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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 Fe 3+ state, not the Fe 2+ of normal hemoglobin.
Methemoglobinemia, or methaemoglobinaemia, is a condition of elevated methemoglobin in the blood. [2] Symptoms may include headache, dizziness, shortness of breath, nausea, poor muscle coordination, and blue-colored skin (cyanosis). [2]
The 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 it can cause blue skin without other symptoms. Most of the Fugates lived long and healthy lives. The "bluest" of the blue Fugates, Luna Stacy, had 13 children and lived to age 84. [6]
Methemoglobin is an oxidized form of hemoglobin attached to a ferric-state iron (Fe3+), which can therefore not carry and deliver oxygen to tissues. [15] The formation of methemoglobin occurs when electrons are not returned to the iron of a normal state hemoglobin, which is not preferred for a functioning organism.
Hemoglobin M disease is a rare form of hemoglobinopathy, characterized by the presence of hemoglobin M (HbM) and elevated methemoglobin (metHb) level in blood. [1] HbM is an altered form of hemoglobin (Hb) due to point mutation occurring in globin-encoding genes, mostly involving tyrosine substitution for proximal (F8) or distal (E7) histidine residues. [2]
Additionally, the oxygen that is already bound is held more tightly to the hemoglobin due to a higher affinity, resulting in less oxygen delivery. A methemoglobin level > 1.5 g/dL causes cyanosis. The most common congenital cause is a deficiency in the enzyme cytochrome b5 reductase which reduces methemoglobin in the blood. [22]
Finger tip carboxyhemoglobin saturation monitor.. A CO-oximeter is a device that measures the oxygen carrying state of hemoglobin in a blood specimen, including oxygen-carrying hemoglobin (O2Hb), non-oxygen-carrying but normal hemoglobin (HHb) (formerly, but incorrectly, referred to as 'reduced' hemoglobin), as well as the dyshemoglobins such as carboxyhemoglobin (COHb) and methemoglobin (MetHb).
Methemoglobin is also formed in small quantities when the dissociation of oxyhemoglobin results in the formation of methemoglobin and superoxide, O 2 −, instead of the usual products. Superoxide is a free radical and causes biochemical damage, but is neutralised by the action of the enzyme superoxide dismutase.