NEW SUBSTRATES FOR OLD ENZYMES - 5-HYDROXY-2'-DEOXYCYTIDINE AND 5-HYDROXY-2'-DEOXYURIDINE ARE SUBSTRATES FOR ESCHERICHIA-COLI ENDONUCLEASE-III AND FORMAMIDOPYRIMIDINE DNA N-GLYCOSYLASE, WHILE 5-HYDROXY-2'-DEOXYURIDINE IS A SUBSTRATE FOR URACIL DNA N-GLYCOSYLASE

5-Hydroxy-2'-deoxycytidine (5-OHdC) and 5-hydroxy-2'-deoxyuridine (5-O HdU) are major products of oxidative DNA damage with mutagenic potenti al. Until now, no enzymatic activity responsible for their removal has been identified. We report here that both 5-OHdC and 5-OHdU are subst rates for Escherichia coli endonuclease III and formamidopyrimidine DN A N-glycosylase (FPG). 5-OHdU is also a substrate for uracil DNA N-gly cosylase. Consistent with their mechanisms of action on previously des cribed substrates, endonuclease III removes 5-OHdC and 5-OHdU via a N- glycosylase/beta-elimination reaction, FPG follows a N-glycosylase/bet a,delta-elimination reaction, and uracil N-glycosylase removes 5-OHdU by N-glycosylase action leaving behind an abasic site. Endonuclease II I removes both lesions more efficiently than FPG, and both endonucleas e III and FPG remove 5-OHdC slightly more efficiently than 5-OHdU. Ura cil DNA N-glycosylase removes 5-OHdU more efficiently than the other t wo enzymes and has no activity on 5-OHdC even when present in great ex cess. Analysis of crude extracts obtained from wild type and endonucle ase III deletion mutants of E. coli correlated well with data obtained with the purified enzymes.