Heme oxygenase cleaves the heme ring at the alpha-methene bridge to form either biliverdin or, if the heme is still attached to a globin, verdoglobin. Biliverdin is subsequently converted to bilirubin by biliverdin reductase.
There are three known isoforms of heme oxygenase.
Heme oxygenase 1 (HO-1) is an inducible isoform in response to stress such as oxidative stress, hypoxia, heavy metals, cytokines, etc.
Heme oxygenase 2 (HO-2) is a constitutive isoform which is expressed under homeostatic conditions.
Heme oxygenase 3 (HO-3) is not catalytically active, but is thought to work in oxygen sensing.
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