Evidence for two DNA repair enzymes for 8-hydroxyguanine (7,8-dihydro-8- oxoguanine) in human cells

Two DNA repair enzymes for 8-hydroxyguanine (also known as 7,8-dihydro-8- oxoguanine; OH⁸Gua, oxo⁸Gua) have been identified in human HeLa cell nuclear extract. One is OH⁸Gua-glycosylase and the other is OH⁸Gua- endonuclease that lacks OH⁸Gua-glycosylase activity. They were separated by heparin-Sepharose column chromatography and characterized by endonuclease nicking assay or by measuring the OH⁸Gua released from substrate DNA using high pressure liquid chromatography-electrochemical detection. Both OH⁸Gua repair enzymes act only on the OH⁸Gua-containing strand of the duplex substrate DNA containing OH⁸Gua/C, OH⁸Gua/T, or OH⁸Gua/G. DNA containing OH⁸Gua/A base pair was very poor substrate for either enzymes. OH⁸Gua- endonuclease simultaneously cleaves phosphodiester bonds on both sides of the OH⁸Gua residue, leaving 5'-hydroxy and 3'-hydroxy groups.