NO may involve metal-independent interactions with other cellular proteins. NO reacts with thiols (compounds containing the –SH group) to form nitrosothiols. In proteins, the thiol moiety is found in the amino acid cysteine. This posttranslational modification, termed S-nitrosylation or S-nitrosation, requires either metals or O2 to catalyze the formation of the nitrosothiol adduct. S-nitrosylation is highly specific, with only certain cysteine residues in proteins becoming S-nitrosylated. S-nitrosylation can alter the function, stability, or localization of target proteins. Major targets of S-nitrosylation include H-ras, a regulator of cell proliferation that is activated by S-nitrosylation, and the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase, which is inhibited when it is S-nitrosylated. Denitrosylation of proteins is poorly understood but may involve enzymes, such as thioredoxin, or chemical reduction by intracellular reducing agents such as glutathione, an abundant intracellular sulfhydryl-containing compound. Glutathione can also be S-nitrosylated under physiologic conditions to generate S-nitrosoglutathione. S-nitrosoglutathione may serve as an endogenous stabilized form of NO or as a carrier of NO. Vascular glutathione is decreased in diabetes mellitus and atherosclerosis, and the resulting deficiency of S-nitrosoglutathione may account for the increased incidence of cardiovascular complications in these conditions.
3. Tyrosine Nitration—Additional reactions of NO are seen, especially in immune system cells producing large amounts of NO. NO undergoes both oxidative and reductive reactions, which allow it to nitrosylate thiols and add nitrate to tyrosines (described below) or lead to stable oxidation products (Table 19–2). NO reacts very efficiently with superoxide to form peroxynitrite (ONOO–), a highly reactive oxidant that leads to DNA damage, nitration of tyrosine, and oxidation of cysteine to disulfides or to various sulfur oxides (SOx). Several cellular enzymes synthesize superoxide, and these enzymes are elevated in concert with NO synthase in numerous inflammatory and degenerative diseases, resulting in an increase in peroxynitrite levels. Numerous proteins
TABLE 19–2 Oxides of nitrogen.
are susceptible to peroxynitrite-catalyzed tyrosine nitration, and this irreversible modification can be associated with either activation or inhibition of protein function. Detection of tyrosine nitration in tissue is often used as a marker of excessive NO production, although a direct causal role of tyrosine nitration in the pathogenesis of any disease has not been definitively established. Peroxynitrite-mediated protein modification is mitigated by intracellular levels of glutathione, which can protect against tissue damage by scavenging peroxynitrite. Factors that regulate the biosynthesis and decomposition of glutathione may be important modulators of the toxicity of NO.
NO is highly labile due to its rapid reaction with metals, O2, and reactive oxygen species. NO can react with heme and hemoproteins, including oxyhemoglobin, which oxidizes NO to nitrate. The reaction of NO with hemoglobin may also lead to S-nitrosylation of hemoglobin, resulting in transport of NO throughout the vasculature. NO is also inactivated by reaction with O2 to form nitrogen dioxide. As noted, NO reacts with superoxide, which results in the formation of the highly reactive oxidizing species, peroxynitrite. Scavengers of superoxide anion such as superoxide dismutase may protect NO, enhancing its potency and prolonging its duration of action.
Inhibitors of Nitric Oxide Synthesis