| Background |
Heme oxygenase (HO) is a microsomal enzyme that oxidatively cleaves heme, a pro-oxidant, into carbon monoxide and biliverdin. Two heme oxygenase isozymes, termed HO-1 and HO-2, have been identified so far. At the amino acid level, HO-1 and HO-2 are 42% homologous. However, each isozyme is evolutionary highly conserved. HO-1 is the inducible form of the enzyme and is therefore also referred to as stress protein Hsp32. Expression of HO-1 can be induced by a variety of stimuli such as heme, metals, hormones, UV radiation and sulfhydryl depleting agents. In contrast, HO-2 is the constituitive form of the enzyme. It is not inducible by exogeneous stimuli and HO-2 is the most prevalent form in most tissues, except the spleen where HO-1 levels are predominant under normal conditions.
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