| Pulmonary nitric oxide in preterm and term infants with respiratory failure | ||
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In biological systems, NO serves as an important intra- and intercellular messenger (42). As a free radical, it is oxidized, reduced or complexed with other biomolecules, depending on the microenvironment (43).
Nitric oxide is an uncharged molecule composed of seven electrons from nitrogen and eight electrons from oxygen (44). This combination results in the presence of an unpaired electron, which makes NO paramagnetic and a radical (41). The majority of biological molecules contain bonds filled with two electrons. NO only reacts rapidly with the select range of molecules that have unpaired electrons in their outer orbital. They are typically other free radicals or transition metals, such as haem iron (44).
Nitrogen oxides of biologic relevance include elemental nitrogen in five oxidation states (NOx: N2O, NO., NO2-, NO2., NO3-) (43). NO is one of the biologically active nitrogen oxides. Therefore, NO does not remain as NO. -radical moiety in biological environment. In aqueous systems and at air-liquid interfaces, NO. -generation yields nitrite (NO2-) and nitrate (NO3-) as end products (45). NO generates a chemiluminescent product upon reaction with ozone (41). The NO. -radical reacts rapidly with the superoxide radical, forming highly reactive peroxynitrite anion (ONOO-) (46). The broader chemistry of NO involves an array of interrelated redox forms implicated in the biochemistry of dioxygen: nitrosonium cation (NO+) and nitroxyl anion (NO-) (45).
In aqueous solution, the calculated half-life of NO at the nanomolar concentrations required for signal transduction would be more than 40 minutes (47). This is clearly longer than the biological half-life of NO, estimated to be close to five seconds (48). Even during this time, the lipophilic nature and small size of NO enable diffusion over several cell diameters and enable it to function as a transcellular messenger (49). NO may diffuse in and out of a cell membrane within a millisecond (44).
Nitric oxide forms complexes with transition metal ions, including those regularly found in metalloproteins (50). The main trap for NO is oxyhemoglobin, which binds NO faster by five to six orders of magnitude than oxygen (45). The reaction with haemoglobin produces nitrate and methaemoglobin (met-Hb) (51). The basis of many biological actions of NO is the activation of guanylyl cyclase through binding to the haem prosthetic group of the enzyme (52). Guanylyl cyclase increases the production of cGMP, modulating endothelium-dependent relaxation (53), platelet function (54) and nitrergic inhibitory transmission (55). Other NO-sensitive metalloproteins are NOS, cytochrome P450 (22), ferritin, ceruloplasmin, myoglobin, cyclo-oxygenase, catalase, ribonucleotide reductase and several components of the mitochondrial respiratory chain (56). These reactions have wide implications for the physiologic and toxic effects of NO.
Low molecular thiols, such as glutathione or cysteine, are likely to control NO homeostasis (57). NO reacts with sulfhydryl groups of thiols to form nitroso-thiols, possibly serving both to stabilize and to activate NO (50, 58). By slowly releasing NO, nitroso-thiols may form a protected way to effectively deliver an additional source of NO.
Nitric oxide was previously known as a pollutant and a noxious gas present in the exhaust fumes from cars, which causes acid rain and destroys the ozone layer (59). The potential toxicity of endogenous NO still warrants consideration.
The direct toxicity of nitric oxide is modest. In activated macrophages, NO serves as an effector molecule, degrading the iron-sulphur centres, which results in the release of iron ions and iron-nitrosyl complexes (60). The same pattern of cytotoxity was found in hepatoma cells (60). In vitro, NO deaminated deoxynucleosides, deoxynucleotides and intact DNA at physiological pH, inducing mutations in the bacterial genome (61). Chronic intermittent (62) or continuous (63) exposure of animals to 0.5 – 2 parts per million (ppm) NO caused apparently nitrogen dioxide (NO2) -independent degeneration of interstitial cells, interstitial matrix and connective tissue and emphysematous changes with large airspaces and destruction of alveolar septa.
The toxicity of NO increases greatly upon reaction with superoxide radical (O2-) (64). In both gas phase and aqueous solution, the rapid reaction with superoxide forms highly reactive peroxynitrite anion (46). At neutral pH, peroxynitrite forms peroxynitrous acid (ONOOH) (64). Although peroxynitrite has important microbicidal and tumoricidal functions, the generation of excess ONOO- leads to oxidative injury and lung damage (65). Specifically, peroxynitrite nitrates phenolic residues of tyrosines, forming nitrotyrosine, a marker of the toxic NO pathway (44). The detection of nitrotyrosine illustrates the site of peroxynitrite production and oxidative stress, providing evidence of the toxicity of NO in a number of diseases (66).
Nitrotyrosine has been detected from lung sections of patients and animals with acute lung injury (67, 68), idiopathic pulmonary fibrosis (69) and acute respiratory distress syndrome (ARDS) (70). Peroxynitrite degrades surfactant by causing nitration of the tyrosine residues of surfactant proteins, formation of lipid peroxides and loss of surface activity (71). Plasma nitrotyrosine was elevated in premature infants who developed chronic lung disease (CLD) (72). The presence of superoxide, transient metal, high concentrations of NO and oxygen and the absence of thiol groups, urate and ascorbate in the airways promote the destructive role of NO, as the generation of ONOO- is accelerated or the defence mechanisms against ONOO- toxicity are weakened (47, 73).
Reaction of peroxynitrous acid with target molecules may result in products characteristic of both NO2 and hydroxyl radical (OH.) as reactive intermediates (74). Furthermore, in the presence of oxygen, NO is rapidly oxidized to NO2, which is a major pollutant. Even at very low concentrations NO2 may acutely injure the distal airways and alveoli and disrupt the vascular endothelium (75, 76). However, as detected by chemiluminescence, the levels of NO2 generated during clinical use of <40 ppm iNO remained below 0.3 ppm and were mostly undetectable (77).
Reaction of NO with oxyhaemoglobin yields methaemoglobin, an inactive oxygen transporter, which decreases the oxygen-carrying capacity of haemoglobin (78). The balance of met-Hb depends on its rate of production and the rate of elimination by methaemoglobin reductase in the erythrocyte (79). In a preterm lamb model, iNO at 80 ppm for 23 hours increased met-Hb up to 3.0%, which was low enough not to affect the oxygen-carrying capacity of blood (80). After lung transplantation in a Native American Indian girl, ten hours of 80 ppm iNO transiently increased the met-Hb level up to 9% (81).
NO modulates platelet function by inhibiting platelet aggregation and adhesion (82). This may increase the bleeding time and the risk of intracranial haemorrhage or other bleeding disorders, especially in premature infants (83-85).
Being a free radical, nitric oxide has both pro- and antioxidant properties (47). NO can be protective against oxidative injury, depending on the specific conditions (86). A nitric oxide radical can both stimulate lipid oxidation and mediate oxidant-protective reactions in membranes (87). At high rates of NO production, the pro-oxidant versus antioxidant outcome depends critically on the relative concentrations of the individual reactive species (88). The pro-oxidant reactions of NO occur with superoxide, whereas the antioxidant effects of NO consequent to direct reactions with alkoyl and peroxyl radical intermediate during lipid peroxidation, terminating the propagation of lipid radical chain reactions (88).
NO limits injury to target molecules or tissues during events associated with excess production of reactive oxygen species. These include inhibition of oxidative killing of murine lung fibroblasts and mesencephalic neurons (89), attenuation of low-density lipoprotein oxidation (90, 91) and modulation (92) and reduction of ischemia-reperfusion injury (93).
Hydrogen peroxide mediates oxidation of different biological molecules that may result in tissue damage (89). NO does not react directly with OH., but is able to protect cells against OH. -mediated toxicity (56). NO induced ferritin, haem oxygenase, superoxide dismutase and endonuclease IV, which are protective proteins against oxidative stress, providing a cellular signal to up-regulate a variety of protective genes (94, 95).
In a premature lamb model of mild respiratory distress syndrome (RDS), 20 ppm iNO for five hours did not change significantly either the malondialdehyde and total antioxidant status levels in blood or malondialdehyde, reduced glutathione, glutathione peroxidase and glutathione reductase in lung parenchyma or amino-imino-propene bond generation, suggesting that short-term iNO did not increase oxidative stress and lung inflammation (96). In preterm rabbits, 14 ppm iNO for 20 hours decreased or prevented hyperoxia-induced oxidant stress and surfactant abnormality (97).