OXIDATIVE DAMAGE TO 5-METHYLCYTOSINE IN DNA

Exposure of pyrimidines of DNA to ionizing radiation under aerobic con ditions or oxidizing agents results in attack on the 5,6 double bond o f the pyrimidine ring or on the exocyclic 5-methyl group, The primary product of oxidation of the 5,6 double bond of thymine is thymine glyc ol, while oxidation of the 5-methyl group yields 5-hydroxymethyluracil . Oxidation of the 5,6 double bond of cytosine yields cytosine glycol, which decomposes to 5-hydroxycytosine, 5-hydroxyuracil and uracil gly col, all of which are repaired in DNA by Escherichia coli endonuclease III. We now describe the products of oxidation of 5-methylcytosine in DNA. Poly(dG-[H-3]dmC) was gamma-irradiated or oxidized with hydrogen peroxide in the presence of Fe3+ and ascorbic acid, The oxidized co-p olymer was incubated with endonuclease III or 5-hydroxymethyluracil-DN A glycosylase, to determine whether repairable products were formed, o r digested to 2'-deoxyribonucleosides, to determine the total compleme nt of oxidative products. Oxidative attack on 5-methylcytosine resulte d primarily in formation of thymine glycol, The radiogenic yield of th ymine glycol in poly(dG-dmC) was the same as that in poly(dA-dT), demo nstrating that 5-methylcytosine residues in DNA were equally susceptib le to radiation-induced oxidation as were thymine residues.