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Title: Methemoglobin  
Author: World Heritage Encyclopedia
Language: English
Subject: Hemoglobin, Hemolysis (microbiology), CO-oximeter, Methemoglobinemia, Photoacoustic imaging
Collection: Cellular Respiration, Hemoproteins
Publisher: World Heritage Encyclopedia


The structure of the enzyme that converts methemoglobin to hemoglobin[1]

(English: methaemoglobin) (pronounced "met-hemoglobin") is a form of the oxygen-carrying 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, unlike oxyhemoglobin.[2] 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 (diaphorase I) 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.[3]


  • Common causes of elevated methemoglobin 1
  • Therapeutic uses 2
  • Methemoglobin saturation 3
  • Blood stains 4
  • See also 5
  • References 6
  • External links 7

Common causes of elevated methemoglobin

Therapeutic uses

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 non-toxic cyanomethemoglobin.[7]

Methemoglobin saturation

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

Blood stains

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).[8]

See also


  1. ^ Bando, S.; Takano, T.; Yubisui, T.; Shirabe, K.; Takeshita, M.; Nakagawa, A. (2004). "Structure of human erythrocyte NADH-cytochromeb5reductase". Acta Crystallographica Section D Biological Crystallography 60 (11): 1929–1934.  
  2. ^
  3. ^ Denshaw-Burke, Mary (2006-11-07). "Methemoglobinema". Retrieved 2008-03-31. 
  4. ^ 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.  
  5. ^
  6. ^
  7. ^ Vale, J. A. (2001). "Cyanide Antidotes: from Amyl Nitrite to Hydroxocobalamin - Which Antidote is Best?".  
  8. ^ Bremmer et al PloS One 2011

External links

  • Methemoglobin at the US National Library of Medicine Medical Subject Headings (MeSH)
  • MetHb Formation
  • The Blue people of Troublesome Creek
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