Affinity of hemoglobin for carbon monoxide Ĭompared 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. 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. Most endogenously produced carbon monoxide is stored as carboxyhemoglobin. Therefore heme can both carry carbon monoxide in the case of carboxyhemoglobin, or, undergo enzymatic catabolism to generate carbon monoxide.Ĭarbon monoxide was characterized as a neurotransmitter in 1993 and has since been subcategorized as a gasotransmitter. 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. In biology, carbon monoxide is naturally produced through many enzymatic and non-enzymatic pathways. 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. 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. The average red blood cell contains 250 million hemoglobin molecules. The FDA has previously set a threshold of 14% COHb in certain clinical trials evaluating the therapeutic potential of carbon monoxide. The biological threshold for carboxyhemoglobin tolerance is 15% COHb, meaning toxicity is consistently observed at levels in excess of this concentration. The average non-smoker maintains a systemic carboxyhemoglobin level under 3% COHb whereas smokers approach 10% COHb. 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. Carboxyhemoglobin is often mistaken for the compound formed by the combination of carbon dioxide ( carboxyl) and hemoglobin, which is actually carbaminohemoglobin. 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.
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