|[Frontiers in Bioscience 2, d189-196, May 1, 1997]|
INTERACTIONS BETWEEN SUPEROXIDE AND NITRIC OXIDE: IMPLICATIONS IN DNA DAMAGE AND MUTAGENESIS|
David Jourd'heuil, David Kang and Matthew B. Grisham
Department of Molecular and Cellular Physiology, Louisiana State University Medical Center, Shreveport, LA 71130, USA
Received 4/3/97; Accepted 4/7/97; On-line 5/1/97
In general, O2- per se is not thought to be highly toxic to cells and tissues since it is a better reducing agent than an oxidant (37). However, O2- will rapidly dismutate to form H2O2. Although H2O2 is an oxidizing agent, most of the H2O2-dependent oxidizing activity is mediated by secondary radicals such as OH. generated from metal (Mn)-catalyzed reactions. It is known that O2- and H2O2 interact with chelates of iron or copper to yield the potent oxidant OH. or OH.-like species via the superoxide-driven Fenton reaction:
O2- + Mn+2 --> O2 + Mn+1
H2O2 + Mn+1 --> OH. + OH- + Mn+2
The interaction of free radicals derived from superoxide-driven Fenton reactions with DNA has received considerable interest (38). Recent reports have focused on the ability of copper to participate in mutagenic reactions in vivo via Fenton-catalyzed reactions. Copper is an important structural metal in chromatin (39) that in fact induces more DNA bases damage in the presence of H2O2 than does iron (40).
There is now evidence to suggest that NO may modulate Fenton-driven oxidative reactions. We have recently investigated the ability of different fluxes of nitric oxide to modulate iron complex (10) and hemoprotein-catalyzed oxidative reactions (41). We found that generation of O2- and H2O2 in the presence of 5 µM Fe+3-EDTA stimulated oxidation of DHR producing approximately 15 µM rhodamine (10). Catalase and superoxide dismutase were both effective at inhibiting this classic Fenton-driven reaction. Addition of NO increased DHR oxidation such that a 2:1 NO/ O2- ratio increased DHR oxidation by 30%. As the ratio NO/ O2- further increased to 4.5, DHR oxidation was reduced by 40%. Kanner et al. (42) proposed that NO may inhibit iron-mediated oxidative reactions by forming nitrosyl complexes with ferrous iron:
Fe+3-EDTA + O2- --> Fe+2-EDTA + O2
Fe+2-EDTA + NO --> NO-Fe+2-EDTA
NO-Fe+2-EDTA+ H2O2 --> Fe+3-EDTA + HNO2 + OH-
The efficiency of such interaction may explain the decrease in DHR oxidation obtained in the presence of high fluxes of NO. We have also assessed the ability of myoglobin to oxidize DHR in the presence or the absence of O2-, H2O2 and /or NO (33). In the presence of equimolar fluxes of H2O2 and O2-, the addition of myoglobin dramatically enhanced DHR oxidation via the formation of ferryl (Fe+4) myoglobin. This oxidative reaction was as expected inhibited by catalase but not Superoxide dismutase. Addition of NO to this system further enhanced DHR oxidation which was inhibited by superosxide dismutase suggesting that O2- reacted with NO to form a potent oxidant such as peroxynitrite in addition to the H2O2-dependent formation of ferryl myoglobin. Further increases in NO flux dramatically inhibited DHR oxidation which was found to be due to the NO-mediated reduction of ferryl heme to the met form. Taken together, these data suggest that NO may enhance or inhibit iron complex and hemoprotein-catalyzed oxidative reactions depending upon the relative fluxes of O2-, H2O2 and NO. In accordance with our results, Pacelli et al. have shown that NO can inhibit DNA strand breaks induced by H2O2 and certain transition metals (43).