How muscarine in mushrooms becomes toxic

Mushrooms exist in a breathtaking variety of shapes, colors and sizes. Especially in autumn, mushroom hunters go into the forests to find the tastiest of them, prepare them in multiple ways and eat them with relish. However, it is well known that there are also poisonous mushrooms among them and it is life-saving to distinguish between them. But are these mushrooms really poisonous?

Researchers at Friedrich Schiller University Jena and the Leibniz Institute for Natural Product Research and Infection Biology—Hans Knöll Institute (Leibniz-HKI) have investigated this question and recently published the results of a study about muscarine in Angewandte Chemie International Edition.

This toxin is found in various mushrooms, the best known of which is the fly agaric mushroom (Amanita muscaria), which also gave the toxin its name. However, considerably higher concentrations of muscarine are found in fiber cap mushrooms and fool’s funnel mushrooms.

A team led by Dirk Hoffmeister, Professor at the Institute of Pharmacy at the University of Jena and associated with his group to the Leibniz-HKI, has now been able to show that muscarine is not only present in mushrooms as such, but it can also be stored as a harmless precursor and only be released when mushrooms got injured.

Muscarine was discovered 150 years ago as the first fungal toxin. The current study was able to prove that it is stored, for example, in the fool’s funnel mushroom Clitocybe rivulosa as 4phosphomuscarin, which is less toxic.

“There are indications that other substances are also present because pure muscarine apparently has a different effect than a mushroom containing muscarine,” says Sebastian Dorner, doctoral researcher in Hoffmeister’s team.

The fool’s funnel mushroom is also known as the false champignon and can easily be confused with the real champignon. Only when the mushroom is damaged by cutting, cooking or digestion, an enzyme releases the poisonous muscarine from this precursor molecule.

In other mushrooms however, muscarine is already present in its active form. It is not uncommon for organisms to show defense and protective reactions when they are damaged, for example by being eaten by animals.

The mixture of free active and “hidden” inactive muscarine, which only becomes active poison when eaten, increases the danger of certain types of mushrooms such as the funnel mushrooms. These results could help doctors and toxicologists to better assess the actual danger of certain types of fungi and treat poisoning more efficiently.

Muscarine interferes with the transmission of signals by the neurotransmitter acetylcholine and leads to permanent excitation. The consequences are increased salivation and lacrimation, sweating, vomiting, diarrhea, circulatory collapse and even fatal cardiac paralysis.

It is irrelevant whether the poison has already been ingested in free form or as a precursor that is only activated in the body. The correct identification of edible mushrooms is therefore still an important prerequisite for an enjoyable and carefree mushroom meal.