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Daniel R. McNeill, Avinash Narayana, Heng-Kuan Wong, and David M. Wilson III

Laboratory of Molecular Gerontology, Gerontology Research Center, National Institute on Aging, Intramural Research Program, National Institutes of Health, Department of Health and Human Services, Baltimore, Maryland, USA

Abstract
Many environmental metals are co-carcinogens, eliciting their effects via inhibition of DNA repair. Apurinic/apyrimidinic (AP) endonuclease 1 (Ape1) is the major mammalian abasic endonuclease and initiates repair of this cytotoxic/mutagenic lesion by incising the DNA backbone via a Mg²⁺-dependent reaction. In this study we examined the effects of arsenite [As(III) ], cadmium [Cd(II) ], cobalt [Co(II) ], iron [Fe(II) ], nickel [Ni(II) ], and lead [Pb(II) ] at concentrations ranging from 0.3 to 100 µM on the incision activity of Ape1 in the presence of 1 mM MgCl₂. Pb(II) and Fe(II) inhibited Ape1 activity at each of the concentrations tested, with an IC₅₀ (half-maximal inhibitory concentration) of 0.61 and 1.0 µM, respectively. Cd(II) also inhibited Ape1 activity but only at concentrations > 10 µM. No inhibition was seen with As(III) , Co(II) , or Ni(II) . A similar inhibition pattern was observed with the homologous Escherichia coli protein, exonuclease III, but no inhibition was seen with the structurally distinct AP endonuclease E. coli endonuclease IV, indicating a targeted effect of Pb(II) , Fe(II) , and Cd(II) on the Ape1-like repair enzymes. Excess nonspecific DNA did not abrogate the metal inactivation, suggesting a protein-specific effect. Notably, Cd(II) , Fe(II) , and Pb(II) [but not As(III) , Co(II) , or Ni(II) ] inhibited AP endonuclease activity in whole-cell extracts but had no significant effect on single nucleotide gap filling, 5´-flap endonuclease, and nick ligation activities, supporting the idea of selective inactivation of Ape1 in cells. Our results are the first to identify a potential DNA repair enzyme target for lead and suggest a means by which these prevalent environmental metals may elicit their deleterious effects. Key words: Ape1 AP endonuclease, base excision DNA repair, environmental heavy metal toxicity, lead, mutagenesis/carcinogenesis. Environ Health Perspect 112:799-804 (2004) . doi:10.1289/txg.7038 available via http://dx.doi.org/ [Online 13 April 2004]

Address correspondence to D. Wilson, Laboratory of Molecular Gerontology, GRC, National Institute on Aging, IRP, NIH, 5600 Nathan Shock Dr., Baltimore, MD 21224-6825. Telephone: (410) 558-8153. Fax: (410) 558-8157. E-mail: wilsonda@grc.nia.nih.gov

We thank E. Silbergeld (Johns Hopkins Bloomberg School of Public Health) , V. Bohr and R. Brosh (NIA) , P. Ellis (Pacific Northwest National Laboratory) , and J. Carney (University of Maryland School of Medicine) for critical reading and helpful discussion of this manuscript.

The authors declare they have no competing financial interests.

Received 17 February 2004 ; accepted 13 April 2004.