Botulinum toxin is a protein and neurotoxin produced by the bacterium Clostridium botulinum. It is the most acutely toxic substance known, with an estimated human median lethal dose of 1.3–2.1 ng/kg intravenously or intramuscularly and 10–13 ng/kg when inhaled. Botulinum toxin can cause botulism, a serious and life-threatening illness in humans and animals. Popularly known by one of its trade names, Botox, it is used for various cosmetic and medical procedures.
The seven serologically distinct toxin types are designated A through G. Additionally, six of the seven toxin types have subtypes with five subtypes of BoNT A having been described. The toxin is a two-chain polypeptide with a 100-kDa heavy chain joined by a disulfide bond to a 50-kDa light chain. This light chain is an enzyme (a protease) that attacks one of the fusion proteins (SNAP-25, syntaxin or synaptobrevin) at a neuromuscular junction, preventing vesicles from anchoring to the membrane to release acetylcholine. By inhibiting acetylcholine release, the toxin interferes with nerve impulses and causes flaccid (sagging) paralysis of muscles in botulism, as opposed to the spastic paralysis seen in tetanus.
Botulinum toxin is denatured at temperatures greater than 80 °C (176 °F).