Methemoglobin (British 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 release bound oxygen, unlike oxyhemoglobin. It is a bluish chocolate-brown in color. The NADH-dependent enzyme methemoglobin reductase (diaphorase I) is responsible for converting methemoglobin back to hemoglobin.

Normally one to two percent of people'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.[1]


Common causes

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.[2]

Methemoglobin saturation

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

  • 1-2% Normal
  • Less than 10% metHb - No symptoms
  • 10-20% metHb - Skin discoloration only (most notably on mucus membranes)
  • 20-30% metHb - Anxiety, headache, dyspnea on exertion
  • 30-50% metHb - Fatigue, confusion, dizziness, tachypnea, palpitations
  • 50-70% metHb - Coma, seizures, arrhythmias, acidosis
  • Greater than 70% metHb - Death

See also


  1. ^ Denshaw-Burke, Mary (2006-11-07). "Methemoglobinema". Retrieved 2008-03-31. 
  2. ^ Vale, J. A. (2001). "Cyanide Antidotes: from Amyl Nitrite to Hydroxocobalamin - Which Antidote is Best?". Toxicology 168 (1): 37–38. 

External links

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Look at other dictionaries:

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  • methemoglobin — methemoglòbīn m <G methemoglobína> DEFINICIJA fiziol. oksidacijski oblik hemoglobina na koji je kisik čvrsto vezan pa je stoga neupotrebljiv za prijenos kisika krvlju ETIMOLOGIJA met + v. hemoglobin …   Hrvatski jezični portal

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  • Methemoglobin reductase — protein Name=cytochrome b5 reductase 3 caption=Ribbon diagram of erythrocytic methemoglobin reductase with FAD bound. From PDB|1UMK. width=220px HGNCid=2873 Symbol=CYB5R3 AltSymbols=DIA1 EntrezGene=1727 OMIM=250800 RefSeq=NM 007326 UniProt=P00387 …   Wikipedia

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  • methemoglobin — noun Etymology: International Scientific Vocabulary Date: 1870 a soluble brown crystalline basic blood pigment that differs from hemoglobin in containing ferric iron and in being unable to combine reversibly with molecular oxygen …   New Collegiate Dictionary

  • methemoglobin — /met hee meuh gloh bin, hem euh , hee meuh gloh , hem euh /, n. Biochem. a brownish compound of oxygen and hemoglobin, formed in the blood, as by the use of certain drugs. Also, methaemoglobin. Also called ferrihemoglobin, hemiglobin. [1865 70;… …   Universalium

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