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Nodularins are potent toxins produced by the cyanobacterium Nodularia spumigena .[1] This aquatic, photosynthetic cyanobacterium forms visible colonies that present as algal blooms in brackish water bodies throughout the world.[2] The late summer blooms of Nodularia spumigena are among the largest cyanobacterial mass occurrences in the world. Cyanobacteria are composed of many toxic substances, most notably of microcystins and nodularins: the two are not easily differentiated. A significant homology of structure and function exists between the two, and microcystins have been studied in greater detail. Because of this, facts from microcystins are often extended to nodularins.[3]

Nodularin
The 2d structure of the Nodularin-R peptide.
Names
Other names
Cyclo[(2S,3S,4E,6E,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoyl-D-γ-glutamyl-(2Z)-2-(methylamino)-2-butenoyl-(3S)-3-methyl-D-β-aspartyl-L-arginyl]
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.150.290
KEGG
Properties
C41H60N8O10
Molar mass 824.98 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Nodularin-R is the predominant toxin variant, though 10 variants of nodularin have been discovered to date. Nodularins are cyclic nonribosomal pentapeptides and contain several unusual non-proteinogenic amino acids such as N-methyl-didehydroaminobutyric acid and the β-amino acid ADDA. These compounds are relatively stable compounds: light, temperature, and microwaves do little to degrade the compounds.[4]

Nodularins are often attributed to gastroenteritis, allergic irritation reactions, and liver diseases.[5] Nodularin-R is most notorious as a potent hepatotoxin that may cause serious damage to the liver of humans and other animals. The WHO drinking water concentration limit for nodularins (extended from microcystins-LR) is 1.5 ug /L.[6]

Contents

Physical PropertiesEdit

Nodularin has a molecular formula C41H60N8O10 and average molecular weight of 824.963 g/mol. The compound has 8 defined stereocenters.[7] It is a solid substance. In methanol, nodularin is soluble 2 mg/mL.[8] Toxins break down slowly at temperatures greater than 104F, pH less than 1 and pH greater than 9.[9] Nodularins are typically resistant to breakdown via hydrolysis and oxidation in aquatic conditions.[10] Hazardous decomposition products of nodularins are carbon monoxide and carbon dioxide.[11]

Mechanism of ActionEdit

MetabolismEdit

Nodularin primarily targets the liver, though nodularins also accumulate in the blood, intestines, and kidneys.[12] In the liver, this targeting leads to cytoskeletal damage, necrosis, and rapid blistering of the hepatocytes. Cell death and rapid blistering also destroys the finer blood vessels of the liver. The damage results in blood pooling in the liver, which can lead to an increase in liver weight of 100%. Death by nodularin poisoning occurs from this hemorrhagic shock. This is fast acting, and occurs within a few hours after a high dose [13]

At the molecular level and in further detail, nodularin is processed in a complex manner to induce toxic effects. During digestion, nodularins diffuse from small intestine into liver due to active uptake by an unspecific organic anion transporter in the bile acid carrier transport system. This transporter is expressed in the gastrointestinal tract, kidney, brain, and liver.[14] Once in the liver, nodularin inhibits three key enzymes, specifically the catalytic units of serine/threonine protein phosphatases: Protein phosphatase 1 (PP-1) and Protein phosphatase 2A. (PP-2A), and Protein phosphatase 3 (PP-3).[15] These enzymes act by removing the phosphate from a protein, inhibiting the function of the protein.

 
Emphasised on nodularin are the key sites for interaction with protein phosphatase, which leads to inhibition of the enzyme.

An initial noncovalent interaction involving the ADDA side chain (specifically where ADDA has a 6E double bond) of the nodularin and a free D-glutamyl carboxyl group off a cyclic structure of the phosphatase is the source of toxicity. The ADDA group blocks enzyme (phosphatase) activity by interacting with hydrophobic groove and obstructing substrate access to active site cleft. The toxin- phosphatase bond interactions (nodularin-PP-1, nodularin-PP-2A) are extremely strong. This leads to inhibition of the enzyme activity. Of note, nodularins differ from microcystins here: nodularins bind noncovalently to protein phosphatases while microcystins bind covalently.[16]

A further interaction involves a Michael-addition covalent linkage of electrophilic α, β unsaturated carbonyl of a methyldehydroalanine residue on the nodularin to a thiol of cysteine 273 on PP-1.[17] Though the covalent bond in step 2 is not essential for inhibition of enzyme activity, it does help mediate activity. Without this covalent bond, there is over a 10-fold reduction of nodularin affinity for the phosphatase.[18] The inhibition of the protein phosphatases results in increased phosphorylation of cytoskeletal proteins and cytoskeletal associated proteins. The hyperphosphorylation of intermediate filaments of the cell, specifically of cytokeratin 8 and cytokeratin 18, is the main cause for protein imbalance. The protein imbalance stimulates redistribution and rearrangement of these proteins, which changes the whole cell morphology and membrane integrity. More specifically, this redistribution leads to collapse of actin microfilaments in the hepatocyte cytoskeleton and dislocation of a-actinin and talin. Contact with neighboring cells is reduced and sinusoidal capillaries lose stability which rapidly leads to intrahepatic hemorrhage and often results in serious liver malfunction or death.[19]

Reactive Oxidative SpeciesEdit

Nodularins are further implicated in the formation of reactive oxidative species (ROS), specifically superoxide and hydroxyl radicals, which consequently cause oxidative DNA damage via peroxidation of lipids, proteins, and DNA via an unknown mechanism.[20]

Tumor Promoting ActivityEdit

Nodularins have received great attention as a carcinogenic threat, as the bacteria have tumor-initiating and tumor-promoting activity. Their tumor-promoting activity level is much stronger than that of microcystins; this is believed to be due to the smaller ring structure of nodularins, which enables them to be more easily taken into hepatocytes. This tumor promoting activity is achieved through induced gene expression of TNF-alpha and proto-oncogenes, though the exact mechanism is unknown. Further, the tumor suppressor gene products retinoblastoma and p53 are inactivated by the phosphorylation (described above). If the tumor suppressor is inactivated, tumor growth is likely to occur.

Considering a more public health and epidemiologic standpoint, there is a correlation of primary liver cancer in areas of China with nodularins and microcystins in the water of ponds, ditches, rivers, and shallow wells.[21]

Experiments in rats, where animals were exposed to non-lethal doses of nodularin, provided evidence as a carcinogen via tumor-initiating and tumor promoting activity. This is achieved by the inhibition of PP-1 and PP-2A. Nodularins have been implicated in the expression of the oncogenes and tumor suppressor genes tumor necrosis factor-alpha, c-jun, jun-B, jun-D, c-fos, fos-B, and fra-1 gene expression. More data is needed to have a better understanding of the carcinogenicity of nodularins .[22]

Medical AspectsEdit

SymptomsEdit

Symptoms of exposure include blistering around the mouth, sore throat, headache, abdominal pain, nausea and vomiting, diarrhea, dry cough, and pneumonia.[23] If non-lethal doses are consumed over time, damage to the liver may present as chronic symptoms of liver disease. These symptoms include jaundice, bleeding easily, swollen abdomen, mental disorientation or confusion, sleepiness, or coma.

Nodularins typically affect aquatic life such as fish and plants. However, in certain cases, nodularins have been cited in the deaths of dogs, sheep, and humans (dawson et al.). Nodularin poisoning is not very prevalent in humans: very few cases have been reported and confirmed as nodularin poisoning.

ExposureEdit

Nodularins can produce symptoms from ingestion, inhalation, and percutaneous contact. Methods of exposure include aspiration of the bacteria, dermal exposure, ingestion, and/or inhalation in recreational sports, professional fishing, or domestic uses such as showering.[24] Interestingly, conventional water treatment processes do not completely remove nodularins and microcystins from raw water.[25] Nodularins can also be ingested via contaminated drinking water or contaminated seafood. Specifically nodularins have been detected in relatively high concentrations in Baltic clams, blue mussel, flounder, cod, and threespine-stickle backs and relatively lower concentrations in herring and salmon.[26] Further, nodularins have been documented as entering the human body during contaminated water in kidney dialysis.[27] Blowing wind can spread substances from cyanobacterial blooms up to 10 km, increasing the area of potential exposure.

ToxicologyEdit

Currently, toxin concentrations are usually referred to as a mass of nodularins within the cells and those dissolved in a defined volume water. The provisional safety guideline of nodularins is 1 microgram / L. LD oral toxicity is estimated from microcystins and reported as 5 mg/kg. The toxicity of nodularins, based on LD and inhaled toxicities, is comparable to that of chemical organophosphate nerve agents.[28]

TreatmentEdit

As nodularin poisoning is rare and it remains difficult to definitively distinguish poisoning from nodularins, there is no standard method of treatment. Further, because nodularins and microcystins have rapid, and irreversible liver damage, therapy has little to no value. Chronic exposure to low concentrations is equally detrimental to the liver.[29] High precautions should be taken to avoid exposure.[30]

Research has indicated that treating during and after with melatonin (dose: 15 mg/kg of body weight) has protective functions against oxidative stress / damage induced by nodularins. (source 121 from (#1).

SafetyEdit

At risk populations for nodularin poisoning are human individuals, animals, and plants living within 10 km radius of seashore and lakefront areas. Additionally, humans 50 years and older are at elevated risk.

Safety guidelines can be implemented to reducing risk, specifically involving the cleanliness standards of drinking water. Microorganisms have been proven as effective in biodegradation and removal of nodularins, which could be useful in controlling cyanobacterial blooms in public water supplies. Protective clothing and physically avoiding areas of visible cyanobacterial blooms help reduce accidental exposures.

SynthesisEdit

Synthesis of nodularins is currently not well understood. The biosynthesis of nodularins is nonribosomal. Synthesis is conducted by multienzyme complexes, such as peptide synthetases, polypetide synthases, and tailoring enzymes.[31]

HistoryEdit

The first documented case of nodularin poisoning was an animal (sheep) in Australia in 1878. The chemical structure of Nodularin-R was identified in 1988. In Caruaru, Brazil in 1996, dialysis solutions from a local reservoir were contaminated with blue-green algae. Patients receiving hemodialysis were exposed to these solutions, 100 of 131 developed acute liver failure and 52 / 131 patients died after developing toxic hepatitis.[32]

See alsoEdit

ReferencesEdit

  1. ^ Sivonen K, Kononen K, Carmichael WW, Dahlem AM, Rinehart KL, Kiviranta J, Niemela SI (1989). "Occurrence of the hepatotoxic cyanobacterium Nodularia spumigena in the Baltic Sea and structure of the toxin". Appl. Environ. Microbiol. 55 (8): 1990–5. PMC 202992 . PMID 2506812. 
  2. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  3. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  4. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  5. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  6. ^ "Nodularin". Substances of Biological Interest, Bacterial Toxin, Natural Toxin. SelfDecode. Retrieved 30 April 2017. 
  7. ^ "Nodularin". ChemSpider. Royal Society of Chemistry. 
  8. ^ "Nodularin". Sigma-Aldrich. Sigma-Aldrich. Retrieved 2017-04-30. 
  9. ^ "Nodularin". Substances of Biological Interest, Bacterial Toxin, Natural Toxin. SelfDecode. Retrieved 30 April 2017. 
  10. ^ "Nodularin". Substances of Biological Interest, Bacterial Toxin, Natural Toxin. SelfDecode. Retrieved 30 April 2017. 
  11. ^ "Nodularin". PubChem Open Chemistry Database. National Institute of Health. 
  12. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  13. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  14. ^ Zegura, Bojana; Straser, Alja; Filipic, Metka (January–April 2011). "Genotoxicity and potential carcinogenicity of cyanobacterial toxins - a review". Reviews in Mutation Research. 727 (1-2): 16–41. doi:10.1016/j.mrrev.2011.01.002. 
  15. ^ Ohta, T; Sueoka, E; Iida, N; Komori, A; Suganuma, M; Nishiwaki, R; Tatematsu, M; Kim, S. J.; Carmichael, W. W.; Fujiki, H (1994). "Nodularin, a potent inhibitor of protein phosphatases 1 and 2A, is a new environmental carcinogen in male F344 rat liver". Cancer research. 54 (24): 6402–6. PMID 7527297. 
  16. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  17. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  18. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  19. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  20. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  21. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  22. ^ Zegura, Bojana; Straser, Alja; Filipic, Metka (January–April 2011). "Genotoxicity and potential carcinogenicity of cyanobacterial toxins - a review". Reviews in Mutation Research. 727 (1-2): 16–41. doi:10.1016/j.mrrev.2011.01.002. 
  23. ^ "How are humans exposed to cyanobacteria and cyanotoxins?". Nutrient Policy Data - Health and Ecological Effects. EPA.gov. 
  24. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  25. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  26. ^ Chen, Yun; Shen, Danfeng; Fang, Danjun (21 October 2013). "Nodularins in Poisoning". Clinica Chimica Acta. 425: 18–29. doi:10.1016/j.cca.2013.07.005. 
  27. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  28. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  29. ^ Zegura, Bojana; Straser, Alja; Filipic, Metka (January–April 2011). "Genotoxicity and potential carcinogenicity of cyanobacterial toxins - a review". Reviews in Mutation Research. 727 (1-2): 16–41. doi:10.1016/j.mrrev.2011.01.002. 
  30. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5. 
  31. ^ Zegura, Bojana; Straser, Alja; Filipic, Metka (January–April 2011). "Genotoxicity and potential carcinogenicity of cyanobacterial toxins - a review". Reviews in Mutation Research. 727 (1-2): 16–41. doi:10.1016/j.mrrev.2011.01.002. 
  32. ^ Dawson, R. M. (1998). "The Toxicology of Microcystins". Elsevier Science. 36 (7): 953–962. doi:10.1016/S0041-0101(97)00102-5.