Hypoalbuminemia in inflammatory disorders is not an infrequent finding. However, little is known about albumin synthesis in these patients. In the present study we have measured the albumin synthesis in four patients with inflammatory diseases using the ['4Cjcarbonate technique. Because inflammation causes a decreased albumin synthesis and this decreased synthesis could not be related to a reduced amino acid supply, we have also examined the possible molecular mechanisms of reduced albumin synthesis during inflammation using in vivo and in vitro experiments in rats. In rats with turpentine-induced inflammation, serum albumin concentration and liver albumin mRNA level were markedly decreased. These changes could not be reproduced by administration of fibrinogen-, or fibrin-degradation products, or several hormones, such as corticosteroids, growth hormone, and adrenaline. However, monocytic products, especially interleukin 1, postulated to be important mediators of the inflammatory response, reduced albumin synthesis and liver albumin messenger RNA content but not total protein synthesis in rats in vivo and in primary cultures of rat hepatocytes. These findings suggest that monocytic products play an important role in reduced albumin synthesis during inflammation.

SummaryTissue factor-factor VIIa catalysed activation of factor X and factor IX is inhibited by the complex of tissue factor pathway inhibitor (TFPI) and factor Xa. At present, no information is available as to what extent the kinetics of complex formation between TFPI and factor Xa during factor X activation contribute to the overall rate of inactivation of the factor X converting complex. We have determined the kinetic parameters of the individual reactions, i. e. factor X activation, formation of the TFPI-factor Xa complex, and inactivation of tissue factor-factor VIIa by the TFPI-factor Xa complex. We modelled the overall reaction by assuming a two-step reaction: factor Xa generated by tissue factor-factor VIIa forms a reversible complex with TFPI and in the second step this complex forms a reversible quaternary complex with tissue factor- factor VIIa. The validity of the model was demonstrated by analysis of factor Xa generation curves in the presence of TFPI. Independently determined constants for factor X activation (kcat= 12 s-1, Km = 70 nM) and inhibition of tissue factor-factor VIIa by TFPI-factor Xa complex (rate constant of inhibition of 1.1 × 108 M-1s-1) were used. The association rate constant of the formation of the TFPI-factor Xa complex was estimated by fitting the model to the data. The rate constants of association of the complex between factor Xa and the variants full length TFPI, TFPI 1-247 and TFPI1-61 were very close to the values determined independently in a kinetic study on the inhibition of factor Xa in the presence of phospholipids, namely 3.4 × 106 M-1s-1, 0.4 × 106 M-1s-1 and 0.3 × 106 M-1s-1, respectively. These results indicate that the factor Xa-dependent inhibition of tissue factor-factor VIIa-catalysed factor X activation by TFPI can be adequately described by the two-step reaction sequence. We found that phospholipids (25 mol % phosphat-idylserine/75 mol % phospatidylcholine) increased the rate constant of association with factor Xa for full length TFPI, but not for the C-ter- minus truncated TFPI. Our results further indicate that optimal inhibition of tissue factor-factor VIIa activity is obtained with full length TFPI because of the higher rate of TFPI-factor Xa complex formation.

We have determined the rate constants of inactivation of factor Xa and thrombin by antithrombin III/heparin during the process of prothrombin activation. The second-order rate constant of inhibition of factor Xa alone by antithrombin III as determined by using the synthetic peptide substrate S-2337 was found to be 1.1 X 10(6) M-1 min-1. Factor Xa in prothrombin activation mixtures that contained prothrombin, and either saturating amounts of factor Va or phospholipid (20 mol % dioleoylphosphatidylserine/80 mol % dioleoylphosphatidylcholine, 10 microM), was inhibited by antithrombin III with a second-order rate constant that was essentially the same: 1.2 X 10(6) M-1 min-1. When both factor Va and phospholipid were present during prothrombin activation, factor Xa inhibition by antithrombin III was reduced about 10-fold, with a second-order rate constant of 1.3 X 10(5) M-1 min-1. Factor Xa in the prothrombin activation mixture that contained both factor Va and phospholipid was even more protected from inhibition by the antithrombin III-heparin complex. The first-order rate constants of these reactions at 200 nM antithrombin III and normalized to heparin at 1 microgram/mL were 0.33 and 9.5 min-1 in the presence and absence of factor Va and phospholipid, respectively. When the prothrombin concentration was varied widely around the Km for prothrombin, this had no effect on the first-order rate constants of inhibition. It is our conclusion that factor Xa when acting in prothrombinase on prothrombin is profoundly protected from inhibition by antithrombin III in the absence as well as in the presence of heparin.(ABSTRACT TRUNCATED AT 250 WORDS)

SummaryTissue factor : factor VIIa induced activation of blood coagulation is inhibited by the complex between factor Xa and tissue factor pathway inhibitor (factor Xa : TFPI). We recently reported that phospholipid-bound factor Xa reduces the high binding affinity of factor Xa : TFPI for negatively charged phospholipids by a partial degradation of TFPI (17). The present study was undertaken to elucidate the factor Xa cleavage sites in TFPI and to delineate the consequences of this proteolysis with respect to the inhibitory activity of factor Xa : TFPI. We found that phospholipid-bound factor Xa cleaves in TFPI the peptide bonds between Lys86-Thr87 and Arg199-Ala200. Interestingly, Arg199 is the P1 residue of the third Kunitz-type protease inhibitor domain. The fast cleavage of the Arg199-Ala200 bond results in a 50-70% reduction of the anticoagulant activity of factor Xa : TFPI, as determined with a dilute tissue factor assay, but is not associated with a diminished inhibitory activity of factor Xa : TFPI towards TF : factor VIIa catalyzed activation of factor X. On the other hand, the slower cleavage of the Lys86-Thr87 peptide bond was associated with both a diminished anticoagulant and anti-TF : factor VIIa activity. Dissociation of factor Xa from the cleaved TFPI was not observed. These data provide evidence for a dual role of factor Xa since it is the essential cofactor in the TFPI-controlled regulation of TF-dependent coagulation as well as a catalyst of the inactivation of TFPI.

SummaryLow molecular weight (LMW) heparin preparations have unknown distributions of ATIII-binding material, so mean molecular weights as such might bear little information on their anti-factor Xa and anti-thrombin activities, and on the neutralization of these activities by platelet factor 4 (PF4). These properties were investigated in pure systems with proteins of human origin. Pseudo-first order rate constants of inactivation of factor Xa and thrombin by antithrombin III were determined as function of heparin concentration, in the presence of 4.0 mM CaCl2. Despite a large variation in the mean molecular weights, the ratios of the anti-factor Xa over the anti-thrombin activities were essentially the same for the 4th International Standard for heparin (0.46), the 1st International Standard for LMW heparin (0.32), CY216 (0.42) and enoxaparin (0.50). The ultra LMW heparin CY222 had only a 2-times higher ratio (0.98). Analysis of CY216 subfractions, obtained by gel filtration, showed that the heparin molecules of the upper region of the molecular weight distribution are responsible for the anti-thrombin, but also to a large extent for the anti-factor Xa activities. The results indicate that depolymerization of unfractionated heparin does not result in an increased anti-factor Xa/anti-thrombin ratio, because in the presence of Ca2+-ions the rate constants of inactivation of factor Xa are lowered as compared to those of native heparin. PF4-dependent neutralization of anti-factor Xa and anti-thrombin activities of fixed concentrations of the LMW heparins was studied by measuring rate constants as function of PF4 concentration. All anti-thrombin and 50% of the anti-factor Xa activities were readily neutralized. Excess PF4 was required to neutralize another 35-50% of the anti-factor Xa activities. At PF4 levels obtained at maximal release of the content of platelet α-granules, all anti-thrombin and most (≥85%) of the anti-factor Xa activities can be neutralized.

The inhibition of prothrombinase by tissue factor pathway inhibitor (TFPI) has been studied in the presence and absence of prothrombin. The rate constant of association of prothrombinase with full-length TFPI was 2.1x10(7) M-1.s-1 and 0.05x10(7) M-1.s-1 for the reaction with C-terminus truncated TFPI (TFPI1-161). The rate constant of dissociation was 0.65x10(-4) s-1 in both cases. The rate constant of inhibition of prothrombinase by TFPI1-161 was similar to that of solution-phase factor Xa. In contrast, phospholipids and factor Va enhanced the association rate of the reaction between factor Xa and full-length TFPI by approx. 20-fold. Although TFPI, and in particular the full-length variant of the molecule, is a potent inhibitor of prothrombinase (overall inhibition constant of 3 pM), we also found that prothrombin competed very effectively with TFPI for the active site of factor Xa in the prothrombinase complex. A 50% reduction of the rate constant of inhibition was measured in the presence of 4 nM prothrombin, i.e. 0.2% of the plasma concentration of prothrombin. The physiological significance of TFPI as an inhibitor of prothrombinase activity is thus questionable.

The physiological inhibitor of tissue factor (TF)⅐factor VIIa (FVIIa), full-length tissue factor pathway inhibitor (TFPI FL ) in complex with factor Xa (FXa), has a high affinity for anionic phospholipid membranes. The role of anionic phospholipids in the inhibition of TF⅐FVIIa-catalyzed FX activation was investigated. FXa generation at a rotating disc coated with TF embedded in a membrane composed of pure phosphatidylcholine (TF⅐PC) or 25% phosphatidylserine and 75% phosphatidylcholine (TF⅐PSPC) was measured in the presence of preformed complexes of FXa⅐TFPI FL Blood coagulation in vivo is initiated when circulating factor VII(a) binds in a calcium-dependent way to its cofactor, tissue factor (TF) 1 (see Refs. 1 and 2 for a review). This complex formation results in enhanced catalytic activity of factor VIIa (FVIIa), which via limited proteolysis, activates factors X (FX) and IX (FIX) (3). TF is a transmembrane glycoprotein, which under normal conditions is expressed only in extravascular tissues (4, 5).The main physiological regulator of TF-induced blood coagulation is tissue factor pathway inhibitor (TFPI) (6, 7), a single chain glycoprotein of 42 kDa and a member of the Kunitz family of serine protease inhibitors. TFPI contains an acidic N terminus, followed by three tandemly repeated Kunitz-type inhibition domains, and a basic C-terminal tail (8). Site-directed mutagenesis has revealed that the first Kunitz domain binds to FVIIa and that the second Kunitz domain interacts with the active site of FXa (9). No such functions could be attributed to the third Kunitz domain (10). Yet, various interactions have been ascribed to this domain, e.g. with lipoproteins and heparin, but their importance for the inhibitory function of TFPI is not clear (11,12). On the other hand, the basic C-terminal region of TFPI (residues 240 -276) has been shown to play a crucial role in the anticoagulant activity of this inhibitor (13,14). Despite numerous studies, it remains unclear how this basic C terminus modulates the anticoagulant activity of TFPI (15)(16)(17)(18)(19). TFPI inhibits the generation of FXa and FIXa by the TF⅐FVIIa complex in a unique, two-step reaction (20). First, TFPI binds Ca 2ϩ independently to FXa, thereby inhibiting the FXa catalytic activity (9). In a second step, the FXa⅐TFPI complex binds in a Ca 2ϩ -dependent way to TF⅐FVIIa. This results in the formation of the quaternary complex TF⅐FVIIa⅐FXa⅐TFPI, in which the proteolytic activity of the TF⅐FVIIa complex is fully neutralized. The effect of TFPI on TF⅐FVIIa activity in the absence of FXa is negligible (21,22), implying that the true inhibitor of TF⅐FVIIa activity is the FXa⅐TFPI complex. The rate of complex formation of FXa and TFPI is enhanced by negatively charged phospholipids for full-length TFPI (TFPI FL ) but not for TFPI 1-161 , a truncated variant lacking the third Kunitz domain and the potential phospholipid binding C-terminal tail (16,23).Recently (24), we demonstrated that TFPI FL in complex with FXa has a much higher affinity for anionic ...

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