Saxitoxin (STX) is a potentneurotoxin and the best-known paralytic shellfish toxin. Ingestion of saxitoxin by humans, usually by consumption of shellfish contaminated by toxicalgal blooms, is responsible for the illness known asparalytic shellfish poisoning (PSP).
The term saxitoxin originates from the genus name of the butter clam (Saxidomus) from which it was first isolated. But the term saxitoxin can also refer to the entire suite of more than 50 structurally related neurotoxins (known collectively as "saxitoxins") produced byprotists,algae andcyanobacteria which includes saxitoxin itself (STX),neosaxitoxin (NSTX),gonyautoxins (GTX) anddecarbamoylsaxitoxin (dcSTX).
Saxitoxin has also been found in at least twelve marinepuffer fish species in Asia and one freshwater fish (tilapia) inBrazil.[4] The ultimate source of STX is often still uncertain. The dinoflagellatePyrodinium bahamense is the source of STX found inFlorida.[5][6] Recent research shows the detection of STX in the skin, muscle, viscera, and gonads of "Indian River Lagoon" southern puffer fish, with the highest concentration (22,104 μg STX eq/100 g tissue) measured in theovaries. Even after a year of captivity,Landsberg et al. found the skin mucus remained highly toxic.[7] The concentrations in puffer fish from the United States are similar to those found in the Philippines, Thailand,[6] Japan,[6][8] and South American countries.[9] Puffer fish also accumulate a structurally distinct toxin,tetrodotoxin.[10]
A diagram of the membrane topology of a voltage gated sodium channel protein. Binding sites for different neurotoxins are indicated by color. Saxitoxin is denoted by red.
The voltage-gated sodium channel is essential for normal neuronal functioning. It exists asintegral membrane proteins interspersed along theaxon of a neuron and possessing fourdomains that span thecell membrane. Opening of the voltage-gated sodium channel occurs when there is a change involtage or someligand binds in the right way. It is of foremost importance for these sodium channels to function properly, as they are essential for the propagation of anaction potential. Without this ability, the nerve cell becomes unable to transmit signals and the region of the body that it enervates is cut off from thenervous system. This may lead to paralysis of the affected region, as in the case of saxitoxin.[3]
Saxitoxin binds reversibly to the sodium channel. It binds directly in the pore of the channel protein, occluding the opening, and preventing the flow of sodium ions through the membrane. This leads to the nervous shutdown described above.[3]
Although thebiosynthesis of saxitoxin seems complex, organisms from two differentkingdoms, indeed two differentdomains, species of marinedinoflagellates and freshwater cyanobacteria are capable of producing these toxins. While the prevailing theory of production in dinoflagellates was through symbioticmutualism with cyanobacteria, evidence has emerged suggesting that dinoflagellates themselves also possess thegenes required for saxitoxin synthesis.[19]
Saxitoxin biosynthesis is the first non-terpenealkaloid pathway described for bacteria, though the exact mechanism of saxitoxin biosynthesis is still essentially a theoretical model. The precise mechanism of howsubstrates bind toenzymes is still unknown, and genes involved in the biosynthesis of saxitoxin are either putative or have only recently been identified.[19][20]
Two biosyntheses have been proposed in the past. Earlier versions differ from a more recent proposal by Kellmann, et al. based on both biosynthetic considerations as well as genetic evidence not available at the time of the first proposal. The more recent model describes a STX gene cluster (Sxt) used to obtain a more favorable reaction. The most recent reaction sequence of Sxt in cyanobacteria[20] is as follows. Refer to the diagram for a detailed biosynthesis and intermediate structures.
This is followed by SxtA-catalyzed methylation of acetyl-ACP, which is then converted to propionyl-ACP, yielding intermediate 2.
Later, another SxtA performs aClaisen condensation reaction between propionyl-ACP andarginine producing intermediate 4 and intermediate 3.
SxtG transfers an amidino group from an arginine to the α-amino group of intermediate 4 producing intermediate 5.
Intermediate 5 then undergoes retroaldol-like condensation by SxtBC, producing intermediate 6.
SxtD adds a double bond between C-1 and C-5 of intermediate 6, which gives rise to the 1,2-H shift between C-5 and C-6 in intermediate 7.
SxtS performs anepoxidation of the double bond yielding intermediate 8, and then an opening of the epoxide to analdehyde, forming intermediate 9.
SxtU reduces the terminal aldehyde group of the STX intermediate 9, thus forming intermediate 10.
SxtIJK catalyzes the transfer of a carbamoyl group to the free hydroxyl group on intermediate 10, forming intermediate 11.
SxtH and SxtT, in conjunction with SxtV and the SxtW gene cluster, perform a similar function which is the consecutive hydroxylation of C-12, thus producing saxitoxin and terminating the STX biosynthetic pathway.
Saxitoxin is highly toxic toguinea pigs, fatal at only 5 μg/kg wheninjected intramuscularly. Themedian lethal dose (LD50) for mice is very similar with varying administration routes:i.v. is 3.4 μg/kg,i.p. is 10 μg/kg andp.o. is 263 μg/kg. The oral LD50 for humans is 5.7 μg/kg, therefore approximately 0.57 mg of saxitoxin is lethal if ingested and the lethal dose by injection is about one-tenth of that (approximately 0.6 μg/kg). The human inhalation toxicity ofaerosolized saxitoxin is estimated to be 5 mg·min/m3. Saxitoxin can enter the body via open wounds and a lethal dose of 50 μg/person by this route has been suggested.[21]
The human illness associated with ingestion of harmful levels of saxitoxin is known asparalytic shellfish poisoning, or PSP, and saxitoxin and its derivatives are often referred to as "PSP toxins".[1]
The medical and environmental importance of saxitoxin derives from the consumption of contaminatedshellfish and certain finfish which can concentrate the toxin from dinoflagellates or cyanobacteria. The blocking of neuronalsodium channels which occurs in PSP produces aflaccid paralysis that leaves its victim calm and conscious through the progression ofsymptoms. Death often occurs fromrespiratory failure. PSP toxins have been implicated in various marine animal mortalities involvingtrophic transfer of the toxin from its algal source up the food chain to higherpredators.[3]
Studies in animals have shown that the lethal effects of saxitoxin can be reversed with4-aminopyridine,[22][23][24] but there are no studies on human subjects. As with any paralytic agent, where the acute concern is respiratory failure, mouth-to-mouth resuscitation orartificial ventilation of any means will keep a poisoned victim alive untilantidote is administered or the poison wears off.[25]
Saxitoxin, by virtue of its extremely low LD50, readily lends itself to weaponization. In the past, it was considered for military use by the United States and was developed as achemical weapon by theUS military.[26] It is known that saxitoxin was developed for both overt military use as well as for covert purposes by theCIA.[27] Among weapons stockpiles were M1 munitions that contained either saxitoxin,botulinum toxin or a mixture of both.[28] The CIA is known to have issued a small dose of saxitoxin to U-2 spy plane pilotFrancis Gary Powers in the form of a small injection hidden within a silver dollar, for use in the event of his capture and detainment.[27][28]
After the 1969 ban onbiological warfare by PresidentNixon, the US stockpiles of saxitoxin were destroyed, and development of saxitoxin as a military weapon ceased.[29] In 1975, the CIA reported to Congress that it had kept a small amount of saxitoxin andcobra venom against Nixon's orders which was then destroyed or distributed to researchers.[27]
^abClark R. F., Williams S. R., Nordt S. P., Manoguerra A. S. (1999)."A review of selected seafood poisonings".Undersea Hyperb Med.26 (3):175–84.PMID10485519. Archived from the original on October 7, 2008. Retrieved2008-08-12.
^Lagos NS, Onodera H, Zagatto PA, Andrinolo D́, Azevedo SM, Oshima Y (1999). "The first evidence of paralytic shellfish toxins in the freshwater cyanobacterium Cylindrospermopsis raciborskii, isolated from Brazil".Toxicon.37 (10):1359–1373.Bibcode:1999Txcn...37.1359L.doi:10.1016/S0041-0101(99)00080-X.PMID10414862.
^Bordner J., Thiessen W. E., Bates H. A., Rapoport H. (1975). "The structure of a crystalline derivative of saxitoxin. The structure of saxitoxin".Journal of the American Chemical Society.97 (21):6008–12.Bibcode:1975JAChS..97.6008B.doi:10.1021/ja00854a009.PMID1176726.
^Schantz E. J., Ghazarossian V. E., Schnoes H. K., Strong F. M., Springer J. P., Pezzanite J. O., Clardy J. (1975). "The structure of saxitoxin".Journal of the American Chemical Society.97 (5):1238–1239.Bibcode:1975JAChS..97.1238S.doi:10.1021/ja00838a045.PMID1133383.
^Fleming J. J., McReynolds M. D., Du Bois J. (2007). "(+)-Saxitoxin: a first and second generation stereoselective synthesis".Journal of the American Chemical Society.129 (32):9964–75.Bibcode:2007JAChS.129.9964F.doi:10.1021/ja071501o.PMID17658800.
^Handbook of toxicology of chemical warfare agents. Gupta, Ramesh C. (Ramesh Chandra), 1949- (Second ed.). London: Academic Press. 21 January 2015. p. 426.ISBN978-0-12-800494-4.OCLC903965588.{{cite book}}: CS1 maint: others (link)
^Chang FC, Spriggs DL, Benton BJ, Keller SA, Capacio BR (1997). "4-Aminopyridine reverses saxitoxin (STX)- and tetrodotoxin (TTX)-induced cardiorespiratory depression in chronically instrumented guinea pigs".Fundamental and Applied Toxicology.38 (1):75–88.doi:10.1006/faat.1997.2328.PMID9268607.S2CID17185707.
^Chen H, Lin C, Wang T (1996). "Effects of 4-Aminopyridine on Saxitoxin Intoxication".Toxicology and Applied Pharmacology.141 (1):44–48.doi:10.1006/taap.1996.0258.PMID8917674.
[2] Neil Edwards. The Chemical Laboratories. School of Chemistry, Physics & Environmental Science. University of Sussex at Brighton. Saxitoxin - from food poisoning to chemical warfare
