Microcystin-LR is a naturally occurring toxin produced by cyanobacteria. It is one of a large family of microcystins. It is considered the most toxic compound of this family. Microcystins are cyclic heptapeptides. The seven peptides that are involved in the structure of a microcystin include a unique β-amino acid (Adda), which molecular formula is (2S,3S,8S,9S,4E,6E)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid. It contains D-alanine (D-ala) and D-methyl-aspartate (Masp). Furthermore, microcystins contain two variable residues, which make the differentiation between variants of microcystins. These two variable elements are always L-amino acids. In microcystin-LR these variable residues are L-arganine and L-leucine. Microcystin-LR inhibits protein phosphatase type 1 and type 2A (PP1 and PP2A) activities in the cytoplasm of liver cells.
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