Nitrite toxicosis in the ascorbic acid-deficient guinea pig

Nitrite Generates an Oxidant Stress and Increases Nitric Oxide in EA.hy926 Endothelial Cells

Nitrite is a breakdown product of nitric oxide that in turn is oxidized to nitrate in cells. In this work, we investigated whether reactive oxidant species mightbe generated during nitrite metabolism in cultured EA.hy926 endothelial cells. Nitrite was taken up by the cells in a time- and concentration-dependent manner and oxidized to nitrate, which accumulated in cells to concentrations almost 10-fold those of nitrite. Conversion of low millimolar concentrations of nitrite to nitrate was associated with increased oxidant stress in the cells. This manifested as increased oxidation of dihydrofluorescein in tandem with depletion of both GSH and ascorbate. Further, loading cells with ascorbate or treatment with desferrioxamine prevented nitrite-induced dihydrofluorescein oxidation. Nitrite within cells also increased the fluorescence of 4-amino-5-methylamino-2',7'-difluorofluorescein and inhibited the activity of cellular glyceraldehyde 3-phosphate dehydrogenase, which are markers of intracellular nitrosation reactions. Intracellular ascorbate partially prevented both of these effects of nitrite. Although ascorbate can reduce nitrite to nitric oxide at low pH, in endothelial cells loaded with ascorbate, its predominant effect at high nitrite concentrations is to prevent potentially damaging nitrosation reactions.

Methylene blue as an inhibitor of superoxide generation by xanthine oxidase. A potential new drug for the attenuation of ischemia/reperfusion injury

Tissue oxidases, especially xanthine oxidase, have been proposed as primary sources of toxic oxygen radicals in many experimental models of disease states. Among these, ischemia-reperfusion injury may be of the greatest clinical interest. In this paper we propose the use of methylene blue as a means of suppressing the production of superoxide radicals O2- by acting as an alternative electron acceptor for xanthine oxidase. Previous work has indicated that methylene blue accepts electrons from xanthine oxidase at the iron-sulfur center. Initial experiments in our laboratory demonstrated that (1) pairs of electrons from each enzymatic oxidation are transferred to methylene blue, (2) the reduction of methylene blue can be achieved by model iron-sulfur centers, similar to the iron-sulfur center of xanthine oxidase, (3) reduced methylene blue auto-oxidizes to produce H2O2 directly, rather than O2-, and (4) methylene blue is effective at non-toxic levels (2-5 mg/kg) in preventing free radical damage to liver and kidney tissues in an in vitro model of ischemia and reoxygenation. Accordingly, we propose that methylene blue may represent a new class of antioxidant drugs that competitively inhibit reduction of molecular oxygen to superoxide by acting as alternative electron acceptors for tissue oxidases. We have termed these agents "parasitic" electron acceptors.

The effect of ascorbic acid on nitrite-induced methemoglobin formation in rats, sheep, and normal human erythrocytes

Interspecies differences were demonstrated with respect to the occurrence of sodium nitrite-induced methemoglobin (METHB) in the erythrocytes of rats, dorset sheep, and normal humans, with the rats displaying approximately 25-33% of the sensitivity of sheep and human erythrocytes. Ascorbic acid incubation along with the nitrite was able to significantly reduce METHB formation in a dose-dependent manner in both rats and humans but not in sheep.