Enhancement of menadione cytotoxicity by bicarbonate: redox cycling and a possible role for the carbonate radical in quinone cytotoxicity.

摘要

We investigated the effect of bicarbonate on quinone redox cycling and cytotoxicity. A cell-free system utilized menadione and ascorbic acid to catalyze a redox cycle, and we utilized murine hepatoma (Hepa1c1c7) cells for in vitro experiments. Experiments were performed using low (2 mM) vs physiological (25 mM) bicarbonate levels in buffer equilibrated to physiological pH. We found that menadione redox cycling was enhanced by bicarbonate using oximetry and ascorbic acid oxidation. Furthermore, we treated Hepa1c1c7 cells with menadione and found that cytotoxicity and oxidative stress (dichlorofluorescin oxidation) was significantly increased with physiological bicarbonate-containing media. Interestingly, the inhibition of superoxide dismutase (SOD) showed a protective effect against menadione cytotoxicity. Using isolated BSA protein, we found a significant increase in protein carbonyls with menadione/ascorbate/SOD with physiological bicarbonate levels; low bicarbonate or SOD-omitted reactions produced less protein carbonyls. In conclusion, these findings suggest that the hydrogen peroxide generated by menadione redox cycling together with bicarbonate are substrates for SOD peroxidase activity that leads to carbonate radical which enhances cytotoxicity. These findings may represent an additional mechanism of quinone-induced toxicity.