Modification of carboxyl groups in botulinum neurotoxin types A and E

Effects of chemical modification of carboxyl groups of botulinum neurotoxin serotypes A and E were studied by using a water soluble carbodiimide-nucleophile reaction that is highly specific for modifying carboxyl groups of proteins. In both types A and E, increasing levels of the reagents, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and norleucine methyl ester or glycine methyl ester, at pH 4.8 caused increased loss of toxicity. More glycine could be incorporated than norleucine. Amino acid analysis did not reveal modification of any amino acid residue other than carboxyl groups (possible reaction of sulfhydryl groups was not studied). Loss of one carboxyl group did not severely affect toxicity, but modification of three carboxyl groups caused greater than 95% detoxification in both types. Complete detoxification could not be achieved with any amount of the reagents. Modification of three to five carboxyl groups did not affect serological activity.