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Enzyme Explorer

Cyanobacterial Toxins

• Microcystins

• References
Toxic cyanobacterial waterbloom
Toxic cyanobacterial waterbloom, courtesy Wayne Carmichael, Wright State University

Cyanobacteria (also referred to as blue-green algae) are a small group of photosynthetic-planktonic bacteria whose evolution dates back more than 3.5 billion years. Although usually unicellular, they often grow in colonies large enough to see. Cyanobacteria that produce algae blooms can be extremely toxic.

Cyanobacteria have proved to be a source for a large quantity of novel organic compounds with biological activity. Several genera of cyanobacteria have been found to be toxic, and have been linked to the death of large numbers of birds and mammals. Human fatalities have occurred from exposure to the toxins from contaminated water supplies.

Toxins from cyanobacteria are released primarily when the cell is lysed or compromised. When substances are added to water to break up the occurrence of blooms of cyanobacteria, the cells die and break open, releasing potentially deadly concentrations of toxins. Cyanobacterial blooms continue to increase in number and the incidence of such blooms is widespread in surface waters throughout the world.


Microcystins
• Microcystin LR
• Microcystin RR
• Microcystin LA
• Microcystin YR

• Cylindrospermopsin

• Nodularin
Electron Micrograph of Cyanobacterial species Microcystis aeruginosa
Electron Micrograph of Cyanobacterial species Microcystis aeruginosa, courtesy Wayne Carmichael, Wright State University

One of the groups of toxins produced and released by cyanobacteria is called microcystin named from the species Microcystis aeruginosa. The microcystins are a group of cyclic heptapeptide hepatotoxins produced by a number of cyanobacterial genera. The peptide ring is made up of two protein amino acids and five non-protein amino acids. It is the two protein amino acids that distinguish microcystin types from each other, while the other amino acids are relatively constant.

By using amino acid single letter code classification, each microcystin is designated a name depending on the variable amino acids which complete their structure. For instance, one of the most common toxins found in water supplies around the world, microcystin-LR contains the amino acids Leucine (L) and Arginine (R) in these variable positions.

The most common of the toxins, microcystins are also the ones most often responsible for poisoning animals and humans who come into contact with toxic blooms. They are extremely stable in water because of their chemical structure, surviving in both warm and cold water and can tolerate radical changes in water chemistry, including pH. To date, approximately 75 different kinds of microcystins have been discovered.

These hepatotoxins inhibit the protein phosphatases inside hepatocytes. They damage the liver by affecting the maintenance of the cytoskeleton by disrupting the balance of phosphate groups on cytoskeletal proteins. The hepatotoxins cause the cytoskeleton to collapse, causing the hepatocytes to collapse inwards. The hepatocytes and also the capillary cells then pull apart, spilling blood into the liver. The blood pools in the liver, causing death.

Another cyanobacterial toxin is Nodularia spumigena, characterized as Nodularin. In 1878, George Francis issued the first scientific report of the potent toxicity of Nodularin cyanobacterial blooms. The structure of nodularin is closely related to that of the potent cyclic heptapeptide hepatotoxins, the microcystins. The difference lies in that nodularin is composed of only five amino acids in the peptide ring. Not only are the structures of these two cyanobacterial toxins similar, but they also show the same hepatotoxic effects through the potent inhibition of protein phosphatases.

It is speculated that low-level exposure to these toxins may promote the development of cancer in the liver and other chronic disorders of the gastrointestinal tract. This is likely to occur because the protein phosphatases that are inhibited by hepatotoxins play an important role in regulating cell division. While not initiating the cancers, these toxins have been shown to hasten the development of cancer in animals. Carmichael suggests that "the extraordinarily high rates of liver cancer in parts of China may be tied to the cyanobacterial toxins in water."

Analytical testing confirms the amount of cyanobacteria toxin in water samples. In recent years, HPLC, ELISA, and the protein phosphatase assay have made the quantification of total and individual toxins possible. Lyophilized cultures and bloom samples are used to determine the quantitative concentration of toxin. Results are expressed as milligrams or micrograms of toxin per gram of dry weight.


Microcystin LR
Microcystin LR, M2912

Potent inhibitor of protein phosphatase types 1 and 2A; has no effect on protein kinase.
Microcystins LR

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Microcystin RR
Microcystin RR , M1537

Inhibitor of protein phosphatase type 2A with lower toxicity than microcystin LR.
Microcystins RR

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Microcystin LA
Microcystin LA , M4194

Inhibitor protein phosphatase type 2A and protein phosphatase type 3 more potently than protein phosphatase type 1 (order of potency PP2A > PP3 > PP1)
Microcystins LA

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Microcystin YR
Microcystin YR , M4069

Inhibitor protein phosphatase type 2A and protein phosphatase type 1.
Microcystins YR

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Cylindrospermopsin
Cylindrospermopsin, C9866

Inhibition of protein synthesis.
Target organ = liver
Cylindrospermopsin

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Nodularin
Nodularin, N5148

Potent inhibitor of protein phosphatases types 1 and 2A.
Nodularin

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References

 

  1. Carmichael, W.W. Health effects of toxin producing cyanobacteria: the cyanoHABs. Human Ecol. Risk Assess. 7, 1393-1407 (2001)
  2. Carmichael, W.W. et al. Human fatalities from cyanobacteria: chemical and biological evidence for cyanotoxins. Environmental Health Perspectives. 109, 663-668 (2001)
  3. Carmichael, W.W., et al., Scientific American, 270, 78-86 (1994)
  4. Chu, F.S. et al. Enzyme-linked immunosorbent assay for microcystins in blue-green algal blooms. J. Assoc. Analyt. Chem., 73, 451-456 (1990)
  5. Claeyssens S. et al., Microcystin-LR induced an inhibition of protein synthesis in isolated rat hepatocytes. Biochem. J., 306, 693–696 (1995)
  6. Gilroy, Duncan J. et al. Assessing Potential Health Risks from Microcystin Toxins in Blue-Green Algae Dietary Supplements. Environmental Health Perspectives. 108, 435-439 (2000)
  7. Lambert, T.W., et al. Quantitation of the microcystin hepatotoxins in water at environmentally relevant concentrations with the protein phosphatase bioassay. Environ. Sci. Technol., 28, 753-755 (1994)
  8. Rinehart, K. L., et. al. Nodularin, microcystin and the configuration of Adda. J. Am. Chem. Soc., 110(25):8557-8558 (1988).
  9. Toxic Cyanobacteria in Water: A guide to their public health consequences, monitoring and management. WHO (1999)

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