Subcellular localization and secretion of factor V from human platelets

Upregulation of coagulation factor V by glucocorticoid in the preovulatory follicles of zebrafish

Increased cortisol levels in the preovulatory follicular fluid suggests a role of glucocorticoid in human ovulation. However, the mechanisms through which cortisol regulates the ovulatory process remain poorly understood. In this study, we examined the upregulation of f5 mRNA by glucocorticoid and its receptor (Gr) in the preovulatory follicles of zebrafish. Our findings demonstrate a significant increase in 11β-hydroxysteroid dehydrogenase type 2 (hsd11b2), a cortisol response gene, in preovulatory follicles. Additionally, hydrocortisone exerts a dose- and time-dependent upregulation of f5 mRNA in these follicles. Importantly, this stimulatory effect is Gr-dependent, as it was completely abolished in gr-/- mutants. Furthermore, site-directed mutagenesis identified a glucocorticoid response element (GRE) in the promoter of zebrafish f5. Interestingly, successive incubation of hydrocortisone and the native ovulation-inducing steroid, progestin (17α,20β-dihydroxy-4-pregnen-3-one, DHP), further enhanced f5 expression in preovulatory follicles. Overall, our results indicate that the dramatic increase of f5 expression in preovulatory follicles is partially attributable to the regulation of glucocorticoid and Gr.

Immunothrombosis and the Role of Platelets in Venous Thromboembolic Diseases

Venous thromboembolism (VTE) is the third leading cardiovascular cause of death and is conventionally treated with anticoagulants that directly antagonize coagulation. However, recent data have demonstrated that also platelets play a crucial role in VTE pathophysiology. In the current review, we outline how platelets are involved during all stages of experimental venous thrombosis. Platelets mediate initiation of the disease by attaching to the vessel wall upon which they mediate leukocyte recruitment. This process is referred to as immunothrombosis, and within this novel concept inflammatory cells such as leukocytes and platelets directly drive the progression of VTE. In addition to their involvement in immunothrombosis, activated platelets can directly drive venous thrombosis by supporting coagulation and secreting procoagulant factors. Furthermore, fibrinolysis and vessel resolution are (partly) mediated by platelets. Finally, we summarize how conventional antiplatelet therapy can prevent experimental venous thrombosis and impacts (recurrent) VTE in humans.

Exploratory Investigation of the Plasma Proteome Associated with the Endotheliopathy of Trauma

BACKGROUND

The endotheliopathy of trauma (EoT) is associated with increased mortality following injury. Herein, we describe the plasma proteome related to EoT in order to provide insight into the role of the endothelium within the systemic response to trauma.

METHODS

99 subjects requiring the highest level of trauma activation were included in the study. Enzyme-linked immunosorbent assays of endothelial and catecholamine biomarkers were performed on admission plasma samples, as well as untargeted proteome quantification utilizing high-performance liquid chromatography and tandem mass spectrometry.

RESULTS

Plasma endothelial and catecholamine biomarker abundance was elevated in EoT. Patients with EoT (n = 62) had an increased incidence of death within 24 h at 21% compared to 3% for non-EoT (n = 37). Proteomic analysis revealed that 52 out of 290 proteins were differentially expressed between the EoT and non-EoT groups. These proteins are involved in endothelial activation, coagulation, inflammation, and oxidative stress, and include known damage-associated molecular patterns (DAMPs) and intracellular proteins specific to several organs.

CONCLUSIONS

We report a proteomic profile of EoT suggestive of a surge of DAMPs and inflammation driving nonspecific activation of the endothelial, coagulation, and complement systems with subsequent end-organ damage and poor clinical outcome. These findings support the utility of EoT as an index of cellular injury and delineate protein candidates for therapeutic intervention.

Platelet Activation via Glycoprotein VI Initiates Thrombin Generation: A Potential Role for Platelet-Derived Factor IX?

Collagen triggers coagulation via activation of factor (F) XII. In a platelet-rich environment, collagen can also trigger coagulation independently of FXII. We studied a novel mechanism of coagulation initiation via collagen-dependent platelet activation using thrombin generation (TG) in platelet-rich plasma. Collagen-induced coagulation is minimally affected by active-site inactivated FVIIa, anti-FVII antibodies, or FXIIa inhibition (corn trypsin inhibitor). Activation of platelets via specific glycoprotein (GP) VI agonists initiates TG, FX activation, and fibrin formation. To determine the platelet-derived trigger of coagulation, we systematically reconstituted factor-deficient plasmas with washed platelets. TG triggered by GPVI-activated platelets was significantly affected in FIX- and FVIII-deficient plasma but not in FVII- and FXII-deficient plasma. In a purified system composed of FX and FVIII, we observed that absence of FIX was compensated by GPVI-activated platelets, which could be inhibited by an anti-FIX antibody, suggesting FIXa activity from activated platelets. Furthermore, with the addition of FVIII in FIX-deficient plasma, TG induced by GPVI-activated platelets was restored, and was inhibited by the anti-FIX antibody. In conclusion, GPVI-activated platelets initiate TG, probably via platelet-derived FIXa activity.

Endocytosed factor V is trafficked to CD42b+ proplatelet extensions during differentiation of human umbilical cord blood-derived megakaryocytes

Plasma- and platelet-derived factor Va are essential for thrombin generation catalyzed by the prothrombinase complex; however, several observations demonstrate that the platelet-derived cofactor, which is formed following megakaryocyte endocytosis and modification of the plasma procofactor, factor V, is more hemostatically relevant. Factor V endocytosis, as a function of megakaryocyte differentiation and proplatelet formation, was assessed by flow cytometry and microscopy in CD34⁺ hematopoietic progenitor cells isolated from human umbilical cord blood and cultured for 12 days in the presence of cytokines to induce ex vivo differentiation into megakaryocytes. Expression of an early marker of megakaryocyte differentiation, CD41, endocytosis of factor V, and the percentage of CD41⁺ cells that endocytosed factor V increased from days 6 to 12 of differentiation. In contrast, statistically significant decreases in expression of the stem cell marker, CD34, and in the percentage of CD34⁺ cells that endocytosed factor V were observed. A statistically significant increase in the expression of CD42b, a late marker of megakaryocyte differentiation, was also observed over time, such that by Day 12, all CD42b⁺ cells endocytosed factor V and expressed CD41. This endocytosed factor V was trafficked to proplatelet extensions and was localized in a punctate pattern in the cytoplasm consistent with its storage in α-granules. In conclusion, loss of CD34 and expression of CD42b define cells capable of factor V endocytosis and trafficking to proplatelet extensions during differentiation of megakaryocytes ex vivo from progenitor cells isolated from umbilical cord blood.

Synergistic effect of a factor Xa inhibitor, TAK-442, and antiplatelet agents on whole blood coagulation and arterial thrombosis in rats

INTRODUCTION

Activated platelets facilitate blood coagulation by providing factor V and a procoagulant surface for prothrombinase. Here, we investigated the potential synergy of a potent factor Xa/prothrombinase inhibitor, TAK-442, plus aspirin or clopidogrel in preventing arterial thrombosis and whole blood coagulation.

METHODS

Thrombus formation was initiated by FeCl(3)-induced rat carotid injury. Bleeding time was evaluated with the rat tail transection model. Whole blood coagulation was assessed by thromboelastographic examination (TEG) for which blood obtained from control, aspirin-, or clopidogrel-treated rats was transferred to a TEG analyzer containing, collagen or adenosine diphosphate (ADP), and TAK-442 or vehicle.

RESULTS

TAK-442 (3mg/kg, po), aspirin (100mg/kg, po) or clopidogrel (3mg/kg, po) alone had no significant effect on thrombus formation, whereas the combination of TAK-442 with aspirin and clopidogrel remarkably prolonged the time to thrombus formation without additional significant prolongation of bleeding time. TEG demonstrated that the onset of collagen-induced blood coagulation were slightly longer in aspirin-treated rats than control; however, when the blood from aspirin-treated rats was subsequently treated in vitro with 100 nM TAK-442, the onset of clotting was significantly prolonged. In contrast, only marginal prolongation was observed with TAK-442 treatment of blood from control animals. The onset time of ADP-induced blood coagulation was slightly longer in clopidogrel-treated rats compared with control, and it was further extended by TAK-442 treatment.

CONCLUSION

These results demonstrate that blood coagulation can be markedly delayed by the addition of TAK-442 to antiplatelets treatment which could contribute to synergistic antithrombotic efficacy in these settings.

The murine platelet and plasma factor V pools are biosynthetically distinct and sufficient for minimal hemostasis

Coagulation factor V (FV) is a central regulator of the coagulation cascade. Circulating FV is found in plasma and within platelet alpha granules. The specific functions of these distinct FV pools are uncertain. We now report the generation of transgenic mice with FV gene expression restricted to either the liver or megakaryocyte/platelet lineage using bacterial artificial chromosome (BAC) constructs. Six of 6 independent albumin BAC transgenes rescue the neonatal lethal hemorrhage of FV deficiency. Rescued mice all exhibit liver-specific Fv expression at levels ranging from 6% to 46% of the endogenous Fv gene, with no detectable FV activity within the platelet pool. One of the 3 Pf4 BAC transgenes available for analysis also rescues the lethal FV null phenotype, with FV activity restricted to only the platelet pool (approximately 3% of the wild-type FV level). FV-null mice rescued by either the albumin or Pf4 BAC exhibit nearly normal tail bleeding times. These results demonstrate that Fv expression in either the platelet or plasma FV pool is sufficient for basal hemostasis. In addition, these findings indicate that the murine platelet and plasma FV pools are biosynthetically distinct, in contrast to a previous report demonstrating a plasma origin for platelet FV in humans.

Biosynthetic origin and functional significance of murine platelet factor V

Factor V (FV), a central regulatory protein in hemostasis, is distributed into distinct plasma and platelet compartments. Although platelet FV is highly concentrated within the platelet alpha-granule, previous analysis of human bone marrow and liver transplant recipients has demonstrated that platelet FV in these individuals originates entirely from the uptake of plasma FV. In order to examine further the biosynthetic origins of the platelet and plasma FV pools, we performed bone marrow transplantations of Fv-null (Fv-/-) fetal liver cells (FLCs) into wild-type mice. Fractionation of whole blood from control mice demonstrated that approximately 14% of total blood FV activity is platelet-associated. Mice that received transplants of Fv-null FLCs displayed a high degree of engraftment and appeared grossly normal, with no evidence for spontaneous hemorrhage. Although total FV levels in Fv-null FLC recipients were only mildly decreased, the FV activity within the platelet compartment was reduced to less than 1% of that in normal mice. We conclude that the murine platelet FV compartment is derived exclusively from primary biosynthesis within cells of marrow origin, presumably megakaryocytes, and that an intact platelet FV pool is not required for protection from spontaneous hemorrhage or bleeding following minor trauma.

Anti-factor V auto-antibody in the plasma and platelets of a patient with repeated gastrointestinal bleeding

Development of autoantibody against coagulation factor V (FV) is a rare clinical condition with hemorrhagic complications of varying severity. The aim of this study was to establish the pathomechanism of an acquired FV deficiency and characterize the FV inhibitor responsible for the clinical symptoms. A 78-year-old female was admitted to hospital with severe gastrointestinal bleeding. General clotting tests and determination of clotting factors were performed by standard methods. FV antigen and FV containing immune complexes were measured by ELISA. The FV molecule was investigated by Western blotting and by sequencing the f5 gene. The binding of patient's IgG to FV and activated FV (FVa) was demonstrated in an ELISA system and its effect on the procoagulant activity of FVa was tested in clotting tests and in a chromogenic prothrombinase assay. Localization of the epitope for the antibody was performed by blocking ELISA. FV activity was severely suppressed both in plasma and platelets. FV antigen levels were normal by ELISA using polyclonal anti-FV antibody or monoclonal antibody against the connecting region of FV, but depressed when HV1 monoclonal antibody against the C2 domain in the FV light-chain was used as capture antibody. The FV molecule was found intact. An IgG reacting with both FV and FVa was present in the patient's plasma and its binding to FV was inhibited by HV1 antibody. FV-containing immune complexes were detected in the patient's plasma and platelet lysate. The patient's IgG inhibited the procoagulant function of FVa. An anti-FV IgG was present in the patient's plasma and platelets. The autoantibody reacted with an epitope in the C2 domain of FV light chain and neutralized the procoagulant function of FVa.

Polymorphisms in coagulation factor genes and their impact on arterial and venous thrombosis

Arterial and venous thromboses, with their clinical manifestations such as stroke, myocardial infarction (MI), or pulmonary embolism, are the major causes of death in developed countries. Several studies in twins and siblings have shown that genetic factors contribute significantly to the development of these diseases. Since the advent of molecular genetics in medicine, it has been a focus of interest to elucidate the role of mutations in various candidate genes and their impact on hemostatic disorders such as arterial and venous thromboses. In this article, we review the current knowledge of the contribution of polymorphisms in coagulation factors to the development of thrombotic diseases. We show that in arterial thrombosis, results are controversial. Only for factor XIII 34Leu a protective effect on the development of myocardial infarction has been demonstrated in several studies. No other single polymorphism in a coagulation factor could be confirmed as a relevant risk factor, although there is evidence for a role of factor V Arg506Gln, factor VII Arg353Gln, and vWF Thr789Ala polymorphisms in patient subgroups. Further studies will be necessary to confirm the value of testing for genetic polymorphisms in arterial thrombosis. A large body of data is available on the role of factor V Arg506Gln and the prothrombin G20210A mutation in venous thrombosis. Some papers already recommend diagnosis and treatment strategies. We will discuss these recent publications on venous thrombosis in our review.

The dynamics of thrombin formation

The central event of the hemostatic process is the generation of thrombin through the tissue factor pathway. This is a highly regulated, dynamic process in which thrombin itself plays many roles, positively and negatively its production and destruction. The hemostatic process is essential to normal physiology and is also the Achilles heel of our aging population. The inappropriate generation of thrombin may lead to vascular occlusion with the consequence of myocardial infarction, stroke, pulmonary embolism, or venous thrombosis. In this review, we summarize our present views regarding the tissue factor pathway by which thrombin is generated and the roles played by extrinsic and intrinsic factor Xa generating complexes in hemostasis and the roles of the stoichiometric and dynamic inhibitors that regulate thrombin generation.

Factor V and thrombotic disease: description of a janus-faced protein

The generation of thrombin by the prothrombinase complex constitutes an essential step in hemostasis, with thrombin being crucial for the amplification of blood coagulation, fibrin formation, and platelet activation. In the prothrombinase complex, the activated form of coagulation factor V (FVa) is an essential cofactor to the enzyme-activated factor X (FXa), FXa being virtually ineffective in the absence of its cofactor. Besides its procoagulant potential, intact factor V (FV) has an anticoagulant cofactor capacity functioning in synergy with protein S and activated protein C (APC) in APC-catalyzed inactivation of the activated form of factor VIII. The expression of anticoagulant cofactor function of FV is dependent on APC-mediated proteolysis of intact FV. Thus, FV has the potential to function in procoagulant and anticoagulant pathways, with its functional properties being modulated by proteolysis exerted by procoagulant and anticoagulant enzymes. The procoagulant enzymes factor Xa and thrombin are both able to activate circulating FV to FVa. The activity of FVa is, in turn, regulated by APC together with its cofactor protein S. In fact, the regulation of thrombin formation proceeds primarily through the upregulation and downregulation of FVa cofactor activity, and failure to control FVa activity may result in either bleeding or thrombotic complications. A prime example is APC resistance, which is the most common genetic risk factor for thrombosis. It is caused by a single point mutation in the FV gene (factor V(Leiden)) that not only renders FVa less susceptible to the proteolytic inactivation by APC but also impairs the anticoagulant properties of FV. This review gives a description of the dualistic character of FV and describes the gene-gene and gene-environment interactions that are important for the involvement of FV in the etiology of venous thromboembolism.

Platelet aggregation through prothrombinase activation induced by non-aggregant doses of platelet agonists

Activation of the prothrombinase complex, which catalyzes the formation of thrombin from prothrombin, is crucial for the (patho)physiological processes of hemostasis and thrombosis. We here report that washed platelets supplemented with prothrombin can be irreversibly aggregated with otherwise non-aggregant doses of adenosine diphosphate (10 micromol/l), thrombin (0.06 U/ml), or collagen (1 microg/ml). Prothrombinase-catalyzed prothrombin to thrombin conversion most probably supports this aggregation response, since inhibitors of thrombin (hirudin or heparin) and an inhibitor of activated factor X (DX-9065a) impair the response. A certain degree of agonist-induced platelet activation seems to be required for this prothrombin-supported aggregation response, since prothrombin alone does not induce aggregation, and blockade of glycoprotein Ia/IIa with a specific antibody inhibits the platelet aggregation response to collagen and prothrombin. These results may suggest that activation of the prothrombinase complex could be a common step of the platelet response to distinct agonists, which may be achieved at low levels of platelet stimulation.

Platelet factor V New York: a defect in factor V distinct from that in factor V Quebec resulting in impaired prothrombinase generation

Studies were performed on a patient with a longstanding bleeding disorder whose major defects were impaired platelet prothrombinase activity in the absence of added factor Va, and a platelet factor V value that was either decreased or at the lower limit of normal when assayed on multiple occasions. In contrast, plasma factor V values were consistently normal. Unlike Scott Syndrome, in which platelet prothrombinase activity is decreased in both the presence and absence of added factor V, her platelets appeared to utilize added factor Va normally in supporting the generation of prothrombinase activity. These findings suggest an intrinsic defect in platelet factor V as the basis of her platelet prothrombinase defect. This defect appears to be different than that described in the Quebec platelet disorder (factor V Quebec). Immunoblot analyses of washed platelet lysates demonstrated a pattern of variably sized factor V molecules that was entirely similar to that observed in normal platelets, and both the heavy and light chains of her factor V after thrombin cleavage were of the same size as that observed in normal platelets. In addition, her platelet multimerin was normal and immunoblot analysis excluded the type of generalized granular protein defect and pathological proteolysis that has been suggested to explain the factor V defect in the Quebec platelet disorder. The findings in this patient thus suggest a new type of platelet factor V defect as the basis for the impaired capacity of her activated platelets to support prothrombinase generation. The findings further support an important role for platelet factor V in hemostasis.

Surface expression and functional characterization of α-granule factor V in human platelets: effects of ionophore A23187, thrombin, collagen, and convulxin

Factor V (FV) present in platelet -granules has a significant but incompletely understood role in hemostasis. This report demonstrates that a fraction of platelets express very high levels of surface-bound, -granule FV on simultaneous activation with 2 agonists, thrombin and convulxin, an activator of the collagen receptor glycoprotein VI. This subpopulation of activated platelets represents 30.7% ± 4.7% of the total population and is referred to as convulxin and thrombin–induced-FV (COAT-FV) platelets. COAT-FV platelets are also observed on activation with thrombin plus collagen types I, V, or VI, but not with type III. No single agonist examined was able to produce COAT-FV platelets, although ionophore A23187 in conjunction with either thrombin or convulxin did generate this population. COAT-FV platelets bound annexin-V, indicating exposure of aminophospholipids and were enriched in young platelets as identified by the binding of thiazole orange. The functional significance of COAT-FV platelets was investigated by demonstrating that factor Xa preferentially bound to COAT-FV platelets, that COAT-FV platelets had more FV activity than either thrombin or A23187–activated platelets, and that COAT-FV platelets were capable of generating more prothrombinase activity than any other physiologic agonist examined. Microparticle production by dual stimulation with thrombin and convulxin was less than that observed with A23187, indicating that microparticles were not responsible for all the activities observed. These data demonstrate a new procoagulant component produced from dual stimulation of platelets with thrombin and collagen. COAT-FV platelets may explain the unique role of -granule FV and the hemostatic effectiveness of young platelets.

Protein C inhibitor secreted from activated platelets efficiently inhibits activated protein C on phosphatidylethanolamine of platelet membrane and microvesicles

Protein C inhibitor (PCI) was detected in human platelets (2.9 ng/10(9) cells) and megakaryocytic cells (1.5 ng/10(6) cells). PCI mRNA was also detected in both platelets and megakaryocytic cells using nested polymerase chain reaction. PCI was found to be located in the alpha-granules of resting platelets. Approximately 30% of the total amount of PCI in platelets was released after stimulation with ADP, collagen, adrenalin, thrombin, or thrombin receptor-activating peptide. Secreted PCI was detected on the surface of activated platelets and phospholipid microvesicles. PCI secreted from thrombin receptor-activating peptide-stimulated platelets inhibited activated protein C (APC) efficiently. PCI significantly inhibited APC in the presence of phospholipid vesicles prepared using rabbit brain cephalin (RBC) or a mixture of 40% phosphatidylethanolamine (PE), 20% phosphatidylserine (PS), and 40% phosphatidylcholine (PC) with a second order rate constant of 1.0 x 10(6) M-1.min-1. Of these phospholipids, PE was critical for this inhibition. The dissociation constants of the binding of APC or PCI to solid phase phospholipids showed that APC binds more preferably to PE than to RBC or PS, and PCI to PE or RBC than to PS or PC. PCI binding to solid phase phospholipids depended on the presence of PE. RBC- or PE-bound PCI inhibited APC significantly but only weakly the gamma-carboxyglutamic acid domainless APC. The gamma-carboxyglutamic acid fragment of protein C suppressed the PCI-mediated inhibition of APC on solid phase RBC or PE. Most of the APC.PCI complex formed on solid phase RBC or PE was released into the soluble phase. These findings suggest that PCI secreted from activated platelets binds preferably to PE of platelet membrane and microvesicles and that it inhibits phospholipid-bound APC efficiently.

A case of coagulation factor V deficiency complicated with intracranial hemorrhage

Factor V deficiency is a relatively uncommon disorder, inherited as an autosomal recessive trait that manifests clinically only in individuals who inherit the defective gene from both parents. The hemorrhage of nasal and oral cavity and ecchymosis are common but intracranial hemorrhage is very rare. We experienced a 53 year old male patient with intracranial hemorrhage due to factor V deficiency. The laboratory tests showed prolongation of APTT and PT, normal bleeding time and normal thrombin time. The levels of the coagulation profiles on the patient revealed a significant decrease factor V, below 1% of normal range (60-140%). Other coagulation factors were normal. He was treated with fresh frozen plasma and completely recovered 3 weeks after treatment.

Factor V is complexed with multimerin in resting platelet lysates and colocalizes with multimerin in platelet alpha-granules

Factor V stored in platelets is an important source of factor Va for the prothrombinase complex. Investigations of potential platelet factor Va-binding proteins, using factor Va light chain affinity chromatography, identified a disulfide-linked multimeric protein with a reduced mobility of 155 kDa in the column eluate. Immunodepletion and immunoblotting indicated that this protein was multimerin. Multimerin specifically bound factors V and Va and the isolated factor Va light chain, but not the heavy chain of factor Va. Factor V stored in platelets, but not plasma factor V, was found to be complexed with multimerin. Multimerin immunodepletion of resting platelet lysates was associated with the removal of factor V and the loss of factor V coagulant activity. Immunoelectron microscopic studies colocalized factor V with multimerin in the alpha-granules of resting platelets. With thrombin-induced platelet activation, we observed dissociation of factor Va-multimerin complexes, multimerin-independent membrane binding of factor Va, and prothrombinase activity that was not inhibitable by multimerin antibodies. This study indicates that platelet factor V is stored as a complex with multimerin and suggests a possible role for multimerin as a carrier protein for factor V stored in platelets.

The assembly of the prothrombinase complex on adherent platelets

Prothrombinase complex assembly, in real time, on platelets adherent to immobilized von Willebrand Factor (vWf) was examined by total internal reflection fluorescence spectroscopy (TIRFS). Electron microscopy showed that the platelets adhered to vWf in a largely unactivated state and could be activated by thrombin. Antibody binding to glycoprotein (GP) Ib and functional GPIIb-IIIa receptor molecules on adherent platelet membranes monitored by TIRFS also indicated that platelets adhered in a largely unactivated state. Maximal expression of the receptor form of GPIIb-IIIa detected by antibody binding was seen only after thrombin stimulation of the adherent platelets. Antibody binding to GPIb was detected on adherent platelets. A reduction in antibody binding was observed after thrombin stimulation of the platelets, indicating a change in GPIb as a consequence of thrombin stimulation of the platelets. The binding of the protein components of the prothrombinase complex to adherent and thrombin-stimulated adherent platelets was then studied individually. Factor Va bound to adherent and thrombin-stimulated adherent platelets was then studied individually. Factor Va bound to adherent and thrombin-stimulated adherent platelets with an estimated Kd of 58 nmol/L. Minimal factor Xa binding was observed on adherent platelets before thrombin stimulation. Factor Xa binding was, however, readily observed on thrombin-stimulated adherent platelets. This factor Xa binding was not saturable, and no Kd value could be estimated. Direct measurement of prothrombinase complex assembly was demonstrated by using an energy transfer phenomenon between fluorescein-labeled factor Va and rhodamine-labeled factor Xa. Prothrombinase complex assembly was observed on both adherent and thrombin-stimulated adherent platelets. The estimated Kd for the factor Va/factor Xa interaction was 4 nmol/L. TIRFS demonstrated that adherent platelets have the ability to support prothrombinase complex assembly, as shown by a direct energy transfer reaction between fluorescently labeled factors Va and Xa.

Disseminated Intravascular Coagulation: Pathogenesis and Management

Disseminated intravascular coagulation (DIC) is a clini-copathological syndrome secondary to an underlying disease. Characteristic laboratory abnormalities of DIC should suggest, much like the recognition of fever, anemia, or congestive heart failure, that an inciting disease process must be searched for. The clinical and laboratory consequences of DIC can be ascribed to the unregulated and unbalanced formation of thrombin, the main clot-forming enzyme, and plasmin, the main clot-lysing enzyme. If too much plasmin is formed in relation to thrombin, a hemorrhagic state, which appears in 60 to 75% of patients with deep vein thrombosis, will occur. Alternatively, if too much thrombin is formed in relation to the degree of secondary fibrinolysis, a thrombotic condition, which appears in 25 to 40% of patients with DIC, will become manifest. The diagnosis of DIC is dependent on the presence of an appropriate clinical situation with concurrent laboratory evidence of thrombin and plasmin formation. Thrombin formation, plasmin formation, or both, can be assessed by detection of fibrin monomer, fibrin/fibrinogen degradation products, and D-dimer or E fragment, respectively. Treatment of DIC should initially be addressed to treatment of the primary, underlying condition inciting the disorder. If treatment for DIC is specifically needed, blood product replacement is the first order of therapy. This replacement should be tailored to each patient's specific needs (i.e., platelets, fibrinogen, or plasma proteins). Heparin has a definite but limited use in conditions associated with acral cyanosis and dermal ischemia. Other specific therapies for DIC may be of use in individualized situations.

Immunoelectron-microscopic studies of human platelet thrombospondin, von Willebrand factor, and fibrinogen redistribution during clot formation

The relative distributions of the human platelet alpha-granule proteins fibrinogen, thrombospondin, and von Willebrand factor were mapped by immunoelectron microscopy in thin cryosections of activated platelets, platelet aggregates, and clots during the first 24 h of in vitro clot formation. In early activated platelets, the results suggest that the canalicular system constitutes a significant component of the external platelet surface, and may act as a compartment for biochemical reactions occurring during granule release. Further, detection of coagulation proteins by various non-morphological procedures may reflect protein contained within canalicular elements. Later in the release process, von Willebrand factor was detected as a major antigen on the platelet canalicular and plasma membranes; thrombospondin, on the other hand, showed minimal binding to platelets and only limited binding to the extensive fibrin network. Comparison of radioimmunoassays of supernatants of thrombin-stimulated platelets in plasma, clotted whole blood, and Triton X-100 platelet releasates indicated that virtually all of the platelet thrombospondin appears in serum. These data confirm the immunocytochemical results indicating that very little platelet thrombospondin binds to the platelet surface, compared with von Willebrand factor, studied here under the same conditions, which binds extensively to the platelet membrane following release and clot formation.

Physiological functions of platelets

Platelets are discoid, anucleate cells with a large number of secretory granules. Physiological agonists (thrombin, collagen, ADP, adrenaline, thromboxane A2, serotonin, vasopressin) interact with specific receptors on the platelet surface which causes the platelet responses shape change, aggregation, secretion of substances from three types of granules and liberation of arachidonate from membrane phospholipids. Some secreted substances and conversion products of arachidonate are platelet agonists and enhance platelet stimulation (positive feedback). The shape change and aggregation responses are of central importance for platelet adhesion to the subendothelium and formation of platelet thrombi. Dense granule secretion and the storage of ADP, ATP, Ca2+ and serotonin, a-granule secretion of platelet-specific, cationic, coagulation and carbohydrate-containing proteins as well as secretion of glycosidases are also shown to be important for platelet participation in haemostasis and thrombosis. Signal transduction mechanisms (phospholipase C activation, polyphosphoinositide metabolism, Ca2+ mobilization) and arachidonate oxygenation are central processes for the physiological functions of platelets.

Cleavage of protein S by a platelet membrane protease

Protein S is a vitamin K-dependent glycoprotein cofactor to the serine protease, activated protein C. In this study we demonstrate that 125I-protein S bound to unstimulated platelets in a time- and calcium-dependent saturable reaction. Half-maximal binding occurred at a protein S concentration of 10 nM, with approximately 1,100 binding sites per platelet. The binding of protein S to platelets was followed by rapid cleavage of the protein mediated by a protease confined to the platelet membrane. The membrane protease was Ca++-dependent, inhibited by high concentrations of diisopropyl fluorophosphate, but was resistant to a variety of other protease inhibitors. Functional studies demonstrated that the cleavage of protein S was associated with complete loss of cofactor anticoagulant activity. We conclude that protein S binds to platelets and is inactivated by a novel Ca++-dependent membrane protease. This may represent a physiological reaction that regulates the activity of protein S.

Formation and regulation of platelet and fibrin hemostatic plug

Formation of a hemostatic plug represents one of the earliest responses to vessel wall injury. Platelets react to any discontinuity in the vascular endothelium through initial contact, spreading, and formation of a thrombus (or aggregate). This development of a primary hemostatic plug requires platelet membrane receptors through which the adhesive macromolecules, von Willebrand factor (vWF) and fibrinogen, anchor platelets to the vessel wall and link them to each other. There are two receptor pathways--classic and alternative--for the binding of vWF to platelets; the latter induced by thrombin, and adenosine diphosphate (ADP) is shared with fibrinogen. Synthetic peptides, patterned after known binding domains of adhesive molecules, have been designed to inhibit their interactions with platelet receptors. A secondary hemostatic plug, composed of platelets enmeshed in fibrin, results from the action of thrombin, which is not only essential for formation of fibrin but also for exposure of platelet receptors for adhesive molecules and for "activation" of factors V and VIII. Thrombin generation is greatly enhanced through the activity of the prothrombinase complex formed on the surface of platelets, perturbed endothelial cells, and leukocytes. A pivotal event is activation of factor X through the intrinsic and extrinsic coagulation pathways. Binding of factors IXa and VIIa to the vascular endothelium represents a localized mechanism for factor Xa generation. Formation of a platelet and fibrin thrombus is controlled by regulatory mechanism: prostacyclin, endogenous heparin-antithrombin III complex, thrombomodulin-protein C-protein S system, and the fibrinolytic system. The balance of all components--vessel wall, platelets, adhesive and coagulation proteins, regulatory mechanisms--determines the effectiveness of the hemostatic plug in maintaining the structural and functional integrity of the circulatory system. An approach to detection of hemostatic derangements in patients at risk evolves from a full understanding of inherited and acquired deficiencies affecting each step of hemostatic plug formation and from selective use of laboratory tests.

Kinetics of thrombin-induced release and activation of platelet factor V

The kinetics of thrombin-induced platelet factor V activation were studied in suspension of washed human platelets. The effect of thrombin in stimulating the release reaction could be separated from its effect on factor V activation by use of a potent inhibitor of the release reaction, the prostacyclin analogue ZK 36374. When platelets were incubated with ZK 36374 prior to stimulation with thrombin, the amount of ZK 36374 required to inhibit 50% of factor Va formation was 15 pM. ZK 36374 at a final concentration of 1 nM was found to block instantaneously and completely the release of factor Va, whereas it has no effect neither on platelet factor V activation nor on the factor Va assay. By varying the time interval between the addition of thrombin (0.5 nM) and ZK 36374 to suspensions of 4.6 X 10(6) platelets/ml the rate of factor V release was found to be 12 pM factor V/min. In the absence of ZK 36374 the total amount of factor V released was 8 pM, whereas Triton X-100-treated platelets gave 13 pM factor V. It appeared that the amount of factor V that could be released was dependent on the thrombin concentration. Maximum release was obtained at 1 nM thrombin. The rate of factor V release increased in proportion to the thrombin concentration. The rate of factor V activation was found to be proportional to the thrombin concentration as well as to the amount of released factor V. When 4.6 X 10(6) platelets/ml were activated by 0.5 nM thrombin, the rates of factor V activation were found to be 0.3 pM and 1.2 pM factor Va/min at 20% and 90% completion of the release reaction. Therefore, the rate of factor V release was at least one order of magnitude faster than the rate of factor V activation. The kinetics of thrombin-induced platelet factor V activation were compared to those of plasma factor V activation in platelet-rich and platelet-free plasma. The results clearly demonstrate that platelets have no effect on the rate of factor V activation and that the kinetics of plasma factor V activation are identical to those of platelet factor V activation.

Biosynthesis of factor V in isolated guinea pig megakaryocytes

Although platelets contain Factor V, localized primarily in the alpha-granules, the origin of this coagulation cofactor in these cells is not known. We therefore explored whether isolated megakaryocytes could biosynthesize Factor V. Guinea pig plasma Factor V coagulant activity was demonstrated to be neutralized by human monoclonal and rabbit polyclonal antibodies directed monospecifically against human Factor V. These antibodies had been used earlier to purify human Factor V. These antibodies had been used earlier to purify human Factor V and to quantify Factor V antigen concentration, respectively (1983. Chiu, H. C., E. Whitaker, and R. W. Colman. J. Clin. Invest. 72:493-503). As determined by a competitive enzyme-linked immunosorbent assay with guinea pig plasma as a standard, Factor V solubilized from guinea pig megakaryocytes was present at 0.098 +/- 0.018 micrograms/10(5) cells. Each megakaryocyte contained about 500 times as much Factor V as is in a platelet (0.234 +/- 0.180 micrograms/10(8) platelets). The content of Factor V antigen in guinea pig plasma was greater (27.0 +/- 3.0 micrograms/ml) than that of Factor V antigen in human plasma (11.1 +/- 0.4 micrograms/ml). In contrast, human platelets contain ninefold more Factor V antigen (2.01 +/- 1.09 micrograms/10(8) platelets) than do guinea pig were 2.85 +/- 0.30 U/ml plasma, 0.022 +/- 0.012 U/10(8) platelets, and 0.032 +/- 0.03 U/10(5) megakaryocytes, compared with human values of 0.98 +/- 0.02 U/ml plasma and 0.124 +/- 0.064 U/10(8) platelets. Isolated megakaryocytes were found to contain Factor V by cytoimmunofluorescence. The megakaryocytes were incubated with [35S]methionine, and radiolabeled intracellular proteins purified were on a human anti-Factor V immunoaffinity column. The purified protein exhibited Factor V coagulant activity and neutralized the inhibitory activity of a rabbit antihuman Factor V antibody, which suggests that megakaryocyte Factor V is functionally and antigenically intact. These results indicate that Factor V is synthesized by guinea pig megakaryocytes. Nonetheless, megakaryocyte Factor V was more slowly activated by thrombin and in the absence of calcium was more stable after activation than was plasma Factor Va. Electrophoresis in sodium dodecyl sulfate and autoradiography of the purified molecule showed a major band of Mr 380,000 and a minor band of Mr 350,000, as compared with guinea pig and human plasma Factor V, where the protein had an Mr of 350,000. Both forms of Factor V were substrates for thrombin. Possible explanations for the higher molecular weight and different thrombin sensitivity and stability observed are that a precursor of Factor V was isolated or that the megakaryocyte Factor V had not been fully processed before isolation.

Platelet C1- inhibitor. A secreted alpha-granule protein

In order to characterize which proteins of the contact phase of coagulation interact with platelets, human platelets were studied immunochemically and functionally to determine if they contain C1- inhibitor. By means of monospecific antibody to C1- inhibitor, a competitive enzyme-linked immunosorbent assay (CELISA) was developed to measure directly platelet C1- inhibitor. With the CELISA, from 33 to 115 ng of C1- inhibitor antigen per 10(8) platelets from 15 normal donors was quantified in lysates of washed human platelets solubilized in nonionic detergent. The mean concentration in 10(8) platelets was 62 +/- 33 ng (SD). Plasma C1- inhibitor either in the platelet suspension medium or on the surface of the platelets could account for only from 6.5 to 16% of the total antigen measured in the solubilized platelets. Upon functional studies, platelets contained 84 +/- 36 ng (SD) of C1- inhibitor activity in 10(8) platelets. As assessed by the CELISA, platelet C1- inhibitor antigen was immunochemically identical to plasma and purified C1- inhibitor. In contrast, the mean concentration of platelet C1- inhibitor antigen in platelets from four patients with classical hereditary angioedema was 8.3 ng/10(8) platelets (range, 5.3 to 11.3 ng/10(8) platelets). 25 and 31% of the total platelet C1- inhibitor was secreted without cell lysis from normal platelets after exposure to collagen (20 micrograms/ml) and thrombin (1 U/ml), respectively, and this secretion was blocked by metabolic inhibitors. Platelet subcellular fractionation showed that platelet C1- inhibitor resided mostly in alpha-granules, similar to the location of platelet fibrinogen. Thus, human platelets contained C1- inhibitor, which became available by platelet secretion. The identification of platelet C1- inhibitor suggests that platelets may modulate the activation of the proteins of early blood coagulation and the classical complement pathways.

Molecular defects in interactions of platelets with the vessel wall

The objectives of this review have been to summarize the recent research on inherited defects involving abnormal platelet function and to illustrate how studies of hemorrhagic syndromes have led to an increased understanding of the molecular events involved in platelet adhesion and aggregation. Emphasis has been placed on the two primary hemostatic reactions: the interaction of platelets with von Willebrand factor to promote adhesion to the subendothelium, and the interaction of platelets with fibrinogen to promote platelet aggregation. Even as these events are more clearly defined, new concepts of molecular structure, function, and heterogeneity are emerging, and the variety of recognized genetic defects is becoming more complex.

Factor V (Quebec): a bleeding diathesis associated with a qualitative platelet Factor V deficiency

Studies were performed on a French-Canadian family afflicted with a bleeding disorder exhibiting an autosomal dominant inheritance pattern and a severe bleeding diathesis after trauma. Clinical laboratory coagulation tests were unimpressive; the only persistent abnormalities include mild thrombocytopenia and moderately reduced Factor V clotting activities. Some individuals had prolonged Stypven times when platelet-rich plasma was used, suggesting that their platelets could not support functional prothrombinase complex assembly. Detailed studies were performed by use of plasma and isolated, washed platelets from a sister and brother. Bioassay data indicate that both individuals had Factor V activities of approximately 40 and 36% of normal, respectively. A comparison of the Factor V radioimmunoassay and bioassay data on the brother's plasma indicated that the circulating amount of Factor V functional activity was low relative to Factor V antigen concentration (approximately 65-75%). In both individuals, the platelet Factor V functional activities were extremely low (2-4%) relative to antigen levels present as determined by radioimmunoassay. These discrepancies between Factor V activities and antigen concentration do not appear to be due to an unstable Factor V molecule or to the presence of a Factor V or Factor Va inhibitor or inactivator. Kinetics of prothrombin activation by use of purified clotting factors indicated that thrombin-activated platelets from both individuals supported prothrombinase complex assembly identical to controls in the presence of added purified Factor Va. Consequently, their bleeding diathesis appears to reflect their platelet, rather than their plasma, Factor V activity. These results suggest that platelet Factor V is an essential component in maintaining stable and prolonged hemostasis after trauma.

Prothrombin is activated on vascular endothelial cells by factor Xa and calcium

Vascular endothelial cells derived from adult bovine aorta (ABAE) treated with factor Xa and calcium were found to activate prothrombin. In contrast, nonvascular cells (human foreskin fibroblasts, bovine corneal endothelial cells, or human fetal lung cells) had either no or very little effect on prothrombin activation. In the presence of 6 X 10(5) ABAE cells, 20 ng of factor Xa converted 90 micrograms of prothrombin into 80 units of thrombin after 45 min at 37 degrees C. Exogenous factor V was not required for prothrombin activation, but thrombin generation was enhanced 2- to 4-fold by the addition of factor V (500-2,500 ng/ml). Treatment of ABAE cells with anti-bovine factor V IgG markedly inhibited prothrombin activation by factor Xa and calcium. In cells grown in serum-free medium for 3 months, the amount of factor V activity was equivalent to that found in cells grown with serum, which suggests that these cells probably synthesize factor V. Sparse ABAE cells increased prothrombin activation by factor Xa 6-fold compared to activation in confluent cells. Although previous thrombin treatment of ABAE cells did not enhance prothrombin activation, addition of dansyl arginine-4-ethyl piperidine amide markedly inhibited activation of 125I-labeled prothrombin by factor Xa, indicating that thrombin formation is necessary for optimal prothrombin activation. These data indicate that aortic endothelium may provide a physiologically important surface for activation of prothrombin as well as a mechanism for optimal formation of clots at sites of vascular injury.

Inhibition of thromboxane A2 synthesis in human platelets by coagulation factor Xa

Factor Xa binds to platelets provided that factor Va is present on the platelet surface, an interaction that results in a striking acceleration of the conversion of prothrombin to thrombin. Thrombin then initiates fibrin formation, induces platelet aggregation, and stimulates the intraplatelet synthesis of thromboxane A2 (TXA2). Addition of thrombin (2.4-14.4 nM) to platelet-rich plasma increased the basal level of TXA2, measured as thromboxane B2, from less than 0.5 pmol per 10(8) platelets to (mean +/- SEM) 100 +/- 22 and 250 +/- 10 pmol per 10(8) platelets, respectively. Treatment of platelet-rich plasma with increasing concentrations of factor Xa (1-12 nM) prior to the addition of thrombin progressively inhibited the production of TXA2. Thrombin (9.6 nM), which produced 93% of the maximal formation of TXA2, was inhibited 70% by factor Xa (10 nM). To identify which of these steps in thromboxane synthesis was inhibited by factor Xa, platelets labeled with [14C]arachidonic acid were exposed to thrombin and products of prostaglandin synthesis were separated by thin-layer chromatography. In contrast to the inhibition of TXA2 synthesis, prostaglandin E2 and prostaglandin F2 alpha synthesis were not inhibited suggesting that neither phospholipase(s) nor cycloxygenase was involved. The inhibition of TXA2 formation by factor Xa could be reversed by increasing the molar ratio of thrombin to factor Xa to 5.5. Incubation of platelets with an IgG fraction of a human monoclonal antifactor V antibody, previously shown to inhibit factor Xa binding, was found to block factor Xa inhibition of TXA2 synthesis. The inhibition of TXA2 synthesis requires the presence of the active site serine of factor Xa and is not specific for TXA2 formation induced by thrombin because it is also demonstrable when the agonist is ADP. Further, factor Xa does not require additional plasma components for its action because its inhibitory effects are detected in gel-filtered platelets. The effect of factor Xa was evident at physiological (1.3 mM) calcium concentrations. These results indicate that factor Xa binding to platelets through factor Va not only stimulates thrombin formation but also has a countervailing effect by inhibiting TXA2 formation.

Heterogeneity of human factor V deficiency. Evidence for the existence of antigen-positive variants

Functional human Factor V has been purified using a rapid immunoaffinity method. Following barium citrate adsorption of plasma, Factor V was precipitated with polyethylene glycol at a concentration between 5 and 14%. The resulting preparation was applied to a column containing an immobilized immunoadsorbent consisting of an IgG fraction containing a naturally occurring human monoclonal (IgG(4)lambda) antibody with inhibitory activity against human Factor V. The solid phase immunoglobulin quantitatively bound Factor V from human plasma. The bound Factor V was effectively eluted with a Tris buffer pH 7.2 containing 1.2 M NaCl and 1 M alpha-methyl-D-mannoside. The isolated native Factor V with high specific activity (92 U/mg) showed a single band (M(r), 350,000) on both reduced and nonreduced sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Factor V was purified 5,100-fold over plasma with an overall yield of 77%. The purified Factor V when subjected to thrombin activation exhibited an 18-fold increase in coagulant activity. The isolated Factor V neutralized the inhibitory activities of the monoclonal antibody that was used to purify it, as well as the rabbit antibodies produced by immunizing the animals with the purified Factor V. Immunoelectrophoresis of purified Factor V against the polyclonal rabbit antiserum resulted in a single precipitin arc of identical mobility to the Factor V in normal human plasma. Analysis by double immunodiffusion showed a line of identity between plasma and purified Factor V and crossed immunoelectrophoresis showed a single species in normal plasma.A competitive enzyme-linked immunosorbent assay using the rabbit antibody against Factor V was applied to quantify Factor V antigen level in human plasma. Reconstitution of congenitally deficient or immunodepleted plasma with normal plasma or purified Factor V gave parallel dose-response curves. In 14 normal plasma the coagulant activity was 0.98+/-0.02 U/ml (mean+/-SEM) and antigen concentration was 11.1+/-0.4 mug/ml. A pool of 14 patients with congenital Factor V deficiency were studied. 10 patients had Factor V antigen ranging from 1.0 to 2.4 mug/ml with corresponding coagulant activities (0-0.17 U/ml) indicating a low concentration of normal Factor V, presumably due to decreased synthesis or increased degradation. When these patient plasmas and the normal plasmas were analyzed together an excellent correlation (r = 0.97, P < 0.01) was obtained. However, four patients with coagulant activity (0-0.08 U/ml) had Factor V antigen concentrations ranging from 4.4 to 6.1 mug/ml, indicating the presence of a reduced concentration of abnormal Factor V protein. The presence of patients with antigen similar in concentration to coagulant activity and antigen in excess of Factor V activity indicates the heterogeneity of congenital Factor V deficiency.

High-molecular weight kininogen. A secreted platelet protein

Human platelets were studied immunochemically to determine if they contain high-molecular weight kininogen. On crossed immunoelectrophoresis with total kininogen antisera (antisera that recognizes both high- and low-molecular weight kininogen) extracts of platelets contained total kininogen antigen. Platelet total kininogen antigen showed complete antigenic identity with plasma total kininogen and displayed the same electrophoretic migration as plasma total kininogen. Using antisera monospecific to high molecular weight kininogen, a competitive enzyme-linked immunosorbent assay (CELISA) was developed to directly measure platelet high-molecular weight kininogen. By CELISA, 27-101 ng of high molecular weight kininogen antigen per 10(8) platelets was quantitated in detergent-soluble lysates of washed human platelets from nine normal donors with a mean level of 60 ng +/- 24/10(8) platelets. Plasma high-molecular weight kininogen, either in the platelet suspending medium or on the surface of the platelets, could only account for 5% of antigen measured in the solubilized platelets. On the CELISA, platelet high-molecular weight kininogen was immunochemically identical to plasma and purified high-molecular weight kininogen. Platelet high-molecular weight kininogen was secreted from platelets after exposure to ionophore A23187 (3-15 microM), collagen (5-150 micrograms/ml), and thrombin (1.6 U/ml). Secreted platelet high-molecular weight kininogen did not become a part of the platelet Triton-insoluble cytoskeleton. On cross immunoelectrophoresis secreted platelet total kininogen antigen had a similar electrophoretic migration to plasma total kininogen. Thus, human platelets contain high-molecular weight kininogen that can be secreted from platelets and that may participate in plasma coagulation reactions.

Activation of coagulation factor V by a platelet protease

Factor V must be converted to Factor V(a) in order to bind to a high affinity platelet surface site and participate in prothrombin activation. Osterud et al. (10) presented data that suggested that human platelets contain an activated form of Factor V and a Factor V activator. We find that the Factor V released when platelets are disrupted by freezing and thawing or sonication is activated 3- to 10-fold by thrombin as determined by coagulation assay and is therefore stored as the relatively inactive procofactor rather than in the active form Factor V(a). We incubated purified Factor V, which had a specific activity of 140+/-30 U/mg, with Factor V-deficient frozen and thawed platelets (10(9) platelets/ml) obtained from a patient with Factor V deficiency. The specific activity of the Factor V increased to a maximum of 740+/-240 U/mg (mean+/-SD of three experiments). When this partially activated Factor V was incubated with thrombin its specific activity increased further to 1,440+/-280 U/mg, which is similar to the activity of Factor V activated with thrombin alone (1,540+/-60 U/mg). The platelet Factor V activator is not inhibited by dansyl arginine-4-ethylpiperidine amide, 93 muM, indicating that it is not thrombin. When thrombin-stimulated platelets, to which dansyl arginine-4-ethylpiperidine amide had been added to inhibit the further action of thrombin, were incubated with (125)-labeled Factor V, there was no detectable proteolysis of the Factor V molecule. Our failure to detect activation of Factor V under these conditions suggests that <4% of the platelet protease is released by thrombin. Subcellular fractionation of platelets indicates that the platelet protease that activates Factor V is in the soluble fraction. When Factor V(a) formed by the action of platelet protease is incubated with platelets, peptides with M(r) = 105,000, 87,000, and 78,000 bind to the platelet surface. All three radiolabeled peptides are displaced from platelets by unlabeled Factor V(a) formed by the action of thrombin. The stoichiometry of binding suggests that the 105,000-dalton peptide is associated with either an 87,000- or a 78,000-dalton peptide. The 78,000-dalton peptide binds with greater affinity and probably accounts for the bulk of the activity of Factor V(a) in coagulation assays. Whether or not the platelet protease serves to activate Factor V before thrombin formation during normal hemostasis remains to be determined.

Factor V binding to human platelets: the role of divalent cations and of platelet preparatory methods

The binding of bovine factor V to human platelets has been studied to ascertain the influence of divalent cations as well as the manner of preparation of platelet suspensions. The binding of factor V to platelets was inhibited by EDTA and EGTA and required calcium (optimum concentration 5mM) but not magnesium. Factor V bound to a single class of low affinity sites in unstimulated gel filtered platelets with a Kd of 3.42 x 10(-8)M. Approximately, 2,340 factor V molecules were bound per platelet. Stimulation with the calcium ionophore A23187 or ADP and fibrinogen and inhibition with prostaglandins E1 (PGE1) or prostaglandins I2 (PGI2) failed to alter these constants. However, washing platelets by repeated centrifugation even in the presence of inhibitors of platelet activation decreased the number of binding sites to 1200 without change in the kd. These studies demonstrate a requirement for calcium in the physiological range for factor V binding to platelets, establish that washed platelets bind fewer factor V molecules than gel filtered platelets, and indicate that stimulation or inhibition of platelet function does not affect factor V binding.