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Toxins

Introduction

Fungi produce a range of secondary metabolites which include compounds that aid their competitive position. LINK Among the compounds are a variety which cause toxicosis in mammals, especially humans, insects and other herbivores. LINK. The compounds of most importance to humans are those which are ingested accidentally with food LINK. Toxins may also be consumed deliberately, either to find out if the mushroom is flavoursome, or because the mushroom has been misidentified (see below). LINK Considering the vast number of toxic fungi, it is surprising that more cases of fungal toxicosis are not observed.

Toxins in Food

Fungi take from their environment nutrients which are used for their growth and development. When the energy resource becomes depleted, the production of secondary metabolites increases, including toxic compounds.

Human and other animal foodstuffs are a major potential source of nutrient for fungi. Spores of a wide range of fungi are common in the air. LINK If conditions are suitable (access, high water activity and moderate temperature), fungi colonise the foodstuffs. Some common air-borne fungi are known to produce extremely toxic compounds; these include Aspergillus, Alternaria, Fusarium and Penicillium.

Toxins

Common toxins include alkaloids, cyclopeptides, and coumarins. The compounds are active at extremely low concentrations and have a rapid effect. The toxins may cause death. In sublethal quantities, ongoing consumption of the toxins may also trigger cancer, and influence the physiology of the consumer. Many of the compounds are heat stable remaining active after cooking or treatment of foodstuff. The potential for damage is particularly important for humans and livestock held in intensive conditions.

The most important group of toxins studied for their toxicological effects are the aflatoxins. Aflatoxins are produced by the group of fungi in the Aspergillus parasiticus group, that is Aspergillus flavus and Aspergillus parasiticus. The fungi grow when humidity is high enough on and in stored grains and animal feeds. They also grow as endophytes in living plants of cotton, peanuts and corn, where colonisation of the host plant may take place prior to seed ripening. The fungi, and probably toxins, are present in the seed at harvest.

The fungi produce aflatoxins, which are difurano-coumarins, in several forms. Aflatoxin B1 and G1 are found most commonly. Their effects are similar. Birds are particularly sensitive to aflatoxins. Feed containing contaminated peanuts given to turkeys in Britain caused widespread death in the early 1960s. The birds died from liver and kidney failure. Aflatoxins also induce cancer of the liver. As the compounds are active at parts per billion, aflatoxins are among the most toxic and dangerous fungal metabolites.

Other toxins produced by fungi have attained notoriety. Vomitoxin or trichothecene, is produced by Fusarium graminearum and other fungi. The fungus can grow on damp grains, and produce the toxin during storage. It is a potentially important problem for housed animals, such as pigs. This fungus also produces zearalenone, which appears to mimic oestrogen in mammals, and thus causes abortions in pregnant livestock, especially pigs.

Each of these fungi, when closely studied, has been found to produce a range of other toxins, active in different doses, and produced under different conditions. The production of secondary metabolite also varies with strain. Further, some toxins are produced by a range of seemingly unrelated fungi. Unfortunately, toxicology is an area of research which is driven by the end effect of the product, and we do not understand much of the basic biology of metabolite production

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Other Metabolites

During the Spanish conquest of the Americas, stories began to filter back to Europe of the use of magic mushrooms, and their place in the cultures of the American Indians. The mushrooms were eventually identified, and subsequently became popular in the Americas during the protests against the Western involvement in the Vietnam war. At this time they were called magic mushrooms, largely because of their hallucinatory effects.

The active ingredients are tryptamine derivatives. They are found widely in two genera, and in a few species of other genera. The fungi are generally saprotrophic, with one species preferentially colonising horse and cow dung. The fungi can be found around the world. Many societies now recognise the use of magic mushrooms as either a problem or an irritant, depending on the cultural conditions.

Psilocybin is colourless, and rapidly oxidises to blue, giving the characteristic feature used in field identification. The concentration of active ingredient in mushrooms varies widely, and some unpleasant effects can be triggered unexpectedly. Onset of signs is usually within a few minutes of consumption, and the signs can last for days, though the effect is usually resolved within hours.


Amanita.

Basidiocarps of other species are also known to produce mood alterations, and some have been used deliberately over the centuries, often associated with religious ceremonies. The use of Amanita muscaria is a particularly odd story. Documents suggest that rich of Russia used to consume the fungus for its halucinatory properties. The wealthy used then to urinate into containers for the peasants to drink. Apparently the active molecules used to pass, almost immediately, through the system and were excreted in the urine. However, no other society has used A. muscaria in this way, and in fact the mushroom is more likely to poison the consumer.

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Specific Effects of Toxins

A number of fungi produce secondary metabolites which are active in mammals in other ways. Several agaric fungi produce toxins. In Europe, the largest number of poisonings is due to consumption of Amanita phalloides. Amatoxin is found in most species of Amanita , some species of Lepiota, Conocybe and Galerina. Amatoxin is a cluster of compounds, including the peptides amanitin and phallotoxin.

Amanitin is an inhibitor of RNA polymerase II, and thus acts in all eukaryotic cells where protein synthesis takes place. The liver is the first organ in the human body exposed to high concentrations of amanitin after the gut, and is thus the first to be affected.

Phallotoxin was once thought to be responsible for the usual symptoms of amatoxins. The compound acts to inhibit F actin in the cell cytoskeleton. It acts immediately, and probably does not move beyond the lining of the gut.

The activity of amatoxins gives some idea of symptoms of poisoning by Amanita. Amatoxins are taken up by cells lining the gut, where protein synthesis is immediately inhibited. The toxins are released to the blood stream and transported to the liver. In the liver, protein synthesis is immediately stopped. Amanitin is excreted in the urine, and most is evacuated from the body within hours of ingestion. However, if sufficient liver tissue is affected, liver failure will ensure death.

The dose that is likely to kill an average human is in the range of 6-7 mg, easily found in the cap of one mature A. phalloides. However, like other fungal toxins, the concentration which is fatal for individuals differs, the concentration in different specimens differs, and the environment influences how much toxin can be produced in one basidiocarp.

Any fungus collected from the field should be identified before it is consumed.

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Australian Toxigenic Fungi

A wide range of exotic and indigenous fungi found in Australia are known to be potentially toxic. Given the lack of understanding of our fungi, it best to be cautious when sampling the culinary potential of unusual or unknown species of fungi. The table from the Sydney Fungal Studies Group lists some fungi known to be toxic and measures that might be necessary if they are to be consumed. LINK to toxic and hallucinogenic fungi.

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Conclusion

Fungi produce a wide range of secondary metabolites, whose role in their ecological interactions is unknown. Among these compounds are a group whose effects on the consumer range from neurological effects to toxicosis. They appear to act directly on the central nervous system, and other primary organs in mammals.

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References

DR Benjamin (1995) Mushrooms: Poisons and Panaceas. WH Freeman, New York.

Blaney BL (1996) Fungal Toxins and Animals. In: Fungi of Australia Vol 1B, Fungi in the Environment. ABRS, Canberra.

Southcott RV (1996) Mechanisms of Macrofungal Poisoning in Humans. In: Fungi of Australia Vol 1B, Fungi in the Environment. ABRS, Canberra.

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