Effect of photooxidation on biological properties of botulinum neurotoxin A

: Photooxidation of botulinic neurotoxin A in the presence of methylene blue is associated with a decrease in toxicity down to complete detoxication. During neurotoxin photooxidation, when the toxicity makes up to 1 to 3% of the original one, the conformation of the neurotoxin molecule and its antigenic properties remain unchanged. Under these conditions, using diethylpyrocarbonate, a specific reagent for histidine, the photooxidized neurotoxin was found to contain 5-6 oxidized histidine residues per molecule of neurotoxin; this was accompanied by changes in the UV absorbance spectrum around 280 nm. It was assumed that the main decrease in neurotoxin toxicity during photooxidation is probably due to oxidation of tryptophane, since the differential UV spectra suggest that the higher the extremum around 280 nm, the greater the decrease of toxicity; chemical modification of histidine residues alone causes no noticeable detoxication.