Heme oxygenase is the rate-limiting enzyme in the heme degradative pathway. There are 2 isozymic forms of heme oxygenase, an inducible heme oxygenase-1 (HMOX1) and a constitutive heme oxygenase-2 (HMOX2). Down-regulation of HMOX2 is associated with the increased expression of HMOX1 in human cell lines.
