Tetrodotoxin

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Tetrodotoxin
Formula	C₁₁H₁₇N₃O₈
LD₅₀	5.0 - 8.0 µg/kg
Molecular mass	319.28 u

Tetrodotoxin (anhydrotetrodotoxin 4-epitetrodotoxin, tetrodonic acid, TTX) is a potent neurotoxin with no known antidote, which blocks action potentials in nerves by binding to the pores of the voltage-gated, fast sodium channels in nerve cell membranes. The binding site of this toxin is located at the pore opening of the voltage-gated Na⁺ channel. Its name derives from Tetraodontiformes, the name of the order that includes the pufferfish, porcupinefish, ocean sunfish or mola, and triggerfish, several species of which carry the toxin. Although tetrodotoxin was discovered in these fish and found in several other animals, it is actually the product of certain bacteria such as Pseudoalteromonas tetraodonis, certain species of Pseudomonas and Vibrio, as well as some others.

1 Tetrodotoxin sources in nature
2 Biochemistry
- 2.1 Physiology
- 2.2 Total synthesis
3 Tetrodotoxin poisoning
4 Geographic frequency of tetrodotoxin toxicity
5 See also
6 External links
7 References

[edit] Tetrodotoxin sources in nature

Tetrodotoxin has also been isolated from widely differing animal species, including western newts of the genus Taricha (where it was termed "tarichatoxin"), parrotfish, toads of the genus Atelopus, several species of blue-ringed octopodes of the genus Hapalochlaena (where it was called "maculotoxin"), several starfish, an angelfish, a polyclad flatworm, several species of Chaetognatha (arrow worms), several nemerteans (ribbonworms) and several species of xanthid crabs. The toxin is variously used as a defensive biotoxin to ward off predation, or as both a defensive and predatory venom (the octopodes, chaetognaths and ribbonworms). Tarichatoxin and maculotoxin were shown to be identical to tetrodotoxin in 1964 and 1978, respectively.

[edit] Biochemistry

Tetrodotoxin binds to what is known as site 1 of the fast voltage-gated sodium channel. Site 1 is located at the extracellular pore opening of the ion channel. The binding of any molecules to this site will temporarily disable the function of the ion channel. Saxitoxin and several of the conotoxins also bind the same site.

The use of this toxin as a biochemical probe has elucidated two distinct types of voltage-gated sodium channels present in humans: the tetrodotoxin-sensitive voltage-gated sodium channel (TTX-s Na⁺ channel) and the tetrodotoxin-resistant voltage-gated sodium channel (TTX-r Na+ channel). Tetrodotoxin binds to TTX-s Na⁺ channels with a binding affinity of 5-15 nanomolar, while the TTX-r Na⁺ channels bind TTX with low micromolar affinity. Nerve cells containing TTX-r Na⁺ channels are located primarily in cardiac tissue, while nerve cells containing TTX-s Na⁺ channels dominate the rest of the body. The prevalence of TTX-s Na⁺ channels in the central nervous system makes tetrodotoxin a valuable agent for the silencing of neural activity within a cell culture.

[edit] Physiology

The toxin blocks the fast Na+ current in human myocytes (the contractile cells of the heart), thereby inhibiting their contraction. By contrast, the sodium channels in pacemaker cells of the heart are of the slow variety, so action potentials in the cardiac nodes are not inhibited by the compound. The poisoned individual therefore dies not because the electrical activity of the heart is compromised, but because the myocytes are effectively paralyzed.

Blocking of fast Na+ channels has medicinal use in treating some cardiac arrhythmias; however, tetrodotoxin is far too potent to have any therapeutic value.

[edit] Total synthesis

Y. Kishi et al Nagoya University, Nagoya, Japan, (now at Harvard University) reported the first total synthesis of D,L-tetrodotoxin in 1972.^[1] M. Isobe et al at Nagoya University, Japan and J. Du Bois et al at Stanford University, USA, reported the asymmetric total synthesis of tetrodotoxin in 2003.^[2]^[3] The two 2003 syntheses used very different strategies, with Isobe's route based on a Diels-Alder approach and Du Bois's work using C-H bond activation.

[edit] Tetrodotoxin poisoning

Fish poisoning by consumption of members of the order Tetraodontiformes is extremely serious. The skin and organs of the pufferfish can contain levels of tetrodotoxin sufficient to produce paralysis of the diaphragm and death due to respiratory failure. Toxicity varies between species and at different seasons and geographic localities, and the flesh of many pufferfish may not usually be dangerously toxic.

[edit] History

The first recorded cases of tetrodotoxin poisoning were from the logs of Captain James Cook. He recorded his crew eating some local tropic fish (pufferfish), then feeding the remains to the pigs kept on board. The crew experienced numbness and shortness of breath, while the pigs were all found dead the next morning. In hindsight, it is clear that the crew received a mild dose of tetrodotoxin, while the pigs ate the pufferfish body parts that contain most of the toxin, thus killing them.

The toxin was first isolated and named in 1909 by Japanese scientist Dr. Yoshizumi Tahara.

[edit] Symptoms and diagnosis

The diagnosis of pufferfish poisoning is based on the observed symptomology and recent dietary history.

The effects of tetrodotoxin poisoning include shortness of breath, numbness, tingling, lightheadedness, paralysis and irregular heartbeat. Symptoms typically onset quickly, minor ones instantaneously. Death is the usual outcome. Although the toxin unbinds from channels as its concentration around nerves diminishes, its molecules are exceptionally potent and unbind only very slowly. Treatment usually consists of respiratory assistance. Nothing equivalent to an antivenom has been developed--presumably because the toxin acts quickly and binds with an affinity that is not easily overcome.

[edit] Course of tetrodotoxin poisoning and complications

The first symptom of intoxication is a slight numbness of the lips and tongue, appearing between 20 minutes to three hours after eating poisonous pufferfish. The next symptom is increasing paresthesia in the face and extremities, which may be followed by sensations of lightness or floating. Headache, epigastric pain, nausea, diarrhea, and/or vomiting may occur. Occasionally, some reeling or difficulty in walking may occur. The second stage of the intoxication is increasing paralysis. Many victims are unable to move; even sitting may be difficult. There is increasing respiratory distress. Speech is affected, and the victim usually exhibits dyspnea, cyanosis, and hypotension. Paralysis increases and convulsions, mental impairment, and cardiac arrhythmia may occur. The victim, although completely paralyzed, may be conscious and in some cases completely lucid until shortly before death. Death usually occurs within 4 to 6 hours, with a known range of about 20 minutes to 8 hours.

[edit] Geographic frequency of tetrodotoxin toxicity

Poisonings from tetrodotoxin have been almost exclusively associated with the consumption of pufferfish from waters of the Indo-Pacific ocean regions. Several reported cases of poisonings, including fatalities, involved pufferfish from the Atlantic Ocean, Gulf of Mexico, and Gulf of California. There have been no confirmed cases of tetrodotoxicity from the Atlantic pufferfish, Sphoeroides maculatus. However, in three studies, extracts from fish of this species were highly toxic in mice. Several recent intoxications from these fishes in Florida were due to saxitoxin, which causes paralytic shellfish poisoning with very similar symptoms and signs. The trumpet shell Charonia sauliae has been implicated in food poisonings, and evidence suggests that it contains a tetrodotoxin derivative. There have been several reported poisonings from mislabelled pufferfish and at least one report of a fatal episode in Oregon when an individual swallowed a Rough-skinned Newt, Taricha granulosa.

[edit] Relative frequency of tetrodotoxin ingestive poisonings

From 1974 through 1983 there were 646 reported cases of pufferfish poisoning in Japan, with 179 fatalities. Estimates as high as 200 cases per year with mortality approaching 50% have been reported. Only a few cases have been reported in the United States, and outbreaks in countries outside the Indo-Pacific area are rare, except in Haiti, where Tetrodotoxin plays a key role in the creation of so called zombie poisons.

[edit] Target populations

Race is not a factor in susceptibility to tetrodotoxin poisoning. This toxicosis may be avoided by not consuming pufferfish or other animal species containing tetrodotoxin. Other animal species known to contain tetrodotoxin are not usually consumed by humans. Poisoning from tetrodotoxin is of significant public health concern primarily in Japan, where "Fugu" is a traditional delicacy. It is prepared and sold in special restaurants where trained and licensed individuals carefully remove the viscera to reduce the danger of poisoning. There is potential for misidentification and/or mislabelling, particularly of prepared, frozen fish products.

[edit] Food analysis

The mouse bioassay developed for paralytic shellfish poisoning (PSP) can be used to monitor tetrodotoxin in pufferfish and is the current method of choice. An HPLC method with post-column reaction with alkali and fluorescence has been developed to determine tetrodotoxin and its associated toxins. The alkali degradation products can be confirmed as their trimethylsilyl derivatives by gas chromatography/mass spectrometry. These chromatographic methods have not yet been validated.

[edit] See also

Clairvius Narcisse, a Haitian alleged to have been buried alive under the effect of the drug

[edit] External links

[edit] References

^ (a) Kishi, Y.; Aratani, M.; Fukuyama, T.; Nakatsubo, F.; Goto, T.; Inoue, S.; Tanino, H.; Sugiura, S.; Kakoi, H. J. Am. Chem. Soc. 1972, 94, 9217-9219. (b) Kishi, Y.; Fukuyama, T.; Aratani, M.; Nakatsubo, F.; Goto, T.; Inoue, S.; Tanino, H.; Sugiura, S.; Kakoi, H. J. Am. Chem. Soc. 1972, 94, 9219-9221.
^ (a) Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003, 125, 8798-8805 (b) Angew. Chem. Int. Ed. 2004, 43, 4782 (DOI 10.1002/anie.200460293). See a free online review
^ Hinman, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510 -11511. (DOI 10.1021/ja0368305)

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Categories: Neurotoxins | Ion channel toxins