Saxitoxin is a
neurotoxinA neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...
naturally produced by certain species of marine
dinoflagellateThe dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
s (Alexandrium sp.,
GymnodiniumGymnodinium is a genus of dinoflagellates. It is one of the few naked dinoflagellates, or species lacking armor . Since 2000, the species which had been considered to be part of Gymnodinium have been divided into several genera, based on the nature of the apical groove and the biochemistry...
sp., Pyrodinium sp.) and
cyanobacteria (
AnabaenaAnabaena is a genus of filamentous cyanobacteria that exists as plankton. It is known for its nitrogen fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanobacteria that produce neurotoxins, which are harmful to...
sp., some
AphanizomenonAphanizomenon is an important genus of cyanobacteria that inhabits freshwater lakes and can cause choking blooms. Studies on the species Aphanizomenon flos-aquae have shown that it can regulate buoyancy through light-induced changes in turgor pressure...
spp., Cylindrospermopsis sp.,
LyngbyaLyngbya is a genus of cyanobacteria, unicellular autotrophs that form the basis of the oceanic food chain.Lyngbya form long unbranching filaments inside a rigid mucilage sheath. Sheaths may form tangles or mats, intermixed with other phytoplankton species...
sp.,
PlanktothrixPlanktothrix is a genus of filamentous blue-green algae. Type species of the genus Planktothrix is Planktothrix agardhii Anagnostidis & Komárek. The genus was scrutinized in 2002 by S. Suda . Before, some species of the taxon were grouped within the genus Oscillatoria.- Species :* Planktothrix...
sp.). Ingestion of saxitoxin (usually through shellfish contaminated by toxic algal blooms) is responsible for the human illness known as
paralytic shellfish poisoningParalytic shellfish poisoning is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks . These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae, such as...
(PSP). In fact, the term saxitoxin originates from the species name of the butter clam (
SaxidomusSaxidomus, common name "Washington clams", is a genus of large edible saltwater clams, marine bivalve mollusks in the family Veneridae, the venus clams.The species Saxidomus giganteus is known as the "butter clam"....
giganteus) in which it was first recognized. Saxitoxin, one of the most potent natural toxins known, acts on the voltage-gated sodium channels of nerve cells, preventing normal cellular function and leading to paralysis. The term saxitoxin can also refer to an entire suite of related neurotoxins (known collectively as "saxitoxins") produced by these microorganisms, which include pure saxitoxin (STX), neosaxitoxin (neoSTX), and the gonyautoxins (GTX) and decarbamoylsaxitoxin (dcSTX). Detection of saxitoxin in shellfish such as mussels, clams and scallops frequently leads to closures of commercial and recreational shellfish harvesting, especially in California, Oregon, Washington, and New England.
STX has been found in at least 12 marine puffer fish fish species in Asia and one freshwater fish
tilapiaTilapia , is the common name for nearly a hundred species of cichlid fish from the tilapiine cichlid tribe. Tilapia inhabit a variety of fresh water habitats, including shallow streams, ponds, rivers and lakes. Historically, they have been of major importance in artisan fishing in Africa and the...
in Brazil. However, the ultimate source of STX is still uncertain. In the United States,
paralytic shellfish poisoningParalytic shellfish poisoning is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks . These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae, such as...
is limited to
New EnglandNew England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
and the
West CoastWest Coast or Pacific Coast are terms for the westernmost coastal states of the United States. The term most often refers to the states of California, Oregon, and Washington. Although not part of the contiguous United States, Alaska and Hawaii do border the Pacific Ocean but can't be included in...
. The dinoflagellate Pyrodinium bahamense is the source of STX found in
FloridaFlorida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
. Recent research shows the detection of STX in the skin, muscle, viscera, and gonads of “
Indian River LagoonThe Indian River Lagoon is a grouping of three lagoons: Mosquito Lagoon, Banana River, and the Indian River, on the Atlantic Coast of Florida. It was originally named Rio de Ais after the Ais Indian tribe, who lived along the east coast of Florida....
” southern puffer fish, with the highest concentration (22,104 µg STX eq/100 g tissue) measured in the ovaries. Even after a year of captivity, the skin mucus remained highly toxic. The various concentrations in puffer fish from the United States are similar to those found in the Philippines, Thailand, Japan, and South American countries.
Biosynthesis
Although STX
biosynthesisBiosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
seems complex, organisms from the two
kingdomsIn biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
, species of marine
dinoflagellateThe dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
s and freshwater
cyanobacteria, are capable of making these toxins by the same biosynthetic pathway. The
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s involved in the biosynthesis of STX have not been identified by previous studies.
Saxitoxin synthesis is the first non-
terpeneTerpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...
alkaloidAlkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Also some synthetic compounds of similar structure are attributed to alkaloids...
pathway described for
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. A complete STX biosynthetic
gene clusterA gene cluster is a set of two or more genes that serve to encode for the same or similar products. Because populations from a common ancestor tend to possess the same varieties of gene clusters, they are useful for tracing back recent evolutionary history...
(sxt) is used to obtain a more favourable reaction. The predicted reaction sequence of suggested SxtA, based on its
primary structureThe primary structure of peptides and proteins refers to the linear sequence of its amino acid structural units. The term "primary structure" was first coined by Linderstrøm-Lang in 1951...
, is the loading of the
ACPThe acyl carrier protein is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4'-phosphopantethiene moiety...
with
acetateAn acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
from
acetyl-CoAAcetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
, followed by SxtA-catalyzed
methylationIn the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...
of acetyl-ACP, which is then converted to
propionylPropanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...
-ACP. Later another SxtA performs a
Claisen condensationThe Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, resulting in a β-keto ester or a β-diketone...
reaction between propionyl-ACP and
arginineArginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
producing 4.
SxtG transfers an amidino group from arginine to the α-amino 4 group producing 5, which later undergoes retroaldol-like
condensationA condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride,...
by SxtB. SxtD adds a
double bondA double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
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
aldehydeAn aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
. SxtU reduces the terminal aldehyde group of the STX precursor 9 forming 10. SxtI catalyzes the transfer of a carbamoyl group to the free
hydroxylA hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
group on 10. SxtH and SxtT perform a similar function which is the consecutive
hydroxylationHydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...
of C-12 terminating the STX biosynthetic pathway. This is only a proposed biosynthetic pathway; the actual mechanism of how
substratesIn biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
bind to the enzymes is still unknown.
Synthesis
The challenge for chemical synthesis comes from the dense arrangement of
heteroatomIn organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...
s 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
glycerolGlycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
-derived
sulfamateSulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, and sulfamidic acid, is a molecular compound with the formula H3NSO3...
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
12. This is oxidized to form a product N,O-
acetalAn acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...
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
azideAzide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...
displacement of the primary tosylate 15. The p-methoxybenzyl (PMB) is used to
protectA protecting group or protective group is introduced into a molecule by chemical modification of a functional group in order to obtain chemoselectivity in a subsequent chemical reaction...
the NH group by
alkylationAlkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene . Alkylating agents are widely used in chemistry because the alkyl group is probably the most common group encountered in...
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
hydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
. 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
AgNO3Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...
resulting in
carbodiimideA carbodiimide or a methanediimine is a functional group consisting of the formula RN=C=NR. Carbodiimides hydrolyze to form ureas, which makes them uncommon in nature.-Carbodiimide formation:...
formation and ring closure 23. Adding trichloroacetyl isocyanate, 23 is converted to
carbamateCarbamates are organic compounds derived from carbamic acid . A carbamate group, carbamate ester, and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes.-Synthesis:Carbamic acids are derived...
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 chainIn organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called "main chain" or backbone. The placeholder R is often used as a generic placeholder for alkyl group side chains in chemical structure diagrams. To indicate other non-carbon...
. The last step of the synthesis is to oxidize on the carbon with hydroxyl group with
DCCN,N-Dicyclohexylcarbodiimide is an organic compound with chemical formula C13H22N2 whose primary use is to couple amino acids during artificial peptide synthesis. Under standard conditions, it exists in the form of white crystals with a heavy, sweet odor. The low melting point of this material...
,
DMSODimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...
, C
5H
5N•HO
2CCF
3. The product can be highly purified using
CH3CNAcetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
, H
2O and 10 mM
heptafluorobutyric acidHeptafluorobutyric acid is an organofluorine compound with the formula C3F7CO2H. As the fluorinated derivative of butyric acid, this colourless liquid is prepared by electrofluorination of the corresponding butyryl fluoride.-Applications:...
, giving overall yield of 1.3%.
Human illness
The human illness associated with ingestion of harmful levels of saxitoxin is known as
paralytic shellfish poisoningParalytic shellfish poisoning is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks . These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae, such as...
, or PSP, and saxitoxin and its derivatives are often referred to as "
PSP toxins".
The medical and ecological importance of saxitoxin lies mainly in effects of harmful algal blooms on
shellfishShellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found only in freshwater...
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 paralysisFlaccid paralysis is a clinical manifestation characterized by weakness or paralysis and reduced muscle tone without other obvious cause .-Polio:...
that leaves its victim calm and conscious through the progression of
symptomA symptom is a departure from normal function or feeling which is noticed by a patient, indicating the presence of disease or abnormality...
s. Death often occurs from
respiratory failureThe term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial...
. PSP toxins have been implicated in various marine animal mortalities involving
trophic transferA food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
of the toxin from its algal source up the food web to higher predators.
Military Interest
It is listed in
schedule 1 of the
Chemical Weapons ConventionThe Chemical Weapons Convention is an arms control agreement which outlaws the production, stockpiling and use of chemical weapons. Its full name is the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction...
. According to the book Spycraft,
U-2The Lockheed U-2, nicknamed "Dragon Lady", is a single-engine, very high-altitude reconnaissance aircraft operated by the United States Air Force and previously flown by the Central Intelligence Agency . It provides day and night, very high-altitude , all-weather intelligence gathering...
spyplane pilots were provided with needles containing saxitoxin to be used for suicide in the event escape was impossible. The United States military isolated saxitoxin and assigned it the
chemical weapon designationChemical, biological, and radiological warfare agents are sometimes assigned what is termed a military symbol. Military symbols evolved out of the First World War from the British in part for secrecy, and to simplify reference to chemicals by something other than a chemical name...
TZ.
For a lung effect by aerosol, the median lethal dosage (LCt
50) of TZ is 5 mg·
minA minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...
/m³. Due to its high aerobiological decay rate (e.g., ~17%/min) and production cost, it was weaponized in tainted
flechetteA flechette is a pointed steel projectile, with a vaned tail for stable flight. The name comes from French , "little arrow" or "dart", and sometimes retains the acute accent in English: fléchette.-Bulk and artillery use:...
s for special operations.
Though its early isolation and characterization were from 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
Paralytic shellfish poisoning is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks . These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae, such as...
- Alexandrium tamarense
Alexandrium tamarense is a dinoflagellate known to produce saxitoxin, a neurotoxin which causes the human illness clinically known as paralytic shellfish poisoning . Multiple species of phytoplankton are known to produce saxitoxin, including at least 10 other species from the genus Alexandrium....
sp.
- Anabaena circinalis
Anabaena circinalis is species of cyanobacteria of genus Anabaena. It is capable of producing saxitoxin, which is poisonous to both human and animals....
- Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
- Tetrodotoxin
Tetrodotoxin, also known as "tetrodox" and frequently abbreviated as TTX, sometimes colloquially referred to as "zombie powder" by those who practice Vodou, is a potent neurotoxin with no known antidote. There have been successful tests of a possible antidote in mice, but further tests must be...
External links