Microcystin-RR is an Arg-Arg analog of microcystin-LR. It is hepatotoxic, although found to be up to 10-fold less toxic than microcystin-LR on intraperitoneal injection injection in mice. It is a potent inhibitor of protein phosphatase 2A (PP2A). Microcystins (also known as cyanoginosins) are a class of toxins produced by certain freshwater cyanobacteria. Microcystins are chemically stable over a wide range of temperature and pH, possibly as a result of their cyclic structure. The toxins are also resistant to enzymatic hydrolysis (in guts of animals) by some general proteases, such as pepsin, trypsin, collagenase, and chymotrypsin.
belongs to the class of organic compounds known as hybrid peptides. Hybrid peptides are compounds containing at least two different types of amino acids (alpha, beta, gamma, delta) linked to each other through a peptide bond.
The site of action of microcystins is the hepatocyte, the commonest cell type in the liver. They act by disrupting the cytoskeleton, the adaptable protein framework that constantly shapes and reshapes the cell as it responds to the environment. The cells die and this destroys the finer blood vessels of the liver leading to massive hepatic bleeding.
The molecular target are a group of enzymes called protein phosphatases that play a role in regulating protein interactions and activities. Very well-defined types of protein phosphatase (type 1 and type 2A) are inhibited very specifically by very low concentrations of microcystins. This enzyme removes phosphate from a protein, a common step in many biochemical pathways. This inhibition, with
subsequent build up of phosphorylated proteins, is believed to be a mechanism by which microcystins destroy livers.
Microcystins also activate the enzyme phosphorylase b, which plays a very important role in the affairs of the hepatocyte. The combination of inhibition and activation is rapidly lethal to the cell. The specificity of some of these toxins makes them valuable research tools.
Metabolism
Microcystins are extremely stable and resist common chemical breakdown such as hydrolysis or oxidation under conditions found in most natural water bodies. These toxins can break down slowly at high temperature (40 °C or 104 o F ) at either very low (<1) or high (>9) pH. The half-life, the time it takes for one-half of the toxin to degrade, at pH 1 and 40 oC is 3 weeks; at typical ambient conditions half-life is 10 weeks.
Toxicity Values
LD50 for rats and mice are in the range 36-122 micrograms/kg with the inhalation toxicity 180 mg/min/m3 or 43 micrograms/kg.
Lethal Dose
In comparing the available indications of hazards from cyanotoxins with other water-related health hazards, it is conspicuous that
cyanotoxins have caused numerous fatal poisonings of livestock and wildlife, but no human fatalities due to oral uptake hav
Microcystis extracts: Group 3, not classifiable as to their carcinogenicity to humans. (1)
Uses/Sources
Microcystins are produced by the cyanobacterial cells. When the algae dies, the cell walls burst,
releasing the toxin into the water.
Minimum Risk Level
WHO developed a drinking water concentration limit of 1.5 μg/L for microcystin LR. They assumed that a 60 kg (132 lbs.) person drinks two liters of water each day and that 80% of the two liters is from a contaminated source.
Health Effects
Liver damage. While microcystin-LR does not cause cancer, microcystin may stimulate the growth of cancer cells.
Symptoms
The most common sign of human poisoning with microcystins is liver damage.
Treatment
Microcystins can be broken down by some bacterial proteases
