Purification and chemical characterization of the major neurotoxin from the venom of Pelamis plautrus

South American snake venoms with abundant neurotoxic components. Composition and toxicological properties. A literature review

In South America there are three snake genera with predominantly neurotoxic venoms: Crotalus, Micrurus and Hydrophis, which include nine species/subspecies, 97 species and a single marine species, respectively. Although accidents with neurotoxic venoms are less frequent than those with anticoagulant, cytotoxic or necrotic venoms (e.g. from Bothrops), they are of major public health importance. Venoms from genus Crotalus have been extensively studied, while data on the venoms from the other two genera are very limited, especially for Hydrophis. The venoms of North and South American Crotalus species show biochemical and physiopathological differences. The former species cause bothrops-like envenomation symptoms, while the latter mainly have neurotoxic and myotoxic effects, leading to respiratory paralysis and, occasionally, renal failure by myoglobinuria and death, often with no local lesions. Micrurus and Hydrophis also cause neurotoxic envenomations. Many studies have isolated, identified and characterized new enzymes and toxins, thus expanding the knowledge of snake venom composition. The present review summarizes the currently available information on neurotoxic venoms from South American snakes, with a focus on protein composition and toxicological properties. It also includes some comments concerning potential medical applications of elapid and crotalic toxins.

Two color morphs of the pelagic yellow-bellied sea snake, Pelamis platura, from different locations of Costa Rica: snake venomics, toxicity, and neutralization by antivenom

The yellow-bellied sea snake, Pelamis platura, is the most broadly distributed snake species. Despite being endowed with a highly lethal venom, a proteomic analysis of its toxin composition was unavailable. The venoms of specimens collected in Golfo de Papagayo and Golfo Dulce (Costa Rica), where two distinctive color morphs occur, were chromatographically compared. The latter inhabits a fjord-like gulf where the transit of oceanic sea snakes into and from the basin is restricted, thus possibly affecting gene flow. RP-HPLC evidenced a conserved venom protein profile in both populations, despite their divergent color phenotypes. Following a trend observed in other sea snakes, P. platura venom is relatively simple, being composed of proteins of the three-finger toxin (3FTx), phospholipase A2 (PLA2), cysteine-rich secretory protein (CRISP), 5'-nucleotidase, and metalloproteinase families. The first three groups represent 49.9%, 32.9%, and 9.1% of total venom protein, respectively. The most abundant component (~26%) is pelamitoxin (P62388), a short-chain 3FTx, followed by a major basic PLA2 (~20%) and a group of three isoforms of CRISPs (~9%). Whereas isolated pelamitoxin was highly lethal to mice, neither the PLA2 nor the CRISP fraction caused death. However, the PLA2 rapidly increased plasma creatine kinase activity after intramuscular injection, indicating its myotoxic action. Differing from myotoxic PLA2s of viperids, this PLA2 was not cytolytic to murine myogenic cells in vitro, suggesting possible differences in its mechanism of action. The median lethal dose (LD50) estimates for P. platura crude venom in mice and in three species of fishes did not differ significantly. The sea snake antivenom manufactured by CSL Ltd. (Australia), which uses Enhydrina schistosa as immunogen, cross-recognized the three major components of P. platura venom and, accordingly, neutralized the lethal activity of crude venom and pelamitoxin, therefore being of potential usefulness in the treatment of envenomations by this species.

BIOLOGICAL SIGNIFICANCE

Integrative analyses of animal venoms that combine the power of proteomics (venomics) with the characterization of their functional and immunological properties are significantly expanding knowledge on these remarkable bioweapons, both from a basic and a medical perspective. Costa Rica harbors a unique population of the yellow-bellied sea snake, Pelamis platura, that is restricted to a fjord-like gulf (Golfo Dulce). This population differs markedly from oceanic populations found elsewhere along the Pacific coast of this country, by presenting a patternless bright yellow coloration, instead of the typical bicolored or tricolored pattern of this species. It has been suggested that the dominance of this yellow-morph in Golfo Dulce might reflect gene flow restrictions, caused by the oceanographic conditions at this location. The present study demonstrates that the remarkable phenotypic variation between the two color morphs inhabiting Golfo Dulce and Golfo de Papagayo, respectively, is not associated with differences in the expression of venom components, as shown by their conserved RP-HPLC profiles. Proteomic analysis revealed the relatively simple toxin composition of P. platura venom, which contains three predominant types of proteins: three-finger toxins (protein abundance: 49.9%), phospholipases A2 (32.9%), and cysteine-rich secretory proteins (9.1%), together with few minor components. Further, the involvement of these most abundant proteins in the toxic effects of the venom, and their cross-recognition and neutralization by a sea snake antivenom produced against the venom of Enhydrina schistosa, were analyzed.

Laser Raman spectroscopy of snake venom neurotoxins: conformation

Laser Raman spectra of neurotoxins of Pelamis platurus (yellow-bellied sea snake) and Laticauda semifasciata (broad-banded blue sea snake) were investigated. The amide I band appeared at 1672 cm-1 for both toxins, which presents an indication of anti-parallel beta structure. Since this agrees well with the result from the CD-ORD studies of snake neurotoxin, it was concluded that snake neurotoxins mainly consist of beta structure. The amide III band appeared at 1245 cm-1 for P. platurus toxin and 1248 cm-1 for L. semifasciata toxin. The four disulfide bonds present in the toxin have a very similar geometry. After vigorous heat treatment, the backbone configuration of the toxin molecule basically remained the same although it was partially denatured. The major peak at 512 cm-1 was not altered by the heat treatment but a new shoulder appeared at 546 cm-1. This suggests that a new type of S-S stretching vibration (trans-gauche-trans) was produced as a result of heat treatment. However, the majority of the S-S vibrations remained in the gauche-gauche-gauche orientation. A substantial change in the interactions between a tyrosine aromatic ring and neighboring residues was apparently the alteration caused by the heat treatment.

Structure-function relationship of lapemis toxin: a synthetic approach

The synthetic approach to the structure-function relationship of lapemis toxin has been very useful in clarifying the important binding regions. To identify the neurotoxic binding domain(s) of lapemis toxin, several peptides were synthesized using the 9-fluorenylmethoxycarbonyl protocols. These peptides were based on the sequence of lapemis toxin, a 60-amino-acid, short-chain postsynaptic neurotoxin found in sea snake (Lapemis hardwickii) venom. The peptides were purified using high-performance liquid chromatography and sequenced to verify the correct synthesis, isolation, and purity. The synthetic peptide names and single letter sequences were Peptide A1 (15 mer) CCNQQSSQPKTTTNC Peptide B1 (18 mer) CYKKTWSDHRGTRIERGC Peptide B2 (16 mer) YKKTWSDHRGTRIERG Peptide C1 (12 mer) CPQVKPGIKLEC Peptide NS (20 mer) EACDFGHIKLMNPQRSTVWY. The peptide NS (nonsense peptide) sequence was arbitrarily determined and used as a control peptide. Biological activities of the synthetic peptides were determined by in vivo as well as by in vitro assay methods. For the in vivo assay, lethality was determined by intravenous injection in mice (Swiss Webster). For the in vitro assay, peptide binding to the Torpedo californica nicotinic acetylcholine receptor was determined. The peptides were found to be nontoxic at approximately 114 times the known LD50 of lapemis toxin. Binding studies with 125I-radiolabeled lapemis toxin and tyrosine-containing peptides indicated that lapemis toxin and peptide B1 bound the receptor, while the other peptides had no detectable binding. The central loop domain of lapemis toxin (peptide B1) plays a dominate role in the toxin's binding ability to the receptor. These results and the hydrophilicity analysis predict peptide B1 may serve as an antagonist or antigen to neutralize the neurotoxin effects in vivo.

Importance of two arginine residues in Lapemis postsynaptic neurotoxins: re-examination using acetylcholine receptor-neurotoxin complex instead of free toxin

The role of the three arginine residues in Lapemis neurotoxin has been re-examined using an acetylcholine receptor-toxin complex. The receptor-bound neurotoxin was treated with phenylglyoxal to modify available arginine residues. The positions of the modified arginine residues were then identified from the amino acid sequences of proteolytically digested fragments of the detached neurotoxin. The result was compared with the result from modification of the free Lapemis toxin (unbound). Arg-31 and Arg-34 were not modified when Lapemis toxin was bound to receptor although they were modified when the toxin was not bound to receptor. It was concluded that Arg-31 and Arg-34 residues are involved in the toxin-receptor interaction.

Isolation and characterization of Pelamis platurus (yellow-bellied sea snake) postsynaptic isoneurotoxin

Pelamis platurus (yellow-bellied sea snake) venom contains several neurotoxins, the major toxin, which is most toxic, and two other isotoxins. The second most toxic neurotoxin (Pelamis toxin b) was isolated and characterized. It contains 60 amino acid residues with only one residue difference from the major toxin, Pelamis toxin a. The difference is at the tenth amino acid residue from the acid terminal. The isoelectric point of toxin b is 8.7. Raman spectroscopic examination of toxin b indicates that the toxin contains a considerable amount of antiparallel beta-structure, beta-turn, and random coil without alpha-helix as the amide I band appears at 1673 cm-1 and the amide III band at 1246 cm-1. Circular dichroic studies also indicate a typical beta-sheet structure. The Pelamis toxin b is a typical postsynaptic neurotoxin as it binds to the acetylcholine receptor competitively with a well known toxin, alpha-bungarotoxin. The LD50 of toxin b is 0.185 microgram g-1 in mice by intravenous injection, indicating high toxicity of a postsynaptic neurotoxin.

Characterization of nicked myotoxin a and its effect on the sarcoplasmic reticulum calcium pump

Myotoxin a, a muscle-necrotizing polypeptide isolated from Crotalus viridis viridis (prairie rattlesnake) venom, was nicked at Met-28 by cyanogen bromide. Amino acid analysis indicated that the methionine content was reduced to zero from the original 1 mol. Judging from circular dichroism, the nicked myotoxin a had a conformation similar to that of original myotoxin. Raman spectra indicated that the conformations of the three disulfide bonds are not affected in nicked myotoxin a. Like the original toxin, nicked myotoxin a was myotoxic and inhibited calcium ion loading activity, although the inhibitory action was slightly lower than that of the original myotoxin a. Both modified and unmodified myotoxin a showed myonecrotic activity as determined by examining histological slides. The modified toxin also inhibited the formation of decavanadate-induced two-dimensional crystalline arrays of the sarcoplasmic reticulum Ca2+-ATPase just as the original myotoxin a does.

Isolation and primary structure of the major toxin from sea snake, Acalyptophis peronii, venom

The major neurotoxin from the venom of Acalyptophis peronii captured in the Gulf of Thailand was isolated. Although there are two toxic fractions in the venom, the most toxic and abundant fraction was selected for purification and chemical characterization. The LD50 of the major toxin is 0.125 micrograms/g mice, indicating an extremely toxic nature. The toxin consists of 60 amino acid residues with methionine as the amino-terminal and asparagine as the carboxy-terminal end. It contains nine half-cystine residues. There is 1 mol each of tryptophan, tyrosine, methionine, valine, aspartic acid, leucine, and alanine, and there is no phenylalanine. The molecular weight calculated from the amino acid sequence determination was 6600. The toxin replaces alpha-bungarotoxin in binding with the acetylcholine receptor, indicating that the A. peronii major neurotoxin competes with alpha-bungarotoxin for the same binding site of the acetylcholine receptor.

Effect of sulfhydryl group modification on the neurotoxic action of a sea snake toxin

Pelamis toxin alpha is a major neurotoxin isolated from the venom of Pelamis platurus (yellow-bellied sea snake). The effect of sulfhydryl group modification by NN'-1,4-phenylenedimaleimide on the neurotoxic action of Pelamis toxin alpha has been investigated. The cross-linked toxin having a molecular weight of 11 000 was formed without significant structural changes in the toxin. Lethality tests on the modified toxin indicated that it retained considerable toxicity, although its potency was weaker than that of the native toxin. Binding studies with the acetylcholine receptor isolated from the electroplax of Torpedo californica indicated that the modified toxin binds to the receptor but less effectively than the native toxin. These results suggest that the decreases in toxicity and binding to the receptor are due to a decrease in accessibility of cross-linked neurotoxin to the receptor. This leads us to the conclusion that the region of the neurotoxin containing the sulfhydryl group is not essential for its biological activity. Analysis of the structure and function relationships of the modified toxin suggests that the neurotoxin-acetylcholine receptor interaction requires the proper orientation of the neurotoxin molecule.

Isolation and characterisation of toxic components from the venom of the common Indian sea snake (Enhydrina schistosa)

Three neurotoxic components, enhydrotoxins a, b and c, were isolated by a three step procedure from the venom of the sea snake Enhydrina schistosa, caught in the Arabian sea. The i.v. LD50 values in mice were 0.042, 0.045 and 0.052 mg/kg, respectively. All three toxins irreversibly blocked neuromuscular transmission. Abolition of responses to acetylcholine indicated a post-synaptic site of action of these toxins. The molecular weight and amino acid composition of enhydrotoxin a are similar to values earlier reported for Enhydrina toxins.

Some pharmacological properties of the venom, venom fractions and pure toxin of the yellow-bellied sea snake Pelamis platurus

The effects of the crude venom, four partially purified venom fractions and pure toxin (Pelamis toxin alpha) from yellow-bellied sea snake, Pelamis platurus, on respiration, blood pressure, heart and skeletal muscle of rabbits have been examined. Results indicated that crude venom, a partially purified toxic fraction and Pelamis toxin alpha caused initial respiratory stimulant effects followed by respiratory paralysis. In most cases, respiratory paralysis occurred before a profound fall in arterial pressure. Depression of the twitch response to nerve stimulation was observed in the tibialis anterior muscle. No significant change in the electrocardiogram was seen. Three partially purified non-toxic fractions of the crude venom induced transient respiratory stimulant effects. It was concluded that the crude venom and Pelamis toxin alpha had an identical mode of action and that they caused respiratory paralysis in rabbits.