The heme ring is cleaved by heme oxygenase EC Number: 1.14.99.3 Heme oxygenase (HO) cleaves the heme ring at the alpha-methene bridge to form bilverdin. This reaction forms the only endogenous source of carbon monoxide. HO-1 is inducible and is thought to have an antioxidant role as it's activated in virtually all cell types and by many types of "oxidative stress" (Poss and Tonegawa, 1997). HO-2 is non-inducible. Pubmed9380736 Reactome Database ID Release 43189398 Reactome, http://www.reactome.org ReactomeREACT_22100 Reviewed: Sassa, S, 2007-01-24 10:18:36 has a Stoichiometric coefficient of 3