However, arachidonate thoroughly metabolites can be synthesized by and have potent biologic effects on other airway cells such as leukocytes, smooth muscle, nerves, mucus glands, and platelets. 8-Isoprostane (8-IP), a prostaglandin (PG)-F2-like compound, belongs to the F2 isoprostane class that is produced in vivo by the free radical-catalyzed peroxidation of arachidonic acid [24]. Because of the transcellular feature of arachidonic acid metabolism and function, airway lining fluid is the critical medium for these actions. Significant amounts of arachidonic acid and its derivatives are present in ALF. 8-Isoprostane, LTs, and prostanoids have been detected in EBC and used as biomarkers for oxidative stress and respiratory infection.

Methods used to detect arachidonic acid derivatives in the EBC include gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), radioimmunoassay (RIA), and enzyme immunoassay (EIA). Figure 2Metabolism of arachidonic acid. Arachidonic acid is released from phospholipids by the action of phospholipase A2 (PLA2). Arachidonic acid is metabolized by cyclooxygenases (COXs), lipoxygenases (LOXs), and cytochrome P450 (CYP). COXs metabolize arachidonic …Most studies measuring 8-IP used commercial EIA kits with a detection limit of 1pg/mL [25]. GC/MS is a more sensitive method for 8-IP detection and has been used to validate EIA results [26]. 8-IP levels in the EBC of healthy subjects were reported in the range of 0�C40pg/mL.

Increased concentrations of 8-IP in the EBC as a marker of oxidative stress has been demonstrated in multiple lung diseases, such as asthma AV-951 [27, 28], COPD [29, 30], interstitial lung disease [31], and cystic fibrosis [32, 33]. LTs can also be measured in the EBC by EIA with a detection limit of 4pg/mL. Other methods such as LC/MS/MS, GC/MS, and high-performance liquid chromatography (HPLC) are also used in LT detection [34]. LTs in EBC samples from healthy subjects range from 0 to 25pg/mL. Elevated LTs were found to be correlated with parameters of inflammation in the lungs [35]. Similarly, prostanoids can be measured by EIA, RIA, and chromatographic techniques and are present in the range of 0�C200pg/mL in EBC [26, 36].3.3. Oxygen and Nitrogen Reactive Species and Redox-Relevant Molecules in EBCInvestigations of reactive oxygen species (ROS) and reactive nitrogen species (RNS) are among the interests of EBC biomarkers in many lung disease studies. Multiple RNS formation starts with nitric oxide (NO). NO is a volatile component of the EBC [3, 13, 37, 38] and is synthesized from the amino acid L-arginine by nitric oxide synthase (NOS) (Figure 3).