Methemoglobin (British: methaemoglobin) (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 Cytochrome-''b''5 reductase is a NADH-dependent enzyme that converts ferricytochrome from a Fe3+ form to a Fe2+ form. It contains FAD and catalyzes the reaction: In its b5-reducing capacity, this enzyme is involved in desaturation and elongatio ...

( 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 Methemoglobinemia, or methaemoglobinaemia, is a condition of elevated methemoglobin in the blood. Symptoms may include headache, dizziness, shortness of breath, nausea, poor muscle coordination, and blue-colored skin (cyanosis). Complications m ...

. 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 Subunit may refer to: *Subunit HIV vaccine, a class of HIV vaccine *Protein subunit, a protein molecule that assembles with other protein molecules *Monomer, a molecule that may bind chemically to other molecules to form a polymer *Sub-subunit, a ...

of hemoglobin being a globular protein with an embedded heme group.

Common causes of elevated methemoglobin

* Reduced cellular defense mechanisms ** Children younger than 4 months exposed to various environmental agents ** Pregnant women are considered vulnerable to exposure of high levels of nitrates in drinking water ** Cytochrome b5 reductase deficiency ** G6PD deficiency **

Hemoglobin M disease Hemoglobin M disease is a rare form of hemoglobinopathy, characterized by the presence of hemoglobin M (HbM) and elevated methemoglobin (metHb) level in blood. HbM is an altered form of hemoglobin (Hb) due to point mutation occurring in globin-enco ...

** Pyruvate kinase deficiency * Various pharmaceutical compounds ** Local anesthetic agents, especially prilocaine and

benzocaine Benzocaine, sold under the brand name Orajel amongst others, is an ester local anesthetic commonly used as a topical pain reliever or in cough drops. It is the active ingredient in many over-the-counter anesthetic ointments such as products f ...

. ** Amyl nitrite, chloroquine,

dapsone Dapsone, also known as 4,4'-sulfonyldianiline (SDA) or diaminodiphenyl sulfone (DDS), is an antibiotic commonly used in combination with rifampicin and clofazimine for the treatment of leprosy. It is a second-line medication for the treatment a ...

nitrates Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble ...

, nitrites,

nitroglycerin Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating g ...

, nitroprusside,

phenacetin Phenacetin (acetophenetidin, ''N''-(4-ethoxyphenyl)acetamide) is a pain-relieving and fever-reducing drug, which was widely used following its introduction in 1887. It was withdrawn from medicinal use as dangerous from the 1970s (e.g., withdrawn ...

, phenazopyridine, primaquine, quinones and

sulfonamides In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactive. ...

* Environmental agents ** Aromatic amines (e.g. ''p''-nitroaniline
patient case
** Arsine ** Chlorobenzene ** Chromates **

Nitrates Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble ...

/ nitrites ** Umbellulone * Inherited disorders ** Some family members of the Fugate family in Kentucky, due to a recessive gene, had blue skin from an excess of methemoglobin. * In cats **Ingestion of

paracetamol Paracetamol, also known as acetaminophen, is a medication used to treat fever and mild to moderate pain. Common brand names include Tylenol and Panadol. At a standard dose, paracetamol only slightly decreases body temperature; it is inferior ...

(i.e. acetaminophen, tylenol)

Therapeutic uses

Amyl nitrite is administered to treat cyanide poisoning. It works by converting hemoglobin to methemoglobin, which allows for the binding of

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...

(CN^–) anions by ferric (Fe³⁺)

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s and the formation of cyanomethemoglobin. The immediate goal of forming this cyanide adduct is to prevent the binding of free cyanide to the cytochrome a₃ group in cytochrome c oxidase.

Methemoglobin saturation

Methemoglobin is expressed as a concentration or a percentage. Percentage of methemoglobin is calculated by dividing the concentration of methemoglobin by the concentration of total hemoglobin. Percentage of methemoglobin is likely a better indicator of illness severity than overall concentration, as underlying medical conditions play an important role. For example, a methemoglobin concentration of 1.5 g/dL may represent a percentage of 10% in an otherwise healthy patient with a baseline hemoglobin of 15 mg/dL, whereas the presence of the same concentration of 1.5 g/dL of methemoglobin in an anemic patient with a baseline hemoglobin of 8 g/dL would represent a percentage of 18.75%. The former patient will be left with a functional hemoglobin concentration of 13.5 g/dL and potentially remain asymptomatic while the latter patient with a functional hemoglobin concentration 6.5 g/dL may be severely symptomatic with a methemoglobin of less than 20%. * 1-2% Normal * Less than 10% metHb - No symptoms * 10-20% metHb - Skin discoloration only (most notably on mucous membranes) * 20-30% metHb - Anxiety,

headache Headache is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of depression in those with severe headaches. Headaches can occur as a result ...

, dyspnea on exertion * 30-50% metHb -

Fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...

, confusion, dizziness, tachypnea,

palpitations Palpitations are perceived abnormalities of the heartbeat characterized by awareness of cardiac muscle contractions in the chest, which is further characterized by the hard, fast and/or irregular beatings of the heart. Symptoms include a rapi ...

* 50-70% metHb -

Coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...

, seizures, arrhythmias, acidosis * Greater than 70% metHb - High risk of death This may be further compounded by the "functional hemoglobin's" decreased ability to release oxygen in the presence of methemoglobin. Anemia, congestive heart failure, chronic obstructive pulmonary disease, and essentially any pathology that impairs the ability to deliver oxygen may worsen the symptoms of methemoglobinemia.

Blood stains

Increased levels of methemoglobin are found in blood stains. Upon exiting the body, bloodstains transit from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO₂) to methemoglobin (met-Hb) and

hemichrome A hemichrome (FeIII) is a form of low-spin methemoglobin (metHb). Hemichromes, which precede the denaturation processes of hemoglobin (Hb), are mainly produced by partially denaturated hemoglobins and form histidine complexes. Hemichromes are usu ...

(HC).Bremmer et al PLoS ONE 2011 http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021845

References

External links

*
MetHb Formation

{{Globins Cellular respiration Hemoglobins Hemoproteins Respiratory physiology