PARENT SESSION
3:45 PM to 5:15 PM
Monday, April 22, 2002
Mini-Symposium 8
DNA Repair
Room: Nevada 6-7
, Co-Chair: Kao, Gary1; Glazer, Peter21University of Pennsylvania, Philadelphia, PA2Yale University, New Haven, CT
(MS08-5) Direct radiation damage in DNA: the yield of immediate strand breaks as a function of site location and base sequence.
Purkayastha, Shubhadeep*,1, Razskazovskiy, Yuriy1, Debije, Michael1, Bernhard, William1, 1 Department of Biochemistry and Biophysics, Rochester, New York
ABSTRACT-
Direct-type damage is damage produced in DNA by direct interaction with ionizing radiation plus damage produced by holes and electrons transferred to the DNA from the hydration layer. The other type of damage, known as indirect, is damage produced by the reactions of water radiolysis products with DNA. Final products and the mechanisms of formation are significantly better understood for indirect-type damage. In this study, we use HPLC to determine the relative probabilities of direct-type cleavage for each of the phosphodiester bonds in three different duplex oligodeoxynucleotides. Individual radiation-chemical yields have been measured for all major cleavage products formed in DNA crystals subjected to X-irradiation. The studied sequences included d(CGCGCG)2 (I), d(CGCACG:CGTGCG) (II) and d(CACGCG:CGCGTG) (III) crystallized as magnesium (I, III) and barium (II) salts. The most abundant products of DNA cleavage under these conditions are the 3′- and 5′-phosphorylated DNA fragments produced by the loss of one of the six sugars on each strand. No oxygen-dependent products such as 3′-phosphoglycolites and 8-oxoguanine were found in the crystals, indicating that the cleavage proceeds under anoxic conditions. The damage yields to the terminal sites in all duplexes is greater than the inner sites. There is, however, no statistically significant difference in the cleavage probabilities at any of the inner positions of the duplexes regardless of the base sequence. The total yield of immediate strand breaks in these crystals, calculated from the product yields, is 0.06 micromole/J for both I and III. Because the exact barium content in II is unknown the absolute yield was not determined; but in I, II, and III, the immediate strand break yields is about 10% of the free radical yield measured at 4 K in the same crystals. A model is considered that takes into account direct ionization of the sugars with subsequent electron transfer to the bases being in competition with chemical transformation of the damaged sugar site. This study was supported by PHS Grant 2-R01-CA32546, awarded by the National Cancer Institute. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Cancer Institute.
KEYWORDS: dna, hplc