The biological effects of NO • are mediated by various actions, either by NO • itself or by secondary RNS, and the overall biochemistry of NO • is deceptively complex. Moreover, the metabolism and chemistry of NO • depend importantly on local concentrations and pH; the recently described acidification of the airway surface in asthmatics [33] might significantly affect NO • metabolism in these patients. It is well known that interactions with the ion centers of iron or other transition metals are responsible for many of the signaling properties of NO • ; the activation of the heme enzyme guanylyl cyclase and the consequent formation of cGMP is involved not only in smooth-muscle relaxation but also in the activation of certain transcription factors, the expression of several pro-inflammatory and anti-inflammatory genes (including cytokines and cyclo-oxygenase), and the production of respiratory mucus [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] . In addition to such direct signaling properties, many actions of NO • might be due largely to secondary RNS that can react with multiple additional targets, in some cases forming nitroso or nitro adducts as potentially unique NO • -mediated signaling mechanisms. As discussed, the formation of protein nitrotyrosine has been postulated as a potential RNSspecific signaling pathway. Even more interest has been given to the reversible S-nitros(yl)ation of protein cysteine residues, which has been proposed to affect a number of redox-sensitive signaling pathways, for example by the activation of p21 ras or the inhibition of protein tyrosine phosphatases [35, 36] . Similar modifications of reactive cysteine residues in transcription factors such as nuclear factor-κB or of caspases contribute to the regulation of gene expression and apoptosis [37] [38] [39] . The precise mechanisms leading to protein S-nitrosylation in vivo are still not clarified, but might involve dinitrogen trioxide (formed during the autoxidation of NO • ), iron-nitrosyl complexes, and perhaps ONOO - [16] ; changes in NO • metabolism during inflammatory lung diseases undoubtedly affect such NO • -dependent signaling pathways. In addition, S-nitrosylation can be reversed by either enzymic (thioredoxin or glutaredoxin) or chemical (metals or oxidants) mechanisms, and evidence is increasing that this reversible modification is complementary to more widely accepted oxidant-dependent redox signaling pathways [40] . The reported alterations in S-nitrosothiol levels in tracheal secretions of patients with asthma or cystic fibrosis further point to altered NO • metabolism in these cases, and might provide new clues to the role of S-nitrosylation in controlling such disease processes [41, 42] . Unfortunately, technical limitations to detect S-nitrosylation in specific protein targets in vivo have limited a full understanding of this potential signaling pathway; further research in these areas can be expected to establish more clearly its significance in the pathophysiological properties of NO • .