5-Formyluracil (fU) is a major oxidative thymine lesion produced by reactive oxygen species and exhibits genotoxic and cytotoxic effects via several mechanisms. In the present study, we have searched for and characterized mammalian fU-DNA glycosylase (FDG) using two approaches. In the first approach, the FDG activity was examined using purified base excision repair enzymes. Human and mouse endonuclease III homologues (NTH1) showed a very weak FDG activity, but the parameter analysis and NaBH4 trapping assays of the Schiff base intermediate revealed that NTH1 was kinetically incompetent for repair of fU. In the second approach, FDG was partially purified (160-fold) from rat liver. The enzyme was a monofunctional DNA glycosylase and recognized fU in single-stranded (ss) and double-stranded (ds) DNA. The most purified FDG fraction also exhibited monofunctional DNA glycosylase activities for uracil (U), 5-hydroxyuracil (hoU), and 5-hydroxymethyluracil (hmU) in ssDNA and dsDNA. The fU-excising activity of FDG was competitively inhibited by dsDNA containing U·G, hoU·G, and hmU·A but not by intact dsDNA containing T·A. Furthermore, the activities of FDG for fU, hmU, hoU, and U in ssDNA and dsDNA were neutralized by the antibody raised against SMUG1 uracil-DNA glycosylase, showing that FDG is a rat homologue of SMUG1.
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