Names | |||
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Preferred IUPAC name Carbonylhemoglobin | |||
Other names Carboxyhemoglobin Carboxyhaemoglobin Kohlenoxyhaemoglobin Kohlenoxyhämoglobin Kohlenoxydhämoglobin Kohlenmonoxyhämoglobin Carbonmonoxyhemoglobin Carbon-monoxide-hemoglobin Carbon-monoxide-Methemoglobin Carbonic oxide hæmoglobin | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Carboxyhemoglobin (carboxyhaemoglobin BrE) (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 (carboxyl) and hemoglobin, which is actually carbaminohemoglobin. Carboxyhemoglobin terminology emerged when carbon monoxide was known by its historic name, "carbonic oxide", and evolved through Germanic and British English etymological influences; the preferred IUPAC nomenclature is carbonylhemoglobin. [2] [3] [4]
The average non-smoker maintains a systemic carboxyhemoglobin level under 3% COHb whereas smokers approach 10% COHb. [4] The biological threshold for carboxyhemoglobin tolerance is 15% COHb, meaning toxicity is consistently observed at levels in excess of this concentration. [5] The FDA has previously set a threshold of 14% COHb in certain clinical trials evaluating the therapeutic potential of carbon monoxide. [6]
The average red blood cell contains 250 million hemoglobin molecules. [7] 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.), [8] therefore a typical red blood cell may carry up to one billion gas molecules. As the binding of carbon monoxide with hemoglobin is reversible, certain models have estimated that 20% of the carbon monoxide carried as carboxyhemoglobin may dissociate in remote tissues. [7]
In biology, carbon monoxide is naturally produced through many enzymatic and non-enzymatic pathways. [7] 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. [4]
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. [4] [7]
Compared to oxygen, carbon monoxide binds with approximately 240 times greater affinity, [9] [4] 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. [4] [10] [11] 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. [5] Other human mutations have been described (see also: hemoglobin variants). [12] [13] Structural variations and mutations across other hemoproteins likewise affect carbon monoxide's interaction with the heme prosthetic group as exemplified by Cytochrome P450 where certain forms of the CYP3A family is relatively less affected by the inhibitory effects of carbon monoxide. [4]
Murinae species have a COHb half-life of 20 minutes compared to 300 minutes for a typical human (see § Toxicokinetics). [4] 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. [4]
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, which may provide benefits against hypoxia. [14] Similarly, the elevated levels in smokers has been suggested to be a basis for the smoker's paradox. [4] Prolonged exposure to carbon monoxide and elevated carboxyhemoglobin, such as in smoking, results in erythremia. [4] Furthermore, humans can acclimate to toxic levels of carbon monoxide based on findings reported by Esther Killick. [4]
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. [4] 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. [4]
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. [4]
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 act 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. [4]
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. [4] Later in 1854, Adrien Chenot similarly suggested carbon monoxide could remove oxygen from blood and be oxidized within the body to carbon dioxide. [4] 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". [4] 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. [4]
Carbon is derived from the Latin term carbo, meaning coal, via the French charbone, which first appeared in print in 1786. [15] The etymology of oxygen is generally accepted mean 'acid' based on Lavoisier's system, which also recognized carbon as a nonmetallic element capable of oxidation, although the original degrees of oxides were based on diamond, graphite, coal and carbonic acid (CO2) as the most oxidized form; [15] Lavoisier's system was superseded by other obsolete oxide nomenclature systems. [16]
Upon discovering carbon monoxide through a series of experiments originating from coke (short for coal-cake [15] ), Cruickshank named the new molecule "gaseous oxide of carbon" which evolved to "carbonic oxide" and was translated into German as "kohlenoxyd". Kohlen is the German word for coal. [4] [17] As carbonic acid (CO2) was considered to be the most highly oxidized form in Lavoisier's system, the name carbonic oxide implied an intermediate oxidized species between coal and carbonic acid (i.e. use of the word acid indicated maximum oxidation).
Haem is derived from Greek meaning blood, [18] [19] and globin is Latin derived from globus typically accepted to mean glob/spherical/round object; the terms are conjoined with an -o- . Regarding haem, the use of "ae / æ" remains prevalent in British English in modern day [20] whereas the American English spelling evolved to heme from hema. [19]
Felix Hoppe-Seyler coined the name "hämoglobin" in 1864. [21] In German, an umlaut such as ä is synonymous with spelling as "ae", therefore hämoglobin is commonly spelled as haemoglobin throughout German literature, hence haemoglobin is the term adopted by English literature.
Hoppe-Seyler likewise coined the name Kohlenoxydhämoglobin [22] which may have similarly been directly translated back into English as "carbonic oxide hæmoglobin". [23] The term carboxyhæmoglobin appeared as early as 1895 in works by John Haldane while the name for CO was still widely regarded as carbonic oxide. [24]
The term "carbon monoxide" was formally introduced in 1879, but the name would not become mainstream for several decades. [4] Variations of COHb terminology, such as carbonmonoxyhemoglobin, [25] [11] followed and eventually evolved and simplified back into "carboxyhemoglobin".
As carboxy is now firmly associated with the CO2 carboxyl group, and carbon monoxide is generally regarded as a carbonyl, IUPAC has recommended "carbonylhemoglobin" as the preferred COHb nomenclature. [4] Despite the IUPAC guidance, carboxyhemoglobin remains the most widely used term (akin to the survival of bicarbonate nomenclature).
Historically, carboxyhemoglobin detection has been achieved by colorimetric analysis, chemical reactivity, spectrophotometry, gasometric and thermoelectric detection methods. [4] Gas chromatography analysis emerged in 1961 and remains a commonly used method. [4]
Modern methods include pulse oximetry with a CO-oximeter, and a variety of other analytical techniques. [26] [27] 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. [28]
Carbon monoxide poisoning, also known as carboxyhemoglobinemia, [29] [30] has plagued humankind since primitive ancestors first harnessed fire. In modern times, carboxyhemoglobin data assist physicians in making a poisoning diagnosis. However, carboxyhemoglobin levels do not necessarily correlate with the symptoms of carbon monoxide poisoning. [31] In general, 30% COHb is considered severe carbon monoxide poisoning. [4] The highest reported non-fatal carboxyhemoglobin level was 73% COHb. [4]
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), [32] 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). [33] In absence of oxygen, cells switch to anaerobic respiration which if prolonged may significantly increase lactic acid leading to metabolic acidosis. [34]
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 cellular region, oxygen is released at the tissue due to a conformational change in hemoglobin as caused by ionization of hemoglobin's surface due to the "acidification" of the tissue's local pH (meaning a relatively higher concentration of 'acidic' protons / hydrogen ions annotated as H+; an acidic pH is commonly referenced to as either low pH based on the acidity of pH 1-7 having a low number, or, referred to as a high pH due to the high concentration of H+ ions as the scale approaches pH 1); the local acidity is 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 hemoglobin's amines transitioning between the deprotonated/unionized Hb-NH2 to the protonated/ionized Hb-NH3+ state) based on the less-acidic pH (arterial blood averages pH 7.407 whereas venous blood is slightly more acidic at pH 7.371 [35] ). 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 [36] (i.e. oxygen is 'ejected' upon arrival at the cell due to H+ ions bombarding the hemoglobin surface residues to convert Hb from "R-state" to "T-state"). Furthermore, the mechanism for formation of carbaminohemoglobin generates additional H+ 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, pH buffers and other physiochemical 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, whereas deoxygenated hemoglobin is dark red and carbaminohemoglobin has a blue hue. [13]
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. [37] 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 (such as inhibition of cytochrome c oxidase). [7] [38]
In common air under normal atmospheric conditions, a typical patient's carboxyhemoglobin has a half-life around 300 minutes. [4] 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. [4] Additionally, treatment in a hyperbaric chamber is a more effective manner of reducing the half-life of carboxyhemoglobin to 30 minutes [4] and allows oxygen to dissolve in biological fluids for delivery to tissues.[ citation needed ]
Supplemental oxygen takes advantage of Le Chatelier's principle to quicken the decomposition of carboxyhemoglobin back to hemoglobin: [39]
As carbon monoxide is now understood to have a therapeutic potential, pharmaceutical efforts have focused on development of carbon monoxide-releasing molecules and selective heme oxygenase inducers. [40]
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. [41]
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells.
Carbon monoxide is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simplest carbon oxide. In coordination complexes, the carbon monoxide ligand is called carbonyl. It is a key ingredient in many processes in industrial chemistry.
Hemoglobin is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin in the blood carries oxygen from the respiratory organs to the other tissues of the body, where it releases the oxygen to enable aerobic respiration which powers an animal's metabolism. A healthy human has 12 to 20 grams of hemoglobin in every 100 mL of blood. Hemoglobin is a metalloprotein, a chromoprotein, and globulin.
Red blood cells (RBCs), referred to as erythrocytes in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cell and the vertebrate's principal means of delivering oxygen to the body tissues—via blood flow through the circulatory system. Erythrocytes take up oxygen in the lungs, or in fish the gills, and release it into tissues while squeezing through the body's capillaries.
Myoglobin is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglobin has a higher affinity for oxygen and does not have cooperative binding with oxygen like hemoglobin does. Myoglobin consists of non-polar amino acids at the core of the globulin, where the heme group is non-covalently bounded with the surrounding polypeptide of myoglobin. In humans, myoglobin is found in the bloodstream only after muscle injury.
Heme, or haem, is a ring-shaped iron-containing molecular component of hemoglobin, which is necessary to bind oxygen in the bloodstream. It is composed of four pyrrole rings with 2 vinyl and 2 propionic acid side chains. Heme is biosynthesized in both the bone marrow and the liver.
An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe and a thin needle, but sometimes the femoral artery in the groin or another site is used. The blood can also be drawn from an arterial catheter.
Carbon monoxide poisoning typically occurs from breathing in carbon monoxide (CO) at excessive levels. Symptoms are often described as "flu-like" and commonly include headache, dizziness, weakness, vomiting, chest pain, and confusion. Large exposures can result in loss of consciousness, arrhythmias, seizures, or death. The classically described "cherry red skin" rarely occurs. Long-term complications may include chronic fatigue, trouble with memory, and movement problems.
The Bohr effect is a phenomenon first described in 1904 by the Danish physiologist Christian Bohr. Hemoglobin's oxygen binding affinity (see oxygen–haemoglobin dissociation curve) is inversely related both to acidity and to the concentration of carbon dioxide. That is, the Bohr effect refers to the shift in the oxygen dissociation curve caused by changes in the concentration of carbon dioxide or the pH of the environment. Since carbon dioxide reacts with water to form carbonic acid, an increase in CO2 results in a decrease in blood pH, resulting in hemoglobin proteins releasing their load of oxygen. Conversely, a decrease in carbon dioxide provokes an increase in pH, which results in hemoglobin picking up more oxygen.
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), as well as the dyshemoglobins such as carboxyhemoglobin (COHb) and methemoglobin (MetHb). The use of 'CO' rather than 'Co' or 'co' is more appropriate since this designation represents a device that measures carbon monoxide (CO) bound to hemoglobin, as distinguished from simple oximetry which measures hemoglobin bound to molecular oxygen—O2Hb—or hemoglobin capable of binding to molecular oxygen—HHb. Simpler oximeters may report oxygen saturation alone, i.e. the ratio of oxyhemoglobin to total 'bindable' hemoglobin. CO-oximetry is useful in defining the causes for hypoxemia, or hypoxia,.
The oxygen–hemoglobin dissociation curve, also called the oxyhemoglobin dissociation curve or oxygen dissociation curve (ODC), is a curve that plots the proportion of hemoglobin in its saturated (oxygen-laden) form on the vertical axis against the prevailing oxygen tension on the horizontal axis. This curve is an important tool for understanding how our blood carries and releases oxygen. Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.
Heme oxygenase, or haem oxygenase, is an enzyme that catalyzes the degradation of heme to produce biliverdin, ferrous iron, and carbon monoxide.
Gasotransmitters is a class of neurotransmitters. The molecules are distinguished from other bioactive endogenous gaseous signaling molecules based on a need to meet distinct characterization criteria. Currently, only nitric oxide, carbon monoxide, and hydrogen sulfide are accepted as gasotransmitters. According to in vitro models, gasotransmitters, like other gaseous signaling molecules, may bind to gasoreceptors and trigger signaling in the cells.
Carbaminohemoglobin (carbaminohaemoglobin BrE) (CO2Hb, also known as carbhemoglobin and carbohemoglobin) is a compound of hemoglobin and carbon dioxide, and is one of the forms in which carbon dioxide exists in the blood. Twenty-three percent of carbon dioxide is carried in blood this way (70% is converted into bicarbonate by carbonic anhydrase and then carried in plasma, 7% carried as free CO2, dissolved in plasma).
The Haldane effect is a property of hemoglobin first described by John Scott Haldane, within which oxygenation of blood in the lungs displaces carbon dioxide from hemoglobin, increasing the removal of carbon dioxide. Consequently, oxygenated blood has a reduced affinity for carbon dioxide. Thus, the Haldane effect describes the ability of hemoglobin to carry increased amounts of carbon dioxide (CO2) in the deoxygenated state as opposed to the oxygenated state. Vice versa, it is true that a high concentration of CO2 facilitates dissociation of oxyhemoglobin, though this is the result of two distinct processes (Bohr effect and Margaria-Green effect) and should be distinguished from Haldane effect.
Water gas is a kind of fuel gas, a mixture of carbon monoxide and hydrogen. It is produced by "alternately hot blowing a fuel layer [coke] with air and gasifying it with steam". The caloric yield of the fuel produced by this method is about 10% of the yield from a modern syngas plant. The coke needed to produce water gas also costs significantly more than the precursors for syngas, making water gas technology an even less attractive business proposition.
Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monoxide tension (PxCO). The subscript x in each symbol represents the source of the gas being measured: "a" meaning arterial, "A" being alveolar, "v" being venous, and "c" being capillary. Blood gas tests (such as arterial blood gas tests) measure these partial pressures.
Carbon monoxide-releasing molecules (CORMs) are chemical compounds designed to release controlled amounts of carbon monoxide (CO). CORMs are being developed as potential therapeutic agents to locally deliver CO to cells and tissues, thus overcoming limitations of CO gas inhalation protocols.
Gaseous signaling molecules are gaseous molecules that are either synthesized internally (endogenously) in the organism, tissue or cell or are received by the organism, tissue or cell from outside and that are used to transmit chemical signals which induce certain physiological or biochemical changes in the organism, tissue or cell. The term is applied to, for example, oxygen, carbon dioxide, sulfur dioxide, nitrous oxide, hydrogen cyanide, ammonia, methane, hydrogen, ethylene, etc.
Esther Margaret Killick was an English physiologist who was a professor of physiology at the London School of Medicine for Women from 1941 until her death in 1960. Her main research interests lay in respiratory physiology and carbon monoxide poisoning.