By Shaw-Fang Yet, Luis G. Melo, Matthew D. Layne, and Mark A. Perrella
Heme oxygenase (HO) enzymes catalyze the initial reaction in heme catabolism.1 Heme is a diverse metalloporphyrin molecule that is incorporated into important hemoproteins, such as hemoglobin and cytochromes, in the form of iron protoporphyrin IX.2 Distinct forms of heme oxygenase have been identified2-4: (1) HO-1, a 32-kDa protein induced by diverse stimuli, including inflammatory cytokines and factors that promote oxidative stress; (2) HO-2, a 36-kDa protein that is predominantly a constitutive enzyme; and (3) HO-3, a 33-kDa protein highly homologous to HO-2 (~90%) at the amino acid level4 HO-1 and HO-2 are clearly products of distinct genes as they differ in chromosomal localization, gene
1 r. Tenhunen, H. S. Marver, and r. Schmid, Proc. Natl. Acad. Sci. U.S.A. 61, 748 (1968).
2 M. D. Maines, Anna. Rev. Pharmacol. Toxicol. 37, 517 (1997).
4 W. K. McCoubrey, Jr., T. J. Huang, and M. D. Maines, Eur. J. Biochem. 247, 725 (1997).
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