Carboxyhemoglobin
Encyclopedia
Carboxyhemoglobin (COHb) is a stable complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...

 of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 and hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 that forms in red blood cell
Blood cell
A blood cell, also called a hematocyte, is a cell normally found in blood. In mammals, these fall into three general categories:* red blood cells — Erythrocytes* white blood cells — Leukocytes* platelets — Thrombocytes...

s when carbon monoxide is inhaled or produced in normal metabolism. Large quantities of it hinder delivery of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 to the body. Tobacco smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...

 (through carbon monoxide inhalation) raises the blood levels of COHb by a factor of several times from its normal concentrations. It is bright, cherry red.

Carbon monoxide as poison

Hemoglobin binds to carbon monoxide preferentially compared to oxygen (approx 240:1), so effectively, COHb will not release the carbon monoxide, and therefore hemoglobin will not be available to transport oxygen from the lungs to the rest of the body.

In large quantities, the effect of COHb is death - known medically as carboxyhemoglobinemia or carbon monoxide poisoning
Carbon monoxide poisoning
Carbon monoxide poisoning occurs after enough inhalation of carbon monoxide . Carbon monoxide is a toxic gas, but, being colorless, odorless, tasteless, and initially non-irritating, it is very difficult for people to detect...

. However in smaller quantities COHb leads to oxygen deprivation of the body causing tiredness, dizziness and unconsciousness.

COHb has a half-life in the blood of 4 to 6 hours, but in cases of poisoning, this can be reduced to 70 to 35 minutes with administration of pure oxygen (the lower number applying when oxygen is administered with 4 to 5% CO2 to cause hyperventiliation).

COHb increases risk of blood clot. It is thought that through this mechanism smoking increases the risk of having an ischemic stroke.

Pregnant smokers may give birth to babies of a lower birth mass. One theory is that, since fetal hemoglobin takes up carbon monoxide more readily than in an adult, therefore the fetus of a smoker will suffer from mild hypoxia, potentially retarding its development.

Carbon monoxide as normal biological metabolite

In biology, carbon monoxide is naturally produced by the action of heme oxygenase
Heme oxygenase
This reaction can occur in virtually every cell; the classic example is the formation of a bruise, which goes through different colors as it gradually heals: red heme to green biliverdin to yellow bilirubin...

 1 and 2 on the heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...

 from hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 breakdown. This process produces a certain amount of carboxyhemoglobin in normal persons, even if they do not breathe any carbon monoxide.

Following the first report that carbon monoxide is a normal neurotransmitter in 1993, as well as one of three gasses which naturally modulate inflammatory responses in the body (the other two being nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

 and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

), carbon monoxide has received a great deal of clinical attention as a biological regulator. In many tissues, all three gases are known to act as anti-inflammatories, vasodilators and encouragers of neovascular growth. However, the issues are complex, as neovascular growth is not always beneficial, since it plays a role in tumor growth, and also the damage from wet macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...

, a disease for which smoking (a major source of carbon monoxide in the blood, several times more than natural production) increases the risk from 4 to 6 times.

Carbon monoxide as beneficially active substance, and potential pharmaceutical

Studies involving carbon monoxide have been conducted in many laboratories throughout the world for its anti-inflammatory and cytoprotective properties. These properties could potentially be used to prevent the development of a series of pathological conditions including ischemia reperfusion injury, transplant rejection, atherosclerosis, severe sepsis, severe malaria or autoimmunity. Clinical tests involving humans have been performed, however the results have not yet been released.
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