A particular type of hemoglobin that is altered so that it is useless for carrying oxygen and delivering it to tissues throughout the human body. Since hemoglobin is the key carrier of oxygen in the blood, its wholesale replacement by methemoglobin can cause cyanosis (a slate gray-blueness) due to lack of oxygen. A small amount of methemoglobin is normally present in blood but the conversion of a larger fraction of hemoglobin into methemoglobin, which does not function reversibly as an oxygen carrier, results in perceptible cyanosis. In technical terms, methemoglobin is a transformation product of normal hemoglobin (oxyhemoglobin) and is produced by the oxidation of the normal ferrous iron contained in the heme part of hemoglobin to ferric iron which, in firm union with water, is chemically useless for respiration. The presence of methemoglobin in the blood is termed methemoglobinemia. It may be acquired anytime in life by exposure to a number of different chemical agents, such as nitrites or it may be congenital due a genetic condition: {{}}Hb M disease — the production of any one of a group of mutant hemoglobins known collectively as hemoglobin M ("M" for methemoglobin), or Deficiency of the enzyme methemoglobin reductase which is required for the reduction of methemoglobin to normal oxyhemoglobin. In both kinds of congenital methemoglobinemia, the cyanosis starts in early infancy with no history of lung or heart disease to account for it. However, the patterns of inheritance deficiency are distinctively different — autosomal dominant with Hb M disease versus autosomal recessive with the enzyme deficiency. The cyanosis from Hb M disease is resistant to treatment with reducing agents such as ascorbic acid or methylene blue, whereas with deficiency of the enzyme, the cyanosis is typically improved by ascorbic acid or methylene blue therapy.
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A transformation product of oxyhemoglobin because of the oxidation of the normal Fe2+ to Fe3+, thus converting ferroprotoporphyrin to ferriprotoporphyrin; it contains water in firm union with ferric iron, thus being chemically different from oxyhemoglobin and useless for respiration; found in sanguineous effusions and in the circulating blood after poisoning with acetanilid, potassium chlorate, and other substances. SYN: ferrihemoglobin.
- m. reductase a flavoenzyme catalyzing the reduction of m. to hemoglobin in the red blood cell.

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met·he·mo·glo·bin or chiefly Brit met·hae·mo·glo·bin (')met-'hē-mə-.glō-bən n a soluble brown crystalline basic blood pigment that is found in normal blood in much smaller amounts than hemoglobin, that is formed from blood, hemoglobin, or oxyhemoglobin by oxidation, and that differs from hemoglobin in containing ferric iron and in being unable to combine reversibly with molecular oxygen called also ferrihemoglobin

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met·he·mo·glo·bin (met-heґmo-glo″bin) [met- + hemoglobin] a brown pigment formed from hemoglobin by oxidation of the ferrous to the ferric state with essentially ionic bonds. A small amount is present in the blood normally, but injury or toxic agents convert a larger proportion of hemoglobin into methemoglobin, which does not function reversibly as an oxygen carrier.

Medical dictionary. 2011.

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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.… …   Wikipedia

  • methemoglobin — methemoglobin. См. гемиглобин. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • 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

  • methemoglobin — [met hē′mō glō΄bin, meth eməglō΄bin; methem′ōglō΄bin, met hem′əglō΄bin] n. [ MET(A) + HEMOGLOBIN] a brownish, crystalline substance containing ferric iron, formed in the blood by the oxidation of hemoglobin, as by the action of certain drugs or… …   English World dictionary

  • methemoglobin — methemoglobinas statusas T sritis chemija apibrėžtis Hemoglobinas, turintis Fe(III) ir nepernešantis O₂. atitikmenys: angl. methemoglobin rus. метгемоглобин …   Chemijos terminų aiškinamasis žodynas

  • methemoglobin — methemoglobinas statusas T sritis ekologija ir aplinkotyra apibrėžtis Hemoglobino darinys, kurio molekulės geležis oksiduota ir jis negali prisijungti ir pernešti deguonies. Susidaro organizme apsinuodijus kai kuriomis medžiagomis. atitikmenys:… …   Ekologijos terminų aiškinamasis žodynas

  • 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

  • methemoglobin reductase (NADPH) — met·he·mo·glo·bin re·duc·tase (NADPH) (met he″mo gloґbin re dukґtās) NADPH methemoglobin reductase …   Medical dictionary

  • 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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