Acute respiratory distress syndrome in adults (ARDS) carries a high mortality. Patients with ARDS experience severe oxidative stress from neutrophil activation, and from treatment with high inspired oxygen concentrations (F(I)O2). Oxidative stress arises from an increased generation of reactive oxygen species (ROS) which overwhelm existing antioxidant defenses. Patients who do not survive ARDS sustain much greater levels of oxidative molecular damage, suggesting that they are less able to protect themselves against increased oxidative stress. We measured plasma levels of pro-oxidant substrates for xanthine oxidase, namely hypoxanthine and xanthine, and correlated them with the loss of plasma protein thiol groups. All patients with ARDS had higher levels of hypoxanthine (37.48 +/- 3.1 microM in nonsurvivors, 15.24 +/- 2.09 microM in survivors) compared with patients undergoing pulmonary resection (9.22 +/- 1.89 microM), patients in intensive care with sepsis but no lung injury (1.12 +/- 0.69 microM) and normal healthy control subjects (1.43 +/- 0.38 microM). The difference in plasma hypoxanthine levels between survivors and nonsurvivors of ARDS was highly significant (p < 0.001) and showed a negative correlation with loss of protein thiol groups. Xanthine levels were also higher in patients with ARDS but were not significantly different between ARDS survivors and nonsurvivors. Nonsurvivors of ARDS appear to experience higher levels of oxidative stress and damage than do survivors.

Overall, our data strongly suggest heightened concentrations of oxidative stress in the lungs of patients with ARDS that lead to significantly increased oxidative protein damage.

Heme oxygenase protein is elevated in the lungs of patients with ARDS and may contribute to the changes in iron mobilization, signaling, and regulation seen in this condition.

During intensive care treatment, patients with ARDS decrease their percentage plasma concentrations of total plasma linoleic acid, but increase their percentage concentrations of oleic and palmitoleic acids. As plasma linoleic acid concentrations decreased, there was usually an increase in plasma 4-hydroxy-2-nonenal values, one of its specific peroxidation products, suggestive of severe oxidative stress leading to molecular damage to lipids.

Inhaled nitric oxide (.NO) is used to improve gas exchange and reduce pulmonary vascular resistance (PVR) in patients with the acute respiratory distress syndrome (ARDS). Although controlled studies have shown no survival benefit, some investigators have suggested that inhaled.NO may have antiinflammatory properties under these circumstances. In contrast, others have speculated that.NO given by inhalation could be cytotoxic, as it combines with superoxide at near diffusion-limited rates to produce the highly reactive oxidant peroxynitrite (ONOO(-)). We therefore quantified levels of 3-nitrotyrosine, a marker for ONOO(-) formation, in bronchoalveolar lavage fluid (BAL) from patients with ARDS receiving inhaled.NO, and from patients with comparable lung injury who were not so treated. We also measured levels of 3-chlorotyrosine as an index of neutrophil activation to assess indirectly the effects of inhaled.NO on lung inflammation. Patients receiving .NO had increased levels of 3-nitrotyrosine (6.76 +/- 2.79 versus 0.4 +/- 0.15 nmol/mg of protein, p < 0.05) and 3-chlorotyrosine (7.97 +/- 2.74 versus 1. 53 +/- 1.09 nmol/mg of protein, p < 0.05) in BAL protein compared with controls. In patients with ARDS, inhaled.NO increases the formation of 3-nitrotyrosine and is accompanied by an increase in levels of 3-chlorotyrosine (a marker of neutrophil activation). The possible long-term consequences of these observations remain to be evaluated.

Haem oxygenase-1 is upregulated by numerous insults, including oxidative stress, and under such circumstances it is considered to be a protective stratagem. We have measured the haem oxygenase-1 expression in heart, lung and liver tissues of control and iron-overloaded rats. Lung tissue from ironoverloaded rats displayed a significant increase in the haem oxygenase-1 protein but no changes in haem oxygenase-1 mRNA. Conversely, heart tissue showed a significant increase in haem oxygenase-1 mRNA but no changes in haem oxygenase-1 protein. We conclude that during oxidative stress caused by iron overload, lung tissue responds with a rapid upregulation of haem oxygenase-1 levels.z 1999 Federation of European Biochemical Societies.

Increased levels of plasma thiol appear to be associated with mortality in patients with ARDS secondary to CPB surgery.

Haem oxygenase-1 (HO-1) is a highly inducible stress protein that removes haem from cells with the release of biliverdin, carbon monoxide and low-molecular-mass iron (LMrFe). Several antioxidant functions have been ascribed to HO; its induction is considered to be a protective event. However, LMrFe produced during haem catabolism might elicit a pro-oxidant response, with deleterious consequences. We therefore investigated the delicate balance between pro-oxidant and antioxidant events with the use of a microsomal lipid peroxidation (LPO) system. By using microsomal-bound HO in an NADPH-dependent LPO system, we assessed the pro-oxidant nature of the released LMrFe and the antioxidant effect of the released bilirubin. Hb, a biologically relevant substrate for HO, was included with the microsomes to supplement the source of haem iron and to promote LPO. We found significant increases in microsomal LPO, by using the thiobarbituric acid (TBA) test, after incubation with Hb. This Hb-stimulated peroxidation was inhibited by HO inhibitors and by iron chelators, suggesting a HO-driven, iron-dependent mechanism. GLC-MS was employed to measure the specific LPO product 4-hydroxy-2-nonenal and to confirm our TBA test results. A HO inhibitor attenuated an increase in intracellular LMrFe that occurred after treatment of rat pulmonary artery smooth-muscle cells with Hb. Additionally, exogenously added bilirubin at an equimolar concentration to the LMrFe present in both microsomal and liposomal systems was unable to prevent the pro-oxidant effect of the iron. Under certain circumstances HO can act as a pro-oxidant and seems to have a role in stimulating microsomal LPO.