Haem oxygenase (1.14.99.3) (HO) [1] is the microsomal enzyme that, in animals, carries out the oxidation of haem, it cleaves the haem ring at the alpha-methene bridge to form biliverdin and carbon monoxide [2]. Biliverdin is subsequently converted to bilirubin by biliverdin reductase. In mammals there are three isozymes of haem oxygenase: HO-1 to HO-3. The first two isozymes differ in their tissue expression and their inducibility: HO-1 is highly inducible by its substrate haem and by various non-haem substances, while HO-2 is non-inducible. It has been suggested [3] that HO-2 could be implicated in the production of carbon monoxide in the brain where it is said to act as a neurotransmitter. In the genome of the chloroplast of red algae as well as in cyanobacteria, there is a haem oxygenase (gene pbsA) that is the key enzyme in the synthesis of the chromophoric part of the photosynthetic antennae [4]. A haem oxygenase is also present in the bacteria Corynebacterium diphtheriae (gene hmuO), where it is involved in the acquisition of iron from the host haem [5]. There is, in the central section of these enzymes, a well-conserved region centred on a histidine residue.

1. Heme oxygenase: function, multiplicity, regulatory mechanisms, and clinical applications. FASEB J. 2, 2557-68
2. Nucleotide sequence and organization of the rat heme oxygenase gene. J. Biol. Chem. 262, 6795-802
3. Carbon monoxide: killer to brain messenger in one step. Science 259, 309
4. The heme oxygenase gene (pbsA) in the red alga Rhodella violacea is discontinuous and transcriptionally activated during iron limitation. Proc. Natl. Acad. Sci. U.S.A. 94, 11736-41
5. Utilization of host iron sources by Corynebacterium diphtheriae: identification of a gene whose product is homologous to eukaryotic heme oxygenases and is required for acquisition of iron from heme and hemoglobin. J. Bacteriol. 179, 838-45

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