Identification of a sequence of human activated protein C (residues 390-404) essential for its anticoagulant activity

Activated protein C (APC) exerts its physiologic anticoagulant role by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. To identify the regions on the surface that mediate anticoagulant activity, 26 synthetic peptides were prepared representing 90% of the human protein C heavy chain primary structure and tested for their ability to inhibit APC anticoagulant activity. Peptide-(390-404) specifically inhibited APC activity in activated partial thromboplastin time and Xa-1-stage coagulation assays in normal, in protein S-depleted and Factor VIII-deficient plasma with 50% inhibition at 5 microM peptide. Polyclonal antibodies raised against this peptide and immunoaffinity-purified on a protein C-Sepharose column inhibited APC anticoagulant activity in activated partial thromboplastin time and Xa-1-stage assays in normal, protein S-depleted, and Factor VIII-deficient plasma with half-maximal inhibition at 30 nM anti-(390-404) antibody. Neither the peptide-(390-404) nor the anti-(390-404) antibodies inhibited APC amidolytic activity or the reaction of APC with recombinant [Arg358] alpha 1-antitrypsin. Furthermore, in a purified system, peptide-(390-404) inhibited APC-catalyzed inactivation of Factor Va in the presence as well as in the absence of phospholipids with 50% inhibition at 4 microM peptide. These data suggest that the region containing residues 390-404 in APC is essential for anticoagulant activity and is available to interact with antibodies or with other proteins such as the macromolecular substrates Factors Va or VIIIa.

Antithrombotic effects of combining activated protein C and urokinase in nonhuman primates

BACKGROUND

We have determined in vivo the relative antithrombotic efficacy and hemostatic safety of combining low-dose activated protein C (APC) and urokinase (urinary plasminogen activator, u-PA), two natural proteins that regulate thrombogenesis.

METHODS AND RESULTS

To model acute thrombotic responses of native blood under conditions of arterial flow, thrombogenic segments of Dacron vascular graft (VG) were incorporated into chronic exteriorized femoral arteriovenous (AV) access shunts in baboons. Thrombus formation on VG was determined by measuring 1) the deposition of autologous 111In platelets using real-time scintillation camera imaging, 2) the accumulation of 125I fibrin, 3) segment patency by Doppler flow analysis, and 4) blood tests for thrombosis, including plasma concentrations of platelet factor 4, beta-thromboglobulin, fibrinopeptide A (FPA), and D-dimer. Treatments consisting of low-dose and intermediate-dose APC (0.07 or 0.25 mg/kg.hr), u-PA (25,000 or 50,000 IU/kg.hr), or the combination were administered for 1 hour by continuous intravenous infusion. In untreated controls, platelets and fibrin accumulated rapidly, reaching plateau values at 1 hour of 15.1 +/- 3.8 x 10(9) platelets and 7.8 +/- 2.2 mg fibrin. Although the low-dose APC or u-PA alone did not decrease either platelet or fibrin deposition significantly, this combination moderately reduced both platelet and fibrin accumulation (7.3 +/- 2.6 x 10(9) platelets, p less than 0.05; 3.9 +/- 0.6 mg fibrin, p less than 0.05). Furthermore, intermediate-dose APC or u-PA reduced thrombus formation by half when administered alone (p less than 0.001 for both platelet and fibrin deposition), and the combination markedly interrupted the accumulation of platelets (3.0 +/- 1.0 x 10(9) platelets, p less than 0.001) and fibrin (1.3 +/- 0.6 mg fibrin, p less than 0.001). During active treatments, all VG segments remained patent. Hemostatic plug forming capability, as measured by template bleeding times, remained normal during all experiments (p greater than 0.05). The T50 clearance time for APC activity was not affected by the concurrent administration of u-PA. u-PA alone increased the plasma levels of D-dimer, FPA, and, interestingly, APC, implying that during pharmacological activation of the fibrinolytic system, thrombin activity was released, and the protein C pathway was activated.

CONCLUSIONS

A combination of intermediate-dose APC and u-PA produce substantial and efficient antithrombotic effects without impairing hemostatic function.

Identification of divalent metal ion-dependent inhibition of activated protein C by alpha 2-macroglobulin and alpha 2-antiplasmin in blood and comparisons to inhibition of factor Xa, thrombin, and plasmin

The half-life of activated protein C (APC) was 31 min in citrated blood and 18 min in whole blood. Immunoblotting analysis of citrated blood identified APC-protein C inhibitor (APC-PCI) and APC-alpha 1-antitrypsin complexes. Whole blood contained two additional APC-inhibitor complexes, one stimulated by Ca2+ and another by Mg2+. The former was identified as APC-alpha 2-macroglobulin (APC-alpha 2M) while the latter was not identified. APC-alpha 2-antiplasmin complexes (APC-alpha 2AP) were identified, comigrating with APC-PCI complexes. Purified alpha 2M and alpha 2AP inhibited APC in the presence of Ca2+ (k2 = 99 and 100 M-1 S-1, respectively. Inhibition of APC and Factor Xa by alpha 2M and inhibition of APC by alpha 2AP was stimulated by Ca2+, Mn2+, and Mg2+. Inhibition of thrombin by alpha 2M and of plasmin by alpha 2AP was not altered by EDTA or Ca2+, suggesting divalent metal ions affect APC and Factor Xa rather than the inhibitors. k2 values for the APC inhibitors and their plasma concentrations suggest that PCI and alpha 1-antitrypsin are the more important APC inhibitors and that alpha 2M and alpha 2AP are metal ion-dependent auxiliary inhibitors. Inhibitors can account for the in vivo half-life of APC.

Evidence of activation of the protein C pathway during acute vascular damage induced by Mediterranean spotted fever

Mediterranean spotted fever (MSF) is a rickettsiosis that induces widespread microvascular injury. To obtain quantitative information on the in vivo activation and inactivation of the protein C system during the acute phase of endothelial damage, several components of the protein C pathway were studied in 28 MSF patients. Upon admission (day 1), patients showed clear evidence of endothelial damage as reflected by the significant decrease in the ratio VIII:C/vWF:Ag (0.36 +/- 0.14, mean +/- SD) compared with normals (0.98 +/- 0.14), and clinical and laboratory signs of hemostatic alterations such as decreased platelet count, positive fibrinogen/fibrin degradation products, and increased thrombin:antithrombin-III complex levels. Antigenic protein C (72% +/- 18%) and protein C inhibitor (PCI) (41% +/- 20%) were significantly decreased (P less than .001). Complexes of activated protein C (APC) with PCI or with alpha 1-antitrypsin (alpha 1AT) and of plasma kallikrein with PCI (KK:PCI) were measured using sandwich enzyme-linked immunosorbent assays. APC:alpha 1AT complex levels were increased in patients at day 1 (27 +/- 13 ng/mL) compared with controls (7 +/- 2 ng/mL), and APC:PCI and KK:PCI complexes, which were not detectable in any of the controls, were present in 57% and 75% of the 28 MSF patients, with mean levels of 11 +/- 5 and 46 +/- 16 ng/mL, respectively. After remission of the disease (day 30), a trend toward normal values in the majority of the parameters studied was found. This study shows that, in the course of endothelial injury, MSF patients experience a generalized activation of the protein C pathway, resulting in consumption of protein C and PCI, and in the appearance of APC:inhibitor complexes. Moreover, these data provide the evidence that KK:PCI circulating complexes occur in vivo.

Urinary protein C inhibitor. Glycosaminoclycans synthesized by the epithelial kidney cell line TCL-598 enhance its interaction with urokinase

Protein C inhibitor (PCI), also known as plasminogen activator inhibitor 3, inhibits a variety of serine proteases by forming sodium dodecyl sulfate-stable 1:1 complexes. In purified systems PCI is only a weak inhibitor of urokinase. Nevertheless, complexes between PCI and urokinase are found in appreciable amounts in native human urine. Since PCI activity is stimulated by heparin and other glycosaminoglycans, we investigated the presence of stimulating glycosaminoglycans on cells lining the urinary tract. We chose the epithelial kidney tumor cell line TCL-598 as a model and isolated metabolically labeled glycosaminoglycans. TCL-598 incorporated [35S] sulfate into high Mr components (Mr greater than 200,000 and approximately 75,000) as judged from sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography of cell extracts; the Mr greater than 200,000 component bound specifically to PCI-Sepharose 4B and was eluted either with heparin (5 mg/ml) or with NaCl (2.0 M). Treatment of this PCI-binding material with chondroitinase ABC, but not with chondritinase AC or heparitinase, abolished binding to PCI-Sepharose, confirming the glycosaminoglycan nature of this material and suggesting the involvement of dermatan sulfate in binding. These glycosaminoglycans eluted from PCI-Sepharose stimulated urokinase inhibition by PCI in a dose-dependent way and enhanced complex formation of 125I-urokinase and PCI as did in control experiments dermatan sulfate from porcine skin and from bovine mucosa. Our results suggest that PCI activity might be regulated also in vivo by the presence or absence of stimulating glycosaminoglycans; dermatan sulfate-containing glycosaminoglycans associated with kidney cells might be responsible for stimulation of the urokinase inhibitory activity of PCI in the urinary tract; the type of glucosaminoclycans might furthermore regulate enzyme specificity of PCI.

Comparison of anticoagulant and procoagulant activities of stimulated platelets and platelet-derived microparticles

Activation of human platelets considerably enhanced their ability to accelerate factor Va inactivation by activated protein C (APC). The anticoagulant activity of platelet suspensions was markedly dependent on the kind of agonist used to activate platelets. APC-catalyzed factor Va inactivation in free solution was characterized by an apparent second-order rate constant of 2 x 10(5) (mol/L)-1 (seconds)-1. Nonstimulated platelets (2.4 x 10(8)/mL) and platelets stimulated with adenosine diphosphate or adrenalin accelerated factor Va inactivation fourfold. Rates of factor Va inactivation were increased 11-fold by thrombin-stimulated platelets, 29-fold after platelet stimulation with the Ca(2+)-ionophore A23187. At low platelet concentrations (3 x 10(7)/mL) only background levels of anticoagulant activity were observed in platelet suspensions that were nonstimulated or stimulated with thrombin or collagen. However, when such reaction mixtures were stirred during the activation procedure, platelet anticoagulant activity was increased more than 10-fold. Independent of platelet stimulation and stirring conditions, exogenously added purified plasma protein S increased platelet-dependent factor Va inactivation approximately twofold. Addition of a neutralizing antiprotein S antibody had little effect on the anticoagulant activity of platelets. This indicates that, under the reaction conditions tested, platelet-released protein S did not contribute to factor Va inactivation. Approximately 25% of the anticoagulant activity of stimulated platelet suspensions appeared to be associated with microparticles that were released on platelet activation. Such microparticles may provide an important source of anticoagulant activity. A similar distribution of procoagulant, ie, prothrombinase, activity between platelets and microparticles was observed for the same platelet suspensions. Because platelet stimulation and stirring also had the same overall effects on the ability of platelets and platelet microparticles to promote prothrombin activation and factor Va inactivation, it appears likely that the generation of potential platelet anticoagulant and procoagulant activities is coupled to the same platelet stimulation reactions.

Complexes of activated protein C with alpha 1-antitrypsin in normal pregnancy and in severe preeclampsia

Protein C is a vitamin K-dependent regulator of blood coagulation. Activated protein C is regulated in plasma in large part by two inhibitors, protein C inhibitor and alpha 1-antitrypsin. Complexes of activated protein C with both inhibitors in plasma samples from subjects with normal or pathologic pregnancy were measured. In normal pregnancy we observed a progressive and significant increase in activated protein C/alpha 1-antitrypsin complex levels, from 9 +/- 3 ng/ml in the first trimester to 16 +/- 3 ng/ml in the third trimester, as well as an increase in alpha 1-antitrypsin plasma levels. In severe preeclampsia, but not in chronic hypertension with superimposed severe preeclampsia, there was a greater increase in activated protein C/alpha 1-antitrypsin levels (25 +/- 10 ng/ml) (p less than 0.001) and a decrease in protein C and protein C inhibitor levels as compared with normal pregnant women at similar gestational ages. These data show an increase in the activation of the protein C pathway in both normal and pathologic pregnancy and provide evidence for an enhancement of thrombin generation in severe preeclampsia compared with chronic hypertension with superimposed severe preeclampsia.

In vivo and in vitro complexes of activated protein C with two inhibitors in baboons

In vivo complex formation of activated protein C with protein C inhibitor (APC-PCI) and with alpha 1-antitrypsin (APC-alpha 1AT) following infusion of 0.25 or 1.0 mg APC/kg in 1 hour into baboons was studied using immunoblotting and sandwich enzyme-linked immunosorbent assay (ELISA)s. Before APC infusion, detectable plasma levels (about 30 ng/mL) of APC-alpha 1AT complex were found in the baboon plasma. At the lower APC dose, APC-PCI and APC-alpha 1AT complex levels were 1.4 +/- 0.3 (mean +/- SD) and 0.8 +/- 0.1 microgram/mL after 1 hour of infusion. At the higher APC dose, the APC-PCI level was similar to the APC-alpha 1AT level during the first 30 minutes, but after 1 hour of infusion the APC-alpha 1AT level was higher than the APC-PCI level, reaching 4.1 +/- 1.2 and 2.9 +/- 1.2 microgram/mL, respectively. After 24 hours, complex levels had returned to basal conditions. During infusion of protein C (1.0 mg/kg in 1 hour), both complexes were detected in low concentrations. Following bolus injection of APC, half-lives (t1/2) for APC and APC-PCI and APC-alpha 1AT complexes of 10, 40, and 140 minutes, respectively, were observed. After 1-hour incubation with 2.5 micrograms/mL APC, baboon plasma contained 1.0 +/- 0.2 and 0.8 +/- 0.1 microgram/mL of APC-PCI and APC-alpha 1AT, respectively. Addition of 10 micrograms/mL APC to baboon plasma yielded 2.5 and 2.4 micrograms/mL APC-PCI and APC-alpha 1AT after 1 hour, respectively. Immunoblotting analysis also showed in vivo formation of complexes of APC with an auxilliary inhibitor but not in vitro in citrated plasma. These data show that both PCI and alpha 1AT are physiologic inhibitors of APC and suggest that when PCI is depleted by a high dose of APC, alpha 1AT becomes the major inhibitor of APC.

Interaction of plasma kallikrein with protein C inhibitor in purified mixtures and in plasma

The interaction between plasma kallikrein (KK) and protein C inhibitor (PCI) and the influence of KK on the complex formation between activated protein C (APC) and PCI was studied in purified systems as well as in plasma in order to assess the significance of these reactions in the plasma milieu. PCI complexed to KK (KK:PCI) or to APC (APC:PCI) was measured by sandwich ELISA's using antibodies directed against each protein in the complexes. The formation of KK:PCI complexes assayed by this method paralleled the inhibition of KK amidolytic activity by PCI in purified system. Incubation of normal plasma (NHP) at 4 degrees C, which can induce prekallikrein activation due to cold activation, resulted in PCI inactivation and appearance of KK:PCI complexes. PCI activity fell to 35% of the NHP and 1.2 micrograms/ml of KK:PCI complex was formed. However, incubation of NHP at room temperature or of prekallikrein deficient plasma at 4 degrees C did not result in significant decrease of PCI activity. Thus the PCI inactivation was associated with prekallikrein activation and complexation to PCI following cold activation. Incubation of exogenous purified KK with NHP resulted in PCI inactivation and complexation with KK in a temperature-dependent manner. Addition of 2.8 micrograms/ml KK to plasma at 4 degrees C resulted in the inactivation of 55% of plasma PCI and the formation of 0.9 microgram/ml KK:PCI which represents 21% of the KK added, whereas at 37 degrees C PCI was inactivated to 30% and only 0.30 microgram/ml KK:PCI complexes were measured. These results indicate that PCI is a major KK inhibitor at 4 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Elucidating the structural chemistry of glycosaminoglycan recognition by protein C inhibitor

Glycosaminoglycans (GAGs) including heparin accelerate the inhibition of serine proteases by serine protease inhibitors (serpins), an essential process in regulating blood coagulation. to analyze the molecular basis for GAG recognition by the plasma serpin protein C inhibitor (PCI; also known as plasminogen activator inhibitor 3), we have constructed a complete, energy-minimized, three-dimensional model of PCI by using the structure of homologous alpha 1-antitrypsin as a template. Sequence analysis, hydrogen-bonding environment, and shape complementarity suggested that the N-terminal residues of PCI, which are not homologous to those of alpha 1-antitrypsin, form an amphipathic alpha-helix, here designated A+ since it precedes the alpha 1-antitrypsin A helix. Electrostatic calculations revealed a single, highly positive surface region arising from both the A+ and H helices, suggesting that this two-helix motif is required for GAG binding by PCI. The dominant role of electrostatic interactions in PCI-heparin binding was confirmed by the strong ionic strength dependence of heparin stimulation. The involvement of the A+ helix in heparin binding was verified by demonstrating that an anti-PCI antibody that specifically binds the A+ peptide blocks heparin binding.

Inhibition of thrombus formation by activated recombinant protein C in a primate model of arterial thrombosis

Activated protein C (APC) is an antithrombotic enzyme. The therapeutic potential of infused human recombinant APC (rAPC) was studied in a primate model of platelet-dependent thrombosis. Eight baboons with chronic femoral arteriovenous shunts received rAPC infusions for 1 hour. The shunts were extended with 5-cm long, 4-mm-i.d. thrombogenic Dacron graft segments for the time of infusion. The plasma level of the enzyme, the blood flow in the shunt, and the deposition of indium-111-labeled platelets and iodine-125 fibrinogen on the graft were measured. The influence of rAPC infused at doses of 0.25 and 1.0 mg/kg-hr was compared with the effects of control infusions of saline. Five of eight control grafts occluded within 60 minutes, whereas there was no change in the blood flow during rAPC infusion. Deposition of platelets was inhibited by 13 +/- 10% and by 42 +/- 13% (mean +/- SEM) after 30 minutes of infusion at the two doses, which gave rise to circulating rAPC plasma concentrations of 0.4 and 1.9 mg/l, respectively. Both doses significantly inhibited fibrin deposition in the graft. Circulating plasma markers of thrombus formation and of fibrinolysis did not increase significantly during rAPC infusion; measurements of bleeding time were also within normal limits. Thus, rAPC, like human plasma-derived APC, inhibited thrombus formation without impairing primary hemostasis.

Determination of plasma protein C inhibitor and of two activated protein C-inhibitor complexes in normals and in patients with intravascular coagulation and thrombotic disease

We developed an ELISA to quantitate complexes of activated protein C (APC) with a major plasma APC inhibitor, alpha 1-antitrypsin (alpha 1AT) in human plasma based on the sandwich principle using two different antibodies directed towards protein C and alpha 1AT, respectively. This ELISA test was specific for APC:alpha 1AT complexes and sensitive to greater than or equal to 150 pg complex. Fifty-one of 56 healthy donors had APC:alpha 1AT complex levels above the detection limit (3 ng/ml) ranging from 4 to 14 ng/ml (mean value +/- SD: 7.6 +/- 2.5 ng/ml). Patients (n = 10) with disseminated intravascular coagulation (DIC) had detectable levels of APC:alpha 1AT complex ranging from 21 to 125 ng/ml (median: 69 ng/ml). Complexes of APC with plasma protein C inhibitor (PCI) were also measured using an ELISA sandwich assay. None of the 30 healthy donors had detectable levels (greater than or equal to 5 ng/ml) of APC:PCI complex, and plasma samples from 9 of 10 DIC patients had detectable concentrations of APC:PCI complex ranging from 10 to 63 ng/ml (median: 22 ng/ml). APC:alpha 1AT complex was detected in 25 of 26 patients with deep venous thrombosis (DVT), with levels ranging from 5 to 136 ng/ml (median: 23 ng/ml), whereas APC:PCI was detected in only 6 DVT patients, with levels between 11 and 105 ng/ml. PCI antigen levels in 70 normals ranged from 56 to 175% (mean +/- SD: 99.1% +/- 24.2%). PCI antigen levels were decreased in DIC patients, in patients with cerebral arterial thrombosis, and in DVT patients undergoing heparin therapy, but not in patients with myocardial infarction. PCI antigen levels were decreased much further in DVT patients receiving heparin compared to those not receiving heparin, showing that heparin therapy is associated with a decrease in PCI levels. The detection in normal subjects and in thrombotic patients of circulating APC:inhibitor complexes supports the view that the protein C pathway is activated during DIC and DVT. Moreover, it emphasizes that both PCI and alpha 1AT are physiologic inhibitors of APC. Thus, measurement of APC complexes may provide sensitive parameters for specific detection of activation of the clotting and protein C pathways.

Orientation of the putative recognition helix in the DNA-binding domain of Hin recombinase complexed with the hix site

On the basis of sequence similarity with other known DNA-binding proteins, the DNA-binding domain of Hin recombinase, residues 139-190, is thought to bind DNA by a helix-turn-helix motif. Two models can be considered that differ in the orientation of the recognition helix in the major groove of DNA. One is based on the orientation of the recognition helix found in the 434 repressor (1-69) and lambda repressor-DNA cocrystals, and the other is based on the NMR studies of lac repressor headpiece. Cleavage by EDTA.Fe attached to a lysine side chain (Ser183----Lys183) near the COOH terminus of Hin(139-184) reveals that the putative recognition helix is oriented toward the center of the inverted repeats in a manner similar to that seen in the 434 and lambda repressor-DNA cocrystals.

Inhibition of activated protein C by recombinant alpha 1-antitrypsin variants with substitution of arginine or leucine for methionine358

alpha 1-Antitrypsin (alpha 1-AT) was recently identified as a major physiologic plasma inhibitor of activated protein C. The reaction with activated protein C of recombinant alpha 1-AT containing amino acid substitutions at the reactive center was studied. The substitution of Arg358 for Met, as observed in a patient with a severe bleeding disorder with the mutant alpha 1-AT Pittsburgh, increased the association rate constant for activated protein C from 1.1 x 10(1) to 4.9 x 10(4) M-1 s-1. The association rate constant of activated protein C with protein C inhibitor, a native plasma serpin that contains Arg354 at the reactive site, is 6 x 10(3) M-1 s-1 in the absence of heparin. Plasma containing 4 microM [Arg358]alpha 1-AT inhibited activated protein C activity by greater than 95% in 15 s, and the inhibited activated protein C was shown by immunoblotting to exist as activated protein C-inhibitor complexes. In controls 50% loss of activated protein C activity in normal plasma occurred in 19 min. Double-substituted [Pro357,Met358]alpha 1-AT----[Ala357,Arg358]alpha 1-AT had similar reactivity toward activated protein C as the single-substituted [Arg358]alpha 1-AT. Thus, replacement of the reactive center Met358 of alpha 1-AT by Arg358, analogous to Arg354 of protein C inhibitor, results in an activated protein C inhibitor that is more potent than either of the native inhibitors. Comparison of the association rate constant of the [Arg358]alpha 1-AT for activated protein C to that for thrombin (4 x 10(4) versus 3 x 10(5) M-1 s-1) suggests that thrombin would be more effectively inhibited than activated protein C, thereby giving an explanation for bleeding rather than thrombosis in the alpha 1-AT Pittsburgh patient.

Determination of functional and antigenic protein C inhibitor and its complexes with activated protein C in plasma by ELISA's

Enzyme-linked immunosorbent assays (ELISA's) were developed for the measurement of protein C inhibitor (PCI) antigen and activity and for its complexes with activated protein C (APC) in plasma. For PCI activity and antigen, APC or anti-PCI, respectively, was immobilized to microtiter plates and PCI bound was detected with labelled anti-PCI antibodies. For APC:PCI complexes, two different antibodies directed against protein C and PCI were used. The assays for PCI were calibrated with pooled normal human plasma (NHP) and with purified PCI, and for APC:PCI complexes with known concentrations of purified pre-formed complexes added to buffer or to plasma. The lower limit of sensitivity of the PCI activity and antigen assays was 10 ng/ml and 0.5 ng/ml, respectively and for plasma APC:PCI complexes 12 ng/ml. Mean coefficients of variation of 1.5% to 5.8% (intra-assay) and 2.1% to 9.8% (inter-assay) were found for the assays. For PCI antigen, a range of 56% to 162% of the NHP value was obtained in samples from 70 healthy donors (mean +/- SD = 98.6% +/- 23.1%). For PCI activity, the range was 59% to 148% (94.3% +/- 20.2). A good correlation (0.92) was obtained when both assays were compared. Plasma levels of APC:PCI complexes in 30 normals were under the detection limit (less than 12 ng/ml). In plasma samples from 10 patients with disseminated intravascular coagulation (DIC) PCI antigen concentrations were decreased (55.6% +/- 20%) and 8 of the patients had APC-PCI complex levels between 32 and 240 ng/ml (median, 35 ng/ml). After addition of 20 micrograms/ml APC to NHP or to protein C depleted plasma, 6.1 micrograms/ml complexes were recovered after 90 min incubation. Incubation of 10 micrograms/ml APC with NHP in the presence of 10 U/ml heparin yielded 11 micrograms/ml complexes after 90 min, which represent more than 90% of the maximum possible value. Thus, the method should be adequate to study complexes of APC in vivo in clinical conditions in which activation of protein C pathway may occur.

Complex formation between urokinase and plasma protein C inhibitor in vitro and in vivo

Protein C inhibitor (PCI) and plasminogen activator inhibitor 3 (PAI-3; urinary urokinase inhibitor) are immunologically identical. The role of PCI for urokinase (uPA) inhibition in vivo was investigated. We therefore developed an enzyme-linked immunosorbent assay (ELISA) specific for uPA-PCI complexes: Rabbit anti-PCI IgG was immobilized on a microtiter plate and following incubation with uPA-PCI complex-containing samples, bound uPA-PCI complexes were quantified with a horseradish-peroxidase-linked monoclonal antibody (MoAb) to uPA. Using this assay, time, dose, and heparin-dependent complexes were detected when uPA was incubated with normal plasma or purified urinary PCI, whereas no complexes were measurable using PCI-immunodepleted plasma. Plasma samples (containing 20 mmol/L benzamidine to prevent complex formation ex vivo) from patients undergoing systemic urokinase therapy (1 x 10(6) IU/60 min intravenously [IV]) after myocardial infarction were also studied. uPA present in these plasma samples (up to 1,200 ng/mL) had only 43% to 70% of the specific activity of purified 2-chain uPA, suggesting that a major portion of uPA is complexed to inhibitors. In these plasma samples uPA-PCI complexes were present in a concentration corresponding to 21% to 25% of inactive uPA antigen. These data suggest that at high uPA concentrations, such as during uPA therapy, plasma PCI might contribute significantly to uPA inhibition in vivo.

Purification and characterization of plasma protein C inhibitor

Plasma protein C inhibitor (PCI) was purified to homogeneity (greater than 95%) with good recovery (greater than 25%) and reproducibility, and the inhibition of a number of blood clotting and fibrinolytic enzymes by purified PCI was studied. PCI inhibited activated protein C (APC), two-chain urokinase (2c-uPA), two-chain tissue plasminogen activator (2c-tPA), thrombin, factor Xa, plasma kallikrein and factor XIa, and this inhibition was accelerated by heparin. The inhibition of each enzyme was accompanied by formation of enzyme inhibitor complexes and by degradation of the inhibitor to lower molecular weight derivatives. Plasma kallikrein and factor XIa cleaved PCI of native Mr = 57,000 into two products with Mr = 54,000 and 52,000 whereas the other enzymes converted the PCI to a product with Mr = 54,000. PCI did not detectably inhibit alpha-factor XIIa or plasmin. Kinetic studies using PCI yielded the following second-order rate constants for inhibition of human APC, 2c-uPA, 2c-tPA, thrombin, factor Xa, kallikrein and factor XIa respectively: 0.65 x 10(4), 0.22 x 10(4), 0.08 x 10(4), 0.61 x 10(4), 2.01 x 10(4), 6.50 x 10(4), and 9.03 x 10(4) M-1s-1 in the absence of heparin and 1.58 x 10(6), 0.43 x 10(6), 0.03 x 10(6), 0.52 x 10(6), 0.09 x 10(6), 0.18 x 10(6) and 0.74 x 10(6) M-1s-1 in the presence of optimal concentrations of heparin. The rate constants for the inhibition of factor XIa and 2c-uPA by PCI suggest a possible role of PCI in the physiologic regulation of these enzymes. The second order rate constants for inhibition of bovine APC and Gla-domainless bovine APC by human PCI were 0.61 x 10(4) and 0.26 x 10(4) M-1s-1 in the absence of heparin and 0.54 x 10(6) and 0.71 x 10(6) M-1s-1 in the presence of heparin, respectively. Calcium ions (0.05 to 4 mM) did not affect these rate constants. The results obtained with normal and Gla-domainless APC indicate that the Gla domain of APC is not required for inactivation by PGI and is not essential for the heparin stimulation of this reaction.

Interaction of type 1 plasminogen activator inhibitor with the enzymes of the contact activation system

The interaction between type 1 plasminogen activator inhibitor (PAI-1), a serine protease inhibitor, and the three serine proteases generated during contact activation of plasma was studied using functional and immunologic approaches. Incubation of Factor XIIa, Factor XIa, and plasma kallikrein with either purified PAI-1 or platelet-derived PAI-1 resulted in the formation of sodium dodecyl sulfate-stable complexes as revealed by immunoblotting techniques. Functional assays indicated that Factor XIa and, to a lesser extent, Factor XIIa and plasma kallikrein neutralized the ability of purified PAI-1 to bind to immobilized tissue-type plasminogen activator (t-PA). Immunoblotting demonstrated that these enzymes also neutralized the ability of PAI-1 to form complexes with fluid-phase t-PA. Clot lysis assays employing purified proteins and 125I-fibrinogen were used to investigate the profibrinolytic effect of these contact activation enzymes. At enzyme concentrations that did not result in direct activation of plasminogen, only Factor XIa was capable of stimulating the lysis of clots supplemented with both t-PA and PAI-1. As a consequence of their interactions with PAI-1, the amidolytic activity of Factor XIIa, Factor XIa, and plasma kallikrein was neutralized by this inhibitor in a time-dependent and concentration-dependent manner. Minimum values estimated for the apparent second-order rate constant of inhibition were 1.6 x 10(4), 2.1 x 10(5), and 6.0 x 10(4) M-1 s-1 for Factor XIIa, Factor XIa, and plasma kallikrein, respectively. These data define new reactions between coagulation and fibrinolysis proteins and suggest that a major mechanism for stimulation of the intrinsic fibrinolytic pathway may involve neutralization of PAI-1 by Factor XIa.

A comparison between activated protein C and des-1-41-light chain-activated protein C in reactions with type 1 plasminogen activator inhibitor

This study investigates the role of the gamma-carboxyglutamic acid (gla) containing domain of activated protein C in interactions with both platelet-derived and purified type 1 plasminogen activator inhibitor (PAI-1). The activity of human platelet PAI-1 was neutralized to the same extent by bovine activated protein C and bovine des-1-41-light chain-activated protein C. Both forms of activated protein C formed SDS-stable, divalent-cation independent complexes with platelet PAI-1, as demonstrated by immunoblotting using antibodies directed to either protein C or PAI-1. Since activated protein C neutralized PAI-1, the potential inhibition of the enzyme by PAI-1 was studied. Purified PAI-1 inhibited the amidolytic activity of bovine-activated protein C and bovine des-1-41-light chain-activated protein C with a k2 of 2.85 X 10(4) M-1 sec-1 for both proteins. These data suggest that the gla domain of activated protein C is not required for neutralization of PAI-1 activity, for complex formation with PAI-1, or for inhibition of the amidolytic activity of activated protein C by PAI-1.

Familial protein S deficiency with a variant protein S molecule in plasma and platelets

A protein S deficient family presenting a variant protein S molecule in plasma and platelets is described. The propositus, age 20, and two brothers suffered from venous thrombotic disease. The propositus, the only family member studied while taking oral anticoagulants, had a protein S antigen (ag) level of 17% and undetectable activity. As demonstrated by immunoblotting both the propositus and one clinically affected brother (42% ag, 7% activity) presented variant protein S molecules of 65,000 molecular weight (mol wt) while the other clinically affected brother (64% ag, 11% activity) had only protein S with normal electrophoretic mobility of 70,000 mol wt. The mother had normal protein S levels (93% ag, 100% activity) but had both normal and variant protein S molecules and based on her functional protein S data a normal anticoagulant activity of the variant molecule is suggested. One asymptomatic but protein S deficient sister (68% ag, 9% activity) as well as the asymptomatic protein S deficient father (59% ag, 10% activity) had only protein S molecules of 70,000 mol wt. The variant protein S bound to C4b-binding protein in plasma, and differed from normal protein S in carbohydrate content. Platelets of each family member contained the same immunoblotting pattern of normal and variant protein S forms as found in plasma, consistent with the hypothesis that protein S gene expression involves codominant expression of two alleles that is similar in cells that control the synthesis of both platelet and plasma forms of protein S.