The toxin was treated with [14C]trimethyloxonium tetrafluoroborate or [3H]glycine methyl ester in the presence of 1-ethyl-3(3-dimethylaminopropyl) carbodiimide. Esterification of separate carboxyl groups with [14C]trimethyloxonium tetrafluoroborate decreased the toxicity no more than two-fold. Blocking of any single carboxyl group with [3H]glycine methyl ester did not cause more than a two-fold decrease of toxicity, and modification of two carboxyl groups caused no more than a six-fold decrease. Partial localization of modified residues in the amino acid sequence was performed. By circular dichroism, it was shown that the decrease of toxicity was not associated with alteration of secondary or tertiary structure. It is concluded that free carboxyl groups are not absolutely essential for toxicity, however they are necessary for expression of the maximum RTX-III toxicity.
