Kinetic studies of the regeneration of recombinant hirudin variant 1 with oxidized and reduced dithiothreitol

Flexible Folding: Disulfide-Containing Peptides and Proteins Choose the Pathway Depending on the Environments

In the last few decades, development of novel experimental techniques, such as new types of disulfide (SS)-forming reagents and genetic and chemical technologies for synthesizing designed artificial proteins, is opening a new realm of the oxidative folding study where peptides and proteins can be folded under physiologically more relevant conditions. In this review, after a brief overview of the historical and physicochemical background of oxidative protein folding study, recently revealed folding pathways of several representative peptides and proteins are summarized, including those having two, three, or four SS bonds in the native state, as well as those with odd Cys residues or consisting of two peptide chains. Comparison of the updated pathways with those reported in the early years has revealed the flexible nature of the protein folding pathways. The significantly different pathways characterized for hen-egg white lysozyme and bovine milk α-lactalbumin, which belong to the same protein superfamily, suggest that the information of protein folding pathways, not only the native folded structure, is encoded in the amino acid sequence. The application of the flexible pathways of peptides and proteins to the engineering of folded three-dimensional structures is an interesting and important issue in the new realm of the current oxidative protein folding study.

Effects of Metal Ions, Temperature, and a Denaturant on the Oxidative Folding Pathways of Bovine α-Lactalbumin

Bovine α-lactalbumin (αLA) has four disulfide (SS) bonds in the native form (N). On the oxidative folding pathways of this protein, two specific SS folding intermediates, i.e., (61–77, 73–91) and des[6–120], which have two and three native SS bonds, respectively, accumulate predominantly in the presence of Ca²⁺. In this study, we reinvestigated the pathways using a water-soluble cyclic selenoxide reagent, trans-3,4-dihydroxyselenolane oxide (DHS^ox), as a strong and quantitative oxidant to oxidize the fully reduced form (R). In the presence of ethylenediaminetetraacetic acid (EDTA) (under a metal-free condition), SS formation randomly proceeded, and N did not regenerate. On the other hand, two specific SS intermediates transiently generated in the presence of Ca²⁺. These intermediates could be assigned to (61–77, 73–91) and des[6–120] having two common SS bonds, i.e., Cys61-Cys77 and Cys73-Cys91, near the calcium binding pocket of the β-sheet domain. Much faster folding to N was observed in the presence of Mn²⁺, whereas Na⁺, K⁺, Mg²⁺, and Zn²⁺ did not affect the pathways. The two key intermediates were susceptible to temperature and a denaturant. The oxidative folding pathways revealed were significantly different from those of hen egg white lysozyme, which has the same SS-bonding pattern as αLA, suggesting that the folding pathways of SS-containing proteins can alter depending on the amino acid sequence and other factors, even when the SS-bond topologies are similar to each other.

A water-soluble selenoxide reagent as a useful probe for the reactivity and folding of polythiol peptides

A water-soluble selenoxide (DHS^ox) having a five-membered ring structure enables rapid and selective conversion of cysteinyl SH groups in a polypeptide chain into SS bonds in a wide pH and temperature range. It was previously demonstrated that the second-order rate constants for the SS formation with DHS^ox would be proportional to the number of the free SH groups present in the substrate if there is no steric congestion around the SH groups. In the present study, kinetics of the SS formation with DHS^ox was extensively studied at pH 4–10 and 25 °C by using reduced ribonuclease A, recombinant hirudin variant (CX-397), insulin A- and B-chains, and relaxin A-chain, which have two to eight cysteine residues, as polythiol substrates. The obtained rate constants showed stochastic SS formation behaviors under most conditions. However, the rate constants for CX-397 at pH 8.0 and 10.0 were not proportional to the number of the free SH groups, suggesting that the SS intermediate ensembles possess densely packed structures under weakly basic conditions. The high two-electron redox potential of DHS^ox (375 mV at 25 °C) compared to l-cystine supported the high ability of DHS^ox for SS formation in a polypeptide chain. Interestingly, the rate constants of the SS formation jumped up at a pH around the pK_a value of the cysteinyl SH groups. The SS formation velocity was slightly decreased by addition of a denaturant due probably to the interaction between the denaturant and the peptide. The stochastic behaviors as well as the absolute values of the second-order rate constants in comparison to dithiothreitol (DTT^red) are useful to probe the chemical reactivity and conformation, hence the folding, of polypeptide chains.

Kinetic and thermodynamic analysis of the conformational folding process of SS-reduced bovine pancreatic ribonuclease A using a selenoxide reagent with high oxidizing ability

Redox-coupled folding pathways of bovine pancreatic ribonuclease A (RNase A) with four intramolecular disulfide (SS) bonds comprise three phases: (I) SS formation to generate partially oxidized intermediate ensembles with no rigid folded structure; (II) SS rearrangement from the three SS intermediate ensemble (3S) to the des intermediates having three native SS linkages; (III) final oxidation of the last native SS linkage to generate native RNase A. We previously demonstrated that DHS^ox, a water-soluble selenoxide reagent for rapid and quantitative SS formation, allows clear separation of the three folding phases. In this study, the main conformational folding phase (phase II) has been extensively analyzed at pH 8.0 under a wide range of temperatures (5–45 °C), and thermodynamic and kinetic parameters for the four des intermediates were determined. The free-energy differences (ΔG) as a function of temperature suggested that the each SS linkage has different thermodynamic and kinetic roles in stability of the native structure. On the other hand, comparison of the rate constants and the activation energies for 3S → des with those reported for the conformational folding of SS-intact RNase A suggested that unfolded des species (desU) having three native SS linkages but not yet being folded are involved in very small amounts (<1%) in the 3S intermediate ensemble and the desU species would gain the native-like structures via X-Pro isomerization like SS-intact RNase A. It was revealed that DHS^ox is useful for kinetic and thermodynamic analysis of the conformational folding process on the oxidative folding pathways of SS-reduced proteins.

Diverse pathways of oxidative folding of disulfide proteins: underlying causes and folding models

The pathway of oxidative folding of disulfide proteins exhibits a high degree of diversity, which is manifested mainly by distinct structural heterogeneity and diverse rearrangement pathways of folding intermediates. During the past two decades, the scope of this diversity has widened through studies of more than 30 disulfide-rich proteins by various laboratories. A more comprehensive landscape of the mechanism of protein oxidative folding has emerged. This review will cover three themes. (1) Elaboration of the scope of diversity of disulfide folding pathways, including the two opposite extreme models, represented by bovine pancreatic trypsin inhibitor (BPTI) and hirudin. (2) Demonstration of experimental evidence accounting for the underlying mechanism of the folding diversity. (3) Discussion of the convergence between the extreme models of oxidative folding and models of conventional conformational folding (framework model, hydrophobic collapse model).

Rapid and quantitative disulfide bond formation for a polypeptide chain using a cyclic selenoxide reagent in an aqueous medium

To elucidate the reaction mechanism of the disulfide (SS) bond formation reaction of a polypeptide molecule with a water-soluble selenoxide reagent, trans-3,4-dihydroxyselenolane oxide (DHS(ox)), short-term oxidation experiments were carried out for the reduced state (R) of a recombinant hirudin CX-397 variant at pH 7.0 and 25 °C. In the reaction, R was oxidized sequentially to one-SS, two-SS, and three-SS intermediate ensembles within 1 min. The kinetic analysis revealed that the three second-order rate constants for the SS formation are proportional to the number of thiol groups existing in the reactant SS intermediates, indicating the stochastic nature of the SS formation. Ab initio calculation at the HF/6-31++G(d,p) level in water by using the polarizable continuum model suggested that the SS formation reaction is highly exothermic and proceeds via a reactive thioselenurane intermediate with a distorted linear O-Se-S linkage. The results clearly demonstrated that the rate-determining step of the SS formation reaction is the first bimolecular process between a thiol substrate and DHS(ox) rather than the subsequent process to release a SS product.

Characterization of kinetic and thermodynamic phases in the prefolding process of bovine pancreatic ribonuclease A coupled with fast SS formation and SS reshuffling

In the redox-coupled oxidative folding of a protein having several SS bonds, two folding phases are usually observed, corresponding to SS formation (oxidation) with generation of weakly stabilized heterogeneous structures (a chain-entropy losing phase) and the subsequent intramolecular SS rearrangement to search for the native SS linkages (a conformational folding phase). By taking advantage of DHS(ox) as a highly strong and selective oxidant, the former SS formation phase was investigated in detail in the oxidative folding of RNase A. The folding intermediates obtained at 25 °C and pH 4.0 within 1 min (1S°-4S°) showed different profiles in the HPLC chromatograms from those of the intermediates obtained at pH 7.0 and 10.0 (1S-4S). However, upon prolonged incubation at pH 4.0 the profiles of 1S°-3S° transformed slowly to those similar to 1S-3S intermediate ensembles via intramolecular SS reshuffling, accompanying significant changes in the UV and fluorescence spectra but not in the CD spectrum. Similar conversion of the intermediates was observed by pH jump from 4.0 to 8.0, while the opposite conversion from 1S-4S was observed by addition of guanidine hydrochloride to the folding solution at pH 8.0. The results demonstrated that the preconformational folding phase coupled with SS formation can be divided into two distinct subphases, a kinetic (or stochastic) SS formation phase and a thermodynamic SS reshuffling phase. The transition from kinetically formed to thermodynamically stabilized SS intermediates would be induced by hydrophobic nucleation as well as generation of the native interactions.

Conformational exchange is critical for the productivity of an oxidative folding intermediate with buried free cysteines

Much has been learned about the folding of proteins from comparative studies of the folding of proteins that are related in sequence and structure. Observation of the effects of mutations helps account for sequence-specific properties and large variations in folding rates observed in homologous proteins, which are not explained by structure-derived descriptions. The folding kinetics of variants of a β-stranded protein, toxin α from Naja nigricollis, depends on the length of their loop lk1. These proteins, named Tox60, Tox61, and Tox62, contain four disulfide bonds. We show that their oxidative refolding pathways are similar. Differences in these pathways are restricted to the last step of the reaction, that is, the closure of the last disulfide. At this step, two species of three-disulfide intermediates are observed: intermediate C lacking the B3 disulfide and intermediate D lacking the B2 disulfide. Surprisingly, D is the most productive intermediate for Tox61 despite the low accessibility of its free cysteines. However, in the case of Tox62, its conversion efficiency drops by 2 orders of magnitude and C becomes the most productive intermediate. NMR was used in order to study the structural dynamics of each of these intermediates. Both three-disulfide intermediates of Tox61 exist in two forms, exchanging on the 1- to 100-ms scale. One of these forms is structurally very close to the native Tox61, whereas the other is always significantly more flexible on a picosecond-to-nanosecond timescale. On the other hand, in the case of Tox62, the three-disulfide intermediates only show a native-like structure. The higher conformational heterogeneity of Tox61 intermediate D allows an increased accessibility of its free cysteines to oxidative agents, which explains its faster native disulfide formation. Thus, residue deletion in loop lk1 probably abrogates stabilizing intramolecular interactions, creates conformational heterogeneity, and increases the folding rate of Tox60 and Tox61 compared to Tox62.

Deciphering the structural basis that guides the oxidative folding of leech-derived tryptase inhibitor

Protein folding mechanisms have remained elusive mainly because of the transient nature of intermediates. Leech-derived tryptase inhibitor (LDTI) is a Kazal-type serine proteinase inhibitor that is emerging as an attractive model for folding studies. It comprises 46 amino acid residues with three disulfide bonds, with one located inside a small triple-stranded antiparallel beta-sheet and with two involved in a cystine-stabilized alpha-helix, a motif that is widely distributed in bioactive peptides. Here, we analyzed the oxidative folding and reductive unfolding of LDTI by chromatographic and disulfide analyses of acid-trapped intermediates. It folds and unfolds, respectively, via sequential oxidation and reduction of the cysteine residues that give rise to a few 1- and 2-disulfide intermediates. Species containing two native disulfide bonds predominate during LDTI folding (IIa and IIc) and unfolding (IIa and IIb). Stop/go folding experiments demonstrate that only intermediate IIa is productive and oxidizes directly into the native form. The NMR structures of acid-trapped and further isolated IIa, IIb, and IIc reveal global folds similar to that of the native protein, including a native-like canonical inhibitory loop. Enzyme kinetics shows that both IIa and IIc are inhibitory-active, which may substantially reduce proteolysis of LDTI during its folding process. The results reported show that the kinetics of the folding reaction is modulated by the specific structural properties of the intermediates and together provide insights into the interdependence of conformational folding and the assembly of native disulfides during oxidative folding.

Oxidative folding pathway of onconase, a ribonuclease homologue: insight into oxidative folding mechanisms from a study of two homologues

The oxidative folding pathways of two four-disulfide proteins of the ribonuclease family, ONC and RNase A, which have similar three-dimensional folds but only 30% sequence homology, are compared. In this study, a mechanism for the oxidative folding pathway of ONC is proposed. In particular, the kinetic roles and thermodynamic characteristics of key intermediates along the oxidative folding pathway, specifically, the structured intermediates, I(1), I(2), and I(3), previously identified as des-[19-68,30-75], des-[30-75], and des-[19-68], respectively, are discussed. In addition, the effects of temperature on the oxidative folding pathway have been examined. Differences in the folding mechanism between ONC and RNase A are attributed to the differences in their amino acid sequences and related inter-residue interactions, including differences in hydrophobic interactions. Compared to RNase A, ONC utilizes more efficient interactions along the oxidative folding pathway to adopt its native fold more rapidly.

Disulfide-linked protein folding pathways

Determining the mechanism by which proteins attain their native structure is an important but difficult problem in basic biology. The study of protein folding is difficult because it involves the identification and characterization of folding intermediates that are only very transiently present. Disulfide bond formation is thermodynamically linked to protein folding. The availability of thiol trapping reagents and the relatively slow kinetics of disulfide bond formation have facilitated the isolation, purification, and characterization of disulfide-linked folding intermediates. As a result, the folding pathways of several disulfide-rich proteins are among the best known of any protein. This review discusses disulfide bond formation and its relationship to protein folding in vitro and in vivo.

Dissimilarity in the oxidative folding of onconase and ribonuclease A, two structural homologues

The oxidative folding of frog onconase (ONC), a member of the ribonuclease A family, was examined and shows markedly different behavior compared to its structural homologue bovine pancreatic ribonuclease A (RNase A) under similar conditions. Application of a reduction pulse (using a small amount of reduced dithiothreitol) during the oxidative regeneration of ONC indicated the survival of the native protein along with three other (structured) species, I(1), I(2) and I(3), with the rest of the unstructured species being converted to fully reduced protein. Mass spectrometry indicates that I(1) has two disulfide bonds, whereas I(2) and I(3) have three disulfide bonds each. A disulfide mapping method, based on cyanylation, was used to identify I(2) and I(3) as des-[30-75] and des-[19-68], respectively. On enzymatic digestion using trypsin, I(1) was identified as des-[19-68, 30-75]. Differences in the intermediates that are generated during the oxidative folding of the two structural homologues, RNase A and ONC, demonstrate that regenerative pathways are not necessarily influenced by tertiary structure. This indicates that the lack of a disulfide bond in ONC, analogous to the (65-72) disulfide bond in RNase A which plays an important role in its oxidative regeneration, does not adversely affect the oxidative folding of ONC.

Elucidation of the disulfide-folding pathway of hirudin by a topology-based approach

A theoretical model for the folding of proteins containing disulfide bonds is introduced. The model exploits the knowledge of the native state to favor the progressive establishment of native interactions. At variance with traditional approaches based on native topology, not all native bonds are treated in the same way; in particular, a suitable energy term is introduced to account for the special strength of disulfide bonds, as well as their ability to undergo intramolecular reshuffling. The model thus possesses the minimal ingredients necessary to investigate the much debated issue of whether the refolding process occurs through partially structured intermediates with native or non-native disulfide bonds. This strategy is applied to a context of particular interest, the refolding process of hirudin, a thrombin-specific protease inhibitor, for which conflicting folding pathways have been proposed. We show that the only two parameters in the model (temperature and disulfide strength) can be tuned to reproduce well a set of experimental transitions between species with different number of formed disulfides. This model is then used to provide a characterization of the folding process and a detailed description of the species involved in the rate-limiting step of hirudin refolding.

Capillary electrophoresis investigation of a partially unfolded conformation of beta(2)-microglobulin

Dialysis-related amyloidosis is a disease in which partial unfolding of beta(2)-microglobulin plays a key pathogenetic role in the formation of the amyloid fibrils. We have recently demonstrated that a partially unfolded conformer of beta(2)-microglobulin is involved in fibrillogenesis and that this species is significantly populated under physiological conditions. In this work capillary electrophoresis has been used to measure the equilibrium between the native protein and this conformer in samples known to have a higher or lower amyloidogenic potential, namely full-length beta(2)-microglobulin, two truncated species and a mutant, created by replacing histidine in position 31 with thyrosine. In addition, for all protein species folding stability experiments have been carried out by monitoring the secondary structure by circular dichroism at increasing concentrations of guanidinium chloride. The values of free energy of unfolding in the absence of denaturant, obtained by elaboration of these experiments, were found to be inversely correlated to the area percent of the partially unfolded conformer, as measured by capillary electrophoresis. Affinity capillary electrophoresis experiments have been also carried out under nondenaturing conditions to assess the affinity of copper and suramin to either the native form or the conformational intermediate of full-length beta(2)-microglobulin.

Structural determinants of oxidative folding in proteins

A method for determining the kinetic fate of structured disulfide species (i.e., whether they are preferentially oxidized or reshuffle back to an unstructured disulfide species) is introduced. The method relies on the sensitivity of unstructured disulfide species to low concentrations of reducing agents. Because a structured des species that preferentially reshuffles generally first rearranges to an unstructured species, a small concentration of reduced DTT (e.g., 260 microM) suffices to distinguish on-pathway intermediates from dead-end species. We apply this method to the oxidative folding of bovine pancreatic ribonuclease A (RNase A) and show that des[40-95] and des[65-72] are productive intermediates, whereas des[26-84] and des[58-110] are metastable dead-end species that preferentially reshuffle. The key factor in determining the kinetic fate of these des species is the relative accessibility of both their thiol groups and disulfide bonds. Productive intermediates tend to be disulfide-secure, meaning that their structural fluctuations preferentially expose their thiol groups, while keeping their disulfide bonds buried. By contrast, dead-end species tend to be disulfide-insecure, in that their structural fluctuations expose their disulfide bonds in concert with their thiol groups. This distinction leads to four generic types of oxidative folding pathways. We combine these results with those of earlier studies to suggest a general three-stage model of oxidative folding of RNase A and other single-domain proteins with multiple disulfide bonds.

Oxidative folding of proteins

The oxidative folding of proteins is reviewed and illustrated with bovine pancreatic ribonuclease A (RNase A). The mutual effects of conformational folding and disulfide bond regeneration are emphasized, particularly the "locking in" of native disulfide bonds by stable tertiary structure in disulfide intermediates. Two types of structured metastable disulfide species are discerned, depending on the relative protection of their disulfide bonds and thiol groups. Four generic pathways for oxidative folding are identified and characterized.

Kinetic folding pathway of a three-disulfide mutant of bovine pancreatic ribonuclease A missing the [40-95] disulfide bond

The oxidative refolding pathway of a three-disulfide mutant of bovine pancreatic ribonuclease A (RNase A) from the fully reduced unfolded form to the native state has been studied by using oxidized and reduced dithiothreitol as the redox reagents at pH 8.0 and 25 degrees C. This mutant was prepared by replacing Cys40 and Cys95 in RNase A with alanines while maintaining the other three native disulfide bonds to mimic one of the two major three-disulfide intermediates (des-[40-95]) observed in the regeneration of wild-type RNase A. The kinetics of refolding of this mutant were measured by quenching the regeneration reaction at various times with a rapid blocking reagent, 2-aminoethyl methanethiosulfonate (AEMTS), fractionating the disulfide intermediates by using cation-exchange HPLC, and analyzing the time course of each group of disulfide species. It was found that the disulfide intermediates formed during regeneration reach a steady-state distribution after a short period of preequilibration similar to that in the regeneration of wild-type RNase A. The experimental data acquired under different redox conditions were fit to a kinetic model with a steady-state treatment. The fitted results indicate that this mutant refolds through a rate-determining step which involves the oxidation of certain two-disulfide species to form a putative three-disulfide species which proceeds rapidly to the native protein. A rough estimation suggests that this pathway could constitute no more than 5% of the major pathway leading to the formation of des-[40-95] (the major three-disulfide intermediate formed) in the regeneration of wild-type RNase A. Several kinetic constants pertaining to the oxidation and reduction of various disulfide intermediates were compared with those obtained in the regeneration studies of wild-type RNase A to gain further understanding about the folding pathways of RNase A. Comparisons are also given for the oxidative refolding studies of several other three disulfide bond proteins, suggesting that the formation of a large number of disulfide-bonded intermediates during oxidative refolding is probably a common feature for most proteins.

Regeneration of three-disulfide mutants of bovine pancreatic ribonuclease A missing the 65-72 disulfide bond: characterization of a minor folding pathway of ribonuclease A and kinetic roles of Cys65 and Cys72

The oxidative regeneration pathways of two three-disulfide mutants of bovine pancreatic ribonuclease A (RNase A) missing the 65-72 disulfide bond, [C65S,C72S] and [C65A,C72A], have been studied by using oxidized dithiothreitol (DTTox) as an oxidizing agent and 2-aminoethylmethanethiosulfonate (AEMTS) as a thiol-blocking agent at 25 degrees C and pH 8.0. These mutants are analogues of the des-[65-72] intermediate, which is one of the two major three-disulfide intermediates that follow after the transition states in the regeneration pathways of wild-type RNase A [Rothwarf, D. M., Li, Y.-J., and Scheraga, H. A. (1998) Biochemistry 37, 3760-3766, 3767-3776.]. Both mutants folded through the same pathway but at a rate lower than that of the wild-type protein. The major rate-determining step in the regeneration of these mutants was determined to be the oxidation from the two-disulfide intermediates (2S) to the post-transition-state three-disulfide intermediate (3S*), suggesting the existence of a minor oxidation pathway (2S --> 3S*, where 3S* is des-[65-72]) in the regeneration of the wild-type protein, in addition to one of the two major disulfide-rearrangement pathways (3S --> des-[65-72]). The regeneration intermediates of these mutants (R, 1S, 2S, and 3S) participate in a steady state with a kinetic behavior resembling that of the wild-type protein. However, the apparent equilibrium constants () in the steady state, averaged with statistical factors for these mutants, are significantly smaller than those for the wild-type protein, indicating that the intermediates in the regeneration of the mutants are relatively less stable by 0.32 kcal/mol. This difference is due to the decrease in the average rate constants for intramolecular disulfide-bond formation () for the mutant proteins. Loop entropy calculations indicate that the increase in the average length of all possible disulfide loops of the mutants due to the replacement of Cys65 and Cys72 is not sufficient to account for the observed reduction of the values of for the mutants. Therefore, it is the removal of energetic factors (arising from the loss of the 65-72 disulfide loop) that leads to deceleration of the regeneration of the mutant proteins. The formation of the 65-72 disulfide loop in the regeneration of wild-type RNase A appears to facilitate the subsequent folding events.

Regeneration of bovine pancreatic ribonuclease A: identification of two nativelike three-disulfide intermediates involved in separate pathways

During the regeneration of bovine pancreatic ribonuclease A (RNase A) from the reduced to the native form with mixtures of oxidized and reduced dithiothreitol at 25 degrees C, pH 8.0, the disulfide-containing protein intermediates achieve a steady-state distribution. By manipulating the redox conditions after the attainment of the steady-state condition, it has been possible to kinetically trap and, thereby, isolate and identify the disulfide-bonded species that follow the rate-determining step in the regeneration pathway. Two three-disulfide species have been identified by peptide mapping. Both species contain three native disulfide-bond pairings, one lacks the 65-72 disulfide bond (des-[65-72]), and the other lacks the 40-95 disulfide bond (des-[40-95]). These species are the same as those identified during the reduction of RNase A. By restarting the regeneration process from isolated des-[65-72] and des-[40-95], it is shown that both intermediates lie directly on regeneration pathways.

Chemical synthesis and structural characterization of the RGD-protein decorsin: a potent inhibitor of platelet aggregation

Decorsin is a 39-residue RGD-protein crosslinked by three disulfide bridges isolated from the leech Macrobdella decora belonging to the family of GPIIb-IIIa antagonists and acting as a potent inhibitor of platelet aggregation. Here we report the solid-phase synthesis of decorsin using the Fmoc strategy. The crude polypeptide was purified by reverse-phase HPLC in its reduced form and allowed to refold in the presence of glutathione. The homogeneity of the synthetic oxidized decorsin was established by reverse-phase HPLC and capillary zone electrophoresis. The results of amino acid analysis after acid hydrolysis of the synthetic protein, NH2-terminal sequencing and mass determination (4,377 Da) by electrospray mass spectrometry were in full agreement with this theory. The correct pairing of the three disulfide bridges in synthetic decorsin was determined by a combined approach of both peptide mapping using proteolytic enzymes and analysis of the disulfide chirality by CD spectroscopy in the near-UV region. Synthetic decorsin inhibited human platelet aggregation with an IC50 of approximately 0.1 microM, a figure quite similar to that determined utilizing decorsin from natural source. In particular, the synthetic protein was 2,000-fold more potent than a model RGD-peptide (e.g., Arg-Gly-Asp-Ser) in inhibiting platelet aggregation. Thermal denaturation experiments of synthetic decorsin, monitored by CD spectroscopy, revealed its high thermal stability (Tm approximately 74 degrees C). The features of the oxidative refolding process of reduced decorsin, as well as the thermal stability of the oxidized species, were compared with those previously determined for the NH2-terminal core domain fragment 1-41 or 1-43 from hirudin. This fragment shows similarity in size, pairing of the three disulfides and three-dimensional structure with those of decorsin, even if very low sequence similarity. It is suggested that the less efficient oxidative folding and the enhanced thermal stability of decorsin in respect to those of hirudin core domain likely can be ascribed to the presence of the six Pro residues in the decorsin chain, whereas none is present in the hirudin domain. The results of this study indicate that decorsin can be obtained by solid-phase methodology in purity and quantities suitable for structural and functional studies and thus open the way to prepare by chemical methods novel decorsin derivatives containing unusual amino acids or even non-peptidic moieties.