  |
|
|
Fungi produce a remarkable array of active compounds. The best known are antibiotics, such as penicillin. Mycotoxins are toxic compounds produced by this group of organisms. Some toadstools are notorious for their potentially lethal poison. Amanita phalloides and Galerina sp. cause fulminant hepatocytolysis. They contain two groups of toxic compounds: amatoxins and phallotoxins. About 10 mg of alpha-amanitin (an amatoxin) is sufficient to induce massive liver necrosis. This is the amount contained in a single toadstool. Muscarine was first isolated from Amanita muscaria, the fly agaric. With its beautiful bright red cap covered with white scales it is the quintessential toadstool of fairytales. Muscarine is also found in the Inocybe and Clitocybe genera. Other toxins may be responsible for effects in the central nervous system. More than a hundred people die each year from eating poisonous toadstools. Cortinarius species cause acute renal failure due to orellanine poisoning. Tricholoma equestre causes rhabdomyolysis. The problems caused by mycotoxins are well known in veterinary medicine. Stachybotryotoxicosis, for example, a disease that results upon ingestion of the toxins produced by Stachybotrys atra, is a known problem in horses
(do not confuse this with botryomycosis, a bacterial infection!). The toxin is believed to cause pulmonary haemorrhage in children but there is currently insufficient evidence to support this. Yellow rice disease has occurred sporadically in Japan. It is caused by eating mouldy rice on which Penicillium citreoviride is present, which produces the toxic metabolite citreoviridine. It is possible that onyalai is caused by exposure to certain mycotoxins, but more study is needed. More than 150 types of mycotoxin are known, including patulin, citrinin, sterigmatocystin, cyclopiazonic acid, ochratoxin, trichothecene mycotoxins, zearalenone, T2 mycotoxins (‘yellow rain’) and others. Not all of them pose the same threat to humans. Other active mycotoxins will probably be discovered in future years.*
Mycotoxins sometime play an unexpected but important role in scientific research. The poisonous toadstool Amanita phalloides contains, among other things, alpha-amanitin and phalloidin. Phalloidin disrupts the cytoskeleton. Alpha-amanitin is a cyclic octapeptide that contains several unusual amino acids. This molecule binds strongly to RNA polymerase II, the enzyme active in the nucleoplasm and responsible for the production of premessenger RNA. RNA polymerase I, active in the nucleolus and responsible for the production of certain of ribosome subunits, is not sensitive to alpha-amanitin. Very similar enzymes differ substantially in their susceptibility to inhibition by mycotoxins, allowing precise dissecetion of biochemical pathways.
