Molecular Mechanism by which Cobra Venom Cardiotoxins Interact with the Outer Mitochondrial Membrane

Cardiotoxin CTII from Najaoxiana cobra venom translocates to the intermembrane space (IMS) of mitochondria to disrupt the structure and function of the inner mitochondrial membrane. At low concentrations, CTII facilitates ATP-synthase activity, presumably via the formation of non-bilayer, immobilized phospholipids that are critical in modulating ATP-synthase activity. In this study, we investigated the effects of another cardiotoxin CTI from Najaoxiana cobra venom on the structure of mitochondrial membranes and on mitochondrial-derived ATP synthesis. By employing robust biophysical methods including 31P-NMR and 1H-NMR spectroscopy, we analyzed the effects of CTI and CTII on phospholipid packing and dynamics in model phosphatidylcholine (PC) membranes enriched with 2.5 and 5.0 mol% of cardiolipin (CL), a phospholipid composition that mimics that in the outer mitochondrial membrane (OMM). These experiments revealed that CTII converted a higher percentage of bilayer phospholipids to a non-bilayer and immobilized state and both cardiotoxins utilized CL and PC molecules to form non-bilayer structures. Furthermore, in order to gain further understanding on how cardiotoxins bind to mitochondrial membranes, we employed molecular dynamics (MD) and molecular docking simulations to investigate the molecular mechanisms by which CTII and CTI interactively bind with an in silico phospholipid membrane that models the composition similar to the OMM. In brief, MD studies suggest that CTII utilized the N-terminal region to embed the phospholipid bilayer more avidly in a horizontal orientation with respect to the lipid bilayer and thereby penetrate at a faster rate compared with CTI. Molecular dynamics along with the Autodock studies identified critical amino acid residues on the molecular surfaces of CTII and CTI that facilitated the long-range and short-range interactions of cardiotoxins with CL and PC. Based on our compiled data and our published findings, we provide a conceptual model that explains a molecular mechanism by which snake venom cardiotoxins, including CTI and CTII, interact with mitochondrial membranes to alter the mitochondrial membrane structure to either upregulate ATP-synthase activity or disrupt mitochondrial function.

Detection of Naja atra Cardiotoxin Using Adenosine-Based Molecular Beacon

This study presents an adenosine (A)-based molecular beacon (MB) for selective detection of Naja atra cardiotoxin (CTX) that functions by utilizing the competitive binding between CTX and the poly(A) stem of MB to coralyne. The 5′- and 3′-end of MB were labeled with a reporter fluorophore and a non-fluorescent quencher, respectively. Coralyne induced formation of the stem-loop MB structure through A₂-coralyne-A₂ coordination, causing fluorescence signal turn-off due to fluorescence resonance energy transfer between the fluorophore and quencher. CTX3 could bind to coralyne. Moreover, CTX3 alone induced the folding of MB structure and quenching of MB fluorescence. Unlike that of snake venom α-neurotoxins, the fluorescence signal of coralyne-MB complexes produced a bell-shaped concentration-dependent curve in the presence of CTX3 and CTX isotoxins; a turn-on fluorescence signal was noted when CTX concentration was ≤80 nM, while a turn-off fluorescence signal was noted with a further increase in toxin concentrations. The fluorescence signal of coralyne-MB complexes yielded a bell-shaped curve in response to varying concentrations of N. atra crude venom but not those of Bungarus multicinctus and Protobothrops mucrosquamatus venoms. Moreover, N. nigricollis venom also functioned as N. atra venom to yield a bell-shaped concentration-dependent curve of MB fluorescence signal, again supporting that the hairpin-shaped MB could detect crude venoms containing CTXs. Taken together, our data validate that a platform composed of coralyne-induced stem-loop MB structure selectively detects CTXs.

DNA Aptamers against Taiwan Banded Krait α-Bungarotoxin Recognize Taiwan Cobra Cardiotoxins

Bungarus multicinctus α-bungarotoxin (α-Bgt) and Naja atra cardiotoxins (CTXs) share a common structural scaffold, and their tertiary structures adopt three-fingered loop motifs. Four DNA aptamers against α-Bgt have been reported previously. Given that the binding of aptamers with targeted proteins depends on structural complementarity, in this study, we investigated whether DNA aptamers against α-Bgt could also recognize CTXs. It was found that N.atra cardiotoxin 3 (CTX3) reduced the electrophoretic mobility of aptamers against α-Bgt. Analysis of the changes in the fluorescence intensity of carboxyfluorescein-labeled aptamers upon binding toxin molecules revealed that CTX3 and α-Bgt could bind the tested aptamers. Moreover, the aptamers inhibited the membrane-damaging activity and cytotoxicity of CTX3. In addition to CTX3, other N. atra CTX isotoxins also bound to the aptamer against α-Bgt. Taken together, our data indicate that aptamers against α-Bgt show cross-reactivity with CTXs. The findings that aptamers against α-Bgt also suppress the biological activities of CTX3 highlight the potential utility of aptamers in regard to the broad inhibition of snake venom three-fingered proteins.

Analysis of ligand binding to proteins using molecular dynamics simulations

This work aims to explore theoretically the molecular mechanisms of ligand binding to proteins through the use of molecular dynamics simulations. The binding of sodium dodecyl sulfate (SDS) to cobra cardio toxin A3 (CTX A3) and thiourea (TOU) to lysozyme have been chosen as the two model systems. Data acquisitions were made by Gromacs software. To begin with, the collisions of ligand molecules with every residue of CTX A3 and lysozyme were evaluated. With this information in hand, the average numbers of collisions with each residue was defined and then assessed. Next, a measure of the affinity of a residue, P(i), referred to as conformational factor, toward a ligand molecule was established. Based on the results provided, all site-making residues for CTX A3 and lysozyme were identified. The results are in good agreement with the experimental data. Finally, based on this method, all site-making residues of bovine carbonic anhydrase (BCA) toward the SDS ligand were predicted.

Age-dependent effect of myostatin blockade on disease severity in a murine model of limb-girdle muscular dystrophy

Myostatin (MSTN) is a muscle-specific secreted peptide that functions to limit muscle growth through an autocrine regulatory feedback loop. Loss of MSTN activity in cattle, mice, and humans leads to a profound phenotype of muscle overgrowth, associated with more and larger fibers and enhanced regenerative capacity. Deletion of MSTN in the mdx mouse model of Duchenne muscular dystrophy enhances muscle mass and reduces disease severity. In contrast, loss of MSTN activity in the dyW/dyW mouse model of laminin-deficient congenital muscular dystrophy, a much more severe and lethal disease model, does not improve all aspects of muscle pathology. Here we examined disease severity associated with myostatin (mstn-/-) deletion in mice nullizygous for delta-sarcoglycan (scgd-/-), a model of limb-girdle muscular dystrophy. Early loss of MSTN activity achieved either by monoclonal antibody administration or by gene deletion each improved muscle mass, regeneration, and reduced fibrosis in scgd-/- mice. However, antibody-mediated inhibition of MSTN in late-stage dystrophic scgd-/- mice did not improve disease. These findings suggest that MSTN inhibition may benefit muscular dystrophy when instituted early or if disease is relatively mild but that MSTN inhibition in severely affected or late-stage disease may be ineffective.

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.

Non-cytotoxic cobra cardiotoxin A5 binds to alpha(v)beta3 integrin and inhibits bone resorption. Identification of cardiotoxins as non-RGD integrin-binding proteins of the Ly-6 family

Severe tissue necrosis with a retarded wound healing process is a major symptom of a cobra snakebite. Cardiotoxins (CTXs) are major components of cobra venoms that belong to the Ly-6 protein family and are implicated in tissue damage. The interaction of the major CTX from Taiwan cobra, i.e. CTX A3, with sulfatides in the cell membrane has recently been shown to induce pore formation and cell internalization and to be responsible for cytotoxicity in cardiomyocytes (Wang, C.-H., Liu, J.-H., Lee, S.-C., Hsiao, C.-D., and Wu, W.-g. (2006) J. Biol. Chem. 281, 656-667). We show here that one of the non-cytotoxic CTXs, i.e. CTX A5 or cardiotoxin-like basic polypeptide, from Taiwan cobra specifically bound to alpha(v)beta3 integrin and inhibited bone resorption activity. We found that both membrane-bound and recombinant soluble alpha(v)beta3 integrins bound specifically to CTX A5 in a dose-dependent manner. Surface plasmon resonance analysis showed that human soluble alpha(v)beta3 bound to CTX A5 with an apparent affinity of approximately 0.3 microM. Calf pulmonary artery endothelial cells, which constitutively express alpha(v)beta3, showed a CTX A5 binding profile similar to that of membrane-bound and soluble alpha(v)beta3 integrins, suggesting that endothelial cells are a potential target for CTX action. We tested whether CTX A5 inhibits osteoclast differentiation and bone resorption, a process known to be involved in alpha(v)beta3 binding and inhibited by RGD-containing peptides. We demonstrate that CTX A5 inhibited both activities at a micromolar range by binding to murine alpha(v)beta3 integrin in osteoclasts and that CTX A5 co-localized with beta3 integrin. Finally, after comparing the integrin binding affinity among CTX homologs, we propose that the amino acid residues near the two loops of CTX A5 are involved in integrin binding. These results identify CTX A5 as a non-RGD integrin-binding protein with therapeutic potential as an integrin antagonist.

The utrophin A 5'-untranslated region confers internal ribosome entry site-mediated translational control during regeneration of skeletal muscle fibers

Utrophin up-regulation in muscle fibers of Duchenne muscular dystrophy patients represents a potential therapeutic strategy. It is thus important to delineate the regulatory events presiding over utrophin in muscle in attempts to develop pharmacological interventions aimed at increasing utrophin expression. A number of studies have now shown that under several experimental conditions, the abundance of utrophin is increased without a corresponding elevation in its mRNA. Here, we examine whether utrophin expression is regulated at the translational level in regenerating muscle fibers. Treatment of mouse tibialis anterior muscles with cardiotoxin to induce muscle degeneration/regeneration led to a large (approximately 14-fold) increase in the levels of utrophin A with a modest change in expression of its transcript (40%). Isolation of the mouse utrophin A 5'-untranslated region (UTR) revealed that it is relatively long with a predicted high degree of secondary structure. In control muscles, the 5'-UTR of utrophin A caused an inhibition upon translation of a reporter protein. Strikingly, this inhibition was removed during regeneration, indicating that expression of utrophin A in regenerating muscles is translationally regulated via its 5'-UTR. Using bicistronic reporter vectors, we observed that this translational effect involves an internal ribosome entry site in the utrophin A 5'-UTR. Thus, internal ribosome entry site-mediated translation of utrophin A can, at least partially, account for the discordant expression of utrophin A protein and transcript in regenerating muscle. These findings provide a novel target for up-regulating levels of utrophin A in Duchenne muscular dystrophy muscle fibers via pharmacological interventions.

Structural Basis of Citrate-dependent and Heparan Sulfate-mediated Cell Surface Retention of Cobra Cardiotoxin A3

Anionic citrate is a major component of venom, but the role of venom citrate in toxicity other than its inhibitory effect on the cation-dependent action of venom toxins is poorly understood. By immobilizing Chinese hamster ovary cells in microcapillary tubes and heparin on sensor chips, we demonstrated that heparan sulfate-mediated cell retention of the major cardiotoxin (CTX) from the Taiwan cobra, CTX A3, near membrane surfaces is citrate-dependent. X-ray determination of a CTX A3-heparin hexasaccharide complex structure at 2.4 A resolution revealed a molecular mechanism for toxin retention in which heparin-induced conformational changes of CTX A3 lead to citrate-mediated dimerization. A citrate ion bound to Lys-23 and Lys-31 near the tip of loop II stabilizes hydrophobic contact of the CTX A3 homodimer at the functionally important loop I and II regions. Additionally, the heparin hexasaccharide interacts with five CTX A3 molecules in the crystal structure, providing another mechanism whereby the toxin establishes a complex network of interactions that result in a strong interaction with cell surfaces presenting heparan sulfate. Our results suggest a novel role for venom citrate in biological activity and reveal a structural model that explains cell retention of cobra CTX A3 through heparan sulfate-CTX interactions.

Evidence that L1AD3, an apoptosis-inducing cyclic peptide, binds a leukemic T-cell membrane protein receptor

Human leukemic T-lymphocytes undergo extensive and rapid apoptosis in the presence of L1AD3, a small cyclic peptide derivative of cobra cardiotoxin. The first step in this process involves its binding to membranes of susceptible cells. By the use of a biotin "handle" synthetically incorporated at the N-terminus of L1AD3, we show that binding is saturable and selective: normal human peripheral blood lymphocytes do not bind this peptide. Fluorescence resonance energy transfer experiments indicate that the binding sites are separated by at least 55 A. Loss of binding occurs if membrane proteins are enzymatically degraded, suggesting that L1AD3's target is a cell-membrane surface protein receptor. Finally, crosslinking of cyclic BTNL1AD3 peptide to a leukemic T-cell membrane surface receptor, as examined using a biotin-avidin blot, indicated a molecular weight of approximately 34,400.

Evidence showing an intermolecular interaction between KChIP proteins and Taiwan cobra cardiotoxins

Direct protein-protein interaction between Taiwan cobra cardiotoxin3 (CTX3) and potassium channel-interacting proteins (KChIPs) was investigated in the present study. It was found that KChIPs bound with CTX3, in which KChIP and CTX3 formed a 1:1 complex as evidenced by the results of chemical cross-linking. Pull-down assay revealed that the intact EF-hands 3 and 4 of KChIP1 were critical for CTX3-binding. Likewise, removal of EF-hands 3 and 4 distorted the ability of KChIP1 to bind with Kv4.2 N-terminal fragment (KvN) as well as fluorescent probe 8-anilinonaphthalene-1-sulfonate (ANS). In contrast to the interaction between KChIP1 and KvN, the binding of CTX3 to KChIP1 showed a Ca(2+)-independent manner. Fluorescence measurement revealed that CTX3 affected the binding of ANS to Ca(2+)-bound KChIP1, but not Ca(2+)-free KChIP1. Alternatively, KChIP1 simultaneously bound with KvN and CTX3, and the interaction between KChIP1 and KvN was enhanced by CTX3. In terms of the fact that KChIPs regulate the electrophysiological properties of Kv K(+) channel, the potentiality of CTX for this biomedical application could be considered.

Modification of Substrate Inhibition of Synaptosomal Acetylcholinesterase by Cardiotoxins

Different types of cardiotoxin (I-V and n) were isolated and purified from the venom of the Taiwan cobra (Naja naja atra). The effects of these cardiotoxins were studied on membrane-bound acetylcholinesterase, which was isolated from a sheep's brain cortex. The results showed that cardiotoxins I-III, V, and n activated the enzyme by modification of substrate inhibition, but cardiotoxin IV's reaction was different. The inhibition and activation of acetylcholinesterase were linked to the functions of the hydrophobicity index, presence of a cationic cluster, and the accessible arginine residue. Our results indicate that Cardiotoxins have neither a cationic cluster nor an arginine residue in their surface area of loop I; therefore, in contrast to fasciculin, cardiotoxins are attached by loop II to the peripheral site of the enzyme. As a result, fasciculin seems to stabilize nonfunctional conformation, but cardiotoxins seem to stabilize the functional conformation of the enzyme. Based on our experimental and theoretical findings, similar secondary and tertiary structures of cardiotoxins and fasciculin seem to have an opposite function once they interact with acetylcholinesterase.

Role of bone marrow cell trafficking in replenishing skeletal muscle SP and MP cell populations

The multipotent nature of skeletal muscle-derived side population cells is demonstrated by their myogenic and hematopoietic potential in vivo. However, whether muscle side population cells are derived from the bone marrow is unclear. To study the long-term contribution of the hematopoietic system to muscle side population, whole bone marrow cells from Ly5.1 males or from e-GFP transgenic male mice were transplanted into lethally irradiated Ly5.2 females. Long-term cell trafficking of donor bone marrow cells to muscle side population was monitored 17 times in a 34-week study. Fluorescence-activated cell sorter analyses were used to detect Ly5.1 and GFP+ donor cells, which were confirmed by fluorescence in situ hybridization of the Y-chromosome. Analyses post-transplantation indicated that whereas cells of donor origin could be found in the muscle, donor bone marrow cells had contributed little to the muscle side population. Attempts to increase cell trafficking by induced muscle damage again confirmed that more than 90% of side population cells present in the muscle were derived from the host. These results demonstrate that muscle side population cells are not replenished by the bone marrow and suggest a non-hematopoietic origin for this cell population.

Cardiotoxin-III selectively enhances activation-induced apoptosis of human CD8+ T lymphocytes

Cardiotoxin-III (CTX-III), a major cardiotoxin isolated from the venom of the Taiwan cobra (Naja naja atra), is a highly basic, hydrophobic, toxic protein, which can induce lysis of mononuclear cells by an unknown mechanism. This study was undertaken to investigate the effects of CTX-III on untreated and PHA-activated peripheral blood mononuclear cells (PBMCs) in vitro. The results show that treatment of PHA-activated lymphocytes with CTX-III (10 microg/ml) induced apoptosis and depletion of the CD8(+) population. In both untreated and PHA-treated lymphocytes, interferon-gamma production was dramatically reduced and interleukin-2 (IL-2) production was moderately reduced by CTX-III treatment. In PHA-activated lymphocytes, CD4 expression was increased, whereas CD8 and IL-2R beta chain (CD25) expression were decreased. In contrast, CTX-III had no effect on the viability of PHA-activated monocytes but significantly enhanced their tumor necrosis factor-alpha production. These results show that CTX-III selectively enhanced activation-induced apoptosis in CD8(+) T cells. CTX-III was found to bind to the cell membrane of PHA-stimulated PBMCs, and three CTX-III-binding proteins, with molecular weights of 92, 77, and 68 kDa, were identified. We therefore propose that CTX-III interacts with one or more cell surface proteins and initiates a signal pathway causing functional changes. These findings provide an insight into the immunomodulatory properties of CTX-III and suggest a novel method for the selective induction of apoptosis in CD8(+) T lymphocytes.

Peripheral binding mode and penetration depth of cobra cardiotoxin on phospholipid membranes as studied by a combined FTIR and computer simulation approach

Cobra cardiotoxin, a cytotoxic beta-sheet basic polypeptide, is known to cause membrane leakage in many cells including human erythrocytes. Herein, we demonstrate that the major cobra cardiotoxin from Naja atra, CTX A3, can cause leakage of vesicle contents in phosphatidylglycerol (PG) and phosphatidylserine containing, but not in pure phosphatidylcholine (PC), membrane bilayers. By the combined polarized attenuated total reflection infrared spectroscopy and computer simulation studies, CTX A3 is shown to peripherally bind to both zwitterionic and anionic monolayers in a similar edgewise manner with a tilted angle of approximately 48 +/- 20 degrees between the beta-sheet plane of the CTX molecule and the normal of the membrane surface. The average surface area expansion induced by CTX A3 binding to the PG monolayer, however, is two times larger than that of the PC monolayer as determined by the Langmuir minitrough method. Interaction energy considerations of CTX A3 on neutral and negatively charged membrane surfaces suggests that the electrostatic interaction between anionic lipid and cationic CTXs plays a role in modulating the penetration depth of CTX molecules on the initial peripheral binding mode and reveals a pathway leading to the formation of an inserted mode in negatively charged membrane bilayers.

Interaction of the P-type cardiotoxin with phospholipid membranes

The cardiotoxin (cytotoxin II, or CTII) isolated from cobra snake (Naja oxiana) venom is a 60-residue basic membrane-active protein featuring three-finger beta sheet fold. To assess possible modes of CTII/membrane interaction 31P- and 1H-NMR spectroscopy was used to study binding of the toxin and its effect onto multilamellar vesicles (MLV) composed of either zwitterionic or anionic phospholipid, dipalmitoylglycerophosphocholine (Pam2Gro-PCho) or dipalmitoylglycerophosphoglycerol (Pam2Gro-PGro), respectively. The analysis of 1H-NMR linewidths of the toxin and 31P-NMR spectral lineshapes of the phospholipid as a function of temperature, lipid-to-protein ratios, and pH values showed that at least three distinct modes of CTII interaction with membranes exist: (a) nonpenetrating mode; in the gel state of the negatively charged MLV the toxin is bound to the surface electrostatically; the binding to Pam2Gro-PCho membranes was not observed; (b) penetrating mode; hydrophobic interactions develop due to penetration of the toxin into Pam2Gro-PGro membranes in the liquid-crystalline state; it is presumed that in this mode CTII is located at the membrane/water interface deepening the side-chains of hydrophobic residues at the tips of the loops 1-3 down to the boundary between the glycerol and acyl regions of the bilayer; (c) the penetrating mode gives way to isotropic phase, stoichiometrically well-defined CTII/phospholipid complexes at CTII/lipid ratio exceeding a threshold value which was found to depend at physiological pH values upon ionization of the imidazole ring of His31. Biological implications of the observed modes of the toxin-membrane interactions are discussed.

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.

Heparin binding stabilizes the membrane-bound form of cobra cardiotoxin

It has been shown previously that the long chain fragments of heparin bind to the beta-strand cationic belt of the three-finger cobra cardiotoxin (or cytotoxin, CTX) and hence enhance its penetration into phospholipid monolayer under physiological ionic conditions. By taking lysophosphatidylcholine (LPC) micelles as a membrane model, we have shown by (1)H NMR study that the binding of heparin-derived hexasaccharide (Hep-6) to CTX at the beta-strand region can induce conformational changes of CTX near its membrane binding loops and promote the binding activity of CTX toward LPC. The Fourier-transform infrared spectra and NMR nuclear Overhauser effect of Hep-6.CTX and CTX.LPC complex in aqueous buffer also supplemented the aforementioned observation. Thus, the detected conformational change may presumably be the result of structural coupling between the connecting loops and its beta-strands. This is the first documentation of results showing how the association of hydrophilic carbohydrate molecules with amphiphilic proteins can promote hydrophobic protein-lipid interaction via the stabilization of its membrane-bound form. A similar mechanism involving tripartite interactions of heparin, protein, and lipid molecules may be operative near the extracellular matrix of cell membranes.

Expression of cardiotoxin-2 gene. Cloning, characterization and deletion analysis of the promoter

This report is the first study of the regulation of expression of a toxin gene and it also demonstrates the novel finding that the cardiotoxin (CTX)-2 gene from Naja sputatrix is expressed in the venom gland as well as in other tissues in the snake, such as liver, heart and muscle. The venom gland produces a 500-bp (spliced) CTX-2 mRNA as the final transcript. However, the liver produces two types of CTX-2 mRNA, of which the unspliced transcript (1 kb) is predominant; the 500 bp spliced transcript is the minor species. This differential expression of the CTX gene has been attributed to the usage of alternative promoter consisting of independent TATA boxes and corresponding transcription initiation sites. Among the several transcription factors that have been identified by a search of the TFIID database, the participation of two glucocorticoid elements in the expression of the CTX gene has been demonstrated by promoter deletion analysis. Putative binding sites for SP-1, C/EBP, CACCC-binding factor and at least two unknown binding factors have also been identified by DNase I footprinting of the promoter.

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.

In situ hybridization and immunohistochemical analysis of the expression of cardiotoxin and neurotoxin genes in Naja naja sputatrix

Secretory processes and their regulation have been extensively studied in mammalian salivary parotid glands. However, little is known regarding the secretory mechanism in the venom glands of snakes, which invariably produce one of the most complex of all animal secretions. The pharmacologically important and toxic components of the Malayan spitting cobra (Naja naja sputatrix) venom include postsynaptic neurotoxins (NTX), presynaptic neurotoxins (phospholipase A2, PLA2), and cardiotoxins (CTX) which, for convenience, have been collectively referred to as "toxins." We report here for the first time the mechanism of toxin gene expression by studying the accumulated mRNA level and protein synthesis rates for the three toxins over a period of 8 days after stimulation of venom synthesis by manual "milking" of the venom gland. Immunofluorescence and in situ hybridization were used to localize the toxins and their mRNAs in venom gland sections. The rate of protein synthesis, as determined by immunofluorescence and liquid chromatography-mass spectrometry (LC-MS) techniques, increased in parallel with the increase in the toxin mRNA content in the secretory epithelial cells, suggesting that transcriptional regulation of the toxin genes is involved. (J Histochem Cytochem 47:551-560, 1999)

Unfolding and refolding of cardiotoxin III elucidated by reversible conversion of the native and scrambled species

Cardiotoxin analogue III (CTX III) isolated from the venom of the Taiwan Cobra (Naja naja atra) is a small molecular weight, all beta-sheet protein, cross-linked by four disulfide bridges. The unfolding and refolding mechanisms of CTX III have been examined by monitoring the reversible conversion of the native and scrambled species. It is found that, in the presence of a denaturant (urea/guanidinium hydrochloride) and a thiol catalyst, CTX III forms a mixture of scrambled species by shuffling its four native disulfide bonds. Complete unfolding of CTX III can be achieved using either 3.0-4.0 M guanidinium hydrochloride (GdmCl) or 5.0-6.0 M urea. It is observed that GdmCl is thermodynamically more potent but kinetically less efficient than urea in unfolding CTX III. The rate constants of unfolding of CTX III in 8 M urea are significantly greater than that obtained in 5.0 M GdmCl and 8.0 M GdmCl. Interestingly, upon removal of the denaturant, scrambled species of CTX III is found to refold spontaneously through dynamic reshuffling of the non-native disulfides to attain the native disulfide linkages. In addition, CTX III contains highly reactive lysines which are modified by trace amounts of cyanate contaminant which exists invariably even in high-grade urea solutions. The reactive lysines of CTX III are modified by cyanate both in the native and unfolded states of the protein. The modification is nonselective, and the modified product is found to consist of highly heterogeneous species. Surprisingly, these heterogeneous species of modified CTX III are observed to display stability and folding/unfolding properties indistinguishable from those of the native CTX III. The knowledge obtained from the present study, on the conditions to convert the scrambled species, could provide useful clues for a rational design for snake venom cardiotoxins with potential therapeutic applications.

Glycosaminoglycans bind to homologous cardiotoxins with different specificity

We have reported that some cardiotoxins (CTXs), homologous basic polypeptides of cobra venom, bind strongly to heparin. Herein we show that CTXs from spitting cobra venom bind avidly to chondroitin-6-sulfate (CS6) and dermatan sulfate (DS), the glycosaminoglycans (GAGs) abounding in the cornea and elsewhere. We compared the binding strength of Tgamma, a major component of spitting cobra, Naja nigricollis, venom with that of CTX A3, a major component of Naja atra venom to various GAGs including CS6, chondroitin-4-sulfate (CS4), DS, keratan sulfate (KS), hyaluronan (HYA), and heparin. The binding strength of Tgamma followed the order CS6 > KS > HYA > DS > CS4 > heparin, whereas that of CTX A3 was heparin > KS > CS4 > DS > CS6 > HYA. The binding specificity displayed by different CTXs toward GAGs is impressive, given the high homology among CTXs and among GAGs. Strong binding of Tgamma to CS6, rather than to the highly anionic and versatile cousin, heparin, implies specific interaction with CS6. Heparin, at high concentration, displaced CS6 from CS6-Tgamma and CS6-A3 complexes. We also show that corneal CS/DS likely allow Tgamma to bind to corneal epithelium. CTXs of spitting cobra venom are known to cause corneal opacity and/or blindness. Taken together with these observations, our results suggest that corneal CS/DS play a role in the action of CTX in the eye. Most importantly, the present results establish CTXs as cationic, readily available, avidly binding ligands of CS/DS.

The role of acetic acid in the prevention of salt-induced aggregation of snake venom cardiotoxins

Snake venom cardiotoxins (CTXs) exhibit a strong tendency to aggregate upon desalting and hence it is extremely difficult to prepare salt-free cardiotoxin(s). In the present study, we describe a new method for preparation of salt-free CTX based on dialysis against acetic acid. Based on experimental observation and the three dimensional solution structure of cardiotoxin analogue III from the Taiwan cobra (Naja naja atra), a molecular mechanism for the prevention of aggregation of cardiotoxins by acetic acid is discussed. In our opinion, the results obtained in the present study would pave way for elucidating the structural basis for the broad spectrum of biological activities exhibited by snake venom cardiotoxins.

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.

Analysis of binding of cobra cardiotoxins to heparin reveals a new beta-sheet heparin-binding structural motif

Heparin and heparan sulfate have recently been shown to bind to snake cardiotoxin (CTX) and to potentiate its penetration into phospholipid monolayer under physiological ionic conditions. Herein we analyze the heparin-binding domain of CTX using 10 CTXs from Taiwan and African cobra venom. We also performed computer modeling to obtain more information of the binding at molecular level. The results provide a molecular model for interaction of CTX-heparin complex where the cationic belt of the conserved residues on the concave surface of three finger beta-sheet polypeptides initiates ionic interaction with heparin-like molecules followed by specific binding of Lys residues near the tip of loop 2 of CTX. The dissociation constants of CTXs differ by as much as 4 orders of magnitude, ranging from approximately 140 microM for toxin gamma to approximately 20 nM for CTX M3, depending on the presence of Lys residues near the tip of loop 2. High affinity heparin binding becomes possible due to the presence of Arg-28, Lys-33, or the so-called consensus heparin binding sequence of XKKXXXKRX near the tip of the loop. The well defined three-finger loop structure of CTX provides an interesting template for the design of high affinity heparin-binding polypeptides with beta-sheet structure. The finding that several cobra CTXs and phospholipase A2 bind to heparin with different affinity may provide information on the synergistic action of the two venom proteins.

Crystal structure of cardiotoxin V from Taiwan cobra venom: pH-dependent conformational change and a novel membrane-binding motif identified in the three-finger loops of P-type cardiotoxin

The crystal structure of cardiotoxin V from Taiwan cobra venom (CTX A5) has been solved at pH 8.5 and refined to an R-factor of 20.7% for 7013 reflections [>2sigma(F)] between 8- and 2.19-A resolution. The refined model shows that CTX A5 exists as a dimer. The assembly consists of 974 non-hydrogen atoms from 124 residues and 73 water molecules. The global monomeric structure is similar to that determined by NMR at pH 3.7, characterized by a core formed by two beta-sheets connected with three-finger loops. However, local conformational differences are detected in two functionally important regions, loops I and II. A disparity between the NMR and X-ray structure of CTX A5 is detected near the tip of loop I and can be attributed to the difference in the protonation state of His4 at different pH, resulting in a reorientation of the His4 imidazole ring. A concerted motion of amino acid side chains located near His4 is detected and possibly contributes to the pH-dependent binding ability of CTX A5 to phospholipid model membranes. The second difference, detected at the tip of loop II, is due to the hydrophobic contact between CTX dimers in the crystal packing and the interaction of water molecules with amino acid residues in the loop II region of the CTX containing Pro31 (P-type CTX). This interaction forces loop II into a more rigid omega shape bridging the main chain at positions 27 and 34, contradictory to the flexible, tapering shape detected by NMR. Thus, a novel continuous hydrophobic column capable of binding to and possibly penetrating the membrane lipid bilayer is formed by the tips of the three-finger loops. In this respect, the X-ray crystal structure of CTX A5 may represent the CTX structure in the membrane-binding mode.

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.

Induction of helical conformation in all beta-sheet proteins by trifluoroethanol

The effect of 2,2,2-Trifluoroethanol (TFE) on the structure of five all beta-sheet proteins, isolated from the venom of the Taiwan cobra (Naja naja atra), is studied. In all the toxins used, it is observed that significant amount of alpha-helix is induced at higher concentrations of TFE. In all these proteins, the induction of helical conformation and disruption of the tertiary structure seem to occur simultaneously. The structural transitions induced by TFE in reduced and denatured protein appear to be different from those observed in the native protein(s). In our opinion, the findings reported herein could have significant implications on research in the area of protein folding.

Conformational change and inactivation of membrane phospholipid-related activity of cardiotoxin V from Taiwan cobra venom at acidic pH

The phospholipid binding activity of cardiotoxin V from Naja naja atra (CTX A5) was studied by use of Langmuir monolayers and found to exhibit pH-dependence in binding to phosphatidylcholine membrane with an apparent pKa around 6.0. Proton NMR investigation of the CTX A5 molecule in the presence of phosphatidylcholine micelles reveals a decrease in association of CTX A5 with membranes at low pH as a result of the protonation of His-4 near the membrane binding site of loop I region of CTX. The pH-dependent binding can be attributed mainly, but not solely, to the change in charge content of the CTX molecule upon His-4 protonation at the membrane/water interface. This is shown by analyzing the pH- and ionic strength dependence of binding of CTXs to phospholipid monolayers according to Gouy-Chapman theory. The protonation of the His-4 residue also results in a local conformational change in the loop I region since the chemical shifts of amide protons for the amino acid residues from Cys-3 to Thr-14 are all found to vary as a function of pH with an apparent pKa similar to that of His-4. Interestingly, the effect is relayed to other amino acid residues in the structural core of the protein such as those in C-terminal (Lys-60, Cys-61, and Asn-62) and triple-stranded antiparallel beta-sheet (Cys-22, Lys-24, Ala-25, Arg-38, and Ala-41) regions. An additional local conformational change in the molecule results around pH 5 as evidenced by circular dichroism spectroscopic studies, although this change does not affect the characteristic beta-sheet and three-finger loop structure of CTX molecule as revealed by two-dimensional NOESY 1H NMR study. The latter conformational change at acidic pH, however, completely inactivates CTX-induced aggregation/fusion activity of sphingomyelin vesicles. The results suggest that deciphering the functional sites of CTXs on the basis of structure and dynamics determined at low pH should be done with caution. Since 19 out of 44 CTX homologues with known amino acid sequence contain His-4, the effect of His-4 on the structure and function of CTX molecules is important and is discussed in terms of the diverse membrane targets of CTX subtypes. Also discussed is the pH-induced activation of snake venom proteins in the victim.

The use of fluoresceinphosphatidylethanolamine (FPE) as a real-time probe for peptide-membrane interactions

The characterization of fluoresceinphosphatidylethanolamine (FPE) as a real-time indicator of the electrostatic nature of a membrane surface is described. The conditions appropriate for the labelling of membranes and the implementation of FPE as a tool to monitor the interactions of various peptides with model membranes are outlined. It is shown that of the membrane-active peptides studied, Naja naja kaouthia cardiotoxin and pyrularia thionin bind to certain model membranes without insertion. Whereas the leader sequence of the nuclear encoded subunit IV of mammalian cytochrome c oxidase (E.C. 1.9.3.1), known as p-25, and melittin appear to bind and then partially insert into the membrane. It seems evident also that melittin does not adopt a fully transmembrane configuration. Melittin is known to promote membrane lysis and by employing a rapid-kinetic technique it is shown that the time-course of such lysis does not appear to correlate with peptide binding, but following binding a significant proportion of melittin must become inserted into the membrane before lysis appears to commence.

Binding properties of Pyrularia thionin and Naja naja kaouthia cardiotoxin to human and animal erythrocytes and to murine P388 cells

Pyrularia thionin and snake venom cardiotoxin are strongly basic peptides which induce hemolysis, depolarization of muscle cells and activation of endogenous phospholipase A2. An earlier study of the hemolysis reaction indicated that the two peptides bind to and compete for the same site on human erythrocytes. A recent study examined the hemolysis induced by both peptides as the phosphate and Ca2+ content of the reaction mixture was varied. The results of the recent study (VERNON, L. P. and ROGERS, A., Toxicon 30, 701-709) agree with this companion study on the binding of 125I-labeled pyrularia thionin and cardiotoxin to erythrocytes under the same conditions. Added phosphate ion at 5 mM and removal of membrane-bound Ca2+ by treatment with 10 mM EGTA make more binding sites of the same affinity available to both peptides, which are shown to bind in a competitive fashion to the same site. Addition of 10 mM Ca2+ to the medium decreases peptide binding due to competitive binding of Ca2+ to the same site on the membrane. For human erythrocytes the number of binding sites/cell for the thionin ranged from 0.7 to 1.7 x 10(5) and for cardiotoxin from 0.82 to 1.6 x 10(5). The calculated dissociation constants (Kd) from the Scatchard plots ranged from 0.43 to 1.1 microM for the thionin and from 0.40 to 0.98 microM for the cardiotoxin. The binding sites for thionin and cardiotoxin with sheep erythrocytes were 1.7 and 2.0 x 10(4) sites/cell, respectively, and both cow and horse erythrocytes demonstrated 2.7 x 10(4) sites/cell for the thionin. Binding studies with murine P388 cells showed 7.0 and 9.5 x 10(6) sites per cell for Pyrularia thionin and cardiotoxin, respectively.

A bioassay for cobra cardiotoxin activity using semi-isolated cockroach heart

A semi-isolated cockroach heart preparation was used to rapidly determine the activity of cobra cardiotoxin, monitored as a direct response on heart rate. This preparation produced a dose-response curve in the presence of active cardiotoxin and demonstrated that cardiotoxin retained its biological activity after boiling, although cardiotoxin activity was destroyed by heating in the presence of dithiothreitol. Experiments that cross-linked radiolabeled cardiotoxin to solubilized cockroach heart membranes suggested that cardiotoxin bound specifically to a 59,000 mol. wt membrane protein in this tissue.

Membrane interactions of amphiphilic polypeptides mastoparan, melittin, polymyxin B, and cardiotoxin. Differential inhibition of protein kinase C, Ca2+/calmodulin-dependent protein kinase II and synaptosomal membrane Na,K-ATPase, and Na+ pump and differentiation of HL60 cells

Interactions of certain naturally occurring, amphiphilic polypeptides with membranes were investigated. Mastoparan (wasp venom toxin), melittin (bee venom toxin), cardiotoxin (cobra venom toxin), and polymyxin B (antibacterial antibiotic) inhibited protein kinase C stimulated by phosphatidylserine bilayer or arachidonate monomer and blocked binding of [3H] phorbol 12,13-dibutyrate to protein kinase C in the presence of phosphatidylserine bilayer, with IC50 values (concentrations causing 50% inhibition) of 1-8 microM. Mastoparan and polymyxin B were much less inhibitory (IC50, 10-20 microM), whereas melittin and cardiotoxin were similarly inhibitory (IC50, 1-4 microM), when protein kinase C was activated instead by synaptosomal membrane. Kinetic analysis indicate that mastoparan inhibited protein kinase C, assayed using phosphatidylserine or synaptosomal membrane as the phospholipid cofactor, competitively with the phospholipid cofactor, in a mixed manner with CaCl2 or diacylglycerol, noncompetitively with histone, and uncompetitively with ATP, with apparent Ki values of 1.6-18.7 microM. Inhibition of Na,K-ATPase in the membrane by these polypeptides had relative potencies different from those for their inhibition of protein kinase C activated by the same membrane preparation; mastoparan and melittin inhibited the two activities with comparable potencies, but polymyxin B and cardiotoxin were far less effective in inhibiting Na,K-ATPase. The same relative inhibitory potencies of the polypeptides (melittin greater than mastoparan greater than polymyxin B) for inhibition of Na,K-ATPase were also noted for their inhibition of Ca2+/calmodulin-dependent protein kinase II, 86Rb uptake (Na+ pump) by HL60 cells and the phorbol ester-induced differentiation of the leukemia cells. These findings were consistent with discrete interactions of the polypeptides with functionally distinct sites on the membrane, leading to differential inhibition of biological activities associated with the membrane. Actions of certain polypeptides appeared to be more specific compared to those of lipid second messengers such as lyso-phosphatidylcholine and sphingosine, and the antineoplastic ether lipid analogs such as 1-O-octadecyl-2-methyl-rac-glycero-3-ophosphocholine.

Role of cationic residues in cytolytic activity: modification of lysine residues in the cardiotoxin from Naja nigricollis venom and correlation between cytolytic and antiplatelet activity

Cardiotoxins and postsynaptic neurotoxins from snake venoms have similar primary, secondary, and tertiary structures. Cardiotoxins, however, in contrast to neurotoxins, exhibit general cytotoxicity. Comparison of the distribution of hydrophobic and charged amino acid residues in the three-dimensional structures of lytic cardiotoxins and nonlytic neurotoxins indicates the presence of a cationic site associated with a hydrophobic surface in cardiotoxins, but not in neurotoxins. A cationic site flanked by a hydrophobic site is a common structural feature shared by a wide variety of unrelated cytolysins and is predicted to determine the lytic activity of a large group of cytolysins. To determine the essential nature of the cationic site in cardiotoxin CTX-1 from Naja nigricollis crawshawii venom, we modified the positive charges of nine Lys residues to negative, neutral, or positive charges by succinylation, carbamylation, or guanidination, respectively. Circular dichroism studies indicated that these modifications did not affect the conformation of the cardiotoxin. Binding of the modified cardiotoxins to phospholipids was demonstrated by changes in the intrinsic fluorescence of native and modified CTX-1 after binding to phospholipid vesicles, and by resonance energy transfer with anthracene-phospholipid vesicles. Phospholipid binding was not affected by these modifications, but their binding preference was determined by the electrostatic interactions between the polypeptide and phospholipid. Both positively charged native and guanidinated CTX-1 showed direct lytic activity on human erythrocytes and platelets, whereas the succinylated or carbamylated derivatives did not show lytic activity. The loss of lytic activity cannot be related to conformational changes or phospholipid binding abilities of the modified cardiotoxins.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of snake venom cardiotoxin (a membrane-disruptive polypeptide) with human erythrocytes

The action of 7.2 microM cardiotoxin on 0.25% human erythrocytes in a plasma extender solution was studied by the interaction of toxin with intact red blood cells and subsequent hemolysis of the cells. The binding of toxin to cells was completed within 10 min, whereas the membrane rigidity was weakened in a non-lytic period for about 25 min. The toxin molecules bound almost exclusively to the membrane. The bound toxin could not be liberated with either 0.5% Triton X-100 or 0.1 N NaOH. The degree of binding was slightly reduced in the presence of 10 mM mono- and divalent inorganic salts. The action of toxin might weaken the in situ association of several proteins that are linked with band 3 protein of the membrane, thus making the cells fragile and altering the shape of the cell to a smooth sphere.

Binding of cardiotoxin analogue III from Formosan cobra venom to FL cells

The binding equilibrium at 37 or 0 degrees C of 125I-cardiotoxin analogue III (CT III) to fetal lung (FL) cells (cultured human amnion cells) was achieved within 1 h, and the binding at 37 degrees C was irreversible. The Scatchard analysis at 37 degrees C on the binding of 125I-CT III indicated that FL cells had two types of binding sites with different association constants. The association constant and the number of high-affinity sites was 1.1 X 10(10) mol-1 or 2.8 X 10(6) per FL cell, respectively. At 37 or 0 degrees C, the cytotoxicity of CT III paralleled the amount of bound CT III to FL cells, and at 37 degrees C was inhibited by the presence of acidic phospholipids.

Circular dichroism study of the unfolding-refolding of a cardiotoxin from Taiwan cobra (Naja naja atra) venom

Circular dichroism spectroscopy has been used to study the unfolding-refolding process of a cardiotoxin from Taiwan cobra (Naja naja atra) venom upon addition of fluoroalcohols or sodium dodecyl sulfate (SDS) to its aqueous solution. In these experiments, the disulfide bridges remained intact. The unfolding process has been found to be reversible both for fluoroalcohols and for SDS unfolding. The reversibility of the unfolding-refolding process of cardiotoxin in aqueous mixtures of fluoroalcohols was dependent on the volume per volume ratio of alcohol to water. SDS did not unfold the secondary structures of cardiotoxin whereas its tertiary structure was affected. If the SDS concentration in aqueous solution exceeded the critical micelle concentration value of SDS, a quasi-refolded state of cardiotoxin was observed. The mechanism of unfolding-refolding is discussed in terms of molecular interactions which might govern the protein conformation in solution.

The binding of snake venom cardiotoxins to heart cell membranes

Cobra venom cardiotoxins have the effect, inter alia, of causing systolic arrest of the heart. We have observed significant binding in vitro of 35S-labelled cardiotoxins to mouse heart cell membranes. Part of the binding was saturable and could be displaced with homologous unlabelled cardiotoxins but not by neurotoxins or cardiotoxins inactivated by chemical modification. The specifically bound component represented more than 70% of total binding at saturation. Inclusion of Triton X-100 and NaCl in the phosphate-buffered incubation medium prevented nonspecific adsorption to centrifuge tube walls, and gave lower but more reproducible specific binding results, respectively. An apparent dissociation constant of 5 . 10(-7) M and a binding density of 500 pmol toxin/mg membrane protein were derived from the saturation isotherms.