PAH under XUV excitation: an ultrafast XUV-photochemistry experiment for astrophysics

Understanding processes induced by XUV excitation of Polycyclic Aromatic Hydrocarbons (PAHs) is at the heart of molecular astrophysics, which aims at understanding molecular evolution in interstellar media. We used ultrashort XUV pulses to produce highly excited PAHs cations. The photo-induced dynamics is probed using a pump-probe XUV-IR spectroscopy. By studying PAH from small (naphthalene) to large (hexabenzocoronene) PAHs, we show that the dynamic is governed by the large density of states, in which many-body quantum effects are dominant.

Control of H_{2} Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of H_{2} molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of ab initio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100 fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations.

Enantioselective synthesis of α-benzylated lanthionines and related tripeptides for biological incorporation into E. coli peptidoglycan

The synthesis of modified tripeptides (S)-Ala-γ-(R)-Glu-X, where X = (R,S) or (R,R) diastereomers of α-benzyl or α-(4-azidobenzyl)lanthionine, was carried out. The chemical strategy involved the enantioselective alkylation of a 4-MeO-phenyloxazoline. The reductive opening of the alkylated oxazolines, followed by cyclization and oxidation, led to four PMB-protected sulfamidates. Subsequent PMB removal, Boc protection and regioselective opening with cysteine methyl ester led to protected lanthionines. These compounds were further converted in a one pot process to the corresponding protected tripeptides. After ester and Boc deprotection, the four tripeptides were evaluated as potential analogues of the natural tripeptide (S)-Ala-γ-(R)-Glu-meso-A2pm. These compounds were evaluated for introduction, by means of the biosynthetic recycling pathway, into the peptidoglycan of Escherichia coli. A successful in vitro biosynthesis of UDP-MurNAc-tripeptides from the tripeptides containing α-benzyl lanthionine was achieved using purified murein peptide ligase (Mpl). Bioincorporation into E. coli W7 did not occur under different tested conditions probably due to the bulky benzyl group at the Cα carbon of the C-terminal amino acid.

In Vitro and In Vivo Analysis of the Gram-Negative Bacteria-Derived Riboflavin Precursor Derivatives Activating Mouse MAIT Cells

Mucosal-associated invariant T (MAIT) cells recognize microbial compounds presented by the MHC-related 1 (MR1) protein. Although riboflavin precursor derivatives from Gram-positive bacteria have been characterized, some level of ligand heterogeneity has been suggested through the analysis of the MAIT cell TCR repertoire in humans and differential reactivity of human MAIT cell clones according to the bacteria. In this study, using Gram-negative bacteria mutated for the riboflavin biosynthetic pathway, we show a strict correlation between the ability to synthesize the 5-amino-ribityl-uracil riboflavin precursor and to activate polyclonal and quasi-monoclonal mouse MAIT cells. To our knowledge, we show for the first time that the semipurified bacterial fraction and the synthetic ligand activate murine MAIT cells in vitro and in vivo. We describe new MR1 ligands that do not activate MAIT cells but compete with bacterial and synthetic compounds activating MAIT cells, providing the capacity to modulate MAIT cell activation. Through competition experiments, we show that the most active synthetic MAIT cell ligand displays the same functional avidity for MR1 as does the microbial compound. Altogether, these results show that most, if not all, MAIT cell ligands found in Escherichia coli are related to the riboflavin biosynthetic pathway and display very limited heterogeneity.

Diaminopimelic Acid Amidation in Corynebacteriales: NEW INSIGHTS INTO THE ROLE OF LtsA IN PEPTIDOGLYCAN MODIFICATION

A gene named ltsA was earlier identified in Rhodococcus and Corynebacterium species while screening for mutations leading to increased cell susceptibility to lysozyme. The encoded protein belonged to a huge family of glutamine amidotransferases whose members catalyze amide nitrogen transfer from glutamine to various specific acceptor substrates. We here describe detailed physiological and biochemical investigations demonstrating the specific role of LtsA protein from Corynebacterium glutamicum (LtsACg) in the modification by amidation of cell wall peptidoglycan diaminopimelic acid (DAP) residues. A morphologically altered but viable ΔltsA mutant was generated, which displays a high susceptibility to lysozyme and β-lactam antibiotics. Analysis of its peptidoglycan structure revealed a total loss of DAP amidation, a modification that was found in 80% of DAP residues in the wild-type polymer. The cell peptidoglycan content and cross-linking were otherwise not modified in the mutant. Heterologous expression of LtsACg in Escherichia coli yielded a massive and toxic incorporation of amidated DAP into the peptidoglycan that ultimately led to cell lysis. In vitro assays confirmed the amidotransferase activity of LtsACg and showed that this enzyme used the peptidoglycan lipid intermediates I and II but not, or only marginally, the UDP-MurNAc pentapeptide nucleotide precursor as acceptor substrates. As is generally the case for glutamine amidotransferases, either glutamine or NH4(+) could serve as the donor substrate for LtsACg. The enzyme did not amidate tripeptide- and tetrapeptide-truncated versions of lipid I, indicating a strict specificity for a pentapeptide chain length.

Structural and biochemical characterization of the β-N-acetylglucosaminidase from Thermotoga maritima: toward rationalization of mechanistic knowledge in the GH73 family

Members of the GH73 glycosidase family cleave the β-1,4-glycosidic bond between the N-acetylglucosaminyl (GlcNAc) and N-acetylmuramyl (MurNAc) moieties in bacterial peptidoglycan. A catalytic mechanism has been proposed for members FlgJ, Auto, AcmA and Atl(WM) and the structural analysis of FlgJ and Auto revealed a conserved α/β fold reminiscent of the distantly related GH23 lysozyme. Comparison of the active site residues reveals variability in the nature of the catalytic general base suggesting two distinct catalytic mechanisms: an inverting mechanism involving two distant glutamate residues and a substrate-assisted mechanism involving anchimeric assistance by the C2-acetamido group of the GlcNAc moiety. Herein, we present the biochemical characterization and crystal structure of TM0633 from the hyperthermophilic bacterium Thermotoga maritima. TM0633 adopts the α/β fold of the family and displays β-N-acetylglucosaminidase activity on intact peptidoglycan sacculi. Site-directed mutagenesis identifies Glu34, Glu65 and Tyr118 as important residues for catalysis. A thorough bioinformatic analysis of the GH73 sequences identified five phylogenetic clusters. TM0633, FlgJ and Auto belong to a group of three clusters that conserve two carboxylate residues involved in a classical inverting acid-base mechanism. Members of the other two clusters lack a conserved catalytic general base supporting a substrate-assisted mechanism. Molecular modeling of representative members from each cluster suggests that variability in length of the β-hairpin region above the active site confers ligand-binding specificity and modulates the catalytic mechanisms within the GH73 family.

Compensating stereochemical changes allow murein tripeptide to be accommodated in a conventional peptide-binding protein

The oligopeptide permease (Opp) of Escherichia coli is an ATP-binding cassette transporter that uses the substrate-binding protein (SBP) OppA to bind peptides and deliver them to the membrane components (OppBCDF) for transport. OppA binds conventional peptides 2-5 residues in length regardless of their sequence, but does not facilitate transport of the cell wall component murein tripeptide (Mtp, L-Ala-γ-D-Glu-meso-Dap), which contains a D-amino acid and a γ-peptide linkage. Instead, MppA, a homologous substrate-binding protein, forms a functional transporter with OppBCDF for uptake of this unusual tripeptide. Here we have purified MppA and demonstrated biochemically that it binds Mtp with high affinity (K(D) ∼ 250 nM). The crystal structure of MppA in complex with Mtp has revealed that Mtp is bound in a relatively extended conformation with its three carboxylates projecting from one side of the molecule and its two amino groups projecting from the opposite face. Specificity for Mtp is conferred by charge-charge and dipole-charge interactions with ionic and polar residues of MppA. Comparison of the structure of MppA-Mtp with structures of conventional tripeptides bound to OppA, reveals that the peptide ligands superimpose remarkably closely given the profound differences in their structures. Strikingly, the effect of the D-stereochemistry, which projects the side chain of the D-Glu residue at position 2 in the direction of the main chain in a conventional tripeptide, is compensated by the formation of a γ-linkage to the amino group of diaminopimelic acid, mimicking the peptide bond between residues 2 and 3 of a conventional tripeptide.

A novel in vivo cell-wall labeling approach sheds new light on peptidoglycan synthesis in Escherichia coli

Peptidoglycan synthesis and turnover in relation to cell growth and division has been studied by using a new labeling method. This method involves the incorporation of fluorescently labeled peptidoglycan precursors into the cell wall by means of the cell-wall recycling pathway. We show that Escherichia coli is able to import exogenous added murein tripeptide labeled with N-7-nitro-2,1,3-benzoxadiazol-4-yl (AeK-NBD) into the cytoplasm where it enters the peptidoglycan biosynthesis route, resulting in fluorescent labels specifically located in the cell wall. When wild-type cells were grown in the presence of the fluorescent peptide, peptidoglycan was uniformly labeled in cells undergoing elongation. Cells in the process of division displayed a lack of labeled peptidoglycan at mid-cell. Analysis of labeling patterns in cell division mutants showed that the occurrence of unlabeled peptidoglycan is dependent on the presence of FtsZ, but independent of FtsQ and FtsI. Accumulation of fluorescence at the division sites of a triple amidase mutant (ΔamiABC) revealed that AeK-NBD is incorporated into septal peptidoglycan. AmiC was shown to be involved in the rapid removal of labeled peptidoglycan side chains at division sites in wild-type cells. Because septal localization of AmiC is dependent on FtsQ and FtsI, this points to the presence of another peptidoglycan hydrolase activity directly dependent on FtsZ.

Structure and function of the first full-length murein peptide ligase (Mpl) cell wall recycling protein

Bacterial cell walls contain peptidoglycan, an essential polymer made by enzymes in the Mur pathway. These proteins are specific to bacteria, which make them targets for drug discovery. MurC, MurD, MurE and MurF catalyze the synthesis of the peptidoglycan precursor UDP-N-acetylmuramoyl-L-alanyl-γ-D-glutamyl-meso-diaminopimelyl-D-alanyl-D-alanine by the sequential addition of amino acids onto UDP-N-acetylmuramic acid (UDP-MurNAc). MurC-F enzymes have been extensively studied by biochemistry and X-ray crystallography. In gram-negative bacteria, ∼30-60% of the bacterial cell wall is recycled during each generation. Part of this recycling process involves the murein peptide ligase (Mpl), which attaches the breakdown product, the tripeptide L-alanyl-γ-D-glutamyl-meso-diaminopimelate, to UDP-MurNAc. We present the crystal structure at 1.65 Å resolution of a full-length Mpl from the permafrost bacterium Psychrobacter arcticus 273-4 (PaMpl). Although the Mpl structure has similarities to Mur enzymes, it has unique sequence and structure features that are likely related to its role in cell wall recycling, a function that differentiates it from the MurC-F enzymes. We have analyzed the sequence-structure relationships that are unique to Mpl proteins and compared them to MurC-F ligases. We have also characterized the biochemical properties of this enzyme (optimal temperature, pH and magnesium binding profiles and kinetic parameters). Although the structure does not contain any bound substrates, we have identified ∼30 residues that are likely to be important for recognition of the tripeptide and UDP-MurNAc substrates, as well as features that are unique to Psychrobacter Mpl proteins. These results provide the basis for future mutational studies for more extensive function characterization of the Mpl sequence-structure relationships.

Drosophila immunity: analysis of PGRP-SB1 expression, enzymatic activity and function

Peptidoglycan is an essential and specific component of the bacterial cell wall and therefore is an ideal recognition signature for the immune system. Peptidoglycan recognition proteins (PGRPs) are conserved from insects to mammals and able to bind PGN (non-catalytic PGRPs) and, in some cases, to efficiently degrade it (catalytic PGRPs). In Drosophila, several non-catalytic PGRPs function as selective peptidoglycan receptors upstream of the Toll and Imd pathways, the two major signalling cascades regulating the systemic production of antimicrobial peptides. Recognition PGRPs specifically activate the Toll pathway in response to Lys-type peptidoglycan found in most Gram-positive bacteria and the Imd pathway in response to DAP-type peptidoglycan encountered in Gram-positive bacilli-type bacteria and in Gram-negative bacteria. Catalytic PGRPs on the other hand can potentially reduce the level of immune activation by scavenging peptidoglycan. In accordance with this, PGRP-LB and PGRP-SC1A/B/2 have been shown to act as negative regulators of the Imd pathway. In this study, we report a biochemical and genetic analysis of PGRP-SB1, a catalytic PGRP. Our data show that PGRP-SB1 is abundantly secreted into the hemolymph following Imd pathway activation in the fat body, and exhibits an enzymatic activity towards DAP-type polymeric peptidoglycan. We have generated a PGRP-SB1/2 null mutant by homologous recombination, but its thorough phenotypic analysis did not reveal any immune function, suggesting a subtle role or redundancy of PGRP-SB1/2 with other molecules. Possible immune functions of PGRP-SB1 are discussed.

Purification and biochemical characterization of Mur ligases from Staphylococcus aureus

The Mur ligases (MurC, MurD, MurE and MurF) catalyze the stepwise synthesis of the UDP-N-acetylmuramoyl-pentapeptide precursor of peptidoglycan. The murC, murD, murE and murF genes from Staphylococcus aureus, a major pathogen, were cloned and the corresponding proteins were overproduced in Escherichia coli and purified as His(6)-tagged forms. Their biochemical properties were investigated and compared to those of the E. coli enzymes. Staphylococcal MurC accepted L-Ala, L-Ser and Gly as substrates, as the E. coli enzyme does, with a strong preference for L-Ala. S. aureus MurE was very specific for L-lysine and in particular did not accept meso-diaminopimelic acid as a substrate. This mirrors the E. coli MurE specificity, for which meso-diaminopimelic acid is the preferred substrate and L-lysine a very poor one. S. aureus MurF appeared less specific and accepted both forms (L-lysine and meso-diaminopimelic acid) of UDP-MurNAc-tripeptide, as the E. coli MurF does. The inverse and strict substrate specificities of the two MurE orthologues is thus responsible for the presence of exclusively meso-diaminopimelic acid and L-lysine at the third position of the peptide in the peptidoglycans of E. coli and S. aureus, respectively. The specific activities of the four Mur ligases were also determined in crude extracts of S. aureus and compared to cell requirements for peptidoglycan biosynthesis.

AER1, a major gene conferring resistance to Aphanomyces euteiches in Medicago truncatula

Aphanomyces euteiches is a major soilborne oomycete pathogen that infects various legume species, including pea and alfalfa. The model legume Medicago truncatula has recently emerged as a valuable genetic system for understanding the genetic basis of resistance to A. euteiches in leguminous crops. The objective of this study was to identify genetic determinants of resistance to a broad host-range pea-infecting strain of A. euteiches in M. truncatula. Two M. truncatula segregating populations of 178 F(5) recombinant inbred lines and 200 F(3) families from the cross F83005.5 (susceptible) x DZA045.5 (resistant) were screened for resistance to A. euteiches. Phenotypic distributions observed suggested a dominant monogenic control of resistance. A major locus associated with resistance to A. euteiches, namely AER1, was mapped by bulk segregant analysis to a terminal end of chromosome 3 in M. truncatula and explained 88% of the phenotypic variation. AER1 was identified in a resistance-gene-rich region, where resistance gene analogs and genes associated with disease resistance phenotypes have been identified. Discovery of AER1 opens up new prospects for improving resistance to A. euteiches in cultivated legumes using a comparative genomics approach.

Biochemical characterization and physiological properties of Escherichia coli UDP-N-acetylmuramate:L-alanyl-gamma-D-glutamyl-meso-diaminopimelate ligase

The UDP-N-acetylmuramate:L-alanyl-gamma-D-glutamyl-meso-diaminopimelate ligase (murein peptide ligase [Mpl]) is known to be a recycling enzyme allowing reincorporation into peptidoglycan (murein) of the tripeptide L-alanyl-gamma-D-glutamyl-meso-diaminopimelate released during the maturation and constant remodeling of this bacterial cell wall polymer that occur during cell growth and division. Mpl adds this peptide to UDP-N-acetylmuramic acid, thereby providing an economical additional source of UDP-MurNAc-tripeptide available for de novo peptidoglycan biosynthesis. The Mpl enzyme from Escherichia coli was purified to homogeneity as a His-tagged form, and its kinetic properties and parameters were determined. Mpl was found to accept tri-, tetra-, and pentapeptides as substrates in vitro with similar efficiencies, but it accepted the dipeptide L-Ala-D-Glu and L-Ala very poorly. Replacement of meso-diaminopimelic acid by L-Lys resulted in a significant decrease in the catalytic efficacy. The effects of disruption of the E. coli mpl gene and/or the ldcA gene encoding the LD-carboxypeptidase on peptidoglycan metabolism were investigated. The differences in the pools of UDP-MurNAc peptides and of free peptides between the wild-type and mutant strains demonstrated that the recycling activity of Mpl is not restricted to the tripeptide and that tetra- and pentapeptides are also directly reused by this process in vivo. The relatively broad substrate specificity of the Mpl ligase indicates that it is an interesting potential target for antibacterial compounds.

Normal responses to specific NOD1-activating peptidoglycan agonists in the presence of the NOD2 frameshift and other mutations in Crohn's disease

Both NOD2/CARD15 alleles are mutated in approximately 10% of Crohn's disease patients, causing loss of functional responses to low-dose muropeptide agonists. We hypothesized that NOD2 mutations may also impair NOD1/CARD4 responses, supported by data suggesting NOD2 1007fs/1007fs patients had reduced responses to a putative NOD1 agonist, diaminopimelic acid-containing muramyl tripeptide (M-TriDAP). We measured peripheral blood mononuclear cell (n = 8 NOD2 wild type, n = 4 1007fs/1007fs, n = 6 702Trp/1007fs, n = 5 702Trp/702Trp, n = 3 908Arg/1007fs) responses to NOD1 agonists alone (IL-8/TNF-alpha), and agonist enhancement of lipopolysaccharide (LPS) responses (IL-1beta). Significant responses were seen with M-TriDAP at 10 nM (as with NOD2 agonists), but only at > or =100 nM with FK565/TriDAP. M-TriDAP induced IL-8/TNF-alpha secretion, and enhancement of LPS IL-1beta responses was significantly reduced between NOD2 double mutation carriers versus healthy controls, whereas there was no difference with FK565 or TriDAP stimulation, or between 1007fs/1007fs cells and other genotypes. M-TriDAP contains both NOD1 (gamma-D-Glu-mesoDAP) and NOD2 (MurNAc-L-Ala-D-Glu) minimal structures whereas FK565/TriDAP contain only NOD1 activating structures. M-TriDAP has dual NOD1/NOD2 agonist activity in primary cells, possibly due to different intracellular peptidoglycan processing compared to the HEK293 cell system typically used for agonist specificity studies. Responses to specific NOD1 agonists are unaffected by NOD2 genotype, suggesting independent action of the NOD1 and NOD2 pathways.

The Drosophila amidase PGRP-LB modulates the immune response to bacterial infection

The Drosophila host defense against gram-negative bacteria is mediated by the Imd pathway upon sensing of peptidoglycan by the peptidoglycan recognition protein (PGRP)-LC. Here we report a functional analysis of PGRP-LB, a catalytic member of the PGRP family. We show that PGRP-LB is a secreted protein regulated by the Imd pathway. Biochemical studies demonstrate that PGRP-LB is an amidase that specifically degrades gram-negative bacteria peptidoglycan. In agreement with its amidase activity, PGRP-LB downregulates the Imd pathway. Hence, activation of PGRP-LB by the Imd pathway provides a negative feedback regulation to tightly adjust immune activation to infection. Our study also reveals that PGRP-LB controls the immune reactivity of flies to the presence of ingested bacteria in the gut. Our work highlights the key role of PGRPs that encode both sensors and scavengers of peptidoglycan, which modulate the level of the host immune response to the presence of infectious microorganisms.

Peptidoglycan molecular requirements allowing detection by the Drosophila immune deficiency pathway

Innate immune recognition of microbes is a complex process that can be influenced by both the host and the microbe. Drosophila uses two distinct immune signaling pathways, the Toll and immune deficiency (Imd) pathways, to respond to different classes of microbes. The Toll pathway is predominantly activated by Gram-positive bacteria and fungi, while the Imd pathway is primarily activated by Gram-negative bacteria. Recent work has suggested that this differential activation is achieved through peptidoglycan recognition protein (PGRP)-mediated recognition of specific forms of peptidoglycan (PG). In this study, we have further analyzed the specific PG molecular requirements for Imd activation through the pattern recognition receptor PGRP-LC in both cultured cell line and in flies. We found that two signatures of Gram-negative PG, the presence of diaminopimelic acid in the peptide bridge and a 1,6-anhydro form of N-acetylmuramic acid in the glycan chain, allow discrimination between Gram-negative and Gram-positive bacteria. Our results also point to a role for PG oligomerization in Imd activation, and we demonstrate that elements of both the sugar backbone and the peptide bridge of PG are required for optimum recognition. Altogether, these results indicate multiple requirements for efficient PG-mediated activation of the Imd pathway and demonstrate that PG is a complex immune elicitor.

A Drosophila pattern recognition receptor contains a peptidoglycan docking groove and unusual L,D-carboxypeptidase activity

The Drosophila peptidoglycan recognition protein SA (PGRP-SA) is critically involved in sensing bacterial infection and activating the Toll signaling pathway, which induces the expression of specific antimicrobial peptide genes. We have determined the crystal structure of PGRP-SA to 2.2-Å resolution and analyzed its peptidoglycan (PG) recognition and signaling activities. We found an extended surface groove in the structure of PGRP-SA, lined with residues that are highly diverse among different PGRPs. Mutational analysis identified it as a PG docking groove required for Toll signaling and showed that residue Ser158 is essential for both PG binding and Toll activation. Contrary to the general belief that PGRP-SA has lost enzyme function and serves primarily for PG sensing, we found that it possesses an intrinsic L,D-carboxypeptidase activity for diaminopimelic acid-type tetrapeptide PG fragments but not lysine-type PG fragments, and that Ser158 and His42 may participate in the hydrolytic activity. As L,D-configured peptide bonds exist only in prokaryotes, this work reveals a rare enzymatic activity in a eukaryotic protein known for sensing bacteria and provides a possible explanation of how PGRP-SA mediates Toll activation specifically in response to lysine-type PG.

The Drosophila protein PGRP-SA is found to have an intrinsic activity to cleave L,D-configured peptide bonds, which exist only in prokaryotes. This work reveals a rare enzymatic activity in a eukaryotic protein known for sensing bacteria

Peptidoglycan molecular requirements allowing detection by Nod1 and Nod2

Nod1 and Nod2 are mammalian proteins implicated in the intracellular detection of pathogen-associated molecular patterns. Recently, naturally occurring peptidoglycan (PG) fragments were identified as the microbial motifs sensed by Nod1 and Nod2. Whereas Nod2 detects GlcNAc-MurNAc dipeptide (GM-Di), Nod1 senses a unique diaminopimelate-containing GlcNAc-MurNAc tripeptide muropeptide (GM-TriDAP) found mostly in Gram-negative bacterial PGs. Because Nod1 and Nod2 detect similar yet distinct muropeptides, we further analyzed the molecular sensing specificity of Nod1 and Nod2 toward PG fragments. Using a wide array of natural or modified muramyl peptides, we show here that Nod1 and Nod2 have evolved divergent strategies to achieve PG sensing. By defining the PG structural requirements for Nod1 and Nod2 sensing, this study reveals how PG processing and modifications, either by host or bacterial enzymes, may affect innate immune responses.

Malaria co-infection in children influences antibody response to schistosome antigens and inflammatory markers associated with morbidity

The epidemiological coexistence of schistosomiasis and malaria is frequently observed in developing countries. Co-infection with malaria in children could influence the development of acquired immunity associated with the resistance or the pathology of schistosomiasis. In the present study, performed during May to June 1996 in Senegal, the humoral immune response to Schistosoma haematobium 28 kDa glutathione S-transferase (Sh28GST) vaccinal antigen and to soluble egg antigens (SEA) has been evaluated in individuals infected by S. haematobium. Specific immunoglobulin G3 (IgG3) and IgE responses were significantly higher in co-infected children with Plasmodium falciparum compared with children infected with S. haematobium only. In addition, circulating levels of interferon-gamma (IFN-gamma), interleukin-10 (IL-10), and soluble tumor necrosis factor receptor II (sTNF-RII), 3 parameters associated with schistosomiasis morbidity, were significantly increased in co-infected children. Taken together, this study indicated that malaria co-infection can both influence the acquired specific immune response to schistosome antigens and unbalance the regulation of inflammatory factors closely involved in schistosomiasis pathology.

Features of the antibody response attributable to plasmid backbone adjuvanticity after DNA immunization

DNA vaccination induces antigen-specific immune responses with characteristics distinct from other vaccination modes. In the present study, the contribution of the plasmid backbone adjuvant effect to the quality of the DNA-raised antibody response was investigated. For this purpose, three intradermal primings were compared in mice using: (1) the recombinant Schistosoma haematobium glutathione S-transferase antigen (rSh28GST): (2) rSh28GST supplemented with a non-coding plasmid; and (3) a Sh28GST-encoding plasmid. In contrast to immunization with the protein, DNA immunization elicited a very stable antibody (Ab) response over a prolonged period of time. This feature was attributed to the plasmid backbone, because co-administration of the non-coding plasmid with rSh28GST allowed the maintenance of the specific Ab response. A strong anamnestic Ab response was induced after intradermal boost with rSh28GST only in the mice primed with pMSh. This indicated that the selective ability of DNA vaccination to induce memory humoral response was independent of the plasmid backbone. In contrast the plasmid backbone was found to strongly participate in the preferential IgG2a Ab production observed. These results suggest that, following DNA immunization, the Th1-biased profile and the maintenance of the long-lived Ab response could be attributed to an adjuvant effect of the plasmid backbone during priming, whereas the strength of B-cell memory was independent of this effect.

HLA-DR restricted peptide candidates for bee venom immunotherapy

T cell epitopes containing peptides have been recently proposed as an alternative to conventional immunotherapy of allergic diseases because they are expected to be better tolerated than allergen extracts. A principal limitation to their clinical use is that they present an important diversity, which primarily results from the polymorphism of HLA class II molecules. In Caucasian populations, however, seven alleles of the most expressed molecules (namely DRB1*0101, DRB1*0301, DRB1*0401, DRB1*0701, DRB1*1101, DRB1*1301, and DRB1*1501) predominate. Peptides from allergens that would efficiently bind to them should be potential candidates for specific immunotherapy. In this paper, we have determined the peptides present in the major bee venom allergen by investigating the capacity of synthetic peptides that encompass its whole sequence to bind to each allele. Several efficient binders have been identified and are either allele-specific or common to several HLA-DR molecules. Interestingly enough, the 81-97 sequence is universal in the sense that it binds to all studied molecules. This sequence is surrounded by several active regions, which make the 76-106 sequence particularly rich of binding determinants and a good candidate for specific immunotherapy. Statistical analyses of the binding data also provide an overview of the preponderant HLA-DR alleles specificity.

On the diversity and heterogeneity of H-2(d)-restricted determinants and T cell epitopes from the major bee venom allergen

One of the main limitations of using synthetic peptides for immunotherapy in allergic patients is the difficulty to delineate the immunodominant T cell epitopes which are necessarily dependent on HLA molecules. We have thus addressed the question of the role of MHC II molecules in immunodominant epitopes selection in the particular case of the major bee venom allergen (API m1). To exhaustively and easily explore it, we used BALB/c mice whose H-2 haplotype is associated with high IgE and IgG responses to API m1. By means of extensive sets of synthetic peptides, we investigated the specificity of polyclonal T cells and monoclonal hybridomas from mice immunized with API m1 and delineated four immunodominant regions, restricted to either the I-E(d) or the I-A(d) molecule. All the peptides were also tested for their capacity to bind to immunopurified MHC II molecules. Eight determinants of high affinity were identified. They clustered into three distinct regions and were largely overlapping. They included all the immunodominant epitopes, but half of them were not capable of stimulating T cells. Strikingly, interacting surfaces with either the TCR or MHC II molecule greatly differed from one determinant to another. In one case, we observed that flanking regions exerted a particular action on T cell stimulation which prevented the fine epitope localization. Our results underline the diversity and complexity of MHC II-restricted determinants and T cell epitopes from the major bee venom allergen, even in a single haplotype. These data also participate in the development of alternative approaches to conventional immunotherapy.

[Exploration of 112 children suspected of amoxicillin allergy. Indications and efficacy of oral provocation test]

BACKGROUND

One to 10% of treatments using betalactams, particularly synthetic penicillin, are complicated by allergic reactions, usually cutaneous, and not easily imputable to immunologic sensitization in children.

PATIENTS AND METHODS

The aim of this study was to identify, using cutaneous and biological tests, those from a group of 112 children suspected of amoxicillin allergy (evidenced by rash) who were actually sensitized, and to confirm the absence of allergy in others by an oral provocation test (OPT) associated to a long-term survey. The cutaneous tests were made by prick test and intra-dermo reaction (IDR) with Allergopen and with amoxicillin or amoxicillin + clavulanic acid. The biological tests included examination for penicillin and amoxicillin antibodies by using various techniques including enzyme-linked immunosorbent assay (ELISA) immunoglobulin G (IgG) and IgE, FARR, radioallergo sorbent test (RAST) and a histaminoliberation. When these tests were negative, an OPT with the suspected antibiotic was subsequently performed.

RESULTS

Thirty-nine children (36.4%) confidently presented at least one positive cutaneous test (38 Allergopen, ten amoxicillin); 25 biological tests were positive (16 ELISA IgE, one ELISA IgG and eight histaminolibarations), seven times with negative cutaneous test. Forty-five children were judged to be sensitized to amoxicillin, with only one who subsequently took amoxicillin again. Among the 67 others, 52 received an OPT, six of them with moderate cutaneous reactions. Fifty-one (45.5%) children were allergic and 46 (41%) were allowed to take amoxicillin again; 17 did, one of them with a benign cutaneous reaction.

CONCLUSION

Efficacy and safety of this type of investigation seems clear; it will have to be confirmed by other studies.

Pseudopeptide ligands for MHC II-restricted T cells

The potential therapeutic use of peptides to activate or anergize specific T cells is seriously limited by their susceptibility to proteolytic degradation. Classically, peptides are stabilized by incorporation of non-natural modifications including main chain modifications. In the case of MHC II-restricted peptides, the peptide backbone actively participates to the interaction with the MHC molecule and hence may preclude the peptidomimetic approach. We thus investigated whether a single amide bond modification influenced the peptide capacity to bind to a MHC II molecule and to stimulate specific T cells. Twenty pseudopeptide analogs of the I-Ed binder 24-36 peptide, whose sequence was derived from a snake neurotoxin, were obtained by replacing each amide bond of the peptide central part, by either a reduced psi[CH2-NH] or N-methylated psi[CO-NMe] peptide bond. In agreement with the major interacting role played by the peptide backbone, several peptides displayed a low, if any, capacity to bind to the MHC II molecule and did not lead to T cell stimulation. However, one-third of the peptides were almost as active as the 24-36 peptide in I-Ed binding assays and one-fifth in T cell stimulation assays. Among them, two pseudopeptides displayed native-like activity. Good binders were not necessarily good at stimulating T cells, demonstrating that main chain modification also affected T cell recognition. We thus showed that a peptidomimetic approach could create a new type of MHC II ligand to control T cell responses.

The specificity of antibodies raised against a T cell peptide is influenced by peptide amidation

Peptides used for immunization are designed on the basis of combination of B and T cell epitopes. They are sometimes acetylated and amidated in order to mimic the protein insertion of the B cell epitope, but to our knowledge the effect of modifying the N- and C-termini is not clearly identified. In this paper, we have investigated in detail the influence of amidation and acetylation on the immunogenic properties of the T cell epitope 24-36 which is derived from a snake neurotoxin. Acetylation enhanced the capacity of the peptides to bind to I-Ed and to stimulate specific T cells in vitro but both modifications did not influence in vivo the T cell priming ability of the peptides. However, amidation of the peptides 24-36 provoked a dramatic effect on the antibody specificity they elicited, whereas acetylation did not. Antibodies recruited by amidated peptides weakly recognized the non amidated ones, while the latter elicited antibodies which hardly bind to the former. These results show how a subtle chemical change of a peptide immunogen modifies the reactivity of the elicited antibodies in an unrelated manner from the peptide MHC II binding ability and T cell stimulating capacity. We thus amplify the previously described polarity of chimeric TB peptides that raise antibodies mainly against their C-terminal part. Finally, these results may also facilitate the choice of the status of N and C termini of the peptides designed for immunization which at present have their extremities indifferently free or modified by acetylation and/or amidation.

On the immunogenic properties of retro-inverso peptides. Total retro-inversion of T-cell epitopes causes a loss of binding to MHC II molecules

Retro-inversion is considered an attractive approach for drug and vaccine design since it provides the modified peptides with higher resistance to proteolytic degradation. We therefore investigated in detail the effect of retro-inversion on the immunological properties of synthetic peptides. We have synthesized retro-inverso analogues of MHC II restricted peptides that thus contained the correct orientation of the side chains but an inverse main chain. Retro-inversion made the peptides unable to compete in I E(d) or I A(d) binding tests, demonstrating a very low, if any, capacity to bind to MHC II molecules. These results confirm previous structural data that hydrogen bonds between residues of MHC II molecules and the main chain of antigenic peptides play a major interacting role. In vito experiments further showed that retro-inversion of a T-cell epitope causes its inability to either sustain in vitro T-cell stimulation or to prime specific T cells. Moreover, the retro-inverso peptide was not recognized by antibodies raised against the native peptide and did not elicit antibodies when injected into BALB/c mice. Retro-inverso peptides appear to be poor immunogens as a result of their weak capacity to bind to MHC II molecules. As an advantage, they are not expected to trigger undesirable humoral responses such as hypersensitivity or allergic disease. These results also provide a molecular explanation regarding the weak immunogenicity of D-amino acids containing polypeptides.

Fine chemical modifications at N- and C-termini enhance peptide presentation to T cells by increasing the lifespan of both free and MHC-complexed peptides

We investigated the effect of modifying the N- and/or C-termini of the snake toxin peptide 24-36 on its presentation to T cells. Acetylation at the N-terminus as well as amidation at the C-terminus enhanced the capacity of the peptide to activate T cells. Simultaneous modifications further increased the stimulating activity, the peptide becoming approximately 100-fold more potent than the unmodified peptide. Clearly, the introduced modifications increased the lifetime of the peptide free in solution, by decreasing its proteolytic degradation, during the T cell stimulation assays. Paradoxically, however, at similar concentrations of free peptides, the modified ones, especially those having an acetylated N-terminus, were much more active than the unmodified peptide, irrespective of the experimental conditions. These observations suggested that components other than protection from proteolytic degradation should be associated with the higher stimulating activities of the modified peptides. Accordingly, chasing experiments with APC revealed that acetylation at N-terminus caused a higher persistence of the peptides at APC surface. Together, our data indicate that (i) the T cell stimulating capacity of a peptide is associated with its lifespans in the free and MHC II bound states; and (ii) these lifespans can be greatly enhanced by introducing fine chemical modifications at N- and C-termini. These data may have some implications in designing more potent peptidic immunomodulators.

Probing immunogenicity of a T cell epitope by L-alanine and D-amino acid scanning

All residues of the I-Ed restricted fragment 24-36 of a snake toxin were individually changed into L-alanine and the corresponding D-enantiomer. Four analogs substituted with L-Ala at positions 25;30, 31 and 33, and nine analogs substituted with a D-residue along the stretch 25-33 lost most (position 28) or all their capacity to stimulate a toxin-specific T hybridoma. None of these analogs stimulated splenocytes from mice immunized with the peptide 24-36. Only the L-A31 and D-W29 modified analogs could prime a T cell response which, however, showed no cross-reactivity with the native peptide, demonstrating that T cell response selectivity can be deeply modified by mutation or configuration inversion of a single residue. Our data suggest that (i) the region 25-33 is the core of the T epitope that binds to I-Ed, and (ii) Y25 R30 and R33 contribute to the peptide binding by anchoring into pockets of I-Ed. In agreement with T cell priming observations, only the L-A31 and D-W29 modified analogs elicited strong antibody responses, just like the peptide 24-36, whereas nearly all other analogs were less immunogenic. All but the L-Ala30 and L-Ala33 modified analogs were recognized by a 24-36 specific antiserum as well as the native peptide. Altogether, our results show that substitution by D-amino acid in a peptide could be particularly well-suited for either minimizing the risk of hypersensitivity or designing peptidic vaccines.

Immunogenicity of a disulphide-containing neurotoxin: presentation to T-cells requires a reduction step

It is known that production in a host of antibodies against a protein is associated with various molecular events. These include the stimulation of specific T-lymphocytes, a step that implies the processing of the protein into peptides by various endosomal/lysosomal enzymes, such as cathepsins. Strikingly, however, we observed in vitro that cathepsins B and D have no degrading effect on toxin alpha from Naja nigricollis, a curaremimetic toxin of 61 amino acids and four disulphides. In sharp contrast, the enzymes exert an efficient cleavage of the toxin polypeptide chain once the toxin disulphides are reduced. We also found that the fully reduced toxin and the native toxin were presented with comparable efficiency to two different T-hybridomas by antigen-presenting cells (APC). Together, the data suggest that presentation of toxin fragments to T-cells requires a reduction step of toxin disulphides and, in agreement with previous findings, that this step may be achieved by APC. We wish to suggest that this phenomenon may commonly occur for any toxic proteins that contain disulphides.

Cross-polarization dynamics and spin diffusion in some aromatic compounds

The inversion-recovery cross-polarization (IRCP) magic-angle spinning experiment has been applied to study the 13C-1H cross-polarization dynamics of protonated aromatic carbons in ferrocene, 5,6-dimethoxyindole (DMI) and some indole derivatives. Using the 13C-detected proton spin diffusion (SD) experiment recently developed by Zhang et al. [Solid State Nucl. Magn. Reson., 1 (1992) 313], the slow decaying or incoherent stage of the IRCP experiment is shown to be controlled by the spin diffusion process at the directly bound proton. Moreover, a simple phenomenological model treating spin diffusion as a relaxation process provides an excellent agreement with both the IRCP and SD experimental data for all the different C-H pairs of DMI and its derivatives. The resulting time constants of the non-exponential spin diffusion decays are related to the local intra- and intermolecular network of dipolar interactions. This model is nevertheless found to be inadequate for ferrocene because intramolecular spin diffusion then has an inhomogeneous character.

Boc-Cys(Npys)-OH (BCNP): an appropriate reagent for the identification of T cell epitopes in cystine and/or cysteine-containing proteins

Some T cell epitopes become inactive when their thiols are blocked with various irreversible reagents (Régnier-Vigouroux, 1988; Maillère, 1992; Maillère et al., 1993). Blocking protein and peptide thiols with BCNP (Boc-Cys(Npys)-OH) constitutes a most appropriate strategy when searching for thiol-containing T cell epitopes. Free cysteines can thus be readily transformed into disulphide-like moieties which not only resist undesirable oxidative reactions but which also remain susceptible to reduction by antigen presenting cells, a prerequisite for the activity of thiol-dependent T cell epitopes. We describe the use of this reagent in a study of the intact disulphide-rich protein, toxin alpha from Naja nigricollis, and also two disulphide-containing toxin fragments.

A 13C solid-state NMR study of the structure and auto-oxidation process of natural and synthetic melanins

This paper presents a 13C CP/MAS NMR study of the melanin pigments obtained through natural and synthetic origins: sepia-melanin from squid ink and three synthetic 5,6-dihydroxyindole-melanins prepared using different non-enzymatic oxidation pathways. The synthetic pigments can be distinguished from natural melanin by the absence of aliphatic carbons, thereby confirming the unreacted 3,4-dihydroxyphenylalanine and the proteinaceous origins of the aliphatic resonances in natural eumelanin. The spectra of selected non-protonated carbon resonances and those with only protonated carbon signals led to a quantitative analysis. An auto-oxidative experiment using a synthetic melanin, over a period of 130 h, has shown an unusually slow disappearance of hydrogen peroxide formed in situ. The 13C-NMR spectrum of the insoluble oxidized synthetic melanin compared to that before auto-oxidation clearly demonstrates that the oxidation process is associated with chemical changes within the pigment; i.e., carbonyl functional group formation and an increase of the non-protonated carbons fraction.

Effects of 2-deoxy-D-glucose on the glucose metabolism in Saccharomyces cerevisiae studied by multinuclear-NMR spectroscopy and biochemical methods

The effects of various concentrations of deoxyglucose (DG) on the aerobic metabolism of glucose in glucose-grown repressed Saccharomyces cerevisiae cells were studied at 30 degrees C in a standard pyrophosphate medium containing 4.5 10(7) cells/ml. 31P-nuclear magnetic resonance (NMR) spectroscopy was used to monitor DG phosphorylation and the formation of polyphosphates. The production of soluble metabolites of glucose was evaluated by 13C- and 1H-NMR and biochemical techniques. The cells were aerobically incubated with 25 mM of glucose and various concentrations of DG (0, 5 and 10 mM) in order to determine the DG concentration leading to optimum of 2-deoxy-D-glucose 6-phosphate (DG6P) formation without over-inhibiting the synthesis of other metabolites. The production of DG6P increased by about 25% when the external DG concentration was doubled (from 5 to 10 mM). The formation of polyphosphates (polyP), on the other hand, was found to be mainly conditioned by the DG concentration. The amount of polyP decreased by a factor of four upon addition of 5 mM DG and became undetectable in the presence of 10 mM DG. The glucose consumption and the production of soluble metabolites of [1-13C]glucose were then evaluated as a function of time in both the absence and presence of 5 mM DG. The effect of DG is to decrease the glucose consumption and the formation of polyphosphates, ethanol, glycerol, trehalose, glutamate, aspartate and succinate while stimulating the formation of arginine and citrate. Upon co-addition of 25 mM glucose and 5 mM DG, the ratio between the initial rates of glucose consumption (0.16 mM/min) and DG6P production (0.027 mM/min) is about (5.9 +/- 1.2), not very different from the ratio of the initial concentration of glucose and DG (= 5.0). Therefore, hexokinase can phosphorylate deoxyglucose as well as glucose. However, after 100 min of incubation, the glucose concentration in the external medium decreased by about 64% while only 10% of DG was phosphorylated. DG6P was formed and quickly reached the limiting value about 30 min after co-addition of glucose and DG. Nevertheless, when the maximum quantity of DG6P was obtained, the DG consumption became negligible. By contrast, the glucose consumption and the production of ethanol and glycerol, although substantially reduced by about 42%, varied linearly with time up to 80 min of incubation. Thus even in the presence of an excess of DG, glycolysis is only slowed but not gradually or completely inhibited by DG.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of glucose on the deoxyglucose metabolism in S cerevisiae: detection and identification of deoxyglucose and trehalose derivatives by 1H- and 13C-NMR spectroscopy

The metabolism of 2-deoxy-D-glucose (DG) in glucose grown repressed Saccharomyces cerevisiae cells was studied in the absence and presence of glucose (Glc) at 30 degrees C in a standard pyrophosphate medium containing 4.5 x 10(7) cells/ml. 1H- and 13C-NMR spectroscopy were successfully used to distinguish and identify several derivatives of DG and trehalose. Using [1-13C]DG, alpha- and beta-DG, alpha- and beta-DG6P, dideoxy-trehalose (DG-DG) and deoxy-trehalose (DG-Glc) can be simultaneously observed in the intracellular medium. The [DG6P]/[DG] ratio is about 5-6. The results seem to indicate the existence of an equilibrium between DG and DG6P, which limits the production of DG6P in cells. Glucose was found to exert a great influence on the metabolism of DG. It favours the formation of DG-DG and DG-Glc.

Role and environment of the conserved Lys27 of snake curaremimetic toxins as probed by chemical modifications, site-directed mutagenesis and photolabelling experiments

The positive charge of Lys27 was suppressed by chemical means in two short-chain curaremimetic toxins, namely erabutoxin a (Ea) from Laticauda semifasciata and toxin alpha from Naja nigricollis. This modification leads to a decrease in the binding affinity of the toxins for the nicotinic acetylcholine receptor, which range 6-15-fold, as judged from both the data reported here and those previously described in the literature. A negatively charged glutamate residue has been introduced at position 27 of erabutoxin a by site-directed mutagenesis. This change provokes a 120-fold decrease in the affinity, which reflects a major alteration of toxin-receptor cognate events. Using toxin-alpha derivative harbouring a photoactive group at Lys27, we probed the toxin local environment in a receptor-bound state by photocoupling experiments. The delta chain was the predominant coupling target, in contrast to previous observations indicating that a photoactive probe on Lys47 predominantly labelled the alpha chain. The toxin derivative weakly labelled the alpha and gamma chains but not the beta chain. The toxin may therefore interact with subunits other than the alpha chain, at least in the vicinity of Lys27.

Effects of amphotericin B on the glucose metabolism in Saccharomyces cerevisiae cells. Studies by 13C-, 1H-NMR and biochemical methods

A new approach is proposed to investigate the metabolic perturbation induced by drugs in cells. The effects of various concentrations of amphotericin B on the aerobic [1-13C]glucose metabolism in glucose-grown repressed Saccharomyces cerevisiae cells were studied as a function of time using 13C-, 1H-NMR and biochemical methods. The 13C enrichment of different compounds such as ethanol, glycerol and trehalose were determined by 1H-NMR spectroscopy. In the absence of amphotericin B, glycerol diffuses slowly from the internal to the external medium, whereas in its presence this diffusion is greatly facilitated by the formation of pores in the cell membrane. Amphotericin B has been found to exert a marked influence on the glucose consumption and the production of all metabolites; for example, at 1 microM, the glucose consumption and the production of ethanol decrease while the production of glycerol and trehalose increases. The 13C relative enrichments of ethanol, glycerol and trehalose are almost the same with and without the drug. Thus it can be concluded that amphotericin B induces a large effect on the production of these compounds in the cytosol but shows no significant influence on the mechanism of their formation. Upon addition of glucose, all the amino acid concentrations decrease continuously with time; this effect is more pronounced in the presence of the drug. The ratio of the integrated resonances of glutamate (C2 + C3)/C4 reflects the activity of pyruvate carboxylase relative to citrate synthase rather than to pyruvate dehydrogenase. Without amphotericin B, this ratio (approximately 1.0) is practically constant upon addition of glucose which suggests that the activities of pyruvate carboxylase and citrate synthase are equivalent. By contrast, upon coaddition of 25 mM glucose and 1 microM amphotericin B, the glutamate C4 resonance remains virtually unchanged while that of glutamate C2 is much smaller than in its absence and continuously decreases with time. It seems likely that amphotericin B induces a reduction in the activity of pyruvate carboxylase in the mitochondria.

Conformational stability of the covalent complex between elastase and alpha 1-proteinase inhibitor

The equilibrium unfolding-refolding process of the elastase-alpha 1-proteinase inhibitor complex, induced by guanidinium chloride, was followed by spectroscopic methods. A reversible transition with a midpoint at 2.04 +/- 0.04 M guanidinium chloride was observed by fluorescence. This transition was attributed to elastase on the basis of circular dichroism and uv absorption difference data obtained for the covalent complex and for the free proteins. The conformational stability of elastase in the complex was analyzed considering the approximation of a two-state transition. The free energy of denaturation delta GH2O was 4.2 kcal.mol-1 for complexed elastase compared to 10.5 kcal.mol-1 for the free enzyme. Such a decrease in the stability of elastase suggests that, after forming the covalent complex with the inhibitor, the enzyme undergoes not only the expected local modifications of the active site, but also an extensive structural reorganization.

Conformational changes in intact and papain-modified alpha 1-proteinase inhibitor induced by guanidinium chloride

Equilibrium unfolding-refolding processes of active and proteolytically modified alpha 1-proteinase inhibitor induced by guanidinium chloride were studied. Spectroscopic methods of ultraviolet absorption, fluorescence emission and circular dichroism were used. The functional inhibitor unfolds following a multistate process: a first transition (midpoint at 0.6 M guanidinium chloride) was observed whatever the method used and was attributed to a limited conformational modification of the region including the two tryptophan residues. At higher denaturant concentrations, two other transitions were observed, one in fluorescence (midpoint at 1.7 M guanidinium chloride), attributed to the unfolding of the polypeptide chain in the same region and the other one, observed in circular dichroism and in ultraviolet absorption (midpoint at 2.3 M guanidinium chloride), leading to the totally unfolded protein. Evidence for several intermediates was also obtained with the proteolytically modified inhibitor. If total unfolding is considered, the modified inhibitor was found to be more stable towards the denaturant than the functional form (obtained at 5.5 M and 3.5 M guanidinium chloride, respectively). The unfolding irreversibility observed was attributed to the C-terminal fragment Ser359-Lys394 associated with the main chain of the cleaved inhibitor.