Methemoglobin
hemoglobin with ferric iron unable to carry oxygen
Methemoglobin (British: methaemoglobin, shortened MetHb) (pronounced "met-hemoglobin") is a hemoglobin in the form of metalloprotein, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Sometimes, it is also referred to as ferrihemoglobin. 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.
Etymology
The word methemoglobin derives from the Ancient Greek prefix μετα- (meta-: behind, later, subsequent) and the word hemoglobin.
The name hemoglobin is itself derived from the words heme and globin, each subunit of hemoglobin being a globular protein with an embedded heme group.
“Methemoglobin” enters the record as hemoglobin with ferric iron unable to carry oxygen. Crown Archives preserves that source wording while asking what Methemoglobin, hemoglobin and ferric can confirm, complicate or overturn.
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This entry incorporates text from “Methemoglobin” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.