Ascorbate mediates the non-enzymatic reduction of nitrite to nitric oxide

Nitric oxide (NO^•) generated by nitric oxide synthases is involved in many physiological and pathophysiological processes. However, non-enzymatic formation of NO^• also occurs in vivo. Here we investigated the production of NO^• from nitrite, as facilitated by ascorbate, over the pH range of 2.4-7.4. Using a nitric oxide electrode, we observed at low pH a rapid generation of NO^• from nitrite and ascorbate that slows with increasing pH. The formation of NO^• was confirmed by its reaction with oxyhemoglobin. In the ascorbate/nitrite system a steady-state level of NO^• was achieved, suggesting that a futile redox cycle of nitrite-reduction by ascorbate and NO^•-oxidation by dioxygen was established. However, at pH-values of around 7 and greater, the direct reduction of nitrite by ascorbate is very slow; thus, this route to the non-enzymatic production of NO^• is not likely to be significant process in vivo in environments having a pH around 7.4. The production of nitric oxide by nitrite and ascorbate would be important only in areas of lower pH, e.g. stomach/digestive system, sites of inflammation, and areas of hypoxia such as tumor tissue. In patients receiving very large doses of ascorbate delivered by intravenous infusion, plasma levels of ascorbate on the order of 20 - 30 mM can be achieved. After infusion, levels of nitrate and nitrite in plasma were unchanged. Thus, in blood and tissue that maintain a pH of about 7.4, the reduction of nitrite to nitric oxide by ascorbate appears to be insignificant, even at very large, pharmacological levels of ascorbate.