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.