Determination of Steady-state Levels of Oxidative DNA Base Modifications in Mammalian Cells by Means of Repair Endonucleases

The alkaline elution technique in combination with various repair endonucleases (Fpg protein, endonuclease III, exonuclease III, T4 endonuclease V) was used to quantify steady-state (background) levels of oxidative base modifications in various types of mammalian cells. In human lymphocytes the number of base modifications sensitive to Fpg protein, which include 8-hydroxyguanine, was 0.25 +/- 0.05 per 10(6) base pairs. Even lower levels (0.07 +/- 0.02 per 10(6) bp) were observed in HeLa cells. The numbers of sites sensitive to the other repair endonucleases were below the detection limit (0.05 per 10(6) bp). In a direct comparison, the background level of Fpg-sensitive modifications determined by alkaline elution was much lower than the background level of 8-hydroxydesoxyguanosine (8-oxodG) determined after enzymatic DNA hydrolysis by HPLC and electrochemical detection. However, the number of additional Fpg-sensitive modifications induced by a photosensitizer plus light was similar to the additional number of 8-oxodG residues determined by HPLC with electrochemical detection. This indicates that the enzyme assay does not systematically underestimate the number of lesions and points to an artefactual generation of 8-oxodG during DNA isolation and hydrolysis.