Heavy metal ions affect the activity of DNA glycosylases of the Fpg family

Prokaryotic enzymes formamidopyrimidine-DNA glycosylase (Fpg) and endonuclease VIII (Nei) and their eukaryotic homologs NEIL1, NEIL2, and NEIL3 define the Fpg family of DNA glycosylases, which initiate the process of repair of oxidized DNA bases. The repair of oxidative DNA lesions is known to be impaired in vivo in the presence of ions of some heavy metals. We have studied the effect of salts of several alkaline earth and transition metals on the activity of Fpg-family DNA glycosylases in the reaction of excision of 5,6-dihydrouracil, a typical DNA oxidation product. The reaction catalyzed by NEIL1 was characterized by values K (m) = 150 nM and k (cat) = 1.2 min(-1), which were in the range of these constants for excision of other damaged bases by this enzyme. NEIL1 was inhibited by Al3+, Ni2+, Co2+, Cd2+, Cu2+, Zn2+, and Fe2+ in Tris-HCl buffer and by Cd2+, Zn2+, Cu2+, and Fe2+ in potassium phosphate buffer. Fpg and Nei, the prokaryotic homologs of NEIL1, were inhibited by the same metal ions as NEIL1. The values of I-50 for NEIL1 inhibition were 7 A mu M for Cd2+, 16 A mu M for Zn2+, and 400 A mu M for Cu2+. The inhibition of NEIL1 by Cd2+, Zn2+, and Cu2+ was at least partly due to the formation of metal-DNA complexes. In the case of Cd2+ and Cu2+, which preferentially bind to DNA bases rather than phosphates, the presence of metal ions caused the enzyme to lose the ability for preferential binding to damaged DNA. Therefore, the inhibition of NEIL1 activity in removal of oxidative lesions by heavy metal ions may be a reason for their comutagenicity under oxidative stress.