Nitric oxide inhibits DNA ligase activity: potential mechanisms for NO-mediated DNA damage

Nitric oxide-induced modifications of DNA occur either by directly altering DNA chemically through reactive nitrogen oxide species (RNOS) or indirectly by inhibiting various repair processes. DNA ligases are enzymes which rejoin single-strand breaks and are critical for DNA integrity during processes such as gene transcription and repair. The eukaryotic and T4 DNA ligases are active in the presence of ATP and act in two steps: the formation of protein-AMP intermediates, then the ligation of DNA breaks. When T4 DNA ligase was exposed to the NO generator DEA/NO (Et2N[NO(NO)]Na), a concentration- and time-dependent inhibition of these two steps, adenylylation of the protein and ligation of the substrate, was observed. This inhibition was abated by the presence of cysteine, suggesting that RNOS, rather than NO, mediated the inhibition of the ligase activity. As mammalian and T4 DNA ligases act by the same mechanism, the inhibition of DNA ligase may explain the increase in single-strand breaks reported for cells exposed to NO and provides a mechanism to increase DNA lesions without direct chemical modification of DNA by NO or RNOS.