|Jmol-3D images||Image 1|
|Molar mass||918.97 g/mol|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
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alpha-Amanitin or α-amanitin is a cyclic peptide of eight amino acids. It is possibly the most deadly of all the amatoxins, toxins found in several species of the Amanita genus of mushrooms, one being the death cap (Amanita phalloides) as well as the destroying angel, a complex of similar species, principally A. virosa and A. bisporigera. It is also found in the mushrooms Galerina marginata and Conocybe filaris. The oral LD50 of amanitin is approximately 0.1 mg/kg.
The structure of the polypeptide is atypical of most polypeptides, due to the branching of the amino acid chain. A cross bridge between 6-hydroxy-tryptophan and cysteine allows the formation of a second "inner loop," as seen in the diagram at right. The "outer loop" is formed by the normal peptide bond of the carboxyl terminus to the amino terminus of the peptide chain.
Unlike other known fungal cyclic peptides, amatoxins (and phallotoxins such as phalloidin) are synthesized on ribosomes. Heather Hallen, working in the laboratory of Jonathan Walton at Michigan State University, first isolated the genes encoding the proproteins for alpha-amanitin and phallacidin
α-Amanitin can also be used to determine which types of RNA polymerase are present. This is done by testing the sensitivity of the polymerase in the presence of α-amanitin. RNA polymerase I is insensitive, RNA polymerase II is highly sensitive (inhibited at 1μg/ml), RNA polymerase III is moderately sensitive (inhibited at 10μg/ml), and RNA polymerase IV is slightly sensitive (inhibited at 50μg/ml).
Symptoms of poisoning
α-Amanitin has an unusually strong and specific attraction to the enzyme RNA polymerase II. Upon ingestion and uptake by liver cells, it binds to the RNA polymerase II enzyme, effectively causing cytolysis of hepatocytes (liver cells). Few effects are reported within 10 hours; it is not unusual for significant effects to take as long as 24 hours after ingestion to appear, with this delay in symptoms making α-amanitin poisoning even more difficult to diagnose and all the more dangerous. By then, it is far past the time in which stomach pumping would yield an efficient result. Diarrhea and cramps are the first symptoms, but those pass, giving a false sign of remission. Typically, on the 4th to 5th day, the toxin starts to have severe effects on the liver and kidneys, leading to total system failure in both. Death usually takes place around a week from ingestion.
Around 15% of those poisoned will die within 10 days, progressing through a comatose stage to renal failure, liver failure, hepatic coma, respiratory failure and death. Those who recover are at risk of permanent liver damage. Diagnosis is difficult, and is established by observation of the clinical symptoms as well as the presence of α-amanitin in the urine. Urine screening is generally most useful within 48 hours of ingestion. Treatment is mainly supportive (gastric lavage, activated carbon, fluid resuscitation) but includes various drugs to counter the amatoxins, including intravenous penicillin and cephalosporin derivatives, and, in cases of greater ingestion, can extend to an orthotopic liver transplant. The most reliable method to treat amanitin poisoning is through having the stomach pumped immediately after ingestion; however, the onset of symptoms is generally too late for this to be an option. Chemically modified silibinin, silibinin dihydrogen disuccinate disodium (trade name Legalon SIL) a solution for injection, is used in treatment of severe intoxications with hepatotoxic substances, such as death cap (Amanita phalloides) poisoning.
Mode of inhibitory action
From the crystal structure solved by Dr. Bushnell et al., α-Amanitin interacts with the bridge helix in RNA polymerase II (pol II). This interaction interferes with the translocation of RNA and DNA needed to empty the site for the next round of RNA synthesis. The addition of α-amanitin can reduce the rate of pol II translocating on DNA from several thousand to a few nucleotides per minute, but has little effect on the affinity of pol II for nucleoside triphosphate, and a phosphodiester bond can still be formed. The bridge helix has evolved to be flexible and its movement is required for translocation of the polymerase along the DNA backbone. Binding of α-amanitin puts a constraint on its mobility, hence slowing down the translocation of the polymerase and the rate of synthesis of the RNA molecule.
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