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Heme oxygenase (HO) is an enzyme that catalyzes the degradation of heme. This produces biliverdin, iron, and carbon monoxide.[1]
[edit] Reaction
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.
The reaction occurs as follows:
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- Heme + NADPH + H+ + 3 O2 → biliverdin + Fe3+ + CO + NADP+ + H2O
This reaction can occur in virtually every cell; the classic example is the formation of a bruise, which goes through different colors as it gradually heals: red heme to green biliverdin to yellow bilirubin. Under normal physiological conditions, the activity of heme oxygenase is highest in the spleen, where old erythrocytes are sequestrated and destroyed.
[edit] Isoforms
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. Both HO-1 and HO-2 are ubiquitously expressed and catalytically active.
A third heme oxygenase (HO-3) is not catalytically active, but is thought to work in oxygen sensing.
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