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Botulinum toxin (trade name Botox) inhibits acetylcholine release in neurons and causes botulism, an acute paralytic disease which leads to nerve degeneration and takes a long time to recover. I've read the statement that Botox is the most potent poison we know a few times now and was wondering why exactly it is. All sources I've found only claim that it is, without explanation.

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do you want to know the mechanism? Or numbers for comparison? –  Memming Nov 20 '12 at 2:14
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A quick search for a list of LD50 (median lethal dose; half of the people will die with that dosage) gave me 1 ng/kg for Botulinum toxin. This is pretty darn small amount, and in fact the smallest amount within that table.

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True. Also look at the potency page on Wikipedia –  nico Nov 20 '12 at 8:19
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This doesn't seem to answer the why, it doesn't explain the reason for the high toxicity. –  Mad Scientist Nov 20 '12 at 9:22
    
@MadScientist I didn't know if OP wanted to know "why it is known for the most potent poision" or mechanism of Botox. I asked OP via comment, but with no answer so far. –  Memming Nov 20 '12 at 14:16
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Thanks for this answer, LD50 is a good criterion for a statement like this and I expect many of the sources where I've read it are probably referring to this. Though the question remains what exactly gives BT such a low LD50 - it's not the only neurotoxin out there, and some members of the BT family have much higher LD50's despite a very similar mechanism of action. I'm afraid that might be a question yet unanswered by science though. –  Armatus Nov 21 '12 at 10:20
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The biochemical mechanism of BT is pretty well laid out at Wikipedia already. The toxin is likely very potent because (1) it is targeted to neurons; (2) neurons endocytose BT; and (3) the BT light chain's protease activity is able to cleave a wide range of SNARE-complex proteins which effectively disrupt the neurons ability to secrete monoamine neurotransmitters, and peptide hormones. Basically, the neurons are completely inhibited from transmitting nerve impulses.

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Many poisons simply bind to and deactivate their target proteins. BT proteases can bind to, damage, and unbind to find other targets. Good answer. –  Jeremy Kemball Jul 31 '13 at 18:55
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