A monoclonal antibody which inhibits the factor Va:factor Xa interaction

An immunoprecipitation technique has been used to determine the subunit specificity of two of the monoclonal antibodies to bovine Factor V(Va) developed by this laboratory. One of the antibodies is specific for the 74,000-dalton subunit (the E chain) of Factor Va, and the other antibody is specific for the 94,000-dalton subunit (the D chain). The binding of Factor Va to phospholipid was studied by light scattering, and the interaction of Factor Xa with phospholipid-bound Factor Va was examined using 5-dimethylaminonaphthalene-1-sulfonyl-glutamyl-glycyl-arginyl-Xa (Dns-EGR-Xa). Neither the antibody specific for the E chain nor the antibody specific for the D chain inhibit the binding of Factor Va to phospholipid vesicles. The antibody specific for the E chain blocks the increase in fluorescence polarization seen when Factor Va is added to a solution of Dns-EGR-Xa, phospholipid vesicles and calcium. This antibody also inhibits the association of Dns-EGR-Xa with phospholipid-bound Factor Va as determined by gel-exclusion high pressure liquid chromatography. The antibody specific for the D chain of Factor Va does not block the increase in polarization seen when Factor Va is added to a solution of Dns-EGR-Xa, phospholipid, and calcium. It was concluded that the antibody specific for the E chain of Factor Va binds at or near the Factor Xa-binding site on the E chain and that the Factor Va E chain plays a significant role in binding Factor Xa.

Proteolytic alterations of factor Va bound to platelets

The coagulation protein Factor Va forms the receptor for the serine protease Factor Xa at the platelet surface. This membrane-bound complex of Factor Va and Factor Xa plus calcium constitutes the enzymatic complex prothrombinase, which effects the conversion of prothrombin to the clotting enzyme, thrombin. Studies were undertaken to investigate the proteolytic events accompanying the inactivation of platelet-bound Factor Va by activated protein C as well as the ability of Factor Xa to protect Factor Va from activated protein C inactivation. During the course of these studies, observations were made which indicated that Factor Va was also cleaved by both a platelet-associated protease, as well as Factor Xa. When Factor Va was incubated with washed platelets, electrophoresis and autoradiography of solubilized platelet pellets indicated that three Factor Va peptides were associated with the platelet: component D (Mr = 94,000), component E (Mr = 74,000), and a 90,000-dalton peptide (component D') which appeared with time as the result of a platelet-associated protease cleavage of component D. The Factor Va peptides bound to platelets were proteolytically inactivated by activated protein C, resulting in five peptide products, all of which remained associated with the platelet-membrane surface. Factor Va was protected from activated protein C proteolysis by complex formation with Factor Xa or active site-blocked Factor Xa. However, active Factor Xa cleaved platelet-bound Factor Va to peptide products which also remained associated with the platelet. Whereas activated protein C rapidly cleaved components D and D' with secondary cleavages occurring in component E, Factor Xa rapidly cleaved component E with secondary cleavages occurring in components D and D'. The Factor Xa-cleaved Factor Va is catalytically functional. To determine whether cleavage was necessary for function, prothrombin conversion reaction mixtures were monitored for thrombin formation and Factor Va cleavage with time in a defined phospholipid vesicle model system. The results indicated that Factor Xa cleavage of Factor Va is not essential for Factor Va activity but may promote its ability to function in the prothrombinase complex.

Monoclonal antibodies selective for the functional states of bovine factor V and factor Va

Hybridoma technology has been used for the production of murine monoclonal antibodies to bovine coagulation Factor V and its thrombin-activated product, Factor Va. Hybrid cell cultures were assayed for the production of anti-Factor V and anti-Factor Va antibodies by a solid-phase radioimmunoassay. Antibody-producing cell lines were selected, cloned and grown as ascites tumors. Gel filtration chromatography (Ultrogel AcA34) and affinity chromatography (protein A-Sepharose) were used to isolate the monoclonal immunoglobulins from the ascites fluids. Thirteen monoclonal antibodies have been characterized with respect to their binding to Factor V and Factor Va and their effect on cofactor bioactivity. Six of these thirteen antibodies react with both Factor V and Factor Va. One of these antibodies is strongly inhibitory, while a second antibody is only moderately inhibitory. The antibody produced by another cell line binds Factor V but not Factor Va and is not inhibitory. The remaining six cell lines each produce an antibody that reacts preferentially with Factor Va, and each of these antibodies is inhibitory to some extent. Both a radioimmunoassay and light scattering have been used to study the interaction of the immunoglobulins with Factor V and Factor Va. The light scattering technique has proven useful to study the interaction of isolated antibodies and antigens and permits the determination of interaction stoichiometries. Each of the interactions studied was characterized by a stoichiometry of two antigens per antibody. These monospecific immunochemical reagents will be useful in the study of structure and function relationships of Factor V, Factor Va and activation fragments.

Radioimmunoassay of factor V in human plasma and platelets

Homogeneous, single-chain human factor V was used to develop a double antibody competition radioimmunoassay to measure factor V concentrations in plasma and platelets. Standard curves were constructed that allow for the detection of as little as 20 ng factor V/ml of plasma. Normal factor V concentrations range from 4 to 14 micrograms/ml of plasma with an average value of 7.0 +/- 2.0 micrograms/ml (n = 64). No correlation was observed between antigen levels and age or sex. The radioimmunoassay data are consistent with factor V clotting assays, providing freshly drawn plasma is used in the bioassay. Radioimmunoassay of washed platelets indicate that 0.63-1.93 microgram of factor V is present per 2.5 X 10(8) platelets (4612-14128 molecules of the factor V platelet). When normalized to individual hematocrits and platelet count, the data indicated that platelets contribute approximately 18%-25% of the factor V found in whole blood. In addition, two individuals with functionally deficient factor V were examined and found to be deficient in both antigen and activity.

Stroma free human platelet lysates potentiate the in vivo thrombogenicity of factor Xa by the provision of coagulant-active phospholipid

A stroma free platelet lysate supernatant (PLS) was prepared by repeatedly freezing and thawing a human platelet suspension separated from platelet rich plasma by gel filtration. The material was potently thrombogenic in a stasis model in rabbits, but only when combined with a purified preparation of factor Xa. The dose of factor Xa used was not thrombogenic when given alone. Initial evaluation suggested the presence of a factor V activator when PLS shortened the clotting time of normal but not a V deficient plasma. Subsequent evaluation, in a system employing purified components of the prothrombinase complex and a marker of thrombin generation, dansyl-arginine-N-(3-ethyl-1,5-pentanediyl)amide (DAPA), failed to confirm the presence of a factor V activator. Further evaluation in this system demonstrated that the procoagulant activity related to the content of coagulant-active phospholipid. Thin layer chromatography confirmed the presence of phospholipids known to have coagulant activity. These components were isolated by a lipid extraction procedure and the phospholipid replacing activity in the in vitro assay with DAPA was quantitatively retained. The extracted material was non-thrombogenic in vivo but augmented the thrombogenicity of a purified preparation of Xa.

Progressive development of a thrombin inhibitor binding site

The studies reported here were undertaken to determine whether the thrombin precursors prothrombin, prethrombin 1, prethrombin 2, and Meizo thrombin interact with the fluorescent, reversible thrombin inhibitor dansylarginine N,N-(3-ethyl-1,5-pentanediyl)amide (DAPA) [Nesheim, M. E., Prendergast, F. G., & Mann, K. G. (1979) Biochemistry 18, 996--1003]. The results indicate that prothrombin and prethrombin 1, in which the cleavage sites at Arg274-Thr275 and Arg323-Ile324 both remain intact, do not bind DAPA, while prethrombin 2 or Meizo thrombin, which results respectively from a single cleavage of prothrombin at Arg274-Thr275 or Arg323-Ile324, do bind the inhibitor. Since prethrombin 2 is a precursor of thrombin without measurable enzymatic activity, a thorough characterization of its interaction with DAPA was undertaken. The interaction of DAPA with bovine thrombin similarly was studied for comparative purposes. The binding of DAPA to either protein is accompanied by changes in the fluorescence properties of the dansyl moiety including increases in emission intensity, excited-state lifetime, polarization, and a slight blue shift in the wavelength of maximum emission intensity. Corrected excitation spectra indicate energy transfer to DAPA from one or more aromatic side chains of both proteins. Values of P0 for both complexes were extrapolated from Perrin plots of polarization vs. temperature and suggest that the dansyl moiety is held more rigidly in thrombin than in prethrombin 2. With excitation at either 280 or 335 nm the emission intensity of DAPA-prethrombin 2 is substantially less than that of the DAPA-thrombin complex. In contrast, the intensity of the Meizo thrombin-DAPA complex is greater than that of the DAPA-thrombin complex. From measurements of intensity changes the dissociation constants and stoichiometry of DAPA binding to thrombin and prethrombin 2 were measured. Prethrombin 2 binds to DAPA with a Kd = 5.9 x 10(-7) M (n = 1) while thrombin binds about 30 times more tightly with a Kd = 2.0 x 10(-8) M (n = 1). The active site directed irreversible thrombin inhibitors diisopropyl phosphorofluoridate and D-phenylalanylprolylarginyl chloromethyl ketone displace DAPA from thrombin but not from prethrombin 2. The results of these studies indicate the binding of a presumed substrate analogue (DAPA) to an inactive zymogen, prethrombin 2. In addition, the lack of DAPA binding to prothrombin and prethrombin 1, under conditions in which it binds to prethrombin 2, implicates events that accompany cleavage at Arg274-Arg275 in the "progressive" formation of an active site, even though further cleavage at Arg323-Ile324 is required for expression of enzymatic activity.

Studies of the capacity of factor Xa to protect factor Va from inactivation by activated protein C

Blood coagulation Factor Xa and activated protein C are both serine proteases derived from circulating, vitamin K-dependent precursors. They express, respectively, procoagulant and anticoagulant properties through Ca2+ and phospholipid-dependent interactions with coagulation Factor Va. The present studies were undertaken to determine whether Factor Xa and activated protein C interact independently or competitively with Factor Va. The interactions were assessed by examining the activated protein C-catalyzed inactivation of Factor Va in the absence and presence of Factor Xa at various concentrations. The results indicated that the two proteins compete for Factor Va and that Factor Xa protects Factor Va from inactivation by activated protein C. The ability of Factor Xa to protect Factor Va was observed either in the absence of a Factor Xa substrate or under conditions in which the complex of Factor Va and Factor Xa was engaged in the conversion of substrate (prethrombin 1) to thrombin.

The coagulant-active phospholipid content is a major determinant of in vivo thrombogenicity of prothrombin complex (Factor IX) concentrates in rabbits

In vitro evaluation of prothrombin complex concentrates in a thrombin generation assay, using DAPA and purified components of the prothrombinase complex, demonstrated significant levels of coagulant-active "phospholipid replacing" activity. Quantification of this activity showed a significant correlation (r = 0.8747, p less than 0.01) with thrombogenicity measured in vivo in a stasis model in rabbits. Extracted lipid material retained full phospholipid replacing activity in the vitro assay. Thin-layer chromatographic characterization confirmed the presence of phospholipids with known coagulant activity in vitro. In vivo, the extracted material was nonthrombogenic but augmented the thrombogenicity of purified factor Xa. Substitution of a synthetic coagulant-active phospholipid (phosphatidylcholine-phosphatidylserine lipid vesicles) for the extracted phospholipid produced a similar augmentation of a factor-Xa-induced thrombogenicity in vivo. It is concluded that the coagulant-active phospholipid content of prothrombin complex concentrates is a major determinant of thrombogenicity but requires the presence of activated clotting factors for its expression in vivo.

Cofactor dependence of factor Xa incorporation into the prothrombinase complex

The blood coagulation serine protease, factor Xa, was dansylated with the active site-directed inhibitor, dansyl-glutamyl-glycyl-arginyl chloromethyl ketone. The Ca2+-dependent interactions of the inactivated factor Xa with its cofactors, phospholipid and activated factor V (factor Va), were studied through alterations of fluorescence polarization values of dansyl moiety of the modified enyme. In the presence of phospholipid and Ca2+, factor Va and factor Xa interacted with 1:1 stoichiometry, an interaction characterized by markedly enhanced polarization. The factor Va-independent interaction of factor Xa with phospholipid was also observed, characterized by dissociation constant Kd=2.7 x 10-6 M and stoichiometry of 66 mol of phospholipid/mol of factor Xa. The interaction of factor Xa with vesicles in the absence of factor Va exhibited considerably lower polarization values than in the presence of factor Va. These data obtained by direct spectral measurements are in agreement with the inferences drawn previously from studies of kinetics that the prothrombinase complex consists of 1:1 stoichiometric complex of factor Xa and phospholipid-bound factor Va, and that the enzymatic complex assembles in the absence of the natural substrate, prothrombin.

Antigenic changes produced by complex formation between thrombin and antithrombin-III

Immunization of goats and mules with human thrombin resulted in an antiserum that reacted only weakly with the parent molecule, prothrombin. Some of the antibodies in this antiserum showed a greater affinity for thrombin complexed to its naturally occurring inhibitor, antithrombin-III, than for active thrombin. An antiserum against the human thrombin inhibitor, antithrombin-III, produced 2 precipitin lines against human serum but only 1 against plasma. The 2nd line in serum was shown to represent precipitation of a complex of thrombin with antithrombin-III. The neoantigens appearing in antithrombin-III after complex formation were also present in complexes prepared with purified clotting factor Xa and antithrombin-III. Since purified host (mule) thrombin was also capable of causing formation of the neoantigenic sites when complexed to human antithrombin-III, it seems likely that these determinants result from interaction in the host between the immunogens (either human thrombin or antithrombin) and the appropriate interacting host protein (mule antithrombin-III or thrombin, respectively). Studies by radioimmunoassay showed that the antibodies formed are not completely specific for the neoantigens since they also react to a lesser extent with the free proteins.

Antigenic changes produced by complex formation between thrombin and antithrombin-III

Immunization of goats and mules with human thrombin resulted in an antiserum that reacted only weakly with the parent molecule, prothrombin. Some of the antibodies in this antiserum showed a greater affinity for thrombin complexed to its naturally occurring inhibitor, antithrombin-III, than for active thrombin. An antiserum against the human thrombin inhibitor, antithrombin-III, produced 2 precipitin lines against human serum but only 1 against plasma. The 2nd line in serum was shown to represent precipitation of a complex of thrombin with antithrombin-III. The neoantigens appearing in antithrombin-III after complex formation were also present in complexes prepared with purified clotting factor Xa and antithrombin-III. Since purified host (mule) thrombin was also capable of causing formation of the neoantigenic sites when complexed to human antithrombin-III, it seems likely that these determinants result from interaction in the host between the immunogens (either human thrombin or antithrombin) and the appropriate interacting host protein (mule antithrombin-III or thrombin, respectively). Studies by radioimmunoassay showed that the antibodies formed are not completely specific for the neoantigens since they also react to a lesser extent with the free proteins.

Coordinate binding of factor Va and factor Xa to the unstimulated platelet

The interaction of Factor Xa and Factor Va at the platelet surface was investigated by direct, simultaneous binding measurements of both proteins to platelets and by immunochemical and kinetic techniques. Binding measurements of 125I-Factor Xa and 131I-Factor Va to unstimulated platelets indicate that the amount of Factor Xa bound is proportional to the amount of Factor Va bound. At saturating concentrations of Factor Xa, the ratio of platelet-bound Factor Xa to platelet-bound Factor Va was unity. At saturating levels of Factor Va (1.2 X 10(-8) M), 2300 molecules of Factor Xa are bound to the platelet, whereas at an equivalent concentration of unactivated Factor V, only 800 molecules of Factor Xa are bound. In the absence of exogenous Factor V or Va, thrombin-treated washed platelets bound only 200 Factor Xa molecules per platelet, suggesting that under these conditions, platelet Factor Va is the limiting component. Monovalent Fab fragments of burro antibovine Factor V inhibit, to the same degree, the rate of thrombin generation and the binding of Factor Va and Factor Xa to the platelet surface. Anti-Factor V Fab decreased the extent of Factor Va and Factor Xa binding equivalently. When the interaction of Factor Xa with platelets is modeled as Factor Xa binding to platelet-bound Factor Va, double reciprocal plots are linear, yielding a stoichiometry of 1.04 and a dissociation constant of 6 X 10(-10) M. Kinetic experiments indicate the presence of approximately 900 functional Factor Va platelet sites (Kd = 1.5 to 2.2 X 10(-10) M). This number of functional sites is equivalent to the number (837 +/- 48) of Factor Va high affinity binding sites (Kd = 4.0 X 10(-10) M). These sites most likely represent the Factor Xa binding sites involved in the function of the prothrombinase complex at the platelet surface.

Molecular weight of undegraded plasma factor V

The molecular size and immunochemical properties of the unfractionated factor V present in plasma collected by venipuncture into a broad-spectrum anticoagulant and platelet-inhibited mixture were compared with those reported for the isolated, single-chain factor V molecular of 330 000 daltons. The anticoagulant--plasma mixture included 0.28% trisodium citrate, 2 mM benzamidine hydrochloride, 0.02% soybean trypsin inhibitor, 2.0 mM diisopropyl phosphorofluoridate, 10 microM dansylarginine N-(3-ethyl-1,5-pentanediyl)amide, and 5 microM prostaglandin E1. The Stokes radius of unfractionated factor V present in highly inhibited plasma (93 A) is virtually identical with the Stokes radius predicted from the hydrodynamic data for the highly asymmetric, single-chain factor V molecule (91 A). With an expression which relates the Stokes radius and the sedimentation coefficient to the molecular weight of hydrodynamic units, the molecular weight obtained for factor V, using gel filtration data, is 336 000, in good agreement with the molecular weight determined from the sedimentation equilibrium, 330 000. In contrast, the Stokes radius for the factor Va present in serum is significantly smaller (50.5 A) and equivalent to the Stokes radius obtained upon activation of isolated factor V with thrombin. Immunochemical comparisons of the factor V present in the inhibited plasma and isolated factor V were conducted by using burro antibovine factor V antibody and the technique of immunoelectrophoresis. The factor V antigen present in both sources is immunochemically identical, as is the electrophoretic mobility of both factor V preparations. These data serve to justify the conclusion that the factor V isolated as a single-chain 330 000-dalton molecule corresponds to the factor V circulating in plasma.

Isolation of functional human coagulation factor V by using a hybridoma antibody

Spleen cells obtained from mice immunized with partially purified human coagulation Factor V were fused with NS-1 mouse myeloma cells, and hybrids were selected. Culture media were screened for anti-Factor V activity, and an antibody-positive clone was obtained and passaged as an ascites tumor in mice. The ascitic fluid from the hybridoma-bearing mouse could be diluted 1:10(6) before losing reactivity in an anti-Factor V radioimmunoassay. When immobilized on agarose, the monoclonal antibody quantitatively removed Factor V activity from human plasma. Factor V activity could be eluted with 1.2 M NaCl at pH 6.5. Homogeneous Factor V was isolated by chromatography of barium citrate-adsorbed, polyethylene glycol 6000 precipitated plasma on the antibody column followed by chromatography on phenyl-Sepharose. The isolated Factor V exhibited a single band upon gel electrophoresis in sodium dodecyl sulfate with an apparent Mr comparable to that of bovine Factor V (330,000). Upon exposure to thrombin, the activity of Factor V increased 53-fold when measured in Factor V-deficient plasma. This increased activity was associated with discrete proteolytic cleavages of the parent molecule.

Identification of a congenital dysthrombin, thrombin Quick

A dysprothrombin designated prothrombin Quick, is isolated from the plasma of an individual with < 2% of normal functional prothrombin activity and 34% of the normal prothrombin level by immunologic assay. With Factor Xa or taipan snake venom as activators, a fragmentation pattern identical to that of normal prothrombin is observed on gel electrophoresis in dodecylsulfate. This evidence combined with the observed barium citrate adsorption of prothrombin Quick and the low activity suggests that the defect in prothrombin Quick is in the thrombin portion of the molecule. Thrombin Quick is isolated and comigrates with thrombin on dodecyl sulfate gel electrophoresis, either reduced or nonreduced. The activity of thrombin Quick on several biological substrates of thrombin is investigated. Relative to normal thrombin, thrombin Quick is 1/200 as active on fibrinogen and 1/20-1/50 as effective in activating Factors V and VIII and aggregating platelets. A complex with antithrombin III is detected by dodecyl sulfate gel electrophoresis. Further investigation with the active site titrant, dansylanginine-N-(3-ethyl-1,5-pentanediyl)amide showed that the thrombin Quick preparation has the same affinity for the titrant as thrombin, but apparently only 40% active sites per mole protein are titrable.

Circulating heparan sulfate proteoglycan anticoagulant from a patient with a plasma cell disorder

A woman, aged 68, with multiple myeloma (immunoglobulin[Ig]A kappa type) developed an anticoagulant with properties suggestive of heparin. The anticoagulant prolonged the thrombin time but not the reptilase time and was resistant to boiling, proteolytic enzyme digestion, and trichloracetic acid precipitation. The thrombin time was corrected by the addition (in vitro) of protamine sulfate or the addition of purified platelet Factor 4 (PF4) to the plasma. The anticoagulant was isolated by PF4-Sepharose affinity chromatography and analyzed in terms of its molecular weight, uronic acid, and amino acid composition. The proteoglycan isolated had a mol wt of 116,000 and appears to consist of two 38,000 dalton polysaccharide units interconnected by peptide material totaling 39,000 daltons. Electrophoretic analysis of the pronase digested peptidoglycan using the lithium acetate-agarose technique suggested the material was of the heparan sulfate type. The peptidoglycan had about one-tenth the specific activity of commercially available heparin on a weight basis. The isolated proteoglycan was indistinguishable from commercial heparin when analyzed in terms of its ability to act as a cofactor in the antithrombin III inhibition of thrombin.

The calcium-binding properties of bovine factor V

The calcium-binding properties of the coagulation cofactor, Factor V, have been investigated using atomic absorption spectrophotometry, equilibrium dialysis, and chemical exchange experiments. Two classes of binding sites have been observed: one site containing a single Ca2+ ion bound with an unusually high affinity (Kd less than 10(-8) M), and a second site in which 2 mol of calcium are bound/mol of Factor V with an association constant Ka = 1.7 +/- 0.52 x 10(4) M-1 (Kd = 5.9 +/- 1.9 x 10(-5) M). The binding of the dissociable Ca2+ ions is apparently noncooperative. The single high affinity Ca2+ ion may be removed readily by EDTA under native conditions with an immediate loss of Factor V activity, and the activity of the Factor V can be restored by the addition of a molar excess of calcium. The high affinity Ca2+ ion will exchange with radiolabeled calcium in solution, and Factor V labeled in this way was used to provide an independent measure of the stoichiometry of this high affinity binding site, and to observe directly the interaction of calcium in this site with EDTA. In addition, elemental analyses of solid lyophilized samples of Factor V revealed that no transition metals are present and that no phosphorus (or phospholipid) is retained by the Factor V after purification.

Decarboxylation of bovine prothrombin fragment 1 and prothrombin

Bovine prothrombin fragment 1 and prothrombin undergo decarboxylation of their gamma-carboxyglutamic acid residues when the lyophilized proteins are heated in vacuo at 110 degrees C for several hours. The fully decarboxylated fragment 1 product has lost its barium-binding ability as well as the calcium-binding function which causes fluorescence quenching in the presence of 2 mM Ca2+. There is no sign of secondary structure alteration in solution upon analysis by fluorescence emission and circular dichroic spectroscopy. A family of partially decarboxylated fragment 1 species generated by heating for shorter periods shows that the initial decrease in calcium-binding ability occurs almost twice as rapidly as the loss of gamma-carboxyglutamic acid. This is consistent with the idea that differential functions can be ascribed to the 10 gamma-carboxyglutamic acid residues in fragment 1, including both high- and low-affinity metal ion binding sites. Prothrombin itself also undergoes total decarboxylation without any apparent alteration in secondary structure. However, in this case the latent thrombin activity is progressively diminished during the heating process in terms of both clotting activity and hydrolysis of the amide substrate H-D-Phe-Pip-Arg-pNA. The present results indicate that in vitro decarboxylation of gamma-carboxyglutamic acid in dried proteins is useful for analyzing the detailed calcium-binding proteins of vitamin K dependent coagulation factors.

The contribution of bovine Factor V and Factor Va to the activity of prothrombinase

The rates of prothrombin activation under initial conditions of invariant concentrations of prothrombin and Factor Xa were studied in the presence of various combinations of Ca2+, homogeneous bovine Factor V, Factor Va, phosphatidylcholine-phosphatidylserine vesicles, and activated bovine platelets. Reactions were monitored continuously through the enhanced fluorescence accompanying the interaction of newly formed thrombin with dansylarginine-N-(3-ethyl-1,5-pentanediyl) amide. The complete prothrombinase (Factor Xa, Ca2+, phospholipid, and Factor Va) behaved as a "typical" enzyme and catalyzed the activation of prothrombin with an apparent Vmax of 2100 mol of thrombin/min/mol of Factor Va or Factor Xa, whichever was the rate-limiting component. Regardless of whether the enzymatic complex was composed of Factor Xa, Ca2+, and plasma Factor Va plus phospholipid vesicles, or activated platelets in the place of the latter components, similar specific activity values were observed. The combination of Factor Va, Ca2+, and phospholipid enhanced the rate of the Factor Xa-catalyzed activation of prothrombin by a factor of 278,000. Factor Va itself when added to Factor Xa, Ca2+, and phospholipid, enhanced the rate of prothrombin activation by a factor of 13,000. Unactivated Factor V appears to possess 0.27% of the procoagulant activity of thrombin-activated Factor Va. From the kinetics of prothrombinase activity, an interaction between Factor Xa and both Factor V and Factor Va was observed, with apparent 1:1 stoichiometries and dissociation constants of 7.3 x 10(-10) M for Factor Va and 2.7 x 10(-9) M for Factor V. The present data, combined with data on the equilibrium binding of prothrombinase components to phospholipid, indicate that the model prothrombinase described in this paper consists of a phospholipid-bound, stoichiometric complex of Factor Va and Factor Xa, with bound Factor Va serving as the "binding site" for Factor Xa, in concert with its proposed role in platelets.