Some lipid-protein interactions involved in prothrombin activation

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.

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.

A catalytic role for heparin. Evidence for a ternary complex of heparin cofactor thrombin and heparin

The interaction of heparin with chemically modified thrombin and heparin cofactor is studied. Amidinated heparin cofactor does not bind to heparin-agarose and the reaction rate of the amidinated inhibitor with unmodified thrombin is not affected by heparin. Likewise, thrombin modified with 1,2--cyclohexanedione does not bind to heparin agarose and the reaction rate of the modified enzyme with unmodified inhibitor is not affected by heparin. In the absence of heparin, the modified and unmodified proteins react at the same rate in all possible combinations. Affinity chromatography of diisopropylphosphoryl thrombin on heparin cofactor coupled to Sephadex G--50 is used to study the binding of heparin cofactor and thrombin to heparin. The thrombin for all experiments is tritium-labeled and then inactivated with diispropylfluorophosphate. Thrombin is not bound to heparin cofactor-Sephadex columns. However, after treatment of the columns with a heparin solution, thrombin binds tightly, and is eluted at high ionic strength. Bound thrombin can also be eluted with either excess non-radioactive thrombin or excess free heparin. Heparin-dependent binding of thrombin does not occur if the heparin cofactor-Sephadex is heat-denatured. The ability of heparin to couple solution-phase thrombin to solid-phase heparin cofactor indicates that a ternary complex is formed. Analysis of the binding of the proteins to heparin by a dye displacement method suggests that at least one site on heparin binds to thrombin but not to heparin cofactor. Further support for a catalytic role for heparin derives from the ability of catalytic concentrations of heparin to enhance the rate of hydrolysis of prothrombin by thrombin, another protein pair which bind mutually to heparin.

Interactions of the components of the prothrombinase complex

The interaction of components of the prothrombinase complex, i.e. bovine Factor X or Factor Xa, bovine Factor V or Factor Va, phospholipid, and Ca2+, in various combinations was studied primarily by a gel filtration technique. In experiments, in which phospholipids ranging from those isolated from naturally occurring sources to those long chain (18 : 1) as well as short chain 6 : 0 and 7 : 0 fatty acids prepared by chemical and enzymatic synthesis were used, it was evident that a net negative surface charge on the lipid dispersions was one of the important requirements for interaction. Though the short chain fatty acid phospholipids interacted with the proteins of the prothrombinase complex, there was invariably a diminution in the activity of the enzyme complex. It was established that Factor V or Va did not bind Ca2+ and that the binding of either of these factors with phospholipids (with a net negative charge) was not dependent on Ca2+. However, the interaction of Factor X or Factor Xa with phospholipids with a negative charge required Ca2+. It was shown that Factor X could bind to the same type of lipid of lipid surface as that noted for Factor Xa. Of interest was the apparent difference in the phospholipid binding characteristics of the two variant forms of bovine plasma Factor X, i.e. X1 and X2, which might in part explain the differences in their specific activities. Of importance was the lack of demonstrable complex formation between Factors II, X and V in the absence of phospholipids and/or in the presence or absence of Ca2+. The significance of these results as they might apply to the configuration of the prothrombinase complex and its interaction with prothrombin plus the usefulness of the short chain fatty acid phospholipid in exploring these lipid-protein interactions are discussed.

Interaction of short chain and long chain fatty acid phosphoglycerides and bile salts with prothrombin

An equimolar mixture of phosphatidylserine and (dioleoyl) phosphatidyl-ethanolamine could substitute for brain cephalin preparations in the single stage prothrombin assay. However, no clot promoting activity was observed on the addition of any of the individual long chain fatty acid-containing phospholipids. Short chain fatty acid-containing phospholipids, such as diheptanoylphosphatidylcholine, diheptanoylphosphatidylethanolamine, diheptanoylphosphatidic acid, and dihexanoylphosphatidylcholine, or dihexanoylphosphatidylethanolamine were inhibitory under all conditions studied. Similar effects of these two general classes of phospholipids were observed in a two-stage thrombin generation system, in which a mixture of bovine Factor Xa, Factor Va, and Ca2+ were interacted with prothrombin. In the presence of 25 mM Ca2+, dioleoylphosphatidic acid or brain phosphatidylserine alone, and with other long chain phospholipids, formed complexes with bovine plasma prothrombin. On the other hand, dioleoyl-, diheptanoyl- or dihexanoylphosphatidylcholine under comparable conditions showed no binding to prothrombin. There appeared to be a small degree of binding of diheptanoylphosphatidic acid to prothrombin, but it was insufficient to cause any significant change in apparent molecular weight of prothrombin. A mixture of prothrombin, Factor V, diheptanoylphosphatidic acid/diheptanoylphosphatidylcholine and Ca2+ eluted in the void volume of Sephadex G-200, but showed a much reduced coagulant activity. Though a net negative charge on the phospholipid surface is required for phospholipid-protein interactions, this does not necessarily promote coagulant activity. Bile acids and bile salts, such as cholic acid, deoxycholic acid, taurocholic acid, glycocholic acid, lithocholic acid and dehydrocholic acid, exerted varying levels of stimulation on the prothrombin assay and thrombin generation system, but were not as effective as the phospholipids. Interestingly, no interaction of these bile acids or salts with prothrombin was noted in the presence of Ca2+. The results of these experiments suggest that negatively charged micelles per se are not sufficient for binding alone and that other chemical and physical characteristics of phospholipids are of prime importance.

A phospholipase A2 with anticoagulant activity. II. Inhibition of the phospholiped activity in coagulation

An anticoagulant factor with phospholipase A2 activity has been isolated from Vipera berus venom. Phospholipase activity was studied on platelet phospholipid and on brain cephalin. The venom factor showed a potent anticoagulant activity: 1 mug impaired the clotting of 1 ml of citrated recalcified platelet-poor plasma. The anticoagulant inhibited clotting by antagonism to phospholipid. The antagonism constant (Kan = 6.8-10(-9) M) demonstrated the high affinity of the inhibitor for phospholipid. As with other phospholipases A2, the venom factor was thermoresistant but very sensitive to photo-oxidation. Both activities (anticoagulant activity and phospholipase activity) were not markedly dissociated by either denaturation or neutralization processes. Slightly different curves of photo-oxidative inactivation of both activities suggested the presence, on the molecule, of two very close sites responsible for phospholipase and anticoagulant activities. The inhibitor effect on coagulation was independent of the hydrolysis process. In fact, lysoderivatives and fatty acids, resulting from complete hydrolysis with the venom factor, were as active as the native phospholipids. Moreover phospholipase A2 from other viperidae venom, which did not have anticoagulant activity, produced similarly active lysoderivatives. This showed that the cleavage of the beta-acyl bond does not interfere with the activity of phospholipid. A possible mechanism of clotting inhibition by the venom factor was proposed. Owing to its high affinity for phospholipid, the inhibitor would complex phospholipid at its protein binding site impairing the normal arrangement of coagulation protein factors and, consequently, their activation. The positive charges of the inhibitor (pI = 9.2) could bind with phosphoryl or carboxyl groups of phospholipid, making them unavailable for protein binding. The complex formation involves a loss of dissociating capacity of the enzyme towards its substrate. This required an additional interaction of the inhibitor with a coagulation protein factor. The inhibitor could be removed from the complex by specific antibodies, permitting recovery of normal phospholipid-protein interaction. The role of calcium in the complex has not yet been elucidated. This venom factor affords a useful tool for investigating the phospholipid-clotting protein interaction.

Ca++ binding properties of human prothrombin

The binding of Ca++ to human prothrombin has been investigated by equilibrium dialysis. The protein exhibited a positive cooperativity phenomenon for the first three Ca++ bound. Eleven to twelve Ca++ binding sites have been found. They could be differentiated in terms of two classes of sites with respect to their Ca++ affinity: 5 strong binding sites (log Kassoc = 3.9) and 7 weak binding sites (log Kassoc = 2.9). We attempted to determine the Hill coefficient of the strong binding sites responsible for cooperativity. Results have been compared to data previously reported for bovine prothrombin.

Mass-spectrometric identification and sequence location of the ten residues of the new amino acid (gamma-Carboxyglutamic acid) in the N-terminal region of prothrombin

The detailed mass-spectrometric evidence for our original findings [Magnusson et al. (1974) FEBS Lett. 44, 189-193] of ten gamma-carboxyglutamic acid residues in the N-terminal calcium-binding polypeptide of prothrombin is presented. The identification and sequence location of gamma-carboxyglutamic acid was made by electron-impact and field-desorption studies on acetyl permethyl peptide derivatives, and on the free amino acid. Details of the derivatives formed, and how this new amino acid may be easily recognized and sequenced from the mass spectrum, are given as a basis for future work.

Procoagulant activity of platelets in recalcified plasma

High-spun platelet-poor plasma (HSPPP) and platelet-rich plasma (PRP), prepared with minimal contact activation and pH shifts, were recalcified with little dilution in the presence of phospholipid prepared from chloroform-extracted, acetone-dried brain tissue. The clotting times of HSPPP and PRP were the same whether or not the platelets in PRP were first incubated or aggregated with ADP or with collagen. This suggests that platelet procoagulant activity in normal recalcified plasma derives only from provision of platelet phospholipid.

Structural studies on bovine prothrombin. Isolation and partial characterization of the Ca2+ binding and carbohydrate containing peptides of the N-terminus region

Three peptides, one of which binds Ca2-+ (calcium binding fragment, CBF) but contains no carbohydrates and two of which bind no Ca2-+ but contain carbohydrates, have been isolated from the N-terminus region of bovine prothrombin. The preparation of these peptides involved (a) thrombin cleavage of prothrombin to intermediate 1 (thrombinogenic) and fragment 1 (nonthrombinogenic), (b) tryptic attack on fragment 1, and (c) separation of the CBF from the latter reaction by addition of a phosphatidylcholine-phosphatidylserine dispersion in the presence of Ca-2+. Further study on the non-calcium-binding peptides from the tryptic digest of fragment 1 revealed the presence of two low molecular weight glycopeptides, GP-1 and GP-2. A detailed examination of the chemical characteristics of CBF provided some insight into this unusual peptide. Whereas fragment 1, as well as prothrombin, exhibited two classes of Ca-2+ binding sites (one of high affinity, 3-4 mol/mol of peptide and the other of low affinity, 10-12 mol/mol of peptide), CBF bound only 3-4 mol of Ca-2+/mol of peptide. This indicated the presence of only the high affinity sites of the parent molecule. CBF contained an unusually high level of glutamic acid (approximately 30% of the total amino acids as determined in an acid hydrolysate) and had an N-terminal glycine. Most likely these glutamyl residues were present originally as the gamma-carboxyglutamyl residue as proposed by Stenflo et al. (Stenflo, J., Ferlung, P., Egan, W., and Roepstorff, P. (1974), Proc. Natl. Acad. Sci. U.S-A 71, 2730). The CBF contained no detectable carbohydrate. Its molecular weight varied inexplicably according to the procedure used and gave the following values; 8500, by gel filtration; 5200, by 6 M guanidine-HCl gel chromatography; 3490, by analytical ultracentrifugation. The glycopeptides, GP-1 and GP-2, were distinguished from each other by differences in their behavior on ion exchange chromatography and in their amino acid composition, and from CBF by their inability to bind calcium under any conditions. On the other hand, GP-1 and GP-2 had nearly identical levels of carbohydrate, 45.1 and 48.0 wt %, and possessed essentially the same percent distribution of carbohydrates: sialic acid, 16.5 plus or minus 0.5; mannose, 10.3 plus or minus 0.4; glucosamine, 11.2 plus or minus 0.1; galactose, 7.9 plus or minus 0.3. Their molecular weights were as follows: GP-1, 70000, by gel filtration; 6500, by 6 M guanidine-HCl gel chromatography; 4600, by ultracentrifugation; GP-2, 6500 by gel filtration; 6900, by 6 M guanidine-HCl gel chromatography; 1960, by analytical ultracentifugation. Though there are some obvious variations depending on method, this could be attributable to a probable error in v measurement on these carbohydrate containing peptides. The significance of these findings as they relate to prothrombin to thrombin conversion is discussed.

Factor-V activation by thrombin and its role in prothrombin conversion

Purified coagulation factors and specific antibodies to factor V and factor X were used to investigate the action of thrombin on factor V and the mechanism by which thrombin-treated factor V influences prothrombin activation. The formation of a complex or complexes between phospholipid, factor V, factor Xa and calcium was demonstrated by column chromatography on Sephadex gel, and by immunological analysis of the column fractions including the use of solid-phase antibodies. Kinetic experiments demonstrated that generation of thrombin from purified prothrombin was accomplished by this complex. Pre-treatment of factor V with trace quantities of thrombin resulted in increased yield and rate of thrombin generation. It was shown that phospholipid became saturated when incubated with increasing concentrations of factor V and that the initial saturating concentration of the latter was reduced by pre-treatment with thrombin. The findings confirm that optimum conversion of prothrombin to thrombin is accomplished by a complex or complexes of phospholipid, factor V, factor Xa and calcium and it is suggested that thrombin plays an autocatalytic role in these reactions.

The localization of a vitamin K-induced modification in an N-terminal fragment of human prothrombin

1. The N-terminal fragment (PF-I) split off from prothrombin during coagulation was purified to homogeneity from human serum. 2. The apparent molecular weight is 27000+/-2000 in sodium dodecyl sulphate-polyacrylamide-gel electrophoresis, whereas a value of about 19600 is obtained by calculation based on amino acid and carbohydrate analyses. The N-terminal sequence is an Ala-Asx bond. The fragment contains about 16% carbohydrate, binds phospholipids in the presence of Ca(2+) and is adsorbed to BaSO(4). The pK(a) of its BaSO(4)-binding group(s) is 3.1-3.5. 3. By CNBr cleavage of fragment PF-I two peptides (C-1 and C-2) were obtained with molecular weights of about 5900 (C-2) and 12400 (C-1) on the basis of amino acid and carbohydrate analyses. Only the smaller (N-terminal) peptide is adsorbed to BaSO(4) and, since the ability of the whole protein to bind to BaSO(4) is known to be absent in samples obtained from patients treated with vitamin K antagonists, this peptide probably contains the site of a modification to the structure of the protein which occurs during biosynthesis and depends on vitamin K. This peptide does not contain hexosamine or sialic acid.

A polypeptide region of bovine prothrombin specific for binding to phospholipids

The blood-clotting protein prothrombin can be converted to thrombin in free solution by the proteolytic enzyme, activated factor X. When prothrombin is bound to the surface of phospholipid vesicles, the rate of thrombin generation is increased more than 30-fold over the rate of unbound prothrombin. If the prothrombin activation process is terminated after a time interval in which less than 10% of the expected thrombin has been produced, two major products are found in the activation mixture. These products have been termed intermediate 1, a precursor of thrombin, and fragment 1. The conversion of intermediate 1 to thrombin is not accelerated by phospholipid nor can binding of this intermediate to phospholipid particles be demonstrated. In contrast, fragment 1, the other activation product derived from prothrombin, binds to phospholipid particles under the same conditions as prothrombin. On the basis of these observations, we propose that the prothrombin molecule contains a specific region of polypeptide chain for binding to phospholipid particles. This specific polypeptide region or lipid interaction site is part of the nonthrombin-forming activation fragment.