DNA STRAND BREAKAGE BY I-125 DECAY IN A SYNTHETIC OLIGODEOXYNUCLEOTIDE .1. FRAGMENT DISTRIBUTION AND EVALUATION OF DMSO PROTECTION EFFECT

A double stranded oligodeoxynucleotide containing a single I-125-dC in a defined location was used to investigate DNA strand breakage result ing from I-125 decay, Samples of a 41 bp oligodeoxynucleotide were inc ubated in 20 mM phosphate buffer (PB), or PB plus 2 M dimethylsulphoxi de (DMSO), at 4 degrees C during 18-20 days, The P-32-5'-end labelled DNA fragments produced by I-125 decays were separated on denaturing po lyacrylamide gels, and the P-32 activity in each fragment was determin ed by scintillation counting after elution of fragments from gel. Most of the breaks, around 90%, occurred within 4-5 nucleotides of the I-1 25-dC, but DNA breaks were detected up to 16 nucleotides from the deca y site. The I-125-dC was located at the 21st nucleotide from the P-32- 5'-end label, and since P-32 was not detected in fragments longer than 20 nucleotides, it was assumed that all I-125 decay events produce at least one break in the I-125-labelled DNA strand, The results show a considerable protection effect of DMSO on DNA breaks at sites >5-6 nuc leotides from the I-125 location. The probability of breaks in this re gion was decreased with DMSO by a factor of 2 to 8-fold, suggesting si gnificant role for radical-mediated DNA breaks at the more distant sit es. 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.