Observations on the interaction of phospholipids and certain clotting factors in prothrombin activator formation

Structure and function of factor X: properties, activation, and activity in prothrombinase. A retrospective in a historical context

The evolution of our understanding of the formation of thrombin from the postulated thrombokinase of Morawitz to activated Factor X and prothrombinase occurred during a period of nearly 100 years. During this time structure-function relationships have emerged and the roles of phospholipid surfaces, the accessory factor, Factor V and its activated form have been clarified. This paper summarizes this story with particular acknowledgement of the seminal contributions of Haskell Milstone.

Characterisation of a novel thrombomodulin c.1487delC,p.(Pro496Argfs*10) variant and evaluation of therapeutic strategies to manage the rare bleeding phenotype

INTRODUCTION

A novel variant in the thrombomodulin (TM) gene, c.1487delC,p.(Pro496Argfs*10), referred to as Pro496Argfs*10, was identified in a family with an unexplained bleeding disorder. The Pro496Argfs*10 variant results in loss of the transmembrane and intracellular segments of TM and is associated with an increase in soluble TM (sTM) in the plasma. The aim of this study was to characterise the effect of elevated sTM on thrombin generation (TG) and fibrinolysis, and to evaluate therapeutic strategies to manage the patients.

METHODS

Plasma samples were obtained from two patients carrying the variant. TG was triggered using 5 pM tissue factor and measured using the Calibrated Automated Thrombogram. A turbidity clot lysis assay was used to monitor fibrinolysis. TM antigen was quantified by ELISA.

RESULTS

Patients with the Pro496Argfs*10 variant had significantly elevated plasma sTM compared to controls (372.6 vs. 6.0 ng/ml). TG potential was significantly lower in patients but was restored by inhibition of activated protein C (APC) or addition of activated Factor VII (FVIIa) or platelet concentrates. In vitro experiments suggested that activated prothrombin complex concentrates (APCC) posed a risk of thrombosis. The time to 50% lysis was significantly prolonged in patients compared to controls, 69.7 vs. 42.3 min. Clot lysis time was shortened by inhibition of activated thrombin activatable fibrinolysis inhibitor (TAFIa).

CONCLUSIONS

Our data demonstrate that increased sTM enhances APC generation and reduces TG. Simultaneously, the rate of fibrinolysis is delayed due to increased TAFI activation by sTM. Treatment with platelet or FVIIa concentrates may be beneficial to manage this rare bleeding disorder.

Glucosylceramide, a neutral glycosphingolipid anticoagulant cofactor, enhances the interaction of human- and bovine-activated protein C with negatively charged phospholipid vesicles

The effect of glucosylceramide (GlcCer) on activated protein C (APC)-phospholipid interactions was examined using fluorescence resonance energy transfer. Human APC, labeled with either fluorescein (Fl-APC) or dansyl (DEGR-APC) donor, bound to phosphatidylcholine/phosphatidylserine (PC/PS, 9:1 w/w) vesicles containing octadecylrhodamine (OR) acceptor with a K(d) (app) = 16 micro g/ml, whereas Fl-APC (or DEGR-APC) bound to PC/PS/GlcCer(OR) (8:1:1) vesicles with a K(d) (app) = 3 micro g/ml. This 5-fold increase in apparent affinity was not species-specific since bovine DEGR-APC also showed a similar GlcCer-dependent enhancement of binding of APC to PC/PS vesicles. From the efficiency of fluorescence resonance energy transfer, distances of closest approach of approximately 63 and approximately 64 A were estimated between the dansyl on DEGR-APC and rhodamine in PC/PS/GlcCer(OR) and PC/PS(OR), respectively, assuming kappa(2) = 2/3. DEGR-APC bound to short chain C8-GlcCer with an apparent K(d) of 460 nm. The presence of C8-GlcCer selectively enhanced the binding of C16,6-NBD-phosphatidylserine but not C16,6-7-nitrobenz-2-oxa-1,3-diazole (NBD)-phosphatidylcholine to coumarin-labeled APC. These data suggest that APC binds to GlcCer, that PC/PS/GlcCer vesicles like PC/PS vesicles bind to the N-terminal gamma-carboxyglutamic acid domain of APC, and that one mechanism by which GlcCer enhances the activity of APC is by increasing its affinity for membrane surfaces containing negatively charged phospholipids.

Atherogenic Lipoproteins Support Assembly of the Prothrombinase Complex and Thrombin Generation: Modulation by Oxidation and Vitamin E

The importance of lipoproteins in the etiology of atherosclerosis is well established. Evidence is now accumulating to implicate thrombin in the pathogenesis of atherosclerosis. We have investigated whether atherogenic lipoproteins can support thrombin generation. In the absence of platelets or endothelial cells, both very low-density lipoprotein (VLDL) and oxidized low-density lipoprotein (LDL) support assembly of the prothrombinase complex and generation of thrombin. Thrombin generation (per μg of apolipoprotein) supported by VLDL was 19.4-fold greater than that supported by high-density lipoprotein (HDL), P < .00001, and 11.7-fold greater than that supported by LDL, P < .00001. Oxidation of LDL increased lipoprotein-supported thrombin generation 12-fold compared to unmodified LDL, P < .0001. We have shown that the phenomenon of lipoprotein-supported thrombin generation is mediated predominantly by specific phospholipids and is enhanced by oxidation of these phospholipids. The addition of vitamin E (α-tocopherol) markedly reduced the increase in thrombin generation observed after oxidation of LDL (822 ± 57 v 138 ± 47 nmol/L;P < .0001). These effects suggest that lipoproteins are important in the production of thrombin and that vitamin E may confer protection from the detrimental effects of lipoprotein oxidation by limiting thrombin formation. These results suggest that atherogenic lipoproteins are linked to the development of atherosclerosis in part by their capacity to support thrombin generation.

Atherogenic Lipoproteins Support Assembly of the Prothrombinase Complex and Thrombin Generation: Modulation by Oxidation and Vitamin E

The importance of lipoproteins in the etiology of atherosclerosis is well established. Evidence is now accumulating to implicate thrombin in the pathogenesis of atherosclerosis. We have investigated whether atherogenic lipoproteins can support thrombin generation. In the absence of platelets or endothelial cells, both very low-density lipoprotein (VLDL) and oxidized low-density lipoprotein (LDL) support assembly of the prothrombinase complex and generation of thrombin. Thrombin generation (per μg of apolipoprotein) supported by VLDL was 19.4-fold greater than that supported by high-density lipoprotein (HDL), P < .00001, and 11.7-fold greater than that supported by LDL, P < .00001. Oxidation of LDL increased lipoprotein-supported thrombin generation 12-fold compared to unmodified LDL, P < .0001. We have shown that the phenomenon of lipoprotein-supported thrombin generation is mediated predominantly by specific phospholipids and is enhanced by oxidation of these phospholipids. The addition of vitamin E (α-tocopherol) markedly reduced the increase in thrombin generation observed after oxidation of LDL (822 ± 57 v 138 ± 47 nmol/L;P < .0001). These effects suggest that lipoproteins are important in the production of thrombin and that vitamin E may confer protection from the detrimental effects of lipoprotein oxidation by limiting thrombin formation. These results suggest that atherogenic lipoproteins are linked to the development of atherosclerosis in part by their capacity to support thrombin generation.

Endothelial Cell-Blood Interface Actions and the Procoagulant Response

Anesthesiologists and surgeons have focused on the problems of hypocoagulability and resulting hemorrhage after cardiopulmonary bypass. Recent work in endothelial cell biology has demonstrated that the interaction of inflammatory processes and coagulation dysfunction with the endothelium may contribute to either hypocoagulability (bleeding) or hypercoagulability (thrombosis). New work with endothelial cell function and intracellular signaling of procoagulant responses may allow for unique therapeutic interventions in the future.

Insights into the complex association of bovine factor Va with acidic-lipid-containing synthetic membranes

The mechanism of binding of blood coagulation cofactor factor Va to acidic-lipid-containing membranes has been addressed. Binding isotherms were generated at room temperature using the change in fluorescence anisotropy of pyrene-labeled bovine factor Va to detect binding to sonicated membrane vesicles containing either bovine brain phosphatidylserine (PS) or 1,2-dioleoyl-3-sn-phosphatidylglycerol (DOPG) in combination with 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC). The composition of the membranes was varied from 0 to 40 mol% for PS/POPC and from 0 to 65 mol % for DOPG/POPC membranes. Fitting the data to a classical Langmuir adsorption model yielded estimates of the dissociation constant (Kd) and the stoichiometry of binding. The values of Kd defined in this way displayed a maximum at low acidic lipid content but were nearly constant at intermediate to high fractions of acidic lipid. Fitting the binding isotherms to a two-process binding model (nonspecific adsorption in addition to binding of acidic lipids to sites on the protein) suggested a significant acidic-lipid-independent binding affinity in addition to occupancy of three protein sites that bind PS in preference to DOPG. Both analyses indicated that interaction of factor Va with an acidic-lipid-containing membrane is much more complex than those of factor Xa or prothrombin. Furthermore, a change in the conformation of bound pyrene-labeled factor Va with surface concentration of acidic lipid was implied by variation of both the saturating fluorescence anisotropy and the binding parameters with the acidic lipid content of the membrane. Finally, the results cannot support the contention that binding occurs through nonspecific adsorption to a patch or domain of acidic lipids in the membrane. Factor Va is suggested to associate with membranes by a complex process that includes both acidic-lipid-specific and acidic-lipid-independent sites and a protein structure change induced by occupancy of acidic-lipid-specific sites on the factor Va molecule.

Prothrombin contributes to the assembly of the factor Va-factor Xa complex at phosphatidylserine-containing phospholipid membranes

The activation of prothrombin is catalyzed by prothrombinase, a complex of factor Xa and factor Va assembled on a negatively charged phospholipid membrane. We used a tubular flow reactor to identify the relative contributions of factor Va, prothrombin, and the negatively charged phosphatidylserine to the assembly of prothrombinase. Perfusion of phospholipid-coated capillaries with a mixture of factor Xa, factor Va, and prothrombin resulted in a steady-state rate of thrombin production that increased with (i) the phosphatidylserine content of the phospholipid bilayer, (ii) the factor Va concentration, and, most interestingly, (iii) the prothrombin concentration of the perfusion solution. Incorporation of 20 mol % phosphoatidylethanolamine, a phospholipid with poor ability to promote prothrombinase activity, into a 5 mol % phosphatidylserine membrane also increased the steady-state rate of thrombin production. Direct measurements of the amount of prothrombinase in the flow reactor demonstrated that increased catalytic activities were the result of an increased steady-state amount of membrane-associated prothrombinase. Thus, similar turnover numbers of prothrombin activation (3100 min-1) were calculated, irrespective of the phosphatidylserine content of the membrane. We established for membranes with low phosphatidylserine content (< 10 mol%) a linear relationship between the prothrombinase activity and the arithematical product of the factor Va concentration in the perfusion solution and the prothrombin concentration near the catalytic surface. Our results indicate that, in addition to factor Va, prothrombin also is essential to the assembly of prothrombinase at macroscopic surfaces with low phosphatidylserine content. The data further suggest that the prothrombin concentration near the surface, controlled by the prothrombinase activity and mass transfer, is an important regulator of the prothrombinase surface density.

Are acidic lipid domains induced by extrinsic protein binding to membranes?

A widely accepted model for the association of extrinsically bound proteins with acidic lipid-containing membranes has been that approach of the protein to the membrane induces a domain of acidic lipids that serves as the protein binding site. This model has been applied to a variety of membrane proteins including those that participate in the proteolytic complex that converts prothrombin to thrombin during the final stages of the blood coagulation cascade. The 'prothrombinase complex' consists of a serine protease (factor Xa), its protein co-factor (factor Va) and the substrate itself (prothrombin), all bound to phosphatidylserine (PS)-containing membranes derived from stimulated platelets. We have used three approaches to test the domain model as it applies to the proteins of this complex. First, phase diagrams describing the mixing of acidic and neutral lipids have failed to provide evidence for extensive acidic lipid domains (on the order of 50 or more lipid molecules) induced by protein biding. Second, pyrene-containing neutral and acidic phospholipids have been used to test for the occurrence of domains of as few as 20-30 lipids associated with binding of the membrane-binding fragment 1 region of prothrombin. Again, no evidence for domains was obtained. Finally, we have shown that binding of these proteins can be described in terms of a generalized model that presumes an acidic-lipid-independent surface adsorption combined with specific binding of acidic lipids to 'm' sites on a protein. Our results suggest that the concept of a protein-induced domain should not be applied indistriminately to explain binding of extrinsic membrane proteins such as the protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel one-step purification of human alpha-thrombin after direct activation of crude prothrombin enriched from plasma

Crude prothrombin enriched from human plasma was directly activated to generate alpha-thrombin without prior exhaustive purification of the proenzyme using a combination of several different types of chromatographic techniques, as in all previously described methodologies. Activated thrombin was separated from other components in a single step by taking advantage of its highly specific affinity to heparin immobilized on a matrix support of Sepharose CL-6B. On the basis of the data presented herein, we have demonstrated the ease with which at least 25 mg of a highly purified enzyme (greater than 97% homogeneous by laser densitometry) can be obtained per litre of plasma. Our product exhibits a specific activity of at least 4000 National Institutes of Health units/mg and is stable after being freeze-dried for the purpose of long-term storage.

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.

The inhibition of clotting complexes of the extrinsic coagulation cascade by the phospholipase A2 isoenzymes from Naja nigricollis venom

Phospholipase A2 (PLA2) isoenzymes from Naja nigricollis venom exhibit anticoagulant activity with varying potencies. To determine which complexes in the extrinsic coagulation cascade are inhibited by these PLA2 enzymes, we examined their effects on the coagulation of bovine plasma initiated by the addition of thromboplastin, Russell's viper venom (RVV) or thrombin. The weakly anticoagulant PLA2 enzymes, CM-I and CM-II, prolonged clotting initiated by thromboplastin, but not that initiated by RVV or thrombin. The strongly anticoagulant enzyme, CM-IV, prolonged clotting initiated by both thromboplastin and RVV, but not clotting initiated by thrombin. To confirm the differences in their inhibitory properties, we examined the effect of these PLA2 enzymes on reconstituted extrinsic tenase and prothrombinase complexes. The weakly anticoagulant enzymes inhibited the tenase complex, but did not inhibit the prothrombinase complex, whereas the strongly anticoagulant enzyme inhibited both complexes. Thus the enzymes showed distinct differences in their inhibition patterns in the extrinsic coagulation cascade. Their dissimilarity in inhibition of the two phospholipid dependent activation steps probably reflects the difference in phospholipid requirements and/or mechanism of inhibition between the two complexes. Inhibition of successive amplification steps in the extrinsic coagulation cascade by CM-IV is consistent with its potency as a strongly anticoagulant PLA2.

The determination of a calcium-dependent binding constant of the bovine prothrombin Gla domain (residues 1-45) to phospholipid vesicles

Calcium-mediated binding of the radioiodinated peptide representing residues 1-45 of bovine prothrombin to single bilayer phospholipid vesicles composed of phosphatidylserine from bovine brain and synthetic 1-palmitoyl-2-oleoyl-phosphatidylcholine (25:75 PS/PC) has been studied over peptide concentrations from 0.33 microM to 3.75 microM and at a calcium concentration of 1.0 mM. The binding isotherm for the interaction between the radioiodinated peptide and PS/PC vesicles fits a model in which there is noncooperative binding of the peptide to non-interacting sites on the phospholipid bilayer. A dissociation constant determined at these conditions is 11.8 microM compared to 1.0 microM for prothrombin fragment 1.

Chemical modification of prothrombin fragment 1: documentation of sequential, two-stage loss of protein function

The amino groups of prothrombin fragment 1 (amino acids 1-156 of prothrombin) were derivatized by acetylation, amidination, and reductive methylation. Conditions that caused complete acetylation of protein amino groups produced a fragment 1 derivative which no longer displayed a metal ion dependent intrinsic fluorescence change and had lost its membrane binding capability as well. However, when derivatized in the presence of calcium ions, extensive acetylation yielded a product that underwent protein fluorescence quenching at metal ion concentrations similar to those observed for the native protein. This derivative bound to membranes in a calcium-dependent manner with only a small reduction in affinity. Several results showed the existence of a partially functional protein that was characterized by a high degree of calcium-dependent protein fluorescence quenching but which had a requirement for 10-fold higher calcium concentration. This derivative was produced by partial acetylation (greater than 3 equiv) of metal-free protein. This partially acetylated protein had greatly diminished membrane binding. The calcium-protected amino group, therefore, was among the most reactive acetylation sites in the metal-free protein. The second site, responsible for abolishing all metal ion induced fluorescence change, was resistant to acetylation and became derivatized at the last stages of amino group acetylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane-dependent coagulation reaction is independent of the concentration of phospholipid-bound substrate: fluid phase factor X regulates the extrinsic system

Negatively charged phospholipids accelerate blood coagulation; one suggestion is that the binding of the gamma-carboxyglutamic acid-containing zymogens to these lipid surfaces increases their effective concentration as substrates. Alternatively, the charged phospholipids could enhance the direct interaction of substrate with the catalytic complex, which is localized at the membrane surface. We distinguished the alternatives by using prothrombin fragment 1 to compete with the substrate for membrane binding sites without interfering with the direct enzyme-substrate interaction. In a tissue factor-factor VIIa system containing neutral phospholipids (to which the substrate does not bind), prothrombin fragment 1 had no significant effect on factor X activation (Km, 877 +/- 111 nM and 791 +/- 103 nM, with and without prothrombin fragment 1, respectively). In contrast, in a system containing 30% phosphatidylserine, prothrombin fragment 1 displaced phospholipid-bound factor X, increasing the free factor X concentration and the reaction velocity in all 22 determinations. As the velocity increases correlated only with the free factor X concentration, we conclude that the free factor X concentration controls this reaction velocity. The Km in the 30% phosphatidylserine system, calculated using free factor X concentration, was 41 +/- 5 nM and 63 +/- 9 nM, with and without prothrombin fragment 1, respectively. Thus, the negatively charged lipids decreased the intrinsic Km by over 90%. The methodology employed should be applicable to ligand-receptor systems in which ligand binds nonspecifically to the membrane surface.

Topology of the binding site of blood-clotting factors in model membranes. A fluorescence study

Factors II, X and IX are blood-clotting proteins which bind to phospholipid interfaces in the presence of Ca2+ to activate coagulation. The topology of their binding site on the membrane was investigated in two ways. First, the transition temperature changes of equimolar mixtures of dipalmitoylglycerophosphocholine/phosphatidylserine and dimyristoylglycerophosphocholine/dipalmitoylglycerophosphoserin e were examined by the fluorescence polarization of 1,6-diphenylhexatriene. Results show that Ca2+ triggers a shift of about 3-4 degrees C and that blood-clotting factors further increase this shift by about 1.5 degree C. This suggests that in the gel phase, Ca2+ induces some aggregation of the phosphatidylserine molecules which is reinforced by blood proteins. Second, isothermal energy transfer experiments were performed with natural lipids in their fluid phase. The tryptophan residues of the factors were the energy donors, and pyrene covalently bound to a fatty acid chain of either phosphatidylcholine or phosphatidic acid was the energy acceptor. These pyrene-phospholipids probe either the neutral or the acidic component of phospholipid mixtures. It is concluded that the binding sites of the factors are constituted by both types of lipids and that their composition depends on the membrane. Factor II exhibits some specificity for acidic phospholipids and seems to be surrounded by non-interacting zwitterionic lipids. Factor IX appears to be surrounded by statistically the same amount of charged and zwitterionic lipids. We also demonstrate that binding can also occur without Ca2+. This Ca2+-independent binding probably involves electrostatic and hydrophobic forces but its physiological significance remains to be elucidated.

Interactions of divalent cations with phosphatidylserine bilayer membranes

The interaction of divalent cations with a homologous series of diacylphosphatidylserines (diacyl-PS) has been studied by differential scanning calorimetry and X-ray diffraction. Hydrated di-C14-PS (DMPS) exhibits a gel leads to liquid-crystal bilayer transition at 39 degrees C (delta H = 7.2 kcal/mol of DMPS). With increasing MgCl2 concentration, progressive conversion to a phase exhibiting a high melting (98 degrees C), high enthalpy (delta H congruent to 11.0 kcal/mol of DMPS) transition is observed. Similar behavior is observed for DMPS with increasing CaCl2 concentration. In this case, the high-temperature transition of the Ca2+-DMPS complex occurs at approximately 155 degrees C and is immediately followed by an exothermic transition probably associated with PS decomposition. For di-C12-, di-C14-, di-C16- (DPPS), and di-C18-PS, the transition temperatures of the Ca2+-PS complexes are in the range 151-155 degrees C; only di-C10-PS exhibits a significantly lower value, 142 degrees C. A different pattern of behavior is exhibited by DPPS in the presence of Sr2+ or Ba2+, with transitions in the range 70-80 degrees C being observed. X-ray diffraction of the Ca2+-PS complexes at 20 degrees C provides evidence of structural homology. All Ca2+-PS complexes exhibit bilayer structures, the bilayer periodicity increasing linearly from 35.0 A for di-C10-PS to 52.5 A for di-C18-PS. Wide-angle X-ray diffraction data indicate that hydrocarbon chain "crystallization" occurs on Ca2+-PS complex formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in membrane phospholipid distribution during platelet activation

Exposure of phospholipids at the outer surface of activated and control platelets was studied by incubation with a mixture of phospholipase A2 from Naja naja and bee venom, solely or in combination with sphingomyelinase from Staphylococcus aureus, using conditions under which cell lysis remained below 10%. Incubation with phospholipase A2 alone revealed a markedly increased susceptibility of the phospholipids in platelets activated by a mixture of collagen plus thrombin, by the SH-oxidizing compound diamide, or by calcium ionophore A23187, as compared to control platelets or platelets activated separately by collagen or thrombin. Collagen plus thrombin, diamide, and ionophore treated platelets revealed an increased exposure of phosphatidylserine at the outer surface accompanied by a decreased exposure of sphingomyelin, as could be concluded from incubations with a combination of phospholipase A2 and sphingomyelinase. These alterations were much less apparent in platelets activated either by thrombin or by collagen alone. The increased exposure of phosphatidylserine in activated platelets is accompanied by an increased ability of the platelets to enhance the conversion of prothrombin to thrombin by coagulation factor Xa, in the presence of factor Va and calcium. It is concluded that the altered orientation of the phospholipids in the plasma membrane of platelets activated by collagen plus thrombin, by diamide, or by calcium ionophore, is the result of a transbilayer movement. Moreover, the increased exposure of phosphatidylserine in platelets stimulated by the combined action of collagen and thrombin might be of considerable importance for the hemostatic process.

Isolation and characterization of lipid-protein particles containing platelet factor 3 released from human platelets

Lipid-protein particles with platelet factor 3 measured by the Stypven clotting-time test [Hardisty & Hutton (1966) Br. J. Haematol. 12, 764-776] have been isolated from platelet-release supernatant. Starting material was washed platelets, which were released by treatment with collagen. Purification of the particles from other components in the release material was accomplished by gel filtration on Sepharose CL-4B followed by affinity chromatography on poly-L-lysine-Sepharose CL-4B gel. Chemical characterization showed that the particles were composed of 40% protein, 42% phospholipids, 13% cholesterol and 5% triacylglycerols. The phospholipid composition was 38% phosphatidylcholine, 25% phosphatidylethanolamine, 9% phosphatidylserine, 2% phosphatidic acid and 26% sphingomyelin. No carbohydrate was detected. Electron-microscopic studies revealed the presence of membranous particles with diameters between 70 and 170 nm.

Contribution of the platelet factor V content to platelet factor 3 activity

The procoagulant activity obtained from bovine thrombocytes has been compared to that of lipids isolated from platelets, with and without the addition of purified bovine factor V. A one-stage assay, which consisted of delipidated bovine plasma containing RVV-activated factor X, was used to assess the activity. At low lipid concentrations no difference in coagulant activity was found between sonicated vesicles of extracted platelet lipid and lysed platelets. At higher lipid concentrations, however, the extracted lipids were found to be less active than lysed platelets. Determination of factor V in suspensions of gel-filtered platelets demonstrated that suspensions containing 2 X 10(9) platelets per ml possessed about 1% of the factor V activity present in a normal bovine plasma pool. Platelet lysis by sonication produced a five-fold increase in factor V activity. Addition of factor V to sonicated vesicles of extracted platelet lipid, so as to produce an identical factor V activity per amount of lipid as found in lysed platelets, decreased the clotting time only in the higher lipid concentration range. A further three-fold increase in the amount of factor V added to the lipid vesicles made the coagulant properties of the lipid vesicles indistinguishable from those of lysed platelets over the whole range of phospholipid concentrations tested. When the conditions of the test were changed by diminishing the concentration of factor Xa in the substrate plasma, the difference between lysed platelets and extracted platelet lipid disappeared completely. It is concluded that the higher coagulant activity of lysed platelets, as compared to that of extracted platelet lipid, can be ascribed to platelet factor V activity. Therefore there is no compelling necessity to postulate the existence of a specific procoagulant factor in the platelet other than factor V or phospholipids.

Zymogens and cofactors of blood coagulation

Blood coagulation is a system in which a series of zymogens of serine proteases are sequentially activated. In this regard, there is little fundamental difference between coagulation and the activation of the homologous pancreatic zymogens. There are, however, several aspects unique to coagulation which are discussed in detail. These are (1) the requirement for a high-molecular-weight protein or lipoprotein cofactor for optimal reaction rates, (2) the requirement for membranes or a membrane-like surface which further distinguishes this system; (3) a metal ion requirement for most reactions (in contrast to the pancreatic serine proteases) relating to the content of the newly described amino acid gamma-carboxyglutamic acid in the four vitamin K-dependent proteins, regarding which recent data relating to the metal binding sites on prothrombin are discussed in detail; and (4) the uniqueness of the initiating reactions in comparison to those which activate the pancreatic zymogens, insofar as no enzyme corresponding to enterokinase has been identified. The implications of this phenomenon are analyzed with particular attention to the potential role of the endogenous activity of certain zymogens in initiating coagulation. The article deals finally with the specific problems attendant on analyzing a system in which many serine proteases lacking absolute specificity are generated and regulated.