Electron microscopy studies of alpha 2-macroglobulin subunit association after limited reduction with dithiothreitol

The subunits of human alpha 2-macroglobulin (alpha 2M) were dissociated by treatment with reductant (0.5 mM dithiothreitol) under mild conditions. Intact tetramers, half-molecules (subunit dimers), and monomers were identified by chromatography on Superose-6. These products were not in rapidly reversible equilibrium since purified half-molecules were completely stable for up to 18 h. Monomers slowly associated to form some half-molecules in the same time period. Negatively stained preparations of monomers and half-molecules demonstrated significant heterogeneity by electron microscopy. This heterogeneity probably reflected structural differences as well as variation in projection and staining. After reaction with methylamine or proteinase, half-molecules and purified monomers reassociated. The principal product was an intact tetramer displaying an "H-like" image which was visually indistinguishable from the structure of intact tetrameric alpha 2M after conformational change. Incompletely reassociated alpha 2M species were also identified after performing chromatography to increase the fraction of these products. Images resembling one-half of the intact tetrameric alpha 2M structure (epsilon-image) or three-quarters of the intact tetramer (chair-image) were observed. These studies demonstrate that reassociation of reductant-dissociated alpha 2M subunits occurs by the formation of appropriate interactions, so that the resulting tetramers are equivalent to nontreated alpha 2M-trypsin. Unlike native alpha 2M, the structure of conformationally transformed alpha 2M is relatively insensitive to the loss of interchain disulfide bonds. We propose that reassembly of alpha 2M subunits occurs by the binding of two half-molecules or by the addition of individual subunits to half-molecules or trimers.

Receptor-recognized alpha 2-macroglobulin-methylamine elevates intracellular calcium, inositol phosphates and cyclic AMP in murine peritoneal macrophages

Human plasma alpha 2-macroglobulin (alpha 2M) is a tetrameric proteinase inhibitor, which undergoes a conformational change upon reaction with either a proteinase or methylamine. As a result, a receptor recognition site is exposed on each subunit of the molecule enabling it to bind to its receptors on macrophages. We have used Fura-2-loaded murine peritoneal macrophages and digital video fluorescence microscopy to examine the effects of receptor binding on second messenger levels. alpha 2M-methylamine caused a rapid 2-4-fold increase in intracellular Ca2+ concentration ([Ca2+]i) within 5 s of binding to receptors. The agonists induced a focal increase in [Ca2+]i that spread out to other areas of the cell. The increase in [Ca2+]i was dependent on the alpha 2M-methylamine concentration and on the extracellular [Ca2+]. Both sinusoidal and transitory oscillations were observed, which varied from cell to cell. Neither alpha 2M nor boiled alpha 2M-methylamine, forms that are not recognized by the receptor, affected [Ca2+]i in peritoneal macrophages under identical conditions of incubation. The alpha 2M-methylamine-induced rise in [Ca2+]i was accompanied by a rapid and transient increase in macrophage inositol phosphates, including inositol tris- and tetrakis-phosphates. Native alpha 2M did not stimulate a rise in inositol phosphates. Finally, binding of alpha 2M-methylamine to macrophages increased cyclic AMP transiently. Thus receptor-recognized alpha-macroglobulins behave as agonists whose receptor binding causes stimulation of signal transduction pathways.

Identification of monomeric alpha-macroglobulin proteinase inhibitors in birds, reptiles, amphibians and mammals, and purification and characterization of a monomeric alpha-macroglobulin proteinase inhibitor from the American bullfrog Rana catesbeiana

The alpha-macroglobulins are classified as broad-spectrum inhibitors because of their ability to entrap proteinases of different specificities and catalytic class. Tetrameric and dimeric alpha-macroglobulins have been identified in a wide variety of organisms including those as primitive as the mollusc Octopus vulgaris; however, monomeric alpha-macroglobulin proteinase inhibitors have been previously identified only in rodents. The monomeric alpha-macroglobulin proteinase inhibitors are believed to be analogous to the evolutionary precursor of the multimeric members of this family exemplified by the tetrameric human alpha 2-macroglobulin. Until now, monomeric alpha-macroglobulin proteinase inhibitors have only been identified in rodents and have therefore been considered an evolutionary anomaly. However, in this report we have utilized several sensitive assays to screen various plasmas and sera for the presence of monomeric alpha-macroglobulins, and our results suggest that monomeric alpha-macroglobulin proteinase inhibitors are present in organisms belonging to the avian, reptilian, amphibian and mammalian classes of the chordate phylum. This indicates that these proteins are more widespread than previously recognized and that their presence in rodents is not an anomaly. To demonstrate further that the identified proteins were indeed monomeric alpha-macroglobulin proteinase inhibitors, we purified the monomeric alpha-macroglobulin from the American bullfrog Rana catesbeiana. We conclude that this protein is a monomer of 180 kDa on the basis of its behaviour on (i) pore-limit gel electrophoresis, (ii) non-reducing and reducing SDS/PAGE and (iii) gel-filtration chromatography. In addition, we demonstrate that this protein is an alpha-macroglobulin proteinase inhibitor by virtue of (i) its ability to inhibit proteinases of different catalytic class, (ii) the presence of a putative internal beta-cysteinyl-gamma-glutamyl thioester and (iii) an inhibitory mechanism characterized by steric protection of the proteinase active site and by sensitivity to small primary amines. The frog monomeric alpha-macroglobulin is structurally and functionally similar to the well-characterized monomeric alpha-macroglobulin proteinase inhibitor rat alpha 1-inhibitor-3.

Streptokinase and human fibronectin share a common epitope: implications for regulation of fibrinolysis and rheumatoid arthritis

Rheumatoid arthritis is a disease characterized by a destructive inflammatory process in joints. Fibronectin (FN) is present at a high concentration in rheumatoid synovial tissue and it is a chemoattractant for inflammatory cells. FN fragments also play significant and specific roles in promoting inflammation. In the present study, we demonstrate that FN and the streptococcal plasminogen activator streptokinase (SK) share a common epitope which is recognized by both a rabbit anti-SK IgG and a human anti-SK IgG isolated from the serum of a rheumatoid arthritis patient. This cross-reactive antibody was present in the plasma of 40 patients with rheumatoid arthritis. The region of homology is present in a 90-kDa FN fragment generated by plasmin (Pm) digestion of FN. Amino terminal sequence analysis of this fragment demonstrates that it contains the cell binding domain of FN and the domain responsible for plasminogen binding. The epitope common to SK and FN is not reactive in native FN and it is exposed as a consequence of Pm digestion. It is, however, exposed in native SK. Examination of the sequences of FN and SK indicates a region of homology containing the sequence LTSRPA. This sequence, moreover, is present in the 90-kDa FN fragment generated by Pm digestion. The sequence is present in the amino terminal domain of SK which is essential for its ability to serve as a plasminogen activator. LTSPRA coupled to a carrier protein also reacts with anti-SK antibodies obtained from rabbit or the plasma of patients with rheumatoid arthritis. These studies suggest that the Pm-generated FN 90-kDa fragment may react with circulating antibodies originally elicited by streptococcal infections. These immune complexes may play a role in the etiology of rheumatoid arthritis.

Receptor-mediated antigen delivery into macrophages. Complexing antigen to alpha 2-macroglobulin enhances presentation to T cells

Macrophages secrete alpha 2-macroglobulin (alpha 2M), a protein that may facilitate early Ag handling. alpha 2M is able to entrap and form covalent linkages with diverse proteins during a transient proteinase-activated state. The resulting complexes are rapidly endocytosed after binding to high affinity receptors. Such a system could be capable of efficiently delivering a multitude of proteins to macrophages. We have used T hybridoma clones that respond only to hen egg lysozyme, in a MHC-restricted manner, to probe the effect of complex formation on Ag uptake and processing by murine macrophages. Radiolabeled lysozyme was internalized more rapidly and to a greater extent when bound to alpha 2M than when unbound. Macrophages pulsed with lysozyme-alpha 2M-elastase complexes required 200 to 250 times less Ag than those pulsed with free lysozyme to achieve effective presentation to T cells. Adding equimolar amounts of alpha 2M-elastase complexes, or of alpha 2M-methylamine, to free lysozyme had no effect on basal lysozyme presentation. Receptor-recognized forms of alpha 2M, but not lysozyme or BSA, competed effectively for both uptake and presentation of lysozyme-alpha 2M-elastase complexes. These results indicate that proteinase-activated alpha 2M can enhance Ag processing by carrying Ag into macrophages through a receptor-mediated process.

Receptor-mediated antigen delivery into macrophages. Complexing antigen to alpha 2-macroglobulin enhances presentation to T cells

Macrophages secrete alpha 2-macroglobulin (alpha 2M), a protein that may facilitate early Ag handling. alpha 2M is able to entrap and form covalent linkages with diverse proteins during a transient proteinase-activated state. The resulting complexes are rapidly endocytosed after binding to high affinity receptors. Such a system could be capable of efficiently delivering a multitude of proteins to macrophages. We have used T hybridoma clones that respond only to hen egg lysozyme, in a MHC-restricted manner, to probe the effect of complex formation on Ag uptake and processing by murine macrophages. Radiolabeled lysozyme was internalized more rapidly and to a greater extent when bound to alpha 2M than when unbound. Macrophages pulsed with lysozyme-alpha 2M-elastase complexes required 200 to 250 times less Ag than those pulsed with free lysozyme to achieve effective presentation to T cells. Adding equimolar amounts of alpha 2M-elastase complexes, or of alpha 2M-methylamine, to free lysozyme had no effect on basal lysozyme presentation. Receptor-recognized forms of alpha 2M, but not lysozyme or BSA, competed effectively for both uptake and presentation of lysozyme-alpha 2M-elastase complexes. These results indicate that proteinase-activated alpha 2M can enhance Ag processing by carrying Ag into macrophages through a receptor-mediated process.

Expression of a functional alpha-macroglobulin receptor binding domain in Escherichia coli

We have expressed receptor-binding domains of human alpha 2-macroglobulin and rat alpha 1-macroglobulin in Escherichia coli. Expression levels of both recombinants were quite high, but the human one was insoluble, probably forming inclusion bodies. The rat domain, which lacks the human disulfide, was produced in a soluble form and readily purified by two simple chromatographic steps. Purified recombinant rat alpha 1-macroglobulin receptor-binding domain was fully functional in binding to the alpha-macroglobulin receptor on human fibroblasts. This 142 residue domain should serve as an excellent template for analyzing the structural requirements for alpha-macroglobulin receptor ligation and dissecting the varied biological functions resulting from such ligation.

Lipoprotein (a) promotes plasmin inhibition by alpha 2-antiplasmin

Plasmin inhibition by alpha 2-antiplasmin (alpha 2AP) is regulated by the vascular components fibrin(ogen) fragments, plasminogen and lipoprotein (a). Kinetic analysis demonstrates that CNBr-derived fibrinogen fragments completely protect plasmin from alpha 2AP. Plasminogen and 6-aminohexanoic acid decrease the rate of inhibition by 5- and 10-fold respectively. These studies show that CNBr-derived fibrinogen fragments and 6-aminohexanoic acid bind plasmin kringle(s) with binding constants of 2 micrograms/ml and 120 microM respectively, and that plasminogen binds to alpha 2AP with an affinity of 0.5 nM. The unmodulated inhibition is not effected by the presence of lipoprotein (a), but in the presence of protective CNBr-derived fibrinogen fragments the rate of inhibition is increased by the presence of the lipoprotein. The kinetics demonstrate that lipoprotein (a) binds to CNBr-derived fibrinogen fragments with an affinity of 4 nM, displacing plasmin from the protective surface. In addition, tissue-type plasminogen activator and trypsin inhibition by alpha 2AP is not slowed by the presence of CNBr-derived fibrinogen fragments or plasminogen (Pg), respectively. These kinetics suggest that the initial reversible interaction between plasmin and alpha 2AP is mediated by binding of the inhibitor to the kringle 1 domain of plasmin, with a reversible inhibition constant (Ki) of 5.0 x 10(-10) M. Under conditions where this kringle-inhibitor interaction is blocked, the reversible inhibition still occurs between the plasmin and alpha 2AP, but the initial Ki is increased to 5.0 x 10(-9) M. These data suggest that, in the circulation, plasmin inhibition by alpha 2AP may be down-regulated by fibrin, fibrin(ogen) fragments and Pg, but up-regulated by lipoprotein (a) in the presence of fibrin or fibrin(ogen) fragments. The lipoprotein (a)-mediated promotion of plasmin inhibition may provide an additional mechanism by which the lipoprotein impairs fibrinolysis and promotes atherosclerosis.

Ionic modulation of the effects of heparin on plasminogen activation by tissue plasminogen activator: the effects of ionic strength, divalent cations, and chloride

Ionic strength, divalent cations, and Cl- modulate the ability of the glycosaminoglycan heparin to stimulate the activation of human plasminogen (Pg) by tissue-type Pg activator. Kinetic analysis of Pg activation indicates that heparin is inhibitory, stimulatory, or nonstimulatory as a function of ionic strength. While increasing ionic strength inhibits Pg activation in the absence of heparin, in it presence an activation phase followed by an inhibitory phase is observed. Divalent cations, inhibitors of activation in the absence of heparin, increase the rate of activation in its presence. Kinetic analysis demonstrates that divalent cations augment the heparin stimulatory effect a maximum of 60-fold due to increases in kcat without changes in Km of the reaction. This effect is heparin-specific, since activation is not affected by Ca2+ in the presence of heparan sulfate or de-N-sulfated heparin. Also, Cl- inhibits Pg activation in the presence of heparin by acting as a competitive inhibitor (Kic of 100 mM). Furthermore, inhibition by Cl- reduces the overall magnitude of heparin stimulation of Pg activation. These results suggest that physiologic ions in combination with heparin may be significant effectors of Pg activation in the vascular microenvironment.

Purification and characterization of an alpha-macroglobulin proteinase inhibitor from the mollusc Octopus vulgaris

The cell-free haemolymph of the mollusc Octopus vulgaris inhibited the proteolytic activity of the thermolysin against the high-molecular-mass substrate hide powder azure. The purified inhibitor was a glycoprotein composed of two identical 180 kDa disulphide-linked subunits. In addition to the inhibition of the metalloproteinase thermolysin, the protein inhibited the serine proteinases human neutrophil elastase, pig pancreatic elastase, bovine chymotrypsin, bovine trypsin and the cysteine proteinase papain. A fraction of the proteinase-inhibitor complex resisted dissociation after denaturation indicating that some of the proteinase molecules became covalently bound. The nucleophile beta-aminopropionitrile decreased the covalent binding of proteinases to the Octopus vulgaris protein, suggesting that this interaction is mediated by an internal thiol ester; the reactivity and the amino acid sequence flanking the reactive residues of the putative thiol ester were consistent with this hypothesis. Bound trypsin remained active against the low-molecular-mass chromatogenic substrate H-D-Pro-Phe-Arg p-nitroanilide and was protected from inhibition by active-site-directed protein inhibitors of trypsin; however, the bound trypsin was readily inhibited by small synthetic inhibitors. This indicates that the inhibition of proteinases is accomplished by steric hindrance. The proteinase-inhibitory activity of this protein is characteristic of inhibition by mammalian alpha-macroglobulins and the presence of a putative thiol ester suggests that the Octopus vulgaris proteinase inhibitor is a homologue of human alpha 2-macroglobulin.

Effect of desialylation on the biological properties of human plasminogen

There are two major isoenzymes of plasminogen (Pg) in human plasma, designated Pg1 and Pg2. Both Pg forms have an identical primary structure, but differ in their extent of glycosylation. Removal of the oligosaccharide chains alters the normal physiological function of the zymogen and decreases the circulation time of both Pg glycoforms. Recent studies in our laboratory demonstrated that Pg2, with one carbohydrate chain, binds to the surface of U937 monocytoid cells considerably better than Pg1, with two carbohydrate chains, indicating a major role for the carbohydrate chains as determinants for differential binding to the cell surface [Gonzalez-Gronow, Grenett, Fuller & Pizzo (1990) Biochim. Biophys. Acta 1039, 269-276]. In this report we provide evidence that removal of terminal sialic acid from the Thr345-linked oligosaccharide chain of Pg2 is accompanied by the appearance of spontaneous amidolytic and fibrinolytic activity in the single-chain zymogen. Kinetic data demonstrate that asialo-Pg hydrolyses peptide substrates approximately 10% as efficiently as Pm. In addition, the change in carbohydrate content also alters Pg binding to U937 cells. Asialo-Pg binds to U937 cells with a decreased capacity but with a greater affinity than native Pg. Furthermore, asialo-Pg does not compete with native Pg for cell binding. These studies directly demonstrate that the oligosaccharide chains contribute to the heterogeneity observed in the physicochemical and biological properties of Pg1 and Pg2.

Binding of human plasminogen to basement-membrane (type IV) collagen

Plasminogen, the zymogen form of the serine proteinase plasmin, has been implicated in numerous physiological and pathological processes involving extracellular-matrix remodelling. We have previously demonstrated that the activation of plasminogen catalysed by tissue plasminogen activator is dramatically stimulated in the presence of basement-membrane-specific type IV collagen [Stack, Gonzalez-Gronow & Pizzo (1990) Biochemistry 29, 4966-4970]. The present paper describes the binding of plasminogen to type IV collagen. Plasminogen binds to both the alpha 1(IV) and alpha 2(IV) chains of basement-membrane collagen, with binding to the alpha 2(IV) chain preferentially inhibited by 6-aminohexanoic acid. This binding is specific and saturable, with Kd,app. values of 11.5 and 12.7 nM for collagen and gelatin respectively. Although collagen also binds to immobilized plasminogen, this interaction is unaffected by 6-aminohexanoic acid. Limited elastase proteolysis of plasminogen generated distinct collagen-binding fragments, which were identified as the kringle 1-3 and kringle 4 domains. No binding of collagen to mini-plasminogen was observed. These studies demonstrate a specific interaction between plasminogen and type IV collagen and provide further evidence for regulation of plasminogen activation by protein components of the extracellular matrix.

Lipoprotein(a) inhibits plasminogen activation in a template-dependent manner

Lipoprotein(a) [Lp(a)] is a low density lipoprotein whose plasma levels strongly correlate with the occurrence of atherosclerotic disease. Structural studies have demonstrated that Lp(a) contains two disulphide bonded subunits, one of which has structural similarity to plasminogen. This subunit, designed apo-lipoprotein(a), contains multiple repeat copies of a kringle homologous to kringle-4 of plasminogen, one copy of a kringle-5-like structure and a domain homologous to the catalytic light chain of plasmin. This subunit, however, lacks the site where plasminogen activators cleave plasminogen to generate the active proteinase. Recent studies demonstrate that Lp(a) competes with plasminogen for binding to endothelial cells and macrophages and thus prevents assembly of the fibrinolytic system on cell surfaces. Lp(a) also inhibits activation of plasminogen by streptokinase, urokinase-type plasminogen activator or tissue-type plasminogen activator (t-PA). Inhibition of plasminogen activation by t-PA requires the presence of a template on which activation occurs. This template can be either fibrin or heparin. This review considers the role of Lp(a) as an inhibitor of template-dependent activation of the fibrinolytic system.

Heparin oligosaccharides enhance tissue-type plasminogen activator: a correlation between oligosaccharide length and stimulation of plasminogen activation

The rate of plasminogen (Pg) activation by tissue-type Pg activator (t-PA) is enhanced by heparin-derived oligosaccharides. Kinetic analysis of the effects of heparin oligosaccharides, ranging in size from di- to dodecasaccharides, on Pg activation demonstrates that stimulation of the reaction is dependent on the size of the heparin oligosaccharides. Di- and tetrasaccharides enhance the activation through 2-fold increases in kcat and 4-fold decreases in Km. Hexasaccharide and larger oligosaccharides stimulate the reaction by increasing the kcat by as much as 4-fold, but do not affect the Km. Previous experiments have shown that lipoprotein(a) [Lp(a)] inhibits Pg activation by t-PA, but only in the presence of a template which enhances t-PA activity such as fibrinogen fragments or intact heparin. Similiarly, Lp(a) inhibits the enhancement of t-PA activity by the larger heparin oligosaccharides but has no effect on t-PA activity in the presence of di- and tetrasaccharides. The results of this study when considered with our previous observations (Edelberg & Pizzo, 1990) suggest that the enhancement in Pg activation by the smaller oligosaccharides is mediated exclusively via binding to t-PA while the larger oligosaccharides may interact with both t-PA and Pg. Furthermore, studies of Pg activation in the presence of both heparin oligosaccharides and fibrinogen fragments demonstrate that t-PA is stimulated preferentially by fibrinogen fragments.

Kinetics and physiologic relevance of the inactivation of alpha 1-proteinase inhibitor, alpha 1-antichymotrypsin, and antithrombin III by matrix metalloproteinases-1 (tissue collagenase), -2 (72-kDa gelatinase/type IV collagenase), and -3 (stromelysin)

Serpins encompass a superfamily of proteinase inhibitors that regulate many of the serine proteinases involved in inflammation and hemostasis. In vitro, many serpins are catalytically inactivated by proteinases that they do not inhibit, leading to the concept of proteolytic down-regulation of serpin inhibitory capacity. The extent to which down-regulation of serpin activity occurs in vivo is debated, since little is known of the rates at which the process occurs. To address this debate, we have measured the rates of inactivation of three serpins, alpha 1-proteinase inhibitor (alpha 1PI), alpha 1-antichymotrypsin (alpha 1ACT), and antithrombin III (ATIII), by three human matrix metalloproteinases (MMPs-1, -2, and -3) thought to be involved in tissue destruction and repair. Our object was to establish a working kinetic model which can be used to predict whether serpin inactivation by these proteinases is likely to occur in vivo. We determined the rates of inactivation of these three serpins by each of the MMPs and compared these to rates of inhibition of the MMPs by an endogenous inhibitor, alpha 2-macroglobulin. An equation designed to predict the extent of substrate hydrolyzed by an enzyme in the presence of an enzyme inhibitor gave the following predictions of the inactivation in vivo: (i) ATIII is unlikely to be inactivated by the MMPs. (ii) MMP-2 (72-kDa gelatinase/type IV collagenase) is unlikely to inactivate any of the three serpins. (iii) MMP-1 (tissue collagenase) will inactivate alpha 1PI and alpha 1ACT only when its concentration saturates that of its controlling inhibitors. (iv) MMP-3 (stromelysin) may inactivate small amounts of alpha 1PI and more significant amounts of alpha 1ACT, even in the presence of its controlling inhibitors. Any physiologic or pathologic inactivation of these serpins by these MMPs that occurs in vivo will probably be due to MMP-3, and will likely only take place in tissues and inflammatory loci where the concentration of MMP inhibitors is depressed.

The exposure of murine macrophages to alpha 2-macroglobulin 'fast' forms results in the rapid secretion of eicosanoids

The exposure of [3H]arachidonate-radiolabelled murine peritoneal macrophages to alpha 2-macroglobulin-methylamine or alpha 2-macroglobulin-trypsin but not native alpha 2-macroglobulin (alpha 2M) results in the rapid secretion of [3H]eicosanoids. Resident peritoneal macrophages stimulated with 0.1 microM alpha 2M-methylamine exhibited an enhanced secretion within 10 min. The ability of alpha 2M 'fast' forms to stimulate secretion of [3H]eicosanoids was similar to that observed in the presence of the murine macrophage chemoattractant platelet-activating factor. As observed for total [3H]eicosanoid secretion, alpha 2M 'fast' forms also rapidly enhanced the secretion of the cAMP-elevating prostanoid, prostaglandin E2, from resident peritoneal macrophages. Stimulated secretion of prostaglandin E2 in response to 0.1 microM alpha 2M-methylamine was less rapid than that observed using 0.1 microM platelet-activating factor. Similar amounts of secreted prostaglandin E2 were present in media of macrophage cultures after 1 h exposure to the two stimuli. In the presence of 0.1 microM alpha 2M-methylamine, secreted prostaglandin E2 remained elevated, compared to the appropriate buffer control, for at least 24 h. The present results indicate that receptor recognition of alpha 2M 'fast' forms by macrophages results in the rapid stimulation of eicosanoid secretion and suggest that secretion of prostaglandin E2 and other eicosanoids may be involved in the ability of alpha 2 M 'fast' forms to regulate various macrophage functional responses.

Limited proteolysis of the alpha-macroglobulin rat alpha 1-inhibitor-3. Implications for a domain structure

Rat alpha 1-inhibitor-3 is a 180-kDa monomeric proteinase inhibitor found in high concentration in rat plasma. By several criteria it has been shown to be a member of the family of alpha-macroglobulin proteinase inhibitors often exemplified by the tetrameric human alpha 2-macroglobulin. We have used limited proteolysis of rat alpha 1-inhibitor-3 to probe the domain structure of this family of proteins. Proteinases of different specificities, including trypsin, chymotrypsin, thermolysin, and Staphylococcus aureus V8 proteinase, were employed and a common fragmentation pattern was observed when the reaction products were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. These fragments were electrotransferred to polyvinylidene difluoride membranes and subjected to NH2-terminal amino acid sequence analysis in order to position them within the context of the primary structure. The fragmentation pattern may define the domain structure of alpha 1-inhibitor-3 and serve as a model for the domain organization of the family of alpha-macroglobulin proteinase inhibitors.

Kinetic analysis of the effects of glycosaminoglycans and lipoproteins on urokinase-mediated plasminogen activation

The glycosaminoglycans (GAGs) heparin, heparan sulphate and chondroitin 6-sulphate stimulate the rate of urokinase activation of human plasminogen. Kinetic analysis of plasminogen activation demonstrates that heparin, heparan sulphate and chondroitin 6-sulphate increased the catalytic rate (Kcat) by 5.3-, 3.5- and 2.5-fold respectively. These stimulatory GAGs had no effect on the affinity of urokinase for plasminogen, since the Km of the reaction is unaltered by the GAGs. The GAGs may enhance the rate of plasminogen activation through an interaction with the catalytic domain of the urokinase, with dissociation constants of approx. 30 nM. Additionally, the lipoproteins, lipoprotein (a) [Lp(a)] and low-density lipoprotein (LDL) inhibit heparin and heparan sulphate stimulation of plasmin formation. Lp(a) is a competitive inhibitor (Kic 20 nM) and LDL is a mixed inhibitor of heparin-enhanced urokinase-mediated plasminogen activation (Kic 24 nM and Kiu 60 nM). These inhibition constants correlate with physiological concentrations of these lipoproteins. These data suggest that these GAGs and lipoproteins may play an important role in vivo in regulating urokinase-mediated plasmin formation.

Plasmin binding to the plasminogen receptor enhances catalytic efficiency and activates the receptor for subsequent ligand binding

Specific cell surface receptors for plasminogen (Pg) are expressed by a wide variety of cell types. The colocalization of receptors for Pg and its activators restricts plasmin (Pm) activity to specific sites and serves to promote fibrinolysis and local Pg activation. These studies show that both Pg and Pm bind to cellular receptors on monocytoid U937 cells. Limited Pm pretreatment of the cells enhances total Pg binding and alters the kinetics of Pm binding. Furthermore, surface-bound Pg is converted to Pm in the absence of exogenous activators. Cell-bound Pm exhibits a 12-fold increase in catalytic efficiency (kcat/Km) relative to Pm free in solution. These studies demonstrate that Pg/Pm receptor occupancy can be regulated by Pm in the microenvironment and may play a significant regulatory role in fibrinolysis and extravascular proteolysis.

The inhibition of tissue type plasminogen activator by plasminogen activator inhibitor-1. The effects of fibrinogen, heparin, vitronectin, and lipoprotein(a)

Plasminogen activator inhibitor-1 (PAI-1) regulates fibrinolysis by inhibiting tissue type plasminogen activator (t-PA). Fibrinogen, heparin, and vitronectin enhance the rate of inhibition of t-PA by PAI-1. Kinetic studies indicate that both fibrinogen and heparin increase the second-order inhibition constant by a maximum of approximately 4-fold, whereas vitronectin increases the rate constant by a maximum of approximately 6-fold. The dissociation constants of fibrinogen, heparin, and vitronectin for the inhibition reaction were 200 nM, 20 nM, and 600 pM, respectively. In addition, PAI-1 inhibition of t-PA may be regulated by the presence of lipoprotein(a) (Lp(a)). Previous studies demonstrated that Lp(a) competes with plasminogen for the active site of fibrinogen- and heparin-bound t-PA. Kinetic studies described here demonstrate that Lp(a) prevents the inhibition of t-PA by PAI-1 in the presence of fibrinogen and heparin, but has no effect on the reaction in the presence of vitronectin or in the absence of either fibrinogen or heparin. The data suggest that fibrinogen and heparin may enhance the rate of inhibition through an interaction with t-PA, and that vitronectin may enhance the inhibition through an interaction with PAI-1. In addition, these experiments indicate that Lp(a) may regulate fibrinolysis by competing with PAI-1 and plasminogen for fibrinogen- and heparin-bound t-PA. These data suggest that PAI-1 inhibition of t-PA in vivo is primarily mediated via interaction with fibrinogen, heparin, vitronectin, and Lp(a), and therefore, the functional levels of PAI-1 activity in the vasculature may be regulated by the presence of these components.