Preventing amyloid formation by catching unfolded transmembrane segments

A subset of protein misfolding diseases, including, for example, Alzheimer's disease, is associated with the formation of highly insoluble amyloid fibrils with a beta-sheet structure. The amyloidogenic human lung surfactant protein C (SP-C) is generated from SP-C precursor, which has a C-terminal domain (CTC) that prevents SP-C amyloid fibril formation. Analysis of the substrate specificity of CTC reveals that it binds to all amino acid residues that promote membrane insertion, provided that they are in a nonhelical conformation. In line with this unexpectedly general substrate specificity, the anti-amyloid function of CTC extends to a transmembrane segment other than that of (pro)SP-C, namely, the amyloid beta-peptide associated with Alzheimer's disease. These findings indicate that CTC is the first known chaperone to be directed towards nonhelical transmembrane segments and that it may be employed for the development of new diagnostics or anti-amyloid therapies.

Total serum immunoglobulin E levels in a case-control study in asthmatic/allergic patients, their family members, and healthy subjects from India

BACKGROUND

Total immunoglobulin E (IgE) is an important indicator of allergic disorders. However, its role in allergic patients in India has not been evaluated in relation to atopic status for a reference range as compared with healthy subjects.

OBJECTIVE

The aim of the study was to establish serum IgE levels in a diseased group, study its relationship with atopy, and to compare the same with healthy volunteers in Indian subjects.

METHODS

Four hundred and eighty asthmatics/allergic patients, 100 first-degree relatives of asthmatics, and 120 unrelated normal healthy volunteers from Delhi region were recruited for the study. Atopy was established by family history and skin test to common indigenous allergens and, total and specific IgE measurements. Statistical analysis was performed with the help of SPSS software program.

RESULTS

The mean IgE levels were the highest in asthmatic patients and the lowest in the control healthy group. IgE was significantly high in the male than the female healthy volunteers (P<0.05), but not in the diseased group. Prosopis juliflora among pollen allergens and Alternaria alternata among fungal allergens were important sensitizers in allergic patients with 34.7% and 17.7% skin positivity, respectively. Atopic status and asthma were found to be the best predictor of IgE, which was highly significant (r(2)=0.239, P<0.00001). However, at 95% confidence interval as many as 50% of asthmatic patients had their IgE values in the normal range.

CONCLUSION

The IgE levels in Indian allergic patients is significantly related to atopy, but due to wide overlap of IgE levels in patients and healthy subjects, its diagnostic significance in Indian population seems to be limited.

A novel anti-angiogenic form of antithrombin with retained proteinase binding ability and heparin affinity

Latent antithrombin, an inactive antithrombin form with low heparin affinity, has previously been shown to efficiently inhibit angiogenesis and tumor growth. We now show that heat treatment similar to that used for preparation of latent antithrombin also transforms antithrombin to another form, which we denote prelatent, with potent anti-angiogenic and anti-tumor activity but with retained proteinase- and heparin-binding properties. The ability of prelatent antithrombin to inhibit angiogenesis is presumably due to a limited conformational change, which may partially resemble that in latent antithrombin. Such a change is evidenced by a different cleavage pattern of prelatent than of native antithrombin by nontarget proteinases. Prelatent antithrombin exerts its anti-angiogenic effect by a similar mechanism as latent antithrombin, i.e. by inhibiting focal adhesion formation and focal adhesion kinase activity, thereby leading to decreased proliferation of endothelial cells. The proteinase inhibitory fractions in commercial antithrombin preparations, which have been heat treated during production, also have anti-angiogenic activity, comparable with that of the prelatent antithrombin form.

A novel strategy to generate biologically active neo-glycosaminoglycan conjugates

Heparin and heparan sulfate are structurally related polysaccharides with a variety of biological effects/functions. Most of these effects are due to interactions, of varying specificity, between the negatively charged polysaccharide chains and proteins. While such interactions generally involve a single saccharide domain of decasaccharide size or less, ternary complexes of two protein molecules binding to separate domains on a single polysaccharide chain are known to occur. To facilitate studies on domain organization and its importance for biological function a strategy was developed to chemically conjugate defined heparin oligomers in linear and chemoselective fashion. The procedure requires that the oligosaccharide to provide the reducing-terminal domain of the conjugate is generated by lyase degradation of a parent polysaccharide, whereas the nonreducing-terminal domain is obtained through deaminative cleavage with nitrous acid. The applicability of the method was demonstrated by constructing a conjugate composed of two heparin 12-mers, of which the reducing-terminal component contained the antithrombin-binding region, whereas the nonreducing-terminal domain did not. Contrary to any of the unconjugated oligomers, the product was found to efficiently promote the inactivation of thrombin by antithrombin.

Inactivation of papain by antithrombin due to autolytic digestion: a model of serpin inactivation of cysteine proteinases

Cross-class inhibition of cysteine proteinases by serpins differs from serpin inhibition of serine proteinases primarily in that no stable serpin-cysteine proteinase complex can be demonstrated. This difference in reaction mechanism was elucidated by studies of the inactivation of the cysteine proteinases, papain and cathepsin L, by the serpin antithrombin. The two proteinases were inactivated with second-order rate constants of (1.6+/-0.1)x10(3) and (8.6+/-0. 4)x10(2) M-1.s-1 respectively. An antithrombin to papain inactivation stoichiometry of approximately 3 indicated extensive cleavage of the inhibitor concurrent with enzyme inactivation, a behaviour verified by SDS/PAGE. N-terminal sequence analyses showed cleavage predominantly at the P2-P1 bond, but also at the P2'-P3' bond of antithrombin. The papain band in SDS/PAGE progressively disappeared on reaction of the enzyme with increasing amounts of antithrombin, but no band representing a stable antithrombin-papain complex appeared. SDS/PAGE with 125I-labelled papain showed that the disappearance of papain was caused by cleavage of the enzyme into small fragments. These results suggest a mechanism in which papain attacks a peptide bond in the reactive-bond loop of antithrombin adjacent to that involved in serine proteinase inhibition. The reaction proceeds, similarly to that between serpins and serine proteinases, to form an inactive acyl-intermediate complex, although with the substrate pathway dominating in the papain reaction. In this complex, papain is highly susceptible to proteolysis and is degraded by still active papain, which greatly decreases the lifetime of the complex and results in liberation of fragmented, inactive enzyme. This model may have relevance also for the inactivation of physiologically or pathologically important cysteine proteinases by serpins.

A three center study on the diagnostic accuracy of 99mTc-MIBI scintimammography

In order to assess specificity and sensitivity of the prone scintimammography (PSM) in a large series with 99m-Tc MIBI, we performed a three-center study; 420 patients were studied; after mammography all the patients were submitted to PSM and biopsy and/or operation. PSM was considered positive if hot spot within the breast was observed. In palpable masses sensitivity was 0.98 and specificity 0.89, non palpable masses showed a sensitivity of 0.62 and a specificity of 0.91. When the cancers were stratified for T category the sensitivity was 0.28 in T1a 0.26 in the group of T1a carcinomas, 0.56 in T1b 0.95 in T1c and 0.97 T2 tumors. Physical factors such as attenuation. Compton scattering from chest, as well as biological factors have a role in breast tumor imaging. In the tumors smaller than 1 cm biological factors are probably involved too.

Identification of an epitope in antithrombin appearing on insertion of the reactive-bond loop into the A beta-sheet

Previous work has shown that insertion of the reactive-bond loop of antithrombin into the main beta-sheet of the inhibitor, the A sheet, leads to exposure of epitopes that are not present in intact antithrombin. Identical epitopes are exposed in antithrombin-proteinase complexes, inferring that the reactive-bond loop is inserted into the A beta-sheet also in these complexes. Loop insertion thus presumably is involved in the mechanism of inhibition of target proteinases. In this work, we have identified a linear epitope in bovine antithrombin that reacts with antibodies specific for loop-inserted forms of the inhibitor. This epitope is a hexapeptide sequence comprising residues 342-347, Glu-Asp-Leu-Phe-Ser-Pro, and is located on the surface of the protein just carboxy-terminal of helix I. The Phe residue of this epitope is highly conserved in members of the serpin superfamily and appears to stabilize the region of the epitope in antithrombin and other serpins by interacting with the protein core. The conformational change involving expansion of the A beta-sheet following insertion of the reactive-bond loop is presumably transmitted through this Phe residue to the epitope region, thereby rendering this region accessible to antibodies.

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.

Kinetic characterization of the substrate reaction between a complex of antithrombin with a synthetic reactive-bond loop tetradecapeptide and four target proteinases of the inhibitor

A tetradecapeptide corresponding to the P1 to P14 region of the reactive-bond loop of antithrombin (AT) binds to the inhibitor, presumably as a middle strand of the A beta-sheet, thereby converting AT from an inhibitor to a substrate of thrombin (Björk, I., Ylinenjärvi, K., Olson, S.T., and Bock, P. E. (1992) J. Biol. Chem. 267, 1976-1982). The kinetics of cleavage of the AT reactive bond in the AT-peptide complex by four target proteinases were quantified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometry. The kcat/Km values for thrombin and factor IXa were indistinguishable from the second-order rate constants for AT inhibition of these enzymes, whereas the values for factor Xa and plasmin were 10-17-fold higher than the inhibition rate constants. Heparin with high affinity for AT accelerated the substrate reaction with thrombin to an extent consistent with the reduced heparin affinity of the AT-peptide complex. These data show that blocking by the peptide of the putative intramolecular association of the P1 to P14 region of the AT reactive-bond loop with the A beta-sheet leads to AT functioning as a substrate of its target enzymes with an efficiency that equals or exceeds the action of uncomplexed AT as an inhibitor and with the expected heparin activation. The results thus suggest that a substrate-like attack of the proteinase on the inhibitor reactive bond in an exposed loop initiates the inhibition reaction. This attack presumably induces the subsequent trapping of the enzyme by the insertion of the reactive-bond loop into the A beta-sheet.

Isolation and phagocytic properties of neutrophils and other phagocytes from nonmastitic bovine milk

A technique for the separation of neutrophils from macrophages-epithelial cells in samples of nonmastitic bovine milk with low cell counts has been developed. The procedure is based on centrifugation in a discontinuous metrizamide gradient and is rapid, taking less than 40 min. The recovery of the neutrophils is about 30% and their viability about 90%. The isolated neutrophils showed an appreciable unstimulated luminol- and lucigenin-dependent chemiluminescence, which was due to NADPH oxidase rather than to xanthine oxidase. The neutrophils had a higher rate of ingestion of C3-opsonized particles than macrophages-epithelial cells, whereas no significant differences in phagocytosis of IgG-opsonized yeast or unopsonized yeast were detected between the two cell populations. The macrophages-epithelial cells produced no luminol-dependent chemiluminescence and induced considerably lower activity in the lucigenin-dependent system than neutrophils, indicating that these cells contain no myeloperoxidase. Analyses of the activity of the neutrophils in response to C3-opsonized yeast particles showed that the luminol-dependent chemiluminescence of cells isolated from residual milk increased significantly over the lactation period. Moreover, a tendency to a higher phagocytosis and chemiluminescence of neutrophils isolated from residual milk than from stripping milk was indicated.

A new technique, requiring small amounts of cells, for the parallel study of chemiluminescence and phagocytosis via different receptors in the same cell population

An assay permitting the parallel assessment of phagocytosis and chemiluminescence in the same cell population has been developed. The method is based on the phagocytosis of fluorescein isothiocyanate-conjugated yeast particles, either unopsonized or opsonized with complement factor C3 or IgG, by purified cells in suspension in a luminometer. Only a small number of cells (2 x 10(4)-1 x 10(5)) is required, and the reproducibility is high. Moreover, the technique permits phagocytosis to be related to oxygen-dependent killing activity in the same cell population. Since phagocytosis, degranulation and oxygen radical formation as a consequence of well-defined receptor recognition mechanisms can be characterized in very small cell populations, the method is suitable for monitoring the phagocytic function of cells from extravascular sites.

Denaturation behavior of antithrombin in guanidinium chloride. Irreversibility of unfolding caused by aggregation

The structural stability of the protease inhibitor antithrombin from bovine plasma was examined as a function of the concentration of guanidinium chloride (GdmCl). A biphasic unfolding curve at pH 7.4, with midpoints for the two phases at 0.8 and 2.8 M GdmCl, was measured by far-ultraviolet circular dichroism. Spectroscopic and hydrodynamic analyses suggest that the intermediate state which exists at 1.5 M GdmCl involves a partial unfolding of the antithrombin molecule that exposes regions of the polypeptide chain through which slow, intermolecular association subsequently takes place. The partially unfolded molecule can be reversed to its fully functional state only before the aggregation occurs. Upon return of the aggregated state to dilute buffer, the partially unfolded antithrombin remains aggregated and does not regain the spectroscopic properties, thrombin-inhibitory activity, or heparin affinity of the native inhibitor. This behavior indicates that the loss of the functional properties of the proteins is caused by the macromolecular association. Comparative experiments gave similar results for the human inhibitor. Analyses of bovine antithrombin in 6 M GdmCl indicated that the second transition reflects the total unfolding of the protein to a disulfide-cross-linked random coil. This transition is spectroscopically reversible; however, on further reversal to dilute buffer, the molecules apparently are trapped in the partially unfolded, aggregated, intermediate state. The results are consistent with the existence of two separate domains in antithrombin which unfold at different concentrations of GdmCl but do not support the contention that the thrombin-binding and heparin-binding regions of the protein are located in different domains [Villanueva, G. B., & Allen, N. (1983) J. Biol. Chem. 258, 14048-14053].

Extension and structural variability of the antithrombin-binding sequence in heparin

Oligosaccharides with different affinities for antithrombin were isolated following partial deaminative cleavage of pig mucosal heparin with nitrous acid. The smallest high-affinity component obtained was previously identified as an octasaccharide with the predominant structure: (Formula: see text). The interaction of this octasaccharide, and of deca- and dodecasaccharides containing the same octasaccharide sequence, with antithrombin was studied by spectroscopic techniques. The near-ultraviolet difference spectra, circular dichroism spectra, and fluorescence enhancements induced by adding these oligosaccharides to antithrombin differed only slightly from the corresponding parameters measured in the presence of undegraded high-affinity heparin. Moreover, the binding constants obtained for the oligosaccharides and for high-affinity heparin were similar (1.0-2.9 X 10(7) M-1 at I = 0.3). In contrast, two hexasaccharides corresponding to units 1-6 and 3-8, respectively, of the above sequence showed about a 1000-fold lower affinity for antithrombin, and also induced considerably different spectral perturbations in antithrombin. Since the 1-6 hexasaccharide contains a reducing-terminal anhydromannose residue instead of the N-sulfated glucosamine unit 6 of the intact sequence, these results strongly support our previous conclusion that the N-sulfate group at position 6 is essential to the interaction with antithrombin. The low affinity of the hexasaccharide 3-8 provides further evidence that a pentasaccharide sequence 2-6 constitutes the actual antithrombin-binding region in the heparin molecule. Structural analysis of the various oligosaccharides revealed natural variants with an N-sulfate group substituted for the N-acetyl group at position 2. The preponderance of N-acetyl over N-sulfate groups at this position may be rationalized in terms of the mechanism of heparin biosynthesis, assuming that the D-gluco configuration of unit 3 is an essential feature of the antithrombin-binding region.

The active site of antithrombin. Release of the same proteolytically cleaved form of the inhibitor from complexes with factor IXa, factor Xa, and thrombin

Reactions between near equimolar amounts of antithrombin and Factors IXa or Xa resulted in the formation of a free proteolytically modified, two-chain form of the inhibitor, in addition to the inactive antithrombin-protease complexes. The modified inhibitor produced by either enzyme was electrophoretically identical with that formed in the reaction with thrombin. As in the latter reaction, the formation of the modified antithrombin by Factor Xa was increased in the presence of heparin, while only small amounts were produced by Factor IXa both in the absence and presence of the polysaccharide. NH2-terminal sequence analyses of the isolated modified inhibitor formed by Factor Xa showed that a single Arg-Ser bond in the COOH-terminal end of the inhibitor had been cleaved. This cleavage site is identical with that identified in free thrombin-modified antithrombin. The purified antithrombin-Factor IXa and antithrombin-Factor Xa complexes were dissociated by ammonia or hydroxylamine into free enzyme and a modified two-chain form of the inhibitor. Electrophoresis studies and NH2-terminal sequence analyses showed that the modified antithrombin obtained from either complex was identical with that produced in free form by the two enzymes and also with the modified inhibitor that is released from the antithrombin-thrombin complex. The fact that identical results were obtained for the reactions between antithrombin and three enzymes with different specificities strongly suggests that the observed Arg-Ser cleavage site is the active site of antithrombin.

Immunological evidence for a proteolytic cleavage at the active site of antithrombin in the mechanism of inhibition of coagulation serine proteases

Previous studies have shown that a modified form of antithrombin, cleaved at a single Arg-Ser bond near the carboxy-terminal end of the chain, is formed during the reaction with thrombin concurrent with the formation of the inactive enzyme-inhibitor complex. A variety of evidence suggests that this cleavage site is the active site of antithrombin. In this work, antisera against intact antithrombin, the modified form of antithrombin and the antithrombin-thrombin complex were used in immunodiffusion analyses to probe the state of the inhibitor in its complexes with coagulation serine proteases. The results show that new antigenic determinants not present in intact antithrombin are created in modified antithrombin by the single peptide-bond cleavage. the same antigenic determinants are found also in complexes between antithrombin and thrombin or factor Xa. No evidence for the exposure of other new determinants in the complexes was obtained. The most likely conclusion from these results is that antithrombin exists in its complexes with the serine proteases as the modified, two-chain form of the inhibitor. This suggests that the mechanism of inhibition involves proteolytic cleavage of the active site of antithrombin by the protease.

Evidence by chemical modification for the involvement of one or more tryptophanyl residues of bovine antithrombin in the binding of high-affinity heparin

Tryptophanyl residues of bovine antithrombin were modified with N-bromosuccinimide at near-neutral pH. The reaction was found to be specific for tryptophan at low levels of modification, i.e. when only up to 1--1.3 mol tryptophan/mol protein were oxidized. Further modification led to extensive side reactions. Modification of an average of about one tryptophanyl residue per protein molecule did not affect antithrombin activity measured in the absence of heparin, but decreased the activity assayed in the presence of heparin to about half the value given by unmodified antithrombin. Addition of an excess of high-affinity heparin to a similarly modified antithrombin sample resulted in much smaller circular dichroism, ultraviolet absorption and fluorescence changes than those observed with the intact protein. Modification experiments in the presence of excess high-affinity heparin gave a definitely lower extent of modification than when heparin was excluded. These studies thus reinforce the conclusion from previous spectroscopic analyses that one or more tryptophanyl residues of antithrombin are involved in the binding of high-affinity heparin, presumably by being located at or close to the heparin binding site.