Non-enzymatic Fast Repair Of DNA Oxidative Damage May Exist In Cells

Many studies showed that natural polyphenols can rapidly repair DNA oxidative damage in chemical system. Here we reported that the non-enzymatic fast repair activity of two natural polyphenols may also exist in cellular system. Treated with H₂O₂, the viability of Chinese hamster ovary AA8 cell line (AA8) with highly expressed XRCC1 gene (a DNA repairing protein) is significantly higher than that of normal Chinese hamster ovary cell line (CHO). The DNA oxidative damage is believed that it is repaired mainly by the base excision repair (BER) mechanism with many repairing enzymes. In the present study, after blockage of the main enzymatic repair system by different inhibitors, methoxyamine (MX), 3-aminobenzamide (3AB), or nicotinamide (NIC), DNA oxidative damage caused by H₂O₂ showed a 2-5 fold increase in both cell lines. However, natural polyphenols, rosmarinic acid (RA) or verbascoside (VER) significantly reduce the DNA oxidative damage. To confirm that the decrease of DNA oxidative damage by RA or VER is not due to their scavenging activity for reactive oxygen species (ROS), we washed out RA/VER outside of the cells before H₂O₂ added, so that the concentration of polyphenol inside the cells is kept at low level. In our group, based on the reaction rate constants and the concentrations of polyphenol, DNA and H₂O₂, the competition probability was calculated. According to the competition probability, polyphenol should repair DNA oxidative damage at first rather than scavenge ROS. Actually, the present results showed that the cellular ROS are kept at a high level throughout the whole experimental process. Therefore the decrease of DNA damage is due to non-enzymatic fast repair mechanism. Previous study showed that H₂O₂ initiates a chain of events in cells, involving mtDNA damage, drop in mitochondrial membrane potential (MMP) and lose of repair activity. The present result showed that H₂O₂ induced drop in MMP, moreover BER inhibitors decreased MMP further, and RA and VER were able to attenuate the drop so that maintaining the repairability. Taken together, we suggest that the non-enzymatic fast repair mechanism not only exists in chemical system but may also in cells.