The role of an invariant tryptophan residue in α‐bungarotoxin and cobrotoxin

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Abstract
Ozone oxidation converted the single, invariant, tryptophan residue to N2-formylkynurenine in .alpha.-bungarotoxin and cobrotoxin. Upon this modification, the lethal toxicity was significantly reduced in cobrotoxin but mostly retained in .alpha.-bungarotoxin. Each neurotoxin containing kynurenine instead of tryptophan retained the same antigenicity as the native toxin. Fluorescence and CD spectroscopy revealed that, although the environment and state of the kynurenine residue were similar, [Kyn29]cobrotoxin was much more sensitive to pH change than .alpha.-[Kyn28]bungarotoxin. In terms of lethal toxicity and conformational stability, the invariant tryptophan residue appears to play a more important role in cobrotoxin, imparting a higher lethal toxicity than that in .alpha.-bungarotoxin, which has a disulfide bond at Cys29-Cys33.