Editing Retinol (section)

==History==
[[File:Frederick Gowland Hopkins nobel.jpg|thumb|left|Frederick Gowland Hopkins, 1929 Nobel Prize for Physiology or Medicine]]
[[File:George Wald nobel.jpg|thumb|right|George Wald, 1967 Nobel Prize for Physiology or Medicine]]
In 1912, [[Frederick Gowland Hopkins]] demonstrated that unknown accessory factors found in milk, other than [[carbohydrate]]s, [[protein]]s, and [[fat]]s were necessary for growth in rats. Hopkins received a Nobel Prize for this discovery in 1929.<ref name=Semba>{{cite journal | vauthors = Semba RD | title = On the 'discovery' of vitamin A | journal = Annals of Nutrition & Metabolism | volume = 61 | issue = 3 | pages = 192–198 | year = 2012 | pmid = 23183288 | doi = 10.1159/000343124 | s2cid = 27542506 }}</ref> One year later, [[Elmer McCollum]], a [[biochemist]] at the [[University of Wisconsin–Madison]], and colleague [[Marguerite Davis]] identified a fat-soluble nutrient in [[butterfat]] and [[cod liver oil]]. Their work confirmed that of [[Thomas Burr Osborne (chemist)|Thomas Burr Osborne]] and [[Lafayette Mendel]], at [[Yale University|Yale]], also in 1913, which suggested a fat-soluble nutrient in butterfat.<ref name=Semba2>{{cite journal | vauthors = Semba RD | title = Vitamin A as "anti-infective" therapy, 1920-1940 | journal = The Journal of Nutrition | volume = 129 | issue = 4 | pages = 783–791 | date = April 1999 | pmid = 10203551 | doi = 10.1093/jn/129.4.783 | doi-access = free }}</ref> The "accessory factors" were termed "fat-soluble" in 1918 and later "vitamin A" in 1920. In 1931, Swiss chemist [[Paul Karrer]] described the chemical structure of vitamin A.<ref name=Semba/> Retinoic acid and retinol were first synthesized in 1946 and 1947 by two Dutch chemists, [[David Adriaan van Dorp]] and Jozef Ferdinand Arens.<ref>{{cite journal |vauthors=Arens JF, Van Dorp DA |title=Synthesis of some compounds possessing vitamin A activity |journal=Nature |volume=157 |issue= 3981|pages=190–191 |date=February 1946 |pmid=21015124 |doi=10.1038/157190a0 |bibcode=1946Natur.157..190A |s2cid=27157783 |url=}}</ref><ref>{{cite journal |vauthors=Van Dorp DA, Arens JF |title=Synthesis of vitamin A aldehyde |journal=Nature |volume=159 |issue=4058 |pages=189 |date=August 1947 |pmid=20256189 |doi=10.1038/160189a0 |bibcode=1947Natur.160..189V |s2cid=4137483 |url=|doi-access=free }}</ref>


In 1967, [[George Wald]] was a co-recipient of the Nobel Prize in Physiology and Medicine "..."for their discoveries concerning the primary physiological and chemical visual processes in the eye."<ref name="nobel-1967">{{cite web|title=The Nobel Prize in Physiology or Medicine 1967 |url=http://www.nobelprize.org/nobel_prizes/medicine/laureates/1967/index.html|publisher=Nobel Foundation|access-date=28 July 2007|archive-url=https://web.archive.org/web/20131204095703/http://www.nobelprize.org/nobel_prizes/medicine/laureates/1967/index.html|archive-date=4 December 2013|url-status=live}}</ref> [[Photoreceptor cell]]s in the eye contain a [[chromophore]] composed of the protein [[opsin]] and [[11-cis retinal]]. When struck by light, 11-cis retinal undergoes photoisomerization to all-trans retinal and via signal transduction cascade sends a nerve signal to the brain. The all-trans retinal is reduced to all-trans retinol and travels back to the retinal pigment epithelium to be recycled to 11-cis retinal and conjugated to opsin.<ref>{{cite journal | vauthors = Ebrey T, Koutalos Y | title = Vertebrate photoreceptors | journal = Progress in Retinal and Eye Research | volume = 20 | issue = 1 | pages = 49–94 | date = January 2001 | pmid = 11070368 | doi = 10.1016/S1350-9462(00)00014-8 | s2cid = 2789591 }}</ref>

Although vitamin A was not confirmed as an essential nutrient and a chemical structure described until the 20th century, written observations of conditions created by deficiency of this nutrient appeared much earlier in history. Sommer classified historical accounts related to vitamin A and/or manifestations of deficiency as follows: "ancient" accounts; 18th- to 19th-century clinical descriptions (and their purported etiologic associations); early 20th-century laboratory animal experiments, and clinical and epidemiologic observations that identified the existence of this unique nutrient and manifestations of its deficiency.<ref name=Sommer/>