[Marine biotoxins on the Tunisian coast: current state of knowledge]. / Les biotoxines marines sur le littoral tunisien: etat de connaissances.

In Tunisia, the most important sector of aquaculture is shellfish farming industry. Unfortunately, in recent years this sub-sector knows many problems of shellfish contamination due to blooms of toxic algae. This phenomenon occurred for the first time in 1994 in the Gulf of Gabes after the proliferation of Karenia cf. selliformis which synthesizes gymnodimine, a potent neurotoxin. Following these effects, several collection sites especially Boughrara lagoon, were closed for extended periods. Recently, an atypical toxicity was detected, initial characterization studies carried out by LC-MS showed that the toxins detected probably belong to the family PSP toxins. The lagoon of Bizerte was also known since 2006, a repeatable events of contamination of mussels and oysters. As a result, many samples were DSP and PSP positive with concentrations up to 2100 microg eq.STX/100 g meat.

[Screening of anti-inflammatory and analgesic activities in marines macroalgae from Mediterranean Sea]. / Criblage de l'effet anti-inflammatoire et analgesique des algues marines de la Mer Mediterranee.

Methanolic extracts of 13 seaweeds collected from the Mediterranean sea (Tunisian, Moroccan and Greek coasts) from different classes (Chlorophycae, Pheopbycae and Rhodophycae) are testedfor their analgesic and antiinflammatory effects. These activities were estimated in vivo, respectively by writhing test and carrageenan test. Nine species among 13 tested seaweeds showed an important analgesic activity. On the other hand only 5 seaweeds showed a significant anti-inflammatory activity (< 0.001 compared to control group). The percentage of inhibition reached 80% for the red algae Laurencia glandulifera but was only 50% for aspirin. The screening showed different pharmacological profiles. The red algae (Laurencia glandulfera and Hypnea musciformis) and brown algae (Cystoseira barbata and Sargassum vulgare) had endowed with the double analgesic and anti-inflammatory activity. The red algae Geliduim sesquipedale have only anti-inflammatory activity and the other one endowed only with an analgesic activity (Enteromorpha compressa, Chaetomorpha linum, Cystoseira ericoidies, Sacchoriza bulbosa et Corralina officinalis). The simultaneous or individual presence of the analgesic and\or anti-inflammatory activities of the various extracts can find its application in the therapeutic domain.

Prediction of overall survival in patients with hepatocellular carcinoma treated with Y-90 radioembolization by imaging response criteria.

PURPOSE: To evaluate the potential of imaging criteria in predicting overall survival of patients with hepatocellular carcinoma (HCC) after a first transcatheter arterial yttrium-90 radioembolization (TARE) MATERIALS AND METHODS: From October 2013 to July 2017, 37 patients with HCC were retrospectively included. There were 34 men and 3 women with a mean age of 60.5±10.2 (SD) years (range: 32.7-78.9 years). Twenty-five patients (68%) were Barcelona Clinic Liver Cancer (BCLC) C and 12 (32%) were BCLC B. Twenty-four primary index tumors (65%) were>5cm. Three radiologists evaluated tumor response on pre- and 4-7 months post-TARE magnetic resonance imaging or computed tomography examinations, using Response Evaluation Criteria in Solid Tumors (RECIST) 1.1, modified RECIST (mRECIST), European Association for Study of the Liver (EASL), volumetric RECIST (vRECIST), quantitative EASL (qEASL) and the Liver Imaging Reporting and Data System treatment response algorithm. Kaplan-Meier survival curves were used to compare responders and non-responders for each criterion. Univariate and multivariate Cox proportional hazard ratio (HR) analysis were used to identify covariates associated with overall survival. Fleiss kappa test was used to assess interobserver agreement. RESULTS: At multivariate analysis, RECIST 1.1 (HR: 0.26; 95% confidence interval [95% CI]: 0.09-0.75; P=0.01), mRECIST (HR: 0.22; 95% CI: 0.08-0.59; P=0.003), EASL (HR: 0.22; 95% CI: 0.07-0.63; P=0.005), and qEASL (HR: 0.30; 95% CI: 0.12-0.80; P=0.02) showed a significant difference in overall survival between responders and nonresponders. RECIST 1.1 had the highest interobserver reproducibility. CONCLUSION: RECIST and mRECIST seem to be the best compromise between reproducibility and ability to predict overall survival in patients with HCC treated with TARE.

Maurotoxin, a four disulfide bridge toxin from Scorpio maurus venom: purification, structure and action on potassium channels.

A new toxin acting on K+ channels, maurotoxin (MTX), has been purified to homogeneity from the venom of the chactoid scorpion Scorpio maurus. MTX is a basic single chain 34 amino acid residue polypeptide, amidated at its C terminal, and crosslinked by four disulfide bridges. It shows 29-68% sequence identity with other K+ channel toxins, and presents an original disulfide pattern, the last two half-cystine residues (31-34) being connected. Although the first three disulfide bonds have not been defined experimentally, modelling based on the structure of charybdotoxin favored two combinations out of six, one of which has two bridges (3-24 and 9-29) in common with the general motif of scorpion toxins. The last bridge would connect residues 13 and 19. MTX inhibits the binding to rat brain synaptosomal membranes of both [125I]apamin, a SK(Ca) channel blocker (IC50 5 nM), and [125I]kaliotoxin, a Kv channel blocker (IC50 30 pM). MTX blocks the Kv1.1, Kv1.2 and Kv1.3 currents expressed in Xenopus oocytes with IC50 of 45, 0.8 and 180 nM, respectively. MTX represents a member of a new class of short toxins with 4 disulfide bridges, active on voltage-dependent K+ channel and also competing with apamin for binding to its receptor.

Chemical synthesis and characterization of maurocalcine, a scorpion toxin that activates Ca(2+) release channel/ryanodine receptors.

Maurocalcine is a novel toxin isolated from the venom of the chactid scorpion Scorpio maurus palmatus. It is a 33-mer basic peptide cross-linked by three disulfide bridges, which shares 82% sequence identity with imperatoxin A, a scorpion toxin from the venom of Pandinus imperator. Maurocalcine is peculiar in terms of structural properties since it does not possess any consensus motif reported so far in other scorpion toxins. Due to its low concentration in venom (0.5% of the proteins), maurocalcine was chemically synthesized by means of an optimized solid-phase method, and purified after folding/oxidation by using both C18 reversed-phase and ion exchange high-pressure liquid chromatographies. The synthetic product (sMCa) was characterized. The half-cystine pairing pattern of sMCa was identified by enzyme-based cleavage and Edman sequencing. The pairings were Cys3-Cys17, Cys10-Cys21, and Cys16-Cys32. In vivo, the sMCa was lethal to mice following intracerebroventricular inoculation (LD(50), 20 microg/mouse). In vitro, electrophysiological experiments based on recordings of single channels incorporated into planar lipid bilayers showed that sMCa potently and reversibly modifies channel gating behavior of the type 1 ryanodine receptor by inducing prominent subconductance behavior.

Structure-activity relationships of scorpion alpha-neurotoxins: contribution of arginine residues.

The role of arginine residues in the structure-activity relationships of alpha-scorpion neurotoxins was studied. Toxins I and II from Androctonus australis Hector (north African scorpion), containing respectively 2 and 3 arginines, were modified by phenylglyoxal or p-hydroxyphenylglyoxal. Modified derivatives were purified by reverse-phase HPLC and/or ion exchange HPLC. Subsequent bioassays showed that toxin I (AaH I) derivatives with single modifications on Arg 2 and Arg 60 had low activity (25 and 14% of residual activity, assessed in receptor binding experiments). Doubly modified (Arg 2, Arg 60) AaH I had 7% residual activity while further derivatization of the alpha-amino group led to an almost inactive derivative. These results agree with the involvement of arginines 2 and 60, as well as the alpha-amino group, of AaH I in the toxin/receptor interaction, probably via electrostatic interactions. Consistent with the role of N-terminal residues, the selective removal of the N-terminal dipeptide Val-Arg of toxin III from the same scorpion resulted in low activity (7% residual activity). The arginine residue in position 56 of toxin II was important for bioactivity since the derivative modified by phenylglyoxal on Arg 56 exhibited low residual activity (20%). Arg 62 and Arg 18, on the other hand, can be modified without any great effect on the pharmacological activity of AaH II. These results furnish a more precise picture of those residues involved in the "toxic region", which appears to be composed of residues belonging to the conserved hydrophobic surface and to the C-terminal and N-terminal sequences.

Molecular characterization, antigenicity and immunogenicity of anatoxic polymeric forms conferring protection against scorpion venoms.

Two polymeric forms of Buthus occitamus tunetanus (Bot) G-50 and Androctonus australis hector (Aah) G-50 were obtained by controlled polymerization with glutaraldehyde. Their mol. wts, determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and exclusion high-performance liquid chromatography using radiolabelled toxin, ranged from 14,000 to 90,000 and 90,000 to 100,000 for Aah G-50 and Bot G-50, respectively. Modification of about 20% of the lysine residues yielded completely detoxified Bot G-50 fractions which retained the antigenic cross-reactivity with the native G-50 fraction, venom and toxins. High levels of specific antibodies against venoms and toxins were generated by immunization with prepared polymeric forms because absorbance values higher than 3 units were regularly observed by enzyme-linked immunosorbent assay with immune sera dilutions of 1/10,000. In vitro protection experiments demonstrated that immune sera from mice, horse and monkeys efficiently neutralized either Aah or Bot venom. Depending on the adjuvant and animal model species used, the neutralizing titre estimated by subcutaneous injection in mice varied between 20 and 40 LD50/ml. The in vivo protection assays showed that immunized mice could resist the challenge by six times the LD50 amounts of the toxic fraction. This protection was found to be long-lived. It was concluded that G-50 polymeric forms are highly detoxified and immunogenic, and should be useful for the production of potent polyvalent antivenom against scorpion toxins. They could also be considered for further studies towards the development of vaccine candidates.

Purification, structure and activity of three insect toxins from Buthus occitanus tunetanus venom.

One contractive and two depressant toxins active on insect were purified by high-performance liquid chromatography from the venom of Buthus occitanus tunetanus (Bot). The two depressant toxins, BotIT4 and BotIT5, differ only at position 6 (Arg for Lys) and are equally toxic to insects (LD50 to Blatella germanica = 110 ng/100 mg body weight). They show a strong antigenic cross-reaction with a depressive toxin from Leiurus quinquestriatus quinquestriatus (LqqIT2). The two toxins are able to inhibit with high affinity (K0.5 between 2 and 3 nM) the specific binding of the radioiodinated excitatory insect toxin (125I-AaHIT) on its receptor site on Periplaneta americana synaptosomal membranes. These toxins depolarize the cockroach axon, irreversibly block the action potential, and slow down and very progressively block the transmembrane transient Na+ current. The contracturant toxin BotIT1 is highly toxic to B. germanica (LD50 = 60 ng/ 100 mg body weight) and barely toxic to mice (LD50 = 1 microgram/20 g body weight) when injected intracerebroventricularly. It does not compete with 125I-AaHIT for its receptor site on P. americana synaptosomal membranes. On cockroach axon, BotIT1 develops plateau potentials and slows down the inactivation mechanism of the Na+ channels. Thus, BotIT1 belongs to the group of alpha insect-selective toxins and shows a strong sequence identity (> 90%) with Lqh alpha IT and LqqIII, two insect alpha-toxins previously purified from the venom of L. q. hebraeus and L. q. quinquestriatus. respectively.

Maurotoxin, a four disulfide bridges scorpion toxin acting on K+ channels.

Maurotoxin, a toxin from the venom of the Tunisian chactoid scorpion Scorpio maurus, has been purified to homogeneity by gel filtration/reversed-phase HPLC, and characterized. It is a basic and C-terminal amidated 34-residue polypeptide cross-linked by four disulfide bridges. From Edman sequencing results, only six different pairings between the first six half-cystines were retained whereas a disulfide bridge was predicted between the two half-cystines in positions 31 and 34. Modelling based on the structure of charybdotoxin favored two different pairings, one of which possessed two disulfides in common with the general motif of scorpion toxins. The solid-phase technique was used to obtain synthetic maurotoxin, sMTX. The half-cystine pairings of sMTX were determined by enzymatic cleavage and were found to be Cys3 Cys24, Cys9-Cys29, Cys13-Cys19, and Cys31-34, in agreement with experimental data obtained with natural maurotoxin. Both natural and synthetic maurotoxins were lethal to mice following intracerebroventricular injection (LD50, 80 ng/mouse). They blocked the Kv1.1, Kv1.2, and Kv1.3 channels expressed in Xenopus oocytes with almost identical half-effects (IC50) in the range of 40, 0.8 and 150 nM, respectively. They also competed with 125I-apamin (SKca channel blocker) and 125I-kaliotoxin (Kv channel blocker) for binding to rat brain synaptosomes with IC50 of about 5 and 0.03 nM. As the natural and synthetic maurotoxins exhibit indistinguishable physicochemical and pharmacological properties, they are likely to adopt the same half-cystine pairing pattern which is unique among known scorpion toxins. However, this disulfide organization is different from those reported for Pandinus imperator and Heterometrus spinnifer toxins 1 (Pi1 and HsTx1), two novel four-disulfide bridged K+ channel-acting scorpion toxin sharing about 50-70% sequence identity with maurotoxin.

[Study of the protective capacity of scorpion venom Buthus occitanus tunetanus polymerised to glutaraldehyde in mice strains with different haplotypes]. / Edude de la capacité protectrice du venin du scorpion Buthus occitanus tunetanus (Bot) polymérisé au glutaraldehyde chez des souches de souris d'haplotypes différents.

The immune response obtained against the toxic fraction of the scorpion venom Buthus occitanus tunetanus detoxified by polymerisation with glutaraldehyde, was analysed for low inbred mice having different haplotypes: C57BL/6 (H-2b) et BALB/c (H-2d) and the SWISS outbred mouse. This three strains of mice, immunized with the polymeric form of Bot-G50 are able to induce an immune response with bumoral mediation. The anti-polymers antibodies obtained from immunized mice, cross-react with the native Bot-G50 fraction. Indeed, in vitro protection experiments demonstrated that immune sera were neutralizing (between 150 and 235 micrograms of Bot-G50 ml). The in vivo protection assays showed that immunized mice could resist the challenge by high amount of toxic fraction (between 70 and 80 micrograms of Bot-G50). This protection was found to be long-lived, since immunized SWISS mice could resist the challenge by 4 DL50 of the toxic fraction (80 micrograms) six month after the start of the immunized program.

[Characterisation of a novel toxin active on potassium apanaine channels, purified from Buthus occitanus tunetanus scorpion venom]. / Caractérisation d'une nouvelle toxine active sur des canaux potassium apamine sensible purifiée à partir du venin de Buthus occitanus tunetanus.

A new peptidyl inhibitor of the small-conductance Ca(2+)-activated K+ channels (SKca) was purified to homogeneity from the venom of the Tunisian scorpion Buthus occitanus tunetanus. The molecular mass determined by SDS-PAGE, shows that it's a short peptide (3300 Da). The primary sequence of this toxin shows that it is a 31-residue polypeptide cross-linked by three disulfide bridges and structurally related to subfamily 5 of short scorpion toxins. This molecule shows similar pharmacological properties with this group of peptides inducing high toxicity in mice after intracerebro-ventricular injection, and competing with iodinated apamin for binding to its receptor site from rat brain synaptosomes (K0.5 = 4 nM).

Inhibition of sensory afferents activation and visceral pain by a brominated algal diterpene.

BACKGROUND: In the search of new therapeutic options for the treatment of pain, isolation, and testing of secondary metabolites from plant extracts has raised significant attention. We have investigated the effects of the brominated diterpene O(11) 15- cyclo-14-bromo-14,15-dihydrorogiol-3,11-diol (that we have named VLC5), extracted from the Mediterranean red algae Laurencia glandulifera. METHODS: The pure extract was tested on primary afferent calcium signals induced by high concentration of KCl, transcient receptor potential vanilloid (TRPV)1 (capsaicin) or TRPV4 agonists, histamine, or protease-activated receptor-2 (PAR(2) ) agonist. It was also tested in mice in a model of mustard oil-induced colonic hypersensitivity. KEY RESULTS: VLC5 was inhibited PAR(2) agonist or histamine-induced calcium mobilization in mouse primary afferents, but did not modify calcium signals induced by high concentrations of KCl, TRPV1 or TRPV4 agonists. The effect of VLC5 on histamine-induced calcium signal in primary afferent was inhibited by pertussis toxin pretreatment and was dependent on the activation of mu- or kappa-opioid receptor agonists, as it was inhibited by selective antagonists of those two receptors, but not by selective antagonist of the delta-opioid receptor. Intraperitoneal treatment of mice with VLC5 (10 mg kg(-1)) significantly reduced visceral pain behaviors induced by the intracolonic administration of mustard oil, in an opioid receptor-dependent manner. CONCLUSIONS & INFERENCES: We have demonstrated significant analgesic properties for the algal metabolite VLC5, which is able to signal directly to primary afferents, through a mechanism dependent on the activation of opioid receptors. This identifies a new natural compound capable of activating peripheral opioidergic systems, exerting analgesic properties.

Structure/activity relationships of scorpion alpha-toxins. Multiple residues contribute to the interaction with receptors.

Chemical modifications of tyrosine and tryptophan residues of scorpion alpha-neurotoxins II and III from Androctonus australis Hector were performed as well as modification of the two arginines and the alpha-amino group of toxin I. The pharmacological potencies of each derivative were assessed in vivo by LD50 measurement and in vitro by competition experiments with 125I-toxin for synaptosomal receptors. Arginine residues in positions 2 and 60 and the alpha-amino group of Androctonus toxin I were derivatized by p-hydroxyphenylglyoxal; the corresponding modified toxins exhibit low pharmacological potencies. Tryptophan 38 of toxin II and tryptophan 45 of toxin III were modified by nitrophenylsulfenyl chloride, leading respectively to a poorly and a fully active derivative. The tetranitromethane modification of tyrosine residues in positions 60, 5 and 14 of toxin III induced respectively 60%, 40% and 30% of loss of biological activity. Circular dichroic analysis indicated that for every derivative, except the nitrophenylsulfenyl derivative of Trp-45 of AaH III, the conformation of the toxin was not altered by derivatization. Conformational integrity was also confirmed by full activity of the derivatives in radioimmunoassays. Taken together, the results suggest that aromatic residues belonging to the conserved hydrophobic surface, to the C-terminal and to the loop region 37-44 are involved in the molecular mechanisms by which scorpion alpha-toxins act. Charged residues in the N-terminal and C-terminal also contribute to the high efficacy of the binding process. It appears that all important residues are clustered on one face of the toxin, suggesting a multipoint interaction with the proteins of the sodium channel.

Solution structure of maurotoxin, a scorpion toxin from Scorpio maurus, with high affinity for voltage-gated potassium channels.

Maurotoxin (MTX), purified from the scorpionid Scorpio maurus is a potent ligand for potassium channels. It shows a broad specificity as being active on Kv1.1 (Kd = 37 nM), Kv1.2 (Kd = 0.8 nM), Kv1.3 (Kd = 150 nM) voltage-gated potassium channels, as well as on small-conductance calcium-activated potassium channels. It has a unique disulfide pairing among the scorpion toxins family. The solution structure of MTX has been determined by 2D-NMR techniques, which led to the full description of its 3D conformation: a bended helix from residues 6 to 16 connected by a loop to a two-stranded antiparallel beta sheet (residues 23 to 26 and 28 to 31). The interaction of MTX with the pore region of the Kv1.2 potassium channel has been modeled according to their charge anisotropy. The structure of MTX is similar to other short scorpion toxins despite its peculiar disulfide pairing. Its interaction with the Kv1.2 channel involves a dipole moment, which guides and orients the toxin onto the pore, toward the binding site, and which thus is responsible for the specificity.

Effect of maurotoxin, a four disulfide-bridged toxin from the chactoid scorpion Scorpio maurus, on Shaker K+ channels.

Maurotoxin is a 34-residue toxin isolated from the venom of the Tunisian chactoid scorpion Scorpio maurus palmatus and contains four disulfide bridges that are normally found in long-chain toxins of 60-70 amino acid residues, which affect voltage-gated sodium channels. However, despite the unconventional disulfide-bridge pattern of maurotoxin, the conformation of this toxin remains similar to that of other toxins acting on potassium channels. Here, we analyzed the effects of synthetic maurotoxin on voltage-gated Shaker potassium channels (ShB) expressed in Xenopus oocytes. Maurotoxin produces a strong, but reversible, inhibition of the ShB K+ current with an IC50 of 2 nM. Increasing concentrations of the toxin induce a progressively higher block at saturating concentrations. At nonsaturating concentrations of the toxin (5-20 nM), the channel block appears slightly more pronounced at threshold potentials suggesting that the toxin may have a higher affinity for the closed state of the channel. At the single channel level, the toxin does not modify the unitary current amplitude, but decreases ensemble currents by increasing the number of depolarizing epochs that failed to elicit any opening. A point mutation of Lys23 to alanine in maurotoxin produces a 1000-fold reduction in the IC50 of block by the toxin suggesting the importance of this charged residue for the interaction with the channel. Maurotoxin does not affect K+ currents carried by Kir2.3 channels in oocytes or Na+ currents carried by the alphaIIa channel expressed in CHO cells.

A new fold in the scorpion toxin family, associated with an activity on a ryanodine-sensitive calcium channel.

We determined the structure in solution by (1)H two-dimensional NMR of Maurocalcine from the venom of Scorpio maurus. This toxin has been demonstrated to be a potent effector of ryanodyne-sensitive calcium channel from skeletal muscles. This is the first description of a scorpion toxin which folds following the Inhibitor Cystine Knot fold (ICK) already described for numerous toxic and inhibitory peptides, as well as for various protease inhibitors. Its three dimensional structure consists of a compact disulfide-bonded core from which emerge loops and the N-terminus. A double-stranded antiparallel beta-sheet comprises residues 20-23 and 30-33. A third extended strand (residues 9-11) is perpendicular to the beta-sheet. Maurocalcine structure mimics the activating segment of the dihydropyridine receptor II-III loop and is therefore potentially useful for dihydropyridine receptor/ryanodine receptor interaction studies. Proteins 2000;40:436-442.

In vivo protection against Androctonus australis hector scorpion toxin and venom by immunization with a synthetic analog of toxin II.

A synthetic peptide mimicking the North African scorpion Androctonus australis hector toxin II was designed and produced by chemical solid-phase synthesis. It contains the entire sequence of toxin II (64 amino acid residues), with each half-cystine being replaced by the isosteric residue a-aminobutyric acid, and was thus devoid of disulfide bridges. This construct was totally nontoxic in mice even if large amounts, equivalent to 1000 times the LD50 of the original toxin, were injected by the intracerebroventricular route. The synthetic peptide, either as a monomer or polymerized by means of glutaraldehyde, induced the production of antitoxin neutralizing antibodies in immunized mice and rabbits. After three injections with either the monomeric or polymerized synthetic peptide, the immunized mice were protected against several lethal doses of the corresponding native toxin or scorpion venom. Six months after immunization, the mice were completely protected against challenge with eight LD50 of the original toxin. The protection was better when the polymerized synthetic peptide was used. One month after the start of the immunization program, it showed a good correlation between antibody titer and protection. However, antibody titer decreased with time but protection remained high. This suggests that additional factors other than circulating antibodies play a role in protective activity.

Maurotoxin and the Kv1.1 channel: voltage-dependent binding upon enantiomerization of the scorpion toxin disulfide bridge Cys31-Cys34.

Maurotoxin (MTX) is a 34-amino acid polypeptide cross-linked by four disulfide bridges that has been isolated from the venom of the scorpion Scorpio maurus palmatus and characterized. Maurotoxin competed with radiolabeled apamin and kaliotoxin for binding to rat brain synaptosomes and blocked K+ currents from Kv1 channel subtypes expressed in Xenopus oocytes. Structural characterization of the synthetic toxin identified half-cystine pairings at Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34 This disulfide bridge pattern is unique among known scorpion toxins, particularly the existence of a C-terminal '14-membered disulfide ring' (i.e. cyclic domain 31-34), We therefore studied structure-activity relationships by investigating the structure and pharmacological properties of synthetic MTX peptides either modified at the C-terminus ¿i.e. MTX(1-29), [Abu31,34]-MTX and [Cys31,34, Tyr32]D-MTX) or mimicking the cyclic C-terminal domain [i.e. MTX(31-34)]. Unexpectedly, the absence of a disulfide bridge Cys31-Cys34 in [Abu 31,34]-MTX and MTX(1-29) resulted in MTX-unrelated half-cystine pairings of the three remaining disulfide bridges for the two analogs, which is likely to be responsible for their inactivity against Kv1 channel subtypes. Cyclic MTX(31-34) was also biologically inactive. [Cys31,34, Tyr32]D-MTX, which had a 'native', MTX-related, disulfide bridge organization, but a D-residue-induced reorientation of the C-terminal disulfide bridge, was potent at blocking the Kv1.1 channel. This peptide-induced Kv1.1 blockage was voltage-dependent (a property not observed for MTX), maximal in the low depolarization range and associated with on-rate changes in ligand binding. Thus, the cyclic C-terminal domain of MTX seems to be crucial for recognition of Kv1.3, and to a lesser extent, Kv1.2 channels and it may contribute to the stabilization and strength of the interaction between the toxin and the Kv1.1 channel.