methemoglobin /met′hēməglō″bin, met·he″məglō′bin/ , a form of hemoglobin in which the iron component has been oxidized from the ferrous to the ferric state. Methemoglobin cannot carry oxygen. It is a product of various oxidative reactions that constitute normal metabolic activity and is normally present in only trace amounts (about 1%) in the blood, but may increase in chronic inflammation. Maintenance of levels occurs by an active enzymatic reducing capability, the nicotinamide-adenine dinucleotide-methemoglobin reductase system present in normal red blood cells. Also spelled methaemoglobin. See also hemoglobin.
