Retinol is part of the vitamin A family, and is known as vitamin A1, and in a dietary context it is a type of preformed vitamin A. As with other preformed vitamin A's, it can be obtained from animal sources, with the highest concentrations coming from animal liver, with other sources being fish and dairy products. Other forms of vitamin A include retinal, its aldehyde form, retinoic acid, its acid form, and reinyl ester, its ester form. Additionally, herbivores and omnivores can obtain provitamin A from things such as alpha-, beta- and gamma-carotene, which can be converted to retinol as needed by the body. Retinol can be used in the body to form retinyl ester via diacylglycerol O-acyltransferase 1 and acyl-CoA wax akcohol acyltransferase 1 which both use acetyl-CoA as a reactant and produce CoA in addition to the retinyl ester. IT can also be produced by lecithin retinol acyltransferase, which uses a phosphatidylcholine molecule, and produces glycerophosphocholine. All of these reactions take place in the endoplasmic reticulum. Retinyl ester can also be converted back to retinol by patatin-like phospholipase domain-containing protein 4 as the enzyme in a reaction that also converts a diacylglycerol to a triacylglycerol. Alternately, retinyl ester can interact with retinoid isomerohydrolase to form 11-cis-retinol. 11-cis-retinol can be converted to retinyl palmitate by either diacylglycerol O-acyltransferase 1 or acyl-CoA wax alcohol acyltransferase 1 in the endoplasmic reticulum, which both add the acetyl group onto 11-cis-retinol, forming CoA as a side product. Alternatively, retinyl palmitate can be formed by lecithin retinol acyltransferase, which takes a molecule of phosphatidylcholine, and produces glycerophosphocholine in addition to the retinyl palmitate. Rhodopsin, a photosensitive protein found in the retina, can be converted to bathorhodopsin, which has previously been known as prelumirhodopsin. This conversion is caused by the absorption of light into the retinal portion of the protein complex, which then isomerizes, forcing the protein to change shape to accomodate this. Bathorhodopsin almost immediately converts to lumirhodopsin, which then converts to metarhodopsin, and at this point, the retinal is in its all-trans configuration. All-trans retinal can also be formed from 11-cis-retinaldehyde, also known as 11-cis-retinal, via dehydrogenase/reductase SDR family member 4 or retinol dehydrogenase 12 in the cell, as well as retinol dehydrogenases 8 and 16, short-chain dehydrogenase/reductase 3 or dehydrogenase/reductase SRD family member 9 in the endoplasmic reticulum. Two molecules of retinal can also be formed from beta-carotene, after its interaction with betabeta-carotene 15,15'-monooxygenase, or from retinol via retinol dehydrogenase 11 in the endoplasmic reticulum. Additionally, 11-cis-retinaldehyde can reversibly form all-trans retinal via interaction with alcohol dehydrogenase 1A. 11-cis-retinaldehyde is also in the conformation found in rhodopsin, and can be used to create more rhodopsin complexes. 11-cis-retinaldehyde can also be converted to 11-cis-retinol by retinol dehydrogenase in the endoplasmic reticulum. Retinol can also isomerize and form 9-cis-retinol, which can then be reversibly oxidized to form 9-cis-retinal by interacting with either retinol dehydrogenase 11 or dehydrogenase/reductase SDR family member 4. 9-cis-retinal can then be further oxidized to 9-cis-retinoic acid by retinal dehydrogenase 1 or 2. 9-cis-retinoic acid can also be formed from the isomerization of all-trans retinoic acid, which in turn is formed by the oxidation of retinol by either of retinal dehydrogenase 1 or 2. All-trans retinoic acid can also be glucuronidated to form retinoyl b-glucuronide, in a reaction catalyzed by a multiprotein chaperone complex including UDP-glucuronosyltransferase 1-1 in the endoplasmic reticulum. Finally, in the endoplasmic reticulum, all-trans-retinoic acid can undergo epoxidation to form all-trans-5,6-epoxyretinoic acid by interaction with a complex of cytochrome P450 proteins, or hydroxylated to either 4-hydroxyretinoic acid or all-trans-18-hydroxyretinoic acid by cytochrome P450 26A1. In one last reqction, 4-hydroxyretinoic acid can be oxidized once again by cytochrome P450 26A1 to form 4-oxo-retinoic acid.

Retinol Metabolism References