The neurotoxins produced by Clostridium botulinum are the most potent acute 
toxins known and are the causative agents of the neuroparalytic disease 
botulism. The toxins act primarily at peripheral cholinergic synapses by 
blocking the evoked release of the neurotransmitter acetylcholine. There are 
seven distinct serotypes of toxin. All are polypeptides of Mr about 150 kDa that 
have similar structure and pharmacological action. In their most active forms 
the toxins exist as dichain molecules in which a heavy (H) chain is linked by 
disulphide bonding to a light (L) chain. The H chain is believed to be 
associated with the highly specific and avid binding of toxin to the motor nerve 
end plates and also with the process of internalisation of the toxin. The toxic 
activity appears to be associated with the L chain which blockades the 
calcium-mediated release of acetylcholine, probably by interfering at the 
molecular level with the mechanisms whereby neurotransmitter-containing vesicles 
merge with the plasmalemma. The type A toxin is now used therapeutically to 
treat a variety of conditions involving involuntary muscle spasm. The 
therapeutic toxin is a neurotoxin-haemagglutinin complex isolated from cultures 
of C. botulinum. A controlled manufacturing process has been developed for the 
therapeutic toxin which is specially formulated to give a freeze-dried product 
having good stability.