Possible mechanisms of action of cobra snake venom cardiotoxins and bee venom melittin

Genotoxic potential of bee venom (Apis Mellifera) on human peripheral blood lymphocytes in vitro using single cell gel electrophoresis assay

Bee venom (BV) has been known to have therapeutic applications in traditional medicine to treat variety of diseases. It is also known that bee venom possesses anti-inflammatory and anticancer effects and that it can inhibit proliferation and induces apoptosis in cancer cells, but there is lack of information regarding genotoxicity of whole bee venom on normal human cells. In the present study, peripheral blood human lymphocytes from healthy donor were exposed in vitro to different concentration (5, 10, 25, 50 and 100 micro g/mL) of whole bee venom at different time periods (1, 6 and 24 hours). The single cell gel electrophoresis (SCGE) assay was used to evaluate the genotoxicity towards human cells. Results showed statistically significant increase in DNA damage caused in BV treated human lymphocytes compared to corresponding control cells for the tail length and tail moment. These results show that the extent of DNA damage, determined by the use of single cell gel electrophoresis is time and dose dependent. Based on the results it is clear that whole bee venom induces DNA damage and has genotoxic potential on human peripheral blood lymphocytes in vitro.

Phospholipase A2 Activation Enhances Inhibitory Synaptic Transmission in Rat Substantia Gelatinosa Neurons

Phospholipase A2 (PLA2) activation enhances glutamatergic excitatory synaptic transmission in substantia gelatinosa (SG) neurons, which play a pivotal role in regulating nociceptive transmission in the spinal cord. By using melittin as a tool to activate PLA2, we examined the effect of PLA2 activation on spontaneous inhibitory postsynaptic currents (sIPSCs) recorded at 0 mV in SG neurons of adult rat spinal cord slices by use of the whole cell patch-clamp technique. Melittin enhanced the frequency and amplitude of GABAergic and glycinergic sIPSCs. The enhancement of GABAergic but not glycinergic transmission was largely depressed by Na+ channel blocker tetrodotoxin or glutamate-receptor antagonists (6-cyano-7-nitroquinoxaline-2,3-dione and/or dl-2-amino-5-phosphonovaleric acid) and also in a Ca2+-free Krebs solution. The effects of melittin on glycinergic sIPSC frequency and amplitude were dose-dependent with an effective concentration of ∼0.7 μM for half-maximal effect and were depressed by PLA2 inhibitor 4-bromophenacyl bromide or aristolochic acid. The melittin-induced enhancement of glycinergic transmission was depressed by lipoxygenase inhibitor nordihydroguaiaretic acid but not cyclooxygenase inhibitor indomethacin. These results indicate that the activation of PLA2 in the SG enhances GABAergic and glycinergic inhibitory transmission in SG neurons. The former action is mediated by glutamate-receptor activation and neuronal activity increase, possibly the facilitatory effect of PLA2 activation on excitatory transmission, whereas the latter action is due to PLA2 and subsequent lipoxygenase activation and is independent of extracellular Ca2+. It is suggested that PLA2 activation in the SG could enhance not only excitatory but also inhibitory transmission, resulting in the modulation of nociception.

Melittin: a membrane-active peptide with diverse functions

Melittin is the principal toxic component in the venom of the European honey bee Apis mellifera and is a cationic, hemolytic peptide. It is a small linear peptide composed of 26 amino acid residues in which the amino-terminal region is predominantly hydrophobic whereas the carboxy-terminal region is hydrophilic due to the presence of a stretch of positively charged amino acids. This amphiphilic property of melittin has resulted in melittin being used as a suitable model peptide for monitoring lipid-protein interactions in membranes. In this review, the solution and membrane properties of melittin are highlighted, with an emphasis on melittin-membrane interaction using biophysical approaches. The recent applications of melittin in various cellular processes are discussed.

Real-time structural investigation of a lipid bilayer during its interaction with melittin using sum frequency generation vibrational spectroscopy

Interactions between membrane bilayers and peptides/proteins are ubiquitous throughout a cell. To determine the structure of membrane bilayers and the associated peptides/proteins, model systems such as supported lipid bilayers are often used. It has been difficult to directly investigate the interactions between a single membrane bilayer and peptides/proteins without exogenous labeling. In this work we demonstrate that sum frequency generation vibrational spectroscopy can be employed to study the interactions between peptides/proteins and a single lipid bilayer in real time, in situ, and without exogenous labeling. Using melittin and a dipalmitoyl phosphatidylglycerol bilayer as a model system, we monitored the C-H and C-D stretching signals from isotopically symmetric or asymmetric dipalmitoyl phosphatidylglycerol bilayers during their interaction with melittin. It has been found that the extent and kinetics of bilayer perturbation induced by melittin are very sensitive to melittin concentration. Such concentration dependence is correlated to melittin's mode of action. Melittin is found to function via the early and late stage of the carpet model at low and high concentrations, respectively, whereas the toroidal model is probable at intermediate concentrations. This research illustrates the potential of sum frequency generation as a biophysical technique to monitor individual leaflet structure of lipid bilayers in real time during their interactions with biomolecules.

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.

Heat stress facilitates the regeneration of injured skeletal muscle in rats

BACKGROUND

Skeletal muscle stem cells, so-called muscle satellite cells, are responsible for the repair and the regeneration of adult skeletal muscle tissues. Heat stress can facilitate the proliferation and the differentiation of myoblasts in vitro and can enhance their proliferative potential, which may stimulate the regrowth of atrophied skeletal muscle. The purpose of this study was to investigate the effect of heat stress on the regeneration of skeletal muscle injury induced by cardiotoxin.

METHODS

Male Wistar rats, aged 7 weeks, were randomly divided into six groups: a nonheated control group that received a physiological saline injection, a group heat stressed before physiological saline injection, a group heat stressed after physiological saline injection, a group injected with cardiotoxin without heat stress, a group heat stressed before cardiotoxin injection, and a group heat stressed after cardiotoxin injection (25 in each group). To initiate muscle injury and regeneration, 0.5 ml of 10 microM cardiotoxin was injected into the left tibialis anterior muscle. Conscious rats in some groups were exposed to environmental heat stress (41 degrees C for 60 min) in a heat chamber 24 h before or immediately after cardiotoxin or physiological saline injection. The heating protocol in the present study causes an increase in the colonic temperature to 41 degrees C. The left tibialis anterior muscles were dissected 1, 3, 7, 14, and 28 days after injection of cardiotoxin or physiological saline.

RESULTS

The wet weight and water content of muscles increased 1 day after cardiotoxin injection regardless of the application of heat stress, but normalized after 7-14 days. The muscle protein content in control rats had increased 7 days after heat stress. Although the muscle protein content decreased on cardiotoxin injection, heat stress caused a significant recovery in protein level. Expression of heat shock protein 72 (HSP72) and the number of Pax7-positive nuclei decreased after cardiotoxin injection but increased on the application of heat stress in both normal control and cardiotoxin-injected groups.

CONCLUSIONS

Heat stress stimulated not only the proliferation of satellite cells but also protein synthesis during the regeneration of injured skeletal muscle. It is thus strongly suggested that the heating of injured skeletal muscle may facilitate recovery. There was no direct relationship between the level of HSP72 expression and muscle protein content, suggesting that HSP72 expression may not be the key signal for protein synthesis in the necrosis-regeneration process.

Glycosylated neuropeptides: a new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.

Phospholipase A2 activation by melittin enhances spontaneous glutamatergic excitatory transmission in rat substantia gelatinosa neurons

In order to know a role of phospholipase A2 in modulating nociceptive transmission, the effect of a secreted phospholipase A2 activator melittin on spontaneous glutamatergic excitatory transmission was investigated in substantia gelatinosa neurons of an adult rat spinal cord slice by using the whole-cell patch-clamp technique. Bath-applied melittin at concentrations higher than 0.5 microM increased both the amplitude and the frequency of spontaneous excitatory postsynaptic current in a manner independent of tetrodotoxin; the latter effect of which was examined in detail. In 80% of the neurons examined (n = 64), melittin superfused for 3 min gradually increased spontaneous excitatory postsynaptic current frequency (by 65+/-6% at 1 microM; n = 51) in a dose-dependent manner (effective concentration for half-maximal effect = 1.1 microM). This effect subsided within 3 min after washout. The spontaneous excitatory postsynaptic current frequency increase produced by melittin was reduced by the phospholipase A2 inhibitor 4-bromophenacryl bromide (10 microM) while being unaffected by the cyclooxygenase inhibitor indomethacin (100 microM) and the lipoxygenase inhibitor nordihydroguaiaretic acid (100 microM). A similar increase in spontaneous excitatory postsynaptic current frequency was produced by exogenous arachidonic acid (50 microM); this effect was also unaffected by the cyclooxygenase or lipoxygenase inhibitor. Melittin failed to increase spontaneous excitatory postsynaptic current frequency in a nominally Ca2+-free or La3+-containing Krebs solution. We conclude that melittin increases the spontaneous release of L-glutamate to substantia gelatinosa neurons by activating secreted phospholipase A2 and increasing Ca2+ influx through voltage-gated Ca2+ channels in nerve terminals, probably with an involvement of arachidonic acid but not its metabolites produced by cyclooxygenase and lipoxygenase. Considering that the substantia gelatinosa plays an important role in regulating nociceptive transmission, it is suggested that this transmission may be positively modulated by secreted phospholipase A2 activation in the substantia gelatinosa.

Analyses of dose-response curves to compare the antimicrobial activity of model cationic alpha-helical peptides highlights the necessity for a minimum of two activity parameters

To assess and compare different model Leu-Lys-containing cationic alpha-helical peptides, their antimicrobial activities were tested against Escherichia coli as target organism over a broad peptide concentration range. The natural cationic alpha-helical peptides magainin 2 and PGLa and the cyclic cationic peptide gramicidin S were also tested between comparison. The dose-response curves differed widely for these peptides, making it difficult to rank them into an activity order over the whole concentration range. We therefore compared five different inhibition parameters from dose-response curves: IC(min) (lowest concentration leading to growth inhibition), IC(50) (concentration that gives 50% growth inhibition), IC(max) (related to minimum inhibition concentration and minimum bactericidal concentration), inhibition concentration factor (IC(F); describing the increase in concentration of the peptide between minimum and maximum inhibition), and activity slope (A(S); related to the Hill coefficient). We found that these parameters were covariant: two of them sufficed to characterize the dose dependence and hence the activity of the peptides. This was corroborated by showing that the dose dependences followed the Hill equation, with a small, constant aberration. We propose that the activity of antimicrobial peptides can readily be characterized by both IC(50) and IC(F) (or A(S)) rather than by a single parameter and discuss how this may relate to investigations into their mechanisms of action.

Cancer cell injury by cytotoxins from cobra venom is mediated through lysosomal damage

Cytotoxins from cobra venom are known to manifest cytotoxicity in various cell types. It is widely accepted that the plasma membrane is a target of cytotoxins, but the mechanism of their action remains obscure. Using the confocal spectral imaging technique, we show for the first time that cytotoxins from cobra venom penetrate readily into living cancer cells and accumulate markedly in lysosomes. Cytotoxins CT1 and CT2 from Naja oxiana, CT3 from Naja kaouthia and CT1 from Naja haje are demonstrated to possess this property with respect to human lung adenocarcinoma A549 and promyelocytic leukaemia HL60 cells. Immobilized plasma membrane binding accompanies the internalization of CT3 from Naja kaouthia in the HL60 cells, but it is very weak for other cytotoxins. Detectable membrane binding is not a property of any of the cytotoxins tested in A549 cells. The kinetics and concentration-dependence of cytotoxin accumulation in lysosomes correlate well with their cytotoxic effects. On the basis of the results obtained, we propose that lysosomes are a primary target of the lytic action of cytotoxins. Plasma membrane permeabilization seems to be a downstream event relative to lysosome rupture. Direct damage to the plasma membrane may be a complementary mechanism, but its relative contribution to the cytotoxic action depends on the cytotoxin structure and cell type.

Cobra cardiotoxin and phospholipase A2 as GAG-binding toxins: on the path from structure to cardiotoxicity and inflammation

Glycosaminoglycans (GAGs) represent the sulfated carbohydrate moieties of proteoglycans which occur abundantly in tissues of the cardiovascular system. Many proteins bind specifically to GAGs and perform an important role in inflammation, cell proliferation, and blood coagulation processes. Recently, in vitro GAG-binding studies of cardiotoxins (CTXs) and basic phospholipase A(2) (PLA(2)) from cobra venom established the toxins as two new families of GAG-binding proteins. In particular, discontinuous basic residues in beta-sheet CTXs may form a cationic cradle suitable for heparin binding, as in the case of fibronectin module III-13. The binding specificity of beta-sheet proteins to different GAGs can be further enhanced by involving other cationic clusters near the flexible loop of the molecule. Since the three-dimensional structures of many CTXs and PLA(2) are available, these two toxins may serve as models for the elucidation of the molecular recognition of GAG-binding proteins and also as polypeptide templates for further improvement of the binding specificity suitable for future biomedical application. Research along the line of GAG-guided toxicity of cobra venom components may help us to understand the functional role of GAGs and the action mechanism of cobra venom components in the cardiovascular system.

Development of specific inhibitors for heparin-binding proteins based on the cobra cardiotoxin structure: an effective synthetic strategy for rationally modified heparin-like disaccharides and a trisaccharide

Recently, a new heparin disaccharide-binding site on the convex side of cobra cardiotoxin (CTX) was identified by NMR spectroscopy and molecular modeling. To further characterize this site two heparin-like disaccharides were synthesized for binding studies with CTX, and a trisaccharide was synthesized for testing the sequence of the disaccharide binding to CTX. Thus six differentially protected monosaccharide building blocks (three l-iduronic acids and three d-glucosamines) were prepared. These include a l-iduronic acid elongation building block namely methyl 2-O-acetyl-4-O-levulinoyl-3-O-pivaloyl-alpha-l-idopyranosyluronate trichloroacetimidate for which a single-crystal X-ray structure was determined to have M(r)=576.79, a=9.3098(11)A alpha=90 degrees , b=10.3967(12)A beta=90 degrees , c=28.026(3)A gamma=90 degrees , V=2712.7(6)A(3), P2(1)2(1)2(1), Z=4, mu=0.71073A, and R=0.0378 for 7586 observed reflections. It shows that the molecular structure of the donor is in the (1)C(4) conformation with significant 1,3-diaxial interactions between O-1 and O-3 as well as O-2 and O-4. The disaccharides and trisaccharide vary in the degree and position of O- and N-sulfation. The pivaloyl group was used as permanent protecting group of hydroxyl. The levulinoyl group was used as the temporary protecting group to protect the hydroxyl for elongation.

Mac-1(low) early myeloid cells in the bone marrow-derived SP fraction migrate into injured skeletal muscle and participate in muscle regeneration

Recent studies have shown that bone marrow (BM) cells, including the BM side population (BM-SP) cells that enrich hematopoietic stem cells (HSCs), are incorporated into skeletal muscle during regeneration, but it is not clear how and what kinds of BM cells contribute to muscle fiber regeneration. We found that a large number of SP cells migrated from BM to muscles following injury in BM-transplanted mice. These BM-derived SP cells in regenerating muscles expressed different surface markers from those of HSCs and could not reconstitute the mouse blood system. BM-derived SP/Mac-1(low) cells increased in number in regenerating muscles following injury. Importantly, our co-culture studies with activated satellite cells revealed that this fraction carried significant potential for myogenic differentiation. By contrast, mature inflammatory (Mac-1(high)) cells showed negligible myogenic activities. Further, these BM-derived SP/Mac-1(low) cells gave rise to mononucleate myocytes, indicating that their myogenesis was not caused by stochastic fusion with host myogenic cells, although they required cell-to-cell contact with myogenic cells for muscle differentiation. Taken together, our data suggest that neither HSCs nor mature inflammatory cells, but Mac-1(low) early myeloid cells in the BM-derived SP fraction, play an important role in regenerating skeletal muscles.

Responses of fish chromatophore-based cytosensor to a broad range of biological agents

A cytosensor based on living chromatophores from Betta splendens Siamese fighting fish was used to test several classes of biologically active agents. Tested agents include neurotransmitters, adenyl cyclase activators, cytoskeleton effectors, cell membrane effectors and protein synthesis inhibitors. Characteristic cell responses were analyzed, and potential cytosensor applications were considered. Streptococcus pyogenes toxins streptolysin S and streptolysin O, Clostridium tetani tetanolysin, Staphylococcus aureus alpha-toxin and Vibrio parahemolyticus hemolysin, all bacterial toxins that act on cell membranes, elicited a strong response from chromatophores. A comparison of purified toxin to actual bacterial culture from Vibrio parahemolyticus demonstrated a nearly identical chromatophore cell response pattern. This suggests that the cytosensor response is reflective of bacterial toxin production.

Most cited papers in Toxicon

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

Melittin induces both time-dependent aggregation and inhibition of Na,K-ATPase from duck salt glands however these two processes appear to occur independently

Using cupric phenanthroline as a cross-linking agent, we have shown that melittin induced time-dependent aggregations of Na,K-ATPase in microsomal fractions and in preparations of purified Na,K-ATPase from duck salt glands. Incubation of melittin with these preparations also led to the progressive loss of Na,K-ATPase activity. At melittin/protein molar ratio of 5:1, we did not observe inhibition of Na,K-ATPase in the microsomal fraction but the process of enzyme aggregation occurred. At higher melittin/protein molar ratios (10:1 and 30:1), the inhibition of the enzyme and its aggregation proceeded simultaneously but the rates of these processes and maximal values achieved were different. At a melittin/protein ratio of 30:1, Na,K-ATPase inhibition may be described as a biexponential curve with the values for pseudo-first order rate constants being 2.7 and 0.15 min(-1). However, the aggregation may be presented by a monoexponential curve with a pseudo-first order rate constant of 0.15 min(-1). In purified preparations of Na,K-ATPase, the maximal aggregation (about 90%) was achieved at a melittin/protein molar ratio of 2:1, and a further increase in the melittin/protein ratio increased the rate of aggregation but did not affect the value of maximal aggregation. The results show that melittin induced both aggregation and inhibition of Na,K-ATPase but these two processes proceeded independently.

The water-soluble fraction (<10 kD) of bee venom (Apis mellifera) produces inhibitory effect on apical transporters in renal proximal tubule cells

Human envenomation caused by bee stings has been reported to cause acute renal failure and the pathogenetic mechanisms of these renal functional changes are still unclear. Bee venom is also a complex mixture of enzymes and proteins. Thus, this study was conducted to examine the effects of bee venom (BV, Apis mellifera) fractions on apical transporters' activity and its related signal pathways in primary cultured renal proximal tubule cells. Whole BV was extracted into three fractions according to solubility [a water-soluble fraction (BVA), an ethylacetate-soluble fraction (BVE), and a hexane-soluble fraction (BVH)]. BVA fraction was further separated to three portions according to molecular weights: BF1 (>20 kD), BF2 (10-20 kD), and BF3 (<10 kD). Each fraction was treated to the PTCs to the ratio of BV (1 microg/ml). BVA (930 ng/ml) significantly decreased cell viability, but BVH (27 ng/ml) and BVE (43 ng/ml) did not. BF3 (710 ng/ml) among BVA fractions predominantly decreased cell viability and inhibited alpha-methyl-D-glucopyranoside (alpha-MG), phosphate (Pi), and Na(+) uptake. In addition, BF3 increased [(3)H] arachidonic acid release, lipid peroxide formation, and Ca(2+) uptake. These effects of BF3 were blocked by mepacrine and AACOCF(3) (phospholipase A(2) inhibitors) or N-acetylcysteine, vitamin C, and vitamin E (antioxidants). In conclusion, BF3 (<10 kD) among BV fractions is the most effective portion in BV-induced inhibition of alpha-MG, P(i), and Na(+) uptake and these effects of BF3 are associated with phospholipase A(2)-oxidative stress-Ca(2+) signal cascade in the primary cultured rabbit renal proximal tubule cells.

Expression profiling of cytokines and related genes in regenerating skeletal muscle after cardiotoxin injection: a role for osteopontin

To examine the roles of cytokines in muscle regeneration, we injected cardiotoxin into mouse tibialis anterior muscle and examined the expression profiles of cytokines and related genes in the regeneration process. Expression of 40, 64, and 7 genes among 522 genes spotted on a cytokine expression array were increased more than fivefold at 48 hours, 96 hours, and 7 days after toxin injection, respectively, when compared with those of the control muscle. Especially the levels of mRNA for chemokines and chemokine receptors, many of which are potent regulators of macrophages, were highly elevated 48 hours after injury. The expression of osteopontin (OPN), a versatile regulator of inflammation and tissue repair, was up-regulated more than 118-fold in regenerating muscle at 48 hours after injury. Northern blotting confirmed that the expression of OPN was highest at 48 hours after cardiotoxin injection and declined sharply thereafter. Immunohistochemistry showed that OPN was detected both in the cytoplasm of macrophages and in necrotic muscle infiltrated with macrophages. Our studies suggest OPN may serve as an adhesion molecule that promotes macrophage binding to necrotic fibers and may be an important mediator in the early phase of muscle regeneration.

Sequence requirements for the activity of membrane-active peptides

Synthetic peptides were constructed with the sequence of the first 20 residues of melittin and terminating with a range of different amino acid amides. These were found to have haemolytic and cytolytic activity similar to that of melittin, provided that certain charge constraints were observed. The nature of the 21st residue was not critical except when the residue introduced a negative charge. The presence of at least two positive charges in the molecule was found to be essential for activity. One of these charges could be the amino-terminal amine. Peptides could be inactivated by the addition of a non-acidic presequence which was acetylated at the N-terminus. Introducing a protease cleavable sequence into an N-terminal extension of the peptides produced analogues with low haemolytic activity that could be activated by proteolytic action. A peptide with extra positive charges introduced on the hydrophilic face of the helix possessed a haemolytic activity that was greater than that of melittin.

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.

Inhibition by melittin of phosphorylation by protein kinase C of annexin I from cow mammary gland

Protein kinase C (PKC) is a phosphotransferase activated by diacylglycerols, phospholipids and Ca(2+), that regulates a wide variety of biological functions by phosphorylating multiple protein substrates such as annexin I. Annexin I is a phospholipid/Ca (2+)-binding protein distributed in various tissues, including the mammary gland, and is thought to mediate the anti-inflammatory actions of glucocorticoids by inhibiting phospholipase A(2). Melittin, a phospholipase A(2) activator in bee venom, is known to inhibit PKC activity when lysine-rich histone is used as the substrate. The purpose of the present study was to examine whether phosphorylation by PKC of annexin I from cow mammary gland was inhibited by melittin. Melittin inhibited annexin I phosphorylation by PKC in a dose-dependent manner, and its IC(50) value (concentration causing 50% inhibition) was 0.8 microM. The phosphorylation of annexin I was also inhibited by the amphiphilic polypeptides mastoparan and polymyxin B, and their inhibitory effects were comparable to that of melittin. The surface-inactive polypeptide bacitracin was less effective. The inhibition by melittin was effectively reversed by the excess addition of phosphatidylserine, but not distinctly by 1-oleoyl-2-acetyl-sn-glycerol or Ca(2+), suggesting that melittin inhibited the phosphorylation of annexin I by interacting with phosphatidylserine. The inhibition by melittin of PKC phosphorylation of annexin I seems to be pathophysiologically important, because a melittin-like phospholipase A(2)-stimulatory protein is present in bovine endothelial cells.

Involvement of oxidative stress in bee venom-induced inhibition of Na+/glucose cotransporter in renal proximal tubule cells

1. The present study was conducted to examine the involvement of oxidative stress in bee venom-induced inhibition of the Na+/glucose cotransporter (alpha-methyl-d-glucopyranoside (alpha-MG) uptake), a typical functional marker of proximal tubules, in primary cultured rabbit renal proximal tubule cells (PTC). 2. Bee venom (> or = 1 microg/mL) increased lipid peroxide (LPO) formation over 30 min. The increase in [(3)H]-arachidonic acid (AA) release and LPO formation and the inhibition of alpha-MG uptake induced by bee venom (1 microg/mL) and melittin (a major component of bee venom; 0.5 microg/mL) were blocked by N-acetyl-l-cysteine, vitamin C and vitamin E, anti-oxidants. 3. Bee venom- and melittin-induced increases in LPO formation and inhibition of alpha-MG uptake were significantly prevented by mepacrine and AACOCF(3), phospholipase A(2) inhibitors. In addition, nordihydroguaiareic acid (a lipoxygenase inhibitor) and econazole (a cytochrome P-450 epoxygenase inhibitor), but not indomethacin (a cyclo-oxygenase inhibitor), prevented bee venom- and melittin-induced increases in LPO formation and inhibition of alpha-MG uptake. 4. Nordihydroguaiareic acid prevented bee venom- and melittin-induced increases in Ca(2+) uptake. Moreover, anti- oxidants significantly prevented bee venom- and melittin-induced increases in Ca(2+) uptake. 5. In conclusion, bee venom inhibits alpha-MG uptake via the phospholipase A(2)-oxidative stress-Ca(2+) signalling cascade in primary cultured rabbit renal proximal tubule cells.

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.

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.

Phenotypic and functional changes in glial cells as a function of age

Both in vivo and in vitro investigations point to an important role for the immune system in the development of age-related neurodegeneration. Microglia isolated from aged female F344 rats, 18-20 months, show a higher percentage of cells with an ameboid morphology indicative of activation, whereas, astrocytes had a quiescent morphology. The ability of astrocytes and microglia to attenuate toxin-induced neuronal injury was examined. Post-natal day 1-3 pup cells optimally rescued neurons from Abeta-induced toxicity, whereas mixed glial cells from 18-20 month old rats were unable to rescue neurons from Abeta-induced toxicity. Our results suggested the appearance of a neurotoxic co-factor, therefore we investigated the basal level of nitric oxide and pro-inflammatory cytokines to determine if altered levels of immune mediators play a role in the toxicity. Mitogen-stimulated nitric oxide production increased 10 fold with age of donor, whereas, only the pup cells expressed an increase in TNF-alpha production. Basal levels of pro-inflammatory cytokines, as measured by RNA protection assays, increased with age. In particular, IL-1beta was increased 2 fold between adult and aged glial cells. The elevated cytokine expression may contribute to enhanced susceptibility to neurodegenerative diseases.

Structures of heparin-derived disaccharide bound to cobra cardiotoxins: context-dependent conformational change of heparin upon binding to the rigid core of the three-fingered toxin

Glycosaminoglycans (GAGs) have been suggested to be a potential target for cobra cardiotoxin (CTX) with high affinity and specificity via a cationic belt at the concave surface of the polypeptide. The interaction of GAGs, such as high-molecular weight heparin, with CTXs not only can induce aggregation of CTX molecules but also can enhance their penetration into membranes. The binding of short chain heparin, such as a heparin-derived disaccharide [DeltaUA2S(1-->4)-alpha-D-GlcNS6S], to CTX A3 from Taiwan cobra (Naja atra), however, will not induce aggregation and was, therefore, investigated by high-resolution (1)H NMR. A novel heparin binding site on the convex side of the CTX, near the rigid disulfide bond-tightened core region of Cys38, was identified due to the observation of intermolecular NOEs between the protein and carbohydrate. The derived carbohydrate conformation using complete relaxation and conformational exchange matrix analysis (CORCEMA) of NOEs indicated that the glycosidic linkage conformation and the ring conformation of the unsaturated uronic acid in the bound state depended significantly on the charge context of CTX molecules near the binding site. Specifically, comparative binding studies of several heparin disaccharide homologues with two CTX homologues (CTX Tgamma from Naja nigricollis and CTX A3) indicated that the electrostatic interaction of N-sulfate of glucosamine with NH(3)(+)zeta of Lys12 and of the 2-O-sulfate of the unsaturated uronic acid with NH(3)(+)zeta of Lys5 played an important role. These results also suggest a model on how the CTX-heparin interaction may regulate heparin-induced aggregation of the toxin via the second heparin binding site.

Effect of bee venom and its melittin on apical transporters of renal proximal tubule cells

Renal failure by bee venom may be related to a malfunction of renal transporters. However, the effects of bee venom on apical membrane transporters of renal proximal tubular cells are not yet known. The aim of this study was to examine the effects of dried bee venom of Apis mellifera and its melittin on apical transporter activity of primary cultured rabbit kidney proximal tubule cells. Bee venom (1 microg/ml) decreased the cell viability and increased lactate dehydrogenase activity over 30-min treatments. Its effect was blocked by mepacrine or AACOCF(3) (10(-6) M; phospholipase A(2) inhibitors). However, there was no effect on cell viability at a concentration of 0.01 microg/ml of bee venom. Thus, we investigated the effect of bee venom (1 microg/ml) on the activity of renal transporters at 30 min. Bee venom inhibited alpha-methyl-D-glucopyranoside, Pi, and Na(+) uptakes, but increased Ca(2+) uptake. These effects of bee venom were blocked by mepacrine or AACOCF(3) (10(-6) M), and bee venom-induced stimulation of Ca(2+) uptake was also blocked by methoxyverapamil and nifedipine (L-type calcium channel blockers). In addition, bee venom increased [(3)H]-arachidonic acid release by 216 % of that of control. In all experiments, bee venom melittin (0.5 microg/ml) had an identical effect to that of bee venom itself. In conclusion, bee venom inhibited, in part, alpha-MG, Pi, and Na(+) uptakes through its melittin which increased Ca(2+) uptake and arachidonic acid release in primary cultured rabbit renal proximal tubule cells.

Melittin-mediated release of [3H]-oleic acid from E. coli cells is dependent upon heat- and trypsin-sensitive factor(s) in human serum

Synthetic melittin mediated the release of [3H]-oleic acid ([3H]-OA) or its acylated lipids from [3H]-OA-labeled E. coli cells exposed to human serum. This phenomenon was not observed in the absence of serum and was calcium independent. The addition of serum was not required for melittin-mediated lysis of erythrocytes, although lysis was greater in the presence of serum than in its absence (P<0.001). Trypsin treatment of human serum reduced the melittin-mediated release of [3H]-OA/acylated lipids, and this effect was more pronounced upon boiling the serum (P<0.01). A kinetic study showed that maximum release of [3H]-OA/acylated lipids occurred within 3-6 min. Thin layer chromatography (TLC) analysis showed the lipids to be phosphatidyl ethanolamine (PE), phosphatidylethanol (PEt) and phosphatidic acid (PA). There was no detectable level of oleic acid (OA), diacylglycerol (DAG), phosphatidyl choline (PC) or phosphatidyl serine (PS). These findings suggested that a trypsin and heat-sensitive enzyme/factor present in the serum had a role in melittin-mediated action. These findings further showed that melittin activated phospholipase D (PLD), without affecting phospholipase A(2) (PLA(2)) or phospholipase C (PLC) activity.

A Pro --> Ala substitution in melittin affects self-association, membrane binding and pore-formation kinetics due to changes in structural and electrostatic properties

Melittin, the main component of bee venom of Apis mellifera, contains a proline at position 14, which is highly conserved in related peptides of various bee venoms. To investigate the structural and functional role of Pro14 a melittin analogue was studied where proline is substituted by an alanine residue (P14A). The investigations were focussed on: (i) the secondary structure in aqueous solution and membranes; (ii) the self-association in solution; (iii) the binding to POPC membranes; and (iv) the P14A-induced leakage and pore formation in membrane vesicles. Circular dichroism and gel filtration experiments showed that P14A exists at concentrations < 12 microM in monomeric form with an alpha-helicity of 28 +/- 7%. A further increase in peptide concentration leads to the formation of large aggregates consisting of 9 +/- 1 monomers. While binding studies with POPC vesicles revealed for P14A a stronger binding affinity towards membranes than for melittin, the peptide-induced leakage of fluorescent markers from vesicles was less efficient for P14A than for melittin. Furthermore, an unexpected efflux behaviour at high values of bound P14A was observed which indicated that the pore formation kinetics for P14A is more complex than it was reported for melittin. The different features of P14A in aggregation, binding and efflux compared to melittin are mainly ascribable directly to structural changes caused by the proline --> alanine substitution. Furthermore, the results indicate an improved screening of the positively charged residues of P14A by counterions which contributes additionally to the observed differences in peptide activities. It is suggested that the presence of proline in melittin is not only of structural importance but also influences indirectly the electrostatic properties of the native peptide.

Computer-assisted infrared thermographic study of axon reflex induced by intradermal melittin

The aim of the present study was to investigate whether melittin, the principal toxin of the honeybee (Apis mellifera) venom, can be used as an algogenic agent in the study of pain in humans. Five micrograms of melittin in 0.5 ml of saline was intradermally injected into the volar aspect of the forearm. Resultant pain was scored by a visual analogue scale (VAS), and skin temperature change was analyzed by means of a computer-assisted infrared thermography. Intradermal melittin temporarily produced severe pain, followed by a sustained increase in skin temperature. The skin temperature increase peaked in about 10 min and outlasted 1 h. Topical application of 10% lidocaine gel did not significantly suppress the melittin-induced pain, but markedly suppressed both the increase in the peak temperature and the area of temperature increase. In conclusion, 5 microg of melittin is sufficient to produce pain in humans and 10% lidocaine gel differentially decreases the melittin-induced axon reflex without any significant analgesic effect.

Melittin activates endogenous phospholipase D during cytolysis of human monocytic leukemia cells

Human monocytic leukemia cells (U937) were challenged with synthetic melittin, and arachidonic acid (AA)/acylated lipids from both cells (pellet) and media (supernatant) were analyzed by thin layer chromatography (TLC). From these data, melittin-mediated activation/inhibition of major phospholipases in U937 cells was related to pore formation, permeabilization and cytolysis as determined by light microscopy. Also, the effect of melittin on acylhydrolase activity in the cell-free sonicated lysates of U937 cells was examined. Here we report that synthetic melittin (1 microM) caused cytolysis of U937 cells within 10-15 min. Cellular hypertrophy (5 min) and aggregation (1 min) preceded cytolysis. TLC analysis of these lipids showed that total levels (cellular + medium) of diacylglycerol (DAG), phosphatidylethanolamine (PE) and phosphatidylcholine (PC) decreased, while that of arachidonic acid (AA) increased continuously (5-30 min). However, levels of phosphatidylethanol (PEt) phosphatidic acid (PA) and phosphatidylserine (PS) were increased transiently at 5-10 min being maximal at 5 min. Taken together, the combined levels of PEt and PA (an end product of phopholipase D, PLD) were about 42-fold higher than the level of AA at 5-10 min. Enhancement of AA levels appeared to result from in vitro reactions of various acylhydrolases and their phospholipid substrates (free/membrane bound) liberated into the medium during pore formation/cell lysis. Incubation of sonicated cell lysates also enhanced release of AA, which decreased upon addition of melittin, indicating that melittin inhibited these acylhydrolases. A consistent decrease in the level of DAG showed that phospholipase C was unaffected. Hence, transient activation of PLD bymelittin at the point of initiation of cytolysis, suggested a role for PLD in melittin-mediated membrane disruption/cytolysis by an uncharacterized signal transduction mechanism.

Role of phospholipase A2 in the cytotoxic effects of oxalate in cultured renal epithelial cells

BACKGROUND

Oxalate, a common constituent of kidney stones, is cytotoxic for renal epithelial cells. Although the exact mechanism of oxalate-induced cell death remains unclear, studies in various cell types, including renal epithelial cells, have implicated phospholipase A2 (PLA2) as a prominent mediator of cellular injury. Thus, these studies examined the role of PLA2 in the cytotoxic effects of oxalate.

METHODS

The release of [3H]-arachidonic acid (AA) or [3H]-oleic acid (OA) from prelabeled Madin-Darby canine kidney (MDCK) cells was measured as an index for PLA2 activity. The cell viability was assessed by the exclusion of ethidium homodimer-1.

RESULTS

Oxalate exposure (175 to 550 microM free) increased the release of [3H]-AA in MDCK cells but had no effect on the release of [3H]-OA. Oxalate-induced [3H]-AA release was abolished by arachidonyl trifluoromethyl ketone (AACOCF3), a selective inhibitor of cytosolic PLA2 (cPLA2), but was not affected by selective inhibitors of secretory PLA2 and calcium-independent PLA2. The [3H]-AA release could be demonstrated within 15 minutes after exposure to oxalate, which is considerably earlier than the observed changes in cell viability. Furthermore, AACOCF3 significantly reduced oxalate toxicity in MDCK cells.

CONCLUSIONS

Oxalate increases AA release from MDCK cells by a process involving cPLA2. In addition, based on the evidence obtained using a selective inhibitor of this isoform, it would appear that the activity of this enzyme is responsible, at least in part, for the cytotoxic effects of oxalate. The finding that oxalate can trigger a known lipid-signaling pathway may provide new insight into the initial events in the pathogenesis of nephrolithiasis.

Acute renal failure in experimental envenomation with Africanized bee venom

Human victims of multiple bee or wasp stings have been reported and develop severe clinical signs and symptoms. Acute renal failure (ARF), usually due to acute tubular necrosis (ATN) was a frequent complication. The pathogenetic mechanisms of ATN occurring in these accidents are still unclear. In the present study, female Wistar rats weighing 150-200 g were injected intravenously with Africanized bee venom at a dose of 0.4 microL/100 g body weight, and the kidney was observed under light and transmission electron microscopy and in immunohistochemical studies. The animals were divided into two groups: an Early group studied 3 to 8 hours after inoculation, and a Late group studied 24 to 30 hours after inoculation. The animals showed ATN mainly in the cortex and outer medulla with cast formation. After 24 hours, frequent mitotic figures were found in the tubular epithelium. Immunohistochemical studies revealed the presence of myoglobin and muscle actin in the tubular casts. Under electron microscopy, proximal tubule segments showed increasing intracytoplasmic vacuoles and attenuation of the brush border and of the basolateral infolding. This segment and the thick ascending limb of Henle's loop showed hydropic degeneration. Dead cells with apoptosis or necrosis due to cellular disintegration resulted in tubular basement membrane denudation. In the Late group, figures of intracytoplasmic myelin could be observed, some of them containing mitochondrial fragments. These changes are likely to be due to interactive effects of venom components, mainly mellitin and enzymes such as phospholipases, both acting on biological membranes. The ATN found was probably due to multiple causes, mainly a direct action of the venom on tubular cells, myoglobinuria, and perhaps ischemic mechanisms.

Dual effects of extracellular Ca2+ on cardiotoxin-induced cytotoxicity and cytosolic Ca2+ changes in cultured single cells of rabbit aortic endothelium

The effects of extracellular Ca2+ on cytotoxicity induced by cardiotoxin (CTX), isolated from Chinese cobra venom, were investigated in cultured rabbit aortic endothelial cells (RAECs). In Hank's buffered saline solution (HBSS) containing 1.2 mM Ca2+, CTX (1-30 microM) caused cell necrosis and cell death in a concentration-dependent manner, as determined by trypan blue exclusion test performed after a 20-min CTX treatment. The concentration of CTX that caused 50% cell death was about 6.5 microM. CTX (10 microM)-induced RAEC damage was also evident but less prominent in Ca2+-free medium and almost completely prevented in medium containing 7-10 mM Ca2+. Therefore, Ca2+ appears to provoke CTX-induced injury at physiological concentrations, but protects against it at high concentrations. The protection of RAECs from CTX-induced injury could also be achieved by high concentrations of Ni2+ and Mg2+. Using the fura-2 fluorescence technique to measure the cytosolic free Ca2+ concentration ([Ca2+]i) of single RAEC, it was shown that in 1.2 mM Ca2+-containing HBSS, treatment of RAECs with 10 microM CTX for 7-35 min resulted in a tremendous and irreversible [Ca2+]i elevation, suggestive of cell membrane damage and extracellular Ca2+ entry. Ni2+ could also enter the cytosol of these damaged RAECs. However, there was no [Ca2+]i elevation or Ni2+ entry in RAECs that were preincubated in HBSS containing 7 mM Ca2+ or Ni2+ before CTX exposure. In RAECs protected with 7 mM Ca2+, the intracellular Ca2+ signals triggered by 100 microM extracellular ATP or 10 microM bradykinin in CTX-treated groups were similar to those in the untreated control groups. Taken together, the results indicate that high extracellular Ca2+ concentrations protected RAECs from CTX-induced injury, and preserved the ability of CTX-treated RAECs to generate Ca2+ signals in response to physiological stimuli.

Melittin-induced inhibition and aggregation of Ca-ATPase in skeletal muscle sarcoplasmic reticulum: a comparative study

Incubation of melittin with sarcoplasmic reticulum membranes at pH 7. 0 and different melittin:Ca-ATPase molar ratios results in the progressive loss of enzyme activity. At high melittin:Ca-ATPase molar ratios (10:1 and 30:1), enzyme inhibition may be described by a biexponential curve. At pH 7.0, the values of the pseudo-first-order rate constants are 1.0 and 0.1 min-1 for the fast and slow phases of inhibition, respectively, at a melittin:Ca-ATPase molar ratio of 30:1. At pH 6.0 and a melittin:Ca-ATPase molar ratio of 30:1, melittin does not inhibit Ca-ATPase. Melittin-induced aggregation of Ca-ATPase molecules was studied using cupric phenanthroline as a chemical cross-linking agent. At a melittin:Ca-ATPase molar ratio of 5:1, aggregation of Ca-ATPase protein was not observed; however, the loss of enzyme activity was about 30% after 30 min. At melittin:Ca-ATPase molar ratios of 10:1 and 30:1, significant aggregation of Ca-ATPase protein takes place. The rate of Ca-ATPase aggregation is much lower than the rate of enzyme inhibition. At melittin:Ca-ATPase molar ratios of 10:1 and 30:1, the rate of Ca-ATPase protein aggregation is close to that for the slow phase of enzyme inhibition. At pH 6.0 and a melittin:Ca-ATPase molar ratio of 30:1, significant aggregation of Ca-ATPase occurs. It is concluded that melittin induces both Ca-ATPase inhibition and aggregation. These two processes may occur simultaneously, but under some conditions either inhibition or aggregation takes place independently of each other. Therefore, the aggregation of Ca-ATPase induced by melittin is not necessary for enzyme inhibition.

Melittin binds to secretory phospholipase A2 and inhibits its enzymatic activity

Synthetic melittin inhibited the enzymatic activity of secretory phospholipase A2 (PLA2) from various sources, including bee and snake venoms, bovine pancreas, and synovial fluid from rheumatoid arthritis patients, irrespective of substrate (e.g., [14C]-phosphatidylcholine or phosphatidylethanolamine vesicles and [3H]-oleic acid-labeled E.coli). A Lineweaver-Burk analysis showed that melittin was a noncompetitive inhibitor of bee venom PLA2, causing a change in Vmax from 200 to 50 units/min/mg of protein. The Km remained unchanged (0.75 nmole). Melittin inhibited approximately 50% of purified bee venom PLA2 activity in a 30:1 molar ratio (melittin:enzyme). Because the enzyme kinetics indicated a PLA2-melittin interaction, a melittin-sepharose affinity column was used to purify a PLA2 from human serum. Further, an enzyme-linked assay was developed to quantitate PLA2 activity in biological fluids using avidin-peroxidase and ELISA plates coated with biotinylated melittin. These observations may have potential therapeutic significance, as well as provide a convenient basis for the isolation and quantitation of PLA2.

Group II phospholipases A2 are indirectly cytolytic in the presence of exogenous phospholipid

Systemic inflammatory response syndromes including septic shock and salicylate poisoning are associated with high circulating levels of secretory phospholipase A2 (sPLA2). In septic shock, sPLA2 has been implicated in the pathogenesis of multisystem organ failure, presumably by a direct cytotoxic effect on cells. The cytotoxicity of recombinant human sPLA2 and a venom PLA2 were examined on human erythrocytes, erythroleukemia cells and U937 cells. Neither the human nor venom PLA2's were directly injurious to target cells. However, in the presence of liposomal phospholipids, both PLA2's induced irreversible cell injury. Whereas venom PLA2 was cytolytic in the presence of either phosphatidylcholine or phosphatidylethanolamine (PE), rh-sPLA2 caused cell death only in the presence of PE. These data show that normal unperturbed cells are resistant to injury from PLA2, and that additional cofactors such as PE are required to induce cell injury.

Changes in the arterial blood pressure, heart rate and normal ECG parameters of rat after envenomation with Egyptian cobra (Naja haje) venom

1. The effect of Egyptian cobra (Naja haje) venom on the normal electrical activity of the cardiac muscles (ECG) and arterial blood pressure of envenomated rats were investigated in this study. 2. Rats were divided into three groups. The first group was injected im with saline and considered as control group. Rats of the second and third groups were injected IM with 0.02 micrograms and 0.04 micrograms cobra venom/gim b.wt, respectively. 3. Mean blood pressure (MBP), heart rate (HR) and four different ECG parameters (PR and QT intervals, R and T wave amplitudes) were measured over 1 h following envenomation. 4. The low dose (0.02 micrograms/g) of N. haje venom caused hypotension accompanied by an increase in the HR, whereas hypertension and bradycardia developed after injection of the high dose (0.04 micrograms/g) of venom. 5. There was a decrease in the P-R interval after administration of the low dose and prolongation of it after the high dose. The Q-T interval and R-wave amplitude were significantly increased after injection of both doses. T-wave amplitude was significantly elevated only after injection of the high dose. 6. The present results indicate that the Egyptian cobra (N. haje) venom significantly alters the arterial blood pressure and ECG parameters of envenomated rats. The suggests that impairment of the electrical activity of cardiac muscle may be one of the reasons why victims of cobra bite die.

The role of the N-terminal leucine residue in snake venom cardiotoxin II (Naja naja atra)

The N-terminal leucine residue of snake venom cardiotoxin II (CTX II) (Naja naja atra) was systematically replaced with D-leucine (CTXII-L1-D-L), glycine (CTXII-L1G) or deleted [CTXII-(2-60)] to study the role of leucine residue in CTX II molecule. CTX II, CTXL1-D-L, CTXL1G and CTX(2-60) were produced by chemical synthesis method and purified by high performance liquid chromatography. Owing to folding problem in CTXII-(2-60), only CTX II, CTXII-L1-D-L and CTXII-L1G were produced in a pure form and characterized by amino acid analysis, mass spectrometry and peptide mapping. In the structural aspect, changing the Leu-1 by D-Leu or Gly causes a drastic alteration in the whole CTX II structure as detected by circular dichroism, 1-anilino-naphthalene-8-sulfonate (ANS) fluorescence assay. In the functional aspect, both CTXII-L1-D-L and CTXII-L1G are still retained substantial biological activity of CTX II. Therefore, the results indicate that both the chirality and the side-chain of the N-terminal leucine residue of CTX II are important elements in maintaining the whole CTX II structure. In addition, this study is the first report in elucidating the reason why the first N-terminal residue of most CTXs (90.3%) is leucine residue.

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.

Melittin and phospholipase A2 from bee (Apis mellifera) venom cause necrosis of murine skeletal muscle in vivo

Melittin and phospholipase A2 (PLA2) from bee (Apis mellifera) venom were rested for their ability to induce necrosis of skeletal muscle cells after intramuscular injection into mice. Light and electron microscopic examination of tissue indicated that both melittin (4 micrograms/g) and bee venom PLA2 (4 micrograms/g) caused necrosis of skeletal muscle cells within 30 min after i.m. injection. Early changes in the cells consisted of delta lesions, indicating a ruptured plasma membrane, and hypercontraction of myofibrils. By 24 hr the affected cells appeared as an amorphous mass of disorganized and disrupted myofibrils contained in an intact basal lamina. To ensure that the myotoxic activity of the melittin preparation was not due to contaminating. PLA2 activity, the preparation was treated with p-bromophenacyl bromide (p-BPB), a known inhibitor of PLA2 activity. The p-BPB-treated melittin was determined to have no detectable PLA2 activity using a sensitive muscle cell culture assay, and it still induced myonecrosis, although to a lesser extent and of a slower onset. Additionally, p-BPB treatment of purified bee venom PLA2 completely inhibited its myotoxic activity. These results indicate that both melittin and bee venom PLA2 are capable of inducing necrosis of skeletal muscle cells upon i.m. injection, and that the catalytic and myotoxic activities of bee venom PLA2 are inihibited by p-BPB. Also, melittin and contaminating PLA2 in the melittin fraction may be acting synergistically to induce a stronger and more rapid myotoxic effect than occurs with either alone.

Similarities and differences in mechanisms of cardiotoxins, melittin and other myotoxins

Myonecrosis induced in vivo by cardiotoxin, melittin, and Asp49 and Lys49 phospholipase A2 (PLA2) myotoxins involves rapid lysis of the sarcolemma, myofibril clumping, and hypercontraction of sarcomeres. In contrast, skeletal muscle necrosis induced by crotamine and myotoxin a is much slower, consisting of mitochondrial and sarcoplasmic reticulum swelling, myofibril degeneration, and lack of sarcolemma or transverse tubule damage. The mechanisms contributing to the myonecrosis induced by these peptides were evaluated. Two cardiotoxins and two Lys49 PLA2 myotoxins lysed primary cultures of human skeletal muscle within 24 hr at a concentration of 0.25 microM, while melittin, crotamine, and myotoxin a, and an Asp49 PLA2 myotoxin were non-cytolytic at concentrations up to 5.0 microM, suggesting that cytolysis is not a good measure of myotoxicity. Crotamine and the Lys49 PLA2 myotoxin altered Ca2+ ion flux in human heavy sarcoplasmic reticulum by opening the ryanocine receptor. Whole-cell patch-clamp studies demonstrated that administrating crotamine intracellularly increased Na+ currents. Free fatty acids, liberated by activation of tissue phospholipase C or by the PLA2 activity of the myotoxins, were monitored for crotamine, myotoxin a and a Lys49 PLA2 myotoxin in cell cultures in which the lipids had been radiolabeled. Only the Lys49 myotoxin produced significant amounts of fatty acid in cell cultures, supporting a potential role for fatty acid production only in the mechanism of sarcolemma-destroying myotoxins. These findings, coupled with those in the literature, support a hypothesis in which the myotoxins and/or products of lipase activity (e.g. fatty acids) are acting at a site existing on both the Na+ channel and a protein involved in Ca2+ release and probably serving a modulatory function for ion regulation. Based on the similarities in mechanisms between the toxins and fatty acids, the most likely site would be a fatty acid binding site on the protein (either similar to that on fatty acid binding proteins, or an acylated cysteine residue) or in the membrane.

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.

Phospholipase A2-induced neurotoxicity in vitro and in vivo in rats

The present study evaluated the neurotoxic potential of phospholipase A2 (PLA2) in in vitro (primary neuronal cultures) and in vivo (EEG and behavior) rat models of CNS excitability. In vitro, PLA2 (0.0038-5.8 nM) or melittin (a potent activator of endogenous PLA2; 100-5000 nM), were highly neurotoxic, causing approximately 500 units/ml LDH release. The neurotoxic EC50s for PLA2 and melittin were 1.8 (1.4-2.3) and 848 (501-1280) nM, respectively. Neurotoxic concentrations of PLA2 stimulated neuronal release of [3H]AA. Preliminary in vitro experiments evaluating changes in neuronal calcium flux indicated that PLA2 caused transient, and melittin sustained, increases in [Ca2+]i. In vivo, PLA2 (0.5-5 micrograms i.c.v.) or melittin (2.5-20 micrograms i.c.v.) produced nonconvulsive EEG seizures, which generalized to status epilepticus. While the onset of seizure development was markedly delayed for PLA2 (1.5-4.5 h), the seizure inducing effects of melittin were evident within 3.5 +/- 0.2 min and more severe. Both PLA2 and melittin were lethal, exhibiting LD50s of 0.62 micrograms and 8.4 micrograms, respectively. Pretreatment with (+)-MK801 (5 micrograms, i.c.v.) significantly attenuated melittin, but not PLA2, in vivo neurotoxicity. PLA2 induced neuropathology in surviving rats revealed extensive cortical and subcortical injury to forebrain neurons and fibre pathways. Collectively, these results demonstrate the potent neurotoxic potential of PLA2, the delayed clinical nature of its in vivo neurotoxicity and the applicability of these model systems to future studies on mechanisms of PLA2 neurotoxicity and the development of potential PLA2 antagonists.

Activation of bee venom phospholipase A2 through a peptide-enzyme complex

Phospholipase A2 activation by membrane-bound peptides was investigated in order to understand the role of the membrane-induced conformation on activation, and to examine the occurrence of a peptide-enzyme complex at the lipid/water interface. For the peptides studies, bee venom phospholipase A2 was stimulated regardless of the membrane-bound conformation (alpha-helix, beta-sheet or random coil). Using antisera raised against melittin, we were able to demonstrate the occurrence of a calcium-dependent complex involving the enzyme, phospholipid substrate, and peptide.