Iodine-125 Decay in a Synthetic Oligodeoxynucleotide. I. Fragment Size Distribution and Evaluation of Breakage Probability

Abstract

Lobachevsky, P. N. and Martin, R. F. Iodine-125 Decay in a Synthetic Oligodeoxynucleotide. I. Fragment Size Distribution and Evaluation of Breakage Probability.Incorporation of 125I-dC into a defined location of a double-stranded oligodeoxynucleotide was used to investigate DNA breaks arising from decay of the Auger electron-emitting isotope. Samples of the oligodeoxynucleotide were also labeled with 32P at either the 5′ or 3′ end of either the 125I-dC-containing (so-called top) or opposite (bottom) strand and incubated in 20 mM phosphate buffer or the same buffer plus 2 M dimethylsulfoxide at 4°C during 18–20 days. The 32P-end-labeled fragments produced by 125I decays were separated on denaturing polyacrylamide gels, and the 32P activity in each fragment was determined by scintillation counting after elution of fragments from the gel. The relative fragment size distributions were then normalized on a per decay basis and converted to a distribution of single-strand break probabilities as a function of distance from the 125I-dC. The results of three to five experiments for each of eight possible combinations of labels and incubation conditions are presented as a table showing the relative numbers of 32P counts in different fragments as well as graphs of normalized fragment size distributions and probabilities of breakage. The average numbers of single-strand breaks per 125I decay are 3.3 and 3.7 in the top strand and 1.3 and 1.5 in the bottom strand with and without dimethylsulfoxide, respectively. Every 125I decay event produces a break in the top strand, and breakage of the bottom strand occurs in 75–80% of the events. Thus a double-strand break is produced by 125I decay with a probability of approximately 0.8.