Bovine endothelial cell plasminogen activator inhibitor. Purification and heat activation

Generation and in vitro characterisation of inhibitory nanobodies towards plasminogen activator inhibitor 1

Plasminogen activator inhibitor 1 (PAI-1) is the principal physiological inhibitor of tissue-type plasminogen activator (t-PA) and has been identified as a risk factor in cardiovascular diseases. In order to generate nanobodies against PAI-1 to interfere with its functional properties, we constructed three nanobody libraries upon immunisation of three alpacas with three different PAI-1 variants. Three panels of nanobodies were selected against these PAI-1 variants. Evaluation of the amino acid sequence identity of the complementarity determining region-3 (CDR3) reveals 34 clusters in total. Five nanobodies (VHH-s-a98, VHH-2w-64, VHH-s-a27, VHH-s-a93 and VHH-2g-42) representing five clusters exhibit inhibition towards PAI-1 activity. VHH-s-a98 and VHH-2w-64 inhibit both glycosylated and non-glycosylated PAI-1 variants through a substrate-inducing mechanism, and bind to two different regions close to αhC and the hinge region of αhF; the profibrinolytic effect of both nanobodies was confirmed using an in vitro clot lysis assay. VHH-s-a93 may inhibit PAI-1 activity by preventing the formation of the initial PAI-1t-PA complex formation and binds to the hinge region of the reactive centre loop. Epitopes of VHH-s-a27 and VHH-2g-42 could not be deduced yet. These five nanobodies interfere with PAI-1 activity through different mechanisms and merit further evaluation for the development of future profibrinolytic therapeutics.

Functional stability of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of plasminogen activators, such as tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), and a major regulator of the fibrinolytic system. PAI-1 plays a pivotal role in acute thrombotic events such as deep vein thrombosis (DVT) and myocardial infarction (MI). The biological effects of PAI-1 extend far beyond thrombosis including its critical role in fibrotic disorders, atherosclerosis, renal and pulmonary fibrosis, type-2 diabetes, and cancer. The conversion of PAI-1 from the active to the latent conformation appears to be unique among serpins in that it occurs spontaneously at a relatively rapid rate. Latency transition is believed to represent a regulatory mechanism, reducing the risk of thrombosis from a prolonged antifibrinolytic action of PAI-1. Thus, relying solely on plasma concentrations of PAI-1 without assessing its function may be misleading in interpreting the role of PAI-1 in many complex diseases. Environmental conditions, interaction with other proteins, mutations, and glycosylation are the main factors that have a significant impact on the stability of the PAI-1 structure. This review provides an overview on the current knowledge on PAI-1 especially importance of PAI-1 level and stability and highlights the potential use of PAI-1 inhibitors for treating cardiovascular disease.

Intracellular production of recombinant serpins in yeast

Yeast are a valuable system for recombinant serpin production due to their ability to synthesize large amounts of heterologous gene products as well as their expression of folding chaperones and lack of endogenous serpin genes. In this chapter, we describe a method for intracellular expression of cytoplasmic serpins in the yeast Pichia pastoris. We also give details on how this system can be exploited to produce polymer-forming mutants of secretory serpins.

Involvement of disulfide bonds and histidine 172 in a unique beta-sheet to alpha-helix transition of alpha 1-acid glycoprotein at the biomembrane interface

Human alpha(1)-acid glycoprotein (AGP), which is comprised of 183 amino acid residues and 5 carbohydrate chains, is a major plasma protein that binds to basic and neutral drugs as well as to steroid hormones. It has a beta-sheet-rich structure in aqueous solution. Our previous findings suggest that AGP forms an alpha-helix structure through an interaction with biomembranes. We report herein on a study of the mechanism of alpha-helix formation in AGP using various modified AGPs. The disulfide reduced AGP (R-AGP) was extensively unfolded, whereas asialylated AGP (A-AGP) maintained the native structure. Intriguingly, reduced and asialylated AGP (RA-AGP) increased the alpha-helix content as observed in the presence of biomembrane models, and showed a significant decrease in ligand binding capacity. This suggests that AGP has an innate tendency to form an alpha-helix structure, and disulfide bonds are a key factor in the conformational transition between the beta-sheet and alpha-helix structures. However, RA-AGP with all histidine residues chemically modified (HRA-AGP) was found to lose the intrinsic ability to form an alpha-helix structure. Furthermore, disulfide reduction of the H172A mutant expressed in Pichia pastoris also caused a similar loss of folding ability. The present results indicate that disulfide bonds and the C-terminal region, including H172 of AGP, play important roles in alpha-helix formation in the interaction of the protein with biomembranes.

Enthalpy measurement using calorimetry shows a significant difference in potential energy between the active and latent conformations of PAI-1

A central feature of the serpin inhibition mechanism is insertion of the reactive center loop into the central beta-sheet (beta-sheet A). This insertion also occurs when the reactive center loop is cleaved without protease inhibition. Using this effect, we have measured the enthalpy (DeltaH) of loop cleavage and insertion for plasminogen activator inhibitor 1 (PAI-1) as -38 kcal/mol. Because loop insertion can be blocked by incorporating a peptide into the central beta-sheet, it was possible to assign -7 kcal/mol to loop cleavage and -31 kcal/mol to loop insertion. These values are lower than values reported for the serpins alpha 1 -proteinase inhibitor and antithrombin of -53 to -63 kcal/mol, respectively, for loop insertion with negligible enthalpy for loop cleavage. A free energy difference of -9 kcal/mol has been reported between the active and spontaneously loop inserted "latent forms" of PAI-1, which is significantly smaller in magnitude than the -31 kcal/mol of enthalpy we measured for loop insertion. Because the enthalpy should relate closely to those regions of PAI-1 that have moved to lower potential energy, a difference distance matrix is presented that identifies regions of PAI-1 that move during loop insertion.

Intramural plasminogen activator inhibitor type-1 and coronary atherosclerosis

Altered expression of plasminogen activator inhibitor type-1 in vessel walls, reviewed here, might affect coronary atherogenesis. Upregulation might exacerbate vasculopathy by potentiating thrombosis and by inhibiting vascular smooth muscle cell migration, resulting in attenuation of thickness of elaborated fibrous caps implicated in the vulnerability of atheroma to rupture.

The reaction of serpins with proteinases involves important enthalpy changes

When active serpins are proteolytically inactivated in a substrate-like reaction, they undergo an important structural transition with a resultant increase in their conformational stability. We have used microcalorimetry to show that this conformational alteration is accompanied by an important enthalpy change. For instance, the cleavage of alpha(1)-proteinase inhibitor by Pseudomonas aeruginosa elastase, Staphylococcus aureus V8 proteinase, or papain and that of antithrombin by leukocyte elastase are characterized by large enthalpy changes (DeltaH = -53 to -63 kcal mol(-1)). The former reaction also has a large and negative heat capacity (DeltaC(p)() = -566 cal K(-1) mol(-1)). In contrast, serpins release significantly less heat when they act as proteinase inhibitors. For example, the inhibition of pancreatic elastase, leukocyte elastase, and pancreatic chymotrypsin by alpha(1)-proteinase inhibitor and that of pancreatic trypsin and coagulation factor Xa by antithrombin are accompanied by a DeltaH of -20 to -31 kcal mol(-1). We observe no heat release upon proteolytic cleavage of inactive serpins or following inhibition of serine proteinases by canonical inhibitors or upon acylation of chymotrypsin by N-trans-cinnamoylimidazole. We suggest that part of the large enthalpy change that occurs during the structural transition of serpins is used to stabilize the proteinase in its inactive state.

Population analyses of kinetic partitioning in protein folding

A simple lattice model of protein folding is studied in order to analyze the kinetic partitioning phenomena in the energy landscape perspective. By restricting the area of conformational space, it becomes possible to follow many Monte Carlo trajectories until they reach equilibrium. Alteration of population of trajectories is monitored and the relations between the energy landscape and kinetics are examined. Kinetic partitioning phenomena are categorized into different types in terms of characteristic time constants and partitioning ratio. In a specific partitioning process, refolding proceeds along the parallel pathways; the time constants have a temperature dependence similar to that observed in hen lysozyme. High-energy conformations are classified into groups according to the probability that the trajectories starting from those conformations will reach each energy valley. The partitioning ratio is determined by the way in which the conformational space is organized into these groups.

Polymerization of plasminogen activator inhibitor-1

The activity of the serine proteinase inhibitor (serpin) plasminogen activator inhibitor-1 (PAI-1) is controlled by the intramolecular incorporation of the reactive loop into beta-sheet A with the generation of an inactive latent species. Other members of the serpin superfamily can be pathologically inactivated by intermolecular linkage between the reactive loop of one molecule and beta-sheet A of a second to form chains of polymers associated with diverse diseases. It has long been believed that PAI-1 is unique among active serpins in that it does not form polymers. We show here that recombinant native and latent PAI-1 spontaneously form polymers in vitro at low pH although with distinctly different electrophoretic patterns of polymerization. The polymers of both the native and latent species differ from the typical loop-A-sheet polymers of other serpins in that they readily dissociate back to their original monomeric form. The findings with PAI-1 are compatible with different mechanisms of linkage, each involving beta-strand addition of the reactive loop to s7A in native PAI-1 and to s1C in latent PAI-1. Glycosylated native and latent PAI-1 can also form polymers under similar conditions, which may be of in vivo importance in the low pH environment of the platelet.

What is the role of non-native intermediates of beta-lactoglobulin in protein folding?

The mechanism of alpha-->beta transition in folding of beta-lactoglobulin is discussed based on free energy landscape analysis of a long lattice model. It is found that helical propensity of beta-lactoglobulin is driven by conformational entropy and is intrinsically coded in its native structure. We propose a view on a role of folding intermediate, which is "on-pathway" but rich in non-native structures. The present results suggest that the native structure topology plays an important role in alpha-->beta transition.

Comparative fourier transform infrared and circular dichroism spectroscopic analysis of alpha1-proteinase inhibitor and ovalbumin in aqueous solution

Alpha1-proteinase inhibitor (alpha1Pi) and ovalbumin are both members of the serpin superfamily. They share about a 30% sequence identity and exhibit great similarity in their three-dimensional structures. However, no apparent functional relationship has been found between the two proteins. Unlike alpha1Pi, ovalbumin shows no inhibitory effect to serine proteases. To see whether or not a conformational factor(s) may contribute to the functional difference, we carried out comparative analysis of the two proteins' secondary structure, thermal stability, and H-D exchange using FT-IR and CD spectroscopy. FT-IR analysis reveals significant differences in the amide I spectral patterns of the two proteins. Upon thermal denaturation, both proteins exhibit a strong low-wavenumber beta-sheet band at 1624 cm(-1) and a weak high-wavenumber beta-sheet band at 1694 cm(-1), indicative of intermolecular aggregate formation. However, the midpoint of the thermal-induced transition of alpha1Pi (approximately 55 degrees C) is 18 degrees C lower than that of ovalbumin (approximately 73 degrees C). The thermal stability analysis provides new insight into the structural changes associated with denaturation. The result of H-D exchange explains some puzzling spectral differences between the two proteins in D2O reported previously.

Distinct physical and structural properties of the ovine uterine serpin

Experiments were performed to examine the relationship between the structure and function of ovine uterine serpin (OvUS). Limited proteolytic digestion of OvUS caused cleavage of the 55-57 kDa OvUS to a 42 kDa product nearly identical in molecular weight to a naturally-occurring breakdown product of OvUS. N-terminal amino acid sequencing and MALDI-MS revealed that, unlike other serpins, OvUS was preferentially cleaved at about 70 amino acids upstream of the putative reactive center loop. Analysis of the partially-digested protein by gel filtration chromatography suggested that the C-terminal fragment of the protein was still associated under nondenaturing conditions. Partial digestion of OvUS had no effect on the protein's secondary structure, thermal stability, ability to bind lymphocytes or pepsin, or inhibitory activity towards pepsin or mitogen-induced lymphocyte proliferation. In contrast, mild denaturation of OvUS with 0.5 M guanidine HCl increased thermal stability. Unlike for other serpins, the increase in thermal stability was lost upon removal of the denaturant. Incubation of OvUS with 100 fold molar excess of a peptide corresponding to the putative P(14)-P(2) region of the RCL for 24 h at 37 degrees C to induce binary complex formation had no effect on its secondary structure and did not alter the biological activity of the protein. Synthetic peptides corresponding to the putative P(14)-P(2) region and the P(7)-P(15') region of the RCL were not inhibitory to pepsin activity or lymphocyte proliferation. Taken together, these results indicate that the conformation of OvUS is distinct from the prototypical serpin because conditions that lead to the large-scale conformational change in other serpins such as antithrombin III and alpha(1)-antitrypsin do not cause similar changes in OvUS. Moreover, the putative RCL does not seem to contain the activity required to inhibit lymphocyte proliferation or pepsin activity.

Inactivation of N-terminal signaling domain of Sonic hedgehog by forming a disulfide bond

The N-terminal domain of mouse Sonic hedgehog (Shh-N) expressed in mammalian cells showed four-fold bands on non-reduced SDS-PAGE, though it was homogeneous under reduced conditions. It contains three cysteine residues, Cys-25, Cys-103, and Cys-184, which may be concerned with this heterogeneity. Therefore, we examined the formation of a disulfide bond in the recombinant Shh-N and identified three kinds of disulfides with a combination of peptide mapping and NH(2)-terminal amino acid sequencing analysis. Among them, one type of the Shh-N containing a disulfide bond of Cys-103/Cys-184 could be separated from the other Shh-Ns using reverse phase HPLC and had no activity of alkaline phosphatase induction in C3H10T1/2 cells. This molecule could also be made by denaturation of the purified Shh-N with guanidine-HCl under non-reduced conditions. On the other hand, the reduced Shh-N and the reduced S-methylated Shh-N at cysteine residues showed approximately 10-fold higher activity compared to the originally purified Shh-N. These results suggested that Shh-N was synthesized as an active form whose three cysteine residues did not form disulfide and inactivated finally by forming a disulfide bond between Cys-103 and Cys-184.

Accelerated conversion of human plasminogen activator inhibitor-1 to its latent form by antibody binding

The serpin plasminogen activator inhibitor-1 (PAI-1) slowly converts to an inactive latent form by inserting a major part of its reactive center loop (RCL) into its beta-sheet A. A murine monoclonal antibody (MA-33B8), raised against the human plasminogen activator (tPA).PAI-1 complex, rapidly inactivates PAI-1. Results presented here indicate that MA-33B8 induces acceleration of the active-to-latent conversion. The antibody-induced inactivation of PAI-1 labeled with the fluorescent probe N, N'-dimethyl-N-(acetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) ethylene diamine (NBD) at P9 in the RCL caused a fluorescence enhancement and shift identical to those accompanying the spontaneous conversion of the P9.NBD PAI-1 to the latent form. Like latent PAI-1, antibody-inactivated PAI-1 was protected from cleavage by elastase. The rate constants for MA-33B8 binding, measured by NBD fluorescence or inactivation, were similar (1.3-1.8 x 10(4) M-1 s-1), resulting in a 4000-fold faster inactivation at 4.2 microM antibody binding sites. The apparent antibody binding rate constant, at least 1000 times slower than one limited by diffusion, indicates that exposure of its epitope depends on an unfavorable equilibrium of PAI-1. Our observations are consistent with this idea and suggest that the equilibrium involves partial insertion of the RCL into sheet A: latent, RCL-cleaved, and tPA-complexed PAI-1, which are inactive loop-inserted forms, bound much faster than active PAI-1 to MA-33B8, whereas two loop-extracted forms of PAI-1, modified to prevent loop insertion, did not bind or bound much more weakly to the antibody.

Reversible folding of Ada protein (O6-methylguanine-DNA methyltransferase) of Escherichia coli

The multifunctional 39 kDa Escherichia coli Ada protein (O6-methylguanine-DNA methyltransferase) (EC 2.1.1.63), product of the ada gene, is a monomeric globular polypeptide with two distinct alkylacceptor activities located in two domains. The two domains are of nearly equal size and are connected by a hinge region. The Ada protein accepts stoichiometrically the alkyl group from O6-alkylguanine in DNA at the Cys-321 residue and from alkyl phosphotriester at the Cys-69 residue. This protein functions in DNA repair by direct dealkylation of mutagenic O6-alkylguanine. The protein methylated at Cys-69 becomes a transcriptional activator of the genes in the ada regulon, including its own. Each of the two domains functions independently as an alkyl acceptor. The purified homogeneous protein is unstable at 37 degrees C and spontaneously loses about 30% of its secondary structure in less than 30 min concomitant with a complete loss of activity. However, sedimentation equilibrium studies indicated that the inactive protein remains in the monomeric form without aggregation. Furthermore, electrospray mass spectroscopic analysis indicated the absence of oxidation of the inactive protein. This temperature-dependent inactivation of the Ada protein is inhibited by DNA. In the presence of increasing concentrations of urea or guanidine, the protein gradually loses more than 80% of its structure. The two alkyl acceptor activities appear to be differentially sensitive to unfolding and the phosphotriester methyltransferase activity is resistant to 7 M urea. The partial or complete unfolding induced by urea or guanidine is completely reversed within seconds by removal of the denaturant. The heat-coagulated protein can also be restored to full activity by cycling it through treatment with 8 M urea or 6 M guanidine. These results suggest that the nascent or unfolded Ada polypeptide folds to a metastable form which is active and that the thermodynamically stable structure is partially unfolded and inactive.

Thermal conversion from low- to high-activity forms of catalase I from Bacillus stearothermophilus

Catalase I from Bacillus stearothermophilus has the interesting property of increasing its enzyme activity on heating. It was confirmed that after heating at 70 degrees C for 10 min or 65 degrees C for 20 min, almost all the enzyme molecules were converted irreversibly to the activated form. The increase in kcat from 1400 to 3930 s-1 and the decrease in Km for H2O2 from 4.4 to 2.7 mM by heat activation indicate changes in the kinetic property of the enzyme molecule. Therefore, it follows that catalase I has two active forms, a high-activity form and a low-activity form. The heat activation process followed the first-order kinetics with an activation enthalpy (DeltaH*) of 191 kJ/mol while the heat denaturation process had a DeltaH* of 545 kJ/mol. The CD spectra of the two enzyme forms had small but marked differences. The conversion of the low-activity form to the high-activity form was an endothermic process with a Tm of 56 degrees C, which is much lower than that of the heat denaturation (Tm = 76 degrees C), and the enthalpy change for the transition was only 5% of that for the denaturation. It has to be noted that the high-activity form of the enzyme was converted back to a low-activity form through the process of denaturation, refolding, and reconstitution with heme. In addition, the newly obtained low-activity form was brought to a high-activity form by heating. These results suggest that the native state of catalase I has two active conformations that are roughly the same but not identical and are separated by a high energy barrier.

Preparation and characterization of latent alpha 1-antitrypsin

Members of the serine proteinase inhibitor or serpin superfamily have a common molecular architecture based on a dominant five-membered A beta-pleated sheet and a mobile reactive center loop. The reactive center loop has been shown to adopt a range of conformations from the three turn alpha-helix of ovalbumin to the cleaved or latent inhibitor in which the reactive center loop is fully inserted into the A sheet of the molecule. While the cleaved state can be achieved in all inhibitory serpins only plasminogen activator inhibitor-1 and, more recently, antithrombin have been shown to adopt the latent conformation. We show here that the archetypal serpin, alpha 1-antitrypsin, can also be induced to adopt the latent conformation by heating at high temperatures in 0.7 M citrate for 12 h. The resulting species elutes at a lower sodium chloride concentration on an anion-exchange column and has a more cathodal electrophoretic mobility on non-denaturing polyacrylamide gel electrophoresis and isoelectric focusing than native M antitrypsin. Latent antitrypsin is inactive as an inhibitor of bovine alpha-chymotrypsin, is stable to unfolding with 8 M urea, and is more resistant to heat-induced loop-sheet polymerization than native but less resistant than cleaved antitrypsin. The reactive center loop of latent antitrypsin is inaccessible to proteolytic cleavage, and its occupancy of the A sheet prevents the molecule accepting an exogenous reactive center loop peptide. The activity of latent antitrypsin may be increased from < 1% to approximately 35% by refolding from 6 M guanidinium chloride.

Theoretical studies of protein folding and unfolding

The mechanism of protein folding is being investigated theoretically by the use of both simplified and all-atom models of the polypeptide chain. Lattice heteropolymer simulations of the folding process have led to proposals for the folding mechanism and for the resolution of the Levinthal paradox. Both stability and rapid folding have been shown in model studies to result from the presence of a pronounced global energy minimum corresponding to the native state. Concomitantly, molecular dynamics simulations with detailed atomic models have been used to analyze the initial stages of protein unfolding. Results concerning possible folding intermediates and the role of water in the unfolding process have been obtained. The two types of theoretical approaches are providing information essential for an understanding of the mechanism of protein folding and are useful for the design of experiments to study the mechanism in different proteins.

Dynamic structural and functional relationships in recombinant plasminogen activator inhibitor-1 (rPAI-1)

The conformational characteristics of active, latent, and denatured recombinant plasminogen activator inhibitor-1 (rPAI-1) were compared using UV spectroscopy, spectrofluorimetry and circular dichroism (CD) techniques. The UV absorbance wavelength maxima in all preparations approximated 280 nm, while the extinction coefficients of active and latent rPAI-1 differed by up to 60%. When incubated at 37 degrees C, the A280 of latent rPAI-1 was quite stable while the A280 of active rPAI-1 spontaneously increased, eventually approximating that of latent rPAI-1. Alkali difference spectroscopy yielded markedly divergent titration patterns for active and latent rPAI-1, suggesting that the tyrosine residues present in active and latent rPAI-1 differ in terms of solvent exposure. At an excitation wavelength of 280 nm, active rPAI-1 exhibited the greatest relative fluorescence quantum yield. The relative fluorescence of latent and denatured rPAI-1 were less than that of active PAI-1, and the emission maxima of both species were slightly red-shifted in comparison to that of active rPAI-1, suggesting that at least one of the four tryptophan residues present in rPAI-1 is less exposed to the aqueous environment in the active form of the molecule. In contrast, the derived secondary structures based on CD of active and latent rPAI-1 were nearly identical, with both moieties exhibiting approx. 40% alpha-helix and 15% beta-sheet. Taken together, these spectroscopic data provide evidence supporting the hypothesis that active and latent PAI-1 differ in terms of their tertiary conformation and aromatic residue exposure, while their secondary structures appear generally comparable. Furthermore, denaturant-induced reactivation of latent rPAI-1 produces a partially active rPAI-1 with spectroscopic properties similar to that of latent rPAI-1, suggesting that denatured rPAI-1 more closely resembles the latent rPAI-1 conformation after refolding. The spontaneous spectroscopic changes observed in rPAI-1 may reflect conformational transitions that are critical to the regulation of endogenous PAI-1 activity.

Synthetic peptides derived from the sequence around the plasmin cleavage site in vitronectin. Use in mapping the PAI-1 binding site

A series of 8 peptides derived from the amino acid sequence accommodating the plasmin cleavage site in vitronectin were synthesized and used to map its binding site for the type I plasminogen activator inhibitor (PAI-1). This mapping assigned the inhibitor binding site to the K348-R370 region with high affinity recognition elements within the K348-R357 sequence. These results account for our previous finding that cleavage of the R361-S362 bond by plasmin significantly reduces the affinity between PAI-1 and vitronectin, since it splits the PAI-1 binding site in two. Furthermore, in the case of the two-chain form of vitronectin, this cleavage detaches the S362-R379 peptide which provides some of the affinity elements for the binding of PAI-1.

Structural basis of latency in plasminogen activator inhibitor-1

Human plasminogen activator inhibitor-1 (PAI-1) is the fast-acting inhibitor of tissue plasminogen activator and urokinase and is a member of the serpin family of protease inhibitors. Serpins normally form complexes with their target proteases that dissociate very slowly as cleaved species and then fold into a highly stable inactive state in which the residues that flank the scissile bond (P1 and P1';) are separated by about 70 A. PAI-1 also spontaneously folds into a stable inactive state without cleavage; this state is termed 'latent' because inhibitory activity can be restored through denaturation and renaturation. Here we report the structure of intact latent PAI-1 determined by single-crystal X-ray diffraction to 2.6 A resolution. The three-dimensional structure reveals that residues on the N-terminal side of the primary recognition site are inserted as a central strand of the largest beta sheet, in positions similar to the corresponding residues in the cleaved form of the serpin alpha 1-proteinase inhibitor (alpha 1-PI). Residues C-terminal to the recognition site occupy positions on the surface of the molecule distinct from those of the corresponding residues in cleaved serpins or in the intact inactive serpin homologue, ovalbumin, and its cleavage product, plakalbumin. The structure of latent PAI-1 is similar to one formed after cleavage in other serpins, and the stability of both latent PAI-1 and cleaved serpins may be derived from the same structural features.

Mobile reactive centre of serpins and the control of thrombosis

Two protease inhibitors in human plasma play a key part in the control of thrombosis: antithrombin inhibits coagulation and the plasminogen activator inhibitor PAI-1 inhibits fibrinolysis, the dissolving of clots. Both inhibitors are members of the serpin family and both exist in the plasma in latent or inactive forms. We show here that the reactive centre of the serpins can adopt varying conformations and that mobility of the reactive centre is necessary for the function of antithrombin and its binding and activation by heparin; the identification of a new locked conformation explains the latent inactive state of PAI-1. This ability to vary conformation not only allows the modulation of inhibitory activity but also protects the circulating inhibitor against proteolytic attack. Together these findings explain the retention by the serpins of a large and unconstrained reactive centre as compared to the small fixed peptide loop of other families of serine protease inhibitors.

Plasmin cleavage of vitronectin. Identification of the site and consequent attenuation in binding plasminogen activator inhibitor-1

Plasmin is shown to specifically cleave vitronectin at the Arg361-Ser362 bond, 18 amino acid residues upstream from the site of the endogenous cleavage which gives rise to the two-chain form of vitronectin in plasma. The cleavage site is established using the exclusive phosphorylation of Ser378 with protein kinase A. As a result of the plasmin cleavage, the affinity between vitronectin and the type-1 inhibitor of plasminogen activator (PAI-1) is significantly reduced. This cleavage is stimulated by glycosaminoglycans, which are known to anchor vitronectin to the extracellular matrix. A mechanism is proposed through which plasmin can arrest its own production by feedback signalling, unleashing PAI-1 from the immobilized vitronectin found in the vascular subendothelium, which becomes exposed at the locus of a hemostatic event.

Cartilage synthesizes the serine protease inhibitor PAI-1: support for the involvement of serine proteases in cartilage remodeling

The work described here demonstrates the synthesis by human articular cartilage of plasminogen activator inhibitor-1 (PAI-1), a potent inhibitor of the serine protease tissue plasminogen activator (tPA). We also present data demonstrating an increase in PAI-1 messenger ribonucleic acid (mRNA) in chondrocytes exposed to the cytokine interleukin-1 (IL-1). Interestingly, this elevation of steady-state mRNA levels does not appear to result in an increase in synthesis of PAI-1 protein. Northern blot analysis reveals that of the two mRNA species (3.4 kb, 2.4 kb) previously reported for PAI-1, only the larger species (3.4 kb) appears to be synthesized by chondrocytes. Our data demonstrate the IL-1-stimulated production by cartilage of tissue plasminogen activator. We also show evidence for the presence of plasminogen in cartilage. A scheme is presented indicating the probable importance of the serine proteases (tPA and plasminogen) and PAI-1 in cartilage degradation.

Phosphoramidon-sensitive endothelin-converting enzyme in the cytosol of cultured bovine endothelial cells

Neutral metalloproteases with endothelin-1 (ET-1) converting activity were detected in membranous and cytosolic fractions of cultured endothelial cells (EC) from bovine carotid artery in a ratio of 5:1, respectively. The cytosolic enzyme specifically and quantitatively converts big ET-1 to ET-1 (Km = 10.7 microM), but does not convert big ET-3. Like the membranous enzyme, the cytosolic enzyme is only active at pH 6.5-7.5, and is competitively inhibited by phosphoramidon (Ki = 0.79 microM). The apparent molecular weight of the cytosolic enzyme is about 540 kD, which is 5-6 times greater than that of the membranous enzyme. These results indicate the presence of two types of phosphoramidon-sensitive neutral ET-converting enzyme in vascular EC.

Conversion of big endothelin-1 by membrane-bound metalloendopeptidase in cultured bovine endothelial cells

We propose a candidate for the "putative" endothelin (ET) converting enzyme in the cultured endothelial cells (ECs) of bovine carotid artery. The enzyme is membrane-bound, soluble in 0.5% Triton X-100, and capable of converting human big ET-1 to ET-1 by a specific cleavage between Trp21 and Val22. The conversion reached 90% after a 5-hr incubation in the presence of DFP, PCMS and pepstatin A, but it was inhibited by EDTA, omicron-phenanthroline or phosphoramidon. The enzyme is very sensitive to pH, and active only between pH 6.6 and pH 7.6. Conversion of big ET-3 by this enzyme was only 1/9 that of big ET-1. From these results, ET-1 converting enzyme in the bovine EC is most likely to be a membrane-bound, neutral metalloendopeptidase, which is much less susceptible to big ET-3.