Why are mushrooms poisonous?

20 June 2013

Interview with

Tom Bruns, University of California, Berkeley

Nicholas Evans' experience of mushroom poisoning was devastating, but how doAmanita muscaria mushroom they produce these toxins and are they really targeted at us? Chris Smith spoke to Tom Bruns from the University of California, Berkeley who's an expert on fungal ecology and evolution.

Chris - So, why do we think that fungi makes these toxins in the first place?

Tom - Well, the quick answer is we don't really know, but the prevalent theory would be that they're making these to prevent them from being eaten by other organisms that would chew up their fruit bodies before they can sporulate. Exactly which organisms are being targeted isn't clear in most cases.

Chris - It's an extraordinary case of overkill though, isn't it? If you listened to what Nick had to say, A.) It wasn't much of a deterrent and it happened much later, so it didn't really stop him destroying the fruiting body. So, humans presumably are not the target of these toxins.

Tom - Probably not, because we're not the major selective force. With few exceptions, we're not scouring the woods and clearing out all the mushrooms. Certainly, in North America, deer would be a very common vertebrate that would eat mushrooms and they eat a lot of them. A toxin like this would probably have a very similar effect on them, I would think.

Chris - Are there any animals that are invulnerable to the toxins? Can any animals eat these things with impunity?

Tom - Certainly, there's a lot of insects, particularly fungus flies, beetles, and so on that chew up lots of different mushrooms, including some of the most deadly. So they seem to have some immunity to it, yes.

Chris - And when they get inside the bodies of target organisms whether it's us, deer, rabbits, whatever, how do they actually tend to work? Do they all work in the same way as the one that Nick had?

Tom - There's a huge number of mushroom toxins that all act in very, very different ways that aren't even related to each other. The one that he encountered here goes straight for the kidneys, but I don't believe it's actually known how it does that. It's been relatively recent years that this has been sorted out that it is this particular mushroom or group of them that have this kidney toxin.

Chris - Once these toxins enter the body, is there any way that we can get rid of them?

Tom - With a lot of mushroom toxins, you basically get rid of it yourself very quickly. The ones that are less lethal, you're usually sick in hours after eating it. And the ones that are really lethal are the ones like we're discussing here that come on many hours after you've eaten them. In that case, the toxins are very, very difficult to eliminate and part of their toxicity is that they continue to cycle through your system and destroy more cells as they're doing that. So, they're tough to get rid of. I think in this particular case, usually what happens is you'd end up in dialysis very quickly and that can help to eliminate the toxin, but usually, by the time you're doing that, the damage to the kidneys is already extensive.

Chris - What about flipping the coin over and asking, well, if these things can have these devastating and dramatic effects in certain target organs, can we use that in some way and come up with some kind of novel therapeutic perhaps for destroying cancers for example?

Tom - The one that's best known is the toxin that's in the amanitis, the alpha-aminitin. In that case, it shuts off a key enzyme in your system that allows you to produce protein. So, it's a very, very general target and the only reason that the liver in that case ends up being the main vulnerable organ is it gets recycled into the liver again and again, and concentrated in it. So, it probably wouldn't be useful for cancer treatment, but it's been very useful for research. They've use it to study the process of transcription and making of proteins because it allows you to shut it off. So, it's actually a useful chemical for molecular biology, but probably not medicine per se.

Chris - And based on your evolution knowledge, where did fungi get the chemical know-how to make these toxins in the first place?

Tom - So, in the case of the alpha-aminitin that I was just talking about, there's been some very recent data on that from genomic sequences of the aminitin mushroom and it looks like the toxin is related to things like spider venoms and so on. So, there are other organisms that make related compounds, but exactly, how the fungi acquired them is not yet clear. What is clear is that fungi are really good at making lots and lots of diverse chemical compounds and some of these end up being toxins, and very useful to the fungi in some cases where they may kill off a competitor or they may prevent themselves from being eaten by it or something, but fungi in general are just really good chemists. They make lots of different compounds.

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