Microcystin

Microcystins — or cyanoginosins — are a class of toxins produced by certain freshwater cyanobacteria; primarily Microcystis aeruginosa but also other Microcystis, as well as members of the Planktothrix, Anabaena, Oscillatoria and genera. Over 50 different microcystins have been discovered so far, of which microcystin-LR is the most common. Chemically they are cyclic heptapeptides produced through nonribosomal peptide synthases.

Microcystins can be produced in large quantities during algal blooms and pose a major threat to drinking and irrigation water supplies, as well as the environment at large.

Microcystin-LR is the most toxic form of over 80 known toxic variants, and is also the most studied by chemists, pharmacologists, biologists, and ecologists. Microcystin-containing 'blooms' are a problem worldwide, including China, Brazil, Australia, South Africa, the United States and much of Europe. Hartebeespoort Dam in South Africa is one of the most contaminated sites in Africa, and possibly in the entire world.

Microcystins contain several uncommon non-proteinogenic amino acids such as dehydroalanine derivatives and the uncommon β-amino acid ADDA. Microcystins covalently bond to and inhibit protein phosphatases PP1 and PP2A and can thus cause pansteatitis.

Microcystins are hepatotoxic (able to cause serious damage to the liver). Once ingested, microcystin travels to the liver, via the bile acid transport system, where most is stored; though some remains in the blood stream and may contaminate tissue. There appears to be inadequate information to assess the carcinogenic potential of microcystins by applying EPA Guidelines for Carcinogen Risk Assessment. A few studies suggest a relationship may exist between liver and colorectral cancers and the occurrence of cyanobacteria in drinking water in China. Evidence is, however, limited due to limited ability to accurately assess and measure exposure.

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