Inhibition of proteases by alpha 2-macroglobulin. The role of lysyl amino groups of trypsin in covalent complex formation

Renoprotective Mechanism of Remote Ischemic Preconditioning Based on Transcriptomic Analysis in a Porcine Renal Ischemia Reperfusion Injury Model

Ischemic preconditioning (IPC) is a well-known phenomenon in which tissues are exposed to a brief period of ischemia prior to a longer ischemic event. This technique produces tissue tolerance to ischemia reperfusion injury (IRI). Currently, IPC’s mechanism of action is poorly understood. Using a porcine single kidney model, we performed remote IPC with renal IRI and evaluated the IPC mechanism of action. Following left nephrectomy, 15 female Yorkshire pigs were divided into three groups: no IPC and 90 minutes of warm ischemia (control), remote IPC immediately followed by 90 minutes of warm ischemia (rIPCe), and remote IPC with 90 minutes of warm ischemia performed 24 hours later (rIPCl). Differential gene expression analysis was performed using a porcine-specific microarray. The microarray analysis of porcine renal tissues identified 1,053 differentially expressed probes in preconditioned pigs. Among these, 179 genes had altered expression in both the rIPCe and rIPCl groups. The genes were largely related to oxidation reduction, apoptosis, and inflammatory response. In the rIPCl group, an additional 848 genes had altered expression levels. These genes were primarily related to immune response and inflammation, including those coding for cytokines and cytokine receptors and those that play roles in the complement system and coagulation cascade. In the complement system, the membrane attack complex was determined to be sublytic, because it colocalized with phosphorylated extracellular signal-regulated kinase. Furthermore, alpha 2 macroglobulin, tissue plasminogen activator, uterine plasmin trypsin inhibitor, and arginase-1 mRNA levels were elevated in the rIPCl group. These findings indicate that remote IPC produces renoprotective effects through multiple mechanisms, and these effects develop over a long timeframe rather than immediately following IPC.

Inhibition of nerve growth factor-stimulated neurite outgrowth by methylamine-modified alpha 2-macroglobulin

alpha 2-Macroglobulin (alpha 2M) is a rather ubiquitous protein in extracellular spaces of mammals. It is an inhibitor of endopeptidases, can be modified by aliphatic amines, and combines with a number of hormones/cytokines such as beta-nerve growth factor (NGF) [Koo PH, Stach RW (1989): J Neurosci Res 22:247]. The objective of this study is to compare the NGF-binding properties of methylamine-modified human alpha 2M (MA-alpha 2M) versus normal alpha 2M and their effects on the biological activity of NGF and neurite extension by embryonic chicken dorsal root ganglia. As determined by gel filtration, polyacrylamide gel electrophoresis, and equilibrium binding studies, these two forms of alpha 2M are similar in their binding affinities, with MA-alpha 2M binding about twice as much NGF as normal alpha 2M. Both normal alpha 2M and MA-alpha 2M combine noncovalently with NGF, and prior modification of alpha 2M is unnecessary for the binding to occur. In contrast to normal alpha 2M, MA-alpha 2M potently inhibits the biological activity of NGF and exerts a dose-dependent inhibition on the NGF-stimulated neurite outgrowth by embryonic chicken dorsal root ganglia in culture. The inhibitory effect of MA-alpha 2M can be overcome by higher NGF concentrations, but is irreversible at lower NGF concentrations. Trypsin-modified alpha 2M combines covalently and noncovalently with more NGF than normal alpha 2M but has very little neurite inhibitory activity. The mechanism of inhibition by MA-alpha 2M is discussed.

Effect of methylamine on the reaction of alpha 2-macroglobulin with enzymes

The kinetics of reaction of alpha 2-macroglobulin (alpha 2M) with thrombin and with trypsin were studied in the presence and absence of methylamine. The rate of enzyme-induced thiol release was found to be the same whether or not amine was present. The result suggests that covalent bond formation and enzyme-catalyzed amine incorporation proceed via a common (enzyme-dependent) rate-determining step. The reaction of lysyl-modified enzymes (which show poor covalent binding with alpha 2M) was similarly unaffected by amine, indicating that enzyme-catalyzed steps were also rate determining for hydrolysis of the thiol ester. The products of the reactions were analyzed by native and denaturing gel electrophoresis. Methylamine did not affect the total binding of enzyme to alpha 2M but did cause a substantial decrease in covalent binding. Surprisingly, not all covalent complexes were affected by the presence of amine: complexes in which enzyme was covalently bound to one half-molecule increased compared to the reaction with no amine; complexes in which two half-molecules are cross-linked by two bonds to a single enzyme were substantially reduced, however. The results are consistent with a mechanism of reaction in which an enzyme-dependent step is rate determining. This step is accompanied by activation of two thiol esters. One of these reacts immediately with the bound enzyme (or may be hydrolyzed if the enzyme amine groups are blocked). The other activated center is capable of reaction with external nucleophiles such as methylamine.

Evidence for active half-molecules of alpha 2-macroglobulin formed by dissociation in urea

Urea caused dissociation of alpha 2-macroglobulin (alpha 2M) into half-molecules (two disulfide-bonded subunits) as revealed by gel electrophoresis. The fraction of whole molecules remaining decreased with increasing urea concentration. Half-dissociation occurred at about 2.2 M. The ability of alpha 2M to inhibit trypsin also decreased with increasing urea concentration, but the activity-urea curve was shifted to the right as compared to the dissociation-urea curve. Thus, at 3 M urea, gel electrophoresis showed only 6.6% whole molecules, whereas the trypsin inhibitory activity was 95% of that in buffer with no urea, suggesting that half-molecules retain activity. In addition, complexes formed in urea with 125I-labeled trypsin were observed to migrate as half-molecules even though only 50% of such complexes were covalent. These results are surprising in light of the report by Gonias and Pizzo [Gonias, S., & Pizzo, S. (1983) Biochemistry 22, 536-546] that half-molecules formed by mild reduction are active; reduction is assumed to divide the molecule along an axis orthogonal to the break caused by urea. This suggests that active half-molecules can be formed by splitting either the covalent or noncovalent bonds that hold the subunits together. A model is proposed that can account for this possibility. It has the same dimensions and symmetry as a previous model of Feldman et al. [Feldman, S.R., Gonias, S.L., & Pizzo, S.V. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 5700-5704] and accounts in a similar way for previous functional studies of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

The reaction of human alpha 2-macroglobulin with alpha-chymotrypsin. A stopped-flow kinetic investigation

The kinetics of the conformational changes of human alpha 2-macroglobulin (alpha 2M) induced by reaction with pure alpha-chymotrypsin, have been analyzed using three fluorescent probes, namely protein tryptophan groups and the dye 6-(4-toluidino)-2-naphthalenesulfonate, to monitor alterations of the alpha 2M structure, and a covalent conjugate of chymotrypsin and fluorescein isothiocyanate (Chy-FITC). The main reaction sequence exhibits a triphasic time course with any of the labels used. Each phase is first-order. The fixation of a single molecule of chymotrypsin to one protease-binding site of alpha 2M (site A) initiates the whole process and determines the access to the second site (site B). Of the three exponential phases of the reaction (20 degrees C), phase I (k1 approximately 19.6 min-1) and phase II (k2 approximately 5.3 min-1) belong to site A. Phase III is related to site B transformation. It contains two steps with different responses from tryptophan (k3 approximately 0.77 min-1) and Chy-FITC (k3 approximately 0.19 min-1) fluorescence measurements. The point to be stressed is that site A and site B, while presumably identical in the native form, are not equivalent with regard to their fluorescence and kinetic properties. However, the activation energy (E = 30.1 +/- 2.7 kJ mol-1) is the same for the three phases of the reaction. When present in sufficient excess, free chymotrypsin or native alpha 2M is able to form reversible complexes with the above-related chymotrypsin-alpha 2M adducts. Only the alpha 2M site A core seems to be involved in this parallel process. In addition the conformational state of the chymotrypsin-alpha 2M complexes is shown to depend on the pH, with a pKa of 6.4.

Changes of the proteinase binding properties and conformation of bovine alpha 2-macroglobulin on cleavage of the thio ester bonds by methylamine

Cleavage of the thio ester bonds of human alpha2-macroglobulin (alpha 2M) by methylamine leads to an extensive conformational change and to inactivation of the inhibitor. In contrast, cleavage of these bonds in bovine alpha 2M only minimally perturbs the hydrodynamic volume of the protein [Dangott, L. J., & Cunningham, L. W. (1982) Biochem. Biophys. Res. Commun. 107, 1243-1251], as well as its spectroscopic properties, as analyzed by ultraviolet difference spectroscopy, circular dichroism, and fluorescence in this work. A conformational change analogous to that undergone by human alpha 2M thus does not occur in the bovine inhibitor. However, changes of several functional properties of bovine alpha 2M are induced by the amine. The apparent stoichiometry of inhibition of trypsin thus is reduced from about 1.2 to about 0.7 mol of enzyme/mol of inhibitor. In spite of this decrease, the interaction with the proteinase induces similar conformational changes in methylamine-treated alpha 2M as in intact alpha 2M, as revealed by spectroscopic analyses, indicating that the mode of binding of the proteinase to the inhibitor is essentially unperturbed by thio ester bond cleavage. The reaction with methylamine also greatly increases the sensitivity of bovine alpha 2M to proteolysis by trypsin at sites other than the "bait" region. Moreover, the second-order rate constant for the reaction with thrombin is reduced by about 10-fold. These results indicate that the thio ester bonds of bovine alpha 2M, although not required per se for the binding of proteinases, nevertheless are responsible for maintaining certain structural features of the inhibitor that are of importance for full activity.

A rational approach to the specific chemotherapy of pancreatitis

Oedematous pancreatitis is pancreatic acinar cell damage with leakage into the peritoneal cavity and circulation of the inactive zymogens of digestive enzymes and active amylase and lipase. Pancreatic oedema and intra-abdominal fat necrosis occur. Necrotising pancreatitis is pancreatic acinar cell damage accompanied by the specific conversion of trypsinogens to trypsins, at a rate, and on a scale, sufficient to overwhelm local defences. Rapid release of the whole spectrum of activated pancreatic enzymes leads to necrosis of parts of the pancreas and blood vessels, and the disseminated enzyme-mediated damage which characterises the molecular pathology of the established severe disease. Chronic pancreatitis, although less well understood, is also associated with trypsinogen activation within the gland. Two mechanisms have emerged as initiators of trypsinogen activation, lysosomal cathepsins and bile-borne enterokinase. Chemotherapeutic strategies against disease initiation include preparation of synthetic enterokinase and Cathepsin B inhibitors. Chemotherapeutic strategies against second-stage mediation of multi-organ damage in the disease, include oligopeptide or organic functionalities with novel catalytic site-directed moieties (such as fluoromethyl ketones) suitable for in vivo use and the specific inhibition of the relevant range of enzymes in complex with alpha 2-macroglobulin. Interference with pancreatic enzyme biosynthesis using proteolysis-resistant constructs mimicking receptor-binding domains of inhibitor peptide hormones as well as inhibitors of pancreatic signal peptidase are promising additional chemotherapeutic approaches worthy of active investigation.

Formation of a stable complex of thrombin and the secreted platelet protein glycoprotein G (thrombin-sensitive protein, thrombospondin) by thiol-disulfide exchange

When 125I-labeled thrombin was incubated with washed human platelets or with the supernatant solution of activated platelets, it formed a NaDodSO4-stable complex of apparent mass greater than 450 000 daltons. Formation of the complex was temperature dependent; with 20 nM thrombin incubated with the supernatant solution of ionophore-activated platelets, the initial rate of formation of the stable complex was 1 nM thrombin/min at 37 degrees C, 50 times the rate at 22 degrees C. Thrombin with all free amino groups methylated was still reactive. Active-site-blocked thrombin formed the complex only slowly. The complex that formed with active thrombin was not dissociated by hydroxylamine in urea. Reduction with 2-mercaptoethanol dissociated the complex, and its formation was blocked by the sulfhydryl-blocking agents iodoacetamide and 4,4'-dithiodipyridine. The complex was thus unlike those of thrombin and alpha 2-macroglobulin or antithrombin III, but it had characteristics of a disulfide-linked complex. Of the secreted proteins, albumin and glycoprotein G adhered to an activated thiol-Sepharose column, indicating that they contained free thiol groups. Purified glycoprotein G and thrombin formed a complex similar to the complex formed when thrombin was incubated with the supernatant solution of activated platelets. The purified glycoprotein bound 2.6 mol of radioactive N-ethylmaleimide/mol of protein, indicating three sulfhydryl groups per mole. After reacting with purified glycoprotein G, thrombin developed a new sulfhydryl group. It is concluded that glycoprotein G (thrombin-sensitive protein, thrombospondin) and thrombin form a dissociable complex that leads to a covalent complex by thiol-disulfide exchange of a thiol group on glycoprotein G and a disulfide on thrombin.

The two alpha 2-macroglobulin-bound trypsin molecules have different affinities for the basic pancreatic trypsin inhibitor

We have investigated the enzymatic properties of alpha 2-macroglobulin-bound porcine trypsin using a substrate: Z-Gly-Gly-Arg-p-nitroanilide and two inhibitors: p-aminobenzamidine and basic pancreatic trypsin inhibitor. The ternary alpha 2-macroglobulin-(trypsin)2 complex behaves like a mixture of two enzymes which bind basic pancreatic trypsin inhibitor with widely different affinities (Ki = 0.11 microM and 23 microM). About one-half of the trypsin molecules of the ternary complex are covalently bound to alpha 2-macroglobulin. Preparation of the complex in the presence of hydroxylamine prevents covalent bond formation, but the two trypsins of this artificial complex still exhibit large differences in affinity for basic pancreatic trypsin inhibitor. The trypsin molecules of the ternary complex also exhibit small differences in their affinity for Z-Gly-Gly-Arg-p-nitroanilide and p-aminobenzamidine.

Covalent thrombin-alpha 2-macroglobulin complexes. Evidence for bivalent cross-linking of inhibitor chains by a single enzyme molecule

Complexes formed between thrombin and alpha 2-macroglobulin (alpha 2M) were studied by polyacrylamide gel electrophoresis. The results provide evidence for the existence of a recently proposed novel enzyme-inhibitor species in which a single thrombin molecule forms two or more covalent bonds to two or more different alpha 2M chains. At least one of several slowly migrating bands (greater than 375K on nonreduced gels) that have previously been observed in the literature but not well characterized can be assigned to the new species. The involvement of the lysyl amino groups of thrombin is shown by the observation that methylation of these groups reduces the higher molecular weight bands. In addition, increasing the thrombin:alpha 2M ratio causes a relative decrease in the higher molecular weight species, suggesting that these complexes arise by intramolecular reactions that are susceptible to competition by solution thrombin. The data provide support for our previous proposal [Wang, D., Yuan, A., & Feinman, R.D. (1983) Ann. N.Y. Acad. Sci. 421, 90-97] that the 260K band seen in reduced gels is composed of two proteolyzed inhibitor subunits linked to one thrombin molecule. This intersubunit link maintains the integrity of the alpha 2M in sodium dodecyl sulfate, accounting for the high molecular weight bands under nonreducing conditions. Comparison with a synthetically cross-linked alpha 2M molecule allows a tentative but not unambiguous assignment of one of the bands to this novel structure.

Structural arrangement of the proteinase binding sites in human alpha 2-macroglobulin

Using singlet-singlet energy transfer measurements with labeled-chymotrypsin-alpha 2-macroglobulin complexes, we find that the two proteinase binding sites of alpha 2-macroglobulin are separated from each other by 44 A. The free thiol groups generated upon reaction of alpha 2-macroglobulin with trypsin or chymotrypsin react with thiopropyl Sepharose, indicating that they are located at the surface of the complexes. Singlet-singlet energy transfer experiments from labeled proteinases to labeled thiols of alpha 2-macroglobulin show that the thiol groups are in close contact with the proteinase molecules whether the latter are covalently or noncovalently bound to alpha 2-macroglobulin. In addition, they are remote from the association interface between the Mr = 360,000 halves of alpha 2-macroglobulin. Using the same approach we demonstrate that the active sites of chymotrypsin molecules are separated by a distance of at least 20 A from the thiols group of each alpha 2-macroglobulin subunit.

The role of enzyme lysyl amino groups in the reaction with alpha 2-macroglobulin

The primary observation, from our laboratory and others, of the effect of blocking the lysyl amino groups of enzymes is the reduction in the fraction of complexes that are resistant to SDS. The blocked enzyme derivatives do cause the specific proteolysis of the alpha 2M subunit to the 85K/100K fragments, and do cause the appearance of new thiol groups. With respect to the sequence of reaction, we may summarize the results by saying that if the reversible DMM-trypsin is, in fact, a model for the native enzyme, proteolysis can precede formation of the presumed covalent bond between bound enzyme and inhibitor. If our preliminary observations are borne out by later experiments, thiol release may precede covalent bond formation or loss of reactivity with amines, suggesting that an intact thiolester need not be the immediate target for amines; another intermediate, possibly the internal pyroglutamate originally proposed by Howard et al. and seen in model studies, may be an additional, or even the primary, target for covalent bonding with native enzymes. With regard to the "trap" hypothesis, the limited release of thiols in a slow phase is suggestive of enzyme activity within the alpha 2M-protease complex, consistent with the theory. Noncovalent irreversible complexes, however, are not a necessary part of associations seen with lysyl-blocked enzymes (which do cause proteolysis and do release thiols); this result is supported by limited data with noncovalently bound native enzymes. Some fraction of irreversible noncovalently bound enzymes may occur, but our results suggest that although alpha 2M-bound enzymes are unusually sterically hindered, the transformation to the presumed covalent state that appears to depend on intact amino groups, may be sufficient to explain the low dissociation of native enzymes. We feel that more experimental evidence is needed to resolve some of the ambiguities on this question but, we feel the existence of a "trapping" reaction has not been proved. In fact, given the possible existence of equilibria between covalent and noncovalent complexes observed, for example, in soybean trypsin inhibitor, and the very low dissociation constants observed with traditional protein-protein complexes, the question of physically encapsulated structures in alpha 2M may not be resolvable without direct evidence from crystal structures.

Structure of alpha 2-macroglobulin-protease complexes

The analysis of thrombin-alpha 2M reaction mixtures by two-dimensional SDS-PAGE has allowed us to assign several probable molecular species to the mixture of complexes formed. These include structures previously described in, or predicted from, the literature, as well as two types of novel species: Divalent cross-linking of two inhibitor chains by a single enzyme molecule. Very high molecular weight species that are attributed to intermolecular cross-linking of more than one inhibitor molecule. Species containing enzyme monovalently linked to an intact subunit are not supported by our data, but are not excluded and additional study will be required to determine if they exist.

The alpha 2 macroglobulin/thrombin interaction in the presence of hydroxylamine

The influence of a primary amine, hydroxylamine, on the interaction between alpha 2 macroglobulin (alpha 2M) and thrombin was analyzed by electrophoretic and enzymatic methods. Hydroxylamine (final concentrations 0.01 M and 0.1 M) was added to the alpha 2M solution 3 to 5 min before thrombin. In these conditions hydroxylamine had no direct influence on alpha 2M itself. The inhibition of thrombin activity by alpha 2M was still possible and alpha 2M/thrombin complexes were observed. However the rate of inhibition of the clotting activity of thrombin was diminished in function of the hydroxylamine concentration. The complexes obtained in the absence of the nucleophylic agent were resistant to SDS dissociation, whereas those obtained in the presence of hydroxylamine were dissociated by SDS. In both cases, the amount of alpha 2M polypeptide chains cleaved by thrombin was the same (50%). In conclusion, hydroxylamine does not prevent the formation of alpha 2M/thrombin complexes, but it reduces the covalent binding of the enzyme to the inhibitor in a concentration dependent fashion, leading to the formation of "abnormal" complexes.

Absorption of human alpha 2-macroglobulin with selected strains of streptococci

A new interaction was observed between human alpha 2-macroglobulin (alpha 2M) and selected strains of streptococci. The streptococci bound alpha 2M. After its elution from the bacteria, alpha 2M was demonstrated by immunoelectrophoresis against anti-human alpha 2M. The binding of alpha 2M to one streptococcal strain of serological group C and two of group G was confirmed by radial immunodiffusion. Treating the streptococci with trypsin reduced their reactivity with alpha 2M. On the other hand, prolonged treatment with 6 M guanidine-HCl had no effect on the alpha 2M binding. Binding sites for alpha 2M were most likely protein structures on the streptococcal surface.

Dissociability of enzyme-alpha 2-macroglobulin complexes

Experiments were performed to measure the extent to which enzymes bound to alpha 2-macroglobulin (alpha 2M) could be dissociated from the complex. Noncovalent complexes are known to exist between alpha 2M and proteases, such as methyl-trypsin that have had their lysyl amino covalently blocked. Complexes between the inhibitor and native enzymes also have a certain fraction noncovalent binding. Because of the severe steric hindrance imposed on enzymes bound to alpha 2M, even in the noncovalent mode, it has been proposed in the literature that they are not dissociable in the usual sense but, rather, are "trapped" in clathrate-like complexes. The results presented here show that lysyl-blocked methyl-thrombin, or native thrombin are released from their alpha 2M complex by an excess of other lysyl-blocked or native proteases. Under conditions where native thrombin is displaced, labeled enzymes can be incorporated, indicating the inhibitor is intact by the criterion of incorporating enzymes. Likewise, native elastase can be released from its alpha 2M complex by excess cold elastase or the inactive anhydrotrypsin, the latter experiment being carried out with an excess of the low-molecular-weight inhibitor diisopropyl phosphofluoridate. In conjunction with previous results showing that lysyl-blocked enzymes are removed from alpha 2M by soybean trypsin inhibitor, the data indicate that, however sterically hindered, alpha 2M-bound enzymes are dissociable and no unique "trapped" intermediate need be postulated.

Localization of the proteinase-induced thiol groups in alpha 2-macroglobulin

Free thiol groups released on proteolytic attack of alpha 2-macroglobulin by trypsin or chymotrypsin bind covalently to thiopropyl-Sepharose, indicating that they are located at the surface of the complexes. These cysteine sulfhydryl groups appear to be in contact with the alpha 2M-bound proteases from singlet-singlet energy transfer measurements between fluorescein isothiocyanate-labeled proteinases and N-(iodoacetylaminoethyl)-5-naphtylamine-1-sulfonic acid-labeled thiols in alpha 2-macroglobulin.