Damage to DNA by cadmium or nickel in the presence of ascorbate

The interactive effects of the anti-oxidant ascorbate (Asc) and the metals cadmium (Cd, as CdCl2) or nickel (Ni, as NiCl2) on the in vitro formation of breaks in double-stranded deoxyribonucleic acid (d/s DNA) were determined. Concentrations of 50 microM Cd or 200 microM Ni were dosed for 4 hours in factorial combinations with 500 microM Asc in RPMI 1640 medium (7 percent bovine serum) in which AHH-1 TK+/- cells (a spontaneously transformed human B lymphoblastoid cell line by Gentest Corp.) were replicating. In combination with Asc, Cd caused significant d/s DNA breaks (p < 0.01, n = 5), while Cd in the absence of Asc produced only a slight (but not significantly different) amount of d/s DNA damage when compared to the cells with no Cd added. The Asc alone was not damaging. The Cd caused damage to the d/s DNA only when Asc was present. The percent of d/s DNA remaining following the respective treatments was: +Cd+Asc, 13 +/- 3; +Cd-Asc, 46 +/- 8; -Cd+Asc, 54 +/- 5; -Cd-Asc, 55 +/- 7. Conversely, the presence of Ni resulted in increased amounts (percent) of d/s DNA compared to control values: +Ni+Asc, 63 +/- 5; +Ni-Asc, 58 +/- 5; -Ni+Asc, 52 +/- 1; -Ni-Asc, 51 +/- 4, (p < 0.05, n = 3). The contrasting results between Cd and Ni in the presence of Asc may reside in the point of action; while Cd acts directly on DNA, Ni is reported to act on heterochromatin. Although Asc is a recognized anti-oxidant, its presence in the media mixture potentiated d/s DNA damage from the Cd. This may be caused by a Fenton-type reaction in which an antioxidant in the presence of metal generates hydroxyl radicals and consequently d/s DNA breaks. Oxidative reactions between metals, oxygen, and antioxidants such as Asc may represent an important mechanism of cell death, toxicity, and transformation.