We previously reported that the highly reactive cell-free
heme (CFH) is increased in the plasma of patients with
chronic lung injury and causes
pulmonary edema in animal model of
acute respiratory distress syndrome (ARDS) post inhalation of
halogen gas. However, the mechanisms by which CFH causes
pulmonary edema are unclear. Herein we report for the first time that CFH and chlorinated
lipids (formed by the interaction of
halogen gas, Cl2, with
plasmalogens) are increased in the plasma of patients exposed to
Cl2 gas. Ex vivo incubation of red blood cells (RBC) with halogenated
lipids caused oxidative damage to RBC cytoskeletal
protein spectrin, resulting in
hemolysis and release of CFH. Patch clamp and short circuit current measurements revealed that CFH inhibited the activity of
amiloride-sensitive epithelial Na+ channel (ENaC) and
cation sodium (Na+) channels in mouse alveolar cells and trans-epithelial Na+ transport across human airway cells with EC50 of 125 nM and 500 nM, respectively. Molecular modeling identified 22 putative
heme-docking sites on ENaC (energy of binding range: 86-1563 kJ/mol) with at least 2 sites within its narrow transmembrane pore, potentially capable of blocking Na+ transport across the channel. A single
intramuscular injection of the
heme-scavenging
protein,
hemopexin (4 μg/kg
body weight), one hour post
halogen gas exposure, decreased plasma CFH and improved lung ENaC activity in mice. In conclusion, results suggested that CFH mediated inhibition of ENaC activity may be responsible for
pulmonary edema post inhalation injury.