Methemoglobin

 'Methemoglobin'  (metHb; also  hemiglobin  or  ferihemoglobin  ) is characterized by the presence of  'trivalent iron' , which arises from the oxidation of ferrous iron in hemoglobin at. Methemoglobin loses the ability to reversibly bind oxygen. In its place, Fe III + binds a water molecule through the sixth coordination bond. The color of methemoglobin is chocolate brown.

Methemoglobin is also present physiologically in small amounts in erythrocytes (about 1-3% of total hemoglobin concentration ). This is mainly due to the effect of nitrite, which is formed from nitrates contained in food. The reverse reduction of methemoglobin to hemoglobin is mainly ensured by NADH-dependent cytochrome-b 5 reductase (also methemoglobin reductase). A minor role is played by NADPH-dependent methemoglobin reductase, which is dependent on the supply of NADPH from the pentose cycle and on the presence of another electron transporter (eg flavin). | type = article | surname1 = Xu | name1 = F  | last name2 = Quandt | name2 = K S | surname3 = Hultquist | name3 = D E | article = Characterization of NADPH-dependent methemoglobin reductase as a heme-binding protein present in erythrocytes and liver | magazine = Proc Natl Acad Sci U S A | year = 1992 | number = 6 | volume = 89 | pages = 2130-4 | url = https://www.ncbi.nlm.nih.gov/pmc/articles/PMC48610/?tool=pubmed | issn = 0027-8424 }} Non-enzymatic mechanisms include the action of glutathione and ascorbic acid.

Elevated blood levels of methaemoglobin are referred to as "methaemoglobinaemia". The causes are different:
 *  'Hereditary methemoglobinemia'  is usually caused by a congenital defect of NADH-dependent methemoglobin reductase or the presence of abnormal hemoglobin M] hemoglobin M.
 *  'Acquired methemoglobinemia'  is the most common form of methemoglobinemia. May be caused by oxidants :
 * * poisoning by certain substances (nitrobenzene, aniline and its derivatives - eg some dyes),
 * * by some drugs (local anesthetics - benzocaine, then phenacetin, sulfonamides),
 * * increased content of nitrates and nitrites in water and food.

Newborns are particularly sensitive to the increased content of these substances due to the immaturity of the reduction systems and the increased proportion of fetal hemoglobin, which is more easily oxidized. Methaemoglobinaemia is manifested by cyanosis with a characteristic grayscale and hypoxia.{| class = wikitable + Symptoms of methaemoglobinaemia ! Methemoglobin values ​​!! Symptoms
 * 0–2% || normal value
 * <10% || cyanosis
 * <35% || cyanosis and other symptoms (headache, shortness of breath)
 * 70% || lethal concentration
 * }
 * <35% || cyanosis and other symptoms (headache, shortness of breath)
 * 70% || lethal concentration
 * }
 * }

Part of the therapy of acquired methaemoglobinaemia is the administration of some reducing agents - methylene blue or ascorbic acid.