EVIDENCE FOR 2 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 cel l nuclear extract. One is OH8Gua-glycosylase and the other is OH8Gua-e ndonuclease that lacks OH8Gua-glycosylase activity. They were separate d by heparin-Sepharose column chromatography and characterized by endo nuclease nicking assay or by measuring the OH8Gua released from substr ate DNA using high pressure liquid chromatography-electrochemical dete ction. Both OH8Gua repair enzymes act only on the OH8Gua-containing st rand of the duplex substrate DNA containing OH8Gua/C, OH8Gua/T, or OH8 Gua/G. DNA containing OH8Gua/A base pair was very poor substrate for e ither enzymes. OH8Gua-endonuclease simultaneously cleaves phosphodiest er bonds on both sides of the OH8Gua residue, leaving 5'-hydroxy and 3 '-hydroxy groups.