Abstract
The first report concerning DNA radiolysis was published in the Journal of 'Nature' in 1962 by B. Ekert. It was the oxidation of thymine base, that is, the production of hydroxide.Later, base modifications and strand breaks were progressively studied. The study of water radiolysis started in and has developed since 1970, by which production of ·OH(G=2.70) and eaq- (G=2.70) by water molecules was confirmed, which became a key to study the mechanism of DNA radiolysis. Most of DNA damage in cells could be due to oxidation or reduction with these main active species produced by water radiolysis, because the reaction rate constant of DNA bases not only with ·OH but also with eaq- was reported as 1010. However, ·OH contributes to the modification or degradation rate of DNA bases much more than eaq-. Hydoxylation at positions 5,6 of pyrimidine bases and at position 8 of purine bases is main reaction, and further oxidation results in base ring breakage. The modification or degradation rate of pyrimidine bases is much higher than that of purine bases (about 4 times higher).
10-20% of ·OH reacts with the deoxyribose group in DNA molecules (the reaction rate constant: 109) and results in strand break. Three processes were presented, that is, extraction of H with ·OH at position 4 and ionic separation, hydrolysis of ester bond at position 3 or 5 after radical formation at position 4, and direct radical formation of position 5. In fact, 30% of strand breaks take place with alkali treatment (Elgaeter et al. , 1976). In addition, crosslinking of DNA with protein is one kind of DNA modification. Sulfur radicals and aromatic ring radicals, easily produced in protein molecules, can bind to DNA base rings.
