Editing Intrinsic factor (section)

{{Short description|Glycoprotein produced in the stomach which binds to vitamin B12}}
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'''Intrinsic factor''' ('''IF'''), also known as  '''cobalamin binding intrinsic factor''',<ref name="uniprot" /> or '''gastric intrinsic factor''' ('''GIF'''), is a [[glycoprotein]] produced by the [[parietal cell]]s (in humans) or chief cells (in rodents) of the [[stomach]]. It is necessary for the absorption of [[vitamin B12|vitamin B<sub>12</sub>]] later on in the distal [[ileum]] of the [[small intestine]].<ref name="Pocock_2006">{{cite book | vauthors = Pocock G, Richards C | title = Human Physiology: The Basis of Medicine | isbn = 978-019-856878-0 | pages = 230 | date = 2006 | publisher = Oxford University Press | edition = 3rd }}</ref> In humans, the gastric intrinsic factor [[protein]] is encoded by the ''CBLIF'' [[gene]].<ref name="uniprot">{{cite web | title = CBLIF - Cobalamin binding intrinsic factor precursor - Homo sapiens (Human) - CBLIF gene & protein | url = https://www.uniprot.org/uniprot/P27352 | website = www.uniprot.org | access-date = 15 March 2022 | language = en }}</ref> [[Haptocorrin]] ([[Transcobalamin|transcobalamin I]]) is another glycoprotein secreted by the [[salivary gland]]s which binds to  vitamin B<sub>12</sub>. Vitamin B<sub>12</sub> is acid-sensitive and in binding to haptocorrin it can safely pass through the acidic stomach to the duodenum.<ref name="Fedosov_2012" />

In the less acidic environment of the [[small intestine]], pancreatic enzymes digest the glycoprotein carrier and vitamin B<sub>12</sub> can then bind to intrinsic factor.<ref name="Fedosov_2012" /> This new complex is then absorbed by the epithelial cells ([[enterocyte]]s) of the [[ileum]].<ref name="Fedosov_2012">{{cite book | vauthors = Fedosov SN | chapter = Physiological and Molecular Aspects of Cobalamin Transport | title = Water Soluble Vitamins | volume = 56 | pages = 347–367 | year = 2012 | pmid = 22116708 | doi = 10.1007/978-94-007-2199-9_18 | department = (review) | series = Subcellular Biochemistry | isbn = 978-94-007-2198-2 }}</ref> Inside the cells, vitamin B<sub>12</sub> dissociates once again and binds to another protein, [[transcobalamin|transcobalamin II]]; the new complex can then exit the epithelial cells to be carried to the liver.<ref name="Alpers_2013">{{cite journal | vauthors = Alpers DH, Russell-Jones G | title = Gastric intrinsic factor: the gastric and small intestinal stages of cobalamin absorption. A personal journey | journal = Biochimie | volume = 95 | issue = 5 | pages = 989–994 | date = May 2013 | pmid = 23274574 | doi = 10.1016/j.biochi.2012.12.006 | department = (review) }}</ref>