Status of tryptophan residue in cobrotoxin and alpha-bungarotoxin.

Acrylamide quenching studies indicated that the exposure degree of the Trp residue in cobrotoxin was higher than that in alpha-bungarotoxin. The Trp residue of cobrotoxin was in a positively charged environment as revealed by iodide quenching, while the Trp residue of alpha-bungarotoxin was not accessible for iodide. Analysis of hydrophilicity profile and local concentration of positively charged residues of toxin molecule also indicated that Trp in cobrotoxin was in a highly hydrophilic and positively charged environment. Measurement of Trp fluorescence with increasing temperature showed that the stability of environment of Trp in alpha-bungarotoxin was higher than in cobrotoxin. Result of competitive binding for nicotinic acetylcholine receptor (AchR) between cobrotoxin and alpha-bungarotoxin revealed that the molecular interaction of the two toxins with AchR was not the same. These, together with the fact that the cationic groups of the two toxins are involved in the binding with AchR, suggest that the observed different environment surrounded Trp residue and different AchR binding mechanism might fulfill a different requirement of the invariant Trp in the lethality of cobrotoxin and alpha-bungarotoxin.