Specific binding of crotoxin to brain synaptosomes and synaptosomal membranes

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Systemic and local myotoxicity induced by snake venom group II phospholipases A2: Comparison between crotoxin, crotoxin B and a Lys49 PLA2 homologue

The patterns of myotoxicity induced in mice by crotoxin, crotoxin B and a Lys49 phospholipase A(2) (PLA(2)) homologue were compared. Lys49 PLA(2)-induced local myotoxicity is reflected by creatine kinase (CK) loss in injected gastrocnemius muscle, and by a profile of CK increase in plasma characterized by a rapid increment and drop after intramuscular injection, and by a lack of CK increase in plasma after intravenous injection. In contrast, crotoxin and crotoxin B, which induce local and systemic myotoxicity, provoked a more prolonged increment in plasma CK activity upon intramuscular injection, and induced increments in plasma CK after intravenous injection. The three toxins promoted a similar extent of local myotoxicity, assessed by the loss of CK in injected gastrocnemius. A method for the quantitative assessment of the ability of toxins to induce systemic myotoxicity is proposed, based on the estimation of the ratio between the area under the curve in the plasma CK activity (total myotoxicity) to the loss of CK in injected gastrocnemius (local myotoxicity). The highest ratio corresponded to crotoxin, and the lowest corresponded to Lys49 PLA(2), the former being a systemic myotoxin and the latter a local myotoxin. Neutralization by antivenoms also differed between the toxins: a drastic reduction in plasma CK, with very poor neutralization of local CK loss, was achieved in the case of crotoxin B when antivenom was injected intravenously, whereas no neutralization was achieved in the case of Lys49 PLA(2). When tested in undifferentiated myoblasts in culture, Lys49 PLA(2) induced cytotoxicity, whereas crotoxin and crotoxin B did not, evidencing that the latter are devoid of widespread cytolytic activity. Molecular modeling analysis showed that Lys49 PLA(2) has a conspicuous cationic face, which is likely to interact with diverse membranes. In contrast, crotoxin B, despite its overall basic pI, has a lower density of positively charged residues at this molecular region. It is suggested that Lys49 PLA(2)s homologues interact, through this cationic face, with many different cell types, thus lacking specificity for muscle cells. In contrast, crotoxin B has a more selective interaction with targets in the muscle cell membrane. This selectivity might be the basis for the ability of crotoxin and crotoxin B to induce systemic myotoxicity.

Inhibition of crotoxin binding to synaptosomes by a receptor-like protein from Crotalus durissus terrificus (the South American rattlesnake)

Crotoxin (Ctx) is a potent neurotoxin of the venom of Crotalus durissus terrificus (the South American rattlesnake). Ctx is a heterodimer composed of CB, a toxic PLA(2) subunit, and CA, a non-toxic and non-enzymatic subunit, that potentiates the neurotoxicity of CB in vivo. The deleterious action of Ctx upon C. d. terrificus snakes themselves is known to be prevented by a PLA(2) inhibitor (CNF) present in their blood serum. CNF acts by replacing CA in Ctx, thus forming a new stable complex CNF-CB. This complex no longer interacts with the target receptor (TR) to deliver CB to cause its lethal effect. Furthermore, CNF-CB seems to be reminiscent of the interaction Ctx-TR at the pre-synaptic site. In the present work, the binding competition between rat brain synaptosomes (TR) and CNF for Ctx was investigated. Radiolabeled Ctx, made of CA and one isoform of CB (CA-(125)ICB(2)), was used as ligand. The competition by unlabeled Ctx was taken as a reference. The potency of CNF as a competitor was evaluated under different incubation conditions with varying time scale addition of reagents (CA-(125)ICB(2), synaptosomes and CA-CB(2) or CNF). CNF was able to inhibit the binding of the toxin to synaptosomes as well as to partially displace the toxin already bound to its membrane target. The mechanisms of competition involved were discussed and a previous schematic model of interactions between Ctx, TR and CNF was updated.

Crotoxin acceptor protein isolated from Torpedo electric organ: binding properties to crotoxin by surface plasmon resonance

Crotoxin, a potent neurotoxin from the South American rattlesnake Crotalus durissus terrificus, is a heterodimeric phospholipase A(2) (EC 3.1.1.4), which blocks the release of acetylcholine from peripheral neurons. We previously have suggested the existence of a 48 kDa crotoxin-binding protein in the presynaptic membranes of the electric organ of Torpedo marmorata. Here, we report the purification and characterization of this protein that we called the crotoxin acceptor protein from Torpedo (CAPT). The membranes of electric organs from Torpedo were solubilized with a detergent (4% (w/v) Triton X-100) and CAPT was isolated by affinity chromatography on a crotoxin column. SDS-PAGE showed that the purified protein was homogeneous and cross-linking studies with radioiodinated crotoxin confirmed that it had retained its toxin-binding properties. The purified CAPT has similar molecular mass as crocalbin, a crotoxin-binding protein isolated from porcine brains, yet anti-crocalbin antiserum failed to recognize CAPT. Surface plasmon resonance biosensor technology was used to measure the specific interaction between crotoxin and solubilized CAPT. Using this method, it was possible to follow CAPT throughout the purification procedure. As well, an apparent dissociation constant (K(d)(app)) of 3.4 nM was calculated for the interaction of pure CAPT and crotoxin from the dissociation rate constant (k(off)=1.2 x 10(-2)s(-1)) and the association rate constant (k(on)=3.5 x 10(6)M(-1)s(-1)).

Inhibition of crotoxin phospholipase A(2) activity by manoalide associated with inactivation of crotoxin toxicity and dissociation of the heterodimeric neurotoxic complex

Crotoxin (CACB complex) is a convulsant heterodimeric neurotoxic phospholipase A(2) (PLA(2)). The role of phospholipid hydrolysis in its epileptogenic properties remains unresolved. We, thus, studied the effect of manoalide (MLD), a PLA(2) inhibitor, on the toxin catalytic activity and its central and peripheral toxicity. Incubation of crotoxin with MLD fully and irreversibly inactivated its enzymatic activity. Interestingly, crotoxin also lost its central neurotoxicity after intracerebroventricular injection and peripheral toxicity after intravenous administration. MLD-treated crotoxin prevented the high affinity binding of [125I]-radiolabeled crotoxin on rat cortex synaptic plasma membranes. Further analysis of MLD-treated crotoxin by non-denaturing PAGE and surface plasmon resonance indicated that the crotoxin complex was dissociated after MLD treatment. Although the loss of MLD-treated crotoxin peripheral neurotoxicity could not be attributed to this dissociation, the presence of free CA subunit might explain the observed competition in binding experiments. In conclusion, the dissociation of the crotoxin complex by MLD, as demonstrated in this study, did not permit to specify the role of the enzymatic activity in crotoxin epileptogenic properties. Other approaches would be required to resolve this question.

Analysis of fatty acids released by crotoxin in rat brain synaptosomes

Crotoxin, the main toxin of Crotalus durissus terrificus venom, exerts its lethal effect by blocking neurotransmission at the neuromuscular junction level through a triphasic mechanism. This effect seems to depend on its phospholipasic activity, suggesting that the mechanism of neurotransmission blockage may be related to fatty acids release in specific sites of the nervous terminal. In this work, we purified the fatty acids released by crotoxin's activity and this outline was compared with other phospholipases A(2), including CB, a subunit of crotoxin. Our results show a higher release of palmitate and arachidonate by crotoxin when compared to other phospholipases A(2). Since palmitate has a role in protein acylation processes and arachidonate participates in signal transduction events, these mechanisms may be related to the neurotoxic actions of crotoxin.

Mojave rattlesnakes (Crotalus scutulatus scutulatus) lacking the acidic subunit DNA sequence lack Mojave toxin in their venom

The venom composition of Mojave rattlesnakes (Crotalus scutulatus scutulatus) differs in that some individuals have Mojave toxin and others do not. In order to understand the genetic basis for this difference, genomic DNA samples from Mojave rattlesnakes collected in Arizona, New Mexico, and Texas were analyzed for the presence of DNA sequences that relate to the acidic (Mta) and basic (Mtb) subunits of this toxin. DNA samples were subjected to PCR to amplify nucleotide sequences from second to fourth exons of the acidic and basic subunits. These nucleotide sequences were cloned and sequenced. The nucleotide sequences generated aligned exactly to previously published nucleotide sequences of Mojave toxin. All DNA samples analyzed generated product using the basic subunit primers, and aligned identically to the Mtb nucleotide sequence. However, only 11 out of the 14 samples generated a product with the acidic subunit primers. These 11 sequences aligned identically to the Mta nucleotide sequence. The venom from the three snakes whose DNA did not amplify with the acidic subunit primers were not recognized by antibodies to Mojave toxin. This suggests that snakes with venom lacking Mojave toxin also lack the productive nucleotide sequence for the acidic subunit in their DNA.

Increasing molecular diversity of secreted phospholipases A(2) and their receptors and binding proteins

Secreted phospholipases A(2) (sPLA(2)s) form a large family of structurally related enzymes which are widespread in nature. Snake venoms are known for decades to contain a tremendous molecular diversity of sPLA(2)s which can exert a myriad of toxic and pharmacological effects. Recent studies indicate that mammalian cells also express a variety of sPLA(2)s with ten distinct members identified so far, in addition to the various other intracellular PLA(2)s. Furthermore, scanning of nucleic acid databases fueled by the different genome projects indicates that several sPLA(2)s are also present in invertebrate animals like Drosophila melanogaster as well as in plants. All of these sPLA(2)s catalyze the hydrolysis of glycerophospholipids at the sn-2 position to release free fatty acids and lysophospholipids, and thus could be important for the biosynthesis of biologically active lipid mediators. However, the recent identification of a variety of membrane and soluble proteins that bind to sPLA(2)s suggests that the sPLA(2) enzymes could also function as high affinity ligands. So far, most of the binding data have been accumulated with venom sPLA(2)s and group IB and IIA mammalian sPLA(2)s. Collectively, venom sPLA(2)s have been shown to bind to membrane and soluble mammalian proteins of the C-type lectin superfamily (M-type sPLA(2) receptor and lung surfactant proteins), to pentraxin and reticulocalbin proteins, to factor Xa and to N-type receptors. Venom sPLA(2)s also associate with three distinct types of sPLA(2) inhibitors purified from snake serum that belong to the C-type lectin superfamily, to the three-finger protein superfamily and to proteins containing leucine-rich repeats. On the other hand, mammalian group IB and IIA sPLA(2)s can bind to the M-type receptor, and group IIA sPLA(2)s can associate with lung surfactant proteins, factor Xa and proteoglycans including glypican and decorin, a mammalian protein containing a leucine-rich repeat.

Gabaergic-benzodiazepine system is involved in the crotoxin-induced anxiogenic effect

The behavioral effects of crotoxin (CTX), the major component of Crotalus durissus terrificus venom, were studied in rats submitted to the open field, holeboard, and social interaction tests. CTX (100, 250, and 500 microg/kg, i.p.) was administered 2 h before the tests. In the open field, CTX reduced ambulation (250 microg/kg) and rearing (250 and 500 microg/kg) and increased grooming (100 and 250 microg/kg) and freezing (250 microg/kg). In the holeboard and social interaction, all the CTX doses evaluated decreased, respectively, head dip and head dipping, and social interaction time. The CTX-induced behavioral alterations could be attributed to its neuromuscular transmission blockade, but this possibility was ruled out because CTX (250 and 500 microg/kg, i.p., 2 h before the rotarod test) was unable to modify the rotarod performance of rats. The involvement of the benzodiazepine receptor in the CTX-induced behavioral alterations was investigated through the pretreatment (30 min before the tests, i.p.) of the animals with diazepam (1.2 mg/kg), or flumazenil (4 and 10 mg/kg). Both diazepam and flumazenil antagonized the CTX-induced behavioral alterations in the open field, holeboard, and social interaction tests. This study demonstrated that: (1) CTX is an anxiogenic compound; and (2) the gabaergic-benzodiazepine system may play a role in the CTX-induced anxiogenic effect.

Re-examination of crotoxin-membrane interactions

The interaction of crotoxin with synaptic membranes from Torpedo marmorata has been re-examined, using radioiodinated toxin. In competition experiments, the 'saturable binding' is usually calculated by subtracting the non-saturable binding, determined in the presence of an excess of unlabelled crotoxin, from total binding. Paradoxically, we observed a notable increase of the 'saturable binding' of 125I-crotoxin, defined in this manner in the presence of a high concentration of certain competitors (e.g. crotoxin subunit B, agkistrodotoxin, ammodytoxin and ammodytin I2). This potentiation effect was analysed by competition and cross-linking experiments. The dissociation of the basic crotoxin subunit CB, which differs markedly from crotoxin in its membrane-binding characteristics, was found to be the main reason for the observed phenomenon. 125I-CB could be released from 125I-crotoxin by exchange with a molecule having sufficient affinity towards CA (e.g. CB or agkistrodotoxin) or it could be dissociated from the specific crotoxin membrane-binding site(s) by a competitor molecule. Our results, therefore, suggest a reinterpretation of previous 125I-crotoxin binding studies.

A phospholipase A2-like pseudogene retaining the highly conserved introns of Mojave toxin and other snake venom group II PLA2s, but having different exons

Mojave toxin is a neurotoxic, heterodimeric phospholipase A2 (PLA2) from the venom of the Mojave rattlesnake (Crotalus scutulatus scutulatus) and is characteristic of all rattlesnake presynaptic neurotoxins. Here, we describe a phospholipase A2 pseudogene (psi-Mtx) located 2,000 nucleotides upstream, and on the opposite DNA strand, from a gene for Mojave toxin acidic subunit (Mtx-a). The pseudogene lacks the first exon and a few segments of noncoding DNA found in functional snake venom PLA2 genes, but does have the coding information for a complete PLA2 protein. psi-Mtx retains the unusual gene sequence similarity pattern found in functional viperid PLA2 genes. When compared to genes from C. s. scutulatus and the Hahn snake (Trimeresurus flavoviridus), psi-Mtx shows strong conservation of nocoding regions and variable protein-coding regions. Although the nocoding regions of psi-Mtx are conserved with respect to other viperid PLA2 genes, the three exons code for a unique PLA2-like protein similar in sequence to ammodytoxin b found in the venom of the western sand viper (Vipera ammodytes ammodytes). The structure of these genes suggests a common ancestor for all viperid PLA2 genes. Phylogenetic analysis of psi-Mtx, Mtx-a, Mtx-b, pgPLA 1a, and pgPLA 1b suggest that psi-Mtx diverged from an ancestral sequence before the presumed gene duplication event leading to Mtx-a and Mtx-b. However, analysis of the basis of coding regions alone gives a conflicting result.

Different sensitivity of fast- and slow-twitch muscles to some snake venoms and myotoxins

We examined the effect of some crude snake venoms, isolated toxins and non-specific cytotoxic agents on isolated extensor digitorum longus (EDL) and soleus (SOL) muscles of the mouse. The muscles were continuously perfused with a physiological saline solution. Crude venoms from Crotalus viridis viridis, Agkistrodon contortrix laticinctus and Notechis scutatus scutatus were tested at a concentration of 25-50 micrograms ml-1. The increase in the rate of creatine kinase (CK) release (above basal levels) induced in each muscle by each venom or toxin was measured. Also, the myotoxic effect of these agents was investigated with the light microscope. EDL and SOL had the same range of basal rate of CK release (0.30 +/- 0.06 U g-1 hr-1, N = 26), weight (7-10 mg) and content of CK (717.18 +/- 80.19 U g-1 and 501.00 +/- 62.28 U g-1, N = 8), but they had a different sensitivity to the myotoxic action of the tested venoms. The rate of CK release in EDL muscles was in the range of 24-60 U g-1 hr-1 after 60 min of exposure to 25 micrograms ml-1 of each crude venom, whereas the increase of rate of CK release in the SOL was in the range of 1.5-4.0 U g-1 hr-1. Crotoxin and myotoxin a (10 and 25 micrograms ml-1, respectively) were also more effective in EDL than in SOL muscles. The non-specific cytotoxic agents Triton X-100 (0.01%) and polylysine (100 micrograms ml-1) induced the same increase of rate of CK release in both muscles. The data presented in this article show that isolated murine EDL muscles are more sensitive than SOL to the myotoxic action of some snake venoms and toxins.

Synaptosomal binding of 125I-labelled daboiatoxin, a new PLA2 neurotoxin from the venom of Daboia russelli siamensis

Daboiatoxin (DbTx), the PLA2 neurotoxin from Daboia russelli siamensis venom, was shown to bind specifically and saturably to rat cerebrocortical synaptosomes and synaptic membrane fragments. Two families of binding sites were detected by equilibrium binding analysis in the presence and absence of Ca2+. Scatchard analysis of biphasic plateaus revealed Kdl 5 nM and Bmax1, 6 pmoles/mg protein, and Kd2 80 nM and Bmax2 20 pmoles/mg protein, respectively, for the high- and low-affinity binding sites. The binding of 125I-DbTx to synaptosomes did not show marked dependence on Ca2+, Mg2+, Co2+ and Sr2+. Native DbTx was the only strong competitor to 125I-DbTx synaptosomal binding (IC50 12.5 nM, KI 5.5 nM). Two other crotalid PLA2 neurotoxins, crotoxin CB and mojave toxin basic subunit, and nontoxic C. Atrox PLA2 enzyme, were relatively weaker inhibitors, while two viperid PLA2 neurotoxins, ammodytoxin A and VRV PL V, were very weak inhibitors. Crotoxin CA was a poor inhibitor even at microM concentrations, whereas no inhibitory effect at all was observed with crotoxin CACB, ammodytoxin C, VRV PL VIIIa, taipoxin, beta-bungarotoxin, or with PLA2 enzymes from N. naja venom, E. schistosa venom, bee venom and porcine pancreas. All other pharmacologically active ligands examined (epinephrine, norepinephrine, histamine, choline, dopamine, serotonin, GABA, naloxone, WB-4101, atropine, hexamethonium and alpha-bun-garotoxin) also failed to interfere with 125I-DbTx binding. As those competitors that showed partial inhibition were effective only at microM concentration range compared to the Kd (5 nM) of 125I-DbTx synaptosomal binding, DbTx could well recognize a different neuronal binding site. Rabbit anti-DbTx polyclonal antisera completely blocked the specific binding. When a range of Ca2+ and K+ channels modulators were examined, Ca2+ channel blockers (omega-conotoxins GVIA and MVIIC, taicatoxin, calciseptine and nitrendiprene) did not affect the binding even at high concentrations, while charybdotoxin was the only K+ channel effector that could partially displace 125I-DbTx synaptosomal binding amongst the K+ channel blockers tested (apamin, dendrotoxin-I, iberiotoxin, MCD-peptide, 4-aminopyridine and tetraethylammonium), suggesting that neither K+ nor Ca2+ channels are associated with DbTx binding sites.

Specificity and binding affinity of an anti-crotoxin combinatorial antibody selected from a phage-displayed library

A crotoxin-specific, monoclonal, high-affinity, single-chain antibody variable region (scFv) was generated by combinatorial methods using Pharmacia's Recombinant Phage Antibody System. A high-affinity clone, designated A10G, was selected, and its DNA sequence was determined. Protein A10G showed high reaction specificity, with only the closely related rattlesnake neurotoxins, concolor toxin and Mojave toxin, showing cross-reactivity out of eleven group II phospholipase A2s (PLA2s) screened. No group I PLA2s cross-reacted in enzyme-linked immunosorbent assays. The gene coding for A10G was subcloned into an expression vector, and the resulting expressed nonfusion protein, designated A10GPE, was renatured and purified to apparent homogeneity. Dissociation constants of A10G with intact crotoxin and crotoxin basic subunit were determined to be 7 x 10(-10) and 6.8 x 10(-9) M, respectively. When A10GPE was preincubated with either the basic subunit or intact crotoxin at molar ratios of up to 5:1, no inhibition of phospholipase activity was observed. Expressed protein, however, could partially neutralize the lethality of Mojave toxin, a crotoxin homolog, in mice.

Amino acid sequences of a heterodimeric neurotoxin from the venom of the false horned viper (Pseudocerastes fieldi)

The main toxic component of the venom of the false horned viper, Pseudocerastes fieldi, is a heterodimeric neurotoxin composed of a basic subunit, Cb II, and one of two acidic subunits, either Cb I alpha or Cb I beta. The nontoxic acidic subunit increases the toxicity of the basic subunit. Both subunits have phospholipase A2 (PLA2) amino acid sequences. Cb I alpha and Cb I beta themselves are inactive towards phosphatidylcholine and when complexed with Cb II promote a delay in the onset of phospholipase activity of Cb II. Cb I alpha and Cb I beta do hydrolyze the synthetic substrate, 3-octanoyloxy-4-nitrobenzoic acid, but at < 1% the rate of Cb II. Comparisons of the amino acid sequences of Cb II and Cb I alpha with the corresponding acidic and basic subunits of other heterodimeric neurotoxins show high amino acid sequence identity. Some of the amino acids which are different between the acidic and basic subunits are in highly conserved sequences in their respective types of PLA2. This suggests that these amino acid changes in the conserved regions are important for the structure and function of the heterodimeric proteins.

Ammodytoxin A acceptor in bovine brain synaptic membranes

Ammodytoxin A, the presynaptic neurotoxin from Vipera ammodytes ammodytes venom, was found to bind specifically and with high affinity to bovine cortex synaptic membrane preparation. The detected ammodytoxin A high-affinity binding was characterized by equilibrium binding analysis which revealed a single high-affinity binding site with Kd 4.13 nM and Bmax 6.67 pmoles/mg of membrane protein. 125I-ammodytoxin A was covalently cross-linked to its neuronal acceptor using a chemical cross-linking technique. As revealed by subsequent SDS-PAGE analysis and autoradiography, 125I-ammodytoxin A specifically attached to membrane components with apparent mol. wts 53,000-56,000. Besides by the native ammodytoxin A, the binding of radioiodinated ammodytoxin A to the neuronal acceptor was highly attenuated, also by other two iso-neurotoxins from V. a. ammodytes venom, ammodytoxins B and C, and neurotoxin crotoxin B from the venom of the South American rattlesnake (Crotalus durissus terrificus). Vipera berus berus phospholipase A2 was a weaker inhibitor, whereas nontoxic phospholipase A2, ammodytoxin I2 and myotoxic phospholipase A2 homologue, ammodytin L, both from V. a. ammodytes venom as well, were very weak inhibitors. No inhibitory effect on 125I-ammodytoxin A specific binding at all was, however, obtained with alpha-dendrotoxin, beta-bungarotoxin and crotoxin A, respectively. Treatment of synaptic membranes with proteinase K and Staphylococcus aureus V-8 proteinase, a combination of PNGase F and neuroaminidase, heat or acid lowered the 125I-ammodytoxin A specific binding to various extents but never completely abolished it. The ammodytoxin A binding site in bovine synaptic membranes is thus most likely a combination of membrane glycoprotein acceptor and membrane phospholipids. As ammodytoxin A reduced the second negative component of the perineural waveform, measured on mouse triangularis sterni preparation, which is very likely a result of an inhibition of a fraction of the terminal K+ currents, the ammodytoxin A acceptor could well be connected with K+ channels.

Binding proteins on synaptic membranes for crotoxin and taipoxin, two phospholipases A2 with neurotoxicity

Crotoxin and taipoxin are both neurotoxic phospholipases A2 capable of affecting the presynaptic activity to bring about ultimate blockade of synaptic transmission. The enzymatic activity has generally been considered to be necessary but not sufficient for the blockade. Since many phospholipases A2 with comparable or even higher enzymatic activity are not toxic, it has been postulated that the difference lies in the affinity of binding to the presynaptic membrane. In confirmation of this proposition, we and others have previously shown that iodinated crotoxin and taipoxin bind specifically with high affinity to the isolated synaptic membrane fraction from guinea-pig brain, whereas specific binding is not detected with the nontoxic pancreatic phospholipase A2. Experiments based on photoaffinity labeling and simple chemical cross-linking techniques have led to the identification of three polypeptides preferentially present in neuronal membranes as (subunits of) the binding protein(s) for crotoxin and/or taipoxin. Some, but not all, other toxic phospholipases A2 also appear to be ligands for the three polypeptides. We now report studies on partial purification of these polypeptides using affinity chromatography and other techniques. In order to learn the normal physiological roles played by the toxin-binding proteins, the phospholipase-independent effects of the toxins on the synaptosomes have been sought. We have found that under Ca(2+)-free condition, taipoxin or crotoxin inhibits with IC50 of 20-1000 nM the Na(+)-dependent uptake of norepinephrine, dopamine and serotonin by the synaptosomes. In contrast, choline uptake is not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Conversion of bovine pancreatic phospholipase A2 at a single site into a competitor of neurotoxic phospholipases A2 by site-directed mutagenesis

A 45-kDa polypeptide preferentially present in neuronal membranes was previously identified as a subunit of a binding (or receptor) protein for several phospholipase A2 variants with neurotoxicity, including crotoxin, by chemical cross-linking experiments (Yen, C.-H., and Tzeng, M.-C. (1991) Biochemistry 30, 11473-11477). The binding of crotoxin to this receptor protein was completely suppressed by sufficient F22Y, a mutated bovine pancreatic phospholipase A2 generated by site-directed mutagenesis of Phe22 of the wild-type enzyme to Tyr. The IC50 of this inhibition was estimated to be 1 microM. In sharp contrast, the wild-type enzyme gave no effect even at 50 microM. This mutation resulted in only minor and localized structural perturbations with little effect on enzymatic activity. Other phospholipase A2 molecules capable of competing with crotoxin for this binding invariably have Tyr at this position. It was concluded that this Tyr residue is an important determinant for the binding of a number of phospholipase A2 variants to the 45-kDa receptor.

Chemical modification of tryptophan residues in alpha-neurotoxins from Ophiophagus hannah (king cobra) venom

Two alpha-neurotoxins, Oh-4 and Oh-7, from the king cobra (Ophiophagus hannah) venom were subjected to Trp modification with 2-nitrophenylsulfenyl chloride (NPS-Cl). One major NPS derivative was isolated from the modified mixtures of Oh-4 and two from Oh-7 by HPLC. Amino acid analysis and sequence determination revealed that Trp-27 in Oh-4, and Trp-30 and Trp-26 and 30 in the two Oh-7 derivatives, were modified, respectively. Sulfenylation of Trp-27 in Oh-4 caused about 70% drop in lethal toxicity and nicotinic acetylcholine receptor-binding activity. Modification of Trp-30 in Oh-7 resulted in the decrease of lethal toxicity by 36% and binding activity by 61%. The activities were further lost when the conserved Trp-26 in Oh-7 was modified. Sulfenylation of the Trp residues did not significantly affect the secondary structure of the toxins as revealed by the CD spectra. These results indicate that the Trp residues in these two long alpha-neurotoxins may be involved in the receptor binding.

Genomic sequences encoding the acidic and basic subunits of Mojave toxin: unusually high sequence identity of non-coding regions

Mojave toxin (Mtx) is a heterodimeric, neurotoxic phospholipase A2 (PLA2) found in the venom of the Mojave rattlesnake, Crotalus scutulatus scutulatus, and is characteristic of all rattlesnake presynaptic neurotoxins. This paper describes the isolation and nucleotide (nt) sequence of the genomic clones encoding both the non-neurotoxic, non-enzymatic acidic subunit (Mtx-a) and the toxic, PLA2-active basic subunit (Mtx-b), and compares their structures. Both cloned genes shared virtually identical overall organization, with four exons separated by three introns, which were inserted in the same relative positions of the genes' coding regions. The exon/intron structure was similar to that reported for mammalian PLA2 genes. Most remarkable was the high degree of nt sequence identity between Mtx-a and Mtx-b. While the exons shared about 70% identity, the introns were greater than 90% identical and the 5' and 3' untranslated and flanking regions were greater than 95% identical. These findings support our earlier suggestion [Aird et al., Biochemistry 24 (1985) 7054-7058] that the genes coding for the two subunits arose from a common ancestor. There has clearly been a strong selection on the nt sequence of the non-coding regions during this evolutionary process. This is the first report of genomic sequences of PLA2-like proteins from snakes.

Comparison of crotoxin isoforms reveals that stability of the complex plays a major role in its pharmacological action

Crotoxin from the venom of the South American rattlesnake Crotalus durissus terrificus is a potent neurotoxin consisting of a weakly toxic phospholipase-A2 subunit (CB) and a non-enzymic, non-toxic subunit (CA). Crotoxin complex (CACB) dissociates upon interaction with membranes: CB binds while CA does not. Moreover, CA enhances the toxicity of CB by preventing its non-specific adsorption. Several crotoxin isoforms have been identified. Multiple variants of each subunit give different crotoxin complexes that can be subdivided into two classes: those of high toxicity and low enzymic activity and those of moderate toxicity and a high phospholipase-A2 activity. In this study, we demonstrate that the more-toxic isoforms block neuromuscular transmission of chick biventer cervicis preparations more efficiently than weakly toxic isoforms. The less-toxic crotoxin complexes have the same Km and Vmax as CB alone. In contrast, the more-toxic isoforms are enzymically less active than CB. These differences correlate with the stability of the complexes: less-toxic isoforms are less stable (Kd = 25 nM) and dissociate rapidly (half-life about 1 min), whereas the more-toxic isoforms are more stable (Kd = 4.5 nM) and dissociate more slowly (half-life 10-20 min). The rate of interaction of crotoxin complexes with vesicles of negatively charged phospholipids paralleled the rate of dissociation of the complexes in the absence of vesicles. The differences of pharmacological and biochemical properties of crotoxin isoforms indicate that the stability of crotoxin complexes plays a major role in the synergistic action of crotoxin subunits: a stronger association between the two crotoxin subunits would account for their slower dissociation rate, a weaker enzymic activity, a slower interaction with phosphatidylglycerol vesicles, a faster blockade of neuromuscular transmission and a higher lethal potency.

Snake-venom phospholipase A2 neurotoxins. Potentiation of a single-chain neurotoxin by the chaperon subunit of a two-component neurotoxin

The venoms from Crotalinae and Viperinae snakes contain only two kinds of phospholipase A2 neurotoxins (beta-neurotoxins): single-chain beta-neurotoxins, such as agkistrodotoxin and ammodytoxin-A, and dimeric beta-neurotoxins, which, in the case of the best studied ones, crotoxin-like toxins, consist of the non-covalent association of a phospholipase A2 (CB) and a non-enzymatic chaperon (CA). Possible evolutionary relationships of these beta-neurotoxins have been investigated by analyzing whether CA could behave as a chaperon toward agkistrodotoxin and ammodytoxin, as it does in the crotoxin complex. CA increased the lethal potency of agkistrodotoxin and modified its pharmacological effect on Torpedo synaptosomes. Sedimentation experiments proved that CA can form an heterocomplex with agkistrodotoxin. Agkistrodotoxin prevented the binding to CA of an anti-CA mAb which recognizes an epitope at the zone of interaction between crotoxin subunits, suggesting the association of CA and agkistrodotoxin implicated the same zone. A 10-fold molar excess of CA over ammodytoxin modified the effect of ammodytoxin on acetylcholine release but did not increase the lethal potency of ammodytoxin. Sedimentation experiments showed CA and ammodytoxin can form an heterocomplex which is less stable than CA.agkistrodotoxin. Ammodytoxin A did not compete with the anti-CA mAb. These observations are in good agreement with the sequence similarities between CB and agkistrodotoxin (80%) and ammodytoxin A (60%).