Vitamin A is a fat-soluble vitamin and an essential nutrient for animals. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinal, retinoic acid, and several provitamin (precursor) carotenoids, most notably beta-carotene. Vitamin A has multiple functions: it is essential for embryo development and growth, for maintenance of the immune system, and for vision, where it combines with the protein opsin to form rhodopsin – the light-absorbing molecule necessary for both low-light and color vision.
Retinal is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).
Retinoic acid (used simplified here for all-trans-retinoic acid) is a metabolite of vitamin A1 (all-trans-retinol) that mediates the functions of vitamin A1 required for growth and development. All-trans-retinoic acid is required in chordate animals, which includes all higher animals from fish to humans. During early embryonic development, all-trans-retinoic acid generated in a specific region of the embryo helps determine position along the embryonic anterior/posterior axis by serving as an intercellular signaling molecule that guides development of the posterior portion of the embryo. It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages.
17β-Hydroxysteroid dehydrogenases, also 17-ketosteroid reductases (17-KSR), are a group of alcohol oxidoreductases which catalyze the reduction of 17-ketosteroids and the dehydrogenation of 17β-hydroxysteroids in steroidogenesis and steroid metabolism. This includes interconversion of DHEA and androstenediol, androstenedione and testosterone, and estrone and estradiol.
The visual cycle is a process in the retina that replenishes the molecule retinal for its use in vision. Retinal is the chromophore of most visual opsins, meaning it captures the photons to begin the phototransduction cascade. When the photon is absorbed, the 11-cis retinal photoisomerizes into all-trans retinal as it is ejected from the opsin protein. Each molecule of retinal must travel from the photoreceptor cell to the RPE and back in order to be refreshed and combined with another opsin. This closed enzymatic pathway of 11-cis retinal is sometimes called Wald's visual cycle after George Wald (1906–1997), who received the Nobel Prize in 1967 for his work towards its discovery.
In enzymology, a retinol dehydrogenase (RDH) (EC 1.1.1.105) is an enzyme that catalyzes the chemical reaction
In enzymology, a retinal dehydrogenase, also known as retinaldehyde dehydrogenase (RALDH), catalyzes the chemical reaction converting retinal to retinoic acid. This enzyme belongs to the family of oxidoreductases, specifically the class acting on aldehyde or oxo- donor groups with NAD+ or NADP+ as acceptor groups, the systematic name being retinal:NAD+ oxidoreductase. This enzyme participates in retinol metabolism. The general scheme for the reaction catalyzed by this enzyme is:
Retinaldehyde-binding protein 1 (RLBP1) also known as cellular retinaldehyde-binding protein (CRALBP) is a 36-kD water-soluble protein that in humans is encoded by the RLBP1 gene.
11-cis retinol dehydrogenase is an enzyme that in humans is encoded by the RDH5 gene.
Retinol dehydrogenase 11 is an enzyme that in humans is encoded by the RDH11 gene.
Alcohol dehydrogenase class 4 mu/sigma chain is an enzyme that in humans is encoded by the ADH7 gene.
Retinol dehydrogenase 12 is an enzyme that in humans is encoded by the RDH12 gene.
Retinol dehydrogenase 14 is an enzyme that in humans is encoded by the RDH14 gene.
Short-chain dehydrogenase/reductase 3 is an enzyme that in humans is encoded by the DHRS3 gene.
Dehydrogenase/reductase SDR family member 9 is an enzyme that in humans is encoded by the DHRS9 gene.
Hydroxysteroid 17-beta dehydrogenase 6 is an enzyme that in humans is encoded by the HSD17B6 gene.
Retinol dehydrogenase 8 is an enzyme that in humans is encoded by the RDH8 gene.
Retinol dehydrogenase 16 (all-trans) is a protein that in humans is encoded by the RDH16 gene. The gene is also known as RODH-4 and SDR9C8.
11-cis-retinol dehydrogenase (EC 1.1.1.315, RDH5 (gene)) is an enzyme with systematic name 11-cis-retinol:NAD+ oxidoreductase. This enzyme catalyses the following chemical reaction
Retinol dehydrogenase 13 (all-trans/9-cis) is a protein that in humans is encoded by the RDH13 gene. This gene encodes a mitochondrial short-chain dehydrogenase/reductase, which catalyzes the reduction and oxidation of retinoids. The encoded enzyme may function in retinoic acid production and may also protect the mitochondria against oxidative stress. Alternatively spliced transcript variants have been described.
