It has been proposed that hyperhomocysteinemia may exert its pathogenic effects largely through metabolic accumulation of S-adenosylhomocysteine (SAH), a strong noncompetitive inhibitor of most methyltransferases. Here, we investigated the effects of SAH on H(2)O(2)-induced cellular DNA damage in comparison with the effects of homocysteine (Hcy) in a mouse endothelial cell line and a human intestinal cell line. Cells were preincubated for 2 h with H(2)O(2) (20 microM) followed by incubation with SAH or Hcy for 3 h. DNA strand breakage was determined using comet assay and DNA repair capacity determined using the same assay over time at 1, 2, and 3 h during SAH incubation. In both types of cells, SAH at 0.25-2 microM strongly and dose dependently enhanced H(2)O(2)-dependent DNA damage and inhibited DNA repair, whereas Hcy had a much weaker effect. SAH markedly increased uracil misincorporation, and this effect was also much stronger than that of Hcy. Taken together, our results show that SAH potentiates H(2)O(2)-induced DNA damage in cell cultures through impaired DNA repair capability and suggest that such effects are related to uracil misincorporation. Although the in vivo relevance of our findings is unclear, the biological significance of SAH-mediated detrimental effect, secondary to elevated intracellular Hcy, is an interesting area awaiting further exploration.