The structure of cytochrome b5 reductase, the enzyme that converts methemoglobin to hemoglobin.[1]
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.[2] 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.
^Bando, S.; Takano, T.; Yubisui, T.; Shirabe, K.; Takeshita, M.; Nakagawa, A. (2004). "Structure of human erythrocyte NADH-cytochromeb5reductase". Acta Crystallographica Section D. 60 (11): 1929–1934. doi:10.1107/S0907444904020645. PMID 15502298.
^NIH (1966). "Methemoglobin MeSH Descriptor Data 2021". meshb.nlm.nih.gov. Retrieved 7 June 2021.
Methemoglobin (British: methaemoglobin, shortened MetHb) (pronounced "met-hemoglobin") is a hemoglobin in the form of metalloprotein, in which the iron...
Methemoglobinemia, or methaemoglobinaemia, is a condition of elevated methemoglobin in the blood. Symptoms may include headache, dizziness, shortness of...
metabolism. This enzyme can also reduce methemoglobin to normal hemoglobin, gaining it the inaccurate synonym methemoglobin reductase. Isoforms expressed in...
was thought to act as an oxidant, to induce the formation of methemoglobin. Methemoglobin in turn can sequester cyanide as cyanomethemoglobin. However...
which is a disease defined by high levels of methemoglobin in the blood. Increased levels of methemoglobin prevent oxygen from being released into the...
Obstructive sleep apnea Note this causes "spurious" cyanosis, in that, since methemoglobin appears blue, the patient can appear cyanosed even in the presence of...
binding oxygen. In such cases, the enzyme methemoglobin reductase will be able to eventually reactivate methemoglobin by reducing the iron center. In adult...
have a higher level of fetal hemoglobin which converts more easily to methemoglobin. Additionally, infants are at an increased risk after an episode of...
or NADPH-dependent methemoglobin reductase enzymes, methemoglobin is reduced back to hemoglobin. When large amounts of methemoglobin occur secondary to...
kidneys. This method has the advantage of avoiding the formation of methemoglobin (see below). This antidote kit is sold under the brand name Cyanokit...
disorder can cause heart abnormalities and seizures if the amount of methemoglobin in the blood exceeds 20 percent, but at levels between 10 and 20 percent...
to hemoglobin and oxidize the Fe2+ ions to Fe3+ ions forming methemoglobin. Methemoglobin in turn binds to cyanide (CN) creating cyanmethemoglobin, effectively...
risk of worsening of cyanide symptoms by displacement of cyanide from methemoglobin, allowing the toxin to bind to tissue electron transport chains. The...
methemoglobinemia, an increase of hemoglobin in its ferric [Fe3+] state, called methemoglobin, which cannot bind oxygen, and thus decreases overall carriage of oxygen...
doses. In at least one case the patient had pre-existing low levels of methemoglobin reductase, which likely predisposed her to the condition. It has also...
taken in people with underlying heart disease. The patient's levels of methemoglobin should be regularly checked during treatment. While not well studied...
act with hemoglobin to form methemoglobin which binds cyanide. Cyanide has a preference to the ferric ion on methemoglobin over the ferric ion on cytochrome...
descriptions as a fallback Methemoglobinemia – Condition of elevated methemoglobin in the blood, another condition known for causing blue skin coloration...
nitrite ions. Nitrite ions then combine with hemoglobin to produce methemoglobin, blocking the transport of oxygen. The result is a form of oxygen deprivation...
A dark brown or reddish color can appear due to hemolysis, in which methemoglobin is released from broken blood cells. Plasma is normally relatively transparent...
hemoglobin, several electrolytes, oxyhemoglobin, carboxyhemoglobin, and methemoglobin. ABG testing is mainly used in pulmonology and critical-care medicine...
skin appear blue – a symptom called cyanosis. If the heme is oxidized, methemoglobin, which is more brownish and cannot transport oxygen, is formed. In the...
silver compounds Methemoglobinemia, the presence of excessive levels of methemoglobin in the blood Cyanosis, a change of skin color due to decreased amounts...
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