Effects of lethal exposure to hyperoxia and to hydrogen peroxide on NAD(H) and ATP pools in Chinese hamster ovary cells

Cell death by oxidative stress has been proposed to be based on suicidal NAD depletion, typically followed by ATP depletion, caused by the NAD-consuming enzyme poly(ADP)ribose polymerase, which becomes activated by the presence of excessive DNA-strand breaks. In this study NAD⁺, NADH and ATP levels as well as DNA-strand breaks (assayed by alkaline elution) were determined in Chinese hamster ovary (CHO) cells treated with either H₂O₂ or hyperoxia to a level of more than 80% clonogenic cell killing. With H₂O₂ extensive DNA damage and NAD depletion were observed, while at a higher H₂O₂ dosage ATP also became depleted. In agreement with results of others, the poly(ADP)ribose polymerase inhibitor 3-aminobenzamide completely prevented NAD depletion. However, both H₂O₂-induced ATP depletion and cell killing were unaffected by the inhibitor, suggesting that ATP depletion may be a more critical factor than NAD depletion in H₂O₂-induced killing of CHO cells. With hyperoxia, only moderate DNA damage (2 × background) and no NAD depletion were observed, whereas ATP became largely (70%) depleted. We conclude that (1) there is no direct relation between ATP and NAD depletion in CHO cells subjected to toxic doses of H₂O₂ or hyperoxia; (2) H₂O₂-induced NAD depletion is not by itself sufficient to kill CHO cells; (3) killing of CHO cells by hyperoxia is not due to NAD depletion, but may be due to depletion of ATP.