Some properties and the complete primary structures of two reduced and S-carboxymethylated polypeptides (S5C1 and S5C10) from Dendroaspis jamesoni kaimosae (Jameson's mamba) venom

Snake Venom Components as Therapeutic Drugs in Ischemic Heart Disease

Ischemic heart disease (IHD), especially myocardial infarction (MI), is a leading cause of death worldwide. Although coronary reperfusion is the most straightforward treatment for limiting the MI size, it has nevertheless been shown to exacerbate ischemic myocardial injury. Therefore, identifying and developing therapeutic strategies to treat IHD is a major medical challenge. Snake venoms contain biologically active proteins and peptides that are of major interest for pharmacological applications in the cardiovascular system (CVS). This has led to their use for the development and design of new drugs, such as the first-in-class angiotensin-converting enzyme inhibitor captopril, developed from a peptide present in Bothrops jararaca snake venom. This review discusses the potential usefulness of snake venom toxins for developing effective treatments against IHD and related diseases such as hypertension and atherosclerosis. It describes their biological effects at the molecular scale, their mechanisms of action according to their different pharmacological properties, as well as their subsequent molecular pathways and therapeutic targets. The molecules reported here have either been approved for human medical use and are currently available on the drug market or are still in the clinical or preclinical developmental stages. The information summarized here may be useful in providing insights into the development of future snake venom-derived drugs.

Three finger toxins of elapids: structure, function, clinical applications and its inhibitors

The WHO lists snakebite as a "neglected tropical disease". In tropical and subtropical areas, envenoming is an important public health issue. This review article describes the structure, function, chemical composition, natural inhibitors, and clinical applications of Elapids' Three Finger Toxins (3FTX) using scientific research data. The primary venomous substance belonging to Elapidae is 3FTX, that targets nAChR. Three parallel β-sheets combine to create 3FTX, which has four or five disulfide bonds. The three primary types of 3FTX are short-chain, long-chain, and nonconventional 3FTX. The functions of 3FTX depend on the specific toxin subtype and the target receptor or ion channel. The well-known effect of 3FTX is probably neurotoxicity because of the severe consequences of muscular paralysis and respiratory failure in snakebite victims. 3FTX have also been studied for their potential clinical applications. α-bungarotoxin has been used as a molecular probe to study the structure and function of nAChRs (Nicotinic Acetylcholine Receptors). Acid-sensing ion channel (ASIC) isoforms 1a and 1b are inhibited by Mambalgins, derived from Black mamba venom, which hinders their function and provide an analgesic effect. α- Cobra toxin is a neurotoxin purified from Chinese cobra (Naja atra) binds to nAChR at the neuronal junction and causes an analgesic effect for moderate to severe pain. Some of the plants and their compounds have been shown to inhibit the activity of 3FTX, and their mechanisms of action are discussed.

Synergistic strategies of predominant toxins in snake venoms

Synergism is a significant phenomenon present in snake venoms that may be an evolving strategy to potentiate toxicities. Synergism exists between different toxins or toxin complexes in various snake venoms, with phospholipaseA₂s (PLA₂s) (toxins or subunits) the main enablers. The predominant toxins, snake venom PLA₂s, metalloproteases (SVMPs), serine proteases (SVSPs) and three-finger toxins (3FTxs), play essential roles in synergistic processes. The hypothetical mechanisms of synergistic effect can be generalized under the effects of amplification and chaperoning. The Toxicity Score is among the few quantitative methods to assess synergism. Selection of toxins involved in synergistically enhanced toxicity as the targets are important for development of novel antivenoms or inhibitors.

From toxins targeting ligand gated ion channels to therapeutic molecules

Ligand-gated ion channels (LGIC) play a central role in inter-cellular communication. This key function has two consequences: (i) these receptor channels are major targets for drug discovery because of their potential involvement in numerous human brain diseases; (ii) they are often found to be the target of plant and animal toxins. Together this makes toxin/receptor interactions important to drug discovery projects. Therefore, toxins acting on LGIC are presented and their current/potential therapeutic uses highlighted.

Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus)

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Arg-Tyr-Asp (RYD) and Arg-Cys-Asp (RCD) motifs in dendroaspin promote selective inhibition of beta1 and beta3 integrins

Arg-Gly-Asp (RGD) is a unique minimal integrin-binding sequence that is found within several glycoprotein ligands. This sequence has also been found in snake-venom anti-platelet proteins, including the disintegrins and dendroaspin, a natural variant of short-chain neurotoxins isolated from the venom of Dendroaspis jamesonii. In the present study, the motifs RYD and RCD were introduced into the dendroaspin scaffold to replace RGD. Both motifs in dendroaspin caused inhibition of ADP-induced platelet aggregation with IC(50) values of 200 and 300 nM respectively, similar to that of the wild-type RGD motif (170 nM). In comparison with wild-type dendroaspin, both RYD- and RCD-containing dendroaspins were more selective in the inhibition of the adhesion of K562 cells to laminin rather than to fibrinogen and fibronectin, even though they were 10-30-fold less potent at inhibiting K562 cell (containing alpha(5)beta(1) integrin) adhesion to laminin compared with wild-type. Interestingly, the RYD motif produced a similar IC(50) value to the RGD motif at inhibiting A375-SM cell (beta(3) integrin) adhesion to collagen, whereas the RCD motif was approx. 2-6-fold less potent compared with either RGD or RYD. These findings show that the selectivity of dendroaspin binding to beta(1) and beta(3) integrins can be modulated by the introduction of alternative cell recognition sequences.

Proline brackets and identification of potential functional sites in proteins: toxins to therapeutics

Protein toxins induce their specific pharmacological effects through protein protein interaction. Identification of these protein-protein interaction sites could lead to prototypes of highly specific therapeutic agents. However, deciphering the structure function relationships of protein toxins and locating the functional sites is a difficult, tedious and cumbersome task. We recently developed a novel predictive method to identify potential protein protein interaction sites directly from the amino acid sequence of a protein (R. M. Kini and H. J. Evans (1996) FEBS Lett. 385, 81-86) based on the presence of proline residues, a common residue found predominantly in the flanking segments of protein-protein interaction sites (R. M. Kini and H. J. Evans (1996) Biochem. Biophys. Res. Commun. 212, 1115-1124). It is a simple and straight-forward method. This review describes the new method and its application to solve structure function relationships of protein toxins. The method is useful in identifying functional sites in toxins, despite the subtle and complex nature of their structure function relationships and saves significant amounts of time and money.

Modulation of RGD sequence motifs regulates disintegrin recognition of alphaIIb beta3 and alpha5 beta1 integrin complexes. Replacement of elegantin alanine-50 with proline, N-terminal to the RGD sequence, diminishes recognition of the alpha5 beta1 complex with restoration induced by Mn2+ cation

Several recent studies have demonstrated that the amino acid residues flanking the RGD sequence of high-affinity ligands modulate their specificity of interaction with integrin complexes. The present study has addressed the role of the residues flanking the RGD sequence in regulating the recognition by disintegrin of the alphaIIb beta3 and alpha5beta1 complexes by construction of a panel of recombinant molecules of Elegantin (the platelet aggregation inhibitor from the venom of Trimerasurus elegans) expressing specific RGD sequence motifs. Wild-type Elegantin (ARGDNP) and several variants including Eleg. AM (ARGDMP), Eleg. PM (PRGDMP) and Eleg. PN (PRGDNP) were expressed as glutathione S-transferase (GST) fusion proteins in Escherichia coli. The inhibitory efficacies of the panel of Elegantin variants were analysed in platelet adhesion assays with substrates immobilized with fibrinogen and fibronectin. Elegantin molecules containing an Ala residue N-terminal to the RGD sequence (wild-type Elegantin and Eleg. AM) showed strong inhibitory activity towards alphaIIbbeta3-dependent platelet adhesion on fibronectin, whereas a Pro residue in this position (Eleg. PM and Kistrin, the inhibitor from the venom of Calloselasma rhodostoma) engendered lower activity. The decreased activity could not be attributed to a decrease in the affinity of the disintegrin for the alphaIIb beta3 complex because both Eleg. AM and Eleg. PM had similar Kd (app) values. In contrast, Elegantin molecules into which a Met residue was introduced in place of the Asn residue C-terminal to the RGD sequence showed 10-13-fold elevated inhibitory activity towards platelet adhesion on fibrinogen and this was maintained with either a Pro or Ala residue N-terminal to the RGD sequence. In experiments with the alpha5 beta1 complex on K562 cells, the inhibitory efficacies of the panel of Elegantin molecules were analysed under two different cation conditions. First, in the presence of Ca2+/Mg2+, K562 cell adhesion on fibronectin was inhibited equally well by Elegantin and Eleg. AM but inhibited poorly by Eleg. PM and Kistrin. In contrast with platelets, the decreased inhibitory efficacy of the PRGDMP disintegrins was due to poor recognition of the alpha5 beta1 complex. In the presence of Mn2+ cation, K562 cell adhesion on fibrinogen was observed in an alpha5 beta1-dependent manner. Under these conditions both PRGD and ARGD containing disintegrins were strong inhibitors of K562 cell adhesion on fibrinogen and this was due to a markedly improved recognition of the alpha5 beta1 complex by the PRGD molecules. These observations demonstrate the pivotal role of the amino acids flanking the RGD sequence for disintegrin recognition of integrin complexes and highlight the subtle nature by which integrin-ligand binding specificity can be modulated by both cation and adhesive motif.

Flanking proline residues identify the L-type Ca2+ channel binding site of calciseptine and FS2

Calciseptine and FS2 are 60-amino acid polypeptides, isolated from venom of the black mamba (Dendroaspis polylepis polylepis), that block voltage-dependent L-type Ca2+ channels. We predicted that these polypeptides contain an identical functional site between residues 43 and 46 by searching for proline residues that mark the flanks of protein-protein interaction sites [Kini, R. M., and Evans, H. J. (1966) FEBS Lett. 385, 81-86]. The predicted Ca2+ channel binding site also occurs in closely related toxins, C10S2C2 and S4C8. Therefore, it is likely that these toxins also will block L-type Ca2+ channels. To test the proposed binding site on calciseptine and FS2, an eight-residue peptide, named L-calchin (L-type calcium channel inhibitor), was synthesized and examined for biological activity. As expected for an L-type Ca2+ channel blocker, L-calchin reduced peak systolic and developed pressure in isolated rat heart Langendorff preparations without affecting diastolic pressure or heart rate. Furthermore, L-calchin caused a voltage-independent block of L-type Ca2+ channel currents in whole-cell patch-clamped rabbit ventricular myocytes. Thus the synthetic peptide exhibits the L-type Ca2+ channel blocking properties of the parent molecules, calciseptine and FS2, but with a lower potency. These results strongly support the identification of a site in calciseptine and FS2 that is important for binding to L-type Ca2+ channels and reinforce the importance of proline brackets flanking protein-protein interaction sites.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

The integrin alpha IIb beta 3 contains distinct and interacting binding sites for snake-venom RGD (Arg-Gly-Asp) proteins. Evidence that the receptor-binding characteristics of snake-venom RGD proteins are related to the amino acid environment flanking the sequence RGD

We have previously demonstrated [Lu, Williams, Deadman, Salmon, Kakkar, Wilkinson, Baruch, Authi and Rahman (1994) Biochem. J. 304, 929-936] the preferential antagonism of the interactions of the integrin alpha IIb beta 3 on activated platelets with three immobilized glycoprotein ligands (fibrinogen, fibronectin and von Willebrand factor) by a selected panel of snake-venom RGD (Arg-Gly-Asp)-containing proteins including the disintegrins kistrin and elegantin, and the neurotoxin variant dendroaspin. Kistrin and dendroaspin, although structurally unrelated, contain similar amino acids flanking the tripeptide RGD and behaved as identical antagonists preferentially inhibiting platelet adhesion to immobilized fibrinogen as opposed to fibronectin. In contrast, elegantin, which shares extensive sequence similarity with kistrin but has different amino acids around the tripeptide RGD, preferentially inhibited platelet adhesion to immobilized fibronectin as opposed to fibrinogen. To develop further insights into the mechanisms underlying the preferential antagonism shown by the venom proteins in the adhesion studies, we, in the present study, sought to determine the binding properties of kistrin, elegantin and dendroaspin to the alpha IIb beta 3 complex by radioligand kinetic and competition studies. In direct binding experiments, both kistrin and dendroaspin were observed to bind to a single class of binding site on ADP-activated platelets with apparent equilibrium dissociation constant (Kdapp) values of 42 +/- 2 nM and 21 +/- 6 nM respectively. In competition studies, dendroaspin blocked the binding of 125I-labelled kistrin to ADP-activated platelets in a simple competitive manner, with an apparent equilibrium inhibition constant (Kiapp) of 143 +/- 14 nM, from which an indirect Kdapp = 22 nM for dendroaspin was determined. This result suggests that kistrin and dendroaspin bind to the same site on the integrin alpha IIb beta 3 consistent with their similar inhibitory properties. In contrast, elegantin recognized two classes of binding sites on the alpha IIb beta 3 complex with Kdapp values of 10.5 +/- 0.8 nM and 175 +/- 10 nM, and, unlike dendroaspin, did not inhibit the binding of 125I-labelled kistrin to ADP-activated platelets. However, in reciprocal experiments both kistrin and dendroaspin inhibited the binding of 125I-elegantin to ADP-activated platelets in a non-competitive manner, with Kiapp values of 34 +/- 3 nM and 21 +/- 2 nM respectively. Thus elegantin appears to interact with distinct but interacting sites on the alpha IIb beta 3 complex from the binding site of kistrin and dendroaspin, consistent with its distinctive inhibitory preferences as shown in platelet adhesion studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the lower-molecular-mass fraction of venoms from Dendroaspis jamesoni kaimosae and Micrurus fulvius using capillary-electrophoresis electrospray mass spectrometry

Capillary electrophoresis (CE) with electrospray ionization (ESI) and selected ion-monitoring mass-spectrometric (SIM-MS) detection has been used to provide as much information as possible about the lower molecular-mass fraction (peptides of molecular masses up to 8500 Da) of the venoms of Dendroaspis jamesoni kaimosae (Jameson's Mamba) and Micrurus fulvius (Eastern Coral Snake). Method development was based on the venom of D. jamesoni kaimosae, which contains some previously described peptides, with subsequent application to the completely unknown venom of M. fulvius. CE-ESI-SIM-MS provides a rapid and extremely sensitive method for the detection and molecular-mass determination of peptides present in venoms. It has been utilized to provide molecular masses and thus, by inference, confirmation of the peptide compositions for those toxins which have been previously described in the literature. Our methodology indicates the presence of 83 peptides in the venom of D. jamesoni kaimosae and 49 peptides in the venom of M. fulvius in the molecular-mass range 6000-8500 Da.

1H-NMR assignments and secondary structure of dendroaspin, an RGD-containing glycoprotein IIb-IIIa (alpha IIb-beta 3) antagonist with a neurotoxin fold

Dendroaspin, also referred to as mambin, was originally isolated from the venom of the Elapidae snake Dendroaspis jamesoni kaimose. It shares a high level of sequence similarity with the short-chain neurotoxins found in other Elapidae but displays approximately 1000-fold lower neurotoxin activity than the closely related protein erabutoxin b. However, unlike neurotoxins, it contains an RGD (Arg-Gly-Asp) motif and functions as an antagonist of platelet aggregation and cell-cell adhesion of comparable potency to the disintegrins from the venoms of Viperidae. We have determined the secondary structure of dendroaspin using 1H-NMR spectroscopy. Its structure resembles that of the short-chain neurotoxins, with three loops extending from a disulphide-bridged core; however, the strands of the triple-stranded beta-sheet are shorter and the loop containing the RGD sequence is moved away from this sheet. The structure bears little resemblance to that of the disintegrins, except in the RGD-containing loop, suggesting that this loop may be of prime importance in its inhibitory function. Comparison of this preliminary structure with that of the neurotoxins and disintegrins furthers our understanding of the mechanism of integrin antagonists and shows how the neurotoxin fold can be manipulated to give a variety of inhibitors.

Preferential antagonism of the interactions of the integrin alpha IIb beta 3 with immobilized glycoprotein ligands by snake-venom RGD (Arg-Gly-Asp) proteins. Evidence supporting a functional role for the amino acid residues flanking the tripeptide RGD in determining the inhibitory properties of snake-venom RGD proteins

The inhibitory properties of a panel of snake-venom-derived RGD (Arg-Gly-Asp) proteins, including the disintegrins kistrin, elegantin and albolabrin, and the neurotoxin homologue dendroaspin, were investigated in a platelet-adhesion assay using three immobilized ligands of the glycoprotein IIb-IIIa complex (alpha IIb beta 3), namely fibrinogen, fibronectin and von Willebrand factor (vWF). The snake-venom proteins preferentially inhibited the adhesion of ADP-treated platelets to one or more of the immobilized ligands. Kistrin and dendroaspin exhibited similar inhibitory characteristics, abrogating platelet adhesion to fibrinogen and vWF at nanomolar concentrations, but poorly inhibiting adhesion to fibronectin. Kistrin and dendroaspin share little overall amino-acid-sequence identity, but a considerable level of sequence similarity exists around the RGD tripeptide. Synthetic cyclic peptides corresponding to these regions of kistrin and dendroaspin inhibited platelet adhesion to both fibrinogen and fibronectin with approximately equal potency, but were 100-fold weaker antagonists of the interactions of the alpha IIb beta 3 complex with fibrinogen than their parent proteins. The disintegrins elegantin and albolabrin, which share approx. 60% overall amino-acid-sequence similarity with kistrin but have different residues around the RGD tripeptide, exhibited different antagonistic preferences. Elegantin inhibited platelet adhesion to immobilized vWF and fibronectin, but was significantly less effective at disrupting adhesion to fibrinogen. Albolabrin selectively inhibited platelet adhesion to immobilized vWF and was less effective with fibrinogen and fibronectin as adhesive ligands. In contrast with the behaviour of these venom proteins, the adhesion of ADP-treated platelets to immobilized fibrinogen, fibronectin and vWF was inhibited non-selectively by a range of monoclonal antibodies with specificity for the alpha IIb beta 3 complex. These observations, therefore, define antagonistic preferences in this panel of venom proteins towards the interactions of the alpha IIb beta 3 complex with three immobilized glycoprotein ligands.

Three-dimensional structure of the RGD-containing neurotoxin homologue dendroaspin

Dendroaspin is a short chain neurotoxin homologue from the venom of Elapidae snakes, which lacks neurotoxicity. Unlike neurotoxins, it contains an Arg-Gly-Asp-(RGD)-motif and functions as an inhibitor of platelet aggregation and platelet adhesion with comparable potency to the disintegrins from the venoms of Viperidae. We have determined the structure of dendroaspin in solution using NMR spectroscopy. The structure contains a core similar to that of short chain neurotoxins, but with a novel arrangement of loops and a solvent-exposed RGD-motif. Dendroaspin is thus an integrin antagonist with a well defined fold different from that of the disintegrins, based on the neurotoxin scaffold.

Synthetic RGD peptides derived from the adhesive domains of snake-venom proteins: evaluation as inhibitors of platelet aggregation

Synthetic peptides based on the RGD domains of the potent platelet aggregation inhibitors kistrin and dendroaspin were generated. The 13-amino-acid peptides were synthesized as dicysteinyl linear and disulphide cyclic forms. In platelet-aggregation studies, the cyclic peptides showed 3-fold better inhibition than their linear equivalents and approx. 100-fold greater potency than synthetic linear RGDS peptides derived from fibronectin. An amino acid substitution, Asp10-->Ala, in the kistrin-based peptide gave a 4-fold decrease in potency in the linear peptide, but produced a 2-fold elevation in the inhibitory activity of the cyclic form, generating a peptide of potency comparable with that of the parent protein.

Application of electrospray mass spectrometry and matrix-assisted laser desorption ionization time-of-flight mass spectrometry for molecular weight assignment of peptides in complex mixtures

Electrospray mass spectrometry (ES/MS) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI/TOF/MS) were used to provide mass spectra from seven elapid snake venoms. Spectral interpretation was much simpler for MALDI/TOF/MS. ES/MS proved more useful for the provision of molecular weight data for very closely related peptides, but suppression of higher molecular weight compounds was seen to occur during flow injection analysis. MALDI/TOF/MS proved useful for providing a complete picture of the venom, but the low resolution led to obscuring of major ions, and the mass accuracy was poorer for known peptides. Suppression also occurred during MALDI/TOF/MS but could be overcome using alternative matrices because the spectra were very dependent on the choice of matrix. ES/MS and MALDI/TOF/MS provide complementary and confirmatory information such that for the anal sis of complex peptide mixtures (snake venoms), the use of both techniques is desirable.

Mambin, a potent glycoprotein IIb-IIIa antagonist and platelet aggregation inhibitor structurally related to the short neurotoxins

The purification, complete amino acid sequence, functional activity, and structural modeling are described for mambin, a platelet glycoprotein GP IIb-IIIa antagonist and potent inhibitor of platelet aggregation from the venom of the Elapidae snake Dendroaspis jamesonii (Jameson's mamba). Mambin is 59 residues in length and contains four disulfide linkages and an RGD amino acid sequence found in protein ligands that bind to GP IIb-IIIa. Mambin inhibits ADP-induced platelet aggregation (IC50 = 172 +/- 22 nM) and inhibits the binding of purified platelet fibrinogen receptor GP IIb-IIIa to immobilized fibrinogen (IC50 = 3.1 +/- 0.8 nM). Mambin has very little sequence similarity to the Viperidae family of platelet aggregation inhibitors, except for the RGD-containing region in the protein. However, mambin does have ca. 47% similarity to the short-chain postsynaptic neurotoxins found in other Elapidae venoms, which do not contain the RGD sequence and do not act as GP IIb-IIIa antagonists. On the basis of its circular dichroism spectrum, mambin has a beta-sheet structure characteristic of the neurotoxins. Molecular modeling of the mambin sequence onto the erabutoxin b structure predicts a very similar structure within the entire protein except for the loop containing the RGD sequence. Mambin may therefore represent a genetic hybrid of neurotoxic and hemotoxic proteins found in snake venoms.

Structure-activity studies of homologues of short chain neurotoxins from Elapid snake venoms

Three neurotoxin homologues (CM10 and CM12 from Naja haje annulifera and S5C10 from Dendroaspis jamesoni kaimosae) and two short neurotoxins (CM14 from Naja haje annulifera and erabutoxin b from Laticauda semifasciata) were examined by circular dichroism (c.d.) and tested for neuromuscular activity on chick biventer cervicis nerve-muscle preparations. All three homologues had acetylcholine receptor blocking activity, as they abolished responses to indirect stimulation, acetylcholine and carbachol but had no effect on responses to direct muscle stimulation. CM10 was only about 5 times less potent than the short neurotoxin CM14; S5C10 and CM12 were respectively 30 and 300 times less active. The block induced by the three homologues, but not by the neurotoxins, was readily reversed by washing. CM10 and CM12 had virtually identical c.d. spectra which were closely similar to those of the neurotoxins. The spectrum of S5C10 indicated changes in the environment of tyrosine-25 and in the position of tryptophan-29. These alterations could distort the 3-dimensional arrangement of the residues postulated to form the receptor binding site. The results with CM10 and CM12 highlight a role for the first loop (residues 6-16) in the binding of neurotoxins to acetylcholine receptors, in addition to the previously postulated reactive site.

Conformational properties of the neurotoxins and cytotoxins isolated from Elapid snake venoms

The review will critically assess the information available on the conformation of homologous neurotoxins and cytotoxins isolated from Elapid snakes. Particular attention will be given to the dynamics of the molecules in solution because there is the possibility that defined intramolecular rearrangements are involved at the sites of action. Such properties will be then reconciled with the known X-ray crystallographic and sequence data in order to derive likely structure-activity relationships.