EPIGALLOCATECHIN GALLATE MARKEDLY ENHANCES FORMATION OF 8-OXO-7,8-DIHYDRO-2′-DEOXYGUANOSINE IN THE REACTION OF 2′-DEOXYGUANOSINE WITH HYPOCHLOROUS ACID

A tea polyphenol, (−)-epigallocatechin gallate (EGCG), which can scavenge a variety of reactive oxygen species, enhances the yield of 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG) up to 20-fold in the reaction of 2′-deoxyguanosine with hypochlorous acid (HOCl), compared with the reaction without EGCG. Certain concentrations of EGCG inhibited HOCl-mediated oxidation of 2′-deoxyguanosine to 8-oxo-dG to a limited extent, but efficiently inhibited further oxidation of 8-oxo-dG to spiroiminodihydantoin nucleoside, resulting in the accumulation of 8-oxo-dG in the reaction mixture. Conversely, EGCG inhibited dose-dependently an increase in 8-oxo-dG levels in calf thymus DNA incubated with HOCl. However, addition of HOCl to the DNA preoxidized with an oxidant-generating system (CuCl2, ascorbate, H2O2), led to the extensive loss of 8-oxo-dG due to its further oxidation. EGCG effectively inhibited this HOCl-mediated loss of 8-oxo-dG in the oxidized DNA, resulting in an apparent increase in 8-oxo-dG levels in the oxidized DNA, compared with the levels found without EGCG. The conversion of 8-oxo-dG into other oxidized lesions will inevitably affect recognition by DNA repair enzymes as well as the rates of mutations and DNA synthesis. Thus, our results suggest that as a biomarker of oxidative DNA damage, not only 8-oxo-dG but also the products of its further oxidation should be analyzed.