This reaction can occur in virtually every cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

; the classic example is the formation of a bruise

Bruise

A bruise, also called a contusion, is a type of relatively minor hematoma of tissue in which capillaries and sometimes venules are damaged by trauma, allowing blood to seep into the surrounding interstitial tissues. Bruises can involve capillaries at the level of skin, subcutaneous tissue, muscle,...

, 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

Spleen

The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...

, where old erythrocytes are sequestrated and destroyed.

Isoforms

There are three known isoforms of heme oxygenase.

Heme oxygenase 1 (HO-1)

HMOX1

HMOX1 is a human gene that encodes for the enzyme heme oxygenase 1 . Heme oxygenase is an essential enzyme in heme catabolism, it cleaves heme to form biliverdin....

 is an inducible isoform in response to stress such as oxidative stress, hypoxia

Hypoxia (medical)

Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

, heavy metals, cytokine

Cytokine

Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s, etc. Heme oxygenase 2 (HO-2)

HMOX2

Heme oxygenase 2 is an enzyme that in humans is encoded by the HMOX2 gene.- Function :Heme oxygenase, an essential enzyme in heme catabolism, cleaves heme to form biliverdin, which is subsequently converted to bilirubin by biliverdin reductase, and carbon monoxide, a putative neurotransmitter...

is a constitutive isoform that 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.