125I-alpha-bungarotoxin. Effects of radiodecomposition on specific activity.

Radiodecomposition of 125I-alpha-bungarotoxin has been followed by a variety of assays, including an ion-exchange assay which reflects the ability of toxin to bind to receptor and a competition assay that is sensitive to changes in specific activity. The rate of degradation of labeled toxin can be much faster than the decay rate of 125I. The radiodecomposition rate increases with increasing specific activity but is dramatically reduced by the addition of bovine serum albumin. Our results indicate that radiodecomposition affects labeled and unlabeled molecules equally and is due mostly to secondary radiodecomposition. As expected from this mechanism, we have found that radiodecomposition does not affect the specific activity of 125I-labeled alpha-bungarotoxin. Our results from iodinated preparations of moderate specific activity (40-500 Ci/mmol), containing a mixture of unlabeled and monolabeled toxin molecules, are also applicable to preparations of pure monoiodo-alpha-bungarotoxin of similar specific activity.