Production, properties, and thrombin inhibitory mechanism of hirudin amino-terminal core fragments

Microneedle patch-assisted transdermal administration of recombinant hirudin for the treatment of thrombotic diseases

The painless microneedle patch (MNP), widely explored for transdermal drug delivery of macromolecules, can overcome the limitations of traditional administrations of protein and polypeptide anticoagulant drugs. We constructed a recombinant hirudin-loaded microneedle patch, suitable for patients with thrombotic diseases requiring long-term preventive medication. The recombinant hirudin-loaded dissoluble microneedle patch (RHDMNP) was created using a mold casting technique and polyvinylpyrrolidone and polyvinyl alcohol were used as the matrix material with a 1:1.2 ratio. We prepared bilayer RHDMNPs with pyramidal appearance and 0.37 N/needle strength. The intradermal dissolution height of the microneedle reached approximately 78.67% of the total height, and 68.12% of the drug was delivered into the skin. The 12-hour cumulative permeation of the MNP was 21.69 ± 3.90 μg/cm². The MNP showed a T_max of 1.5 h, C_max of 144 ± 71 ng/mL, and area under curve (AUC) of 495 ± 66 ng/mL·min compared to T_max of 0.5 h, C_max of 249 ± 89 ng/mL, and AUC of 944 ± 65 ng/mL·min for the subcutaneous injection group. Both in vivo and in vitro experiments showed that the RHDMNP exerted effective anticoagulant effects, prevented the incidence of acute pulmonary embolism, and revealed the potential for venous thrombosis prevention.

Efficient targeted anticoagulant with active RGD motif

Three anticoagulants combining large peptide recombinant hirudin variants (rHV2-K47) and Arg-Gly-Asp (RGD) motif related to platelet aggregation were generated, i.e. sequences CRFPRGDADPYCE and CNPRGDFRCI were added to the C-terminus of hirudin to obtain RGD-hirudin 1 and 2, respectively, and the sequence RGDSE was inserted between residues 53-54 of hirudin to obtain RGD-hirudin 3. All products exhibited antithrombin and antiplatelet activities, especially IC50 of RGD-hirudin 1 and 2 were much lower with respect to previously reported similar peptides. Our data suggested that RGD-hirudin 1 and 2 would be promising anticoagulants in clinic. Moreover, the triangular structure of active RGD was shown by computer simulation, which might contribute to our understanding on integrin-related peptides and proteins.

Design of a novel plasminogen activator based on the structure of hirudin

Using a phage library, seven peptide sequences with high affinity to human microplasminogen were obtained. Caseinolytic assay indicated that only the synthesized peptide P07 had slight fibrinolytic activity. To enhance its plasminogen activation ability, peptide P07 was fused into loop 32-35 of hirudin. In vitro assay demonstrated that this hirudin-like fusion protein can activate human plasminogen and retain the function of thrombin inhibition. Fusing the sequence ''SPDASRL'' into hirudin generated a plasminogen activation activity 100 times higher than peptide P07 in chromogenic and radial caseinolytic assay. This significant functional improvement might originate from a more specific active structure due to the hirudin scaffold.

Functional properties of a recombinant chimeric plasminogen activator with platelet-targeted fibrinolytic and anticoagulant potential

The construction, purification, and characterization of dscuPA33khC, a bifunctional protein designed for thrombosis treatment is described. The chimera was designed to consist of a decorsin (platelet aggregation inhibitor), a low molecular mass (33kDa) single-chain urokinase (scuPA-33k), and a thrombin inhibitory domain. We have successfully produced this recombinant protein in the Escherichia coli expression system, in which the target protein exists in the form of inclusion bodies. After refolding by dilution in vitro, the chimeric protein was purified to homogeneity by immobilized metal affinity chromatography, ion-exchange chromatography, and gel filtration chromatography. The dscuPA33khC could directly activate plasminogen following Michaelis-Menten kinetics with K(m) = 1.52 microM and K(2) = 0.0024 s(-1). The specific activity of the chimera detected by fibrin plate determination was 11,000 IU/mg, which suggested a high thrombolysis effect. However, the chimeric dscuPA33khC bound the activated platelet and significantly increased affinity to platelet clots as compared to fibrin clots. It was found to inhibit ADP-induced platelet aggregation in a concentration-dependent manner as well as it exhibits antithrombin activity. These results suggest that the chimeric protein not only has platelet-targeted thrombolytic activity but also obtains anti-thrombus function.

Directed evolution towards protease-resistant hirudin variants

Hirudin, a thrombin-specific inhibitor, is efficiently digested and inactivated by proteases with pepsin- and chymotrypsin-like specificity. Using a combination of phage display selection and high-throughput screening methods, several variants of recombinant hirudin were generated. Only very few variants comprising amino acid substitutions in the amino-terminal domain (residues 1-5) and in the carboxyl-terminal tail (residues 49, 50, and/or 56, 57, 62-64) were identified that showed thrombin inhibition activities similar to those of the wild-type polypeptide. Analysis of protease susceptibility, however, revealed that mutations, which conferred protease resistance, simultaneously diminish thrombin inhibition activity. This is particularly apparent for substitutions in the region of residues 56-64, which forms a large number of electrostatic and hydrophobic interactions with thrombin in the crystal structure of the complex. Unlike wild-type hirudin, the variant comprising Pro(50)- ...-His(56)-Asp(57)- ...-Pro(62)-Pro(63)-His(64) is completely resistant to pepsin and chymotrypsin cleavage; however, this is at the expense of thrombin inhibition activity where there is a 100-fold increase in the IC50 value. The frequent replacement of wild-type amino acids by proline at major protease cleavage sites indicates that at least pepsin- and chymotrypsin-like enzymes may exhibit a (conformational) specificity concerning the P1 and P2 positions. On the basis of these results, proline substitutions appear to be a general strategy to design polypeptides that are not susceptible to digestion by a broader range of different proteases.

Construction and characterization of trifunctional single-chain urokinase-type plasminogen activators

Two chimeric proteins have been constructed. One consists of four parts: a portion of the low molecular mass single-chain urokinase-type plasminogen activator (scu-PA-32K, residues 144-411), a 15-mer linker sequence, the C-terminal amino-acid sequence (residues 53-65) of hirudin (Hir), and an RGD sequence derived from the leech protein decorsin, i.e. scu-PA(32 k)-linker-Hir (residues 53-65)-RGD peptide. The other comprises two main segments: scu-PA(32 k) and hirudin into which RGDSP is inserted between its residues 33 and 34, i.e. hirudin (residues 1-33)-RGDSP-hirudin (residues 34-65)-scu-PA(32 k). These two chimeric genes were expressed in Escherichia coli, and the products were purified by Zn2+-chelating Sepharose 4B chromatography and benzamidine Sepharose 6B chromatography. Our results suggested that these two chimeric proteins not only had plasminogen-dependent fibrinolytic activity, but also possessed platelet aggregation inhibitory activity and antithrombin activity.

Theromin, a novel leech thrombin inhibitor

We purified the most potent thrombin inhibitor described to date from the rhynchobdellid leech Theromyzon tessulatum. Designated theromin, it was purified to apparent homogeneity by gel permeation and anion exchange chromatography followed by two reverse-phase steps of high performance liquid chromatography. The primary sequence of theromin (a homodimer of 67 amino acid residues including 16 cysteine residues) was determined by a combination of reduction and s-beta-pyridylethylation, Edman degradation, trypsin enzymatic digestion, and matrix-assisted laser desorption mass spectrometry measurement. Theromin exhibits no sequence homology with any other thrombin inhibitors. Furthermore, theromin significantly diminishes, in a dose-dependent manner, the level of human granulocyte and monocyte activation induced by lipopolysaccharides. In summary, this potent thrombin inhibitor promises to have high biomedical significance.

Incorporation of noncoded amino acids into the N-terminal domain 1-47 of hirudin yields a highly potent and selective thrombin inhibitor

Hirudin is an anticoagulant polypeptide isolated from a medicinal leech that inhibits thrombin with extraordinary potency (Kd = 0.2-1.0 pM) and selectivity. Hirudin is composed of a compact N-terminal region (residues 1-47, cross-linked by three disulfide bridges) that binds to the active site of thrombin, and a flexible C-terminal tail (residues 48-64) that interacts with the exosite I of the enzyme. To minimize the sequence of hirudin able to bind thrombin and also to improve its therapeutic profile, several N-terminal fragments have been prepared as potential anticoagulants. However, the practical use of these fragments has been impaired by their relatively poor affinity for the enzyme, as given by the increased value of the dissociation constant (Kd) of the corresponding thrombin complexes (Kd = 30-400 nM). The aim of the present study is to obtain a derivative of the N-terminal domain 1-47 of hirudin displaying enhanced inhibitory potency for thrombin compared to the natural product. In this view, we have synthesized an analogue of fragment 1-47 of hirudin HM2 in which Val1 has been replaced by tert-butylglycine, Ser2 by Arg, and Tyr3 by beta-naphthylalanine, to give the BugArgNal analogue. The results of chemical and conformational characterization indicate that the synthetic peptide is able to fold efficiently with the correct disulfide topology (Cys6-Cys14, Cys16-Cys28, Cys22-Cys37), while retaining the conformational properties of the natural fragment. Thrombin inhibition data indicate that the effects of amino acid replacements are perfectly additive if compared to the singly substituted analogues (De Filippis V, Quarzago D, Vindigni A, Di Cera E, Fontana A, 1998, Biochemistry 37:13507-13515), yielding a molecule that inhibits the fast or slow form of thrombin by 2,670- and 6,818-fold more effectively than the natural fragment, and that binds exclusively at the active site of the enzyme with an affinity (Kd,fast = 15.4 pM, Kd,slow = 220 pM) comparable to that of full-length hirudin (Kd,fast = 0.2 pM, Kd,slow = 5.5 pM). Moreover, BugArgNal displays absolute selectivity for thrombin over the other physiologically important serine proteases trypsin, plasmin, factor Xa, and tissue plasminogen activator, up to the highest concentration of inhibitor tested (10 microM).

Staphylokinase as a plasminogen activator component in recombinant fusion proteins

The plasminogen activator staphylokinase (SAK) is a promising thrombolytic agent for treatment of myocardial infarction. It can specifically stimulate the thrombolysis of both erythrocyte-rich and platelet-rich clots. However, SAK lacks fibrin-binding and thrombin inhibitor activities, two functions which would supplement and potentially improve its thrombolytic potency. Creating a recombinant fusion protein is one approach for combining protein domains with complementary functions. To evaluate SAK for use in a translational fusion protein, both N- and C-terminal fusions to SAK were constructed by using hirudin as a fusion partner. Recombinant fusion proteins were secreted from Bacillus subtilis and purified from culture supernatants. The rate of plasminogen activation by SAK was not altered by the presence of an additional N- or C-terminal protein sequence. However, cleavage at N-terminal lysines within SAK rendered the N-terminal fusion unstable in the presence of plasmin. The results of site-directed mutagenesis of lysine 10 and lysine 11 in SAK suggested that a plasmin-resistant variant cannot be created without interfering with the plasmin processing necessary for activation of SAK. Although putative plasmin cleavage sites are located at the C-terminal end of SAK at lysine 135 and lysine 136, these sites were resistant to plasmin cleavage in vitro. Therefore, C-terminal fusions represent stable configurations for developing improved thrombolytic agents based on SAK as the plasminogen activator component.

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

The regeneration of native recombinant hirudin variant 1 (rHV1) from the reduced unfolded form to the fully oxidized native state has been carried out with mixtures of oxidized and reduced dithiothreitol at pH 8.3 and 12 degrees C. The regeneration reaction was quenched at various times by the addition of 2-aminoethyl methanethiosulfonate to block unreacted sulfhydryl groups. The quenched protein-folding intermediates were fractionated by both capillary electrophoresis and a combination of anion exchange and reverse phase HPLC and characterized by mass spectrometry, amino acid analysis, and disulfide analysis. These intermediates (before quenching) were found to interconvert rapidly so as to achieve a steady-state distribution early in the regeneration process. The experimental data were fitted to a steady-state kinetic scheme. The analysis reveals that the rate-determining step in the regeneration of rHV1 with oxidized and reduced dithiothreitol involves the oxidation of one or more two-disulfide-containing species, most likely those already containing two native disulfide bonds. This regeneration mechanism is different from one that has been proposed by Chatrenet and Chang [(1993) J. Biol. Chem. 268, 20988]. The differences are discussed, and possible explanations for the differences are presented.

Size of hirudin sequence required to fold into an active core domain

The active core domain of hirudin contains three native disulfides (Cys6-Cys14,Cys16-Cys28, and Cys22-Cys39) and 49 amino acid residues (Hir1-49). This compact structure folds spontaneously, and its folding pathway has been elucidated recently [Chatrenet and Chang (1993) J. Biol. Chem. 268, 20988-20996]. The folding mechanisms of Hir1-35 and Hir1-43 were investigated in order to determine the minimum structural elements required to fold into this active core structure. Hir1-35 includes two native disulfides (Cys6-Cys14 and Cys16-Cys28) and an extra Cys22. When reduced/denatured Hir1-35 was allowed to fold, it folded into a collection of equilibrated 2-disulfide isomers. At least eight fractions of the 2-disulfide species have been observed. Structural analysis revealed that out of the 10 possible disulfide pairings, only five were detected to exist in the 2-disulfide isomers, and all have their half-cystines separated by less than 8-10 amino acid residues. One of the native disulfides, Cys16-Cys28, has not been found in any of those 2-disulfide species. On the other hand, the C-terminal extension of an octapeptide permitted reduced/denaturated Hir1-43 to fold into a defined active structure possessing the three native disulfides. These results also demonstrate that the folding mechanism of Hir1-35 resembles what occurs during the early stage of Hir1-43 (and Hir1-49) folding, which involves a process of nonspecific packing of unfolded polypeptide chains.

Core domain of hirudin from the leech Hirudinaria manillensis: chemical synthesis, purification, and characterization of a Trp3 analog of fragment 1-47

Hirudin is a small (approximately 7 kDa) disulfide-cross-linked polypeptide known as the most potent and specific thrombin inhibitor. We have previously shown that the N-terminal proteolytic fragment 1-47 of hirudin HM2 from Hirudinaria manillensis maintains inhibitory action toward thrombin [Vindigni, A., et al. (1994) Eur. J. Biochem. 226, 323-333]. Here we report the solid-phase chemical synthesis of an analog of fragment 1-47 bearing a Tyr3-->Trp exchange (Y3W analog). The crude, reduced peptide was purified by reverse-phase HPLC and subjected to oxidative folding to the disulfide-cross-linked species. The folding process of the Y3W analog was slower than that of the natural fragment 1-47, but nevertheless still occurred almost quantitatively as the natural species. The overall final yield of the synthetic product was approximately 35%, and its identity and homogeneity was established by a number of analytical techniques, including electrospray mass spectometry. The unique alignment of the three disulfide bridges of the Y3W analog was established by peptide mapping as Cys6-Cys14, Cys16-Cys28, and Cys22-Cys37 and shown to be identical to that of the natural fragment. The results of far- and near-ultraviolet circular dichroism and fluorescence emission measurements provided evidence that the Y3W analog retains the structural features of the natural species. The thermodynamic quantities (delta GD, delta Hm, delta Sm, and delta Cp) characterizing the reversible and cooperative thermal unfolding processes of the Y3W analog (Tm = 60.5 degrees C) and the natural fragment species (Tm = 62.5 degrees C) were evaluated. Despite the relatively high Tm values, the stability of both fragment species at 37 degrees C was only approximately 10 kJ mol-1, well below the average 50 kJ mol-1 typical of single-domain globular proteins. The synthetic Y3W species was found to be approximately 5-fold more active (KI = 30 +/- 5 nM) than the natural fragment 1-47 (KI = 150 +/- 20 nM) in inhibiting thrombin. Of interest was that the difference in the free energies of binding to thrombin at 37 degrees C, delta delta Gb, between the Y3W analog and natural species (4.2 kJ mol-1) was that expected for the difference in hydrophobicity between the two polypeptides resulting from the Tyr-->Trp exchange. The results of this study indicate that solid-phase chemical synthesis represents a convenient and high-yield procedure to prepare analogs of the biologically active, N-terminal core domain of hirudin with improved functional properties.

The disulfide structures of scrambled hirudins

Scrambled hirudins consist of a collection of equilibrated isomers and serve as essential folding intermediates during the in vitro renaturation of hirudin (Chatrenet, B., and Chang, J.-Y. (1993) J. Biol. Chem. 268, 20988-20996). Ten fractions of scrambled hirudins have been isolated. Their disulfide structures were deduced from the analysis of thermolysin-digested peptides by amino acid sequencing and mass spectrometry. The results reveal 9 fractions of pure scrambled species, and, together, 11 species of scrambled structures have been identified. About all possible disulfide isomers of hirudin have been found to exist. The three native disulfides, Cys6-Cys14, Cys16-Cys28, and Cys22-Cys39, are detected in five different scrambled species and constitute 18% of the total disulfide bonds found in scrambled hirudins.

Design of potent bivalent thrombin inhibitors based on hirudin sequence: incorporation of nonsubstrate-type active site inhibitors

Hirudin from medicinal leech is the most potent and specific thrombin inhibitor from medicinal leech with a K(i) value of 2.2 x 10(-14) M. It consists of an active site blocking moiety, hirudin1-48, a fibrinogen-recognition exo-site binding moiety, hirudin55-65, and a linker, hirudin49-54, connecting these inhibitor moieties. Synthetic inhibitors were designed based on the C-terminal portion of hirudin. The bulky active site blocking moiety, hirudin1-48, was replaced by small nonsubstrate-type active site inhibitors of thrombin, e.g., dansyl-Arg-(D-pipecolic acid). The linker moiety was replaced by omega-amino acids of (12-aminododecanoic acid)-(4-aminobutyric acid), and hirudin55-65 was used as a fibrinogen-recognition exo-site binding moiety in most of the inhibitors. The crystal structure of the inhibitor in complex with human alpha-thrombin showed that dansyl, Arg, and D-pipecolic acid of the active site blocking moiety occupy S3, S1, and S2 subsites of thrombin, respectively, and were therefore designated as P3, P1, and P2 residues. The use of dansyl-Arg-(D-pipecolic acid) improved the affinity (K(i)) of the inhibitor 10-100-fold (down to 1.70 x 10(-11) M) compared to that of the similar compounds having D-Phe-Pro-Arg as their substrate-type inhibitor moiety (K(i) = 10(-9)-10(-10) M). The linker connected to P2 residue eliminated the scissile peptide bond. The inhibitor was also stable against human plasma proteases. Further inhibitor design revealed that the toxic dansyl group could be replaced by 4-tert-butylbenzenesulfonyl group and 1- or 2-naphthalenesulfonyl group for in vivo studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Probing the structure of hirudin from Hirudinaria manillensis by limited proteolysis. Isolation, characterization and thrombin-inhibitory properties of N-terminal fragments

Hirudin is the most potent and specific inhibitor of the blood-clotting enzyme thrombin so far known. Several hirudin variants were isolated mostly from Hirudo medicinalis and shown to be polypeptide chains of approximately 7 kDa with three internal disulfide bridges. In this study, limited proteolysis has been used to probe aspects of the structure and dynamics of a hirudin variant HM2 isolated from Hirudinaria manillensis. Proteolysis of the polypeptide chain of 64-amino-acid residues of hirudin HM2 by protease from Staphylococcus aureus V8, trypsin, thermolysin and subtilisin occurs at region 41-49 of the chain. The N-terminal fragments 1-41 and 1-47 were isolated to homogeneity and shown to maintain inhibitory action on thrombin, though much lower than the intact protein. The results were interpreted on the basis of a proposed three-dimensional structure of hirudin HM2 deduced by protein modelling the known structure of hirudin variant HV1 from Hirudo medicinalis (75% sequence similarity between HM2 and HV1). Both proteolysis experiments and protein modelling provide evidence for the existence in hirudin HM2 of a N-terminal well-structured domain (core) and a C-terminal flexible polypeptide segment. Determination of the accessible surface area of the three-dimensional model of hirudin HM2 showed that the sites of preferential cleavages are at the surface of the polypeptide molecule.

Production of disulfide-linked hirudin dimer by in vitro folding

A simple process of in vitro folding has been developed for the preparation of hirudin dimer. A variant of recombinant hirudin with Asp33 replaced by Cys was expressed in yeast and isolated by HPLC. Crude Cys33-hirudin contains heterogeneous products that are made of one species of primary sequence. They were together reduced/denatured, and allowed to re-fold in the sodium bicarbonate buffer (pH 8.3) alone. Active, homogeneous Cys33-hirudin monomer folded spontaneously with a first order rate constant of 0.05 +/- 0.01 min-1, followed by the oxidation of two Cys33 to produce the pure dimer. The folding yield was 90%. On an equal weight basis, both Cys33-hirudin monomer and the dimer exhibit thrombin inhibitory activity comparable to that of wild-type hirudin. Due to the presence of an extra cysteine, the folding of active hirudin monomer (formation of three native disulfides) was accelerated by at least 12-fold.

Comparison of hirudin and heparin as adjuncts to streptokinase thrombolysis in a canine model of coronary thrombosis

Recombinant desulfatohirudin (HI), a potent and specific thrombin inhibitor, was compared with heparin (HE) as an adjunct to streptokinase thrombolysis. In pentobarbital-anesthetized dogs, an occlusive thrombus (whole blood+thrombin) was introduced into the left anterior descending coronary artery (LAD) with superimposed endothelial damage and distal high-grade stenosis. Intravenous infusion of saline (vehicle), HI (0.3 mg/kg followed by 0.3 mg/kg per hour, 1 mg/kg followed by 1 mg/kg per hour, or 2 mg/kg followed by 2 mg/kg per hour), or HE (60 units/kg followed by 40 units/kg per hour or 100 units/kg followed by 60 units/kg per hour) was initiated 15 minutes before streptokinase (750,000 units for 60 minutes) administration. Vessel patency was monitored for 180 minutes after streptokinase administration with a volume flow probe on the proximal LAD. In dogs treated with no adjunctive agent (saline control), none of the vessels were recanalized with streptokinase. Both HI and HE promoted reperfusion, inhibited reocclusion, and reduced the residual thrombus mass in a dose-dependent fashion. However, at comparable levels of therapeutic anticoagulation (activated partial thromboplastin time [APTT] = 1.5-2.0 times baseline) HI exhibited a higher incidence of reperfusion (eight of eight dogs [100%] versus one of eight dogs [12%]), a shorter time to reperfusion (33 +/- 6 versus 59 minutes), a longer duration of initial reperfusion (106 +/- 21 versus 10 minutes), and a smaller residual thrombus mass than did HE. Likewise, the slope of the relation between the APTT prolongation and the total reperfusion time ("anticoagulation/antithrombosis profile") was almost five times higher for the combined HI data than for the HE data. Our results indicate that HI is more effective than HE in enhancing and sustaining coronary recanalization with streptokinase at a HI dose that modestly prolongs coagulation time and does not alter bleeding times.

Conformationally restricted thrombin inhibitors resistant to proteolytic digestion

A new type of thrombin exo-site inhibitor has been designed with enhanced inhibitory potency and increased metabolic stability. With the aid of the model of the structure of the thrombin-hirudin fragment complex [Yue, S.-Y., DiMaio, J., Szewczuk, Z., Purisima, E. O., Ni, F., & Konishi, Y. (1992) Protein Eng. 5, 77-85], cyclic analogs of the hirudin fragment (hirudin55-65) were designed and synthesized. In these analogs, the side chains of appropriately substituted residues, 58 and 61, were joined in order to restrict the conformation of the inhibitor. An analog with an 18-membered lactam ring showed higher antithrombin activity (IC50 = 0.57 microM) than the corresponding analogs with 17- or 16-membered rings and was 2-fold more potent than its linear counterpart. Even 4-fold greater enhancement was obtained when a shorter fragment, hirudin 55-62, was cyclized. This cyclization not only improved the potency but, more importantly, dramatically increased the resistance to proteolytic digestion. Remarkable enhancement of stability to proteolysis was observed for peptide bonds located in the exocyclic linear peptide segments. These results are discussed using molecular modeling.

Importance of the C-terminal region of big endothelin-1 for specific conversion by phosphoramidon-sensitive endothelin converting enzyme

Based on our previous findings that phosphoramidon-sensitive endothelin (ET) converting enzyme (ECE) converts human big ET-1 but does not big ET-3, we investigated structural requirement for substrate peptide. We prepared shorter peptides of big ET-1 and measured hydrolysis of the Trp-Val bond of these peptides. Relative hydrolysis ratios of big ET-1(1-38), (1-37), (16-37), (1-31) and (17-26) were 1, 1.15, 3.71, 0.01 and 0, respectively. In addition, big ET-2 and big ET-3 were not significantly converted by ECE. These results suggest that the carboxyl-terminal sequence at residues 32-37 of big ET-1 is important for conversion, whereas the amino-terminal disulfide loop structure appears to interfere with access of ECE to big ET-1.