NITRIC-OXIDE INHIBITS DNA-LIGASE ACTIVITY - POTENTIAL MECHANISMS FOR NO-MEDIATED DNA-DAMAGE

Nitric oxide-induced modifications of DNA occur either by directly alt ering DNA chemically through reactive nitrogen oxide species (RNOS) or indirectly by inhibiting various repair processes, DNA ligases are en zymes which rejoin single-strand breaks and are critical for DNA integ rity during processes such as gene transcription and repair, The eukar yotic and T4 DNA ligases are active in the presence of ATP and act in two steps: the formation of protein-AMP intermediates, then the ligati on of DNA breaks, When T4 DNA ligase was exposed to the NO generator D EA/NO (Et(2)N[NO(NO)]Na), a concentration- and time-dependent inhibiti on of these two steps, adenylylation of the protein and ligation of th e substrate, was observed, This inhibition was abated by the presence of cysteine, suggesting that RNOS, rather than NO, mediated the inhibi tion of the ligase activity, As mammalian and T4 DNA ligases act by th e same mechanism, the inhibition of DNA ligase may explain the increas e in single-strand breaks reported for cells exposed to NO and provide s a mechanism to increase DNA lesions without direct chemical modifica tion of DNA by NO or RNOS.