Heme oxygenase-1 in lung disease

Heme oxygenase (HO) was first described in 1968 by Raimo Tenhunen as the enzyme responsible for degrading heme, a highly conserved molecule that is essential for most forms of life (1). HO targets the α-methene bridge of the heme ring to catalyze the oxidative breakdown of heme into equimolar amounts of carbon monoxide (CO), biliverdin, and iron (Fe) (Fig. 1). In most mammals, biliverdin is subsequently reduced to bilirubin by biliverdin reductase. Under physiological conditions HO activity is highest in the spleen, where senescent erythrocytes are sequestered and destroyed, but this activity is observed in virtually all organ systems.