Evidence for similar conformational changes in alpha 2-macroglobulin on reaction with primary amines or proteolytic enzymes

Frozen fresh blood plasma preserves the functionality of native human α2-macroglobulin

Human α₂-macroglobulin (hα₂M) is a large homotetrameric protein involved in the broad inhibition of endopeptidases. Following cleavage within a bait region, hα₂M undergoes stepwise transitions from its native, expanded, highly flexible, active conformation to an induced, compact, triggered conformation. As a consequence, the peptidase is entrapped by an irreversible Venus flytrap mechanism. Given the importance of hα₂M, biochemical studies galore over more than seven decades have attempted to ascertain its role, typically using authentic hα₂M purified from frozen and non-frozen fresh blood plasma, and even outdated plasma. However, hα₂M is sensitive once isolated and purified, and becomes heterogeneous during storage and/or freezing, raising concerns about the functional competence of frozen plasma-derived hα₂M. We therefore used a combination of native and sodium dodecylsulfate polyacrylamide gel electrophoresis, affinity and ion-exchange chromatography, multi-angle laser light scattering after size-exclusion chromatography, free cysteine quantification, and peptidase inhibition assays with endopeptidases of two catalytic classes and three protein substrates, to characterize the biochemical and biophysical properties of hα₂M purified ad hoc either from fresh plasma or frozen fresh plasma after thawing. We found no differences in the molecular or functional properties of the preparations, indicating that protective components in plasma maintain native hα₂M in a functionally competent state despite freezing.

Structural and functional insight into pan-endopeptidase inhibition by α2-macroglobulins

Peptidases must be exquisitely regulated to prevent erroneous cleavage and one control is provided by protein inhibitors. These are usually specific for particular peptidases or families and sterically block the active-site cleft of target enzymes using lock-and-key mechanisms. In contrast, members of the +1400-residue multi-domain α2-macroglobulin inhibitor family (α2Ms) are directed against a broad spectrum of endopeptidases of disparate specificities and catalytic types, and they inhibit their targets without disturbing their active sites. This is achieved by irreversible trap mechanisms resulting from large conformational rearrangement upon cleavage in a promiscuous bait region through the prey endopeptidase. After decades of research, high-resolution structural details of these mechanisms have begun to emerge for tetrameric and monomeric α2Ms, which use 'Venus-flytrap' and 'snap-trap' mechanisms, respectively. In the former, represented by archetypal human α2M, inhibition is exerted through physical entrapment in a large cage, in which preys are still active against small substrates and inhibitors that can enter the cage through several apertures. In the latter, represented by a bacterial α2M from Escherichia coli, covalent linkage and steric hindrance of the prey inhibit activity, but only against very large substrates.

alpha-Macroglobulins are present in some gram-negative bacteria: characterization of the alpha2-macroglobulin from Escherichia coli

alpha-Macroglobulins (alphaMs) are large glycoproteins that have been identified in a wide range of vertebrate and invertebrate species and are mostly thiol ester containing proteinase inhibitors. A recent analysis of bacterial genomes ( Budd, A., Blandin, S., Levashina, E. A., and Gibson, T. J. (2004) Genome Biol. 5, R38 ) identified many alpha-macroglobulin-like sequences that appear to have been acquired by Gram-negative bacteria from their metazoan hosts. We report the first expression and characterization of such a bacterial alpha-macroglobulin, that from Escherichia coli. This is also the first alpha-macroglobulin to be characterized that is predicted to be membrane-anchored. We found that the 183-kDa protein contains an intact thiol ester, is monomeric, and is localized to the periplasmic space. Reaction with proteinase results in limited cleavage within a bait region, rapid activation of the thiol ester, cross-linking to the attacking proteinase or other available nucleophiles, and partial protection of the proteinase against macromolecular substrates. Given these properties and the co-occurrence of the alphaM gene with one for a repair transglycosylase, this suggests a possible role for bacterial alphaMs in cell defense following host attack. Such a role would make bacterial alphaMs appropriate novel targets for antibiotic drugs.

A derivative of the plasma protease inhibitor alpha(2)-macroglobulin regulates the response to peripheral nerve injury

Peripheral nerve injury induces endoneural inflammation, controlled by diverse cytokines and extracellular mediators. Although inflammation is coupled to axonal regeneration, fulminant inflammation may increase nerve damage and neuropathic pain. alpha(2)-Macroglobulin (alpha2M) is a plasma protease inhibitor, cytokine carrier, and ligand for cell-signaling receptors, which exists in two well-characterized conformations and in less well-characterized intermediate states. Previously, we generated an alpha2M derivative (alpha(2)-macroglobulin activated for cytokine binding; MAC) similar in structure to alpha(2)M conformational intermediates, which binds tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta), and inhibits endotoxin toxicity. In this study, we report that the continuum of cytokines that bind to MAC includes IL-6 and IL-18. MAC inhibited TNF-alpha-induced p38 mitogen-activated protein kinase activation and cell death in cultured Schwann cells. When administered by i.p. injection to mice with sciatic nerve crush injury, MAC decreased inflammation and preserved axons. Macrophage infiltration and TNF-alpha expression also are decreased. MAC inhibited TNF-alpha expression in the chronic constriction injury model of nerve injury. When MAC was prepared using a mutated recombinant alpha2M, which does not bind to the alpha2M receptor, low-density lipoprotein receptor-related protein-1, activity in the chronic constriction injury model was blocked. These studies demonstrate that an alpha2M derivative is capable of regulating the response to peripheral nerve injury by a mechanism that requires low-density lipoprotein receptor-related protein-1.

Limited Mutations in Full-length Tetrameric Human α2-Macroglobulin Abrogate Binding of Platelet-derived Growth Factor-BB and Transforming Growth Factor-β1

alpha2-Macroglobulin (alpha2M) inhibits diverse extracellular proteases, binds growth factors such as platelet-derived growth factor-BB (PDGF-BB) and transforming growth factor-beta1 (TGF-beta1), and carries beta-amyloid peptide. alpha2M may also trigger cell signaling by binding to the low density lipoprotein receptor-related protein (LRP-1) and/or other cell surface receptors. Based on studies with recombinant alpha2M fragments expressed in bacteria and synthetic peptides, we previously localized a growth factor-binding site near the center of the alpha2M subunit. However, because intact alpha2M forms a hollow cylinder structure, an alternative model for growth factor binding involves nonspecific entrapment within the alpha2M core. To distinguish between these two models, we engineered mutations in the putative growth factor binding sequence of full-length alpha2M. These mutations did not perturb the tetrameric structure of alpha2M, reaction with proteases, the thiol ester bonds, or binding to LRP-1. A single mutation (E730R) completely blocked binding of platelet-derived growth factor-BB to intact alpha2M. E730R did not alter TGF-beta1 binding; however, this mutation in combination with mutations at Glu714 and Asp719 eliminated the increase in TGF-beta1 binding associated with alpha2M conformational change. These studies demonstrate that growth factor binding to intact alpha2M is specific, involving a defined region of the alpha2M subunit. The exact sequences required for binding different growth factors may be non-identical, mimicking the model of the bait region in which different proteases target adjacent and sometimes overlapping sequences.

The properties of rabbit alpha1-macroglobulin upon activation are distinct from those of rabbit and human alpha2-macroglobulins

We have characterized native and activated forms of rabbit alpha1M and compared them to rabbit and human alpha2M. Similar to human alpha2M, rabbit alpha1M is a tetramer associated via disulfide bonds and non-covalent interactions that exhibits autolysis into two fragments when heated. Like human alpha2M, rabbit alpha1M is cleaved by trypsin at one site; however, rabbit alpha1M shares characteristics with rabbit alpha2M that are different from the properties of human alpha2M. Amine or trypsin treatment of rabbit alpha-macroglobulins does not result in a significant conformational change or cleavage of four thiolester bonds. Full thiolester cleavage is only observed for rabbit alpha1M after exposure to both trypsin and a small amine. Additionally, amine-treated rabbit alpha-macroglobulins retain trypsin inhibitory potential and do not fully shield bound proteinases. Methylamine and trypsin treatment of rabbit alpha1M results in two dissimilar conformations that display differing exposure of the receptor-recognition site. While ammonia- and methylamine-modified rabbit alpha1M bind to macrophages with similar affinity to that of human alpha2M, trypsin-treated rabbit alpha1M exhibits dramatically lower affinity. This suggests that rabbit alpha1M may not play the same proteinase-inhibiting physiological role as human alpha2M.

Growth factor-binding sequence in human alpha2-macroglobulin targets the receptor-binding site in transforming growth factor-beta

alpha(2)-Macroglobulin (alpha(2)M) binds transforming growth factor-beta1 (TGF-beta1) and TGF-beta2, forcing these growth factors into a state of latency. The mechanism by which this occurs remains unclear. In this paper, we demonstrate that peptides, derived from the structure of human alpha(2)M (amino acids 714-729), bind directly to TGF-beta1 and block the binding of TGF-beta1 to the type I and II TGF-beta receptors. The alpha(2)M-derived peptides are notable for hydrophobic tripeptide sequences (WIW or VVV) and acidic residues (Glu(714) and Asp(719) in the mature alpha(2)M subunit), which may function analogously to the structural elements that mediate TGF-beta-binding in the type II receptor. Mutating Glu(714) and Asp(719) in the alpha(2)M-peptide-GST fusion protein, FP3, which contains the putative growth factor-binding site, significantly decreased the binding affinity of FP3 for TGF-beta1. The alpha(2)M-derived peptides, which bind TGF-beta1, inhibited the interaction of TGF-beta1 with its receptors in fetal bovine heart endothelial cells. The same peptides also inhibited the activity of TGF-beta1 in endothelial cell proliferation assays. These results demonstrate that alpha(2)M-derived peptides target the receptor-binding sequence in TGF-beta.

Purification and characterization of alpha 2-macroglobulin from the white shrimp (Penaeus vannamei)

alpha(2)-Macroglobulin (alpha(2)M) is a broad-spectrum protease-binding protein abundant in plasma from vertebrates and several invertebrate phyla. This protein was purified from cell-free hemolymph of the white shrimp, Penaeus vannamei, using Blue-Sepharose and Phenyl-Sepharose chromatography. The shrimp alpha(2)M is a 380 kDa protein, a homodimer of two apparently identical subunits of approximately 180 kDa linked by disulphide bridges. The amino acid sequence of the N-terminus is similar to the Limulus alpha(2)M counterpart. The shrimp alpha(2)M has a wide inhibition spectrum against different proteinase types including trypsin, leucine amino peptidase, chymotrypsin, elastase and papain. The secondary structure of shrimp alpha(2)M is mainly beta-sheet (36%), with a characteristic minimum elipticity at 217 nm. Evidence for a thiolester-mediated inhibition mechanism of proteases by alpha(2)M was provided by inactivation with methylamine.

A slow form of alpha-2-macroglobulin in diseased and healthy dogs

Alpha-macroglobulins (AMs) function as non-specific protease inhibitors by using a so-called trapping mechanism, which is a compaction of the molecule that can be seen as a "fast" form in native polyacrylamide gel electrophoresis (PAGE). AMs also play a role in the transport and clearance of cytokines and growth factors from the circulation. In the dog, two AMs are known, alpha-1-macroglobulin (A1M) and alpha-2-macroglobulin (A2M). Using agarose and polyacrylamide gel electrophoresis of canine serum or plasma, we detected a cathodal, slow form of A2M. Upon activation with elastase, slow A2M resembled normal A2M in agarose gel electrophoresis, showing decreased negative charge at semi-saturation but not at full saturation with enzyme. In PAGE, however, slow A2M, unlike normal A2M, did not exhibit a "fast" form after short-term incubation with elastase. After incubation overnight, the "fast" form was seen, indicating a retarded reaction. Incubation of slow A2M with ammonium sulphate, a known activator of AMs, resulted in decreased negative charge in agarose gel electrophoresis and no reaction or partial reaction in PAGE. Slow A2M is present in fresh blood samples or may develop from partial alterations within a few days of storage. Moreover, it is sometimes reversible. Our findings may indicate that slow A2M is a result of an instability of the molecule, leading to a conformational change, which affects electrical charge and impairs the ability to develop into the "fast" form upon activation. This may lead to a delayed clearance of protease and inflammatory mediators from the circulation. Slow A2M was predominantly found in diseased dogs, especially in the Labrador retriever.

Role of alpha2-macroglobulin in regulating amyloid beta-protein neurotoxicity: protective or detrimental factor?

alpha2-Macroglobulin (alpha2M) has been identified as a carrier protein for beta-amyloid (Abeta) decreasing fibril formation and affecting the neurotoxicity of this peptide. The alpha2-macroglobulin receptor/low density lipoprotein receptor related protein (LRP) is involved in the internalization and degradation of the alpha2M/Abeta complexes and its impairment has been reported to occur in Alzheimer's disease. Previous studies have shown alpha2M to determine an enhancement or a reduction of Abeta toxicity in different culture systems. In order to clarify the role of alpha2M in Abeta neurotoxicity, we challenged human neuroblastoma cell lines with activated alpha2M in combination with Abeta. Our results show that in neuroblastoma cells expressing high levels of LRP, the administration of activated alpha2M protects the cells from Abeta neurotoxicity. Conversely, when this receptor is not present alpha2M determines an increase in Abeta toxicity as evaluated by MTT and TUNEL assays. In LRP-negative cells transfected with the full-length human LRP, the addition of activated alpha2M resulted to be protective against Abeta-induced neurotoxicity. By means of recombinant proteins we ascribed the neurotoxic activity of alpha2M to its FP3 fragment which has been previously shown to bind and neutralize transforming growth factor-beta. These studies provide evidence for both a neuroprotective and neurotoxic role of alpha2M regulated by the expression of its receptor LRP.

Uric acid mediates photodynamic inactivation of caprine alpha-2-macroglobulin

Uric acid (2,6,8 trioxopurine), the end product of purine metabolism in mammalian systems, has shown a wide range of antioxidant properties including scavenging of hydroxyl radical and singlet oxygen. In this study we show that in the presence of visible light, uric acid disrupted caprine alpha-2-macroglobulin (alpha(2) M) structure and antiproteolytic function in vitro. Proteinase cleaves the bait region of caprine inhibitor inducing major conformational changes and entrapping the enzyme within its molecular cage. In contrast to native alpha(2) M, modified antiproteinase lost half of its antiproteolytic potential within 4 hours of uric acid exposure. The changes in uv-absorption spectra of the treated protein suggested possible spatial rearrangement of subunits or conformational change. Analysis of the mechanism by which alpha(2) M was inactivated revealed that the process was dependent on generation of superoxide anion and hydrogen peroxide. Our findings suggest that antiproteolytic activity of caprine alpha(2) M could be compromised via oxidative modification mediated by uric acid. Moreover, low concentrations of alpha(2) M were found to stimulate superoxide production by some unknown mechanism.

Identical or overlapping sequences in the primary structure of human alpha(2)-macroglobulin are responsible for the binding of nerve growth factor-beta, platelet-derived growth factor-BB, and transforming growth factor-beta

alpha(2)-Macroglobulin (alpha(2)M) functions as a proteinase inhibitor and as a carrier of diverse growth factors. In this study, we localized binding sites for platelet-derived growth factor-BB (PDGF-BB) and nerve growth factor-beta (NGF-beta) to a linear sequence in the 180-kDa human alpha(2)M subunit which includes amino acids 591-774. A glutathione S-transferase fusion protein containing amino acids 591-774 (FP3) bound PDGF-BB and NGF-beta in ligand blotting assays whereas five other fusion proteins, which collectively include amino acids 99-590 and 775-1451 did not. The K(D) values for PDGF-BB and NGF-beta binding to immobilized FP3 were 300 +/- 40 and 180 +/- 30 nM, respectively; these values were comparable with those determined using methylamine-modified alpha(2)M, suggesting that higher-order alpha(2)M structure is not necessary for PDGF-BB and NGF-beta binding. PDGF-BB and NGF-beta blocked the binding of transforming growth factor-beta1 (TGF-beta1) to FP3. Furthermore, murinoglobulin, which is the only known member of the alpha-macroglobulin family that does not bind TGF-beta, also failed to bind PDGF-BB and NGF-beta. These results support the hypothesis that either a single linear sequence in human alpha(2)M or overlapping sequences are responsible for the binding of TGF-beta, PDGF-BB, and NGF-beta, even though there is minimal sequence identity between these three growth factors. FP3 blocked the binding of PDGF-BB to a purified chimeric protein, in which the extracellular domain of the PDGF beta receptor was fused to the IgG(1) Fc domain, and to PDGF receptors on NIH 3T3 cells. Thus, FP3 may inhibit the activity of PDGF-BB.

Interleukin-4 and IL-10 bind covalently to activated human alpha2-macroglobulin by a mechanism that requires Cys949

Alpha2-macroglobulin (alpha2M) functions as an extracellular carrier of diverse cytokines, including transforming growth factor-beta1 (TGF-beta1), that expresses anti-inflammatory activities. The results presented here demonstrate that interleukin-10 (IL-10) and IL-4, which also regulate the inflammatory response, bind to alpha2M. Unlike TGF-beta, IL-4 and IL-10 bind almost exclusively to the receptor-recognized, or activated, form of alpha2M. Purified IL-4-alpha2M complexes were predominantly covalent due to thiol disulfide exchange involving Cys949 in the alpha2M subunit. Blocking Cys949 with iodoacetamide significantly inhibited IL-4- and IL-10 binding. Bovine serum albumin (BSA), which possesses a free Cys residue and undergoes thiol disulfide exchange reactions, did not compete with alpha2M for the binding of IL-4 or IL-10. These results suggest a model in which IL-4 and IL-10 associate with activated alpha2M to form complexes that are initially noncovalent but unstable. In these complexes, Cys949 is properly aligned to undergo thiol disulfide exchange and generate stable, covalent IL-4-alpha2M and IL-10-alpha2M complexes.

Interaction of transforming growth factor-beta-1 with alpha-2-macroglobulin from normal and inflamed equine joints

Binding between equine plasma alpha-2-macroglobulin (alpha 2M) and several cytokines known to participate in inflammatory reactions in other species was initially examined. Plasma was obtained from 5 horses with various abnormalities. Samples, both untreated and after reaction with methylamine, were incubated with exogenous, radiolabeled, porcine-derived transforming growth factor-beta-1 (125I-TGF-beta 1), recombinant human interleukin-1-beta (125I-IL-1 beta), and recombinant human tumor necrosis factor-alpha (125I-rhTNF-alpha). They were then subjected to nondenaturing polyacrylamide gel electrophoresis (PAGE). Binding of the native (slow) and activated (fast) forms of alpha 2M to each cytokine was subjectively evaluated with autoradiography. Equine alpha 2M bound 125I-TGF-beta 1. However, poor or no binding was observed between alpha 2M and either of 125I-rhTNF-alpha or 125I-IL-1 beta. Synovial fluid was then obtained from 6 normal horses, 6 horses with septic arthritis, and 6 horses with degenerative joint disease. Untreated and methylamine-reacted samples were quantitatively examined for binding with 125I-TGF-beta 1, using the autoradiographic techniques described above and densitometry. Native and activated alpha 2M were also quantified by densitometry of PAGE gels. Native alpha 2M was significantly elevated in septic arthritis (6.4% to 29.5% of total protein detected) and degenerative joint disease (2.8% to 12.3%), compared to normal joints (0.9% to 4.2%). Activated alpha 2M, however, was not detected in untreated synovial fluid samples. In all plasma and joint fluid samples, whether untreated or reacted with methylamine, 125I-TGF-beta 1 bound predominantly to alpha 2M, and preferentially to the activated form of alpha 2M. In synovial fluid, the amount of 125I-TGF-beta 1 binding was proportional to the quantity of alpha 2M present. These results indicate that: 1) equine alpha 2M binds TGF-beta 1; 2) the native form of alpha 2M is present in both equine plasma and synovial fluid, and 3) alpha 2M is a major binding protein for TGF-beta 1 in equine synovial fluid. Therefore, alpha 2M may play a role in regulating this mediator of inflammation in equine joints.

Localization of the binding site for transforming growth factor-beta in human alpha2-macroglobulin to a 20-kDa peptide that also contains the bait region

alpha2-Macroglobulin (alpha2M) functions as a major carrier of transforming growth factor-beta (TGF-beta) in vivo. The goal of this investigation was to characterize the TGF-beta-binding site in alpha2M. Human alpha2M, which was reduced and denatured to generate 180-kDa subunits, bound TGF-beta1, TGF-beta2, and NGF-beta in ligand blotting experiments. Cytokine binding was not detected with bovine serum albumin that had been reduced and alkylated, and only minimal binding was detected with purified murinoglobulin. To localize the TGF-beta-binding site in alpha2M, five cDNA fragments, collectively encoding amino acids 122-1302, were expressed as glutathione S-transferase (GST) fusion proteins. In ligand blotting experiments, TGF-beta2 bound only to the fusion protein (FP3) that includes amino acids 614-797. FP3 bound 125I-TGF-beta1 and 125I-TGF-beta2 in solution, preventing the binding of these growth factors to immobilized alpha2M-methylamine (alpha2M-MA). The IC50 values were 33 +/- 5 and 26 +/- 6 nM for TGF-beta1 and TGF-beta2, respectively; these values were comparable with or lower than those determined with native alpha2M or alpha2M-MA. A GST fusion protein that includes amino acids 798-1082 of alpha2M (FP4) and purified GST did not inhibit the binding of TGF-beta to immobilized alpha2M-MA. FP3 (0.2 microM) neutralized the activity of TGF-beta1 and TGF-beta2 in fetal bovine heart endothelial (FBHE) cell proliferation assays; FP4 was inactive in this assay. FP3 also increased NO synthesis by RAW 264.7 cells, mimicking an alpha2M activity that has been attributed to the neutralization of endogenously synthesized TGF-beta. Thus, we have isolated a peptide corresponding to 13% of the alpha2M sequence that binds TGF-beta and neutralizes the activity of TGF-beta in two separate biological assays.

Native conformations of human complement components C3 and C4 show different dependencies on thioester formation

The thioester bond in complement components C3 and C4 and the protease inhibitor alpha2-macroglobulin have traditionally been thought of as fulfilling the dual roles of mediating covalent attachment and maintaining the native conformational states of these molecules. We previously reported that several human C3 thioester-region mutants, including variants E1012Q and C1010A, in the latter of which thioester-bond formation is precluded, display an unexpected phenotype. Despite the lack of a thioester bond in these mutants, they appear to adopt a native-like conformation as suggested by the finding that they are cleavable by the classical pathway C3 convertase, C4b2a, whereas the C3b-like C3(H2O) species is not. Subsequently, a species referred to as C3(NH3)* was described which potentially could account for the observations with the above mutants. C3(NH3)* is a transient species formed on aminolysis of native C3 that can spontaneously re-form the thioester bond. Importantly, it has a mobility on cation-exchange HPLC that is distinct from both native C3 and C3(H2O), but like the native molecule, it is cleavable by an alternative-pathway C3 convertase. In this study we showed by using cation-exchange HPLC as an additional conformational probe that C3 C1010A and E1012Q mutant proteins did not resemble C3(NH3)*. Instead they displayed a chromatographic behaviour that was indistinguishable from that of native C3. To assess the general applicability of these observations, we engineered the equivalent mutations into human C4, specifically C4 C1010A and C4 E1012Q. As expected, thioester-bond formation did not occur in either of these C4 mutants, but in contrast with the results with C3 we found no evidence for the formation of a stable native-like conformation in either C4 mutant, as assessed using cleavability by C1s as the conformational probe. A possible interpretation of our data is that the adoption of the native conformational state during biosynthesis of C3 and C4 is an energetically permissible process, even if it is not locked in via thioester-bond formation. Whereas this conformational state is stable in mature C3, it is unstable in mature C4, perhaps reflecting the additional post-translational cleavage of C4 before its secretion.

Expression of alpha 2 macroglobulin receptor/low density lipoprotein receptor-related protein is cell culture density-dependent in human breast cancer cell line BT-20

alpha 2Macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR/LRP) is a multifunctional cell plasma membrane receptor that binds and facilitates the endocytosis of a number of ligands involved in protease regulation and lipoprotein metabolism. In the invasive breast cancer cell line BT-20 we show that the expression of alpha 2MR/LRP can be strongly affected by cell culture density. By comparing measurements of mRNA, total cellular alpha 2MR/LRP antigen, and cell surface alpha 2MR/LRP expression we have demonstrated a marked cell density-dependent regulation of this receptor expression. Increasing the cell density results in a 3.4-fold increase in cell surface alpha 2MR/LRP expression. This corresponds to a marked increase in alpha 2MR/LRP mRNA in Northern blots of total RNA from cells cultured at high density and to consistent increases in alpha 2MR/LRP heavy chain in Western blots of cell lysates from high density cultures. These studies together demonstrate the significant up-regulation of alpha 2MR/LRP expression in BT-20 by increased cell density.

Chemical modification of alpha2-macroglobulin to generate derivatives that bind transforming growth factor-beta with increased affinity

alpha2-Macroglobulin (alpha2M) binds a number of cytokines, including transforming growth factor-beta1 (TGF-beta1) and TGF-beta2. The affinity of these interactions depends on the alpha2M conformation. In this investigation, we treated human alpha2M with cis-dichlorodiammineplatinum (II) (cis-Pt), a crosslinking reagent that partially 'locks' the alpha2M conformation, and then with methylamine to generate a preparation (alpha2M-P/M) consisting of stable alpha2M conformational intermediates. alpha2M-P/M bound TGF-beta1 and TGF-beta2 with higher affinity than any other form of alpha2M studied to date. The equilibrium dissociation constants were 14 and 2 nM for TGF-beta1 and TGF-beta2, respectively. alpha2M-P/M, at 100 nM, neutralized the activity of TGF-beta1 by about 75% in an endothelial cell proliferation assay. The equivalent concentration of native alpha2M or methylamine-modified alpha2M had no effect. These studies demonstrate that the potential of alpha2M as a cytokine carrier and neutralizer may not be fully realized in either the native or completely activated conformations.

The contact zones in human alpha2-macroglobulin--functional domains important for the regulation of the trapping mechanism

A functional domain termed the contact zone, which is the region of a subunit interacting with another non-covalently bound subunit, is suggested to play a decisive role in the trapping mechanism of human alpha2-macroglobulin. Tetrameric alpha2-macroglobulin can be dissociated into stable dimers with intact thiol esters by sodium thiocyanate, whereby the contact zones are disrupted. The dissociation leads to significant conformational changes, as studied by ultraviolet-difference spectroscopy, CD, fluorescence and affinity partitioning. The conformation of the dimers is similar to that of MeNH2-treated alpha2-macroglobulin, in which the thiol esters are cleaved, a conformational state with a closed trap occurs, and receptor-recognition sites are exposed. The receptor-binding domain is at least partly exposed in the dimer, as judged by binding of specific mAbs. The bait region in the dimers can be cleaved by proteases, and activation of the thiol esters ensues without binding of the protease. When the dimers were treated with MeNH2, no conformational changes could be detected by ultraviolet-difference spectroscopy or CD. The conformational changes occurring on dissociation into dimers are suggested to be related to trap closure and receptor-recognition-site exposure without cleavage of the thiol esters. The model presented here suggests that two separate conformational changes occur in alpha2-macroglobulin upon activation. The first involves changes at the contact zones as a result of the thiol-ester cleavage, and the second causes exposure of the receptor-recognition sites and closure of the trap.

Molecular cloning of Limulus alpha 2-macroglobulin

The American horseshoe crab Limulus polyphemus contains alpha 2-macroglobulin (alpha 2M) in the hemolymph plasma and hemocytes. alpha 2M from Limulus shows many of the typical characteristics of mammalian alpha 2M, including the presence of an internal thiol-ester, reactivity with a diversity of endopeptidases, a unique proteinase-trapping mechanism, and reactivity with the mammalian alpha 2M receptor. Additionally, Limulus alpha 2M has the unique property that it regulates the limulin-based hemolytic system of the plasma. A cDNA encoding Limulus alpha 2M has been obtained from a hemocyte cDNA library. The open reading frame encodes an N-terminal signal sequence of 25 amino acid residues and a mature protein of 1482 residues. The entire amino acid sequence is similar to those of the mammalian alpha 2Ms (28-29% identity) and contains common features found in mammalian alpha 2Ms. a bait region, an internal thiol-ester site, and a receptor-binding domain. However, the N-terminal portion (positions 24-105) has no sequence similarity with those of mammalian alpha 2Ms, and it is structurally related to that of the human complement factor C8 chain, consistent with a role for Limulus alpha 2M in host defense. The component sugar analysis of Limulus alpha 2M showed the existence of a complex type of oligosaccharide chain similar to those of mammalian alpha 2M. However, unlike mammalian alpha 2M, no sialic acid was detected in Limulus alpha 2M and it contained approximately 3 mol/mol N-acetylgalactosamine, suggesting the presence of O-linked sugar chains, which have not been found in mammalian alpha 2M. Expression of alpha 2M was detected in hemocytes, but not in hepatopancreas, heart, stomach, intestine, coxal gland, brain and skeletal muscle. Furthermore, immunoblotting of large and small granules of the hemocytes with antiserum against alpha 2M indicated the presence of the alpha 2M in large granules. Trypsin-treated Limulus alpha 2M, but not the native alpha 2M, displaced methylamine-treated human 125I-alpha 2M from the human alpha 2M receptor with a Kd of 30 nM, suggesting conservation of the proteinase-clearance mechanisms between mammalian and arthropod evolutionary lineages.

Proteinases are isoform-specific regulators of the binding of transforming growth factor beta to alpha 2-macroglobulin

alpha 2-Macroglobulin (alpha 2M) regulates growth and gene expression in many cell types by binding and neutralizing transforming growth factor beta (TGF-beta). In this study we characterized the effects of the serine proteinase, plasmin, on the interaction of alpha 2M with TGF-beta 1 and TGF-beta 2. Binding of both TGF-beta isoforms to purified alpha 2M-plasmin complex was primarily non-covalent and reversible. The binding affinity of alpha 2M for TGF-beta 1 was increased by plasmin; the Kd values were 320 and 84 nM for native alpha 2M and alpha 2M-plasmin respectively. In contrast the affinity of alpha 2M for TGF-beta 2 was decreased by plasmin; the Kd values were 14 and 80 nM for native alpha 2M and alpha 2M-plasmin respectively. Thrombin decreased the affinity of alpha 2M for TGF-beta 2 in a similar manner to plasmin. In assays of DNA synthesis in fetal bovine heart endothelial cells, native alpha 2M neutralized the activity of exogenously added TGF-beta 2, whereas alpha 2M-plasmin, at equivalent concentrations, had almost no effect. Native alpha 2M and methylamine-modified alpha 2M increased platelet-derived growth factor alpha-receptor expression in vascular smooth-muscle cells, an activity attributed to the neutralization of autocrine TGF-beta activity, whereas alpha 2M-plasmin was less effective at the same concentration. These studies demonstrate that the effects of proteinases on the cytokine-binding and cytokine-neutralizing activities of alpha 2M are cytokine-dependent. By reacting with alpha 2M, proteinases might regulate not only the availability of cytokines in the extracellular spaces but also the composition of the cytokine milieu.

Similar architectures of native and transformed human alpha2-macroglobulin suggest the transformation mechanism

The refined three-dimensional structure of native human alpha2-macroglobulin (alpha2M) has been determined by cryoelectron microscopy and three-dimensional reconstruction. New features corresponding to "sigmoid arches," "basal bodies," and "apical connections" were observed in the molecule. Since similar elements are found in the architecture of transformed alpha2M, the 2 volumes were aligned in three dimensions. In their common orientations, they show many similarities except near the openings of the central chamber. In the native conformation, these apertures are fully opened, allowing the proteases to access the central chamber of the molecule, while in the transformed structure, they are partially closed. These structures suggest that alpha2M conformational change involves a strong lateral compression and a vertical stretching of the native particle seen in its four-petaled flower view to produce the H view of the transformed form. A model of structural transformation, in which all the parts of the alpha2M molecule seem involved in the entrapment of the proteinases is proposed.

Activated alpha 2-macroglobulin promotes mitogenesis in rat vascular smooth muscle cells by a mechanism that is independent of growth-factor-carrier activity

Vascular smooth muscle cell (vSMC) proliferation is important in atherosclerosis. We previously demonstrated that methylamine-activated alpha 2-macroglobulin (alpha 2M) and transforming growth factor beta 1 (TGF-beta 1) cause a synergistic proliferative response in quiescent rat aortic vSMCs [Stouffer, G. A., La-Marre, J., Gonias, S. L. & Owens, G. K. (1993) J. Biol. Chem. 268, 18,340-18,344]. The first goal of this study was to determine whether the synergy is due to the ability of alpha 2M-methylamine (alpha 2M-MeNH2) to bind TGF-beta 1 and target the growth factor to vSMCs that express the alpha 2M receptor. Receptor-recognized alpha 2M derivatives without TGF-beta 1-binding activity, including ternary alpha 2M-trypsin, an 18-kDa proteolytic fragment of the alpha 2M subunit, and the corresponding recombinant receptor-binding fragment (rRBF) increased vSMC [3H]thymidine incorporation and cell number in a manner similar to alpha 2M-MeNH2. In combination with TGF-beta 1, each alpha 2M derivative caused a synergistic vSMC proliferative response. vSMCs responded comparably when treated with alpha 2M-MeNH2 and TGF-beta 1 simultaneously or in sequence. Furthermore, alpha 2M-MeNH2-TGF-beta 1 complexes increased [3H]thymidine incorporation no more than alpha 2M-MeNH2 alone. These results indicate that TGF-beta 1 binding to alpha 2M is not responsible for the synergistic mitogenic activity. Additional studies were undertaken to determine whether activated alpha 2M independently induces a signal-transduction response in vSMCs. alpha 2M-MeNH2 and rRBF caused a rapid, transient increase in vSMC inositol 1,4,5-trisphosphate. This response was pertussis-toxin insensitive. Receptor-associated protein (RAP; 170 nmol/L) inhibited 91-95% of the specific binding of 125I-alpha 2M-MeNH2 and 125I-rRBF to vSMC; however, RAP did not affect the inositol 1,4,5-trisphosphate response or the mitogenic response. These studies suggest that vSMCs express a receptor, other than low-density-lipoprotein-receptor-related protein, that transduces a signal in response to activated alpha 2M. This receptor may mediate the mitogenic activity of alpha 2M in vSMC culture.

Differences in the binding of transforming growth factor beta 1 to the acute-phase reactant and constitutively synthesized alpha-macroglobulins of rat

Human alpha 2-macroglobulin (alpha 2M) is a proteinase inhibitor and carrier of certain growth factors, including transforming growth factor beta 1 (TGF-beta 1). The constitutively synthesized homologue of human alpha 2M in the adult rat is alpha 1M. Rat alpha 2M is an acute-phase reactant, expressed at high levels in experimental trauma, pregnancy and in certain pathological conditions. The physiological role of rat alpha 2M is not known. In this investigation, we demonstrated that rat alpha 1M and rat alpha 2M bind TGF-beta 1. The equilibrium dissociation constants (KD) for the binding of TGF-beta 1 to the native forms of alpha 1M and alpha 2M were 257 and 109 nM respectively. alpha 1M underwent conformational change when it reacted with methylamine. The resulting product bound TGF-beta 1 with higher affinity (32 nM). Methylamine-treated rat alpha 2M did not undergo conformational change and did not bind TGF-beta 1 with increased affinity. Previous studies suggest that the native conformation may be the principal form responsible for the cytokine-carrier activity of alpha 2M in plasma and serum-supplemented cell culture medium. To confirm that native rat alpha 2M is a more efficient TGF-beta 1 carrier than native alpha 1M, fetal bovine heart endothelial cell (FBHE) proliferation assays were performed. TGF-beta 1 (5 pM) inhibited FBHE proliferation, and native alpha 2M (0.3 microM) counteracted this activity whereas alpha 1M (0.3 microM) had almost no effect. Rat alpha 2M underwent conformational change when it reacted with plasmin incorporating 1.1 mol of plasmin/mol. alpha 2M-plasmin bound TGF-beta 1; the KD (61 nM) was lower (P < 0.01) than that determined for the native alpha 2M-TGF-beta 1 interaction. These studies demonstrate that both rat alpha-macroglobulins are carriers of TGF-beta 1. The native form of rat alpha 2M probably has a predominant role, compared with native alpha 1M, as a TGF-beta 1 carrier in the plasma during the acute-phase response.

Low resolution X-ray structure of human methylamine-treated alpha 2-macroglobulin

The structure of methylamine-treated human alpha 2-macroglobulin (alpha 2M-Ma), a 720-kDa tetrameric inactivated proteinase inhibitor from plasma, has been determined to a resolution of 10 A. Data were collected with synchrotron radiation at 120 K, and phases were calculated by multiple isomorphous replacement and solvent flattening. A novel feature of the structure of alpha 2-M is present in its proteinase-binding cavity, dividing it into two compartments. The potential sites for proteinase entrapment in these compartments are sterically restricted. The positions of the thiol groups appearing from the functional important thiol esters upon their cleavage have been determined. They are found at the walls of the compartments at the center of the structure. The overall structure of alpha 2M-MA is much more sphere-like than previously inferred from electron microscopy studies. However, several aspects of the structure are well described by recent three-dimensional reconstructions. Possible models for the monomer, the disulfide bridged dimer, and native alpha 2M are discussed.

Alpha 2-macroglobulin functions as a cytokine carrier to induce nitric oxide synthesis and cause nitric oxide-dependent cytotoxicity in the RAW 264.7 macrophage cell line

Nitric oxide (NO) is an important mediator of macrophage activities. We studied the regulation of macrophage NO synthesis by alpha 2-macroglobulin (alpha 2M), a proteinase inhibitor and carrier of certain growth factors, including transforming growth factor-beta (TGF-beta). Native alpha 2M and the alpha 2M receptor-recognized derivative, alpha 2M-methylamine (alpha 2M-MA), increased nitrite generation by the RAW 264.7 murine macrophage cell line. The level of nitrite accumulation, which is an index of NO synthesis, was comparable to that observed with interferon-gamma. Native alpha 2M and alpha 2M-MA also increased inducible nitric oxide synthase (iNOS) mRNA levels and substantially reduced the number of viable cells, as determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium/succiny l dehydrogenase assay or trypan blue exclusion. At slightly higher alpha 2M concentrations, [3H]thymidine incorporation was inhibited. All of these activities were counteracted nearly completely when the iNOS competitive inhibitor NG-monomethyl-L-arginine was included. By in situ nick translation, native alpha 2M and alpha 2M-MA increased the percentage of cells with detectable single strand chromatin nicks from 4 to 12 and 17%, respectively. This change suggested apoptosis; however, electron microscopy studies demonstrated variability in the morphology of injured cells. To determine the mechanism by which alpha 2M increases macrophage NO synthesis, we studied proteolytic alpha 2M derivatives that retain partial activity. A 600-kDa derivative that retains growth factor binding activity increased RAW 264.7 cell NO synthesis and iNOS mRNA levels comparable to native alpha 2M and alpha 2M-MA. The purified 18-kDa alpha 2M receptor-binding fragment had no effect on NO synthesis or iNOS expression. Thus, the growth factor-carrier activity of alpha 2M and not its receptor-binding activity is essential for NO synthesis regulation. A TGF-beta-neutralizing antibody mimicked the activity of alpha 2M, increasing RAW 264.7 cell NO synthesis and decreasing cellular viability. These studies demonstrate that alpha 2M can regulate macrophage NO synthesis and profoundly affect cellular function without gaining entry into the cell and without binding specific plasma membrane receptors.

Identification of the serum factor required for liposome-primed activation of mouse peritoneal macrophages. Modified alpha 2-macroglobulin enhances Fc gamma receptor-mediated phagocytosis of opsonized sheep red blood cells

Culture supernatant, prepared by incubating fetal calf serum and liposome-treated B cells, augments mouse peritoneal macrophage Fc gamma receptor-mediated phagocytosis of IgG-opsonized sheep red blood cells. The activation process was hypothesized to be as follows. B-cell membranous glycosidases stimulated by liposomes, convert serum factor to a macrophage-stimulating active serum factor. As the active serum factor loses its activation potential by the addition of mannose or by digestion with alpha-mannosidase, mannose residues at the terminal present in the active serum factor are hypothesized to contribute to macrophage activation. The active serum factor was purified on a concanavalin A-Sepharose 4B column, and identified as a modified form of alpha 2-macroglobulin by immunochemical analysises. On non-denaturing polyacrylamide gel electrophoresis, modified alpha 2-macroglobulin showed a slow form, while alpha 2-macroglobulin-trypsin and alpha 2-macroglobulin-methylamine complexes, which bind specifically to alpha 2-macroglobulin receptors, showed a fast form and did not activate macropahges. These findings demonstrate that alpha 2-macroglobulin is the essential serum factor in liposome-primed macrophage activation, and that modified alpha 2-macroglobulin with mannose residues at the terminal sugar chain binds to macrophage mannose receptors, but not alpha 2-macroglobulin receptors, and increases Fc gamma receptor-mediated phagocytosis of IgG-opsonized sheep red blood cells.

Intracranial infusion of monoamine-activated alpha 2-macroglobulin decreases dopamine concentrations within the rat caudate putamen

Monoamine-activated alpha 2-macroglobulin (alpha 2M) has been shown to inhibit choline acetyltransferase in basal forebrain neurons as well as neurotrophin-dependent neuronal functions. The objective of this study was to determine whether monoamine-activated alpha 2M can affect the caudate putamen (CP) dopaminergic system in vivo. Male rats received intracranial infusions of methylamine-activated alpha 2M (0.6 nmole) and contralateral infusions of its vehicle, phosphate-buffered saline (PBS). Five days following infusion, the animals were killed, the CP dissected into three rostral-caudal segments, and assayed for dopamine (DA) using a high-performance liquid chromatography system. Within the two rostral CP segments (the approximate site of cannula placement), statistically significant (26%) reductions of DA concentrations were obtained on the alpha 2M-infused side of the CP with 90-100% of the animals showing decreases. At a more distal (caudal) site of the CP, DA concentrations showed only an insignificant (12%) reduction. No differences in DA concentrations between sides infused with bovine serum albumin versus PBS or from olfactory tubercle samples were obtained in these animals. These results demonstrate that monoamine-activated alpha 2M is capable of producing significant degeneration of the nigrostriatal dopaminergic system in vivo and suggest that this factor may play a role in age-related neurodegenerative disorders such as Parkinson's disease.

Use of hydrophobic affinity partitioning as a method for studying various conformational states of the human alpha-macroglobulins

The serum proteins alpha 2-macroglobulin and pregnancy zone protein undergo major conformational changes when complexed with proteinases. It is shown that the changes in delta log Kmax determined by hydrophobic affinity partitioning is a measure of the extent of changes in the conformation of these alpha-macroglobulins. We introduce a new term for the changes of surface hydrophobicity in a protein as delta log Kacc. This defines the difference of delta log Kmax between a modified and an unmodified conformational state of a specific protein and can be useful as a parameter to describe the apparent conformational changes in the protein.

The kinetics of conformational changes of alpha 2-macroglobulin determined by time resolved X-ray solution scattering

The rate of gross conformational change of alpha 2-macroglobulin (alpha 2M) during its proteinase trapping was directly determined for the first time using time-resolved X-ray solution scattering. Decrease of radius of gyration was observed under pseudo-first-order conditions with excess proteinases, which exhibited a monophasic time-course. The rate constants were 0.5 +/- 0.1 s-1 and 0.8 +/- 0.2 s-1 for the reaction with chymotrypsin and trypsin, respectively. There was no concentration dependence of the observed rate constants. Therefore, the rate-limiting step of the gross conformational change was not the bimolecular encounter reaction between alpha 2M and proteinases, which requires a new proposal of pre-trapping of proteinases before the gross conformational change.

Binding of platelet-derived growth factor-BB and transforming growth factor-beta 1 to alpha 2-macroglobulin in vitro and in vivo: comparison of receptor-recognized and non-recognized alpha 2-macroglobulin conformations

alpha 2-Macroglobulin (alpha 2M) undergoes a major conformational change when reacting with proteinases or primary amines. This conformational change has been referred to as the 'slow' to 'fast' transformation based on the increase in alpha 2M mobility shown by non-denaturing PAGE. Previous studies demonstrated that many cytokines, including transforming growth factor beta 1 (TGF-beta 1) and interleukin-1 beta, bind preferentially or exclusively to alpha 2M which has undergone conformational change. In this study, we demonstrate that platelet-derived growth factor-BB (PDGF-BB) also binds preferentially to conformationally transformed alpha 2M (alpha 2M-methylamine, alpha 2M-trypsin) in vitro. Purified 125I-PDGF-BB-alpha 2M-methylamine complex cleared rapidly from the circulation of mice via the alpha 2M receptor/low-density-lipoprotein-receptor-related protein (alpha 2M-R/LRP). In order to determine whether PDGF-BB or TGF-beta 1 binds to native alpha 2M, we defined the native conformation by lack of interaction with alpha 2M-R/LRP instead of electrophoretic mobility. 125I-PDGF-BB was incubated with 4.3 microM native alpha 2M and 0.47 microM alpha 2M-methylamine. The 125I-PDGF-BB distributed evenly between slow-form and fast-form alpha 2M without shifting the electrophoretic mobility of either species. When the mixed preparation was injected intravenously in mice, 125I-PDGF-BB-fast-form-alpha 2M cleared rapidly and selectively from the circulation; 125I-PDGF-BB which was bound to slow-form alpha 2M was stable in the blood (apparently not recognized by alpha 2M-R/LRP). Therefore, while conformationally transformed alpha 2M binds PDGF-BB preferentially in vitro, non-alpha 2M-R/LRP-recognized alpha 2M binds PDGF-BB as well. Binding of 125I-PDGF-BB and 125I-TGF-beta 1 to alpha 2M was demonstrated in vivo by injecting the free growth factors intravenously into mice. Plasma samples which were subjected to non-denaturing PAGE and autoradiography demonstrated binding of both growth factors exclusively to the slow-form of alpha 2M. Therefore, under normal physiological conditions, native alpha 2M (non-alpha 2M-R/LRP-recognized) is the primary form of the proteinase inhibitor functioning as a carrier of PDGF-BB and TGF-beta 1 in the blood.

Direct kinetics of bait region cleavage of alpha-2-macroglobulin by a rapid quenching method

The rate of bait region cleavage of human alpha-2-macroglobulin by chymotrypsin was determined by a rapid quenching method under conditions where the bimolecular encounter between the two reactants was not rate-limiting. alpha 2M was first mixed with a 30 molar excess of chymotrypsin in a sequential stopped-flow apparatus and after programmed time intervals the activity of chymotrypsin was quenched with 1 N HCl. The fraction of uncleaved subunits was quantitated by SDS-PAGE under reducing conditions. The result indicated that the bait region cleavage proceeded following a two-exponential decay curve with respective rate constants of k1 = 40 s-1 and k2 = 2 s-1.

Methylamine-treated low density lipoproteins elicit different responses in HepG2 cells and macrophages

Recent results from this laboratory have demonstrated the existence of labile thiolester bonds in apolipoprotein B (ApoB). Thiolester bonds can be cleaved with nucleophiles such as methylamine, resulting in conformational change. The purpose of this study was to explore whether the cellular interactions would be altered after methylamine treatment of low density lipoproteins (LDL). Human hepatoma cells, HepG2, and human monocyte derived macrophages were used for these studies. Fresh LDL were incubated with methylamine under mild alkaline conditions under N2 and with preservatives for 24 h. The methylamine-treated LDL showed particle size and net charge identical to fresh native LDL. In addition, no oxidative modification of LDL occurred under the experimental conditions. The methylamine-treated LDL were indistinguishable from native LDL in HepG2 cells as judged by binding, degradation, cholesterol accumulation and de novo sterol synthesis. However, methylamine-treated LDL caused an increased accumulation of cholesteryl esters in macrophages which was comparable to the accumulation caused by acetylated LDL. Dual color digital imaging fluorescence microscopy revealed no competition between acetylated and methylamine-treated LDL, suggesting that the excessive uptake of methylamine-treated LDL was not mediated by the 'scavenger' receptor. The increased accumulation of cholesteryl ester in macrophages also did not appear to stem from the classical LDL receptor. These results suggest that a new receptor binding domain is exposed due to the conformational change upon treatment of LDL with methylamine.

Immunologic evidence for insertion of the reactive-bond loop of antithrombin into the A beta-sheet of the inhibitor during trapping of target proteinases

Identical or highly similar antigenic determinants, not present in the intact inhibitor, were induced in antithrombin on cleavage of the reactive bond, on formation of a complex between antithrombin and a synthetic reactive-loop tetradecapeptide, and on partial denaturation of antithrombin at low concentrations of guanidinium chloride. Previous studies indicate that the common structural feature of these three modified forms of antithrombin is that the region of the reactive-bond loop on the amino-terminal side of the reactive bond, or the corresponding synthetic peptide, is inserted as a middle strand in the main beta-sheet of the inhibitor, the A sheet. The new epitopes in the three modified antithrombin forms therefore most likely are exposed as a result of this insertion. Identical or highly similar epitopes were exposed also in complexes between antithrombin and thrombin or factor Xa, strongly suggesting that a substantial segment of the reactive-bond loop is inserted into the A sheet also in these complexes. In contrast, the new epitopes were not exposed in antithrombin on binding of heparin, implying that the conformational change induced by heparin does not involve such loop insertion. These results provide the first experimental verification of recent hypotheses that insertion of the reactive-bond loop of serpins into the A beta-sheet is involved in the binding of target proteinases.

alpha 2-Macroglobulin bait region integrity. Role in determining fast-form structure

To determine whether integrity of the bait region affects the structure of the remainder of human alpha 2-macroglobulin (alpha 2M), we have determined the separation between cysteine residues in a methylamine-reacted fast-form of alpha 2M. From reduction in fluorescence intensity of covalently-bound donor fluorophore caused by proximity to an acceptor, a separation of 35 +/- 8 A was calculated, which is identical to a previously determined value for proteinase-treated fast-form alpha 2M. This indicates that although bait region cleavage is the physiological route to conformational change in alpha 2M, bait region integrity per se does not significantly affect the structure of fast-form alpha 2M.

Changes in structure of alpha 2-macroglobulin upon reaction with trypsin as assessed by light scattering and differential scanning calorimetry

Employing a combination of static and dynamic light scattering, as well as differential scanning calorimetry (DSC), the structural changes which appear in alpha 2-macroglobulin (alpha 2M) upon trypsin binding have been further characterized. Light-scattering measurements suggest that a 15% reduction in both the hydrodynamic radius and radius of gyration occurs when two molecules of trypsin complex to alpha 2M. Approx. 85% of this trypsin-induced compaction results from the binding of the first proteinase. A complementary result was obtained from DSC measurements in which the major fraction of the trypsin-induced conversion of alpha 2M to a single more thermally stable form results from interaction with the first proteinase molecule. These observations support a functionally asymmetric model of trypsin binding to alpha 2M in which the significant reduction in size of the complex is primarily due to the initial interaction of alpha 2M with a single proteinase molecule.

Role of the scavenger receptor in the uptake of methylamine-activated alpha 2-macroglobulin by rat liver

Alpha 2-Macroglobulin (alpha 2M) requires activation by small nucleophiles (e.g. methylamine; giving alpha 2M-Me) or proteolytic enzymes (e.g. trypsin; giving alpha 2M-Tr) in order to be rapidly removed from the circulation by the liver. Separation of rat liver cells into parenchymal, endothelial and Kupffer cells at 10 min after injection indicates that liver uptake of alpha 2M-Me is shared between parenchymal and endothelial cells, with relative contributions of 51.3% and 48.3% respectively of total liver-associated radioactivity. In contrast, alpha 2M-Tr is almost exclusively taken up by the parenchymal cells (90.1% of liver-associated radioactivity). A preinjection of 5 mg of poly(inosinic acid) decreased liver uptake of alpha 2M-Me to 39.9% of the control value, while it had no effect on liver uptake of alpha 2M-Tr. It appears that poly(inosinic acid) specifically reduces the uptake of alpha 2M-Me in vivo by endothelial cells, leaving uptake by parenchymal cells unaffected. In vitro studies with isolated liver cells indicate that the association of alpha 2M-Me with endothelial cells is 21-fold higher per mg of cell protein than with parenchymal cells. The capacity of endothelial cells to degrade alpha 2M-Me appears to be 46 times higher than that of parenchymal cells. Competition studies show that poly(inosinic acid) or acetylated low-density lipoprotein effectively competes with the association of alpha 2M-Me with endothelial and Kupffer cells, but association with parenchymal cells is unaffected. It is suggested that activation of alpha 2M by methylamine induces a charge distribution on the protein which triggers specific uptake by the scavenger receptor on endothelial cells. It is concluded that the uptake of alpha 2M-Me by the scavenger receptor might function as an additional system for the uptake of activated alpha 2M.

Binding of transforming growth factor-beta 1 to methylamine-modified alpha 2-macroglobulin and to binary and ternary alpha 2-macroglobulin-proteinase complexes

The binding of 125I-labelled transforming growth factor-beta 1 (TGF-beta 1) to human alpha 2-macroglobulin (alpha 2M) was studied by native PAGE and autoradiography. TGF-beta 1 bound preferentially to alpha 2M-methylamine and minimally, if at all, to native alpha 2M. Preparations of alpha 2M-proteinase complex were generated by incubating a standard concentration of alpha 2M (0.4 microM) with different concentrations of trypsin, chymotrypsin or neutrophil elastase (0.04-2.0 microM). The 125I-TGF-beta 1-binding activity depended on the initial ratio of active proteinase to alpha 2M, or r value, used to form the alpha 2M-proteinase complex. With all three proteinases, r values of 2 or greater yielded preparations with unchanged or decreased TGF-beta 1-binding activity relative to native alpha 2M. By contrast, r values near 1 yielded preparations with significantly increased TGF-beta 1-binding activity. The results of [3H]thymidine-incorporation studies performed in mouse keratinocytes were consistent with the 125I-TGF-beta-binding experiments. alpha 2M-trypsin and alpha 2M-chymotrypsin prepared at an r value of 1.0 counteracted the activity of TGF-beta 1, whereas the equivalent complexes prepared at an r value of 3.0 had no effect. As determined by SDS/PAGE, 125I-TGF-beta 1 binding to alpha 2M-methylamine was at least 80% non-covalent. Reaction of alpha 2M-methylamine with iodoacetamide or 5,5'-dithiobis-(2-nitrobenzoic acid) decreased the percentage of covalent binding but had no effect on total binding. Neuraminidase treatment had no effect on the binding of 125I-TGF-beta 1 to alpha 2M-methylamine. Cleavage of the 'bait regions' in alpha 2M-methylamine by prolonged treatment with trypsin also had no effect. These studies suggest that TGF-beta 1 binding to alpha 2M is enhanced by conformational change in the proteinase inhibitor resulting from reaction with proteinase or amine. If both proteinase-binding sites in a single alpha 2M molecule are occupied, TGF-beta 1-binding activity is decreased or perhaps eliminated.

Ultrastructure of alpha 2-macroglobulins

New results concerning the ultrastructure of human alpha 2-macroglobulin (alpha 2M) molecules are presented in connection and comparison with the historical, the current and our own most recent, even unpublished results on the structure and function of alpha 2M and related proteins. The electron microscopic approach uses classical negative staining, combined with the new imaging mode "Electron Energy Loss Spectroscopy", which provides unusual contrast, resolution and readability of the electron micrographs. Immuno- and cryoelectron microscopy, as well as image processing has provided new data necessary to the building of tentative 3D models of the molecule. A model for the native tetrameric alpha 2M is described for the first time, and tries to explain and gather the various observations, sometimes contradictory, taken from different laboratories. A revised version for a model of the methylamine- and proteinase-transformed forms of alpha 2M is also shown. The probable positions of the bait regions and the thiol esters are given on both models. We confirm that alpha 2M is a twin trap capable of inactivating one or two proteinases by partial immobilization. Preliminary results on the production of crystals of alpha 2M-chymotrypsin complexes are also presented. A critical analysis of our models is presented in comparison with others. The technical limitations reached with some techniques and some possible extensions of future research in the field are also presented.

Crystallization and preliminary X-ray analysis of methylamine-treated alpha 2-macroglobulin and 3 alpha 2-macroglobulin-proteinase complexes

Crystals of methylamine-treated alpha 2-macroglobulin (alpha 2M-MA), alpha 2-macroglobulin in complex with two molecules of trypsin, alpha 2M-T2, one molecule of plasmin, alpha 2M-PL, and one molecule of plasmin followed by methylamine-treatment, alpha 2M-PL(MA), have reproducibly been obtained using ammonium sulfate or magnesium sulfate as precipitants. The crystals are fragile tetragonal bipyramids of up to 1.5 mm in length. Crystals of alpha 2M-MA diffracted to at least 9 A resolution, crystals of alpha 2M-T2 diffracted to 10 A resolution and crystals of alpha 2M-PL and alpha 2M-PL(MA) diffracted to 11 A resolution. For alpha 2M-MA the cell parameters were determined as: a=b=257 A, c=555 A; and for alpha 2M-T2 as: a=b=247 A, c=559 A. For both preparations the space group was I4(1)22. As estimated from density measurements, the crystals of alpha 2M-MA and alpha 2M-T2 contain one 360 kDa alpha 2M dimer per asymmetric unit. The volume of the asymmetric unit/molecular weight, Vm, was estimated at 5.6 A3/Da. The crystal parameters of alpha 2M-PL and alpha 2M-PL(MA) were not determined.