

Factor V gene polymorphisms were investigated to detect components that may contribute to the activated protein C (APC) resistance phenotype in patients with venous thromboembolism. A specific factor V gene haplotype (HR2) was defined by six polymorphisms and its frequency was found to be similar in normal subjects coming from Italy (0.08), India (0.1), and Somalia (0.08), indicating that it was originated by ancestral mutational events. The relationship between the distribution of normalized APC ratios obtained with the functional assay and haplotype frequency was analyzed in patients heterozygous for factor V R506Q (factor V Leiden). The HR2 haplotype was significantly more frequent in patients with ratios below the 15th percentile than in those with higher ratios or in normal controls. Moreover, the study of 10 patients with APC resistance in the absence of the factor V R506Q mutation showed a 50-fold higher frequency of HR2 homozygotes. The HR2 haplotype was associated with significantly lower APC ratios both in patients with venous thromboembolism and in age- and sex-matched controls. However, the two groups showed similar HR2 haplotype frequencies. Plasma mixing experiments showed that an artificially created double heterozygote for the factor V R506Q mutation and the HR2 haplotype had an APC ratio lower than that expected for a simple R506Q heterozygote. Time-course experiments evaluating the decay of factor V in plasma showed the normal stability of the molecule encoded by the factor V gene marked by the HR2 haplotype, which ruled out the presence of a pseudo-homozygous APC resistance mechanism. Our results provide new insights into the presence of factor V genetic components other than the factor V R506Q that are able to contribute to the APC resistance phenotype in patients with venous thromboembolism.

Activated protein C cleavage of factor Va leads to dissociation of the A2 domain

The products of cleavage of bovine factor Va by activated protein C (APC) in the presence and absence of phospholipid (25% phosphatidylserine, 75% phosphatidylcholine, PCPS) were evaluated using sedimentation velocity/equilibrium methods in the analytical ultracentrifuge and by immunoprecipitation using an antibody directed against the light chain of the factor Va molecule. The molecular weight and sedimentation coefficient of the associated heavy and light chains of factor Va, 173,000 (7.9 S) is reduced to 132,000 (7.1 S) by APC cleavage at Arg505 and Arg662. Complete cleavage of the factor Va heavy chain (with APC-PCPS) at Arg505, Arg662 and Arg306 results in a drastic change in the molecular weight observed for the product. Two products are resolved with sedimentation coefficients of 3.3 and 6.3 S with estimated molecular weights of 48,000 and 114,000, respectively. Immunoprecipitation studies showed that the products of factor Va cleavage at Arg505 and Arg662 (A1A2N.A2C.LC) are mostly noncovalently associated and consequently immunoprecipitated with an antibody directed against the light chain of the factor Va molecule. In contrast, for factor Va cleaved at Arg505, Arg662, and Arg306 the precipitated complex consisted of the A1 domain (residues 1-306) and the light chain (residues 1537-2183) of factor Va (A1.LC). The fragments corresponding to residues 307-505 (A2N) and 506-662 (A2C) are found in the supernatant. The combined mass of these two products (48,000) is similar to the estimated mass of the 3.3 S fragment estimated from sedimentation velocity/equilibrium studies; while the combined mass of the 1-306 + 1537-2183 products corresponds to 114,000, the estimated mass of the 6.3 S fragment. These data lead to the conclusion that cleavages at Arg306, Arg505, and Arg662 of the factor Va molecule resulted in the dissociation of the entire A2 domain as two noncovalently associated fragments (A2N.A2C). Enzyme kinetic and light scattering data suggest that the complete inactivation of the factor Va molecule involves not only cleavage at Arg306 but also the dissociation of the A2 domain. These data also suggest that the complete APC inactivation of the factor Va molecule is analogous to the spontaneous inactivation of factor VIIIa, which occurs via the dissociation of the A2 domain.

A Factor V Genetic Component Differing From Factor V R506Q Contributes to the Activated Protein C Resistance Phenotype

Factor V gene polymorphisms were investigated to detect components that may contribute to the activated protein C (APC) resistance phenotype in patients with venous thromboembolism. A specific factor V gene haplotype (HR2) was defined by six polymorphisms and its frequency was found to be similar in normal subjects coming from Italy (0.08), India (0.1), and Somalia (0.08), indicating that it was originated by ancestral mutational events. The relationship between the distribution of normalized APC ratios obtained with the functional assay and haplotype frequency was analyzed in patients heterozygous for factor V R506Q (factor V Leiden). The HR2 haplotype was significantly more frequent in patients with ratios below the 15th percentile than in those with higher ratios or in normal controls. Moreover, the study of 10 patients with APC resistance in the absence of the factor V R506Q mutation showed a 50-fold higher frequency of HR2 homozygotes. The HR2 haplotype was associated with significantly lower APC ratios both in patients with venous thromboembolism and in age- and sex-matched controls. However, the two groups showed similar HR2 haplotype frequencies. Plasma mixing experiments showed that an artificially created double heterozygote for the factor V R506Q mutation and the HR2 haplotype had an APC ratio lower than that expected for a simple R506Q heterozygote. Time-course experiments evaluating the decay of factor V in plasma showed the normal stability of the molecule encoded by the factor V gene marked by the HR2 haplotype, which ruled out the presence of a pseudo-homozygous APC resistance mechanism. Our results provide new insights into the presence of factor V genetic components other than the factor V R506Q that are able to contribute to the APC resistance phenotype in patients with venous thromboembolism.

Cleavage requirements for activation of factor V by factor Xa

Coagulation factor V circulates in plasma as a single chain protein which expresses little procoagulant activity. After its activation by limited proteolysis by thrombin or factor Xa, factor Va functions as cofactor to factor Xa in the activation of prothrombin. Thrombin cleaves human factor V at Arg709, Arg1018 and Arg1545 and factor Va is formed by the heavy and light chains, which correspond to the N-terminal and C-terminal fragments, respectively. Factor Xa has been shown to cleave factor V at Arg1018 and at a second undefined position close to Arg709. The factor-Xa-mediated cleavage at Arg1018 has been proposed to be sufficient for expression of full factor Va activity. To study the activation of factor V by factor Xa, site-directed mutagenesis was used to convert Arg709 to Gln, Arg1018 to Ile, and Arg1545 to Gln. Constructs containing all possible combinations of native and mutated residues in these positions were expressed transiently in COS 1 cells. The various factor-V mutants were incubated with factor Xa or thrombin. The proteolytic cleavage pattern was analyzed by Western blotting, and the specific factor-Va activities determined in a prothrombinase assay. Control experiments using thrombin gave results which were in agreement with those on record, i.e. cleavages at both Arg709 and Arg1545 were required for expression of full factor-Va activity, whereas the cleavage at Arg1018 enhanced the rate of cleavage at Arg1545. Factor Xa was found to cleave factor V at all three thrombin cleavage sites, i.e. at Arg709, Arg1018 and Arg1545. An additional factor-Xa-cleavage site was found in the light chain region at Arg1765. Cleavage at Arg1018 by factor Xa was not sufficient for expression of full factor-Va activity. Full factor-Va activity was only obtained after cleavage at both Arg709 and Arg1545. The factor-Xa-mediated cleavage at Arg709 was kinetically favourable over that at Arg1545. Factor V which was mutated at all three sites (at positions 709, 1018 and 1545) was resistant to activation by thrombin. However, treatment with factor Xa yielded an increased factor-Va activity which was associated with the cleavage at Arg1765. Our study extends previously results on thrombin activation of factor V and elucidates the relative importance of the different cleavage sites for activation of factor V by factor Xa.

Binding sites for blood coagulation factor Xa and protein S involving residues 493-506 in factor Va

Inactivation due to cleavage of Factor Va (FVa) at Arg 506 by activated protein C (APC) helps to downregulate blood coagulation. To identify potential functional roles of amino acids near Arg 506, synthetic overlapping pentadecapeptides comprising FVa heavy chain residues 481-525 were tested for their ability to inhibit prothrombin activation by prothrombinase complexes [Factor Xa (FXa):FVa:phospholipids:Ca2+]. The most potent inhibition was observed for peptide VP493 (residues 493-506), with 50% inhibition at 2.5 microM. VP493 also inhibited FXa in plasma in FXa-1-stage clotting assays by 50% at 3 microM. When the C-terminal carboxamide group of VP493 was replaced by a carboxyl group, most prothrombinase inhibitory activity was lost. VP493 preincubated with FXa inhibited prothrombinase with a pattern of mixed inhibition. Homologous peptides from Factor VIII sequences did not inhibit prothrombinase. Affinity-purified antibodies to VP493 inhibited prothrombinase activity and prolonged FXa-1-stage clotting times. VP493 also blocked the ability of protein S to inhibit prothrombinase independently of APC. Immobilized VP493 bound specifically with similar affinity to both FXa and protein S (Kd approximately 40 nM), but did not measurably bind prothrombin or APC. These studies suggest that FVa residues 493-506 contribute to binding sites for both FXa and protein S, providing a rationale for the ability of protein S to negate the protective effect of FXa toward APC cleavage of FVa. Possible loss of this FVa binding site for FXa due to cleavage at Arg 506 by APC may help explain why this cleavage causes 40% decrease in FVa activity and facilitates inactivation of FVa.

Activated protein C-catalyzed proteolysis of factor VIIIa alters its interactions within factor Xase

Factor VIIIa, the cofactor for the factor IXa-dependent conversion of factor X to factor Xa, is proteolytically inactivated by activated protein C (APC). APC cleaves at two sites in factor VIIIa, Arg336, near the C terminus of the A1 subunit; and Arg562, bisecting the A2 subunit (Fay, P., Smudzin, T., and Walker, F. (1991) J. Biol. Chem. 266, 20139-20145). Factor VIIIa increased the fluorescence anisotropy of fluorescein-Phe-Phe-Arg factor IXa (Fl-FFR-FIXa; Kd = 42.4 nM), whereas cleavage of factor VIIIa by APC eliminated this property. Isolation of the APC-cleaved A1/A3-C1-C2 dimer (A1336/A3-C1-C2), and the fragments derived from cleaved A2 subunit (A2N/A2C), permitted dissection of the roles of individual cleavages in cofactor inactivation. Intact A1/A3-C1-C2 dimer increased Fl-FFR-FIXa anisotropy and bound factor X in a solid phase assay, while these activities were absent in the A1336/A3-C1-C2. However, the residues removed by this cleavage, Met337 Arg372, did not directly participate in these functions since neither a synthetic peptide to this sequence nor an anti-peptide polyclonal antibody blocked these activities using intact dimer. CD spectral analysis of the intact and truncated dimers indicated reduced alpha and/or beta content in the latter. The A1/A3-C1-C2 dimer plus A2 subunit reconstitutes cofactor activity and produced a factor VIIIa-like effect on the anisotropy of Fl-FFR-FIXa. However, when A2 was replaced by the A2N/A2C fragments, the resulting fluorescence signal was equivalent to that observed with the dimer alone. These results indicate that APC inactivates the cofactor at two levels within the intrinsic factor Xase complex. Cleavage of either subunit modulates the factor IXa active site, suggesting an essential synergy of interactive sites in factor VIIIa. Furthermore, cleavage of the A1 site alters the conformation of a factor X binding site within that subunit, thereby reducing the affinity of cofactor for substrate.

Structure of membrane-bound human factor Va

Coagulation factor Va is an essential cofactor which combines with the serine protease factor Xa on a phospholipid surface to form the prothrombinase complex. In the present study, the structure of factor Va interacting with lipid surfaces containing phosphatidylserine was studied by electron microscopy. Two-dimensional crystals of factor Va were obtained on planar lipid films under quasi-physiological conditions. The two-dimensional projected structure of factor Va was calculated at a resolution of 2 nm, revealing dimers of factor Va arranged on the surface lattice with the symmetry of the plane group p2. Average unit cell dimensions are a = 14.4 nm, b = 8.8 nm, gamma = 107 degrees. Each factor Va molecule presents two distinct domains of protein density consisting of one small domain, of 3 nm in diameter, connected to a larger domain of about 6 nm x 4.5 nm. The projected structure of factor Va covers an area equivalent to about fifty phospholipid molecules. In addition, edge-on views of factor Va molecules bound to liposomes reveal a globular structure connected through a thin stem to the liposome surface. A three-dimensional model of membrane-bound factor Va is proposed.

Mutation in blood coagulation factor V associated with resistance to activated protein C

Activated protein C (APC) is a serine protease with potent anticoagulant properties, which is formed in blood on the endothelium from an inactive precursor. During normal haemostasis, APC limits clot formation by proteolytic inactivation of factors Va and VIIIa (ref. 2). To do this efficiently the enzyme needs a nonenzymatic cofactor, protein S (ref. 3). Recently it was found that the anticoagulant response to APC (APC resistance) was very weak in the plasma of 21% of unselected consecutive patients with thrombosis and about 50% of selected patients with a personal or family history of thrombosis; moreover, 5% of healthy individuals show APC resistance, which is associated with a sevenfold increase in the risk for deep vein thrombosis. Here we demonstrate that the phenotype of APC resistance is associated with heterozygosity or homozygosity for a single point mutation in the factor V gene (at nucleotide position 1,691, G-->A substitution) which predicts the synthesis of a factor V molecule (FV Q506, or FV Leiden) that is not properly inactivated by APC. The allelic frequency of the mutation in the Dutch population is approximately 2% and is at least tenfold higher than that of all other known genetic risk factors for thrombosis (protein C (ref. 8), protein S (ref. 9), antithrombin10 deficiency) together.

The effect of phospholipids, calcium ions and protein S on rate constants of human factor Va inactivation by activated human protein C

Rate constants for human factor Va inactivation by activated human protein C (APC) were determined in the absence and presence of Ca2+ ions, protein S and varying concentrations of phospholipid vesicles of different lipid composition. APC-catalyzed factor Va inactivation in free solution (in the presence of 2 mM Ca2+) was studied under first-order reaction conditions with respect to both APC and factor Va and was characterized by an apparent second-order rate constant of 6.1 x 10(5) M-1 s-1. Stimulation of APC-catalyzed factor Va inactivation by phospholipids was dependent on the concentration and composition of the phospholipid vesicles. Optimal acceleration (230-fold) of factor Va inactivation was observed with 10 microM phospholipid vesicles composed of 20 mol% dioleoylglycerophosphoserine (Ole2GroPSer) and 80 mol% dioleoylglycerophosphocholine (Ole2GroPCho). At higher vesicle concentrations and at higher molar fractions of Ole2GroPSer some inhibition of APC-catalyzed factor Va inactivation was observed. Membranes that contained anionic phospholipids other than phosphatidylserine also promoted factor Va inactivation. The ability of different anionic lipids to enhance factor Va inactivation increased in the order phosphatidylethanolamine less than oleic acid less than phosphatidic acid less than phosphatidylglycerol less than phosphatidylmethanol less than phosphatidylserine. APC-catalyzed factor Va inactivation in the presence of phospholipid vesicles could be saturated with respect to factor Va and the reaction obeyed Michaelis-Menten kinetics. Both the Km for factor Va and the Vmax of factor Va inactivation were a function of the phospholipid concentration. The Km increased from 1 nM at 2.5 microM phospholipid (Ole2GroPSer/Ole2GroPCho 20:80, mol/mol) to 65 nM at 250 microM phospholipid. The Vmax increased from 20 mol factor Va inactivated.min-1.mol APC-1 at 2.5 microM phospholipid to 62 mol factor Va inactivated.min-1.mol APC-1 at 10 microM phospholipid and remained constant at higher phospholipid concentrations. Protein S appeared to be a rather poor stimulator of APC-catalyzed factor Va inactivation. Protein-S-dependent rate enhancements were only observed in reaction mixtures that contained negatively charged phospholipid vesicles. Independent of the concentration and the lipid composition of the vesicles, protein S caused a twofold stimulation of APC-catalyzed factor Va inactivation. This suggests that, in the human system, enhancement of APC binding to phospholipid vesicles by protein S is of minor importance. Considering that protein S is a physiologically essential antithrombotic agent, it is likely that other factors or phenomena contribute to the in vivo antithrombotic action of protein S.

Normal haemostasis and its regulation

Regulation of normal haemostasis and blood flow involves complex interactions between plasma proteins and blood cells, including platelets, leukocytes and the endothelial lining of blood vessels. Thrombin acts as a pivot in the maintenance of the haemostatic balance; the vascular endothelial cell in particular limits the generation of thrombin by localisation of anticoagulant processes on its luminal membrane. The endothelial cell synthesises key molecules in this process and also binds exogenously derived molecules, as well as releasing proteins of the fibrinolysis cascade. The thromboresistance of the luminal surface is further regulated by lipoxygenase and cyclo-oxygenase metabolites of unsaturated fatty acids synthesised by the endothelial cell. In response to trauma, inflammatory reactions, normal wound healing and in association with a variety of disease states, the anticoagulant and fibrinolytic mechanisms are downregulated and the procoagulant and thrombotic mechanisms predominate with resultant generation of thrombin, fibrin clot formation and subsequent platelet adhesion and aggregation. Pro-inflammatory and prothrombotic cytokines downregulate the fibrinolytic and activated protein C pathways as well as inducing synthesis of specific procoagulant and prothrombotic mediators by platelets and leukocytes as well as endothelium.

Factor V: a prototype pro-cofactor for vitamin K-dependent enzyme complexes in blood clotting

The relative abundance of factor V, factor X and prothrombin has enabled detailed analyses of the prothrombinase complex. Determination of the primary structure for factor V has provided the basis for examination of structure-function relationships. The imminent in vitro expression of recombinant factor V will provide the opportunity for site-specific mutagenesis and a verification of these structure-function relationships. A comparison of the physical properties and primary structures for factors V and VIII has revealed extensive similarities in these two cofactor proteins. This observation indicates that a direct application of the technology developed for the analysis of prothrombinase will lead to an equal understanding of the factor Xase complex. Whether similar relationships exist for other blood coagulation enzyme complexes remains to be determined.

Factor VIII structure and function

The relatively recent ability to obtain highly purified factor VIII (FVIII) preparations from plasma products, the cloning of the FVIII gene, and the expression of recombinant FVIII have provided the basis for significant advancements in the understanding of the structure-function relationships of FVIII. Evaluation of the molecular structure of FVIII has revealed the presence of domains of significant internal amino acid sequence homology as well as homology with similar structural domains of factor V. Specific proteolytic cleavage sites have been identified in the molecule and the use of site directed mutagenesis has identified those proteolytic cleavage sites required for the activation of FVIII. Deletion and substitution variants of FVIII as well as the precise epitope mapping of FVIII antibodies which inhibit the procoagulant function of the protein or its binding to von Willebrand factor have provided insight into the identification of regions of FVIII which are required for normal function.

Activated protein C is not regulated by alpha-2-macroglobulin in plasma

Alpha-2-macroglobulin (a2M) is a wide spectrum plasma inhibitor which functions by a unique mechanism and is a secondary inhibitor of coagulation and fibrinolytic enzymes. Human activated protein C (APC) is the central enzyme of a major regulatory system of coagulation and fibrinolysis. APC is primarily regulated (inhibited) by a specific plasma inhibitor. We undertook this study to investigate the role of a2M as a secondary inhibitor of APC. APC did not interact with a2M by any of the known mechanisms of interaction. APC failed to bind and form the classic proteinase-a2M complex as seen with similar serine proteases, thrombin and trypsin. APC also failed to cleave the a2M molecule. Experiments, using purified APC and either purified a2M or plasma, failed to demonstrate APC binding to a2M in gel filtration chromatography. No enzymatic activity of APC or radiolabeled APC was demonstrated in the a2M peak. Using an immuno-enzymatic assay (Harpel, J Biol Chem 260:4257, 1985) for an a2M and enzyme complex, the amount of APC bound to a2M was less than 3% of the added APC (non-specific binding only); whereas in similar experiments with thrombin, 75-86% of the added trypsin or thrombin bound. These data demonstrate that APC is one of a small number of unique serine proteases that do not interact with a2M. The absence of sequence homology between the a2M 'bait region' and the APC substrate cleavage sequences appear to be the reason APC is not inhibited by a2M.