The OGG1 gene of Saccharomyces cerevisiae encodes a DNA glycosylase activity that is a functional analog of the Fpg protein from Escherichia coli and excises 7,8-dihydro-8-oxoguanine (8-oxoG) from damaged DNA. The repair of this ubiquitous kind of oxidative damage is essential to prevent mutations both in bacteria and in yeast. A human cDNA clone carrying an ORF displaying homology to the yeast protein was identified. The predicted protein has 345 amino acids and a molecular mass of 39 kDa. This protein shares a 38% sequence identity with the yeast Ogg1 protein, adding this novel human gene product to the growing family of enzymes that the repair of oxidatively damaged bases and are related to the E. coli endonuclease III. Northern blot analysis indicates that this gene, localized to chromosome 3p25, is ubiquitously expressed in human tissues. The cloned coding sequence was expressed in an E. coli strain that carried a disrupted fpg gene, the bacterial functional analog of OGG1. Cell-free extracts from these cultures displayed a specific lyase activity on duplex DNA that carried an 8-oxoG͞C base pair. The products of the reaction are consistent with an enzymatic activity like the one displayed by the yeast Ogg1. Analysis of the substrate specificity reveals a very strong preference for DNA fragments harboring 8-oxoG͞C base pairs. The pattern of specificity correlates well with the one found for the yeast enzyme. Moreover, when the human coding sequence was expressed in a yeast strain mutant in OGG1 it was able to complement the spontaneous mutator phenotype. These results make this novel gene (hOGG1) a strong candidate for the human homolog of the yeast OGG1 and suggest an important role of its product in the protection of the genome from the mutagenic effects of the oxidatively damaged purines.Reactive oxygen species (ROS) formed in cells either as by-products of aerobic metabolism or as a consequence of exposure to environmental mutagens can attack DNA or its precursors, yielding oxidatively damaged bases and strand breakage (1, 2). Unrepaired oxidative damage to DNA has been suggested to play a role in carcinogenesis and aging through mutations in genes controlling these biological processes (3-5). Several lines of evidence suggest that an oxidatively damaged form of guanine, 7,8-dihydro-8-oxoguanine (8-oxoG), is critical in terms of mutagenesis (6, 7). In Escherichia coli, two DNA glycosylases cooperate to prevent mutagenesis by 8-oxoG: the Fpg protein, which excises 8-oxoG in damaged DNA (8-10) and the MutY protein, which excises the adenine residues incorporated by DNA polymerases opposite 8-oxoG (11-13). Inactivation of both the fpg (mutM) and mutY (micA) genes of E. coli results in a strong G⅐C 3 T⅐A mutator phenotype (14-17).In Saccharomyces cerevisiae, the OGG1 gene encodes an 8-oxoG DNA glycosylase activity that reduces the mutator phenotype of the fpg mutY mutant of E. coli (18). The Ogg1 protein contains 376 amino acids, and, although it was cloned by functional complementation of the E. co...

We have investigated the substrate specificity of the major nuclear form of the human Ogg1 protein, referred as alpha-hOgg1, for excision of damaged bases from DNA exposed to gamma-irradiation. Excision products were identified and quantified using gas chromatography/isotope dilution mass spectrometry (GC/IDMS). The GST-alpha-hOgg1 protein used in this study is a fusion of alpha-hOgg1 to the C-terminus of the GST protein. The results show that GST-alpha-hOgg1 protein excises 8-hydroxyguanine (8-OH-Gua) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) from DNA exposed to gamma-irradiation in a solution saturated with N(2)O or air. Fourteen other lesions, including oxidised purines and pyrimidines, were not excised from these substrates. Catalytic constants were measured for the excision of 8-OH-Gua and FapyGua from DNA gamma-irradiated under N(2)O. The k (cat)/ K (m)values for excision of 8-OH-Gua and FapyGua were 4.47 x 10(-5)and 8.97 x 10(-5)(min(-1)nM(-1)), respectively. The substrate specificity and the catalytic parameters of the wild-type GST-alpha-hOgg1 protein were compared to that of a polymorphic form of alpha-hOgg1 harbouring a Ser-->Cys mutation at codon 326. In the Japanese population, 47.6% of individuals possess both alleles coding for the wild-type alpha-hOgg1-Ser(326)and mutant alpha-hOgg1-Cys(326)proteins. The GST-alpha-hOgg1-Cys(326)protein was purified and its substrate specificity was determined by GC/IDMS analysis. The results show that the GST-alpha-hOgg1-Cys(326)protein efficiently excises 8-OH-Gua and FapyGua from gamma-irradiated DNA. The k (cat)/ K (m)values for excision of 8-OH-Gua and FapyGua were 2. 82 x 10(-5)and 4.43 x 10(-5)(min(-1)nM(-1)), respectively. Furthermore, we compared the capacity of these two forms of alpha-hOgg1 to act on substrates containing 2,6-diamino-4-hydroxy-5- N -methylformamidopyrimidine (Me-FapyGua). The k (cat)/ K (m)values for excision of Me-FapyGua were 278 x 10(-5)and 319 x 10(-5)(min(-1)nM(-1)), respectively. Cleavage of 34mer oligodeoxyribonucleotides containing 8-OH-Gua, 8-hydroxyadenine or an apurinic/apyrimidinic site paired with a cytosine was also investigated. The results show that both GST-alpha-hOgg1-Ser(326)and GST-alpha-hOgg1-Cys(326)catalyse the various cleavage reactions at very similar rates. Furthermore, both proteins efficiently complement the mutator phenotype of the fpg mutY mutant of Escherichia coli.