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Microcystins are hepatotoxic cyclic heptapeptide toxins produced by cyanobacteria. They are responsible for periodic poisonings of humans and livestock following consumption of freshwater in areas where the blue-green algae are endemic. The toxicity of microcystins is initiated by the inhibition of the catalytic activity of protein phosphatase (PP) 1 and PP2A. Microcystin-LA differs from the major microcystin variant LR with the substitution of alanine in place of the arginine. It inhibits both PP1C and PP2A with an IC₅₀ value of 0.3 nM.

Microcystin LA from algae has been used as a standard for liquid chromatography/mass spectrometry (LCMS) for toxin analysis.

ChEBI: Microcystin la is a peptide.

Microcystin is a small, monocyclic peptide, composed of seven amino acids connected via peptide bonds. It is a cyanotoxin with hepatotoxic and tumor promoting properties. Several algal genera like Anabaena, Aphanizomenon, Microcystis and Planktothrix produce microcystin. Microcystin has four congeners namely, -LR, -LA, -YR and -RR.

Microcystin LA inhibits protein phosphatase type 2A and protein phosphatase type 3 more potently than protein phosphatase type 1 (order of potency PP2A > PP3 > PP1). Induces neutrophil-activating chemokines and extracellular reactive oxygen species, possible mechanisms of its hepatotoxic effects.