Mutagenicity of nitric oxide-releasing compounds in Escherichia coli: effect of superoxide generation and evidence for two mutagenic mechanisms.

The mutagenicity of three nitric oxide (NO) donors, 3-morpholinosydnonimine (SIN-1), a compound generating the precursors of peroxynitrite NO and superoxide, diethylamine/NO (DEA/NO) and spermine/NO (SPER/NO), both releasing authentic NO was analyzed using Escherichia coli tester strains IC203, carrying a deletion of the oxyR gene, and its oxyR(+) parent IC188 (the alternative name of WP2 uvrA/pKM101). The OxyR protein is a redox-sensitive transcriptional activator of genes encoding antioxidant enzymes. Strains IC203 and IC188 contain error-prone DNA polymerases polV, encoded by the chromosomal umuDC genes, and polRI, encoded by mucAB genes carried by pKM101. SIN-1 was determined to be an oxidative mutagen giving a positive response only in IC203, whereas DEA/NO and SPER/NO induced similar positive responses in IC203 and IC188 and were considered as non-oxidative mutagens. The spectrum of ochre suppressors in Trp(+) revertants induced by SIN-1 in IC203 was characterized by a higher number of TA-->AT transversions and GC-->AT transitions, and a lower number of GC-->TA transversions, with respect to the untreated control. The mutagenicity of SIN-1 in IC203, probably induced by peroxynitrite through reactive derivatives, was enhanced in the presence of plumbagin (PLB), a superoxide generator. Superoxide generation by PLB, as well as formation of peroxynitrite in cells treated with SIN-1, evaluated by monitoring the oxidation, respectively, of dihydroethidium and dihydrorhodamine 123, were greater in IC203 than in IC188. Formation of peroxynitrite in IC203 treated with SIN-1 was stimulated by PLB. After treatment with DEA/NO and SPER/NO the number of revertants scored in IC188 was higher than in strains IC187, containing only polV, and IC204, deficient in both polV and polRI. For these compounds, induced suppressor revertants in IC187 and IC204 were almost exclusively GC-->AT transitions, whereas in IC188 significant levels of GC-->TA and TA-->AT transversions were also induced. Mutagenesis by both DEA/NO and SPER/NO was partially inhibited in the presence of PLB. The results show the usefulness of the new tester strain IC203 to differentiate NO-promoted mutagenic mechanisms that involve or do not involve oxygen radicals.