Nitric oxide nitrosothiol production

The reaction in water at pH 7.4 has been much studied since the discovery of the importance of nitric oxide. The products are as for the thermal and photochemical reactions, except that the final product is nitrite ion. This is to be expected since nitric oxide in aerated water at pH 7.4 also yields quantitatively nitrite ion25, by it is believed the series of equations 7-9, which involves oxidation to nitrogen dioxide, further reaction to give dinitrogen trioxide which, in mildly alkaline solution, is hydrolysed to nitrite ion. Under anaerobic conditions it is possible to detect nitric oxide directly from the decomposition of nitrosothiols using a NO-probe electrode system26. Solutions of nitrosothiols both in... 

Possibly, cellular thiols may be oxidized by the inactive adduct of nitric oxide and oxygen to regenerate a nitrosothiol or related species with EDRF activity. Some of the inconsistent results observed in bioassay systems may be due to the secondary and nonenzymatic formation of a nitrosothiols or other species capable of regenerating nitric oxide, which are leached into perfusion cascades. Consequently, bioassay systems should not be the gold standard to distinguish whether nitric oxide is the EDRF, because secondary reactions of nitric oxide decomposition products may regenerate nitric oxide. 

The capacity of furoxan derivatives to behave as NO-donors was first demonstrated by Feelisch et al. [19], who showed that furoxan derivatives produce nitric oxide when dissolved in physiological solution in the presence of thiols. Among the reaction products, they isolated nitrite and, in lesser amounts, nitrate, which are the final oxidation products of nitric oxide in aerobic water solution, as well as dioxime derivatives, which are the reduction products of the furoxans. They also evidenced a marked p H -dependent production of S-nitrosothiols. Working with N, AF-diisopropylfuroxan-3,4-dicarboxamide (29, Ipramidil) and an excess of glutathione (GSH), the amount of S-nitrosoglutathione formed increased with increasing pH until pH 9, above which it... 

Nitrosothiols decompose photochemically and thermally to give the corresponding disulphides and nitric oxide18 23 14 24 (equation 6). In most cases the nitric oxide has not been identified as the primary product but rather as its oxidized form, nitrogen dioxide. 

The spontaneous reaction of nitric oxide with thiols is slow at physiological pH and the final product under anaerobic conditions is not a nitrosothiol (Pryor et al., 1982). The reaction is slow because it involves the conjugate base of the thiol (R—S"). At pH 7.0, the oxidation of cysteine by nitric oxide required 6 hr to reach completion and yields RSSR and N 2O as the products. The synthetic preparation of nitrosothiols usually involves the addition of nitrosonium ion from acidified nitrite to the thiol, or oxidation of the thiol with nitrogen dioxide under anaerobic conditions in organic solvents. Nitric oxide will form nitrosothiols by reaction with ferric heme groups, such as found in metmyoglobin or methemoglobin (Wade and Castro, 1990). It is also possible that nitrosyldioxyl radical also reacts with thiols to form a nitrosothiol. 

Normal endothelial cells modulate the effects of homocysteine by facilitating the S-nitrosilation of homocysteine by nitric oxide. The formed S-nitrosothiol adduct is a potent vasorelaxing substance [2,108]. So, when high levels of homocysteine occur, they may overcome or impair the endothelial capacity for NO synthesis. Endothelial cell damage may result from increased production of reactive oxygen species or from impaired production of nitric oxide [3,102]. In endothelial cells, total homocysteine reduces the levels of tetra-hydrobiopterin (BH4), relative to dihydrobiopterin (BH ), thereby creating a dysfunctional eNOS causing a reduced amount of nitric oxide [1,101],... 

Although the overall yield is small, 4-nitrosophenol is a significant product from addition of ONOO" alone (Fig. 3). ONOO"-mediated formation of nitroso adducts in biological systems, particularly nitrosothiols, likely accounts for the ability of ONOO" to produce nitric oxide-like effects such as vascular relaxation (Wu et al., 1994 Liu et ah, 1994). The yield of 4-nitrosophenol from ONOO" is not significantly affected by lowering the pH to 6.0 but does increase at pH 8.0 (Fig. 3). At pH 7.4 the yield of nitrosophenol increased severalfold when ONOO" and nitric oxide were added simultaneously. The yield of nitrosophenol from ONOO plus nitric oxide was essentially no different from that of nitric oxide alone at pH 6.0 or pH 8.0. 

Marzinzig M, Nussler A K, Stadler J, et al. (1997). Improved methods to measure end products of nitric oxide in biological fluids nitrite, nitrate and S-nitrosothiols. Nitric Oxide Biol. Chem. 1 177-189. 

However, the in vivo sources of nitroxyl production remain uncertain. Some authors suggested that nitroxyl anion might be generated by NO synthases [81,82] or during the decomposition of nitrosothiols [83]. It has also been proposed [81] that the primary product of NO synthase is not nitric acid but nitroxyl anion, which is next oxidized by SOD to NO ... 

---