Carboxyhemoglobin

Carboxyhemoglobin, or carboxyhaemoglobin, (symbol COHb or HbCO) is a stable complex of carbon monoxide and hemoglobin (Hb) that forms in red blood cells upon contact with carbon monoxide. Carboxyhemoglobin is often mistaken for the compound formed by the combination of carbon dioxide and hemoglobin, which is actually carbaminohemoglobin. Carboxyhemoglobin terminology emerged when carbon monoxide was known by its ancient name carbonic oxide; the preferred IUPAC nomenclature is carbonylhemoglobin.
The average non-smoker maintains a systemic carboxyhemoglobin level under 3% COHb whereas smokers approach 10% COHb. The biological threshold for carboxyhemoglobin tolerance is 15% COHb, meaning toxicity is consistently observed at levels in excess of this concentration. The FDA has previously set a threshold of 14% COHb in certain clinical trials evaluating the therapeutic potential of carbon monoxide.
Endogenous carbon monoxide production.
In biology, carbon monoxide is naturally produced through many enzymatic and non-enzymatic pathways. The most extensively studied pathway is the metabolism of heme by heme oxygenase which occurs throughout the body with significant activity in the spleen to facilitate hemoglobin breakdown during erythrocyte recycling. Therefore heme can both carry carbon monoxide in the case of carboxyhemoglobin, or, undergo enzymatic catabolism to generate carbon monoxide.
Carbon monoxide was characterized as a neurotransmitter in 1993 and has since been subcategorized as a gasotransmitter.
Most endogenously produced carbon monoxide is stored as carboxyhemoglobin. The gas primarily undergoes pulmonary excretion, however trace amounts may be oxidized to carbon dioxide by certain cytochromes, metabolized by resident microbiota, or excreted by transdermal diffusion.
Affinity of hemoglobin for carbon monoxide.
The average red blood cell contains 250 million hemoglobin molecules. Hemoglobin contains a globin protein unit with four prosthetic heme groups (hence the name heme-o-globin); each heme is capable of reversibly binding with one gaseous molecule (oxygen, carbon monoxide, cyanide, etc.), therefore a typical red blood cell may carry up to one billion gas molecules. As the binding of carbon monoxide with hemoglobin is reversible, it is estimated that 20% of the carbon monoxide carried as carboxyhemoglobin may dissociate in remote tissues.
Compared to oxygen, carbon monoxide binds with approximately 240 times greater affinity, however the affinity of carbon monoxide for hemoglobin varies both across species and within a species. In the 1950s, Esther Killick was among the first to recognize a difference in carbon monoxide affinity between adult and foetal blood, and a difference between humans and sheep. Murinae species have a COHb half-life of 20 minutes compared to 300 minutes for a typical human (see toxicokinetics below). As a result, the metabolic kinetics, blood saturation point, and tolerance for carbon monoxide exposure vary across species, potentially leading to data inconsistencies pertaining to the toxicology of carbon monoxide poisoning and pharmacology of low-dose therapeutic protocols. 
In humans, the Hb-Kirklareli mutation has a relative 80,000 times greater affinity for carbon monoxide than oxygen resulting in systemic carboxyhemoglobin reaching a sustained level of 16% COHb. Some deep-diving marine mammal species are known to contain concentrations of carbon monoxide in their blood that resembles levels seen in chronic cigarette smokers. It is believed these levels of carbon monoxide could help the animals with the prevention of injuries associated ischemia/reperfusion events associated with the physiological dive response, and similarly the elevated levels in smokers has been suggested to be a basis for the smoker's paradox. Prolonged exposure to carbon monoxide and elevated carboxyhemoglobin, such as in smoking, results in erythremia. Furthermore, humans can acclimate to toxic levels of carbon monoxide based on findings reported by Esther Killick. 
Structural variations and mutations across other hemoproteins likewise affects carbon monoxide's interaction with the heme prosthetic group as exemplified by Cytochrome P450 where the CYP3A family is relatively less affected by the inhibitory effects of carbon monoxide.
History.
A bright red skin complexion is commonly associated with elevated carboxyhemoglobin levels. Trace evidence for an endogenous presence of carbon monoxide dates back to Marcellus Donato circa 1570 who noted an unusually red complexion upon conducting an autopsy of victims who died from charcoal fumes in Mantua. Similar findings pertaining to red complexion later emerged as documented by Johann Jakob Wepfer in the 1600s, and M. Antoine Portal in the late 1700s.
Phlogiston theory is a trace origin for the first chemical explanations of endogenous carboxyhemoglobin exemplified by the work of Joseph Priestley in the eighteenth century who suspected phlogiston to be a cellular waste product carried by the blood of animals which was subsequently exhaled. 
Thomas Beddoes, James Watt, Humphry Davy, James Lind, and many others investigated the therapeutic potential of inhaling factitious airs in the late eighteenth century (see also: Pneumatic Institution). Among the gases experimented with, hydrocarbonate had received significant attention. Hydrocarbonate is water gas generated by passing steam over coke, the process of which generates carbon monoxide and hydrogen, and some considered it contain phlogiston. Beddoes and Watt recognized hydrocarbonate brightened venous blood in 1793. Watt suggested coal fumes could at as an antidote to the oxygen in blood, and Beddoes and Watt likewise speculated hydrocarbonate has a greater affinity for animal fiber than oxygen in 1796.
After the discovery of carbon monoxide by William Cruickshank in 1800, Johann Dömling (1803) and John Bostock (1804) developed hypotheses suggesting blood returned to the heart loaded with carbon monoxide to subsequently be oxidized to carbon dioxide in the lung prior to exhalation. Later in 1854, Adrien Chenot similarly suggested carbon monoxide could remove oxygen from blood and be oxidized within the body to carbon dioxide. The mechanism for carbon monoxide poisoning in the context of carboxyhemoglobin formation is widely credited to Claude Bernard whose memoirs beginning in 1846 and published in 1857 notably phrased, "prevents arterials blood from becoming venous". Felix Hoppe-Seyler independently published similar conclusions in the following year.
The first analytical method to detect carboxyhemoglobin emerged in 1858 with a colorimetric method developed by Felix Hoppe-Seyler, and the first quantitative analysis method emerged in 1880 with Josef von Fodor.
Analytical detection methods.
Historically, carboxyhemoglobin detection has been achieved by colorimetric analysis, chemical reactivity, spectrophotometry, gasometric and thermoelectric detection methods. Gas chromatography analysis emerged in 1961 and remains a commonly used method.
Modern methods include pulse oximetry with a CO-oximeter, and a variety of other analytical techniques. Most methods require laboratory equipment, skilled technicians, or expensive electronics therefore rapid and economical detection technologies remain in development.
Breath carbon monoxide is another detection method that may correlate with carboxyhemoglobin levels.
Carbon monoxide poisoning.
Carbon monoxide poisoning, also known as carboxyhemoglobinemia, has plagued humankind since primitive ancestors first harnessed fire. In modern times, carboxyhemoglobin data assists physicians in making a poisoning diagnosis. However, carboxyhemoglobin levels do not necessarily correlate with the symptoms of carbon monoxide poisoning. In general, 30% COHb is considered severe carbon monoxide poisoning. The highest reported non-fatal carboxyhemoglobin level was 73% COHb.
Respiration.
Gas exchange is an essential process for many organisms to maintain homeostasis. Oxygen accounts for about 20% of Earth's atmospheric air. While inhaling air is critical to supply cells with oxygen for aerobic respiration via the Bohr effect and Haldane effect (and perhaps local low oxygen partial pressure e.g. active muscles), exhaling the cellular waste product carbon dioxide is arguably the more critical aspect of respiration. Whereas the body can tolerate brief periods of hypoxia (as commonly occurs in anaerobic exercise, although the brain, heart, liver and kidney are significantly less tolerant than skeletal muscle), failure to expel carbon dioxide may cause respiratory acidosis (meaning bodily fluids and blood become too acidic thereby affecting homeostasis). In absence of oxygen, cells switch to anaerobic respiration which if prolonged may significantly increase lactic acid leading to metabolic acidosis.
To provide a simplified synopsis of the molecular mechanism of systemic gas exchange, upon inhalation of air it was widely thought oxygen binding to any of the heme sites triggers a conformational change in the protein unit of hemoglobin which then enables the binding of additional oxygen to each of the other heme sites. Upon arrival to the cell, oxygen is released into the tissue due to "acidification" of the local pH (meaning a relatively higher concentration of ‘acidic’ protons / hydrogen ions) caused by an increase in the biotransformation of carbon dioxide waste into carbonic acid via carbonic anhydrase. In other words, oxygenated arterial blood arrives to cells in the "hemoglobin R-state" which has deprotonated/unionized amino acid residues (regarding amines) based on the less-acidic pH (arterial blood averages pH 7.407 whereas venous blood is slightly more acidic at pH 7.371). The "T-state" of hemoglobin is deoxygenated in venous blood partially due to protonation/ionization as caused by the acidic environment hence causing a conformation unsuited for oxygen-binding (i.e. oxygen is 'ejected' upon arrival in the cell). Furthermore, the mechanism for formation of carbaminohemoglobin generates additional hydrogen ions that may further stabilize the protonated/ionized deoxygenated hemoglobin. Upon return of venous blood into the lung and subsequent exhalation of carbon dioxide, the blood is "de-acidified" (see also: hyperventilation) for the deprotonation/unionization of hemoglobin to re-enable oxygen binding as part of the transition to arterial blood (note this process is complex due to involvement of chemoreceptors and other physiological functionalities). Carbon monoxide poisoning disturbs this physiological process hence the venous blood of poisoning patients is bright red akin to arterial blood since the carbonyl/carbon monoxide is retained. 
At toxic concentrations, carbon monoxide as carboxyhemoglobin significantly interferes with respiration and gas exchange by simultaneously inhibiting acquisition and delivery of oxygen to cells, and preventing formation of carbaminohemoglobin which accounts for approximately 30% of carbon dioxide exportation. Therefore a patient suffering from carbon monoxide poisoning may experience severe hypoxia and acidosis in addition to the toxicities of excess carbon monoxide binding to numerous hemoproteins, metallic and non-metallic targets which affect cellular machinery.
Toxicokinetics.
In common air under normal atmospheric conditions, in a typical patient's carboxyhemoglobin has a half-life around 300 minutes. This time can be reduced to 90 minutes upon administration of high-flow pure oxygen, and the time is further reduced when oxygen is administered with 5% carbon dioxide as first identified by Esther Killick. Additionally, treatment in a hyperbaric chamber is a more effective manner of reducing the half-life of carboxyhemoglobin to 30 minutes and allows oxygen to dissolve in biological fluids for delivery to tissues.
Supplemental oxygen takes advantage of Le Chatelier's principle to quicken the decomposition of carboxyhemoglobin back to hemoglobin:
HbCO + O2 ⇌ Hb + CO + O2 ⇌ HbO2 + CO
Carboxyhemoglobin pharmaceuticals.
As carbon monoxide is now understood have a therapeutic potential, pharmaceutical efforts have focused on development of carbon monoxide-releasing molecules and selective heme oxygenase inducers.
An alternative method for drug delivery consists of carbon monoxide immobilized on polyethylene glycol (PEG)-lyated bovine carboxyhemoglobin which is currently in late clinical development. Similarly, maleimide PEG conjugated human carboxyhemoglobin had previously been the subject of pharmaceutical development.