Cytolytic Activity and Molecular Feature of Cardiotoxin and Cardiotoxin-like Basic Protein: The Electrostatic Potential Field Is an Important Factor for Cell Lytic Activity

BACKGROUND/AIM

Cardiotoxin (CT) is a well-known cell lytic protein and has been purified from cobra venom. Cardiotoxin-like basic protein (CLBP) has two amino acid insertions and does not exhibit cell lytic activity. The molecular features of these CT family proteins were examined in the present study using molecular modeling and molecular simulation techniques.

MATERIALS AND METHODS

Molecular models of CT and CLBP were constructed based on the X-ray data of Naja mossambica mossambica CT VII4 (Protein Data Bank ID: 1CDT). The structural features of these models were examined using molecular orbital and electrostatic potential parameters.

RESULTS

The stereo-hydrophobicities and molecular torsions of CT and CLBP, which are indexes of structural features, were similar. Electrostatic potential fields (ESP) differed between CT and CLBP and this was considered one of the critical factors in molecular titer.

CONCLUSION

The distribution of ESP fields may affect the cytolytic activity of the CT family.

Structures of heparin-derived tetrasaccharide bound to cobra cardiotoxins: heparin binding at a single protein site with diverse side chain interactions

Cobra cardiotoxins (CTXs) are three-fingered polypeptides with positively charged domains that have been shown to bind to anionic ligands of snake venom citrate, glycosaminoglycans, sulfoglycosphingolipid, and nucleotide triphosphate with various biochemical effects including toxin dimerization, cell surface retention, membrane pore formation, cell internalization and blocking of enzymatic activities of kinase and ATPase. The reported anionic binding sites, however, are found to be different among different CTX homologues for potentially different CTX activities. Herein, by NMR studies of the binding of inorganic phosphate, dATP (stable form of ATP), and heparin-derived tetrasaccharide to Naja atra CTX A1, a novel CTX molecule exhibiting in vivo necrotic activity on skeletal muscle, we demonstrate that diverse ligands binding to CTXs could also occur at a single protein site with flexible side chain interactions. The flexibility of such an interaction is also illustrated by the available heparin-CTX A3 complex structures with different heparin chain lengths binding at the same site. Our results provide a likely structural explanation on how the interaction between heparan sufate and proteins depends more on the overall charge cluster organization rather than on their fine structures. We also suggest that the ligand binding site of CTX homologues can be fine-tuned by nonconserved residues near the binding pocket because of their flexible side chain interaction and dimerization ability, even for the rigid CTX molecules tightened by four disulfide bonds.

Involvement of c-jun N-terminal kinase in G2/M arrest and caspase-mediated apoptosis induced by cardiotoxin III (Naja naja atra) in K562 leukemia cells

Cardiotoxin III (CTX III), a basic polypeptide with 60 amino acid residues isolated from Naja naja atra venom, may have a potentiality as a structural template for rational drug design in killing cancer cells. Treatment of K562 cells with 0.3 microM of CTX III resulted in G2/M phase cell cycle arrest that was associated with a marked decline in protein levels of G2/M regulatory proteins including cyclin A, cyclin B1, Cdk2 and Cdc25C. In contrast to no effect on the phosphorylation of ERK, p38 MAPK and Akt, an activation of JNK was noted when K562 cells were exposed to CTX III. CTX III-mediated G2/M phase arrest and apoptosis were reduced by treatment with the JNK-specific inhibitor SP600125, but not by ERK and p38MAPK inhibitors. Further investigation showed that the specific JNK inhibitor, SP600125, reduced the activation of caspase-3, caspase-9, and reversed the decline in the expression of cyclin B1. Taken together, our data show for the first time that JNK, but not ERK, p38MAPK or Akt signaling, plays an important role in CTX III-mediated G2/M arrest and apoptosis in K562 cancer cells.

Purification, partial characterization, crystallization and preliminary X-ray diffraction of a novel cardiotoxin-like basic protein from Naja naja atra (South Anhui) venom

A novel cardiotoxin-like basic protein was isolated from the venom of the Chinese cobra (Naja naja atra) from the south of Anhui in China. The protein inhibits the expression of vascular endothelial growth factor and basic fibroblast growth factor in human lung cancer cell line H1299 and induces the haemolysis of rabbit erythrocytes under low-lecithin conditions. After a two-step chromatographic purification, the resultant 7 kDa protein was crystallized by the hanging-drop vapour-diffusion method at room temperature. A complete data set was collected to 2.35 A resolution using an in-house X-ray diffraction system. The crystal belongs to space group P4(1)2(1)2, with unit-cell parameters a = b = 43.2, c = 147.9 A. There are two molecules in the crystallographic asymmetric unit.

Purification and characterization of Ophiophagus hannah cytotoxin-like proteins

Three cytotoxin-like proteins from the venom of Ophiophagus hannah were isolated by a combination of ion exchange chromatography and reverse phase HPLC. Amino acid sequence analysis revealed that these proteins all consisted of 63 amino acids and shared approximate 50% and 56% sequence identity with Naja naja atra cardiotoxins and cardiotoxin-like basic proteins (CLBPs), respectively. CD spectra revealed that their secondary structure was dominated with beta-sheet as those noted with cardiotoxins and CLBPs. O. hannah cytotoxin-like protein exhibited a cell-lytic activity on SK-N-SH cells, but its activity was more weak than that noted for N. naja atra cardiotoxin 3. Alternatively, apoptotic cell death was induced by the addition of N. naja atra CLBP. Based on the sequence information with the toxin molecules, the functional residues and regions related to the differential activity with O. hannah cytotoxin-like protein, cardiotoxin and CLBP are discussed.

Occurrence of non-protein low molecular weight cardiotoxin in Indian King Cobra (Ophiophagus hannah) Cantor 1836, venom

Pathophysiology due to snakebite is a combined effect of various actions of the complex venom constituents. Importance of protein toxins in snake envenomation is well known. The present investigation reports the existence of nonprotein/nonpetide low molecular weight toxin in Indian King Cobra venom, which plays an important role in envenomation consequences in experimental animal models. A group of non-peptidic toxins (OH-NPT1) was isolated from Indian King Cobra Ophiophagus hannah by thin layer chromatography and silica gel column chromatography. UV, IR, NMR and (ESI) TOF-MS studies characterized the OH-NPT1 as a mixture of aliphatic acids having molecular weights 256, 326 and 340Da. The minimum lethal dose of OH-NPT1 was found to be 2.5 microg/20g (iv) and 4microg/20g (ip) in male albino mice. The cardiotoxic property of OH-NPT1 was established through studies on isolated guinea pig heart and auricle preparations, ECG studies in albino rat and estimation of LDH1/LDH and CPK-MB/CPK ratio in Swiss albino mice. Commercial antiserum failed to neutralize the lethality and cardiotoxicity of the toxin. However, calcium and magnesium effectively neutralized the lethal action.

Structural Difference between Group I and Group II Cobra Cardiotoxins: X-ray, NMR, and CD Analysis of the Effect ofcis-Proline Conformation on Three-Fingered Toxins†

Natural homologues of cobra cardiotoxins (CTXs) were classified into two structural subclasses of group I and II based on the amino acid sequence and circular dichroism analysis, but the exact differences in their three-dimensional structures and biological significance remain elusive. We show by circular dichroism, NMR spectroscopic, and X-ray crystallographic analyses of a newly purified group I CTX A6 from eastern Taiwan cobra (Naja atra) venoms that its loop I conformation adopts a type VIa turn with a cis peptide bond located between two proline residues of PPxY. A similar "banana-twisted" conformation can be observed in other group I CTXs and also in cyclolinopeptide A and its analogues. By binding to the membrane environment, group I CTX undergoes a conformational change to adopt a more extended hydrophobic domain with beta-sheet twisting closer to the one adopted by group II CTX. This result resolves a discrepancy in the CTX structural difference reported previously between solution as well as crystal state and shows that, in addition to the hydrophobicity, the exact loop I conformation also plays an important role in CTX-membrane interaction. Potential protein targets of group I CTXs after cell internalization are also discussed on the basis of the determined loop I conformation.

Immobilized betulinic acid column and its interactions with phospholipase A2 and snake venom proteins

Betulinic acid (BA) is a plant-derived pentacyclic triterpenoid. Although BA has been found to have diverse pharmacological effects, including anti-tumor and anti-inflammatory actions and potential as inhibitor of phospholipase A2 (PLA2), its cellular targets remain unclear. In this study, BA was immobilized onto an acrylamide matrix. The immobilized-BA column could retain the purified PLA2 of bovine pancreas or the PLA2 of snake venom from Naja nigricollis. The bound PLA2 were not eluted by high salt concentrations but were eluted by either acid or calcium free buffer. Besides the PLA2, a group of basic proteins of snake venom with molecular weights of about 7 kDa were also strongly bound by immobilized BA. One of these proteins was identified as gamma-cardiotoxin. The usefulness of immobilized BA for exploring the cellular targets of BA is discussed.

Effect of calcium on the vascular contraction induced by cobra venom cardiotoxin

1. The cytotoxic effects of cardiotoxin (CTX) purified from Cobra venom were tested in endothelium-denuded rat aortic ring preparations in tissue organ baths and the effect of extracellular Ca2+ on the cytotoxic effect of CTX was investigated using a digital dynamic calcium imaging technique. 2. At 10 micromol/L, CTX induced a slowly developing and sustained contraction that amounted to approximately 50% of the maximal contraction induced by 80 mmol/L KCl. At high concentrations (> 15 micromol/L), CTX caused irreversible damage to the smooth muscle contractile function. However, washout of CTX at its peak contraction did not affect the subsequent contraction to either KCl or phenylephrine. 3. Contraction induced by CTX was dependent on the Ca2+ concentration in the external solution. A maximal contractile response to CTX was obtained in medium containing 1-2.5 mmol/L Ca2+. This contractile response induced by CTX decreased with higher Ca2+ concentrations and was completely diminished when 7 mmol/L Ca2+, 3 mmol/L Ni2+ or 30 micromol/L tetrandrine (a non-selective calcium channel blocker) was present in the external solution before addition of CTX to the bath. 4. The above observations were supported by the calcium imaging work performed with cultured aortic smooth muscle cells from Wistar-Kyoto rats, in which CTX was shown to induce the elevation of cytosolic Ca2+ in the presence, but not in the absence, of 2.5 mmol/L extracellular Ca2+. Increasing the extracellular Ca2+ concentration to 7 mmol/L, the addition of 3 mmol/L Ni2+ or inclusion of 30 micro mol/L tetrandrine inhibited the elevation of cytosolic Ca2+ induced by CTX. 5. These results suggest that: (i) a CTX-sensitive internal calcium store does not exist in rat aortic smooth muscle; (ii) the contractile effect CTX is associated with a Ca2+ influx process; and (iii) CTX interacts extracellularly with the plasma membrane at the level of the calcium channels, as well as anionic sites to which Ca2+ and other inorganic cations bind.

Separation and structure-function studies of Taiwan cobra cardiotoxins

Six cardiotoxins (CTXs) and one cardiotoxin-like basic protein (CLBP) from Naja naja atra (Taiwan cobra) venom were separated by a SP-Sephadex C-25 column. CTXn and CTXI were well separated by eluting with ammonium acetate buffer, and the separation of CLBP from CTXIV and CTXV mixtures was achieved using sodium phosphate buffer. These findings suggest a differential interaction of CTXs with the chromatographic matrix using different buffer systems. Chemical modification studies on cationic residues of CTXI suggested that there was no single lysine or arginine residue exclusively responsible for its biological activity. Moreover, it was found that the cytotoxicity and hemolytic sites of CTXI could be dissociated by chemical modifications. It suggests the potentiality for preparing toxin derivatives in which a specific activity is retained.

Elucidation of the solution structure of cardiotoxin analogue V from the Taiwan cobra (Naja naja atra)--identification of structural features important for the lethal action of snake venom cardiotoxins

The aim of the present study is to understand the structural features responsible for the lethal activity of snake venom cardiotoxins. Comparison of the lethal potency of the five cardiotoxin isoforms isolated from the venom of Taiwan cobra (Naja naja atra) reveals that the lethal potency of CTX I and CTX V are about twice of that exhibited by CTX II, CTX III, and CTX IV. In the present study, the solution structure of CTX V has been determined at high resolution using multidimensional proton NMR spectroscopy and dynamical simulated annealing techniques. Comparison of the high resolution solution structures of CTX V with that of CTX IV reveals that the secondary structural elements in both the toxin isoforms consist of a triple and double-stranded antiparallel beta-sheet domains. Critical examination of the three-dimensional structure of CTX V shows that the residues at the tip of Loop III form a distinct "finger-shaped" projection comprising of nonpolar residues. The occurrence of the nonpolar "finger-shaped" projection leads to the formation of a prominent cleft between the residues located at the tip of Loops II and III. Interestingly, the occurrence of a backbone hydrogen bonding (Val27CO to Leu48NH) in CTX IV is found to distort the "finger-shaped" projection and consequently diminish the cleft formation at the tip of Loops II and III. Comparison of the solution structures and lethal potencies of other cardiotoxin isoforms isolated from the Taiwan cobra (Naja naja atra) venom shows that a strong correlation exists between the lethal potency and occurrence of the nonpolar "finger-shaped" projection at the tip of Loop III. Critical analysis of the structures of the various CTX isoforms from the Taiwan cobra suggest that the degree of exposure of the cationic charge (to the solvent) contributed by the invariant lysine residue at position 44 on the convex side of the CTX molecules could be another crucial factor governing their lethal potency.

Heparin and heparan sulfate bind to snake cardiotoxin. Sulfated oligosaccharides as a potential target for cardiotoxin action

Cardiotoxins (CTXs) from cobra venom show cytotoxicity toward several cell types. They cause systolic heart arrest and severe tissue necrosis. Their interaction with phospholipids is established but by itself fails to explain the specificity of these toxins; other component(s) of membrane must, therefore, intervene to direct them toward their target. We herein show, for the first time, that sulfated glycosaminoglycans, heparin, heparan sulfate (HS), chondroitin sulfate (CS), and dermatan sulfate (DS), interact with CTX A3, a major component of Taiwan cobra venom, by use of affinity chromatography, circular dichroism, absorbance, and fluorescence intensity and anisotropy measurements. The relative strength of binding, determined by the NaCl concentration required to dissociate the CTX-glycosaminoglycan complex, varied as follows: heparin > DS > CS > HS. In physiological buffer (8 mM Na2HPO4, 2.7 mM KCl, 1.8 mM KH2PO4, 138 mM NaCl, pH 7.4), however, only heparin and HS bound to CTX, with respective dissociation constants of 1.4 and 16 microM, while CS and DS failed to exhibit well defined binding behavior, as indicated by fluorescence measurements. We estimate that CTX makes 3-4 ionic contacts with heparin based on a salt-dependent binding study and that approximately 40% of binding free energy is derived from purely electrostatic interactions under physiological conditions. Sulfated pentasaccharide may be sufficient to bind to CTX. We also found that heparin accentuates the penetration of CTX into phospholipid membranes as analyzed by Langmuir monolayer measurement. In view of these results we propose that heparin-like moieties of the cell surface may modulate the action of CTX.

cDNA sequence analysis of a novel cardiotoxin-like protein from Taiwan banded krait

The cDNAs encoding a novel protein was constructed from the cellular RNA isolated from the venom glands of Bungarus multicinctus by reverse transcription-polymerase chain reaction. The deduced amino acid sequence of this novel protein contained 65 amino acid residues with 8 cysteine residues. Comparative sequence analysis showed that it was structurally related to cardiotoxin rather than neurotoxins. These results suggest that the venom of Bungarus multicinctus may contain cardiotoxin(s) which was not noticed before.

Respiratory toxicity of direct lytic factor in the venom of the southern Chinese cobra (N. naja atra) in dogs

The effects of direct lytic factor (DLF) on respiratory ventilation, gas exchange as well as hemodynamics were studied in anesthetized dogs. After an intravenous DLF dose of 1 mg/kg, the initial manifestation of intoxication was observed as follows: (1) Increase in airway impedance characterized by slowed air flow rate and increased negative transpulmonary pressure. (2) Decrease in dynamic compliance. (3) Progressive increase in venoarterial shunt (Qs/Qt) and decrease in PaO2, (4) Elevation of pulmonary artery blood pressure and fall of mean systemic blood pressure and maximal left ventricular pressure. Above actions reached the peak values at 15 min and thereafter all respiratory functional parameters, except Qs/Qt and hypoxemia, returned gradually to approach the normal levels at 50 min. The tidal volume, PaCO2 and LVEDP remained unchanged until another DLF dose of 1.5 mg/kg was given. After a second dose of DLF (total 2.5 mg/kg), the respiratory functions and the cardiac performance deteriorated as follows: (1) Further increase in Qs/Qt and hypoxemia. (2) Appearance of hypercapnea and acidosis. (3) Fall of dP/dtmax and elevation of LVEDP, widening of QRS complex of ECG. (4) Blood pressure run a downhill course. From above experimental evidence, we came to the conclusion that as well as the basic cardiotoxicity, respiratory toxicity of DLF must be considered as the primary, because of broad spectrum of action of DLF and early effect on respiratory function.

Inhibition by multivalent cations of contraction induced by Chinese cobra venom cardiotoxin in guinea pig papillary muscle

The effects of cardiotoxin (CTX), purified from the venom of Chinese Cobra (Naja naja atra) by a three-step chromatography, on the contractile responses of isolated guinea pig papillary muscle preparation and its antagonism by lanthanum ion (La3+) and divalent cations were examined. CTX induced tonic contraction following a transient augmentation of electrically evoked rhythmic contractions, which is similar to that seen in perfused heart preparation. Multivalent cations, La3+, Ca2+, Mn2+ and Mg2+, concentration-dependently blocked CTX-induced contraction. In Ca(2+)-free medium, CTX did not induce contraction and CTX-induced contraction was not modified in Na(+)-free medium. Nifedipine (1 mumol/L), effectively blocked KCl-induced contracture, but only partially inhibited CTX-induced contraction; thus suggesting that Ca2+ influx induced by CTX utilizes channels other than L-type Ca2+ channels. These cations may compete with CTX for the negatively charged membrane binding site which is responsible for the modulation of Ca2+ movement.

Cloning, direct expression, and purification of a snake venom cardiotoxin in Escherichia coli

The cardiotoxin analogue III (CTX III), isolated from the Taiwan cobra (Naja naja atra) venom, is a sixty-amino acid, all beta-sheet protein. We report the direct expression of CTX III from its synthetic gene as inclusion bodies in Escherichia coli. The yield of the expressed protein is about 40 mg/liter of the culture. CTX III trapped as inclusion bodies is dissolved and refolded by the slow refolding technique. The refolded protein is purified by reverse phase high performance liquid chromatography. The purified and refolded CTX III sample is further characterized by SDS-PAGE, circular dichroism, two-dimensional NMR spectroscopy and haemolytic activity. To our knowledge, this is the first report of the direct expression and purification of snake venom cardiotoxins.

cDNA sequence analysis and expression of cardiotoxins from Taiwan Cobra

The cDNAs encoding cardiotoxins I, III and N were constructed from the cellular RNA isolated from the venom glands of Naja naja atra by reverse transcription-polymerase chain reaction. A high degree of sequence homology was observed with the three cardiotoxins. The cardiotoxins were subcloned in the expression vector pET 20b(+) and transformed in BL 21(DE3) E. coli strain. The expressed protein was isolated from the inclusion bodies of E. coli and purified by reversed phase high performance liquid chromatography. The purified recombinant cardiotoxin III showed an immunoreactivity with anti-cardiotoxin III antibodies as revealed by immunoblot analysis.

Identification of 'molten globule'-like state in all beta-sheet protein

The cardiotoxin analogue III (CTX III), isolated from the Taiwan Cobra venom (Naja naja atra), is a sixty amino acid, all beta-sheet protein. The 2,2,2-trifluoro ethanol (TFE) induced unfolding of CTX III is studied under acidic conditions (pH 2.5). Using circular dichroism, 1-anilino-8-napthalene sulphonic acid binding and NMR experiments, it is shown that stable, partially structured state(s) ['molten globule'-like state] is formed between 50 and 80% TFE concentrations. The protein was found to exist in an unfolded state in 80% TFE containing 2M urea. The TFE induced unfolding process is shown to be completely reversible. In the 'molten globule' state of CTX III in 80% TFE, though portion(s) of the backbone of the protein assume helical conformation, most of the original beta-sheet secondary structural elements in the protein are intact. In our opinion, this is the first report of the identification of a 'molten globule'-like state in the unfolding pathway of an all beta-sheet monomeric protein.

Sequence comparison and computer modelling of cardiotoxins and cobrotoxin isolated from Taiwan cobra

Six cardiotoxins and one neurotoxin isolated and purified from the Taiwan cobra venom (Naja naja atra) possess distinct pharmacological and biochemical properties despite the existence of a grossly similar tertiary structure among these toxins, i. e., a core consisting of a series of short loops and four disulfide bridges. A systematic structure comparison of these major toxin isoforms was made by the secondary-structure predictions together with computer model-building based on the primary sequences and the established X-ray and NMR structures of one published cardiotoxin isoform and cobrotoxin. It is of interest to find that some defined and subtle differences can be detected upon the superposition of these three-dimensional polypeptide chains, which may reflect the intrinsic differences in the surface hydrophobicity of cardiotoxins and cobrotoxin as revealed by hydropathy profiles of these toxins in one of three major loops. The differences seem to correlate with different inhibitory activities exhibited by cardiotoxins in contrast to the lack of activity by cobrotoxin on protein kinase C (PKC).

Two distinct types of cardiotoxin as revealed by the structure and activity relationship of their interaction with zwitterionic phospholipid dispersions

Cardiotoxins (CTXs) are a group of homologous proteins found in cobra snake venom and consist of 60-62 amino acid residues. Although CTXs are known to consist of three extended beta-sheet loops similar to neurotoxins, the target and interaction of CTXs with membranes unlike those of neurotoxins are not well understood. Herein, we report comparative studies of 10 CTXs purified from Taiwan cobra (Naja naja atra) and Mozambique spitting cobra (Naja mossambica mossambica) snake venoms with respect to their interactions with zwitterionic phospholipids. Based on the CTX-induced mixing of sphingomyelin vesicles and the binding of CTX to lysophosphatidylcholine micelles, two distinct types of CTX, i.e. P- and S-type CTX, are identified. P-type CTXs are characterized by the presence of Pro-31 within a putative phospholipid binding site near the tip of loop 2; whereas S-type CTXs are characterized by the presence of Ser-29 within the same but more hydrophilic region. Although binding of all CTXs to phospholipid membranes involves a phospholipid binding site at loop 1, P-type CTXs exhibit higher fusion and binding activity than S-type CTXs, presumably due to the additional phospholipid binding site at loop 2. The binding modes of P- and S-type CTX are thus different. Analysis of the primary structures of 46 CTXs from the genus Naja indicates that these two types of CTXs exist in all species examined. Reasonable structure/activity correlation can be detected for the effects of CTXs on muscle and red blood cells, although notable exceptions are also found. S-type CTXs are generally found to exhibit higher muscle cell depolarization activity, whereas P-type CTXs are found to possess a higher hemolytic activity. Thus the mechanism of action of CTXs seems to involve CTX-membrane interactions and depends on the type of the cell membrane and CTX molecules under study. The two lipid binding sites in P-type CTXs and one lipid binding site in S-type CTXs show large variation in their amino acid residues, but they do display some common distribution of residue type. Analogous to the signal sequences for protein import, these regions are characterized by the coexistence of an exposed hydrophobic surface flanked on either side by a cationic residue. A hypothesis is proposed to explain the general cytotoxic and specific cardiotoxic effect of CTXs based on the two CTX subtypes in snake venom.

Sequence characterization of cardiotoxins from Taiwan cobra: isolation of a new isoform

Cardiotoxins (or called cytotoxins) are a group of very basic polypeptides present in some snake venoms, which are especially abundant in Formosan cobras. Several cardiotoxins with distinct pharmacological and biochemical properties were isolated and purified from the Taiwan cobra venom (Naja naja atra) by employing sequentially preparative-scale cation-exchange chromatography on TSK CM-650 coupled with the improved separation of toxin components on reversed-phase HPLC based on their hydrophobicity. The complete sequences of six cardiotoxin analogues, including one novel isoform never reported previously in this venom, were determined by N-terminal Edman degradation with the automatic pulsed-liquid phase sequencer and comparison of amino-acid compositions of purified toxin components. This novel cardiotoxin, a basic polypeptide of pI 10.13, comprises about 5% of total crude venom. It consists of 60 amino-acid residues with four disulfide bonds and shares about 80-92% sequence identity with several cytotoxins isolated from varied sources of different snakes. A systematic structure comparison of six major cardiotoxin isoforms was made by the secondary-structure predictions and hydropathy profiles based on their primary sequences. It is of interest to find that some subtle differences among these natural cardiotoxin analogues may lie in the intrinsic differences of the surface hydrophobicity of these isotoxins.

The ocular effects of spitting cobras: II. Evidence that cardiotoxins are responsible for the corneal opacification syndrome

Fractionation of H. haemachatus, N. nigricollis, N. nivea and N. melanoleuca venoms using Amberlite CG-50 and (NH4)HCO3 elution gradient chromatography yielded 11-13 fractions for each venom. One fraction, F X, from H. haemachatus, two fractions, F X and F XI, from N. nigricollis and one fraction, F VIII, from N. melanoleuca venoms possessed the whole of ocular activity of the venoms. The fractions were the only venom fractions that caused cardiac depressant activity; their effect was reversed by raising Ca++ concentration in the physiological solution; they did not influence the twitches of the phrenic nerve hemidiaphragm and guinea-pig ileum preparations. Further purification of the fractions on Sephadex G-50 followed by fractionation on Amberlite CG-50 yielded fractions free from phospholipase A2 activity but possessing the same ocular effects. Similarly, the cardiotoxin from commercial N. nigricollis venom caused the same ocular effects as the crude venom and its purified cardiotoxic fractions. All cardiotoxic fractions as well as N. nigricollis cardiotoxin, caused extensive chemosis, blepharitis and corneal opacification with corneal and subconjunctival neovascularization. On a weight basis, the cardiotoxins were weaker in their oculotoxic activity than the corresponding parent crude venoms possibly because of the potentiating effect of phospholipase A2 in the crude venoms. It is postulated that in spitting cobras the cardiotoxins are responsible for the corneal opacification syndrome. In other cobra venoms the stable binding of cardiotoxins with acidic proteins limits their possible ocular effects. Only in the venoms of the spitting species are the cardiotoxins present in an appropriately free form to cause the ocular opacification syndrome.

[Channel-forming activity at planar lipid bilayer of the membrane active polypeptide B form venom of Bungarus fasciatus]

Using planar lipid bilayer formed by lecithin and cholesterol (20 and 5 mg/ml respectively in N-decane) the channel-forming activity of the membrane active polypeptide B(BMAP B) from the venom of Bungarus fasciatus was investigated. Under the existence of a voltage or a salt concentration gradient between two sides of the bilayer, unit conductance fluctuation and a decrease in steady state resistance accompanying BMAP B incorporation and channel formation were observed. By measuring the reversal potential in an asymmetric solution, the selectivity of the BMAP B-channel was estimated having a value of PK/PC1 = 1.4. Divalent cations, such as Ba2+, Ca2+ inhibited the channel activity as they did in biomembranes. These data might provide an explanation for the depolarizing effect of the membrane active polypeptide on the native membranes.

Structural analysis and comparison of cobrotoxin and cardiotoxins by near-IR Fourier transform Raman spectroscopy

Venom toxins were isolated from Formosan cobra (Naja naja atra) by cation-exchange chromatography. The near-IR FT-Raman analytical method has been applied to the characterization and classification of the toxin components in their lyophilized forms. Structural analysis and comparison of various purified toxin fractions were made with respect to their amino acid compositions and near-IR Fourier-transform Raman spectra. The results indicate that the major secondary structure of cobra toxins including cobrotoxin and various cardiotoxins is mainly anti-parallel beta-pleated sheet as judged by the Raman signals at 1238 cm-1 (amide III) and 1671 cm-1 (amide I). It is also found that the relative Raman signal intensities of Tyr, Phe, Trp and Met residues in purified toxins correlate very well with the structural data obtained from amino acid analysis. The advantage and improvement of applying the near-IR FT-Raman spectroscopy to the unambiguous classification and comparison of venom toxins are evident and the discrepancies with previous Raman studies on these venom toxins are also revealed and discussed.

Positive inotropic and toxic action of direct lytic factor on isolated working guinea pig hearts

The positive inotropic and the toxic effects of direct lytic factor (DLF) on the isolated working guinea pig hearts were studied. As compared with baseline values, DLF 1-10 micrograms.ml-1 increased aortic flow up to 138% cardiac output 116%, left ventricular pressure volume work 136%, left ventricular pressure 114%, dP/dtmax 130%, Vmax 128%, and mean aortic systolic pressure 114%, but coronary flow was decreased by 16% on an average (n = 6). However, heart rate remained constant, myocardial oxygen consumption and efficiency were changed little. The cardiotonic effect of DLF was also observed by recording the isometric contractions of the isolated guinea pig papillary muscles and by determining the left ventricular pressure and dP/dtmax in anesthetized dogs. Neither spontaneously beating rate of right atrium nor the excitability of left atrium in vitro was affected by DLF. The results show that DLF is one of the cardiotonic agents without chronotropic effect and its coronary vasoconstriction effect plays an important part in heart failure.

Non-enzymatic inhibitors of coagulation and platelet aggregation from Naja nigricollis venom are cardiotoxins

Four non-enzymatic polypeptides from Naja nigricollis crawshawii venom were recently isolated and shown to inhibit plasma coagulation and platelet aggregation. We have now determined the amino acid compositions, amino terminal sequences and direct lytic activity of these anticoagulants. The results of these studies allow us to identify the anticoagulants as cardiotoxins. The anticoagulant activity of these cardiotoxins is far more potent than that of other cardiotoxins previously reported to have anticoagulant activity.

Reversed-phase and hydrophobic-interaction high-performance liquid chromatography of elapid cardiotoxins

The separation of proteins by hydrophobic-interaction HPLC and reversed-phase HPLC depends upon differences in the hydrophobicity of accessible surface groups. The elution order of a group of snake venom cardiotoxins was found to vary between these two HPLC methods. Circular dichroism spectroscopy showed that the eluant acetonitrile-trifluoroacetic acid used for reversed-phase HPLC altered the conformation of the toxins, whereas the salt-buffer eluting medium used for hydrophobic-interaction HPLC did not affect toxin conformation. The retention times of cardiotoxins on reversed-phase HPLC are therefore influenced by their conformational instability in the eluting medium which causes partial or complete unfolding. Hydrophobic interaction is clearly the preferred method with which to correlate the "surface hydrophobicity" of cardiotoxins and their biological effects.

Amino acid sequence of a cytotoxin-like basic protein with low cytotoxic activity from the venom of the Thailand cobra Naja naja siamensis

A cytotoxin-like basic protein (CLBP) was isolated from the venom of the Thailand cobra (Naja naja siamensis). The cytotoxicity of CLBP toward FL cells was one order of magnitude lower than those of cytotoxins. The amino acid sequence was determined by a combination of conventional methods. The total number of amino acid residues was 62, giving a molecular mass of 6977 Da. The sequence at residues 25-30 in the CLBP molecule was found to be significantly different from those of cytotoxins. This region might play an important role in the cytotoxic activity of cytotoxins.

Cobra cardiotoxins. Purification, effects on skeletal muscle and structure/activity relationships [published errtum appears in Eur J Biochem 1988 Feb 1;171(3):727]

A new preparative method for isolating homogeneous cardiotoxins from cobra venoms is described. The technique, based on reverse-phase high-pressure liquid chromatography, was used to isolate eight cardiotoxins of known sequence from four different venoms. In each case the method was found to be particularly efficient at removing trace quantities of contaminating phospholipase. Cardiotoxins isolated in this manner were found to retain their full biological activity. Without exception the purified cardiotoxins lacked powerful haemolytic activity at concentrations up to 0.01 mM (about 100 micrograms ml-1), although some lysis of human erythrocytes was induced at higher concentrations. The cardiotoxins displayed a wide range of depolarizing activity on cultured skeletal muscle, the lowest activity being associated with the highest LD50 value. Correlating variations in amino acid sequence and variations in depolarization potency revealed the importance of residues in the second and third loops, especially lysine-46, serine-48 and lysine-52, together with a number of hydrophobic residues. Further modifications of pharmacological activity were associated with the presence of additional basic residues in the first and second loops and to minor differences in secondary structure.

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.

Contracture induction by snake venom cardiotoxin in skeletal muscle from humans and rats

Contracture responses to cardiotoxin (CTX) from Naja naja kaouthia venom were investigated in rat and human skeletal muscle of similar fiber type distribution to determine species differences in mechanism of action. Rat diaphragm strips and human vastus lateralis preparations were directly stimulated in a tissue bath. The calcium dependence of toxin action, synergism between CTX and phospholipase A2 (PLA2) activity and roles of Na+ + K+-ATPase activity and the sarcoplasmic reticulum Ca2+ stores in contracture induction were examined. The threshold of contracture to CTX was decreased in human and rat muscle when Sr2+ was substituted for Ca2+ in the bathing medium. In rat, but not in human muscle the threshold of contracture to CTX was decreased in a medium in which Ca2+ had been omitted. The decrease in contracture threshold may relate to toxin binding. The maximum height of contracture for preparations from humans, but not for those from rats was considerably depressed in a medium in which Ca2+ had been omitted. Exogenously added bee venom PLA2 acts synergistically with CTX in skeletal muscle in a manner similar to that in erythrocytes. Ouabain (100 microM) did not elicit contractures in any of the media tested nor affect CTX-induced contractures in Sr2+-containing medium. Dantrolene antagonized CTX-induced contractures, suggesting a role for Ca2+ derived from the sarcoplasmic reticulum in CTX action. The species difference in CTX action may reflect differences in the relative contribution of Ca2+ from the sarcolemma and sarcoplasmic reticulum to the contracture.

Effects of cardiotoxin D from Naja naja siamensis snake venom upon murine splenic lymphocytes

Cardiotoxin D from Naja naja siamensis is cytotoxic to T-lymphocytes above 150 femtomoles/10(6) cells. Equivalent lysis of macrophages or B-lymphocytes requires at least 1000 times more toxin. Reduction and carboxamidomethylation of cardiotoxin D does not effect T cell lysis. At higher toxin concentrations, 50% T-cell lysis occurs within 10 min. Splenocytes cultured with mitogens are up to five times more susceptible to toxin than unstimulated cells. Cardiotoxin D may directly disrupt the plasma membrane, since lysis is unaltered at 4 degrees C.

Characterization of the anticoagulants from Taiwan cobra (Naja naja atra) snake venom

Taiwan cobra (Naja naja atra) snake venom was separated into 19 fractions by means of CM-Sephadex C-50 column chromatography. Anticoagulant Fractions V-VII were refractionated by gel filtration on Sephadex G-50 and the purified component possessed phospholipase A2 activity and an inhibitory effect on collagen-induced platelet aggregation. The anticoagulant action could be antagonized by phospholipid or platelet factor 3. Anticoagulant Fraction XVII was also further refractionated by gel filtration on Sephadex G-50 and the purified component was shown to be cardiotoxin. It was a weak anticoagulant, caused direct hemolysis and potentiated collagen-induced platelet aggregation. Thromboelastographic studies showed that the anticoagulant action of cobra venom is due to the synergistic effects of phospholipase A2 and cardiotoxin.

Amino acid sequence of a cardiotoxin-like basic polypeptide (CLBP) with low cytotoxic activity isolated from the venom of the Formosan cobra (Naja naja atra)

A cardiotoxin-like basic polypeptide, designated as CLBP, was isolated from the venom of Naja naja atra by gel filtration on Sephadex G-50 followed by CM-cellulose chromatography. The cytotoxicity toward Yoshida sarcoma cells and lethal toxicity toward mice of CLBP were both one-order lower than those of cardiotoxins and cobrotoxin, respectively. CLBP is a single polypeptide consisting of 61 amino acid residues with four intramolecular disulfide linkages. The amino acid sequence of CLBP shows a high degree of homology with those of cardiotoxins from the same venom, but differs in the 19 to 23 positions.

On the interaction of cobra venom protein cardiotoxins with erythrocytes

The principally active hemolytic toxin (cardiotoxin) previously purified from the venom of the Thailand cobra, Naja naja siamensis, was shown to produce spontaneous twitching, contractures and membrane depolarization in sartorius muscles from the frog, Rana pipiens. Spontaneous twitching, observed at concentrations greater than 0.1 uM was completely abolished by addition of tetrodotoxin and not affected by d-tubocurarine. Dose and time dependent membrane depolarization of muscle fibers was observed to occur within 10-30 min at 0.2 to 1.0 uM concentrations of the toxin. These observations, taken together with an amino acid analysis characteristic of previously described cobra venom cardiotoxins, characterized this hemolytic toxin as a cardiotoxin. In the absence of EDTA the initial velocities of erythrocyte hemolysis for this toxin showed a sigmoidal concentration dependence which became hyperbolic in the presence of EDTA. The largest increases in hemolysis rates on addition of 1 mM EDTA were observed at low toxin concentrations. In the presence of EDTA extracellular and membrane associated divalent cations are complexed, thus alleviating their competition with toxin for binding to the membrane, a key and apparently rate-determining initial step which leads to hemolysis. In the presence of EDTA hemolysis rates increased linearly at low toxin concentration and reached an extrapolated maximum value at toxin concentrations at which, given its molecular dimensions, there are just sufficient toxin molecules to cover the entire membrane surface area provided by the erythrocytes.

Are interactions with phospholipids responsible for pharmacological activities of cardiotoxins?

Cardiotoxins are small basic proteins (7 000 daltons) that are found in the venoms of Elapidae snakes. Although they are structurally close to alpha-neurotoxins present in the same secretions, their activity is related to their ability to interact with every cell membrane inducing, near micromolar concentration, the modification of its biological properties and/or physical structure. The mode of action of cardiotoxins, on a molecular level, is still under investigation. However, lipid-protein interactions are more and more involved in their binding to membrane and in their activities. Using new experimental data a better definition of phospholipid-cardiotoxin interaction is arrived at and a tentative molecular explanation of the pharmacological activities of these proteins is presented and discussed.

Unfolding processes of small globular proteins: the two-state vs multi-state model

1. The alcohol-induced unfolding of two homologous proteins, neurotoxin and cardiotoxin from Taiwan cobra (Naja naja atra) venom has been analysed. 2. It is postulated that the unfolding process for both proteins is a multi-state conformational transition. 3. It has been hypothesized that between the compact native state of the protein and its fully unfolded state there exists a quasi-continuous spectrum of conformational metastates of protein species. 4. The population distribution of these metastates is partially dependent on the nature of unfolding factors as well as the amino acid composition and sequence. 5. The sum of all transient conformational states and the protein species being in the folded and unfolded states respectively, can be detected by means of circular dichroism spectroscopy since the absorption of circularly polarized light is rapid relative to the rate of fluctuations of the protein structure.

Biochemical and pharmacological properties of cardiotoxins isolated from cobra venom

It has been established that cardiotoxins isolated from Elapidae snakes tend to be contaminated with phospholipase. After a thorough comparison of the methods available for the separation of these two components, both hydrophobic and immunoaffinity chromatography have been found to be convenient and effective methods. With cardiotoxins isolated from Naja naja siamensis we observed that as the contaminating phospholipase was removed, the cardiotoxins were converted from relatively powerful haemolytic agents to essentially nonlytic agents. In contrast, removal of the contaminating phospholipase did not abolish the ability of the cardiotoxins to contract smooth muscle. Homologous toxins isolated from Naja melanoleuca and Haematchatus haematchates have been isolated with powerful lytic activity and weak or low cardiotoxicity. This data is interpreted in the light of the homologous group of cardiotoxins possessing two different properties (1) cardiotoxicity and (2) lytic activity; some molecules in the group possess both properties and other possess only one or the other. Structure activity relationships are presented in support of this concept.

Use of erythrocyte hemolysis kinetics in the purification of complex cardiotoxin mixtures

A human erythrocyte hemolysis kinetic method provides a useful way to follow the purification of cobra venom cardiotoxins or other hemolytic factors. Initial rates of hemolysis, measured as hemoglobin released with time for the separated cardiotoxins from the venom of the Thailand cobra Naja naja siamensis, vary over a greater range than do other commonly used measures of their biological activity. Recovery of the hemolytic activity of the gross cardiotoxin fraction in the subsequently separated fractions has been demonstrated. The method employs submilligram quantities of mixed or purified cardiotoxins.

Effects of purified cardiotoxins from the Thailand cobra (Naja naja siamensis) on isolated skeletal and cardiac muscle preparations

Four polypeptides were isolated by non-exchange chromatography from the crude cardiotoxin fraction of Thailand cobra (Naja naja siamensis) venom. Fraction I and 71 amino acid residues including 1 tryptophan, while fractions, II, III and IV had 60 amino acids and no tryptophan. When tested on isolated skeletal muscle (chick biventer cervicis, chick embryo muscle in culture, guinea pig hemidiaphragm) and cardiac muscle (guinea pig and cat left atria, cat papillary muscle) preparations, fractions II, III and IV, but not fraction I, caused contracture and depolarization. The cardiotoxin-induced contractions could be prevented by pretreatment with raised concentrations of calcium, but were not influenced by a wide range of pharmacological agents which modify nerve-muscle transmission or muscle contractility. The results suggest that cardiotoxins do not act at a specific step in normal excitation-contraction coupling but directly on cell membranes, where they probably cause the formation of pores which result in depolarization and in the influx of calcium.

1H nuclear-magnetic-resonance studies of the conformation of cardiotoxin VII2 from Naja mossambica mossambica

The membrane toxin VII2 from the venom of Naja mossambica mossambica was investigated in aqueous solution by one-dimensional and two-dimensional high-resolution nuclear magnetic resonance (NMR) techniques at 360 MHz. The spectral characterization included identification of the complete spin systems for several amino acid residues, nuclear Overhauser effect measurements, the use of chemically induced dynamic nuclear polarization and studies of the pH dependence of the NMR spectrum. Data from homologous toxins, in particular direct lytic factor 12B from Haemachatus haemachatus, were used to establish assignments of aromatic and methyl proton resonances. From these experiments a short, triple-stranded fragment of antiparallel beta structure could be determined, which includes the residues 23-27, 43-46 and 60-62. Furthermore, the nuclear Overhauser effect measurements indicate close proximity in the protein conformation of the aromatic rings of Trp-14, Tyr-25 and Tyr-59, and the side chain of Ile-46.