The False Morel Mushroom: Intriguing Chemistry and Biological Mimicry.

The false morel, Gyromitra esculenta

False morels are mushrooms that look like morels but which contain a toxin. One of the species, Gyromitra esculenta, for example, is a false morel that is apparently delicious. But if it’s not cooked properly, it can lead to vomiting and diarrhea. On occasion it could even kill consumers by affecting the heart, given that its poison affects nerves which control muscle coordination.

The first problematic compound is gyromitrin, named after the genus of Gyromitra esculenta. There are several reasons why the effects of eating the false morel vary so much, from having no effect to being mortal. For starters, the amount of the toxin can vary greatly from one mushroom to the other. The amount of gyromitrin in false morels can lie anywhere between 40 to 732 milligrams per kilogram of mushrooms (wet weight). Technically, gyromitrin isn’t poisonous in itself until it gets broken down in the stomach into acetaldehyde & monomethylhydrazine(MMH). Presumably not everyone metabolizes it the same way. But when enough MMH is formed, that compound, which coincidentally is a a rocket propellant that spontaneously ignites when it comes into contact with an oxidizer like N₂O₄, reduces pyridoxine and makes it ineffective. That’s a problem because pyridoxine is a form of Vitamin B6, which among other things, is needed to make neurotransmitters. That explains the poison’s ability to stop the heart.

Monomethylhydrazine. Each blue sphere represents a nitrogen atom; grey, carbon and white, hydrogen.
gyromitrin

 

In the 1950s, US pediatricians observed cases of an unusual seizure disorder in young infants. The usual anticonvulsants did nothing to abate the symptoms, but there was a dramatic improvement when vitamin B6 was given to them. Eventually, someone figured out that the babies affected were fed a commercial formula that contained only one-third the vitamin B6 found in other baby formulas. A manufacturing process reduced the pyridoxine content of the synthetic milk, just like mushroom-derived MMH does.

The toxic false morel, Gyromitra esculenta, and an edible true morel mushroom. Source: Wikipedia.

How one prepares the mushrooms also makes an immense difference to how toxic false morels can be. Airdrying is a good preliminary treatment, as that in itself destroys some toxin.  The bulk of the poison is then destroyed by heating. The ‘safest’ way to prepare Gyromitra species is to boil the mushrooms a couple of times (being careful not to inhale any of the vapors, which could contain the volatile toxin). The cooking water has to be thrown out, and then the mushrooms are to be fried in a separate pan. This removes the majority of the monomethylhydrazine.

In the early stages of growth, the true morel not only has the characteristic cavities, brain-like texture and color of the false morel, but it is also more spherical in shape, like its “impostor”. Is it an interesting example of Batesian mimicry, in which an organism, which is not poisonous, has evolved the appearance similar to those of non-poisonous species? The organism without the poison enjoys the benefits of carrying a poison and avoids being eaten but without having to invest the metabolic energy it takes to produce the poison.

Batesian mimicry: on the left is the non-poisonous mimic  Papilio polytes, which resembles the unpalatable Pachliopta aristolochiae (right). Source: Wikipedia

Several sources point out that “this mushroom is still consumed, despite its known carcinogenic properties”. But IARC has classified it as a group 3 carcinogen since 1987, which means that although it has shown some carcinogenic tendencies in some animals, there is no evidence that it causes cancer in humans. It’s best to focus on how to cook them, if one insists on eating them.

Other Sources:

Deficiency Diseases of the Nervous System Joseph Jankovic MD, in Bradley and Daroff’s Neurology in Clinical Practice, 2022

Journal of Chromatography A, 1125 (2006) 229–233. Mehrdad Arshadi and al.  Gas chromatography–mass spectrometry determination of the pentafluorobenzoyl derivative of methylhydrazine in false morel
(Gyromitra esculenta) as a monitor for the content of the toxin gyromitrin

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