Reaction of antithrombin with proteases. Nature of the reaction with trypsin

Anaphylatoxin C5a creates a favorable microenvironment for lung cancer progression

The complement system contributes to various immune and inflammatory diseases, including cancer. In this study, we investigated the capacity of lung cancer cells to activate complement and characterized the consequences of complement activation on tumor progression. We focused our study on the production and role of the anaphylatoxin C5a, a potent immune mediator generated after complement activation. We first measured the capacity of lung cancer cell lines to deposit C5 and release C5a. C5 deposition, after incubation with normal human serum, was higher in lung cancer cell lines than in nonmalignant bronchial epithelial cells. Notably, lung malignant cells produced complement C5a even in the absence of serum. We also found a significant increase of C5a in plasma from patients with non-small cell lung cancer, suggesting that the local production of C5a is followed by its systemic diffusion. The contribution of C5a to lung cancer growth in vivo was evaluated in the Lewis lung cancer model. Syngeneic tumors of 3LL cells grew slower in mice treated with an antagonist of the C5a receptor. C5a did not modify 3LL cell proliferation in vitro but induced endothelial cell chemotaxis and blood-vessels formation. C5a also contributed to the immunosuppressive microenvironment required for tumor growth. In particular, blockade of C5a receptor significantly reduced myeloid-derived suppressor cells and immunomodulators ARG1, CTLA-4, IL-6, IL-10, LAG3, and PDL1 (B7H1). In conclusion, lung cancer cells have the capacity to generate C5a, a molecule that creates a favorable tumor microenvironment for lung cancer progression.

Binding of alkaloids into the S1 specificity pocket of α-chymotrypsin: evidence from induced circular dichroism spectra

Non-covalent binding of planar aromatic molecules into the S1 specificity pocket of the serine protease α-chymotrypsin (αCHT) can be detected by measuring induced circular dichroism (CD) spectroscopic signals. Utilizing this phenomenon, αCHT association of proflavine (PRF), the well known serine protease inhibitor has been investigated together with plant-derived compounds including isoquinoline, pyridocarbazole and indoloquinoline alkaloids, of which αCHT binding has never been reported. Non-degenerate exciton coupling between π-π* transitions of the ligand molecules and two tryptophan residues (Trp172 and Trp215) near to the binding site is proposed to be responsible for the induced CD activity. The association constants calculated from CD titration data indicated strong αCHT association of sanguninarine, ellipticine, desmethyl-isocryptolepine and isoneocryptolepine (K(a) ≈ 10(5) M(-1)) while berberine, coptisine and chelerythrine bind to the enzyme with lower, PRF-like affinity (K(a) ≈ 10(4) M(-1)). PRF-trypsin and ellipticine-trypsin binding interactions have also been demonstrated. The binding of the alkaloids into the S1 pocket of αCHT has been confirmed by CD competition experiments. Molecular docking calculations showed the inclusion of PRF as well as the alkaloid molecules in the S1 cavity where they are stabilized by hydrophobic and H-bonding interactions. These novel nonpeptidic scaffolds can be used for developing selective inhibitors of serine proteases having chymotrypsin-like folds. Furthermore, the results provide a novel, CD spectroscopic based approach for probing the ligand binding of αCHT and related proteases.

The asparagine-stabilized beta-turn of apamin: contribution to structural stability from dynamics simulation and amide hydrogen exchange analysis

Molecular dynamics simulations of bee venom apamin, and an analogue having an Asn to Ala substitution at residue 2 (apamin-N2A), were analyzed to explore the contribution of hydrogen bonds involving Asn2 to local (beta-turn residues N2, C3, K4, A5) and global stability. The wild-type peptide retained a stable conformation during 2.4 ns of simulation at 67 degrees C, with high beta-turn stability characterized by backbone-side chain hydrogen bonds involving beta-turn residues K4 and A5, with the N2 side chain amide carbonyl. The loss of stabilizing interactions involving the N2 side chain resulted in the loss of the beta-turn conformation in the apamin N2A simulations (27 or 67 degrees C). This loss of beta-turn stability propagates throughout the peptide structure, with destabilization of the C-terminal helix connected to the N-terminal region by two disulfide bonds. Backbone stability in a synthetic peptide analogue (apamin-N2A) was characterized by NMR and amide hydrogen exchange measurements. Consistent with the simulations, loss of hydrogen bonds involving the N2 side chain resulted in destabilization of both the N-terminal beta-turn and the C-terminal helix. Amide exchange protection factors in the C-terminal helix were reduced by 9-11-fold in apamin N2A as compared with apamin, corresponding to free energy (deltaDeltaG(uf)) of around 1.5 kcal M(-1) at 20 degrees C. This is equivalent to the contribution of hydrogen bond interactions involving the N2 side chain to the stability of the beta-turn. Together with additional measures of exchange protection factors, the three main contributions to backbone stability in apamin that account for virtually the full thermodynamic stability of the peptide have been quantitated.

Changes in the localization of brain prion proteins during scrapie infection

Prion proteins (PrP) were localized in the brains of normal and scrapie-infected hamsters by immunohistochemistry and Western blotting. PrP monoclonal antibodies and monospecific anti-PrP peptide sera, which react with both the cellular (PrPC) and scrapie (PrPSc) isoforms of the prion protein, were used to locate PrP in tissue sections. In normal hamsters, PrPC was located primarily in nerve cell bodies throughout the CNS; whereas, in the terminal stages of scrapie, PrP immunoreactivity was shifted to the neuropil and was absent from most nerve cell bodies. Prion proteins were not uniformly dispersed throughout the gray matter of scrapie-infected hamster brains; rather, they were concentrated in those regions that exhibited spongiform degeneration and reactive astrogliosis. Since earlier studies showed that the level of PrPC remains constant during scrapie infection as measured in whole brain homogenates and no antibodies are presently available that can distinguish PrPC from PrPSc, we analyzed individual brain regions by Western blotting. Analysis of proteinase K-digested homogenates of dissected brain regions showed that most of the regional changes in PrP immunoreactivity that are seen during scrapie infection are due to the accumulation of PrPSc. These observations indicate that the tissue pathology of scrapie can be directly correlated with the accumulation of PrPSc in the neuropil, and they suggest that the synthesis and distribution of the prion protein has a central role in the pathogenesis of this disorder.

Heparin-induced structural and functional alterations of bovine trypsin

To investigate the mechanism whereby heparin can modulate the activity of serine proteinases, bovine trypsin was chosen as reference and treated with heparin at 10, 100 and 200 micrograms/ml, in buffer solvents, with and without incubation at 37 degrees C. Heparin caused rapid, buffer- and pH-dependent decrease in trypsin solubility due to the generation of insoluble fragments from proteinase. Desalting treatments variously restored solubility by removing insoluble material. UV absorption and fluorescence emission spectra revealed significant heparin-induced conformational alterations in the trypsin molecule, the maximal effect being apparent at a proteinase-to-heparin molar ratio ranging from 1.6 to 1.0. The involvement of the catalytic sites of trypsin by heparin was further confirmed by the significant reduction in the difference absorption spectra of proflavine. Both proteolytic and esterolytic activities of trypsin were shown to be markedly decreased by heparin, especially after 5 h incubation at 37 degrees C. However, when the proteolytic and esterolytic activities of trypsin were measured on fresh solutions not submitted to desalting treatments, variable activation instead of inhibition of both activities was observed in the presence of heparin, this effect waning spontaneously in time or after desalting treatment. The paradoxical increase in functional activities was not inhibited by soybean trypsin inhibitor and was accompanied by denaturation and fragmentation of the proteinase as demonstrated by spectroscopic analyses and SDS-PAGE of fresh solutions. The results obtained indicated that heparin causes a rapid, time- and temperature-dependent conformational alteration of trypsin with irreversible denaturation and degradation of the proteinase. The underlying mechanism appears to be heparin-catalyzed oxidative degradation of trypsin due to liberation of oxygen radicals which are also responsible for the temporary increase in catalytic functions.

Evaluation of the proteolytic potential of in vitro-cultivated hybridoma and recombinant mammalian cells

The proteolytic potential of culture supernatants derived from recombinant baby hamster kidney (BHK) 21 and mouse-mouse hybridoma cells have been characterized. Several assays using enzyme specific chromogenic artificial peptides, as well as a radioactive test for the detection of the total activity, have been established and were adapted to the special conditions existing in culture media of mammalian cells. Proteolytic activity was detected in human serum albumin containing media which was specific for peptides ending with a terminal arginine. The addition of N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer to the culture media resulted in a significant peptide cleavage potential, supporting the fact that this compound is not recommended as a supplement in animal cell culture media. Medium shock protease activity has been detected in culture supernatants of BHK cells when medium was changed completely, caused by a switch from a serum containing state of growth to a serum-free state of growth which is often used in processes with microcarriers. However, this proteolytic activity showed a transient behaviour whereby its secretion stopped when the cells had adapted to the serum-free medium conditions. Characterization of the proteolytic activities using different specific inhibitors and activators supported the assumption that the proteolytic activity reflects a cell specific composition of proteases which can also change dependent on the culture conditions used.

Antithrombin: structure, genomic organization, function and inherited deficiency

Antithrombin is a major plasma protein inhibitor of proteinases generated during blood coagulation; it plays an important role in the regulation of thrombin in blood. The anticoagulant heparin greatly accelerates the rate of inactivation of proteinases by antithrombin, predominantly through its well defined, highly specific binding reaction with the inhibitor, but also through a less strictly defined interaction with some of the proteinases (such as thrombin). There is evidence for an analogous acceleratory mechanism in vivo, that functions by the binding of antithrombin to a subpopulation of heparan sulphate proteoglycans intercalated in the surface of endothelial cells. The location and structure of the gene for antithrombin are known. Both its overall organization and the structure of the subdomains of the expressed protein can be considered in terms of their relationships to a serine proteinase inhibitor superfamily, which is believed to have evolved from a common ancestor. The region of the antithrombin gene 5' to the coding region has been characterized. Unlike other members of the serpin family, there is no TATA-like promoter sequence. Two enhancer sequences have been identified that are homologous to enhancer regions of other genes. There are two polymorphisms: an intragenic polymorphism arising from a translationally silent A to G transition in codon 305, and a length polymorphism arising from the presence of 32 bp or 108 bp non-homologous sequences 345 bp upstream from the translation initiation codon. Inherited deficiency of antithrombin is associated with familial thromboembolism. The molecular genetic basis of some subtypes of deficiency is increasingly yielding to investigation. It is interesting to note that a number of mutations have been identified in CpG dinucleotides, supporting the suggestion that this dinucleotide sequence may represent a mutation hotspot in the human genome.

Binding of the bovine basic pancreatic trypsin inhibitor (Kunitz) to human alpha-, beta- and gamma-thrombin; a kinetic and thermodynamic study

Kinetic and thermodynamic parameters for the binding of the bovine basic pancreatic trypsin inhibitor (BPTI, Kunitz inhibitor) to human alpha-, beta- and gamma-thrombin have been determined, between 5 and 45 degrees C, at pH 7.5. BPTI-binding properties to human thrombins have been analyzed in parallel with those of serine (pro)enzymes acting on cationic and non-cationic substrates, with particular reference to the bovine beta-trypsin/BPTI system. The observed binding behaviour of BPTI to human alpha-, beta- and gamma-thrombin has been related to the inferred stereochemistry of the enzyme/inhibitor contact region(s).

Interaction of human alpha1-antichymotrypsin with chymotrypsin

Human alpha 1-antichymotrypsin reacts with bovine chymotrypsin to form an equimolar complex and this reaction is accompanied by the formation of a free, modified form of the inhibitor. Time-course studies, performed on mixtures containing an excess of native inhibitor and kept at 0 degree C or at 25 degrees C, show that the equimolar complex dissociates spontaneously; this dissociation results in the release of inactive modified alpha 1-antichymotrypsin and of some active enzyme, which is able to recycle with active inhibitor in excess. When all the native inhibitor is used up, the released active enzyme degrades the remaining intact complex into intermediate forms. At the endpoint of the reaction only inactive modified inhibitor and some active chymotrypsin remain. Immunochemical data indicate that, in the complex, a steric hindrance of the antigenic determinants of the inhibitor prevents the formation of the precipitate with specific antiserum. Inactive modified inhibitor, which has dissociated from the complex, has retained antigenic determinants of the native alpha 1-antichymotrypsin.

Hepatocyte receptors for antithrombin III-proteinase complexes

The in vivo clearance of antithrombin III-proteinase complexes occurs via a specific and saturable pathway located on hepatocytes. We now report studies of the catabolism of antithrombin III-proteinase complexes in vitro using rat hepatocytes in primary culture. Antithrombin III-thrombin and trypsin complexes were prepared and purified to homogeneity. Ligand uptake by hepatocytes was concentration, temperature, and time dependent. Initial rate studies were performed to characterize the maximum rate of uptake, V, and apparent Michaelis constant Kapp. These studies yielded a V of 12.8 fmol/mg cell protein/min and a Kapp of 144 nM for antithrombin-trypsin complexes. Competition experiments with antithrombin III, antithrombin III-proteinase complexes, alpha 2-macroglobulin-methylamine, asialoorosomucoid and the neoglycoproteins, fucosyl-bovine serum albumin (BSA), N-acetylglucosaminyl-BSA, and mannosyl-BSA indicated that only antithrombin III-proteinase complexes were recognized by the hepatocyte receptor. Uptake studies were performed at 37 degrees C with 125I-antithrombin III-trypsin and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in conjunction with autoradiography. These studies demonstrate time-dependent uptake and degradation of the ligand to low molecular weight peptides. In addition, there was a time-dependent accumulation of a high molecular weight complex of ligand and a cellular protein. This complex disappeared when gels were performed under reducing conditions.

Modulatory effects on proteinase kinetics caused by association of both enzyme and substrate to heparin

Heparin is shown to produce modulatory effects on the amidolytic activity of trypsin, thrombin and plasmin with various synthetic peptide substrates. Simple Michaelis-Menten kinetics are observed in the absence of heparin. In its presence an enhancement effect is observed at low substrate concentrations, and an inhibitory effect is observed at high substrate concentrations. Other polyanions like dextran sulphate, phosvitin and inositol hexakisphosphate produces a similar effect. The modulatory effect of heparin is abolished when it binds cations. Co-binding of both substrate and enzyme to heparin seems to be a necessary requirement for the effect to occur. A model is proposed which can account semiquantitatively for the kinetics observed. It is suggested that the mechanism, which involves co-binding of substrate and enzyme in an competitive manner to a macromolecular structure, may be of primary importance as a regulatory mechanism in blood coagulation and fibrinolysis.

Electrostatic interactions in the heparin-enhanced reaction between human thrombin and antithrombin

Binding of heparin to thrombin is monitored by means of an aqueous two-phase partition system, and binding of heparin to antithrombin is monitored by means of heparin induced enhancement of the intrinsic fluorescence of the protein. Both types of binding are studied at various electrolyte compositions of the medium. Heparin is displaced from thrombin at lower concentrations of electrolyte than those necessary for its displacement from antithrombin. K+ is more efficient than Na+, which is again more efficient than Li+ in displacing heparin from these proteins. The kinetics of the reaction between thrombin and antithrombin in the presence of heparin were studied by using an assay where synthetic peptide substrate is present in the reaction mixture during the reaction between proteinase and inhibitor. The kinetics are studied at various electrolyte compositions of the medium and the results are compared with those obtained from the binding studies performed under similar conditions. The results are consistent with a model where binding of heparin to antithrombin causes enhancement of the reaction rate, and where this enhancement is abolished again when additional binding of heparin to thrombin takes place on further addition of heparin.