Saxitoxin

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Saxitoxin
IUPAC name	(3aS-(3a-α,4-α,10aR*))2,6-diamino- 4-(((amino-carbonyl)oxy)methyl)-3a,4,8,9-tetrahydro- 1H,10H-pyrrolo(1,2-c)purine-10,10-diol
Identifiers
CAS number	35523-89-8
PubChem	37165
SMILES	N=C1N[C@@H](COC(N)=O)[C@H]3[C@]2(N=C(N)N3)N1CCC2(O)O
Properties
Molecular formula	C10H17N7O4
Molar mass	299.29 g mol−1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Saxitoxin (STX) is a neurotoxin naturally produced by certain species of marine dinoflagellates (Alexandrium sp., Gymnodinium sp., sp.) and cyanobacteria (Anabaena sp., some Aphanizomenon spp., sp., Lyngbya sp., Planktothrix sp.).^[1][2] The term saxitoxin originates from the butter clam (Saxidomus giganteus) in which it was first recognized.

This dinoflagellate prefers warm water of low salt concentrated environment as well as abundant sunlight. It grows in blooms and adds redness to the sea water, a phenomenon known as “red tide.” During winter, it becomes encysted and sinks to the bottom of the bay.^[3]STX has been found in at least 12 marine and freshwater puffer fish species in Asia. However, the ultimate source of STX is still uncertain. In the United States, paralytic shellfish poisoning is limited to New England and the West Coast. The dinoflagellate Pyrodinium bahamense is the source of STX found in Florida.^[4],^[5] Recent research shows the detection of STX in the skin, muscle, viscera, and gonads of “Indian River Lagoon.” puffer fish with the highest tissue concentration, 22,104 µg STX eq/100 g tissue, measured in the ovaries of a southern puffer fish. Even after a year of captivity, the skin of mucus of southern puffer fish remained at highly toxic concentrations.^[6] The various concentrations in puffer fish from the United States are similar to those found in Philippine, Thailand,^[7] Japan,^[8] and South American countries.^[9]

Mechanism

Saxitoxin is a neurotoxin that acts as a selective sodium channel blocker. ^[10]

Biosynthesis

Biosynthesis

Although STX biosynthesis seems complex, organisms from the two kingdoms, species of marine dinoflagellates and freshwater cyanobacteria, are capable of making these toxins by the same biosynthetic pathway.^[11] The enzymes involved in the biosynthesis of STX have not been identified by previous studies.^[12],^[13],^[14]

Saxitoxin synthesis is the first non-terpene alkaloid pathway described for bacteria. A complete STX biosynthetic gene cluster (sxt) is used to obtain a more favourable reaction. The predicted reaction sequence of suggested SxtA, based on its primary structure, is the loading of the ACP with acetate from acetyl-CoA, followed by SxtA-catalyzed methylation of acetyl-ACP, which is then converted to propionyl-ACP. Later another SxtA performs a Claisen condensation reaction between propionyl-ACP and arginine producing 4.

SxtG transfers an amidino group from arginine to the α-amino 4 group producing 5, which later undergoes retroaldol-like condensation by SxtB. SxtD adds a double bond between C-1 and C-5 of 6, which gives rise to the 1,2-H shift between C-5 and C-6 in 7. SxtS performs an epoxidation of the double bond and opening of the epoxide to an aldehyde. SxtU reduces the terminal aldehyde group of the STX precursor 9 forming 10. SxtI catalyzes the transfer of a carbamoyl group to the free hydroxyl group on 10. SxtH and SxtT perform a similar function which is the consecutive hydroxylation of C-12 terminating the STX biosynthetic pathway. This is only a proposed biosynthetic pathway, the actual mechanism of how substrates bind to the enzymes is still unknown.

Synthesis

The challenge for chemical synthesis comes from the dense arrangement of heteroatoms on the tricyclic structure and the dicationic nature of STX further complicates the purification of the target molecule. The starting material of this synthesis is a commercially available compound, a glycerol-derived sulfamate ester 12. This is oxidized to form a product N,O-acetal 13 and is alkynylated with zinc reagent and BF3•OEt2, producing 14 and a subsequent reaction of tosylation at the C10 of the substituted [1,2,3]-oxathiazinane-2,2-dioxide heterocycle, which later undergoes azide displacement of the primary tosylate 15. The p-methoxybenzyl (PMB) is used to protect the NH group by alkylation 16 before performing a reduction of azide with Me3P and a p-methoxybenzenesulfonyl (Mbs) containing compound to produce isothiourea 17. With the PMB and Mbs protecting groups, another azide is introduced at C6, losing PMB under oxidative condition 19. An imidoyl chloride, MbsN=CCl2, is used to re-protect the nitrogen near the tosylate site, before activating the oxathiazinane heterocycle by hydrolysis. At this point, 20, all the required carbon in tricyclic structure of STX is obtained. Next, Me3P is used to reduce azide which is then treated with AgNO3 resulting in carbodiimide formation and ring closure 23. Adding trichloroacetyl isocyanate, 23 is converted to carbamate derivative of STX 24 which can be easily isolated. The 4 double bonds on 24 are then oxidized which shows the efficiency of this synthetic route. An addition of another bicycle reagent of B(O2CCF3) in acid produces beta-STXol, while stabilizing the carbamate side chain. The last step of the synthesis is to oxidize on the carbon with hydroxyl group with DCC, DMSO, C5H5N•HO2CCF3. The product can be highly purified using CH3CN, H2O and 10 mM heptafluorobutyric acid, giving overall yield of 1.3%. ^[15]

Human illness

The human illness associated with ingestion of harmful levels of saxitoxin is known as paralytic shellfish poisoning, or PSP, and saxitoxin and its derivatives are often referred to as "PSP toxins".^[1]

The medical and ecological importance of saxitoxin lies mainly in effects of harmful algal blooms on shellfish and certain finfish which can concentrate the toxin, making it available both for human consumption as well as by various marine organisms. The blocking of neuronal sodium channels which occurs in PSP produces a flaccid paralysis that leaves its victim calm and conscious through the progression of symptoms. Death often occurs from respiratory failure. PSP toxins have been implicated in various marine animal mortalities involving trophic transfer of the toxin from its algal source up the food web to higher predators.

Military use

It is listed in schedule 1 of the Chemical Weapons Convention. According to the book Spycraft, U-2 spyplane pilots were provided with needles containing saxitoxin to be used in the event escape was impossible. The United States military isolated saxitoxin and assigned it the chemical weapon designation TZ. Though its early isolation and characterization were related to military efforts, saxitoxin has been more important to cellular research in delineating the function of the sodium channel.

See also

• Harmful algal bloom
• Paralytic shellfish poisoning
• Alexandrium tamarense sp.
• Anabaena circinalis
• Action potential
• Tetrodotoxin

References

External links

• Molecule of the month Saxitoxin - from food poisoning to chemical warfare
• Paralytic Shellfish Poisoning