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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/07/18 23:13:18」(JST)

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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 bind oxygen, unlike oxyhemoglobin.^[1] It is bluish chocolate-brown in color. In human blood a trace amount of methemoglobin is normally produced spontaneously. But when it is 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.

The structure of the enzyme that converts methemoglobin to hemoglobin^[2]

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[edit]

Reduced cellular defense mechanisms
- Children younger than 4 months exposed to various environmental agents
- Pregnant women are considered vulnerable to exposure of high levels of nitrates in drinking water^[4]
- Cytochrome b5 reductase deficiency
- G6PD deficiency
- Hemoglobin M disease
- Pyruvate kinase deficiency
Various pharmaceutical compounds
- Local anesthetic agents, especially prilocaine as used in the Bier block
- Oral pain relief gels, liquids and lozenges containing benzocaine.^[5]
- Amyl nitrite, chloroquine, dapsone, nitrates, nitrites, nitroglycerin, nitroprusside, phenacetin, phenazopyridine, primaquine, quinones and sulfonamides
Environmental agents
- Aromatic amines (e.g. p-nitroaniline, patient case)
- Arsine
- Chlorobenzene
- Chromates
- Nitrates/nitrites
Inherited disorders
- Some family members of the Fughate family in Kentucky, due to a recessive gene, had blue skin from an excess of methemoglobin.^[6]
In cats
- Ingestion of Paracetamol (i.e. acetaminophen, tylenol)^{[citation needed]}

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 non-toxic cyanomethemoglobin.^[7]

Methemoglobin saturation[edit]

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

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).^[8]

References[edit]

^ http://www.rtso.ca/themes/acquia_marina/student/Methemoglobi1.htm
^ 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. doi:10.1107/S0907444904020645. PMID 15502298. edit
^ Denshaw-Burke, Mary (2006-11-07). "Methemoglobinema". Retrieved 2008-03-31.
^ 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. edit
^ http://www.fda.gov/Drugs/DrugSafety/ucm250024.htm
^ http://abcnews.go.com/Health/blue-skinned-people-kentucky-reveal-todays-genetic-lesson/story?id=15759819#.T2oS0HqV2So
^ Vale, J. A. (2001). "Cyanide Antidotes: from Amyl Nitrite to Hydroxocobalamin - Which Antidote is Best?". Toxicology 168 (1): 37–38.
^ Bremmer et al PloS One 2011 http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021845

External links[edit]

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

Japanese Journal

In vivo Photoacoustic Spectroscopic Imaging of Hemoglobin Derivatives in Thermally Damaged Tissue

Aizawa Kazuya,Sato Shunichi,Saitoh Daizoh [他],Ashida Hiroshi,Obara Minoru
Jpn J Appl Phys 48(6), 062302-062302-4, 2009-06-25
… We observed PA signals at around 500 and 633 nm, which are center wavelengths of methemoglobin (MetHb) absorption peaks, at depths corresponding the uppermost dermis (${\sim}0.16$ mm) and hair follicles (${\sim}0.62$ mm), indicating formation of MetHb in these tissue regions. …
NAID 150000052508

パルスオキシメータ機種の精度について--問題は?解決は? (イブニングセミナー3(第16回日本呼吸管理学会学術集会) 呼吸管理における非侵襲的モニタリング)

福島春樹
日本呼吸ケア・リハビリテーション学会誌 17(3), 219-224, 2007-12
NAID 40016003036

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