Differential protective effects of O‐phenanthroline and catalase on H 2 O 2 ‐induced DNA damage and inhibition of protein synthesis in endothelial cells

The respective roles of H 2 O 2 and ·OH radicals was assessed from the protective effects of catalase and the iron chelator o‐phenanthroline on (1) the inhibition of protein synthesis, and (2) DNA damage and the related events (activation of the DNA repairing enzyme poly(ADP) ribose polymerase with the associated depletion of NAD and ATP stores) in cultured endothelial cells exposed to the enzyme reaction hypoxanthine‐xanthine oxidase (HX‐XO) or pure H 2 O 2 . Catalase added in the extracellular phase completely prevented all of these oxidant‐induced changes. O‐phenanthroline afforded a complete protective effect against DNA strand breakage and the associated activation of the enzyme poly(ADP) ribose polymerase. By contrast, iron chelation was only partially effective in maintaining the cellular NAD and ATP contents, as well as the protein synthetic activity. In addition, the ATP depletion following oxidant injury was much more profound than NAD depletion. These results indicate that: (1) ·OH radical was most likely the ultimate O 2 species responsible for DNA damage and activation of poly‐(ADP) ribose polymerase; (2) both H 2 O 2 and ·OH radicals were involved in the other cytotoxic effects (inhibition of protein synthesis and reduction of NAD and ATP stores); and (3) NAD and ATP depletion did not result solely from activation of poly(ADP) ribose polymerase, but other mechanisms are likely to be involved. These observations are also compatible with the existence of a compartmentalized intracellular iron pool.