Uranium is a radioactive heavy metal with isotopes that decay on the geological time scale. People are exposed to uranium through uranium mining, processing, the resulting mine tailings, and the use of depleted uranium in the military. Acute exposures to uranium are chemically toxic to the kidney; however, little is known about chronic exposures, for example, if there is a direct chemical genotoxicity of uranium. The hypothesis that is being tested in the current work is that hexavalent uranium, as uranyl ion, may have a chemical genotoxicity similar to that of hexavalent chromium. In the current study, reactions of uranyl acetate (UA) and ascorbate (vitamin C) were observed to produce plasmid relaxation in pBluescript DNA. DNA strand breaks increased with increasing concentrations of a 1:1 reaction of UA and ascorbate but were not affected by increasing the ratio of ascorbate. Plasmid relaxation was inhibited by coincubation of reactions with catalase but not by coincubation with the radical scavengers mannitol, sodium azide, or 5,5-dimethyl-1-pyrroline-N-oxide. Reactions of UA and ascorbate monitored by (1)H NMR spectroscopy showed formation of a uranyl ascorbate complex, with no evidence of a dehydroascorbate product. A previous study inferred that hydroxyl radical formation was responsible for oxidative DNA damage in the presence of reactions of uranyl ion, hydrogen peroxide, and ascorbate [Miller et al. (2002) J. Bioinorg. Chem. 91, 246-252]. Current results, in the absence of added hydrogen peroxide, were not completely consistent with the interpretation that strand breaks were produced by a Fenton type generation of reactive oxygen species. Data were also consistent with the interpretation that a uranyl ascorbate complex was catalyzing hydrolysis of the DNA-phosphate backbone, in a manner similar to that known for the lanthanides. These data suggest that uranium may be directly genotoxic and may, like chromium, react with DNA by more than one pathway.