Comparison of plasminogen activator inhibitor activity and antigen in plasma samples

Fibrinolytic system in normotensive subjects and hypertensive patients

The aim of this study was to evaluate the fibrinolytic system by measurement of fibrinogen, plasminogen, tissue-type plasminogen activator (t-PA), and plasminogen activator inhibitor-1 (PAI-1) in healthy normotensive subjects and in patients with essential hypertension. A group of 21 healthy normotensive subjects [age, 39.2 +/- 1.8 years; 8 males, 13 females; body mass index (BMI) = 27.9 kg/m] and 42 patients with untreated essential hypertension (age, 47.6 +/- 1.7 years; 19 males, 23 females; BMI = 28.3 kg/m) were studied. Blood samples and clinical measurement were taken between 7 am and 9 am by an observer in a blind fashion. The systolic/diastolic blood pressure of normotensive subjects was 121.3 +/- 2.5/78.4 +/- 1.3 mm Hg and that of hypertensive patients was 166.4 +/- 4.3/102.9 +/- 1.83 mm Hg, measured in the sitting position. Plasma fibrinogen levels in the normotensive and hypertensive individuals were 295.7 +/- 9.4 mg/dL and 305.67 +/- 10.9 mg/dL, respectively (P = 0.456). The corresponding values for plasminogen were 71.4 +/- 3.8% and 89.5 +/- 2.5%, (P = 0.0031), for t-PA were 6.3 +/- 0.5 ng/mL and 7.6 +/- 0.4 ng/mL (P = 0.0487), and for PAI-1 were 46.9 +/- 5.1 ng/mL and 63.0 +/- 5.6 ng/mL (P = 0.0324), respectively. In conclusion, patients with essential hypertension have disequilibrium in the fibrinolytic system with a tendency toward a hypercoagulability state when compared with normotensive subjects. This state could explain, in part, the thrombotic complications that occur with a higher frequency in hypertensive patients as compared with normotensive subjects.

Association of blood pressure with fibrinolytic potential in the Framingham offspring population

BACKGROUND

Hypertension is an established risk factor for acute coronary events. Because fibrinolytic and hemostatic factors are also associated with cardiovascular disease, we examined the relations of systolic and diastolic blood pressures (SBP and DBP) to levels of plasminogen activator inhibitor antigen, tissue plasminogen activator antigen, fibrinogen, factor VII, von Willebrand factor, fibrinogen, and plasma viscosity in subjects of the Framingham Offspring Study.

METHODS AND RESULTS

We studied 1193 men and 1459 women after the exclusion of subjects with known cardiovascular disease and those receiving anticoagulant or antihypertensive therapy. Linear regression models were used to evaluate SBP and DBP as predictors of fibrinolytic and hemostatic factor levels in separate sex models, with adjustment for age, body mass index, smoking, diabetes, total cholesterol, HDL, triglycerides, alcohol intake, and estrogen use (in women). In both sexes, levels of plasminogen activator inhibitor and tissue plasminogen activator antigen were positively related to SBP and DBP (P<0.001). Plasma viscosity was positively related to SBP (P=0.008) and DBP (P=0.001) in women only. There was no association between SBP or DBP and fibrinogen, factor VII, or von Willebrand factor in either sex.

CONCLUSIONS

These data suggest that impaired fibrinolysis may play an important role in the pathogenesis of cardiovascular disease in hypertensive patients.

Fibrinolytic potential is significantly increased by oestrogen treatment in postmenopausal women with mild dyslipidaemia

OBJECTIVE

To study the effects of oestrogen replacement treatment on fibrinolytic potential in postmenopausal women.

DESIGN

Randomised, double blind, placebo controlled trial of oral 17 beta-oestradiol.

SETTING

Subjects were evaluated in the outpatient setting.

PATIENTS

Nineteen postmenopausal women with mild dyslipidaemia, aged 44 to 69 years (mean (SD) 55.7 (6.7)).

MAIN OUTCOME MEASURES

Fibrinolytic activity (fibrin plate assay) and tissue plasminogen activator (t-PA) antigen were measured at baseline and after three, six, and nine weeks of each treatment.

RESULTS

After nine weeks of 2 mg oestradiol treatment, there was a significant increase in fibrinolytic potential compared with placebo, as indicated by an increase in fibrinolytic activity (mean (SEM), 80 (9) v 54 (5) mm2 of lysis in the fibrin plate, 2 mg v placebo, p = 0.002) and a decrease in t-PA antigen (5.8 (0.9) v 8.4 (1.2) ng/ml, 2 mg v placebo, p < 0.001). There was a similar trend with the 1 mg dose but the changes were less noticeable.

CONCLUSIONS

Hormone replacement treatment with 17 beta-oestradiol for nine weeks significantly increased fibrinolytic potential in postmenopausal women with mild dyslipidaemia. This suggests that the cardioprotective effect of oestrogen may be mediated, in part, by an increase in fibrinolytic potential.

Hypobetalipoproteinemia is associated with low levels of hemostatic risk factors in the Framingham offspring population

BACKGROUND

Given the importance of thrombosis in causation of acute coronary syndromes, it is possible that the beneficial effect of low lipid levels on the risk of coronary events is achieved by lowering thrombotic potential of the blood. Hypobetalipoproteinemia is characterized by plasma concentrations of apolipoprotein B and LDL cholesterol that are one third of those observed in the general population. The aim of this study was to utilize subjects with hypobetalipoproteinemia to examine the relation between thrombotic potential and low levels of LDL cholesterol.

METHODS AND RESULTS

Hemostatic risk factors were measured in 1878 individuals (1003 women and 875 men) participating in cycle 5 of the Framingham Offspring Study. The subjects were divided into five groups on the basis of LDL cholesterol level. Subjects with hypobetalipoproteinemia (LDL cholesterol < 70 mg/dL) had the lowest levels of fibrinogen, plasminogen activator inhibitor-1 antigen, and tissue plasminogen activator antigen. As LDL cholesterol increased, there was a significant increase in the levels of the hemostatic risk factors, with the exception of von Willebrand factor antigen. Adjustment with multivariate regression analyses for the covariates age, sex, body mass index, diabetes mellitus, smoking, alcohol intake, triglyceride level, and use of antihypertensive medication did not materially alter the results.

CONCLUSIONS

Decreasing levels of LDL cholesterol are associated with decreasing levels of hemostatic risk factors. Subjects with hypobetalipoproteinemia have the lowest levels of hemostatic risk factors and may be protected against thrombotic complications of atherosclerotic cardiovascular disease because of reduced thrombotic potential. One mechanism by which lipid-lowering therapy may decrease clinical cardiac events is through a reduction in thrombotic tendency.

Quantification of plasminogen activators and their inhibitors in the aortic vessel wall in relation to the presence and severity of atherosclerotic disease

Increased expression of plasminogen activator inhibitor-1 (PAI-1) has been demonstrated in the human atherosclerotic vessel wall and may contribute to the impaired plasma fibrinolytic capacity in patients at high risk of atherothrombotic events. In addition, the mural PA/plasmin system may have important pathobiologic functions during atherogenesis. We quantitatively analyzed PAs of the tissue type (TPA) and urokinase type (UPA), PAIs, and plasminogen in protein extracts from different layers of human aorta in relation to the presence and severity of atherosclerotic lesions. In comparison with normal control vessels, intimal and neointimal TPA concentrations were reduced in atherosclerotic aortas except in the necrotic core areas of advanced plaques, where TPA was mainly complexed to PAI-1 in extracellular matrix deposits. In the media, TPA antigen was higher in lesional segments and closely associated with smooth muscle cells. UPA antigen was increased in the intima of atherosclerotic lesions and colocalized with tissue-infiltrating macrophages and neointimal smooth muscle cells. By spectrophotometric assay, neither TPA nor UPA activity could be detected in intimal or medial extracts. PAI-1 concentrations increased significantly in the intima of atherosclerotic segments compared with adjacent uninvolved areas or control aortas. The immunohistochemical distribution of PAI-1 was similar to that observed for TPA. A large excess of PAI-1 over PA concentrations, particularly in the intimal layer, characterizes atherosclerotic lesions of the human aorta and suggests that PA action is locally confined and counterbalanced by enhanced PAI expression and accumulation.

Association between increased estrogen status and increased fibrinolytic potential in the Framingham Offspring Study

BACKGROUND

Although extensive evidence indicates that estrogen is responsible for the markedly decreased cardiovascular risk of premenopausal women, the mechanism through which estrogen might exert its protective effect has not been adequately explained. Since thrombosis is now recognized to play an important role in the onset of cardiovascular disease, we investigated the relation between estrogen status and fibrinolytic potential, a determinant of thrombotic risk.

METHODS AND RESULTS

We determined levels of plasminogen activator inhibitor (PAI-1) antigen and tissue plasminogen activator (TPA) antigen in 1431 subjects from the Framingham Offspring Study. Fibrinolytic potential was compared between subjects with high estrogen status (premenopausal women and postmenopausal women receiving hormone replacement therapy) and low estrogen status (men and postmenopausal women not receiving hormone replacement therapy). In all comparisons, subjects with high estrogen status had greater fibrinolytic potential (lower PAI-1 levels) than subjects with low estrogen status. First, postmenopausal women receiving estrogen replacement therapy had lower levels of PAI-1 than those not receiving therapy (13.0 +/- 0.5 versus 19.5 +/- 1.0 ng/mL, P < .001). Second, premenopausal women had lower levels of PAI-1 than men of a similar age (14.8 +/- 0.6 versus 20.3 +/- 0.8 ng/mL, P < .001); this sex difference diminished when postmenopausal women not receiving hormone replacement therapy were compared with men of a similar age (19.6 +/- 0.7 versus 21.1 +/- 0.7 ng/mL, P = .089). Third, premenopausal women had markedly lower levels of PAI-1 antigen than postmenopausal women not receiving estrogen therapy (14.8 +/- 0.6 versus 19.5 +/- 1.0 ng/mL, P < .001). The between-group differences observed for TPA antigen were similar to those for PAI-1 antigen.

CONCLUSIONS

Each of these comparisons indicates that the cardioprotective effect of estrogen may be mediated, in part, by an increase in fibrinolytic potential. These findings might provide at least a partial explanation for the protection against cardiovascular disease experienced by premenopausal women, and the loss of that protection following menopause.

Identification of a PAI-1 binding site in vitronectin

Active PAI-1 (plasminogen activator inhibitor 1) is bound to vitronectin in plasma and in the extracellular matrix. In this study we aimed at identifying the PAI-1 binding site in vitronectin, which at present is a matter of dispute. Vitronectin was cleaved with trypsin and the fragments were tested for inhibitory effect on the PAI-1/vitronectin interaction using vitronectin-coated microtiter plates. Intact vitronectin and the tryptic digest of vitronectin both caused a 50% reduction in PAI-1 binding at a concentration of about 2 nmol/I. Gel-filtration on Sephadex G-50 superfine of the tryptic peptides resulted in one main peak of inhibitory activity. The elution volume, Kav, was 0.55 indicating (a) medium-size peptide(s). The peptide was further purified by reverse-phase HPLC. Structural analysis revealed that it constituted the 45 NH2-terminal amino-acid residues in vitronectin. The NH2-terminal vitronectin peptide caused a 50% decrease in PAI-1 binding to the vitronectin-coated microtiter plates at a concentration of about 13 nmol/l. Thus, the peptide is a little less effective in this respect than intact vitronectin. Reduced and S-carboxymethylated peptide had no effect on the interaction. The NH2-terminal vitronectin fragment increased the stability of active PAI-1 by about 60%, which is a little less than with intact vitronectin. The peptide also prevented PAI-1 from oxidation with chloramine T. The half-life was prolonged about 4-fold as compared to about 30-fold with intact vitronectin.

High plasma levels of plasminogen activator inhibitor 1 (PAI-1) in polycythemia vera and essential thrombocythemia are associated with thrombosis

We attempted to determine if a hypercoagulability state exists in patients with polycythemia vera (PV) and essential thrombocythemia (ET). We studied the hematocrit level, platelet count, use of any antiaggregant drugs, thrombotic or bleeding accidents and plasma levels of antithrombin III, protein C, total protein S, free protein S, vWF:Ag (Von Willebrand's factor related antigen), thrombin-antithrombin complexes, D-dimer, fibrinolytic activity, tissue plasminogen activator, plasminogen and PAI-1 in 33 patients (19 with ET and 14 with PV). PAI-1 plasma concentration was significantly higher in, both ET and PV patients than in the control group, and were higher in those patients with previous thrombotic episodes than in asymptomatic patients or with previous bleeding episodes. Increasing age was associated to more thrombotic episodes while younger patients presented with more hemorrhagic complications. A linear correlation between platelet count and PAI-1 levels in PV patients (r = 0.44, p < 0.05) and ET patients (r = 0.30, p < 0.05) was found. Fibrinolytic activity in patients with ET was significantly decreased when compared to the control group. A hypofibrinolytic state could be an additional factor which could be used as a predictive index of the thrombotic or bleeding tendency in each patient.

Predictive value for thrombotic disease of plasminogen activator inhibitor-1 plasma levels

Plasminogen activator inhibitor-1 plays a major role in the fibrinolytic system as the main physiological inhibitor of both tissue-type and urinary-type plasminogen activators. The inhibitor is present in plasma in small amounts and derives mainly from endothelial cells. Positive correlations have been reported between plasma levels and different parameters, such as serum triglycerides, insulin plasma levels and body mass index. Moreover, high plasma inhibitor concentrations have been observed in different disease states, but it must be stressed that plasminogen activator inhibitor-1 behaves as an acute-phase reactant and measurement of plasma levels is not significant in the acute phase of the disease. A possible predictive value of inhibitor levels for thrombotic events such as deep vein thrombosis and ischemic heart disease has been studied. On the basis of available studies, the predictive value is not clear for venous thrombosis, whereas plasminogen activator inhibitor-1 levels can predict some coronary events, at least in subgroups of young patients with a first myocardial infarction. It remains to be established if treatments able to reduce plasma inhibitor levels lead to a decrease in the risk of thromboembolic events.

Plasminogen activator inhibitor activity in diabetic and nondiabetic survivors of myocardial infarction

Recent studies suggest that plasminogen activator inhibitor (PAI-1) may be a risk factor for recurrent myocardial infarction. We measured PAI-1 activity and antigen and tissue-type plasminogen activator (t-PA) antigen in 35 (20 nondiabetic and 15 diabetic) subjects with no clinical or electrocardiographic evidence of ischemic heart disease and in 74 (50 nondiabetic and 24 diabetic subjects) who had survived a myocardial infarction in the preceding 6-24 months. Levels of PAI-1 activity (18.7 +/- 5.6 versus 12.0 +/- 3.8 arbitrary units [AU] per milliliter, p = 0.001) and t-PA antigen (7.0 +/- 1.9 versus 4.6 +/- 2.0 ng/mL, p = 0.001) were significantly higher in diabetic compared with nondiabetic control subjects. Survivors of myocardial infarction had higher levels of PAI-1 activity and antigen and t-PA antigen than control subjects, and the diabetic survivors had higher levels of PAI-1 activity (25.3 +/- 6.7 versus 20.1 +/- 7.1 AU/mL, p = 0.004) and t-PA antigen (10.6 +/- 4.3 versus 8.4 +/- 3.3 ng/mL, p = 0.03) than the nondiabetic survivors. No difference in PAI-1 antigen levels was found between the diabetic subjects and either the nondiabetic control subjects or survivors of myocardial infarction. After venous occlusion in control subjects, there was a significant increase in PAI-1 antigen (mean 26.7%, range 14.1-58.1% in nondiabetics and mean 25.2%, range 6.2-39.7% in diabetics) and t-PA antigen (mean 78.3%, range 13.6-186.2% for nondiabetic and mean 40.7%, range 17.5-76.2% for diabetic subjects), but in the survivors of myocardial infarction, no significant effect of venous occlusion was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased release of tissue plasminogen activator and its inhibitor in human parotid saliva upon stimulation

Parotid saliva from 12 healthy volunteers was collected prior to and after 5 and 25 min of stimulation at a constant flow rate of 0.25 or 1.0 ml min-1. In the salivary samples the concentrations of tPA (tissue-type plasminogen activator), PAI-1 (plasminogen activator inhibitor type-1), albumin and total protein were determined and the activity of amylase, tPA and PAI assessed. Presence of both tPA and PAI-1 antigen was demonstrated in all samples, and in unstimulated saliva the ratio between the activator and its inhibitor was 1:7. Upon stimulation we found a significantly increased concentration of PAI-1, a less pronounced increase in tPA concentration, unchanged amylase and total protein levels and significantly decreased albumin concentration. tPA activity was significantly reduced after prolonged stimulation which had no effect on PAI activity. In stimulated saliva a significant positive correlation between concentration of tPA and PAI-1 was demonstrated. Stimulation with citric acid had no effect on output of albumin which is passively filtered from blood, whereas the increase in flow rate corresponded to the significantly increased secretion rate of total protein and amylase which is secreted by gland cells. The secretion pattern of tPA and PAI-1 differed significantly from that of albumin in showing markedly increased output rate during the stimulation period, and the relative increase in output of PAI-1 was significantly higher than that of amylase and total protein. Thus, the results from this study suggest an active release of both tPA and its main inhibitor PAI-1 into saliva.

Correlations between fibrinolytic function and acute myocardial infarction

The diurnal variation of tissue plasminogen activator and plasminogen activator inhibitor--2 important factors in regulation of fibrinolysis--is well established, but its clinical implications are not yet fully understood. Fibrinolytic function was measured in a group of 133 patients less than 45 years of age, 3 months after they presented with myocardial infarction, and they were compared with an equal group of carefully matched control subjects. A sub-sample of 71 patients and 50 control subjects was examined 3 years later. Tissue plasminogen activator inhibitor plasma levels were higher in patients versus control subjects at 3 months and at 3 years after myocardial infarction, with a high degree of correlation between the 2 determinations (r = 0.87). Of all hemostatic parameters studied, only low tissue plasminogen activator activity and increased tissue plasminogen activator inhibitor levels were significantly different between patients with and without recurrent infarction during the 3-year period. In those patients with a second infarction greater than 3 years later, however, tissue plasminogen activator inhibitor concentration was not significantly related to reinfarction.

Secretion of plasminogen activator inhibitor-1 from cultured human umbilical vein endothelial cells is induced by very low density lipoprotein

Clinical studies have demonstrated an impaired fibrinolytic function in patients with angiographically ascertained coronary artery disease or previous myocardial infarction. This decreased fibrinolytic function is to a major extent explained by the presence of high plasma levels of plasminogen activator inhibitor-1 (PAI-1) and is most common in patients with hyperlipoproteinemias type IIB and IV. To further investigate the association between hypertriglyceridemia and elevated plasma levels of PAI-1, cultured human umbilical vein endothelial cells were exposed to purified lipoproteins isolated from normo- and hypertriglyceridemic (NTG and HTG) individuals. We found that very low density lipoprotein (VLDL) from both NTG and HTG subjects stimulated the secretion of PAI-1 from endothelial cells in a dose-dependent manner. HTG-VLDL at a concentration of 100 micrograms/ml gave rise to a 73% increase in PAI-1 secretion as compared to control cultures, whereas NTG-VLDL only gave rise to a 30% increase (p less than 0.05), indicating that HTG-VLDL is a more potent stimulus to PAI-1 secretion than is NTG-VLDL. Experiments in which endothelial cells were exposed to VLDL subfractions indicated that large VLDL particles, in particular, induce PAI-1 release. Binding experiments demonstrated a specific cellular binding of both NTG- and HTG-VLDL to the cells, but HTG-VLDL bound about four times more effectively than NTG-VLDL. Exposure of the endothelial cells to an LDL receptor antibody was found to block 75% (p less than 0.005) of the VLDL-induced secretion of PAI-1 from the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Complex formation between plasminogen activator inhibitor 1 and vitronectin in purified systems and in plasma

Functionally active PAI-1 is bound to a discrete binding or carrier protein in plasma, which was recently identified as vitronectin. In the present study, the interaction between PAI-1 and vitronectin has been studied in purified systems and in plasma by agarose gel electrophoresis using non-denaturing conditions and by crossed immunoelectrophoresis using an antiserum produced towards purified PAI-1/vitronectin complex. Both methods revealed a clearly distinguishable complex with electrophoretic mobility in between the parent molecules. Virtually all of the purified vitronectin, which did not contain any appreciable amounts of polymerized material, and almost all of the vitronectin in plasma, had the capacity to form a complex with PAI-1. The results suggested a stoichiometry of 1:1 as the most likely ratio between the two molecules in the complex. In contrast to functionally active PAI-1, latent or chloramine T-inactivated PAI-1 did not form such a complex with vitronectin.

The mechanism of the reaction between human plasminogen-activator inhibitor 1 and tissue plasminogen activator

The structural events taking place during the reaction between PAI-1 (plasminogen-activator inhibitor 1) and the plasminogen activators sc-tPA (single-chain tissue plasminogen activator) and tc-tPA (two-chain tissue plasminogen activator) were studied. Complexes were formed by mixing sc-tPA or tc-tPA with PAI-1 in slight excess (on an activity basis). The complexes were purified from excess PAI-1 by affinity chromatography on fibrin-Sepharose. Examination of the purified complexes by SDS/polyacrylamide-gel electrophoresis (SDS/PAGE) and N-terminal amino acid sequence analysis demonstrated that a stoichiometric 1:1 complex is formed between PAI-1 and both forms of tPA. Data obtained from both complexes revealed the amino acid sequences of the parent molecules and, in addition, a new sequence: Met-Ala-Pro-Glu-Glu-. This sequence is found in the C-terminal portion of the intact PAI-1 molecule and thus locates the reactive centre of PAI-1 to Arg346-Met347. The proteolytic activity of sc-tPA is demonstrated by its capacity to cleave the 'bait' peptide bond in PAI-1. The complexes were inactive and dissociated slowly at physiological pH and ionic strength, but rapidly in aq. NH3 (0.1 mol/l). Amidolytic tPA activity was generated on dissociation of the complexes, corresponding to 0.4 mol of tPA/mol of complex. SDS/PAGE of the dissociated complexes indicated a small decrease in the molecular mass of PAI-1, in agreement with proteolytic cleavage of the 'bait' peptide bond during complex-formation.

Purification of high and low molecular weight plasminogen activator inhibitor 1 from fibrosarcoma cell-line HT 1080 conditioned medium

Functionally active (high-Mr) and inactive (low-Mr) plasminogen activator inhibitor 1 (PAI) have been purified from fibrosarcoma cell-line HT 1080 conditioned medium, containing 1% fetal calf serum. The two forms were first purified by affinity chromatography on heparin-Sepharose and then separated from each other by gel filtration on Sephadex G-150. The final purification was achieved by affinity chromatography on insolubilized monoclonal antibodies towards human PAI. Alternatively, the low-Mr form was purified by chromatography on carboxymethyl-cellulose. Low-Mr PAI purified in this way, could be almost fully reactivated by treatment with guanidinium chloride. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis followed by immunoblotting revealed that the low-Mr form contained nothing but PAI at an Mr of about 50,000. In addition to PAI, the high-Mr form contained a component, which was not antigenically related to PAI. This compound had a molecular weight of about 75,000 and its NH2-terminal amino acid sequence corresponded to that of human vitronectin. We conclude that the high-Mr form of PAI constitutes a complex between 50,000 Mr PAI and vitronectin from fetal calf serum.

Plasminogen activator inhibitor 1 (PAI) is bound to vitronectin in plasma

Functionally active plasminogen activator inhibitor 1 (PAI) is bound to a discrete binding protein in plasma [(1988) Thromb. Haemost. 59, 392-395]. The binding protein has now been partially purified using conventional chromatographic techniques. After addition of active PAI its complex with the binding protein was purified by chromatography on insolubilized monoclonal antibodies towards PAI. Dodecylsulphate (polyacrylamide gel electrophoresis revealed two main compounds with molecular masses of 50 and 75 kDa respectively. NH2-terminal amino acid sequence analysis and immunoblotting analysis suggested that the two compounds were PAI (50 kDa) and vitronectin (75 kDa). We conclude that the PAI-binding protein is identical to vitronectin.

Plasminogen activator inhibitor (PAI-1) in plasma and platelets

The distribution of PAI-1 in the plasma and platelets of normal individuals and of patients with platelet abnormalities was studied. An ELISA, capable of measuring PAI-1 in plasma at 1.5 ng/ml, and a functional assay of t-PA inhibition were used to assay platelet-free plasma (PFP), platelet-rich plasma in which the platelets were lysed (PRP) and serum. The PAI-1 concentration of normal PFP was 21.0 +/- 7.2 ng/ml (mean +/- SD) and those of PRP and serum were 282.6 +/- 68.0 and 270.3 +/- 71.9 ng/ml. The concentration of PAI-1 in PRP was proportional to the platelet count with 0.67 +/- 0.18 ng/10(6) platelets. Patients with thrombocytopenia had approximately normal PAI-1 concentrations in PFP; the extremely low concentrations in serum or PRP reflected the platelet count. A patient with grey platelet syndrome showed a comparable pattern, confirming that PAI-1 occurs in the platelet alpha-granules and indicating that the plasma concentration of PAI-1 is independent of the platelet pool of PAI-1. The median inhibitory activities towards t-PA were 1.6, 8.7 and 8.3 units/ml in normal PFP, PRP and serum respectively. PAI-1 in PFP had a median specific activity (units/mg PAI-1) about 5-fold higher than platelet PAI-1. Plasma and platelets represent two distinct pools of PAI-1, both of which should be considered in studies on the relationship between circulating PAI-1 and thrombotic disease.

Kinetics of the inhibition of plasminogen activators by the plasminogen-activator inhibitor. Evidence for 'second-site' interactions

The reactions between plasminogen-activator inhibitor (PAI) and different plasminogen activators were studied in the presence of chromogenic peptide substrates for the enzymes. Our findings suggest that the rate constants for the reactions of PAI with single-chain tissue plasminogen activator (tPA), two-chain tPA, high-Mr urokinase and low-Mr urokinase are high and quite similar (1.6 X 10(7)-3.9 X 10(7) M-1.s-1). A free active site in the enzymes seems to be necessary for their reaction with PAI. Amino acids with antifibrinolytic properties did not interfere with the reactions. However, di-isopropyl phosphorofluoridate-inactivated tPA inhibited the reaction between PAI and all plasminogen activators in a similar way. These findings clearly demonstrated that a 'second-site' interaction, in addition to that between the enzyme active site and the inhibitor 'bait' peptide bond, is of importance for the high reaction rate. The reaction rate between PAI and single-chain tPA in the presence of an activator substrate (D-Ile-Pro-Arg p-nitroanilide) was decreased in the presence of fibrin. Fibrin caused a decrease in the Km for the single-chain tPA-substrate reaction. As a consequence, the 'free' concentration of single-chain tPA in the system decreased in the presence of fibrin, affecting the reaction rate between PAI and single-chain tPA. The phenomenon might be of physiological relevance, in the sense that single-chain tPA bound to fibrin in the presence of plasminogen would be protected against inactivation by PAI.