The high name recognition of microcystins is due to their toxicity. The toxic effects for humans and animals unfolds in the liver, where they get specifically incorporated via the liver-specific bile acid transport system. Once inside the hepatocytes they act as irreversible inhibitors of protein phosphatases I and IIa. (see lead structures, liver transporter). The recent past saw several casualties from incorporation of contaminated water. Long term exposure to sub-lethal microcystin concentrations is thought to promote primary liver cancer. Microcystin producing cyanobacteria are spread worldwide and present a considerable health risk, especially in the warmer summer periods, when they often form blooms with high concentrations of toxins. Health authorities usually monitor cyanobacterial growth and toxin production during summer months, in order to prevent a threat to public health. The WHO guideline for microcystin-LR is currently 1 µg/Litre.

Microcystins consist of 7 amino acids (aa). All positions are variable with positions X and Z showing the highest variability. Shown here is microcystin-LR (X = leucine, Z = arginine)

aa 1	aa 2 (X)	aa 3	aa 4 (Z)	aa 5	aa 6	aa 7
D-Ala	L-Leu	D-MeAsp	L-Ala	Adda	D-Glu	Mdha
D-Ser	L-Ala	D-Asp	Aba	DM-Adda	D-Glu(OCH3)	Dha
D-Leu	L-Tyr		L-Leu	(6Z)Adda		L-Ser
	L-Glu		L-Arg	ADM-Adda		L-MeSer
	L-Val		L-Glu			Dhb
	L-Glu(OMe)		L-Glu(OMe)			MeLan
	L-Arg		L-Phe
	L-Phe		L-Tyr
	L-Met(O)		L-LHar
	L-H-Phe		L-Trp
	L-H-Tyr		L-Met(O)
	L-Trp		L-HArg
	L-H-Arg
	L-H-Ile
	L-H4Y

Click here to enlarge graphic

Aba: amino-isobutyric acid, Dha: dehydro-alanine, Mdha: N-methyl-dehydroalanine, Met(O): methionine-S-oxid, Glu(OMe): glutamate-methylester, H4Y: 1,2,3,4-tetrahydro-tyrosine, DM-Adda: Desmethyl-Adda, Dhb: Dehydro-butyric acid, MeLan: N-Methyl-Lanthionine, ADM-Adda: O-Acetyl-Adda.

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