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A heme unit of human carboxyhaemoglobin, showing the carbonyl ligand at the apical position, trans to the histidine residue.^[1]

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 (CO). Exposure to small concentrations of CO hinder the ability of Hb to deliver oxygen to the body, because carboxyhemoglobin forms more readily than does oxyhemoglobin (HbO₂). CO is produced in normal metabolism and is also a common chemical. Tobacco smoking (through carbon monoxide inhalation) raises the blood levels of COHb by a factor of several times from its normal concentrations.

Affinity of Hb for CO

Hemoglobin contains four heme groups each capable of reversibly binding to one oxygen molecule.^[2] Oxygen binding to any of these sites causes a conformational change in the protein, facilitating binding to each of the other sites. Carbon monoxide binds to hemoglobin at the same sites as oxygen, but approximately 200 times more tightly.^[3] Normally, oxygen would bind to hemoglobin in the lungs and be released in areas with low oxygen partial pressure (e.g. active muscles).^[4] When carbon monoxide binds to hemoglobin, it cannot be released as easily as oxygen. The slow release rate of carbon monoxide causes an accumulation of CO-bound hemoglobin molecules as exposure to carbon monoxide continues. Because of this, fewer hemoglobin particles are available to bind and deliver oxygen, thus causing the gradual suffocation associated with carbon monoxide poisoning.

Carbon monoxide as a poison

Since COHb releases carbon monoxide slowly, less hemoglobin will be available to transport oxygen from the lungs to the rest of the body. Conversion of most Hb to COHb results in death - known medically as carboxyhemoglobinemia^[5] or carbon monoxide poisoning.^[6] Smaller amounts COHb lead to oxygen deprivation of the body causing tiredness, dizziness, and unconsciousness.

COHb has a half-life in the blood of 4 to 6 hours. This time 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 hyperventilation). Additionally, treatment in a Hyperbaric Chamber is a more effective manner of reducing the half-life of COHb than administering oxygen alone. This treatment involves pressurizing the chamber with pure oxygen at an absolute pressure close to three atmospheres allowing the body's fluids to absorb oxygen and to pass free oxygen on to hypoxic tissues instead of the crippled hemoglobin bonded to CO. In effect, bypassing the need for hemoglobin in the blood.

COHb increases risk of blood clot. It is thought that through this mechanism, smoking increases the risk of having thromboembolic disease.

Pregnant smokers may give birth to babies of a lower birth mass. In addition to vasoconstriction of the placenta, another 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 and potential pharmaceutical

In biology, carbon monoxide is naturally produced by the action of heme oxygenase 1 and 2 on the heme from hemoglobin 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,^[7]^[8] as well as one of three gasses which naturally modulate inflammatory responses in the body (the other two being nitric oxide and hydrogen sulfide), 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.^[9] 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, 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.

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 ischemic reperfusion injury, transplant rejection, atherosclerosis, severe sepsis, severe malaria or autoimmunity. Clinical tests involving humans have been performed. As of 2009^[update], however, the results had not yet been released.^[10]

References

^ Gregory B. Vásquez; Xinhua Ji; Clara Fronticelli; Gary L. Gilliland (1998). "Human Carboxyhemoglobin at 2.2 Å Resolution: Structure and Solvent Comparisons of R-State, R2-State and T-State Hemoglobins". Acta Crystallogr. D 54 (3): 355–366. doi:10.1107/S0907444997012250.
^ Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.
^ Berg, Jeremy. Biochemistry. 7. W.H. Freeman Company, 2011.
^ Schmidt-Nielsen K. 1997. Animal Physiology, fifth ed. Cambridge University Press, Cambridge, UK.
^ López-Herce J, Borrego R, Bustinza A, Carrillo A (September 2005). "Elevated carboxyhemoglobin associated with sodium nitroprusside treatment". Intensive Care Med 31 (9): 1235–8. doi:10.1007/s00134-005-2718-x. PMID 16041521.
^ Roth D, Hubmann N, Havel C, Herkner H, Schreiber W, Laggner A (July 2009). "Victim of Carbon Monoxide Poisoning Identified by Carbon Monoxide Oximetry". J Emerg Med 40 (6): 640–2. doi:10.1016/j.jemermed.2009.05.017. PMID 19615844.
^ New York Times article. Accessed May 2, 2010
^ Verma, A; Hirsch DJ; Glatt CE; Ronnett GV; Snyder SH (1993). "Carbon monoxide: A putative neural messenger". Science 259: 381–384. doi:10.1126/science.7678352. PMID 7678352.
^ Li, L; Hsu, A; Moore, PK (Sep 2009). "Actions and interactions of nitric oxide, carbon monoxide and hydrogen sulphide in the cardiovascular system and in inflammation--a tale of three gases!". Pharmacol Ther. 123 (3): 386–400. doi:10.1016/j.pharmthera.2009.05.005. PMID 19486912.
^ Johnson, Carolyn Y. (October 16, 2009). "Poison gas may carry a medical benefit". The Boston Globe. Retrieved October 16, 2009.

External links

Carboxyhemoglobin at the US National Library of Medicine Medical Subject Headings (MeSH)

UpToDate Contents

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1. 一酸化炭素中毒 carbon monoxide poisoning
2. 煙の吸入 smoke inhalation
3. 動脈血液ガス arterial blood gases
4. 一酸化炭素拡散能 diffusing capacity for carbon monoxide
5. 赤血球増加症患者に対する診断的アプローチ diagnostic approach to the patient with polycythemia

English Journal

Use of postmortem computed tomography to reveal an intraoral gunshot injuries in a charred body.

Sano R, Hirawasa S, Kobayashi S, Shimada T, Awata S, Takei H, Otake H, Takahashi K, Takahashi Y, Kominato Y.SourceDepartment of Legal Medicine, Gunma University, Graduate School of Medicine, Maebashi 371-8511, Japan.
Legal medicine (Tokyo, Japan).Leg Med (Tokyo).2011 Nov;13(6):286-8. Epub 2011 Oct 7.
A 53-year-old man was found dead after a fire at his residence had been extinguished. Although a pistol was recovered beside the body, external examination was unable to indicate any gunshot wound because of severe charring of the body. Postmortem computed tomography (CT) scan performed prior to aut
PMID 21982878

Quantitative evaluation of volatile hydrocarbons in post-mortem blood in forensic autopsy cases of fire-related deaths.

Yonemitsu K, Sasao A, Oshima T, Mimasaka S, Ohtsu Y, Nishitani Y.SourceDepartment of Forensic Medicine, Graduate School of Medical Sciences, Kumamoto University, Kumamoto 860-8556, Japan.
Forensic science international.Forensic Sci Int.2011 Oct 21. [Epub ahead of print]
Volatile hydrocarbons in post-mortem blood from victims of fires were analyzed quantitatively by headspace gas chromatography mass spectrometry. The benzene and styrene concentrations in the blood were positively correlated with the carboxyhemoglobin (CO-Hb) concentration, which is evidence that the
PMID 22019392

Japanese Journal

臨床研究・症例報告カルボキシヘモグロビン測定により早期診断し得た遺伝性球状赤血球症の新生児2例

小児科臨床 68(5), 1022-1026, 2015-05
NAID 40020429593

The Use of Cerebral Oximetry in Acute Carbon MonoxideIntoxication

The Keio Journal of Medicine 64(4), 57-61, 2015
NAID 130005115291

The Use of Cerebral Oximetry in Acute Carbon MonoxideIntoxication: A Preliminary Study

The Keio Journal of Medicine advpub(0), 2015
NAID 130005113581

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