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DNA repair is of fundamental importance for protection of the genetic material against mutations in an interplay with mechanisms that regulate the cell cycle, gene expression, and programmed cell death. Defects in DNA repair, or in processes in tegrated with DNA repair, may give cells a hyper mutable phenotype that increases the likelihood of mutations in genes controlling cell growth. Two principally different DNA repair mechanisms are known; (a) direct repair of a damaged base by a single enzyme without using information from the complementary strand, and (b) excision repair, in which DNA containing the damage is removed and replaced by new DNA using DNA repair synthesis. Mechanisms for excision repair are complex and comprise base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and recombination repair. In addition, the cell has mechanisms for repair of strand breaks. It has recently become clear that defective MMR is the cause of hereditary nonpolyposis colon cancer (HNPCC), and probably some 15% of the cases of sporadic colon cancer. There is also evidence that defective repair may be a primary cause of certain other forms of cancer.

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.