The synthesis of sulfated dextran beads for isolation of human plasma coagulation factors II, IX, and X

Determination of total protein in highly purified factor IX concentrates

Protein determination by the methods of Kjeldahl, Biuret, Bradford and UV absorbance at 280 nm have been studied in regard to accuracy, precision and simplicity. A reference preparation of a highly purified factor IX concentrate, Nanotiv, reconstituted to 1/5 of ordinary volume was used in the study in order to make a comparison between the different procedures. The Kjeldahl method resulted in a protein concentration of 3.7 mg/ml, whereas the Biuret, Bradford (BSA) and UV absorbance at 280 nm resulted in protein concentrations of 3.6, 2.5 and 2.8 mg/ml, respectively. The corresponding values for specific activity were 136, 140, 200 and 179 IU/mg, respectively. These results demonstrate a great variation in the response obtained by different methods for determination of total protein.

A rapid separation of alpha- and beta-lipoproteins by affinity chromatography

A rapid and inexpensive separation of alpha- and beta-lipoproteins has been achieved using sulfated dextran beads as an affinity chromatography medium. The separation is completed in less than 15 min and the cholesterol content of the lipoprotein fractions can then be determined in a separate procedure.

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.

Proteolysis of factor VIII heavy chain polypeptides in plasma and concentrates

Factor VIII heavy chain (FVIII HC) polypeptides have been studied in both normal plasma and FVIII concentrates on exposure to three coagulation proteases. FVIII samples were incubated with labelled affinity-purified anti-FVIII Fab' fragments, immunocomplexes formed were visualized by autoradiography after sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), and apparent relative molecular masses (Mr) of each band assigned. FVIII HC polypeptides were detected in all types of samples, including plasma, without further purification. Normal plasma contained a range of polypeptides with the largest dominant band at a net apparent Mr of 250-300 kD, and the smallest at 80-90 kD: the bands visualized correspond to the 90-210 kD HC species seen on conventional analysis of purified FVIII. No bands were produced from samples of haemophilic plasma. Treatment of plasma or FVIII concentrate with low concentrations (1 IU/ml) of thrombin removed the 250-300 kD and other intermediate bands, intensified then removed the 80-90 kD polypeptide and produced a band at 40-50 kD. Thrombin-associated rise and fall in FVIII clotting activity by one-stage assay correlated with intensity of the 80-90 kD polypeptide. A polypeptide of Mr 40-50 kD was also produced after incubation with activated factor X: activated factor VII plus thromboplastin had no effect on HC structure. FVIII polypeptides were visualized in prothrombin complex concentrates, with a more degraded profile seen in a deliberately 'activated' product.

Studies of natural anticoagulant proteins and anticardiolipin antibodies in patients with the lupus anticoagulant

Components of the natural anticoagulant system (NAS) and anticardiolipin antibodies were examined in 21 patients with lupus anticoagulant (LA), 13 of whom had past histories of thrombotic episodes. No relationship could be shown between the antigenic levels of protein C and S (PC, PS) and a history of thrombosis. Inhibition of the anticoagulant activity of activated protein C (APC) was observed using plasma from 20/21 patients when phospholipid vesicles were used as the surface for the coagulation reaction. This effect was not affected by the addition of PS. When platelet membranes were employed only 2/21 patients demonstrated inhibition of APC. Under the latter condition, PS functional activity was inhibited in 7/21 patients, six of whom had a past history of thrombosis. Reduced antithrombin III or heparin cofactor II levels were observed in a total of 4/21 patients and may have contributed to the development of thrombosis in three of these patients. Antibodies specifically directed against these proteins were not detected suggesting the possibility of an associated constitutional deficiency. Anticardiolipin antibodies, though elevated in 17/21 patients, did not serve as a useful marker for an increased risk of thrombosis, and the level did not correlate with inhibition of the activity of APC or PS. We conclude that the mechanism of thrombosis in patients with LA is multi-factorial. A subset of patients in whom LA specifically inhibits PS function may represent patients who are at significant risk from thrombosis.

Purification and preliminary characterization of rabbit vitamin K-dependent coagulation proteins

We describe herein a procedure for the purification of protein C, protein S, prothrombin, factor VII, and factor X to apparent homogeneity from rabbit plasma. The initial steps, which are common to the purification of vitamin K-dependent proteins from other mammalian species, include adsorption onto and elution from barium followed by anion exchange chromatography. Proteins were further purified using a variety of techniques, including affinity chromatography, gel filtration, and anion exchange chromatography in a Fast Protein Liquid Chromatography system. Significant structural homologies exist between rabbit, human, and bovine vitamin K-dependent proteins. As is true for protein C and factor X in human and bovine plasma, rabbit protein C and factor X are two-chain proteins which can be converted to active proteases by specific venom activators. Rabbit factor VII is also a two-chain protein and can restore coagulant activity to human or bovine plasma deficient in factor VII. In contrast, rabbit protein S and prothrombin are single chain proteins. In view of the well-described species specificity of many of the vitamin K-dependent proteins, purified rabbit coagulant and anticoagulant proteins should be useful in the development of animal models of coagulation and/or thrombosis.

Dissection of laminin by cathepsin G into its long-arm and short-arm structures and localization of regions involved in calcium dependent stabilization and self-association

Native laminin-nidogen complex isolated from mouse Engelbreth-Holm-Swarm tumor was treated with purified cathepsin G or leucocyte elastase, two neutral serine proteases which play a role in inflammatory processes accompanied by degradation of basement membranes. Both enzymes were found to be more active than porcine pancreatic elastase. In the absence of Ca2+, laminin fragments produced by leucocyte elastase resembled those formed by the pancreatic enzyme but at physiological concentrations of Ca2+ cleavage by cathepsin G was much more selective. Initially laminin (900 kDa) was cleaved at two major sites only with similar rates leading to three fragments. Fragment C1-4 (about 550 kDa) comprises the intact three short arms of the molecule and fragment C8-9 (about 350 kDa) contains the entire triple-coiled region by which its three chains are assembled and the major part of the terminal globular domain of the long arm. The remaining C-terminal region of this domain was recovered as fragment C3 of about 50 kDa. Stabilization against proteolytic attack was restricted to the region of fragment C1-4 and only this fragment exhibited strong Ca2+ dependent self-association similar to that of intact laminin or of its complex with nidogen. The associative properties of fragment C1-4 were dramatically diminished upon removal of the tip of one of the short arms comprising fragment 4. In addition, this provides a clear assignment of the important laminin function to a distinct domain in one of its short arms. The new fragment C8-9 may be employed for exploring the properties and possible functions of the upper long-arm region which so far has not been available as a fragment.

Transcription of thrombomodulin mRNA in mouse hemangioma cells is increased by cycloheximide and thrombin

We have measured mRNA levels for thrombomodulin, an endothelial membrane cofactor for the activation of protein C by thrombin, in a mouse hemangioma cell line. Cycloheximide, an inhibitor of protein synthesis, increased levels of thrombomodulin mRNA, as measured in an S1 nuclease protection assay, to 2.5-4.0 times control levels. Thrombomodulin transcription in response to cycloheximide treatment, as determined by nuclear run-on analysis, was 3.9 +/- 1.3 (mean +/- SD) times that found in untreated cells. Thrombin also increased thrombomodulin mRNA levels to 151 +/- 21% (mean +/- SD) of control levels after 2 hr. Transcription increased in response to thrombin by 2.1- to 7.3-fold. The combination of thrombin and cycloheximide had no additive effect on thrombomodulin mRNA levels. Thrombin treatment of hemangioma cells also caused an increase in thrombomodulin protein synthesis to 142 +/- 17% (mean +/- SD) of control levels as determined by immunoprecipitation of [32S]methionine-labeled thrombomodulin. We conclude that thrombomodulin expression is determined in part by the rate of transcription and that thrombomodulin mRNA levels in hemangioma cells are increased by treatment with cycloheximide or thrombin. The increased transcription in response to cycloheximide suggests the existence of a labile protein repressor of thrombomodulin transcription.

Isolation of human blood coagulation alpha-factor Xa by soybean trypsin inhibitor-sepharose chromatography and its active-site titration with fluorescein mono-p-guanidinobenzoate

A method based on active-site affinity chromatography on soybean trypsin inhibitor (SBTI)-Sepharose was developed for isolation of human factor Xa in primarily the undergraded alpha-form. The chromatography procedure separated factor Xa from factor X, the Russel's viper venom proteinase used to activate factor X, and traces of contaminating thrombin. alpha-Factor Xa was unstable at pH 7.6 and 25 degrees C, undergoing slow proteolytic degradation to functionally heterogeneous products as evidenced by the greater loss of coagulation assay activity compared to activity measured with a chromogenic substrate. The results of monitoring factor Xa degradation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were consistent with proteolysis of the light chain as a major component reaction occurring in parallel with slower proteolysis of the heavy chain. The decreased rates of these reactions at pH 6.0 enabled isolation and storage of factor Xa in greater than or equal to 88% alpha-form and minimized the heterogeneity due to proteolytic degradation. Characterization of the reaction of fluorescein mono-p-guanidinobenzoate (FMGB) with human and bovine factor Xa isolated by SBTI-Sepharose chromatography demonstrated its utility as a sensitive reagent for continuous fluorometric active-site titration. Analysis of the reaction kinetics as a function of FMGB and human factor Xa concentrations in G/2 0.3, pH 7.4, buffer at 25 degrees C indicated that the ratio of acylation to deacylation rate constants was greater than 200 and that the Km for FMGB was 0.06-0.11 microM, predicting pre-steady-state burst amplitudes of greater than or equal to 96-98% of the active-site concentration at FMGB concentrations greater than or equal to 5 microM. Human factor Xa active-site concentrations were consistent with 82-99% active preparations when compared with the protein concentrations determined from the 280-nm absorbance. Concentrations of human alpha-factor Xa as low as 20 nM could be measured with FMGB, indicating a sensitivity approximately 50 times greater than that measured by spectrophotometric active-site titration with p-nitophenyl p'-guanidinobenzoate.

Rapid purification of factor IX, factor X and prothrombin by immunoaffinity and ion exchange chromatography

This study describes a rapid purification of factor IX, factor X and prothrombin by immunoaffinity and ion exchange chromatography. Human factor IX was purified from plasma in 34% yield using barium sulfate adsorption and immunoaffinity purification. The specific clotting activity of purified factor IX was 269 units/mg, with an apparent molecular weight of 57,000 in the presence of sodium dodecyl sulfate on polyacrylamide gels. The immunodepleted, factor-IX deficient plasma was chromatographed on a dextran sulfate agarose column which resolved prothrombin and factor X in highly purified states and with approximately 50% yield. The specific activities of prothrombin and factor X obtained by this procedure were 24 units/mg and 147 units/mg respectively. Both proteins isolated by this method showed a single component on SDS gel electrophoresis and the molecular weights of intact prothrombin and factor X were 72,000 and 67,000 respectively.

Inhibition of activated protein C by platelets

Activated protein C (APC), an anticoagulant that acts by inactivating Factors Va and VIIIa, is dependent on a suitable surface for its action. In this study we examined the ability of human platelets to provide this surface and support APC-mediated anticoagulant effects. The activity of APC was examined in three systems: the Factor Xa recalcification time of Al(OH)3 adsorbed plasma, studies of thrombin generation in recalcified plasma, and assessment of the rate of inactivation of purified Factor Va. In comparison with phospholipid, intact platelets required significantly greater concentrations of APC to achieve a similar degree of anticoagulation. When washed platelet membranes were substituted for intact platelets, adequate support of APC was observed and the anticoagulant effect was similar to that obtained with phospholipid. Platelet releasate obtained by stimulation of platelets with thrombin and epinephrine contained an inhibitor that interfered with the ability of phospholipid and washed platelet membranes to catalyze the anticoagulant effects of APC. A noncompetitive inhibition was suggested by Dixon plot analysis of the interaction between platelet releasate and APC. The activity of the platelet APC inhibitor was immediate and was not enhanced by heparin, distinguishing it from the circulating protein C inhibitor. The presence of this inhibitor in the platelet and its release with platelet stimulation emphasizes the procoagulant role of this cell.

Inhibition of the anticoagulant activity of protein S by prothrombin

Protein S is a vitamin K-dependent protein cofactor to the anticoagulant, activated protein C (APC). This study examines the inhibition of human protein S anticoagulant activity by prothrombin. In the absence of protein S, the anticoagulant activity of APC measured in a Factor Xa recalcification time, was comparable using normal or plasma adsorbed with Al(OH)3. Protein S was an effective cofactor to APC in Al(OH)3-adsorbed plasma, but was significantly less active in normal plasma. Analysis of the difference in the two plasmas revealed that normal plasma contained an inhibitor to the anticoagulant activity of protein S that was removed by Al(OH)3 adsorption. Purification of this inhibitory activity demonstrated that it was mediated by the vitamin K-dependent protein, prothrombin. Prothrombin purified by conventional techniques caused immediate, dose-dependent inhibition of the cofactor activity of protein S in the presence of phospholipids or platelets, but had no effect on the anticoagulant activity of APC. The inhibition was demonstrable using a Factor Xa recalcification time, and studies of the rates of inactivation of purified Factor Va. Increasing concentrations of protein S overcame the inhibition by prothrombin and kinetic analysis of the interaction demonstrated that prothrombin acted as a competitive inhibitor to protein S. Immunoabsorption of prothrombin from plasma using immobilized antiprothrombin antibodies was associated with the complete removal of the protein S inhibitory activity. We conclude that the anticoagulant activity of protein S is modulated by prothrombin and that this may represent another regulatory mechanism of the natural anticoagulant system.

An investigation of the coagulant activity of the venom of the saw-scaled viper (Echis carinatus) from Saudi Arabia

Unlike the venom of Echis carinatus from India, Pakistan, Nigeria, Kenya, Iran and Oman, Saudi Arabian E. carinatus venom is a poor activator of prothrombin. However, it possesses similar defibrinogenating activity to the other venoms. This is because the venom from Saudi Arabian snakes contains a calcium-dependent factor X activator. It is suggested that in future studies of the coagulant activity of venoms, the determination of plasma coagulant activity should be carried out in the presence of added calcium ions. This applies particularly to those venoms which do not act on plasma or fibrinogen, but which do cause in vivo defibrinogenation.

Large scale purification of factor X by hydrophobic chromatography

Factor X is a critical enzyme in the blood coagulation cascade, however, in recent years the coagulation zymogen factor X has received additional interest as a selective proteinase to allow production of functional eukaryotic proteins in a prokaryotic expression system. Traditional factor X purification schemes suffer from low yields, low capacity, lengthy dialysis steps, and contamination by the autoproteolytic activated enzyme factor Xa. By incorporating a reversible inhibitor of factor X activation, we were able to recover 67% of the factor X present without any detectable activated enzyme. Six liters of plasma could be processed onto a 50 mL phenylalanine-Sepharose hydrophobic chromatography column without saturating the matrix. The final product is devoid of detectable proteolytic activity. At time of use, the zymogen is specifically activated with a Sepharose-bound activating enzyme isolated from Russell's Viper Venom, resulting in factor Xa free of other detectable proteinases.

Formation of factor Va by atherosclerotic rabbit aorta mediates factor Xa-catalyzed prothrombin activation

Vascular cell procoagulant activity may be important in the pathogenesis of atherosclerosis. In previous studies, we described the ability of the atherogenic metabolite homocysteine to activate endothelial cell Factor V, a key coagulation cofactor for thrombin generation. The present study was designed to investigate Factor V activity and Factor Xa-catalyzed prothrombin activation by control and atherosclerotic aorta from normal and hypercholesterolemic rabbits. Factor Xa generated ninefold more thrombin on atherosclerotic aortic segments than on control segments. Atherosclerotic segments activated 125I-prothrombin with Factor Xa in the presence of the thrombin inhibitor dansyl arginine-4-ethylpiperidine amide and cleaved 125I-Factor V. This suggests that increases in vessel-wall Factor V activity and Factor Xa-catalyzed prothrombin activation result from activation of vessel-wall Factor V. 125I-Factor Va peptides generated by atherosclerotic aorta were very similar in molecular weight to those generated by homocysteine-treated cells. When vascular endothelium was mechanically removed by brushing, atherosclerotic vessels still generated four- to fivefold more thrombin than control vessels. These data and results from immunocytochemical studies suggest that Factor V in atherosclerotic vessels is associated with both endothelium and other cells of the lesion. In contrast, Factor V in control vessels is associated primarily with endothelium. The increases in Factor V activity and thrombin formation in the blood vessel wall of hypercholesterolemic rabbits may contribute to the development of atherosclerosis and its complications.

Differential effects of activation of prothrombin by streptokinase compared with urokinase and tissue-type plasminogen activator (t-PA)

We have previously reported paradoxical prothrombotic effects manifest by elevations of fibrinopeptide A (FPA) after administration of streptokinase to patients with acute myocardial infarction. To characterize mechanisms responsible and their dependence on streptokinase (SK) as opposed to other activators of the fibrinolytic system, the present study was performed to compare effects of streptokinase, tissue-type plasminogen activator (t-PA), and urokinase on plasma and on purified prothrombin in concentrations similar to those achieved pharmacologically. The effects of plasmin were assessed to determine the extent to which the elevations of FPA seen could be attributed to activation of plasminogen. Elevations of FPA were observed after incubation of each of the activators with citrated plasma at 37 degrees C for 60 minutes. However, they were most marked with streptokinase (64.5 +/- 4.6 pmol FPA/ml (mean +/- SE) with 100 IU SK/ml, and 77.6 +/- 5.0 pmol/ml with 500 IU SK/ml). Elevations of FPA induced by streptokinase were attenuated by 100 IU/ml heparin [15.2 +/- 1.9 pmol/ml after 100 IU of SK (p less than 0.001 compared with results with streptokinase without heparin)]. Human plasmin, 2.5 CTA/ml, caused changes similar to those induced by streptokinase. The minimal elevations of FPA induced by t-PA or urokinase (less than 10 pmol/ml without heparin) were not significantly attenuated by heparin. Incubation of barium citrate adsorbed plasma (vitamin K factor depleted) with streptokinase markedly attenuated elevation of FPA. Addition of prothrombin (1.5 microM) and streptokinase (100 IU/ml) to the barium citrate adsorbed plasma elicited elevations of FPA similar to those induced by streptokinase in citrated plasma. Amidolytic activity with the "thrombin" substrate H-D-phenylalanyl-L-pipecolyl-L-arginine-p-nitroanilide dihydrochloride (S-2238) was evident when streptokinase, plasminogen (0.24 microM), and prothrombin (1.5 microM) were incubated in buffer. Thus, concentrations of streptokinase that are low in terms of therapeutic blood levels activate prothrombin in plasma, likely due to activation of plasminogen. Neither tissue-type plasminogen activator nor urokinase in pharmacologically comparable concentrations increase thrombin activity appreciably perhaps because of less intense activation of plasminogen. Consideration of the prothrombotic effects observed may be relevant to selection of specific agents for therapeutic thrombolysis, to appropriate titration of dose, and to the need for the use of heparin conjointly with particular activators of the fibrinolytic system such as streptokinase.

Automated fluorogenic methods for the evaluation of the extrinsic coagulation reactions in human plasma

Highly sensitive automated methods for the evaluation of the extrinsic coagulation reactions in human plasma were developed by the combination of fluorogenic peptide substrate (MCA) for thrombin and a centrifugal autoanalyzer (Cobas Bio). Prothrombin time (PT) was measured by the reaction time to reach 0.1 relative fluorescence which was caused by the action of thrombin generated after the activation of 3 microliters plasma with human placental tissue factor (Thromborel S). Factors X and VII contents in plasma were measured by the same method after mixing diluted plasma with each factor deficient plasma, tissue factor, calcium and MCA in which 10-800% of each factor was quantitatively measured. Prothrombin content in plasma was quantitated by measuring thrombin activity after the activation with human activated Factor X in the presence of phospholipid and calcium in which 10-160% of prothrombin was measured. By the application of these fluorogenic methods to the patients with cardiovascular diseases, it was demonstrated that these methods are highly sensitive not only to hypocoagulable state, but also to hypercoagulable state, particularly to the increase of Factors X and VII concentrations in plasma.

Vascular smooth muscle cells synthesize, secrete and express coagulation factor V

Expression of cellular procoagulant activity may be one of the more important responses to vascular injury. Because factor V, a coagulation cofactor in the prothrombinase complex, catalyzes the conversion of prothrombin to thrombin, it may be a key to understanding this response. Therefore, we have investigated the synthesis, secretion and expression of factor V by vascular smooth muscle cells, which proliferate at sites of vascular injury. Cultured aortic vascular smooth muscle cells constitutively secreted Factor V activity, as determined by a functional assay. Labeled factor V was immunoprecipitated from conditioned medium of [35S]methionine-labeled cells, indicating that the secreted factor V was synthesized by vascular smooth muscle cells. Treatment of vascular smooth muscle cells with tunicamycin prevented secretion of factor V, suggesting that its secretion was dependent on the presence of N-linked carbohydrate. Factor V activity was also expressed on the vascular smooth muscle cell surface, as indicated by the ability of cultured cells to promote factor Xa-catalyzed prothrombin activation. These data suggest that the proliferation of smooth muscle cells in response to vascular injury may be one mechanism that links vascular disease with thrombosis.

Regulation of factor XIa activity by platelets and alpha 1-protease inhibitor

We have studied the complex interrelationships between platelets, Factor XIa, alpha 1-protease inhibitor and Factor IX activation. Platelets were shown to secrete an inhibitor of Factor XIa, and to protect Factor XIa from inactivation in the presence of alpha 1-protease inhibitor and the secreted platelet inhibitor. This protection of Factor XIa did not arise from the binding of Factor XIa to platelets, the presence of high molecular weight kininogen, or the inactivation of alpha 1-protease inhibitor by platelets. The formation of a complex between alpha 1-protease inhibitor and the active-site-containing light chain of Factor XIa was inhibited by activated platelets and by platelet releasates, but not by high molecular weight kininogen. These results support the hypothesis that platelets can regulate Factor XIa-catalyzed Factor IX activation by secreting an inhibitor of Factor XIa that may act primarily outside the platelet microenvironment and by protecting Factor XIa from inhibition, thereby localizing Factor IX activation to the platelet plug.

A comparison of six DNA bending models

The predictions of six DNA bending models were compared with experimental relative mobility data. The study showed that all the models are reasonably accurate in predicting bending in synthetic sequences and in a natural sequence. The least accurate of these models is the Calladine-Dickerson model. The most consistent model is the ApA Wedge, possibly because it distributes the bends into base-roll and base-tilt components.

Enzyme-linked coagulation assay. IV. Sensitive sandwich enzyme-linked immunosorbent assays using Russell's viper venom factor X activator-antibody conjugates

We have applied the enzyme-linked coagulation assay (ELCA) system to the development of an amplified immunoassay using the clotting cascade to enhance sensitivity of detection of immune complexes. The factor X-activating enzyme of Russell's viper venom was detectable using ELCA in amounts as low as 0.25 fg per assay. Monoclonal antibodies to beta-hCG, placental alkaline phosphatase (PLAP), and the P-24 antigen of HTLV-III were labeled with this enzyme or peroxidase and used for "sandwich" immunoassays using another monoclonal antibody (beta-hCG, PLAP) or polyclonal patient IgG (P-24 antigen) bound to a polylysine-glutaraldehyde-coated plate as a "capture" reagent. After the immunobinding step, the plate was washed and substrate consisting of a mixture of factors X, V, and II in buffer containing calcium and lipid was incubated for various lengths of time. The mixture was transferred to another plate coated with fibrinogen and containing peroxidase-fibrinogen in EDTA solution to measure the amount of thrombin generated. Using this protocol, we were able to measure the presence of 2-10 pg/ml of beta-hCG and PLAP (5-30 amol per sample). All three model antigens were detectable at concentrations 2-3 orders of magnitude less using RVV-XA-labeled antibodies and ELCA than they were using peroxidase-labeled antibodies. The assay has considerable potential as a general immunoassay amplification system, yielding a "color test" for antigens of interest with a detection limit not readily attainable using other chromogenic methodologies.

Purification of a proteinase inhibitor from bovine serum with C1-inhibitor activity

This report describes the purification of a novel proteinase inhibitor from bovine serum. This protein was purified to apparent homogeneity employing affinity binding to sulfated dextran and precipitation by ammonium sulfate, followed by sequential chromatography on DEAE-cellulose, heparin-Sepharose and Sephacryl S-200. Quantitative enzyme-linked immunosorbent assays revealed that the concentration of this inhibitor is approximately 3 microM in bovine serum. The inhibitor is a single polypeptide chain with an estimated Mr of 83,000 as determined by SDS-polyacrylamide gel electrophoresis. An aspartic acid was found at the amino terminus of the protein; N-terminal amino acid sequence data indicated that there was no significant homology with other reported amino acid sequences. This bovine inhibitor covalently complexed the human proteinases C1-r, C1-s, factor XIIa and plasma kallikrein, which are also complexed and inactivated by human C1-inhibitor. In addition, the bovine inhibitor complexed and inactivated bovine chymotrypsin, a feature which functionally distinguishes it from human C1-inhibitor. Although the bovine inhibitor appears functionally very similar to C1-inhibitor, we found no evidence for structural homology with the human counterpart.

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.

Enhancement of prothrombin activation on platelets by endothelial cells and mechanism of activation of factor V

Factor Xa-catalyzed prothrombin activation occurs on cellular surfaces, including those of vascular endothelial cells. In a reconstituted model, endothelial cells and platelets acted synergistically to maximally activate prothrombin in the presence of Factor Xa. Synergism was observed at platelet concentrations less than 1 X 10(8)/ml. Thrombin formation was required for optimal prothrombin activation by endothelial cells and Factor Xa, with thrombin serving as an activator of Factor V. These data provide additional support for the hypothesis that vascular endothelium is a physiologic surface for hemostasis.

Effect of limited modification of amino groups on the reactivity of human factor Xa

The basis of the specificity of human coagulation factor Xa has been probed with a reagent that reacts with nucleophiles, N-succinimidylpropionate. At pH 8.0 and 0.25 mM N-succinimidylpropionate, 0.4 microM factor Xa lost approx. 90% of its activity toward prothrombin in 4 min. The decay was first-order, k = 0.64 min-1, which increased to 0.98 min-1 in 1 mM Ca2+, and the dependence of k upon pH was consistent with primary amines being the target. The rate of modification was unaffected by the presence of a tetrapeptide substrate during modification; likewise, activity toward a tripeptide p-nitroanilide was unaltered during exposure of factor Xa to N-succinimidylpropionate with or without Ca2+. In addition, inhibition by antithrombin III was retained with a somewhat enhanced rate after modification; however, the acceleration of this by heparin was significantly less. Kinetic determination of the number of residues modified gave a reaction order of 2.0, while reaction with N-succinimidyl[3H]propionate yielded labeled factor Xa containing 1.0 mol N-succinimidylpropionate/mol factor Xa and 50% normal clotting activity, or 2.0 mol N-succinimidylpropionate/mol and 1% activity, respectively. Thus, one nucleophilic group is required for the reaction of factor Xa with prothrombin but not for the hydrolysis of peptides or recognition of antithrombin III. The decay of clotting activity of the factor X zymogen in N-succinimidylpropionate was much slower though still Ca2+-dependent. Conversely, the reaction of a related compound--N-succinimidyl(4-hydroxyphenyl)propionate or Bolton-Hunter reagent--with factor Xa broadly resembled that of N-succinimidylpropionate but the decay curves indicated more complex kinetics. Therefore, the target groups vary in their accessibility to modification according to the structural characteristics of both the protein and the reagent.

Protein kinase C phosphorylates human platelet inositol trisphosphate 5'-phosphomonoesterase, increasing the phosphatase activity

Phosphoinositide breakdown in response to thrombin stimulation of human platelets results in the formation of the calcium-mobilizing messenger molecules inositol 1,4,5-trisphosphate and inositol 1,2-cyclic-4,5-trisphosphate and of diglyceride, which activates protein kinase C. We find that protein kinase C phosphorylates and thereby increases the activity of inositol 1,4,5-trisphosphate 5'-phosphomonoesterase, a phosphatase that hydrolyzes these molecules to inert compounds. The 5'-phosphomonoesterase phosphorylated using [gamma-32P]ATP comigrates on SDS-polyacrylamide gels with a protein (40 kd) phosphorylated rapidly in response to thrombin stimulation of 32PO4-labeled platelets. Peptide maps of proteolytic digests of these two phosphorylated proteins indicate that they are the same. We propose that platelet Ca2+ mobilization is regulated by protein kinase C phosphorylation of the inositol 1,4,5-trisphosphate 5'-phosphomonoesterase. These results explain the observation that phorbol ester treatment of intact human platelets results in decreased levels of inositol trisphosphate and decreased Ca2+ mobilization upon subsequent thrombin addition.

Kinetics of the inhibition of thrombin by hirudin

The dissociation constant for hirudin was determined by varying the concentration of hirudin in the presence of a fixed concentration of thrombin and tripeptidyl p-nitroanilide substrate. The estimate of the dissociation constant determined in this manner displayed a dependence on the concentration of substrate which suggested the existence of two binding sites at which the substrate was able to compete with hirudin. A high-affinity site could be correlated with the binding of the substrate at the active site, and the other site had an affinity for the substrate that was 2 orders of magnitude lower. Extrapolation to zero substrate concentration yielded a value of 20 fM for the dissociation constant of hirudin at an ionic strength of 0.125. The dissociation constant for hirudin was markedly dependent on the ionic strength of the assay; it increased 20-fold when the ionic strength was increased from 0.1 to 0.4. This increase in dissociation constant was accompanied by a decrease in the rate with which hirudin associated with thrombin. This rate could be measured with a conventional recording spectrophotometer at higher ionic strength and was found to be independent of the binding of substrate at the active site.

Inhibition by human thrombomodulin of factor Xa-mediated cleavage of prothrombin

Human thrombomodulin significantly inhibited the rate of prothrombin conversion to thrombin by Factor Xa in the presence of phospholipid or platelets, calcium, and Factor Va. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of 125I-prothrombin activation revealed that thrombomodulin reduced the rate of prothrombin activation but did not alter the cleavage pattern. The inhibition was reversed by the inclusion of a highly specific rabbit antithrombomodulin antibody. If thrombomodulin was replaced by hirudin, the rate of thrombin generation was not decreased excluding the possibility that the inhibition by thrombomodulin was secondary to the binding of small amounts of thrombin formed early in the reaction and the prevention of feedback breakdown of prothrombin by thrombin. The inhibitory activity of thrombomodulin was overcome by increasing the concentration of Factor Xa and specific, saturable binding of thrombomodulin to Factor Xa was demonstrated. These results indicate that thrombomodulin binds to Factor Xa and thereby inhibits the activity of the prothrombinase complex.

Effect of thrombomodulin on the kinetics of the interaction of thrombin with substrates and inhibitors

Thrombomodulin decreased by 20-30% the Michaelis constant of two tripeptidyl p-nitroanilide substrates of thrombin. Thrombomodulin increased the rate of inactivation of thrombin by two peptidyl chloromethane inhibitors by a similar amount. This effect appeared to be due to a decrease in the dissociation constants of the inhibitors. An improved method for the separation of fibrinopeptides A and B by h.p.l.c. was developed, and this method was used to study the effect of thrombomodulin on the thrombin-catalysed cleavage of fibrinogen. In this reaction, thrombomodulin was a competitive inhibitor with respect to the A alpha-chain of fibrinogen. The release of fibrinopeptide B was also inhibited by thrombomodulin. Analysis of the inhibition caused by thrombomodulin with respect to fibrinopeptides A and B yielded the same dissociation constant for the thrombin-thrombomodulin complex. In the presence of thrombomodulin, the rate of inactivation of thrombin by antithrombin III was stimulated 4-fold. This stimulation showed saturation kinetics with respect to thrombomodulin. Thrombomodulin was found to compete with hirudin for a binding site on thrombin. As a result of this competition, hirudin became a slow-binding inhibitor of thrombin at high thrombomodulin concentrations. Estimates of the dissociation constant for thrombomodulin were obtained in several of the above experiments, and the weighted mean value was 0.7 nM.

Activation of endogenous factor V by a homocysteine-induced vascular endothelial cell activator

Vascular endothelium possesses multiple procoagulant properties, including synthesis and expression of Factor V. We studied the effects of homocysteine on the regulation of endothelial cell Factor V activity. Elevated levels of homocysteine are associated with the congenital thrombotic disorder homocystinuria. Treatment of cultured endothelial cells with 0.5-10 mM homocysteine had no effect on cell morphology, but did increase Factor V activity and prothrombin activation by Factor Xa. A radioimmunoassay for endothelial cell Factor V demonstrated that homocysteine treatment did not increase Factor V antigen levels. 125I-prothrombin was activated by treated endothelial cells and Factor Xa in the presence of thrombin inhibitors. Exogenous 125I-Factor V was cleaved by homocysteine-treated but not control endothelial cells. 125I-Factor V cleavage products distinct from those generated by thrombin and Factor Xa were identified. These data provide evidence for regulation of endothelial cell Factor V activity, and indicate that increased Factor V activity associated with homocysteine-treated vascular endothelium results primarily from induction of an activator of Factor V.

A human osteosarcoma cell line secretes a growth factor structurally related to a homodimer of PDGF A-chains

Platelet-derived growth factor (PDGF), as purified from fresh human platelets, is a protein of relative molecular mass (Mr) 30,000 composed of two disulphide-linked subunit chains of similar size, named A and B (ref. 1). The dimer structure of PDGRF seems to be important for its biological effects, as reduction irreversibly inactivates the factor; it is not known, however, whether PDGF exists as a heterodimer or as a mixture of homodimers. Amino-acid sequence analysis has revealed that the A- and B-chains of human PDGF are related to each other, and that the B-chain is almost identical to part of the v-sis gene product of simian sarcoma virus (SSV). There is experimental evidence that a PDGF-like protein is indeed operational in SSV-induced transformation and the biologically active v-sis product is probably structurally similar to a putative dimer of PDGF B-chains. PDGF-like growth factors and/or a 4.2-kilobase (kb) c-sis transcript are present in several transformed mammalian cell lines and in certain nontransformed cells; cloned c-sis complementary DNA from human T cells transformed with human T-lymphotropic virus (HTLV) or from human endothelial cells contains the coding sequence for a putative PDGF B-chain precursor, but apparently lacks PDGF A-chain sequences. We have previously partially purified and characterized a PDGF-like growth factor from U-2 OS cells (osteosarcoma-derived growth factor, ODGF) and shown that this factor has structural, functional and immunological characteristics in common with PDGF. We describe here a procedure for the preparation of homogeneous ODGF, and provide evidence that this factor, which binds to the PDGF receptor, has a structure similar to a homodimer of PDGF A-chains.

Enzyme-linked coagulation assay. II. A sensitive assay for tissue factor and factors II, VII, and X

We have developed a solid-phase clotting assay which uses peroxidase-fibrinogen in solution and fibrinogen bound to microtiter plates as a substrate for the thrombin generated from the clotting cascade. We have developed this assay for measurement of the extrinsic pathway factors thromboplastin (tissue factor, factor III), VII and VIIa, X, and II. Using long incubation times (40-90 min), thromboplastin could be measured in extracts of human brain at very low concentrations. Specificity for thromboplastin was demonstrated by showing a requirement for factors II, V, X, and VII but not for VIII, IX, XI, or XII; both substrate plasmas monodeficient in single factors and mixtures of the pure factors were used in demonstrating this specificity. The assay was modified to measure factors II, VII, VIIa, and X using appropriate deficient plasmas. The limit of detection was 2-3 orders of magnitude lower than a one-stage clotting test for all factors assayed. This assay has the advantages of convenience, specificity comparable to standard clotting tests, and high sensitivity.