Dna Strand Breakage by ¹²⁵I-Decay in A Synthetic Oligodeoxynucleotide: Fragment distribution and evaluation of DMSO protection effect

Abstract

A double stranded oligodeoxynucleotide containing a single ¹²⁵I-dC in a defined location was used to investigate DNA strand breakage resulting from ¹²⁵ I decay. Samples of a 41 bp oligodeoxynucleotide were incubated in 20 mM phosphate buffer (PB), or PB plus 2 M dimethylsulphoxide (DMSO), at 4°C during 18-20 days. The ³²P-5'-end labelled DNA 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 gel. Most of the breaks, around 90%, occurred within 4-5 nucleotides of the ¹²⁵I-dC, but DNA breaks were detected up to 16 nucleotides from the decay site. The ¹²⁵I-dC was located at the 21st nucleotide from the ³²P-5'-end label, and since ³²P was not detected in fragments longer than 20 nucleotides, it was assumed that all ¹²⁵I decay events produce at least one break in the ¹²⁵I-Iabelled DNA strand. The results show a considerable protection effect of DMSO on DNA breaks at sites >5-6 nucleotides from the ¹²⁵I location. The probability of breaks in this region was decreased with DMSO by a factor of 2 to 8-fold, suggesting significant role for radical-mediated DNA breaks at the more distant sites. However, the total protection effect of DMSO is rather small: 1.1, because of the small contribution of breakage at distant sites to the total yield.