The heme oxygenase system and cellular defense mechanisms. Do HO-1 and HO-2 have different functions?

Heme oxygenase isozymes, HO-1, HO-2 and HO-3, are HSP32 protein cognates, with a known function of catalyzing the isomer specific oxidation of the heme molecule, including that of NO synthase. Unknown until recent years was that the system is a central component of the cellular defense mechanisms; this can be attributed to a combination of many factors. In biological systems HO activity is responsible for production of equimolar amounts of CO, biliverdin and free Fe. The serine/threonine kinase, biliverdin reductase, catalyzes reduction of biliverdin to bilirubin. Bilirubin is a potent antioxidant and CO is a signal molecule. Although both active HO isozymes catalyze the same reaction, HO-1 and HO-2 may function in a rather distinct fashion in protection against tissue injury. HO-1 is the stress responsive cognate that is rapidly induced by free and stable radicals as well as by hypoxia. Supra induction of HO-1 completely protects ischemic kidney against tissue pathology. This involves rapid inactivation of the pro-oxidant heme of denatured hemoproteins and converting it to bilirubin and CO. In the case of severe tissue injury, such as compression injury, HO-1 is induced and colocalizes with cGMP and pro-apoptotic oncogenes. HO-2, which is the constitutive form, in addition to maintaining cell heme homeostasis, inactivates NO derived radicals. The isozyme binds the free radical at its "heme regulatory motifs" and is "suicide" inactivated at the protein and transcript levels. Data are shown that provide evidence for role of the HO system in the cellular defense mechanism against free radical-mediated tissue damage, and are consistent with the forwarded concept that HO isozymes have common, as well as distinct, roles in cellular defense mechanisms.