Structure-function relationships for cardiotoxins interacting with phospholipids

The myth of cobra venom cytotoxin: More than just direct cytolytic actions

Cobra venom cytotoxin (CTX) is a non-enzymatic three-finger toxin that constitutes 40-60% of cobra venom. Thus, it plays an important role in the pathophysiology of cobra envenomation, especially in local dermonecrosis. The three-finger hydrophobic loops of CTX determine the cytotoxicity. Nevertheless, the actual mechanisms of cytotoxicity are not fully elucidated as they involve not only cytolytic actions but also intracellular signalling-mediated cell death pathways. Furthermore, the possible transition cell death pattern remains to be explored. The actual molecular mechanisms require further studies to unveil the relationship between different CTXs from different cobra species and cell types which may result in differential cell death patterns. Here, we discuss the biophysical interaction of CTX with the cell membrane involving four binding modes: electrostatic interaction, hydrophobic partitioning, isotropic phase, and oligomerisation. Oligomerisation of CTX causes pore formation in the membrane lipid bilayer. Additionally, the CTX-induced apoptotic pathway can be executed via death receptor-mediated extrinsic pathways and mitochondrial-mediated intrinsic pathways. We also discuss lysosomal-mediated necrosis and the occurrence of necroptosis following CTX action. Collectively, we provided an insight into concentration-dependent transition of cell death pattern which involves different mechanistic actions. This contributes a new direction for further investigation of cytotoxic pathways activated by the CTXs for future development of biotherapeutics targeting pathological effects caused by CTX.

Snake cytotoxins bind to membranes via interactions with phosphatidylserine head groups of lipids

The major representatives of Elapidae snake venom, cytotoxins (CTs), share similar three-fingered fold and exert diverse range of biological activities against various cell types. CT-induced cell death starts from the membrane recognition process, whose molecular details remain unclear. It is known, however, that the presence of anionic lipids in cell membranes is one of the important factors determining CT-membrane binding. In this work, we therefore investigated specific interactions between one of the most abundant of such lipids, phosphatidylserine (PS), and CT 4 of Naja kaouthia using a combined, experimental and modeling, approach. It was shown that incorporation of PS into zwitterionic liposomes greatly increased the membrane-damaging activity of CT 4 measured by the release of the liposome-entrapped calcein fluorescent dye. The CT-induced leakage rate depends on the PS concentration with a maximum at approximately 20% PS. Interestingly, the effects observed for PS were much more pronounced than those measured for another anionic lipid, sulfatide. To delineate the potential PS binding sites on CT 4 and estimate their relative affinities, a series of computer simulations was performed for the systems containing the head group of PS and different spatial models of CT 4 in aqueous solution and in an implicit membrane. This was done using an original hybrid computational protocol implementing docking, Monte Carlo and molecular dynamics simulations. As a result, at least three putative PS-binding sites with different affinities to PS molecule were delineated. Being located in different parts of the CT molecule, these anion-binding sites can potentially facilitate and modulate the multi-step process of the toxin insertion into lipid bilayers. This feature together with the diverse binding affinities of the sites to a wide variety of anionic targets on the membrane surface appears to be functionally meaningful and may adjust CT action against different types of cells.

Antioxidants: promising neuroprotection against cardiotoxin-4b-induced cell death which triggers oxidative stress with early calpain activation

Cardiotoxin-4b (CTX-4b), isolated from Naja naja sputatrix venom, shows lethality in several cell types. Employing murine primary cortical neurons, this study was undertaken to investigate the molecular mechanisms of CTX-4b in the induction of neuronal death. CTX-4b induced a dose- and time-dependent neuronal death. Strong induction of calpains as early as 4h post-CTX-4b 75 nM treatment was detected in neurons with negligible caspase 3 activation. For the first time in cultured murine primary cortical neurons, it was noted that CTX-4b-mediated cell death triggered oxidative stress with an increase in reactive oxygen species (ROS) levels, and that application of antioxidants showed effective attenuation of cell death. Taken together, these results indicate that CTX-4b-mediated neuronal death is associated with (i) early calpain activation and (ii) oxidative stress. Most importantly, antioxidants have proved to be a promising therapeutic avenue against CTX-4b-induced neuronal death.

Most cited papers in Toxicon

Citation of a published work is one of the parameters considered in the analysis of relevance and importance of scientific contributions. In 2002, for the first time the Impact Factor of Toxicon has risen above 2.0, placing it at the 17th position among 76 journals in the 'toxicology' field. The aim of this article was to identify the most cited articles in Toxicon, that have contributed to the steady increase of its Impact Factor. The number of citations, complete reference and type of all documents appearing in Toxicon in the period 1963-2003 were retrieved from the ISI Web-of-Science homepage. The documents retrieved were sorted by the number of citations received. A 'citation index', defined as the number of citations divided by the number of years since publication, was calculated for each document. It was clearly seen that reviews in Toxicon received 4.4-fold more citations than articles. Unexpectedly, it was found that recent papers were proportionally more cited than old ones. A decrease in the proportion of papers dealing on 'snake*' through out the period and the broadened range of subjects of the most cited papers recently published in Toxicon reflects an increased 'visibility' in other fields of toxinology. Research on plant toxins gained its own space in Toxicon with newer publications showing high citation indexes. It can be postulated that these facts helped to increase Toxicon's Impact Factor from 1.248 in 1999 to 2.003 in 2002. With the increased number of issues in Toxicon as well as publications of subject-dedicated volumes containing mostly reviews, the Impact Factor of Toxicon is expected to keep rising in the near future.

Cytotoxic potency of cardiotoxin from Naja sputatrix: development of a new cytolytic assay

The possible involvement of specific regions/loops of cardiotoxin from Naja sputatrix venom in mediating its cytolytic activity is evaluated using a new cytolytic assay. In this assay, the amount of chloramphenicol acetyltransferase (CAT) that is released upon lysis of the cellular membranes by the cytotoxin has been measured as an index of cytolysis. This newly developed CAT system is more sensitive than the traditional haemolysis method utilizing red blood cells or the lactate dehydrogenase assay for cytolysis. Series of chimaeric toxin molecules have been constructed by swapping the loops between highly hydrophilic neurotoxin and highly hydrophobic cardiotoxin molecules from Naja sputatrix, which are known to exhibit structural similarity (three-finger conformation) but to have different functional properties. Comparison of the cytolytic activities of the recombinant chimaeric toxins demonstrated the possible involvement of all three loops of cardiotoxin in its cytolytic potency. However, the first two loops of the protein appear to make the major contribution to its lytic activity. cDNAs encoding cardiotoxin and the chimaeric toxins, when expressed in transfected cultured Chinese hamster ovary cells, resulted in cell lysis, indicating that these cDNAs can be developed as useful cytolytic agents.

NMR structure of bucandin, a neurotoxin from the venom of the Malayan krait (Bungarus candidus)

A high-resolution solution structure of bucandin, a neurotoxin from Malayan krait (Bungarus candidus), was determined by (1)H-NMR spectroscopy and molecular dynamics. The average backbone root-mean-square deviation for the 20 calculated structures and the mean structure is 0.47 A (1 A=0.1 nm) for all residues and 0.24 A for the well-defined region that spans residues 23-58. Secondary-structural elements include two antiparallel beta-sheets characterized by two and four strands. According to recent X-ray analysis, bucandin adopts a typical three-finger loop motif and yet it has some peculiar characteristics that set it apart from other common alpha-neurotoxins. The presence of a fourth strand in the second antiparallel beta-sheet had not been observed before in three-finger toxins, and this feature was well represented in the NMR structure. Although the overall fold of the NMR structure is similar to that of the X-ray crystal structure, there are significant differences between the two structures that have implications for the pharmacological action of the toxin. These include the extent of the beta-sheets, the conformation of the region spanning residues 42-49 and the orientation of some side chains. In comparison with the X-ray structure, the NMR structure shows that the hydrophobic side chains of Trp(27) and Trp(36) are stacked together and are orientated towards the tip of the middle loop. The NMR study also showed that the two-stranded beta-sheet incorporated in the first loop, as defined by residues 1-22, and the C-terminus from Asn(59), is probably flexible relative to the rest of the molecule. On the basis of the dispositions of the hydrophobic and hydrophilic side chains, the structure of bucandin is clearly different from those of cytotoxins.

Membrane binding motif of the P-type cardiotoxin

Carditoxins (CTXs) from cobra snake venoms, the basic 60-62 residue all-beta sheet polypeptides, are known to bind to and impair the function of cell membranes. To assess the membrane induced conformation and orientation of CTXs, the interaction of the P-type cardiotoxin II from Naja oxiana snake venom (CTII) with perdeuterated dodecylphosphocholine (DPC) was studied using ( 1 )H-NMR spectroscopy and diffusion measurements. Under conditions where the toxin formed a well-defined complex with DPC, the spatial structure of CTII with respect to the presence of tightly bound water molecules in loop II, was calculated using the torsion angle dynamics program DYANA. The structure was found to be similar, except for subtle changes in the tips of all three loops, to the previously described "major" form of CTII in aqueous solution illustrated by the "trans" configuration of the Val7-Pro8 peptide bond. No "minor" form with the "cis" configuration of the above bond was found in the micelle-bound state. The broadening of the CTII backbone proton signals by 5, 16-doxylstearate relaxation probes, together with modeling based on the spatial structure of CTII, indicated a periphery mode of binding of the toxin molecule to the micelle and revealed its micelle interacting domain. The latter includes a hydrophobic region of CTII within the extremities of loops I and III (residues 5-11, 46-50), the basement of loop II (residues 24-29,31-37) and the belt of polar residues encircling these loops (lysines 4,5,12,23,50, serines 11,46, histidine 31, arginine 36). It is suggested that this structural motif and the mode of binding can be realized during interaction of CTXs with lipid and biological membranes.

Two forms of cytotoxin II (cardiotoxin) from Naja naja oxiana in aqueous solution: spatial structures with tightly bound water molecules

1H-NMR spectroscopy data, such as NOE intraprotein and (bound water)/protein contacts, 3J coupling constants and deuterium exchange rates were used to determine the in-solution spatial structure of cytotoxin II from Naja naja oxiana snake venom (CTII). Exploiting information from two 1H-NMR spectral components, shown to be due to cis/trans isomerization of the Val7-Pro8 peptide bond, spatial structures of CTII minor and major forms (1 : 6) were calculated using the torsion angle dynamics algorithm of the DYANA program and then energy refined using the FANTOM program. Each form, major and minor, is represented by 20 resulting conformers, demonstrating mean backbone rmsd values of 0.51 and 0.71 A, respectively. Two forms of CTII preserve the structural skeleton as three large loops, including two beta-sheets with bend regions, and demonstrate structural differences at loop I, where cis/trans isomerization occurs. The CTII side-chain distribution constitutes hydrophilic and hydrophobic belts around the protein, alternating in the trend of the three main loops. Because of the Omega-shaped backbone, formed in participation with two bound water molecules, the tip of loop II bridges the tips of loops I and III. This ensures the continuity of the largest hydrophobic belt, formed with the residues of these tips. Comparison revealed pronounced differences in the spatial organization of the tips of the three main loops between CTII and previous structures of homologous cytotoxins (cardiotoxins) in solution.

Snake venom cardiotoxins-structure, dynamics, function and folding

Snake cardiotoxins are highly basic (pI > 10) small molecular weight (approximately 6.5 kDa), all beta-sheet proteins. They exhibit a broad spectrum of interesting biological activities. The secondary structural elements in these toxins include antiparallel double and triple stranded beta-sheets. The three dimensional structures of these toxins reveal an unique asymmetric distribution of the hydrophobic and hydrophilic amino acids. The 3D structures of closely related snake venom toxins such as neurotoxins and cardiotoxin-like basic proteins (CLBP) fail to show similar pattern(s) in the distribution of polar and nonpolar residues. Recently, many novel biological activities have been reported for cardiotoxins. However, to-date, there is no clear structure-function correlation(s) available for snake venom cardiotoxins. The aim of this comprehensive review is to summarize and critically evaluate the progress in research on the structure, dynamics, function and folding aspects of snake venom cardiotoxins.

Effects of a toxin from the mucus of the Caribbean sea anemone (Bunodosoma granulifera) on the ionic currents of single ventricular mammalian cardiomyocytes

The effects were studied of a toxin (Bainh) isolated from the secretion of the Caribbean sea anemone Bunodosoma granulifera on electrical and mechanical activities of rat ventricular muscle. The effects on the ionic currents of single rat and dog ventricular cardiomyocytes were studied using the whole-cell recording patch-clamp technique. In the concentration range from 1 to 10 mg/ml, Bainh increased the force of contraction and induced an increase in action potential duration of ventricular multicellular preparations. In single cardiomyocytes, at concentrations up to 10 mg/ml Bainh showed no significant effects on the sodium current. However, at 0.5-1 mg/ml it increased the L-type Ca current (ICaL) by 25-50%. This increase in ICaL was not voltage dependent and was reversible after washout. The transient outward current was not significantly affected by Bainh (1-10 mg/ml). In this concentration range, Bainh markedly (approximately 75%) increased the inward-going rectifier current, IKI. This effect that was not voltage dependent and was fully reversible upon returning to control solution. It is suggested that these effects on ionic currents could explain the positive inotropic action of Bainh on cardiac multicellular preparations.

Membrane structure and dynamics as viewed by solid-state NMR spectroscopy

The purpose of the present study is the investigation of the structure and dynamics of biological membranes using solid-state nuclear magnetic resonance (NMR) spectroscopy. Two approaches are used in our laboratory. The first involves the measurement of high-resolution 13C and 1H spectra obtained by the magic angle spinning (MAS) technique while the second approach involves the measurement of 31P and 2H powder spectra in static samples. This paper will present some recent results obtained by high-resolution solid-state 1H NMR on the conformation of gramicidin A incorporated in a phosphatidylcholine bilayers. More specifically, we were able to observe changes in the gramicidin spectra as a function of the cosolubilization solvent initially used to prepare the samples. The interaction between lipid bilayers and an anticancer drug derived from chloroethylurea was also investigated using proton NMR spectroscopy. Finally, we have studied the interaction between cardiotoxin, a toxic protein extracted from snake venom, and negatively charged lipid bilayers using 31P solid-state NMR spectroscopy.

Model of interaction between a cardiotoxin and dimyristoylphosphatidic acid bilayers determined by solid-state 31P NMR spectroscopy

The interaction of cardiotoxin IIa, a small basic protein extracted from Naja mossambica mossambica venom, with dimyristoylphosphatidic acid (DMPA) membranes has been investigated by solid-state 31P nuclear magnetic resonance spectroscopy. Both the spectral lineshapes and transverse relaxation time values have been measured as a function of temperature for different lipid-to-protein molar ratios. The results indicate that the interaction of cardiotoxin with DMPA gives rise to the complete disappearance of the bilayer structure at a lipid-to-protein molar ratio of 5:1. However, a coexistence of the lamellar and isotropic phases is observed at higher lipid contents. In addition, the number of phospholipids interacting with cardiotoxin increases from about 5 at room temperature to approximately 15 at temperatures above the phase transition of the pure lipid. The isotropic structure appears to be a hydrophobic complex similar to an inverted micellar phase that can be extracted by a hydrophobic solvent. At a lipid-to-protein molar ratio of 40:1, the isotropic structure disappears at high temperature to give rise to a second anisotropic phase, which is most likely associated with the incorporation of the hydrophobic complex inside the bilayer.

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.

Proton NMR studies of the structural and dynamical effect of chemical modification of a single aromatic side-chain in a snake cardiotoxin. Relation to the structure of the putative binding site and the cytolytic activity of the toxin

This paper presents the comparative comprehensive analysis of NMR structural parameters (NOEs, scalar coupling, chemical shifts) of toxin gamma, a cardiotoxin isolated from the venom of Naja nigricollis, and three chemical derivatives, i.e. the 2-nitrophenylsulphonyl (NPS)-Trp11, 3-nitro-Tyr22 and 3-nitro-Tyr51 derivatives. In previous work, the chemical modifications of single side chains have suggested that these aromatic residues, in association with several lysine residues, contributed to the cytotoxicity of toxin gamma. Analysis of these results based on the refined solution structure of the toxin has resulted in the proposal of a conserved phospholipid binding site through which cardiotoxins are likely to interact with the membrane of target cells. The present work shows that modifications of either the tryptophan residue or the tyrosine residues, which are within or near the proposed binding site, have no influence on the three-dimensional structure of the protein. On the other hand, the proton exchange study of the backbone amides indicates that the structural core of the protein is destabilized in the three derivatives. This corresponds to a decrease of the overall stability of the protein as indicated by the comparative solvent denaturation study of the unmodified toxin gamma and the Trp11 derivative. More specifically, the dynamics of the three-stranded beta sheet, a part of the structural core, are highly perturbed by the chemical modifications. This sheet was previously proposed as a part of the phospholipid binding site of cardiotoxins. The dynamical perturbation of this site appears to be correlated with the decrease in toxicity of the chemical derivatives.

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.

Refined three-dimensional solution structure of a snake cardiotoxin: analysis of the side-chain organization suggests the existence of a possible phospholipid binding site

The solution structure of toxin gamma (60 residues, 4 disulfides) from Naja nigricollis was determined by proton nmr and molecular modeling with DIANA and X-PLOR. The structures were calculated using 489 distance and 81 dihedral angle constraints. The average atomic rms deviation between the nine refined structures and the average structure is 0.118 nm for the backbone atoms. Toxin gamma has an overall folding consisting of three loops stabilized by the four disulfides and forming a two- and a three-stranded beta-sheet (loop I and loops II, III, respectively). The same type of folding has been observed for two homologous cardiotoxins. The very close similarity of the solution structure of toxin gamma and the crystal structure of toxin VII4 includes details of the topological arrangement of numerous side chains. Among these are the conserved residues K12, K18, K35, and Y22, known to be critical for the cytolytic activity of toxin gamma. A cluster of hydrophobic side chains organized around Y22 is found on one side of the three-stranded beta-sheet and is spatially close to a group of three lysines (K12, K18, K35). The side chains of these lysines form a cationic site that can accommodate the binding of a phosphate ion as found in the crystal structure of toxin VII4. The hydrophobic cluster constitutes a possible binding site for the hydrophobic moiety of phospholipids. Together with the complementary cationic site, this hydrophobic surface can form a conserved site by which cardiotoxins bind to membrane phospholipids.

Determination of the nuclear-magnetic-resonance solution structure of cardiotoxin CTX IIb from Naja mossambica mossambica

The NMR structure of cardiotoxin CTX IIb from Naja mossambica mossambica in aqueous solution was determined from a total of 593 nuclear Overhauser enhancement distance constraints and 135 dihedral angle constraints, which were collected using two-dimensional homonuclear 1H-NMR experiments. Structure calculations were performed with the program DIANA, using the redundant dihedral angle constraints strategy for improved convergence, followed by restrained energy minimization with the program FANTOM and a modified version of the program AMBER. The CTX IIb structure is represented by a group of 20 conformers with an average root-mean-square deviation relative to the mean solution structure of 0.072 nm for the backbone atoms, and 0.116 nm for all heavy atoms. The molecular structure of CTX IIb is characterized by a three-stranded beta-sheet made up of residues 20-26, 32-39 and 48-54, and a two-stranded beta-sheet composed of residues 1-5 and 10-14. A cluster of four disulfide bonds, 3-21, 14-38, 42-53 and 54-59, form the core of the molecule and crosslink the individual polypeptide strands. The NMR structure is similar to the previously reported X-ray crystal structure of the cardiotoxin CTX VII4 from the same species. Differences between the two structures were noted in the tips of the two loops formed by residues 6-9 and 27-31, which connect the beta-strand 1-5 with 10-14, and 20-26 with 32-39, respectively. For these loops the NMR data also indicate significantly increased dynamic disorder in the solution structure. These observations are discussed with respect to earlier suggestions by others that these two loops are essential structural elements for function and specificity of a wide variety of homologous toxins.

Characterization by infrared spectroscopy of the interaction of a cardiotoxin with phosphatidic acid and with binary mixtures of phosphatidic acid and phosphatidylcholine

The effect of cardiotoxin IIa from Naja mossambica mossambica, a small basic protein extracted from snake venom, on dimyristoylphosphatidic acid (DMPA) and on equimolar mixtures of DMPA and dimyristoylphosphatidylcholine (DMPC) has been studied by Fourier transform infrared spectroscopy. The interaction of cardiotoxin with DMPA dispersions decreases both the cooperativity of the phase transition of the lipid and the molecular order of the lipid acyl chains in the gel phase. This effect increases with the proportion of the toxin in the complexes and leads to the total abolition of the phase transition of DMPA at a lipid-to-protein molar ratio of 5. Small-angle X-ray results demonstrate that the structure of the lipid-protein complexes is poorly ordered and gives rise to broad diffusion peaks rather than to well-resolved diffraction patterns. Infrared spectra of oriented cardiotoxin-DMPA films show that the protein is not homogeneously oriented with respect to the bilayer surface. The destabilization of the gel-phase structure of DMPA by cardiotoxin also results in a deeper water penetration in the interfacial region of the lipid since more carbonyl ester groups appear to be hydrogen bonded in the presence of the toxin. The infrared results on the phosphate group vibrations also indicate clearly that the basic residues of cardiotoxin interact strongly with the phosphate group of DMPA that becomes partly ionized at a pH as low as 6.5. The results obtained on the interaction of cardiotoxin with an equimolar mixture of DMPA and DMPC clearly demonstrate the ability of this toxin to induce lateral phase separation in this mixture with one phase containing DMPA-rich domains perturbed by cardiotoxin while the second phase is composed of regions enriched in DMPC. Comparison of the results of the current study with those obtained on other basic proteins and polypeptides suggests that charge-induced phase separation occurs only when the charge density on certain regions of the protein structure is high enough to lead to efficient electrostatic interactions with anionic phospholipids. This condition occurs only when the conformation of the protein or polypeptide is well-ordered at the lipid interface.

Binding of cytotoxin P4 from Naja nigricollis nigricollis to B16F10 melanoma and WEHI-3B leukemia cells

Cobra venoms cause irreversible destruction of cells cultured in vitro [1,2]. The venom of Naja nigricollis nigricollis possessed the most potent cytotoxic activity towards B16F10 melanoma cells among various examined venoms [2]. The main cytotoxic factor (P4) isolated from this venom showed preferential activity on tumor cell lines and caused lysis at concentrations of 10(-7) M (0.8-1 micrograms/ml) [3]. The present study examined the binding of cytotoxin P4 to melanoma B16F10 and WEHI-3B leukemia cell lines and found that, like cytotoxicity, it depended on concentration, temperature and incubation time. Cytotoxin concentrations that elicited no apparent damage to cells during the first hour of incubation caused lysis after a longer period of incubation, suggesting that a critical number of bound molecules is required in order to cause cell death. Bivalent ions, such as Mg2+, Ca2+ or Sr2+, which decreased binding to the cells also inhibited cytotoxicity. Competition experiments as well as the displacement of 75% of the bound radiolabelled cytotoxin with 'cold' cytotoxin, suggest the presence of specific binding sites for the toxin in the examined tumor cells. The non-specific binding of the cytotoxin P4 to sea urchin ova and sperm cells without affecting their fertility, even at high concentrations of 10(-5) M, indicates that the specific binding to cells is probably a necessary condition for cell lysis.

Characterization of recombinant antistasin secreted by Saccharomyces cerevisiae

Secretion from recombinant yeast was used as a potential source of large quantities of the leech protein antistasin (ATS), a potent and highly specific inhibitor of the serine protease coagulation factor Xa. Mature recombinant ATS (r-ATS) is obtained after intracellular cleavage by the yscF protease of the mating factor alpha-1 pre-proleader from the fusion protein at the Lys-Arg sequence junction. Production levels are relatively low (ca. 1 mg/liter). Purification of the secreted product from a complex growth medium involved cell removal by microfiltration and diafiltration, cation-exchange capture and concentration on S-Sepharose Fast Flow, C-4 reverse-phase high-performance liquid chromatography (RP-HPLC), and HPLC cation-exchange chromatography step, and RP-HPLC concentration and desalting. The process was scaled up from the 16- to the 250-liter level with a corresponding increase in amount of r-ATS. From the 250-liter fermentation two major forms, r-ATS-I and r-ATS-II, distributed approximately 60:40, and a minor form, r-ATS-minor (ca. 1% of the purified r-ATS), were characterized. Limited N-terminal sequence analysis by Edman degradation indicated that r-ATS-I has the predicted mature N-terminus starting with Gln, that r-ATS-II is N-terminally blocked with pyroglutamate, and that r-ATS-minor is an incompletely processed form. RP-HPLC, hydrophilic-interaction HPLC, cation-exchange HPLC analysis, and electrophoresis results are consistent with the differences observed by sequencing. Preliminary in vitro characterization by intrinsic Ki determination for factor Xa inhibition indicated that the yeast r-ATS forms are indistinguishable from each other as well as from r-ATS expressed by the insect baculovirus host-vector system. Nevertheless, r-ATS-I and r-ATS-II appear less potent than insect-derived r-ATS in the activated partial thromboplastin time clotting assay. Further characterization indicated that C-terminal cleavage at Pro-116 had occurred in r-ATS-I and r-ATS-II as well as oxidation of methionine residues to methionine sulfoxide. The possible role of the C-terminus in inhibition of the prothrombinase complex is discussed.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Hemolysis of erythrocytes and fluorescence polarization changes elicited by peptide toxins, aliphatic alcohols, related glycols and benzylidene derivatives

Hemolysis rates of human erythrocytes induced by C2 and C8-C14 straight chain 1-alkanols, 1,2-alkanediols and the corresponding benzylidene derivatives (benzaldehyde acetals) have been studied and compared with hemolysis rates obtained by three peptide toxins. The peak of activity occurs at C12 for the alkanols and glycols and at C10 for the benzylidene derivatives. The most active compound is 1-dodecanol, followed by 1,2-dodecanediol and the C10 benzylidene acetal, which show 50% hemolysis at 15, 99 and 151 microM, respectively, at 37 degrees C. A few lysolecithins and longer chain cis-unsaturated alcohols were studied for comparison purposes, and were found to be more active than 1-dodecanol. The most active were the 16:0 lysolecithin and cis-9-tetradecene-1-ol, which gave 50% hemolysis at concentrations of 2.8 and 5.6 microM respectively. The hemolytic activities of 1-dodecanol, 1,2-dodecanediol and the C10 benzylidene acetal were compared to activities of Pyrularia thionin and melittin with cow, horse, sheep, pig and human erythrocytes. Whereas the peptide toxins showed clear specificity for human erythrocytes, no selectivity was shown by any of the other compounds tested. Addition of the thionin or Naja naja kaouthia cardiotoxin to erythrocyte ghosts caused a slight but reproducible increase in the order of the phospholipid bilayer, as measured with the fluorescent probe NBD-PC. Cardiotoxin gave a greater response than did the P thionin, and extensively iodinated P thionin gave a smaller change than did P thionin. Similar results were obtained with melittin, but this peptide gave a markedly greater response than all other peptides. Addition of dodecanol or the C10 benzylidene acetal caused a marked increase in membrane fluidity. All of these data indicate that the organic compounds interact directly with and are incorporated nonspecifically into the membrane lipid bilayer, but the peptide toxins interact specifically with some component on the surface of the membrane, either a protein or specific phospholipid domain, followed by insertion into the membrane and decreasing phospholipid movement.

Do cardiotoxins possess a functional site? Structural and chemical modification studies reveal the functional site of the cardiotoxin from Naja nigricollis

Examination of the literature has revealed that regarding the amino acid sequences, cardiotoxins constitute a family of homogeneous compounds. In contrast, cardiotoxins appear heterogeneous as far as their biological and spectroscopic properties are concerned. As a result, comparison between these molecules with a view to establishing structure-activity correlations is complicated. We have therefore reviewed recent works aiming at identifying the functional site of a defined cardiotoxin, ie toxin gamma from the venom of the spitting cobra Naja nigricollis. The biological and structural properties of toxin gamma are first described. In particular, a model depicting the 3-dimensional structure of the toxin studied by NMR spectroscopy is proposed. The toxin polypeptide chain is folded into 3 adjacent loops rich in beta-sheet structure connected to a small globular core containing the 4 disulfide bonds. A number of derivatives chemically modified at a single aromatic or amino group have been prepared. The structure of each derivative was probed by emission fluorescence, circular dichroism and NMR spectroscopy. Also tested was the ability of the derivatives to kill mice, depolarize excitable cell membranes and lyse epithelial cells. Modification of some residues in the first loop, in particular Lys-12 and at the base of the second loop substantially affected biological properties, with no sign of concomitant structural modifications other than local changes. Modifications in other regions much less affected the biological properties of the toxin. A plausible functional site for toxin gamma involving loop I and the base of loop II is presented. It is stressed that the functional site of other cardiotoxins may be different.

Delineation of the functional site of a snake venom cardiotoxin: preparation, structure, and function of monoacetylated derivatives

Toxin gamma, a cardiotoxin from the venom of the cobra Naja nigricollis, was modified with acetic anhydride, and the derivatives were separated by cation-exchange and reverse-phase chromatography. Nine monoacetylated derivatives were obtained, and those modified at positions 1, 2, 12, 23, and 35 were readily identified by automated sequencing. The overall structure of toxin gamma, composed of three adjacent loops (I, II, and III) rich in beta-sheet, was not affected by monoacetylation as revealed by circular dichroic analysis. Trp-11, Tyr-22, and Tyr-51 fluorescence intensities were not affected by modifications at Lys-12 and Lys-35, whereas Trp-11 fluorescence intensity slightly increased when Lys-1 and Lys-23 were modified. The cytotoxic activity of toxin gamma to FL cells in culture was unchanged after modification at positions 1 and 2, whereas it was 3-fold lower after modification at Lys-23 and Lys-35. The derivative modified at Lys-12 was 10-fold less active than native toxin. Using two isotoxins, we found that substitutions at positions 28, 30, 31, and 57 did not change the cytotoxic potency of toxin gamma. A good correlation between cytotoxicity, lethality, and, to some extent, depolarizing activity on cultured skeletal muscle cells was found. In particular, the derivative modified at Lys-12 always had the lowest potency. Our data show that the site responsible for cytotoxicity, lethality, and depolarizing activity is not diffuse but is well localized on loop I and perhaps at the base of loop II. This site is topographically different from the AcChoR binding site of the structurally similar snake neurotoxins.

Cardiotoxin VII4 from Naja mossambica mossambica. The refined crystal structure

The crystal structure of cardiotoxin VII4 from Naja mossambica mossambica was refined to 2.5 A resolution. Fifty ordered solvent sites were localized and included in the refinement. The final R factor is 0.197 (lambda/(2sin theta) less than 5 A; F greater than 3 sigma). The three-dimensional structure is characterized by two beta-sheets. Of particular interest is the two-stranded beta-sheet in the N-terminal region. This shows a large right-handed twist and, though strongly connected to the core of the molecule, and in particular to the C-terminal end, protrudes out of the bulk of the molecule. The segment of four amino acid residues connecting the two strands of this sheet is particularly exposed. It contains an invariant proline residue that has probably an important structural role, and is completely hydrophobic. Two other conserved hydrophobic zones were identified; the largest extends over the second and third loops, on one side only of the molecule. All side-chains of invariant hydrophobic character (except proline residues) belong to one of these three zones. Also discussed are the dimeric assembly and the rather loose packing in the crystal. The three-dimensional structure is compared with that of short and long alpha-neurotoxins. Comparison with two-dimensional nuclear magnetic resonance results on the 68% homologous cardiotoxin CT X IIb shows an excellent overall agreement. A few differences are probably genuine.

Characterization of a cytotoxin-like basic protein from the cobra (Naja naja naja) venom

A cytotoxin-like basic protein has been isolated from the venom of the nominate race of cobra (Naja naja naja from Pakistan) by a single step of high-performance liquid chromatography. The primary structure was determined and consists of 62 amino acid residues in a single polypeptide chain. It is highly similar to that of the cytotoxin-like basic proteins isolated from other Naja species, but differs in two of the SS-loop structures from that of cytotoxins.

Adsorption of plasma proteins onto anticoagulant polystyrene derivatives: a fluorescence study

Quenching of fluorescence was used to monitor adsorption of thrombin (T), antithrombin (AT) and their inactive complex (T-AT) onto three anticoagulant biomaterials made of polystyrene beads bearing the functional groups of heparin. An adsorption capacity of 0.12 mumol of T per mg of polymer allowed the formation of a monolayer of protein at the polymer surface. An affinity constant of 3 x 10(7) l.mol-1 between thrombin and polymer was estimated, whatever the polymer used. The affinity of T-AT was similar although weaker. Desorption of proteins from the polymeric interface by means of polycations (polybrene and polylysine) showed that the inactive complex T-AT is more quantitatively and easily released than thrombin.

Localization of the toxic site of Naja mossambica cardiotoxins: small synthetic peptides express an in vivo lethality

Cardiotoxins are small basic proteins which cause heart failure when they are injected in vivo. In order to better understand their molecular mode of action, short peptides designed on the model of the first loop of the molecule of cardiotoxin IV from Naja mossambica mossambica venom have been synthetized by the solid-phase procedure of Merrifield. These peptides express lethality in mouse when they are injected intravenously. Taking into account the respective molecular weights, they are 3.5 to 5% as toxic as the cardiotoxin. Furthermore, the symptomatology they induce is undistinguishable from that induced by cardiotoxins. These results strongly support our previous hypothesis that the first loop of the molecule is the toxic site of cardiotoxins.

Role of tyrosine and tryptophan residues in the structure-activity relationships of a cardiotoxin from Naja nigricollis venom

This paper is an attempt to localize the critical area determining toxicity in a snake cardiotoxin. Toxin gamma is a single-chain polypeptide of 60 amino acids, which has been isolated from the venom of the African spitting cobra, Naja nigricollis. Three aromatic residues, namely, Trp-11, Tyr-22, and Tyr-51, have been individually modified by chemical means. The structure of the native toxin and of each derivative has been carefully investigated by circular dichroism, fluorescence, proton magnetic resonance spectroscopy, and two specific monoclonal antibodies. None of the chemical modifications alters the overall structure of the toxin, which in all cases remains folded into three adjacent loops (I, II, and III) rich in beta-pleated sheet emerging from a small globular region containing four disulfide bridges. A number of subtle changes, however, have been detected in the structure of each derivative compared with that of the native toxin. In particular, nitration of Tyr-51 provoked a structural perturbation in the globular region. Nitration of Tyr-22 induces a more substantial change in the beta-sheet area of the molecule. Thus, the strong inter-ring NOE that is observed in the native toxin between Tyr-22 and Tyr-51 vanishes in the Tyr-22 derivative, and significant changes are observed in the globular region. In contrast, no alteration of the beta-sheet structure of loops II and III has been detected after modification of Trp-11. All changes observed for this derivative remain located in the vicinity of the indole side chain of Trp-11 in loop I. The biological consequences of the modifications were measured: the lethal potency in vivo in mice and the cytotoxic activities in vitro on FL-cells. Lethal activities correlate with cytotoxicity: Tyr-51 modified toxin is equally potent as native toxin, whereas Tyr-22 and Trp-11 derivatized toxins are characterized by substantially lesser activities, the Trp-11 derivatized toxin being the least potent. We conclude that (1) Tyr-51 is not involved in the functional site of the toxin, although it is in interaction with the core of the molecule, (2) Tyr-22 may play a dual structural and functional role, and (3) Trp-11 is in, or in close proximity to, the functional site of the toxin. These data indicate the importance of loop I in determining toxicity of the cardiotoxin.

Crystal structure of a snake venom cardiotoxin

Cardiotoxin VII4 from Naja mossambica mossambica crystallizes in space group P61 (a = b = 73.9 A; c = 59.0 A) with two molecules of toxin (molecular mass = 6715 Da) in the asymmetric unit. The structure was solved by using a combination of multiple isomorphous replacement and density modification methods. Model building and least-squares refinement led to an agreement factor of 27% for a data set to 3-A resolution prior to any inclusion of solvent molecules. The topology of the molecule is similar to that found in short and long snake neurotoxins, which block the nicotinic acetylcholine receptor. Major differences occur in the conformation of the central loop, resulting in a change in the concavity of the molecule. Hydrophobic residues are clustered in two distinct areas. The existence of stable dimeric entities in the crystalline state, with the formation of a six-stranded antiparallel beta sheet, may be functionally relevant.

Mode of inhibitory action of melittin on Na+-K+-ATPase activity of the rat synaptic membrane

The effects of melittin from bee venom, cardiotoxin from Formosan cobra venom, and ouabain on Na+-K+-ATPase activity of the synaptic membrane isolated from rat cerebral cortex were studied. Melittin was the most potent in inhibiting Na+-K+-ATPase activity. Mg2+-ATPase was less susceptible than Na+-K+-ATPase to the inhibitory action of toxins. High K+ (30 mM) reversed the inhibitory action of melittin on Na+-K+-ATPase but did not affect that of cardiotoxin. A comparison between the effects of ouabain and melittin was studied, using double-reciprocal plots of Na+-K+-ATPase activity against K+. It was shown that both were competitive with K+ for binding to the K+ site. Moreover, a median-effect plot revealed that ouabain and melittin antagonized each other when inhibiting Na+-K+-ATPase. Phosphatidylcholine (PC) was the only one of the phospholipids tested capable of protecting Na+-K+-ATPase from the inhibitory action of melittin but not that of ouabain. However, the inhibitory action of cardiotoxin on this enzyme was decreased by phosphatidylserine and sphingomyelin, in addition to PC. All of these findings suggest that the melittin polypeptide potently inhibits Na+-K+-ATPase, possibly by binding to the K+ 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.