Anatoxin-a
Names | |
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IUPAC name
1-(9-azabicyclo[4.2.1]non-2-en-2-yl)ethan-1-one
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Other names
Anatoxin A
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Identifiers | |
3D model (
JSmol ) |
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ChEMBL | |
ChemSpider | |
ECHA InfoCard
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100.215.761 |
KEGG | |
PubChem CID
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UNII | |
CompTox Dashboard (EPA)
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Properties | |
C10H15NO | |
Molar mass | 165.232 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Anatoxin-a, also known as Very Fast Death Factor (VFDF), is a secondary, bicyclic
History
Anatoxin-a was first discovered by P.R. Gorham in the early 1960s, after several herds of cattle died as a result of drinking water from
Occurrence
Anatoxin-a is a neurotoxin produced by multiple genera of freshwater cyanobacteria that are found in water bodies globally.[3] Some freshwater cyanobacteria are known to be salt tolerant and thus it is possible for anatoxin-a to be found in estuarine or other saline environments.[4] Blooms of cyanobacteria that produce anatoxin-a among other cyanotoxins are increasing in frequency due to increasing temperatures, stratification, and eutrophication due to nutrient runoff.[5] These expansive cyanobacterial harmful algal blooms, known as cyanoHABs, increase the amount of cyanotoxins in the surrounding water, threatening the health of both aquatic and terrestrial organisms.[6] Some species of cyanobacteria that produce anatoxin-a don't produce surface water blooms but instead form benthic mats. Many cases of anatoxin-a related animal deaths have occurred due to ingestion of detached benthic cyanobacterial mats that have washed ashore.[7]
Anatoxin-a producing cyanobacteria have also been found in soils and aquatic plants. Anatoxin-a sorbs well to negatively charged sites in clay-like, organic-rich soils and weakly to sandy soils. One study found both bound and free anatoxin-a in 38% of aquatic plants sampled across 12 Nebraskan reservoirs, with much higher incidence of bound anatoxin-a than free.[8]
Experimental studies
In 1977, Carmichael, Gorham, and Biggs experimented with anatoxin-a. They introduced toxic cultures of A. flos-aquae into the stomachs of two young male calves, and observed that muscular fasciculations and loss of coordination occurred in a matter of minutes, while death due to respiratory failure occurred anywhere between several minutes and a few hours. They also established that extensive periods of
In the same year, Devlin and colleagues discovered the bicyclic secondary amine structure of anatoxin-a. They also performed experiments similar to those of Carmichael et al. on mice. They found that anatoxin-a kills mice 2–5 minutes after
Electrophysiological experiments done by Spivak et al. (1980) on frogs showed that anatoxin-a is a potent agonist of the muscle-type (α1)2βγδ nAChR. Anatoxin-a induced depolarizing neuromuscular blockade, contracture of the frog's rectus abdominis muscle, depolarization of the frog sartorius muscle, desensitization, and alteration of the action potential. Later, Thomas et al., (1993) through his work with chicken α4β2 nAChR subunits expressed on mouse M 10 cells and chicken α7 nAChR expressed in oocytes from
Toxicity
Effects
Laboratory studies using mice showed that characteristic effects of acute anatoxin-a poisoning via intraperitoneal injection include muscle fasciculations, tremors, staggering, gasping, respiratory paralysis, and death within minutes. Zebrafish exposed to anatoxin-a contaminated water had altered heart rates.[11]
There have been cases of non-lethal poisoning in humans who have ingested water from streams and lakes that contain various genera of cyanobacteria that are capable of producing anatoxin-a. The effects of non-lethal poisoning were primarily gastrointestinal: nausea, vomiting, diarrhea, and abdominal pain.[12] A case of lethal poisoning was reported in Wisconsin after a teen jumped into a pond contaminated with cyanobacteria.[13]
Exposure routes
Oral
Ingestion of drinking water or recreational water that is contaminated with anatoxin-a can pose fatal consequences since anatoxin-a was found to be quickly absorbed through the gastrointestinal tract in animal studies.[14] Dozens of cases of animal deaths due to ingestion of anatoxin-a contaminated water from lakes or rivers have been recorded, and it is suspected to have also been the cause of death of one human.[15] One study found that anatoxin-a is capable of binding to acetylcholine receptors and inducing toxic effects with concentrations in the nano-molar (nM) range if ingested.[16]
Dermal
Dermal exposure is the most likely form of contact with cyanotoxins in the environment. Recreational exposure to river, stream, and lake waters contaminated with algal blooms has been known to cause skin irritation and rashes.[17] The first study that looked at in vitro cytotoxic effects of anatoxin-a on human skin cell proliferation and migration found that anatoxin-a exerted no effect at 0.1 µg/mL or 1 µg/mL, and a weak toxic effect at 10 µg/mL only after an extended period of contact (48 hours).[18]
Inhalation
No data on inhalation toxicity of anatoxin-a is currently available, though severe respiratory distress occurred in a water skier after they inhaled water spray containing a fellow cyanobacterial neurotoxin, saxitoxin.[19] It is possible that inhalation of water spray containing anatoxin-a could pose similar consequences.
Mechanism of toxicity
Anatoxin-a is an agonist of both neuronal α4β2 and α4
In normal circumstances, acetylcholine binds to nAchRs in the post-synaptic neuronal membrane, causing a conformational change in the extracellular domain of the receptor which in turn opens the channel pore. This allows Na+ and Ca2+ ions to move into the neuron, causing cell depolarization and inducing the generation of action potentials, which allows for muscle contraction. The acetylcholine neurotransmitter then dissociates from the nAchR, where it is rapidly cleaved into acetate and choline by acetylcholinesterase.[21]
Anatoxin-a binding to these nAchRs cause the same effects in neurons. However, anatoxin-a binding is irreversible, and the anatoxin-a nAchR complex cannot be broken down by acetylcholinesterase. Thus, the nAchR is temporarily locked open, which leads to overstimulation due to the constant generation of action potentials.[20]
Two enantiomers of anatoxin-a, the positive
Cases of toxicity
Many cases of wildlife and livestock deaths due to anatoxin-a have been reported since its discovery. Domestic dog deaths due to the cyanotoxin, as determined by analysis of stomach contents, have been observed at the lower North Island in New Zealand in 2005,[22] in eastern France in 2003,[23] in California of the United States in 2002 and 2006,[24] in Scotland in 1992, in Ireland in 1997 and 2005,[2] in Germany in 2017[25] an 2020[26] In each case, the dogs began showing muscle convulsions within minutes, and were dead within a matter of hours. Numerous cattle fatalities arising from the consumption of water contaminated with cyanobacteria that produce anatoxin-a have been reported in the United States, Canada, and Finland between 1980 and the present.[2]
A particularly interesting case of anatoxin-a poisoning is that of
Synthesis
Laboratory synthesis
Cyclic expansion of tropanes
The first biologically occurring initial substance for tropane expansion into anatoxin-a was cocaine, which has similar stereochemistry to anatoxin-a. Cocaine is first converted into the endo isomer of cyclopropane, which is then photolytically cleaved to obtain an alpha, beta unsaturated ketone. Through the use of diethyl azodicarboxylate, the ketone is demethylated and anatoxin-a is formed. A similar, more recent synthesis pathway involves producing 2-tropinone from cocaine and treating the product with ethyl chloroformate producing a bicyclic ketone. This product is combined with trimethylsilyldiazylmethane, an organoaluminum Lewis acid and trimethylsinyl enol ether to produce tropinone. This method undergoes several more steps, producing useful intermediates as well as anatoxin-a as a final product.[2]
Cyclization of cyclooctenes
The first and most extensively explored approach used to synthesize anatoxin-a in vitro, cyclooctene cyclization involves 1,5-cyclooctadiene as its initial source. This starting substance is reacted to form methyl amine and combined with hypobromous acid to form anatoxin-a. Another method developed in the same laboratory uses aminoalcohol in conjunction with mercuric (II) acetate and sodium borohydride. The product of this reaction was transformed into an alpha, beta ketone and oxidized by ethyl azodicarboxylate to form anatoxin-a.[2]
Enantioselective enolization strategy
This method for anatoxin-a production was one of the first used that does not utilize a chimerically analogous starting substance for anatoxin formation. Instead, a racemic mixture of 3-tropinone is used with a chiral lithium amide base and additional ring expansion reactions in order to produce a ketone intermediate. Addition of an organocuprate to the ketone produces an enol triflate derivative, which is then lysed hydrogenously and treated with a deprotecting agent in order to produce anatoxin-a. Similar strategies have also been developed and utilized by other laboratories.[2]
Intramolecular cyclization of iminium ions
Iminium ion cyclization utilizes several different pathways to create anatoxin-a, but each of these produces and progresses with a pyrrolidine iminium ion. The major differences in each pathway relate to the precursors used to produce the imium ion and the total yield of anatoxin-a at the end of the process. These separate pathways include production of alkyl iminium salts, acyl iminium salts and tosyl iminium salts.[2]
Enyne metathesis
Enyne metathesis of anatoxin-a involves the use of a ring closing mechanism and is one of the more recent advances in anatoxin-a synthesis. In all methods involving this pathway, pyroglutamic acid is used as a starting material in conjunction with a Grubb's catalyst. Similar to iminium cyclization, the first attempted synthesis of anatoxin-a using this pathway used a 2,5-cis-pyrrolidine as an intermediate.[2]
Biosynthesis
Anatoxin-a is synthesized in vivo in the species Anabaena flos-aquae,[2] as well as several other genera of cyanobacteria. Anatoxin-a and related chemical structures are produced using acetate and glutamate. Further enzymatic reduction of these precursors results in the formation of anatoxin-a. Homoanatoxin, a similar chemical, is produced by Oscillatoria formosa and utilizes the same precursor. However, homoanatoxin undergoes a methyl addition by S-adenosyl-L-methionine instead of an addition of electrons, resulting in a similar analogue.[1] The biosynthetic gene cluster (BGC) for anatoxin-a was described from Oscillatoria PCC 6506 in 2009.[28]
Stability and degradation
Anatoxin-a is unstable in water and other natural conditions, and in the presence of UV light undergoes photodegradation, being converted to the less toxic products dihydroanatoxin-a and epoxyanatoxin-a. The photodegradation of anatoxin-a is dependent on pH and sunlight intensity but independent of oxygen, indicating that the degradation by light is not achieved through the process of photo-oxidation.[20]
Studies have shown that some microorganisms are capable of degrading anatoxin-a. A study done by Kiviranta and colleagues in 1991 showed that the bacterial genus Pseudomonas was capable of degrading anatoxin-a at a rate of 2–10 μg/ml per day.[29] Later experiments done by Rapala and colleagues (1994) supported these results. They compared the effects of sterilized and non-sterilized sediments on anatoxin-a degradation over the course of 22 days, and found that after that time vials with the sterilized sediments showed similar levels of anatoxin-a as at the commencement of the experiment, while vials with non-sterilized sediment showed a 25-48% decrease.[20]
Detection
There are two categories of anatoxin-a detection methods. Biological methods have involved administration of samples to mice and other organisms more commonly used in ecotoxicological testing, such as brine shrimp (Artemia salina), larvae of the freshwater crustacean Thamnocephalus platyurus, and various insect larvae. Problems with this methodology include an inability to determine whether it is anatoxin-a or another neurotoxin that causes the resulting deaths. Large amounts of sample material are also needed for such testing. In addition to the biological methods, scientists have used chromatography to detect anatoxin-a. This is complicated by the rapid degradation of the toxin and the lack of commercially available standards for anatoxin-a.[20]
Public health
Despite the relatively low frequency of anatoxin-a relative to other cyanotoxins, its high toxicity (the lethal dose is not known for humans, but is estimated to be less than 5 mg for an adult male[30]) means that it is still considered a serious threat to terrestrial and aquatic organisms, most significantly to livestock and to humans. Anatoxin-a is suspected to have been involved in the death of at least one person.[15] The threat posed by anatoxin-a and other cyanotoxins is increasing as both fertilizer runoff, leading to eutrophication in lakes and rivers, and higher global temperatures contribute to a greater frequency and prevalence of cyanobacterial blooms.[20]
Water regulations
The World Health Organization in 1999 and EPA in 2006 both came to the conclusion that there was not enough toxicity data for anatoxin-a to establish a formal tolerable daily intake (TDI) level, though some places have implemented levels of their own.[31][32]
United States
Drinking water advisory levels
Anatoxin-a is not regulated under the
State | Concentration (µg/L) |
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Minnesota | 0.1 |
Ohio | 20 |
Oregon | 0.7 |
Vermont | 0.5 |
Recreational water advisory levels
In 2008 the state of Washington implemented a recreational advisory level for anatoxin-a of 1 µg/L in order to better manage algal blooms in lakes and protect users from exposure to the blooms.[35]
Canada
The Canadian province of Québec has a drinking water Maximum Accepted Value for anatoxin-a of 3.7 µg/L.[36]
New Zealand
New Zealand has a drinking water Maximum Accepted Value for anatoxin-a of 6 µg/L.[37]
Water treatment
As of now, there is no official guideline level for anatoxin-a,[38] although scientists estimate that a level of 1 μg l−1 would be sufficiently low.[39] Likewise, there are no official guidelines regarding testing for anatoxin-a. Among methods of reducing the risk for cyanotoxins, including anatoxin-a, scientists look favorably on biological treatment methods because they do not require complicated technology, are low maintenance, and have low running costs. Few biological treatment options have been tested for anatoxin-a specifically, although a species of Pseudomonas, capable of biodegrading anatoxin-a at a rate of 2–10 μg ml−1 d−1, has been identified. Biological (granular) activated carbon (BAC) has also been tested as a method of biodegradation, but it is inconclusive whether biodegradation occurred or if anatoxin-a was simply adsorbing the activated carbon.[38] Others have called for additional studies to determine more about how to use activated carbon effectively.[40]
Chemical treatment methods are more common in drinking water treatment compared to biological treatment, and numerous processes have been suggested for anatoxin-a.
have not shown great efficacy.Directly removing the cyanobacteria in the water treatment process through physical treatment (e.g., membrane filtration) is another option because most of the anatoxin-a is contained within the cells when the bloom is growing. However, anatoxin-a is released from cyanobacteria into water when they senesce and lyse, so physical treatment may not remove all of the anatoxin-a present.[42] Additional research needs to be done to find more reliable and efficient methods of both detection and treatment.[40]
Laboratory uses
Anatoxin-a is a very powerful nicotinic acetylcholine receptor agonist and as such has been extensively studied for medicinal purposes. It is mainly used as a pharmacological probe in order to investigate diseases characterized by low acetylcholine levels, such as
Genera of cyanobacteria that produce anatoxin-a
- Anabaena (Dolichospermum)[43]
- Aphanizomenon[31]
- Cylindrospermopsis[3]
- Cylindrospermum
- Lyngbya[44]
- Microcystis[45]
- Nostoc[3]
- Oscillatoria[44]
- Microcoleus (Phormidium)[44]
- Planktothrix[44]
- Raphidiopsis[44]
- Tychonema[46]
- Woronichinia[44]
See also
References
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- ^ "Health Effects Support Document for the Cyanobacterial Toxin Anatoxin-A" (PDF). United States Environmental Protection Agency. June 2015. Retrieved October 25, 2020.
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- ^ "Cyanobacterial toxins: Anatoxin-a" (PDF). World Health Organization. November 2019. Retrieved October 25, 2020.
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- ^ a b Toxicological Reviews of Cyanobacterial Toxins: Anatoxin-A. National Center for Environmental Assessment (Report). U.S. Environmental Protection Agency. November 2006. Archived from the original on 2018-09-23. Retrieved 2018-09-22.
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- ^ Patockaa J, Stredab L (2002). "Brief review of natural nonprotein neurotoxins". ASA Newsletter. 89 (2): 16–24. Archived from the original on 2013-01-04.
- ^ a b c "2015 Drinking Water Health Advisories for Two Cyanobacterial Toxins" (PDF). United States Environmental Protection Agency. June 2015. Retrieved October 25, 2020.
- OCLC 40395794.
- ^ "Rules and Regulations: Drinking Water HABs Response Plan". Utah Department of Environmental Quality. 2018-02-12. Retrieved 2020-10-14.
- ^ "Drinking Water Contaminant Candidate List 3-Final". Federal Register. 2009-10-08. Retrieved 2020-09-27.
- ^ "Washington State Recreational Guidance for Microcystins (Provisional) and Anatoxin-a (Interim/Provisional)" (PDF). Washington State Department of Health. July 2008. Retrieved October 25, 2020.
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Further reading
- Wood SA, Rasmussen JP, Holland PT, Campbell R, Crowe AL (2007). "First Report of the Cyanotoxin Anatoxin-A from Aphanizomenon issatschenkoi (cyanobacteria)". Journal of Phycology. 43 (2): 356–365. S2CID 84284928.
- Wonnacott S, Gallagher T (April 2006). "The Chemistry and Pharmacology of Anatoxin-a and Related Homotropanes with respect to Nicotinic Acetylcholine Receptors". Marine Drugs. 4 (3): 228–254. PMC 3663412.
External links
- Very Fast Death Factor (Anatoxin-a) at The Periodic Table of Videos(University of Nottingham)
- Molecule of the Month: Anatoxin at the School of Chemistry, Physics, and Environmental Studies, University of Sussex at Brighton