The impact of radiodecay on 125I-Tyr-54-alpha-bungarotoxin

The intramolecular consequences, for the structural and functional integrity of labeled proteins, of the radiodecomposition of covalently bound 125I were analyzed, using the 125I-Tyr-54 derivative of the curarimimetic protein alpha-bungarotoxin (alpha BuTx) as a specific example. Mono- and di-iodinated alpha BuTx derivatives of high specific radioactivity were prepared and monitored for up to ten 125I half-lives. Analysis of the specific activity of a mono-125I-alpha BuTx as a function of storage time was carried out and revealed that at least 95.3% of all radiolabeled alpha BuTx molecules are inactivated by the decay event. Generation of radioactive fragments of low molecular weight was studied in several preparations of di-125I-alpha BuTx; it was found that approximately 89% of all decaying 125I-alpha BuTx molecules undergo fragmentation of the labeled tyrosyl moiety. It has long been known that covalently bound 125I cleaves and inactivates double-stranded DNA with extreme efficiency (see Halpern, A., and Stöcklin, G. (1977) Radiat. Environ. Biophys. 14, 167-183; 257-274); the present findings establish that alpha BuTx, a protein, is similarly disabled by 125I decay.