Characterization of iodinated derivatives of alpha-bungarotoxin

The iodination of alpha-bungarotoxin and the reactivity of iodinated derivatives towards nicotinic acetylcholine receptor are described. 125I2- and 125I-alpha-bungarotoxin can be resolved, but the latter was not separated from unreacted alpha-bungarotoxin. A study of the reactivities of the various forms of the toxin towards nicotinic acetylcholine receptor indicated that di-iodination had modified its reactivity. The 125I2-form bound with a slower rate constant than alpha-bungarotoxin to the receptor. 125I-alpha-bungarotoxin showed no modification of reactivity towards the receptor. Apart from the A280, two methods for calibrating 125I-alpha-bungarotoxin are described. They may be employed in the presence of other proteins. The first of these is an immunological assay using the complex formed between toxin and antitoxin antibodies. The second is a dilution assay, where competition between iodinated and noniodinated toxins for binding sites on nicotinic acetylcholine receptor is exploited.