Substrate specificity of human endonuclease III (hNTH1). Effect of human APE1 on hNTH1 activity

Base excision repair of oxidized pyrimidines in human DNA is initiated by the DNA N-glycosylase/apurinic/apyrimidinic (AP) lyase, human NTH1 (hNTH1), the homolog of Escherichia coli endonuclease III (Nth). In contrast to Nth, the DNA N-glycosylase activity of hNTH1 is 7-fold greater than its AP lyase activity when the DNA substrate contains a thymine glycol (Tg) opposite adenine (Tg:A) (Marenstein, D. R., Ocampo, M. T. A., Chan, M. K., Altamirano, A., Basu, A. K., Boorstein, R. J., Cunningham, R. P., and Teebor, G. W. (2001) J. Biol. Chem. 276, 21242-21249). When Tg is opposite guanine (Tg:G), the two activities are of the same specific activity as the AP lyase activity of hNTH1 against Tg:A (Ocampo, M. T. A., Chaung, W., Marenstein, D. R., Chan, M. K., Altamirano, A., Basu, A. K., Boorstein, R. J., Cunningham, R. P., and Teebor, G. W. (2002) Mol. Cell. Biol. 22, 6111-6121). We demonstrate here that hNTH1 was inhibited by the product of its DNA N-glycosylase activity directed against Tg:G, the AP:G site. In contrast, hNTH1 was not as inhibited by the AP:A site arising from release of Tg from Tg:A. Addition of human APE1 (AP endonuclease-1) increased dissociation of hNTH1 from the DNA N-glycosylase-generated AP:A site, resulting in abrogation of AP lyase activity and an increase in turnover of the DNA N-glycosylase activity of hNTH1. Addition of APE1 did not abrogate hNTH1 AP lyase activity against Tg:G. The stimulatory protein YB-1 (Marenstein et al.), added to APE1, resulted in an additive increase in both activities of hNTH1 regardless of base pairing. Tg:A is formed by oxidative attack on thymine opposite adenine. Tg:G is formed by oxidative attack on 5-methylcytosine opposite guanine (Zuo, S., Boorstein, R. J., and Teebor, G. W. (1995) Nucleic Acids Res. 23, 3239-3243). It is possible that the in vitro substrate selectivity of mammalian NTH1 and the concomitant selective stimulation of activity by APE1 are indicative of selective repair of oxidative damage in different regions of the genome.