Sea snake neurotoxins

Neurotoxins present in sea snake venoms are summarized. All sea snake venoms are extremely toxic, with low LD5Q values. Most sea snake neurotoxins consist of only 60-62 amino acid residues with 4 disulOde bonds, while some consist of 70 amino acids with 5 disulfide bonds. The origin of toxicity is due to the attachment of 2 neurotoxin molecules to 2 a subunits of an acetylcholine receptor that is composed of a2 6 subunits. The complete structure of several of the sea snake neurotoxins have been worked out. Through chemical modification studies the invariant tryptophan and tyrosine residues of post-synaptic neurotoxins were shown to be of a critical nature to the toxicity function of the molecule. Lysine and arginine are also believed to be important. Other marine vertebrate venoms are not well known. 

It is a supposition that the )9-sheet structure of neurotoxin is an essential structural element for binding to the receptor. The presence of -sheet structure was found by Raman spectroscopic analysis of a sea snake neurotoxin (2). The amide I band and III band for Enhydrina schistosa toxin were at 1672 cm and 1242 cm" respectively. These wave numbers are characteristic for anti-parallel -sheet structure. The presence of -sheet structure found by Raman spectroscopic study was later confirmed by X-ray diffraction study on Laticauda semifasciata toxin b. 

The one residue most extensively studied is tryptophan. It is very easily modified, indicating that tryptophan residue is exposed 5-8). Raman spectroscopic analysis of a sea snake neurotoxin indicated that a single tryptophan residue is indeed exposed (2). The tryptophan residue lies in the important loop consisting of segment 4. Modification of the tryptophan residue induces the loss of AChR binding ability as well as the loss of toxicity 5-8). 

There is only one tyrosine residue in some sea snake neurotoxins. This residue is usually quite difficult to modify, but once it is modified, the toxicity is lost (9). Histidine seems not to be essential as the chemical modification of this residue does not affect the toxicity 10). 

Argine and lysine are believed to be important, but results are not clear because sea snake neurotoxins contain several residues of these amino acids (7). 

When a nerve-muscle preparation is stimulated in the presence of a sea snake neurotoxin, there is no twitch. However, when the muscle itself is stimulated directly in the presence of a neurotoxin, the muscle contracts. This means that neurotoxin does not inhibit the muscle itself. Moreover, postsynaptic neurotoxin does not inhibit the release of acetylcholine from the nerve ending. Therefore, the site of snake toxin inhibition must be in the postsynaptic site 20). Later it was shown that a neurotoxin strongly binds to the acetylcholine receptor (AChR). 

Table II. Amino Acid Sequence of Sea Snake Neurotoxins (Type I or Short Chain)...

Table III. Amino Acid Sequences of Sea Snake Neurotoxin (T>pe II, Long Chain)...

While most investigations show that sea snake neurotoxins are postsynaptic type, Gawade and Gaitonde (23) stated that Enhydrina schistosa major toxin has dual actions or postsynaptic as well as presynaptic toxicity. E, schistosa venom phospholipase A is both neurotoxic and myotoxic. Neurotoxic action of the enzyme is weak so that there is sufficient time for myonecrotic action to take place (24), Sea snake, L. semifasciata toxin also inhibits transmission in autonomic ganglia, but has no effect on transmission in choroid neurons. 

Neurotoxins from Sea Snake and Other Vertebrate Venoms... 

All sea snakes are poisonous and their venoms are extremely toxic. The LD q for crude sea snake venom can be as low as 0.10 fig/g mouse body weight (i). For purified toxin the LD q is even lower, suggesting the high toxicity of sea snake toxins and venoms. This toxicity is derived from the presence of potent neurotoxins. Compared to snake venoms of terrestrial origin, sea snake venoms have been studied less. Different enzymes reported to be present or absent are summarized in Table I. 

Before discussing the structure of the neurotoxins, it is necessary to define the types of neurotoxins. Three types of neurotoxins have been found so far in snake venoms. The first one is a postsynaptic neurotoxin, the second is a presynaptic neurotoxin, and the last is a cholinesterase inhibiting neurotoxin. Most sea snake venoms seem to contain only the postsynaptic neurotoxin. Only in Enhydrina... 

Another type of neurotoxin found in sea snake venoms is a hybrid type structurally situated between the short-chain and long-chain types. As can be seen in Table IV, two toxins shown here have a long stretch of segment 4, yet there is no disulfide bond in this portion. 

The similarity of the primary structure of different sea snake venoms has already been discussed. Postsynaptic neurotoxins from Elapidae venom have been extensively studied. Elapidae include well-known snakes such as cobra, krait, mambas, coral snakes, and all Australian snakes. Like sea snake toxins, Elapidae toxins can also be grouped into short-chain (Type I) and long-chain (Type II) toxins. Moreover, two types of neurotoxins are also similar to cardiotoxins, especially in the positions of disulfide bonds. However, amino acid sequences between cardiotoxins and sea snake and Elapidae neurotoxins are quite different. In comparing the sequence of sea snake and Elapidae neurotoxins, there is a considerable conservation in amino acid sequence, but the difference is greater than among the various sea snake toxins. 

Lee CY, Chang CC, Chen YM (1972) Reversibility of neuromuscular blockade by neurotoxins from elapid and sea snake venoms. Taiwan Yi Xue Hui Za Zhi 71 344-9 Lee CY, Tsai MC, Chen YM, Ritonja A, Gubensek F (1984) Mode of neuromuscular blocking action of toxic phospholipases A2 from vipera ammodytes venom. Arch Int Pharmacodyn Ther 268 313-24... 

G. A. Petsko and D. Tsernoglou, PEES Lett., 68, 1 (1976). The Crystal Structure of a Post-Synaptic Neurotoxin from Sea Snake at 2.2 A Resolution. 

The derived solution conformation of short neurotoxins (Figures 8 and 9) is in general agreement with the X-ray crystal structure of erabutoxins(46,48), although the amino acid composition of these sea snake toxins differs by ca 30% from that of the terrestrial snake neurotoxins mostly studied by NMR. The conformational reorientation of the fragment 30-34 (3 turn in the central loop. Figure 9)... 

Tu, A. T. (1990). Neurotoxins from sea snake and other vertebrate venoms. In Marine Toxins Origin, Structure, and Molecular Pharmacology (S. Hall and G. Strinchartz, eds.). ACS Symp. Ser. 418 336-346. 

Yu, N., Lin, T, and Tu, A. T. (1975). Laser Raman scattering of neurotoxins isolated from the venoms of sea snakes Lapemis hardwickii schistosa. J. Biol. Chem. 250 1782-1785. 

Snake neurotoxins are the main toxic proteins of cobra, krait, tiger snake and sea snake venoms which block neuromuscular transmission and cause animals death of respiratory paralysis. Snake neurotoxins are classified into two distinct types, postsynaptic and presynaptic neurotoxins, in relation to the neuromuscular junction. Postsynaptic neurotoxins bind specifically to nicotinic acetylcholine receptor (AChR) at the motor endplate and produce a nondepolarizing block of neuromuscular transmission. Presynaptic neurotoxins block the release of acetylcholine from the presynaptic motor nerve terminals. 

To date, more than 120 toxins with neurotoxic activity have been isolated in pure state from elapid and hydrophid (sea-snake) venoms. Over 100 highly homologous postsynaptic neurotoxins belonging to two distinct size groups, short and long neurotoxins, have been sequenced (Yang, 1974,1984 Mebs, 1988 Endo and Tamiya, 1991). Short neurotoxins contain 60-62 amino acid residues with four disulfide bonds, and long neurotoxins comprise... 

Ducancel, F., Guignery-Frelat, G., Boulain, J.C. and Menez, A. (1990) Nucleotide sequence and structure analysis of cDNA encoding short-chain neurotoxins from venom glands of sea snake (Aipysurus laevis). Toxicon 2S 119-123. 

Bacteria, protozoa, and venomous animals synthesize numerous toxins that are used to kill their prey or to defend themselves. Sea anemones, jellyfish, cone snails, insects, spiders, scorpions, and snakes all make potent and highly specific neurotoxins. Plants form a host of alkaloids and other specialized products, some of which are specifically neurotoxic and able to deter predators. More than 500 species of marine cone snails of the genus Conus synthesize a vast array of polypeptide toxins (conotoxins), 487-489 some with unusual posttranslational modifications.490 491 The slow-moving snails are voracious predators that use their toxins, which they inject with a disposible harpoonlike tooth,492 to paralyze fish, molluscs, or worms.493... 

Not only the Kraits produce a-neurotoxins. The Indian Cobra Cobra naja naja) also utilises one of fhe mosf pofenf of all fhe snake toxins, cobratoxin, which is also an a-neurotoxin. Cobratoxin is phenomenally toxic only 4.5 mg is needed to kill a human. In fact, a single Cobra can produce sufficient toxin to kill ten men. The Sea Kraits (e.g. Laticauda semifasciata from Malaysia) are the most toxic of all snakes they produce erabutoxin which is an a-neurotoxin of unbelievable potency, but fortunately, as discussed earlier, they have small jaws, which makes it difficult for them to bite a human. 

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