microcystin-LR

Microcystin-LR (MC-LR) is a toxin produced by cyanobacteria. It is the most toxic of the microcystins.
Microcystins are cyclic heptapeptides. The seven amino acids that are involved in the structure of a microcystin include a unique β-amino acid (ADDA). It contains alanine (D-ala), D-β-methyl-isoaspartate (D-β-Me-isoAsp), and glutamic acid (D-glu). Furthermore, microcystins contain two variable residues, which make the differentiation between variants of microcystins. These two variable elements are always standard L-amino acids. In microcystin-LR these are leucine and arginine.
Eighty microcystins have been identified to date, representing differences in the two variable residues and some modifications in the other amino acids. These modifications include demethylation of Masp and Mdha and methylesterification of D-Glu. Different microcystins have different toxicity profiles, with microcystin-LR found to be the most toxic.
Toxicity of cyanotoxins is very diverse and include neurotoxicity, hepatotoxicity, cytotoxicity and dermatotoxicity. Microcystins are generally associated with hepatotoxicity. The toxic effect of microcystins is due to their inhibition of protein phosphatases.