Novel Activities of Human Uracil DNA N-glycosylase for Cytosine-derived Products of Oxidative DNA Damage

Uracil DNA N-glycosylase is a repair enzyme that releases uracil from DNA. A major function of this enzyme is presumably to protect the genome from pre-mutagenic uracil resulting from deamination of cytosine in DNA. Here, we report that human uracil DNA N-glycosylase also recognizes three uracil derivatives that are generated as major products of cytosine in DNA by hydroxyl radical attack or other oxidative processes. DNA substrates were prepared by gamma-irradiation of DNA in aerated aqueous solution and incubated with human uracil DNA N-glycosylase, heat-inactivated enzyme or buffer. Ethanol-precipitated DNA and supernatant fractions were then separated. Supernatant fractions after derivatization, and pellets after hydrolysis and derivatization were analyzed by gas chromatography/isotope-dilution mass spectrometry. The results demonstrated that human uracil DNA N-glycosylase excised isodialuric acid, 5-hydroxyuracil and alloxan from DNA with apparent K(m) values of approximately 530, 450 and 660 nM, respectively. The excision of these uracil analogues is consistent with the recently described mechanism for recognition of uracil by human uracil DNA N-glycosylase [Mol,C.D., Arval,A.S., Slupphaug,G., Kavil,B., Alseth,I., Krokan,H.E. and Tainer,J.A. (1995) Cell, 80, 869-878]. Nine other pyrimidine- and purine-derived products that were identified in DNA samples were not substrates for the enzyme. The results indicate that human uracil DNA N-glycosylase may have a function in the repair of oxidative DNA damage.