Heme oxygenase (HO) was initially described as the regulatory enzyme in the heme degradative pathways. It catalyzes the conversion of heme into carbon monoxide (CO), iron, and biliverdin, which acts as an antioxidant. Three isoforms of HO have been described to date: HO-1, which is inducible; HO-2, which is constitutively expressed; and the recently evidenced HO-3, probably involved in heme binding.
Casanas-Roux, F., Van Langendonckt, A., Dolmans, M.-M., & Donnez, J. (2002). Expression of inducible heme oxygenase in human endometrium. Fertility and Sterility, 78(6), 1327-1328. https://doi.org/10.1016/S0015-0282(02)04395-9 (Original work published 2002)