From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
The structure of the enzyme that converts methemoglobin to hemoglobin[1]

Methemoglobin (English: methaemoglobin) (pronounced "met-hemoglobin") is a form of metalloprotein hemoglobin, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Methemoglobin cannot bind oxygen, which means it cannot carry oxygen to tissues. It is bluish chocolate-brown in color. In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. The NADH-dependent enzyme methemoglobin reductase (a type of diaphorase) is responsible for converting methemoglobin back to hemoglobin.

Normally one to two percent of a person's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation. An abnormal increase of methemoglobin will increase the oxygen binding affinity of normal hemoglobin, resulting in a decreased unloading of oxygen to the tissues. [2]

Common causes of elevated methemoglobin[edit]

Therapeutic uses[edit]

Amyl nitrite is administered to treat cyanide poisoning. It works by converting hemoglobin to methemoglobin, which allows for the binding of cyanide and the formation of cyanomethemoglobin. The immediate goal of forming this cyanide adduct is to prevent the binding of free cyanide to the cytochrome a3 group in cytochrome c oxidase.[8]

Methemoglobin saturation[edit]

Methemoglobin saturation is expressed as the percentage of hemoglobin in the methemoglobin state; That is MetHb as a proportion of Hb.

Blood stains[edit]

Increased levels of methemoglobin are found in blood stains. Upon exiting the body, bloodstains transit from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO2) to methemoglobin (met-Hb) and hemichrome (HC).[9]

See also[edit]


  1. ^ Bando, S.; Takano, T.; Yubisui, T.; Shirabe, K.; Takeshita, M.; Nakagawa, A. (2004). "Structure of human erythrocyte NADH-cytochromeb5reductase". Acta Crystallographica Section D. 60 (11): 1929–1934. doi:10.1107/S0907444904020645. PMID 15502298.
  2. ^ Denshaw-Burke, Mary (2006-11-07). "Methemoglobinema". Retrieved 2008-03-31.
  3. ^ Manassaram, D. M.; Backer, L. C.; Messing, R.; Fleming, L. E.; Luke, B.; Monteilh, C. P. (2010). "Nitrates in drinking water and methemoglobin levels in pregnancy: A longitudinal study". Environmental Health. 9: 60. doi:10.1186/1476-069X-9-60. PMC 2967503. PMID 20946657.
  4. ^
  5. ^ a b "Survival after severe methemoglobinemia secondary to sodium nitrate ingestion". Toxicology Communications. 11 May 2018.
  6. ^
  7. ^ Dowers, Kirsty (2011-08-01). "Don't eat that! Toxicities in cats (Proceedings)". Retrieved 2018-11-02.
  8. ^ Vale, J. A. (2001). "Cyanide Antidotes: from Amyl Nitrite to Hydroxocobalamin - Which Antidote is Best?". Toxicology. 168 (1): 37–38.
  9. ^ Bremmer et al PLoS ONE 2011

External links[edit]