Methemoglobin (MetHb)

Methemoglobin is an oxidized form of hemoprotein derived from normal adult hemoglobin. In this molecule, the central iron atom within each heme group exists in the ferric (Fe³⁺) rather than the functional ferrous (Fe²⁺) state. This oxidation prevents binding and transport of oxygen by red blood cells. Normally, only about one to two percent of total circulating hemoglobin exists as methemoglobin due to continuous reduction by NADH-dependent cytochrome b5-methemoblogin reductase systems. Elevated levels—caused either by genetic defects affecting these reducing enzymes or exposure to certain chemicals/drugs—lead to clinical syndromes collectively termed "methemoglobinemia," characterized primarily by cyanosis and hypoxic symptoms despite adequate environmental oxygen availability. Treatment focuses on removing causative agents and restoring normal redox balance using agents such as methylene blue when indicated.

Other names

Methaemoglobin (British spelling)FerrihemoglobinHemoglobin M (in the context of certain genetic variants)MHb (less common abbreviation)

For drugs causing increased methemoglobinemia: Oxidation of ferrous iron (Fe²⁺) in heme to ferric iron (Fe³⁺), converting functional hemoglobin into non-functional methemoglobin. For antidotes/treatments like methylene blue: Reduction of ferric iron back to ferrous state via enzymatic pathways.