Fast migrating protein, immunochemically related to human factor VIII, studied by crossed immunoelectrophoresis in agarose

S ummary . A fast migrating protein (FMP) was detected by agarose radio-crossed immunoelectrophoresis, in addition to factor VIII antigen (VII:RAg), using antiserum to human factor VIII (FVIII). FMP had partial immunochemical identity with FVIII, migrated as an α-protein, and was distinct from α-2 macroglobulin, fibronectin or IgM. FMP was precipitated by concanavalin A and was separable from the bulk of VII:RAg by ammonium sulphate fractionation. A significant amount of FMP was seen in normal serum ( n = 12), plasma from patients with: (a) disseminated intravascular coagulation ( n = 12) and (b) severe haemophilia A ( n = 6). Trace amounts of FMP were observed in plasma from normal donors ( n = 12), but neither VIII: RAg nor FMP was detectable in the plasma or serum from patients with severe von Willebrand's disease ( n = 3). Freshly prepared cryoprecipitate contained trace amounts of FMP, similar to normal plasma, but increased levels were observed in antihaemophilic factor concentrates prepared for patient use. Significant levels of FMP were also seen in cryoprecipitate after storage at 4°C for 7 d and this generation of FMP was diminished by the addition of protease inhibitors. The presence of significant levels of FMP in situations where proteolytic enzymes may be activated and inhibition of its generation by protease inhibitors, suggest that this protein is produced by proteolytic action of enzyme(s) on the FVIII molecule.     

Analysis of factor VIII coagulant antigen in normal, thrombin-treated, and hemophilic plasma.

The relationship between Factor VIII coagulant antigen (VIII:CAg) and Factor VIII-associated von Willebrand factor (VIII:vWF), and the effect of thrombin on VIII:CAg have been determined in plasma by using complexes of VIII:CAg and 125I-labeled human anti-VIII:CAg-Fab. Antibody-treated plasma samples were electrophoresed on NaDodSO4/polyacrylamide agarose gels and analyzed by autoradiography. The major VIII:CAg-125I-labeled Fab complex that persisted in NaDodSO4 had Mr 3.2 x 10(5). This Mr value was confirmed by column chromatography and sucrose density centrifugation and is presumed to reflect a free VIII:CAg of Mr 2.7 x 10(5). Minor bands were also present on autoradiograms of normal plasma corresponding to Mr values of 2.5, 1.85, and 1.7 x 10(5) (free VIII:CAg related proteins with Mr values of 2.0, 1.35, and 1.2 x 10(5), respectively). None of the VIII:CAg bands was present in plasma samples from five patients with severe hemophilia A. No radioactivity was associated with VIII:vWF multimers on NaDodSO4 gels. Thrombin treatment of normal plasma eliminated the radioactive band at 3.2 x 10(5) and increased the intensity of a band of Mr 1.7 x 10(5). Generation of this presumed VIII:CAg fragment of Mr is approximately equal to 1.2 x 10(5) coincided with a thrombin-induced increase in Factor VIII coagulant activity. These data demonstrate that the form of VIII:CAg detected in normal plasma is not covalently linked to VIII:vWF multimers and is absent in plasma from five hemophilia A patients. Thrombin-induced proteolysis of VIII:CAg can be detected in microliter quantities of normal plasma.     

An immunoradiometric assay for human factor VIII/von Willebrand factor (VIII:vWF) using a monoclonal antibody that defines a functional epitope

A murine monoclonal antibody has been produced (RFF-VIII:R/2) that binds specifically to human factor VIII-related antigen (VIII:RAg) in plasma and in vascular endothelial cells but has no reactivity with factor VIII procoagulant antigen (VIII:cAg). This antibody is a potent inhibitor of von Willebrand factor activity (VIII:vWF) in that it can totally neutralize ristocetin-induced aggregation of platelet rich plasma and inhibit platelet adhesion at high flow rates. RFF-VIII:R/2 can be used in a one-stage, fluid phase immunoradiometric assay that can detect VIII:RAg at concentrations of 0.001 u/ml. This method has been used to analyse plasma from patients with von Willebrand's disease (vWD). Results obtained in these patients showed a high degree of correlation between the monoclonally-defined epitope and VIII:vWF levels measured by ristocetin-induced aggregation of washed platelets. This correlation was maintained in those patients with the 'variant' types of vWD who exhibit highly disparate VIII:vWF and VIII:RAg levels when the latter is determined using polyclonal antisera. It appears that this monoclonal antibody recognizes a site on the VIII:RAg molecule which is associated with its interaction with the platelet membrane. Immunoradiometric assays using RFF-VIII:R/2 offer a simplified, reproducible means of detecting functionally-active VIII:RAg as an alternative or supplement to techniques involving platelet interactions.     

Multimeric structure of platelet factor VIII/von Willebrand factor: the presence of larger multimers and their reassociation with thrombin- stimulated platelets

The multimeric structure of platelet factor VIII/von Willebrand factor (FVIII/vWF) in cell extracts and in collagen and thrombin releasates has been analyzed by SDS polyacrylamide gel electrophoresis followed by detection with 125I-anti-FVIII/vWF. Platelets contained larger multimers than those normally present in plasma. When secreted FVIII/vWF was analyzed, all platelets. In contrast, in thrombin releasates the larger multimers were lost in a manner dependent on divalent cations, time, and thrombin dose. This loss could not be accounted for by modification of FVIII/vWF by thrombin or platelet enzymes since no effect of thrombin on the multimeric structure of FVIII/vWF in the absence of platelets or in the presence of platelet lysates was observed. Large multimers of 125I-labeled purified FVIII/vWF underwent divalent cation-dependent association with platelets in the presence of thrombin, indicating that the loss of FVIII/vWF from thrombin releasates was due to reassociation with the platelet. These studies show a structural difference between platelet and plasma FVIII/vWF that suggests a specific role for platelet FVIII/vWF in hemostasis.     

The oligosaccharide of human thrombin: investigations of functional significance

Functional properties of the carbohydrate chain of human thrombin were examined by quantitating the activity of the enzyme before and after partial removal of its oligosaccharide by exoglycosidases. The following activities were studied: fibrinogen clotting, factor VIII coagulant (VIII:C) activation, inhibition by defibrinated plasma (anti- thrombin-III and alpha 2-macroglobulin), binding to polymerized fibrin, and stimulation of platelet release and aggregation. In general, the published information about the activity of native thrombin in these interactions was confirmed, though differences were observed in the association constants describing thrombin binding to fibrin. Partial deglycosylation had no apparent effect on any of these activities. It is concluded, therefore, that the oligosaccharide of human thrombin is located outside the major protein and platelet-binding regions of the molecule.     

Direct radioimmune detection in human plasma of the association between factor VIII procoagulant protein and von Willebrand factor, and the interaction of von Willebrand factor-bound procoagulant VIII with platelets

The predominant procoagulant factor VIII (VIII:C) form in normal human plasma containing various combinations of anticoagulants and serine/cysteine protease inhibitors is a protein with mol wt 2.6 +/- 0.2 X 10(5). This protein can be detected by 125I-anti-VIII:C Fab binding and gel electrophoresis in the presence and absence of sodium dodecylsulfate (SDS) and is distinct from the subunit of factor VIII/von Willebrand factor (VIII:vWF) multimers. No larger VIII:C form is present in plasma from patients with severe congenital deficiencies of each of the coagulation factors, other than VIII:C. The mol wt approximately 2.6 X 10(5) VIII:C form is, therefore, likely to be the in vivo procoagulant form of VIII:C, rather than a partially proteolyzed, partially activated derivative of a larger precursor. About 60% of this procoagulant mol wt approximately 2.6 X 10(5) VIII:C form in plasma is present in noncovalent complexes with larger VIII:vWF multimers, which attach reversibly to platelet surfaces in the presence of ristocetin. This VIII:vWF-bound protein of mol wt approximately 2.6 X 10(5) may be the plasma procoagulant form of VIII:C which, after proteolytic activation, accelerates the IXa-mediated cleavage and activation of X postulated to occur on platelet surfaces.     

The effect of lipoproteins on the synthesis of prostacyclin, von Willebrand factor and apoliproteins A-I and A-II in cultured human endothelial cells

Primary cultures of confluent human endothelial cells (ECM) were grown in media containing the major lipoproteins (LP) and lipoprotein deficient serum (LDS). The release of 6-keto-PGF₁, von Willebrand factor (VIII RAg) and apolipoproteins (apo) A-I and A-II were investigated by radioimmunoassay. The cell-associated VIII RAg, apo A-I and apo A-II were also confirmed by fluorescein antibodies, and the synthesis of the apolipoproteins was examined by incorporation of [^3H]leucine.

Apo A-I and apo A-II were located and synthesized in ECM, yet only apo A-I was released into the medium. Very low density (VLDL) and low density lipoproteins (LDL) in concentrations of 50–600 μg/ml stimulated release of apo A-I. Stimulation of ECM for 5 min with thrombin (T) or arachidonic acid (A) did not induce apo A-I release.

VIII RAg was always released into the media from ECM. The release was not affected by the lipoproteins. VIII RAg was also localized on the cell surface (VIII RAgC) and approximately 80% was released by trypsin. LDL stimulated the occurrence of factor VIII RAg on the cell surface.

6-Keto PGF₁ was always released into the medium and the production was stimulated by T and AA. The main lipoproteins (50–600 μg/ml) and apo A-I and A-II did not affect the release of 6-keto-PGF₁

This study shows that endothelial cells synthesize and release proteins important for thrombogenesis and atherosclerosis. The release of apolipoproteins A-I was stimulated by VLDL and LDL, and the concentration of cell-related factor VIII RAg was stimulated by LDL.     

In vivo platelet hyperactivity and factor VIII related antigen increase long after myocardial infarction. A controlled effect of sulfinpyrazone

In the Anturane Reinfarction Italian Study the trend of some specific and quantitative haemostatic parameters is being investigated in different series of patients balanced for sulfinpyrazone and placebo. In a series of young male patients who had had myocardial infarction 6 months previously, it has been shown that the placebo treatment subsample presented shortened platelet production time (PPT), increased levels of plasma beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4), and increased factor VIII related antigen (VIII:RAg) compared with a matched control group. A close correlation between plasma concentration of VIII:RAg and PPT was evidenced in the same treatment subsample. The sulfinpyrazone treatment subsample presented normalized PPT accompanied by reduction of VIII:RAg but not of beta-TG or PF4 levels.     

Modulation of the enzymatic activity of alpha-thrombin by polyanions: consequences on intrinsic activation of factor V and factor VIII.

The polyanions heparin and dermatan sulfate catalyze alpha-thrombin inhibition and can delay the onset of factor VIII and factor V necessary for intrinsic prothrombin activation to begin in plasma. These polyanions bind alpha-thrombin at its anion-binding exosite(s), structural domain(s) occupancy of which may alter the properties of the fibrin(ogen) recognition exosite of alpha-thrombin. We compared how such four polyanions influenced factor VIII and factor V activation during intrinsic coagulation. A pentasaccharide with high affinity for antithrombin III and the C-terminal dodecapeptide fragment of hirudin (hirugen) which occupy the anion-binding and fibrin(ogen) recognition exosites of alpha-thrombin, respectively, could not significantly inhibit factor VIII and factor V activation. In contrast, heparin and a bis-lactobionic acid, both of which catalyzed alpha-thrombin inhibition, could effectively inhibit factor VIII and factor V activation. These results suggest that occupancy of fibrin(ogen) or anion-binding exosites by itself does not provide a necessary and sufficient condition for catalysis of thrombin inhibition or the inhibition thrombin-mediated amplification reactions.     

A monoclonal antibody binding to human medulloblastoma cells and to the platelet glycoprotein IIb-IIIa complex

S ummary. A monoclonal antibody, designated M148, produced by the hybridoma technique from spleen cells of mice immunized with human medulloblastoma, was found by indirect immunofluorescence to bind to normal human platelets (both Pl^Al positive and Pl^Al negative) and megakaryocytes, as well as to some medulloblastoma and neuroblastoma cells and cell lines and certain other solid tumours. No binding was observed to other marrow constituents, nor to any other normal tissue examined. The antibody bound to platelets from a patient with the Bernard-Soulier syndrome but not to thrombasthenic platelets. It immunoprecipitated glycoproteins IIb and IIIa from ¹²⁵I-labelled normal platelet membranes, and completely inhibited ADP-induced fibrinogen binding and aggregation of platelets. Aggregation was also inhibited in response to adrenaline, collagen, thrombin, sodium arachidonate and the ionophore A23187; clot retraction was partially inhibited. The antibody was without effect on thromboxane formation or 5-hydroxytryptamine (5HT) secretion in response to thrombin, but inhibited 5HT secretion in response to arachidonate. It did not inhibit factor VIII binding or agglutination in response to ristocetin, but completely inhibited factor VIII binding in response to thrombin. These findings suggest that the epitopes are close to the fibrinogen and factor VIII binding sites on glycoproteins IIb/IIIa, and that the lack of these glycoproteins is sufficient explanation for the pattern of dysfunction observed in thrombasthenic platelets, without invoking any other membrane abnormality.     

Thrombin-mediated release of factor VIII antigen from human umbilical vein endothelial cells in culture

We have examined the effects of purified human alpha-thrombin on factor VIII antigen (FVIII-Ag) release by human umbilical vein endothelial cells in culture. Alpha-thrombin induced a time and dose-dependent release of FVIII-Ag into supernatant medium. Alpha-thrombin-mediated FVIII-Ag release was not dependent on protein synthesis and was observed in both serum-free and serum-containing media. FVIII-Ag release, however, was prevented when the serine esterase activity of thrombin was inhibited. Pretreatment of human endothelial cells with alpha-thrombin, but not diisofluorophosphate-thrombin, prevented subsequent FVIII-Ag release by alpha-thrombin. Thrombin-mediated FVII- Ag release was not associated with significant 51Cr release from prelabeled endothelial monolayers. We conclude that alpha-thrombin induces release of preformed FVIII-Ag from human umbilical vein endothelial cells by a receptor-independent, nonlytic mechanism requiring serine esterase activity.     

Thrombin Activation of Factor VIII

The current experiments examine the reaction between purified human alpha-thrombin and purified human factor VIII and compare this with the reaction between alpha-thrombin and the low molecular weight factor VIII procoagulant activity obtained by high ionic strength dissociation. The reaction patterns of both procoagulants are similar, demonstrating in initial increase in factor VIII activity followed by a decay of procoagulant activity. The extent of activation which is observed in the initial phase decreases with lower temperatures and with lower thrombin concentrations. A pseudo-first order relationship is demonstrated for the activation phase. The labile factor VIII procoagulant activity produced by the initial action of thrombin appears to be intrinsically unstable, since the addition of hirudin or diisopropylfluorophosphate does not prevent the subsequent decay of the procoagulant activity. The similar activation and decay patterns of purified factor VIII and the dissociated low molecular weight factor VIII procoagulant support the concept that the low molecular weight procoagulant itself represents the functional coagulant moiety of the factor VIII complex.     

Histidine-rich glycoprotein binding to activated human platelets

Specific binding of purified histidine rich glycoprotein (HRGP) to human platelets stimulated with either bisdiazoniumbenzidine-crosslinked immunoglobulin G (BDB-IgG), with thrombin or with collagen was dose- and divalent cation dependent. A 5-10-fold increase of platelet bound 125I-HRGP was obtained when 0.5-0.8 x 10(9) platelets/ml were activated with 100 micrograms BDB-IgG/ml, 0.1 U thrombin/ml or 15 micrograms collagen/ml. At maximal binding tested 16,000 molecules of HRGP became bound per platelet, but saturation was not achieved. Such platelet inhibitors as acetylsalicylic acid, prostaglandin E1 and cytochalasin B reduced the capacity of platelets to bind ligand, and by kinetic experiments involving enzymatic digestion of radiolabelled bound HRGP the ligand revealed to remain surface bound rather than being taken up to inner parts of the cell.     

High molecular weight kininogen inhibits thrombin-induced platelet aggregation and cleavage of aggregin by inhibiting binding of thrombin to platelets.

In this study we show that high molecular weight kininogen (HK) inhibited alpha-thrombin-induced aggregation of human platelets in a dose-dependent manner with complete inhibition occurring at plasma concentration (0.67 mumol/L) of HK. HK (0.67 mumol/L) also completely inhibited thrombin-induced cleavage of aggregin (Mr = 100 Kd), a surface membrane protein that mediates adenosine diphosphate (ADP)-induced shape change, aggregation, and fibrinogen binding. The inhibition of HK was specific for alpha- and gamma-thrombin-induced platelet aggregation, because HK did not inhibit platelet aggregation induced by ADP, collagen, calcium ionophore (A23187), phorbol myristate acetate (PMA), PMA + A23187, or 9,11-methano derivative of prostaglandin H2 (U46619). These effects were explained by the ability of HK, at physiologic concentration, to completely inhibit binding of 125I-alpha-thrombin to washed platelets. As a result of this action of HK, this plasma protein also completely inhibited thrombin-induced secretion of adenosine triphosphate, blocked intracellular rise in Ca2+ in platelets exposed to alpha- and gamma-thrombin, inhibited thrombin-induced platelet shape change, and blocked the ability of thrombin to antagonize the increase in intracellular cyclic adenosine monophosphate (cAMP) levels induced by iloprost. Because elevation of cAMP is known to inhibit binding of thrombin to platelets, we established that HK did not increase the intracellular concentration of platelet cAMP. Finally, HK did not inhibit enzymatic activity of thrombin. To study the role of HK in the plasma environment, we used gamma-thrombin to avoid fibrin formation by alpha-thrombin. Platelet aggregation induced by gamma-thrombin was also inhibited by HK in a dose-dependent manner. The EC50 (concentration to produce 50% of the maximum rate of aggregation) of gamma-thrombin for washed platelets was 7 nmol/L and increased to 102 nmol/L when platelets were suspended in normal human plasma. The EC50 for platelet aggregation induced by alpha-thrombin in plasma deficient in total kininogen was 40 nmol/L. When supplemented with HK at plasma concentration (0.67 mumol/L), the EC50 increased to 90 nmol/L, a value similar to that for normal human plasma. These results indicate that (1) HK inhibits thrombin-induced platelet aggregation and cleavage of aggregin by inhibiting binding of thrombin to platelets; (2) HK is a specific inhibitor of platelet aggregation induced by alpha- and gamma-thrombin; and (3) HK plays a role in modulating platelet aggregation stimulated by alpha-thrombin in plasma.     

Binding of thrombin-activated human factor VIII to platelets

To study association of platelets with factor VIII, the purified protein was 125I-labelled with Bolton-Hunter reagent to a specific activity of 243,000-360,000 cpm/U. Autoradiographs of SDS polyacrylamide gels revealed polypeptides of VIII at Mr about 240 kDa, 90 kDa and intermediate values, as well as some radioactive contaminants, but the light chain (Mr 78/76) seen with silver stain was not labelled. After 2.5-5-fold activation with thrombin, the higher radioactive Mr band disappeared, the band at 90 kDa became more intense, and a band appeared at about 45 kDa. The radioactivity associated with platelets, studied in the presence of haemophilic BaSO4-treated plasma, was maximal after 6-8 min and increased 3-15-fold on activation with thrombin. With activated VIII, autoradiographs of platelet pellets showed only VIIIa but results are expressed as units of unactivated VIII bound. At 0.3-0-0.7 U/ml, 10(8) platelets bound 0.0008-0.004 U VIIIa. The amount bound was not affected by the ratio of unlabelled VIII to VIII labelled in the presence of a 50-fold molar excess of unlabelled Bolton-Hunter reagent. Binding increased to 1.5 U VIIIa/10(8) platelets (about 13,600 molecules per platelet) at 140 U/ml, with no evidence of saturation. Binding was not affected by monoclonal antibodies to platelet glycoproteins IIb/IIIa or Ib, quenching the thrombin before adding platelets, or aggregating the platelets with A23187 in the presence of thrombin. Qualitatively, binding of labelled VIIIa and factor Va studied by others are similar. Binding of 125I-VIIIa to aggregated and unaggregated platelets was normal in patient M.S. whose platelets were shown by others to be deficient in their ability to bind radiolabelled factor Xa and generate coagulant activity. This difference. and the fact that platelet coagulant activity is increased by platelet activation and/or aggregation, suggest that binding of Va or VIIIa alone does not determine the assembly of active proteolytic complexes on the platelet surface.     

Synergistic induction of plasminogen activator inhibitor type-1 in HEP G2 cells by thrombin and transforming growth factor-beta.

Plasminogen activator inhibitor type-1 (PAI-1) is a physiologic modulator of the fibrinolytic system. Its activity in plasma increases in diverse thrombotic states. The large synthetic capacity of the liver make it a source of potentially large amounts of PAI-1. Because thrombin activity increases in association with thrombotic disorders and because specific binding sites for thrombin have been identified on hepatocytes, we characterized the effect of thrombin on hepatocyte PAI-1 production. Incubation of Hep G2 cells with human alpha-thrombin resulted in a dose- and time-dependent increase in the concentration of PAI-1 in conditioned media. This effect was inhibited completely by hirudin and by antithrombin III. Steady-state levels of both the 3.2-kb and 2.2-kb forms of PAI-1 mRNA increased after stimulation of the cells with thrombin, indicating that thrombin influences PAI-1 expression in Hep G2 cells at the pretranslational level. Incubation of Hep G2 cells with alpha-thrombin and either platelet lysates or purified transforming growth factor-beta (TGF-beta), both previously shown to augment hepatocyte PAI-1 expression, resulted in a synergistic increase in the concentration of PAI-1 in conditioned media. PAI-1 mRNA appeared to be synergistically increased as well. Thus, thrombin increases expression of both PAI-1 protein and mRNA in Hep G2 cells and exerts synergistic effects with TGF-beta. These results underscore the potential importance of inhibition of thrombin under conditions in which thrombolysis is induced pharmacologically.     

Activation of factor XI in plasma is dependent on factor XII [see comments]

The activation of factor XI initiates the intrinsic coagulation pathway. Until recently it was believed that the main activator of factor XI is factor XIIa in conjunction with the cofactor high molecular weight kininogen on a negatively charged surface. Two recent reports have presented evidence that in a purified system factor XI is activatable by thrombin together with the soluble polyanion dextran sulfate. To assess the physiological relevance of these findings we studied the activation of factor XI in normal and factor XII-deficient plasma. We used either kaolin/cephalin or dextran sulfate as a surface for the intrinsic coagulation pathway, tissue factor to generate thrombin via the extrinsic pathway, or the addition of alpha-thrombin directly. 125I-factor XI, added to factor XI-deficient plasma at physiologic concentrations (35 nmol/L), is rapidly cleaved on incubation with kaolin. The kinetics appear to be exponential with half the maximum cleavage at 5 minutes. Similar kinetics of factor XI cleavage are seen when 40 nmol/L factor XIIa (equal to 10% of factor XII activation) is added to factor XII-deficient plasma if an activating surface is provided. Tissue factor (1:500) added to plasma did not induce cleavage of factor XI during a 90-minute incubation, although fibrin formation within 30 seconds indicated that thrombin was generated via the extrinsic pathway. Adding 1 mumol/L alpha-thrombin (equivalent to 50% prothrombin activation) directly to factor XII deficient or normal plasma (with or without kaolin/cephalin/Ca2+ or dextran sulfate) led to instantaneous fibrinogen cleavage, but again no cleavage of factor XI was observable. We conclude that in plasma surroundings factor XI is not activated by thrombin, and that proposals of thrombin initiation of the intrinsic coagulation cascade are not supportable.     

Effectiveness, specificity and safety of intranasal 1-deamino-8-D-arginine vasopressin treatment of normal blood donors

A study of the absorption of 300 micrograms of 1-deamino-8-D-arginine vasopressin (DDAVP) given intranasally to normal blood donors was carried out to determine (a) the correlation between plasma levels of DDAVP and the percent rise of factor VIII procoagulant activity (VIII:C) and (b) the efficacy, specificity and safety of this treatment in increasing the recovery of factor VIII:C in donated blood. The maximum drug concentration was highly correlated to the maximum percent rise of VIII:C (r = 0.858, p less than 0.01). A differentiated effect of DDAVP on increases of VIII:C, VIII:Ag, vWF:Ag and vWF multimers was observed. A transient rise of fibrinopeptide A from 5 to 16 ng/ml, 30 min post-DDAVP, was not accompanied by changes in fibrinogen levels or generation of detectable factor Xa or thrombin. DDAVP had no effect on the factor XII-dependent pathway of plasminogen activation, or on the donor's vital signs and hematological parameters. Side effects were minor and of short duration. Intranasal DDAVP treatment of blood donors is considered to be a practical means of improving the recovery of VIII:C from normal donors.     

In Vivo Platelet Hyperactivity and Factor VIII Related Antigen Increase Long after Myocardial Infarction

In the Anturane Reinfarction Italian Study the trend of some specific and quantitative haemostatic parameters is being investigated in different series of patients balanced for sulfinpyrazone and placebo. In a series of young male patients who had had myocardial infarction 6 months previously, it has been shown that the placebo treatment subsample presented shortened platelet production time (PPT), increased levels of plasma β-thrombo-globulin (β-TG) and platelet factor 4 (PF₄), and increased factor VIII related antigen (VIII:RAg) compared with a matched control group. A close correlation between plasma concentration of VIILRAg and PPT was evidenced in the same treatment subsample. The sulfinpyrazone treatment subsample presented normalized PPT accompanied by reduction of VIII:RAg but not of β-TG or PF₄ levels.     

Requirements of von Willebrand factor to protect factor VIII from inactivation by activated protein C

The interaction of factor VIII with von Willebrand factor (vWF) was investigated on a quantitative and qualitative level. Binding characteristics were determined using a solid phase binding assay and protection of factor VIII by vWF from inactivation by activated protein C (aPC) was studied using three different assays. Deletion mutants of vWF, a 31-kD N-terminal monomeric tryptic fragment of vWF that contained the factor VIII binding site (T31) and multimers of vWF of different size were compared with vWF purified from plasma. We found that deletion of the A1, A2, or A3 domain of vWF had neither an effect on the binding characteristics nor on the protective effect of vWF on factor VIII. Furthermore, no differences in binding of factor VIII were found between multimers of vWF with different size. Also, the protective effect on factor VIII of vWF was not related to the size of the multimers of vWF. A 20-fold lower binding affinity was observed for the interaction of T31 with factor VIII, and T31 did not protect factor VIII from inactivation by aPC in a fluid-phase assay. Comparable results were found for a mutant of vWF that is monomeric at the N- terminus (vWF-dPRO). The lack of multimerization at the N-terminus may explain the decreased affinity of T31 and vWF-dPRO for factor VIII. Because of this decreased affinity, only a small fraction of factor VIII was bound to T31 and to vWF-dPRO. We hypothesized that this fraction was protected from inactivation by aPC but that this protection was not observed due to the presence of an excess of unbound factor VIII in the fluid phase. Therefore, vWF, T31, and vWF-dPRO were immobilized to separate bound factor VIII from unbound factor VIII in the fluid phase. Subsequently, the protective effect of these forms of vWF on bound factor VIII was studied. In this approach, all forms of vWF were able to protect factor VIII against inactivation by aPC completely. We conclude, in contrast with earlier work, that there is no discrepancy between binding of factor VIII to vWF and protection of factor VIII by vWF from inactivation by aPC. The protective effect of T31 was not recognized in previous studies due to its low affinity for factor VIII. The absence of multimerization observed for T31 and vWF- dPRO may explain the low affinity for factor VIII. No other domains than the binding site located at the D' domain were found to be involved in the protection of factor VIII from inactivation by aPC.