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Retinol

Retinol


Retinol, or vitamin A, is a necessary nutrient in all higher animals. It plays an important role in vision, in the maintenance of epithelial cell layers, in spermatogenesis, and in fetal development. Retinol must be obtained from

the diet, either from organic compounds that are in animal products, such as retinyl esters , or from compounds in plants, most notably β- carotene (see Figure 1). Retinol and its derivatives are collectively called retinoids.

Chemically, retinol is an isoprenoid; isoprenoids are a group of compounds that includes vitamins E and K and cholesterol, which are synthesized from isoprene units. As shown in Figure 1, retinol is a hydrocarbon molecule with a single hydroxyl group at one end. This hydroxyl group can be oxidized to form an aldehyde group (yielding retinal), or to a carboxylic acid group (yielding retinoic acid). Retinal and retinoic acid are the biologically active forms of retinol. In addition to these different oxidation states, retinol occurs as a number of different isomers . The retinol structure shown in Figure 1 is all-trans -retinol, as all of the double bonds are in the trans configuration. Many cis isomers also occur. Two important examples are 11-cis -retinal, which is required for vision (see Figure1), and 13-cis -retinoic acid (or isotretinoin), which is used as an anti-acne drug.

One of the symptoms of vitamin A deficiency is night blindness. If the deficiency persists over time, the eyes will eventually deteriorate, and permanent blindness will result. In specialized cells of the eye, 11-cis -retinal is linked covalently to the enzyme opsin. Opsin is also a membrane protein; it comprises approximately 95 percent of the membrane protein of disks in the outer segments of the eye's rod cells. The disks in a rod cell are circular layers of membrane stacked on top of each other. Each cell contains hundreds to thousands of these disks. This greatly increases the membrane surface area, and thus the amount of opsin, in these light-detecting cells. The combination of 11-cis -retinal and opsin is called rhodopsin. Visible light is absorbed by the 11-cis -retinal, causing it to isomerize to all-trans -retinal (see Figure 1). The resulting shape change in the rhodopsin initiates a series of reactions that ultimately sends signals to the brain.

The other active form of vitamin A is retinoic acid. Retinoic acid exerts its physiological functions via its interactions with a family of nuclear receptors called the retinoic acid receptors (RARs). RARs are members of a superfamily of nuclear receptors that include steroid hormone and thyroid hormone receptors. RARs are involved in regulating the transcription of many genes, including genes involved in the cell growth and cell differentiation of developing organisms. Because of this link between retinoic acid and development, female patients taking isotretinoin (13-cis -retinoic acid) must not become pregnant due to the considerable risk to offspring of severe birth defects.

see also Acne Medication.

Stephanie E. Dew

Bibliography

Maden, Malcolm (1998). "Vertebrate Development: A Nervous Vitamin." Current Biology 8(23): R846849. Also available from <http://www.bmn.com>.

Zubay, Geoffrey (1998). Biochemistry, 4th edition. Dubuque, IA: Wm. C. Brown Publishers.

Internet Resources

Centers for Disease Control. Information available from <http:www.cdc.gov>.

National Institutes of Health. Information available from <http://www.nih.gov>.

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retinol

retinol A diterpene compound, composed of four isoprene units structurally related to carotenes, which is involved in the synthesis of the visual pigments. A deficiency of this compound results in a number of symptoms, including cessation of growth, cornification of epithelial structures, and, most notably, night blindness.

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retinol

ret·i·nol / ˈretnôl; -ˌōl/ • n. Biochem. a yellow compound, C20H29OH, found in green and yellow vegetables, egg yolk, and fish-liver oil. It is essential for growth and vision in dim light. Also called vitamin A.

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retinol

retinol: see Vitamin A under vitamin.

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retinol

retinol (ret-in-ol) n. see vitamin A.

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retinol

retinol See vitamin A.

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retinol

retinol See vitamin A.

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Retinol

Retinol

OVERVIEW

Retinol (RET-uh-nol) is the scientific name for vitamin A, a vitamin found only in animals. It occurs as a yellowish to orange powder with a slight brownish cast and is a relatively stable compound. Retinol is converted in the body from an alcohol to the corresponding aldehyde, retinal (C20H28O), one of the primary chemical compounds involved in the process by which light is converted to nerve impulses in the retina of the eye. Vitamin A is also required for a number of other biochemical reactions in the body, including growth and development of tissue and maintenance of the immune system.

KEY FACTS

OTHER NAMES:

Vitamin A

FORMULA:

C20H30O

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Alcohol (organic)

STATE:

Solid

MOLECULAR WEIGHT:

286.45 g/mol

MELTING POINT:

63.5°C (146°F)

BOILING POINT:

Not applicable; decomposes

SOLUBILITY:

Practically insoluble in water; soluble in ethyl alcohol and methyl alcohol

HOW IT IS MADE

Vitamin A is synthesized in animal bodies through a variety of pathways. One important source of vitamin A is a group of related compounds called the carotenes, substances responsible for the yellowish or orangish appearance of fruits and vegetables such as carrots, sweet potatoes, squash, cantaloupe, apricots, pumpkin, and mangos. Some leafy green vegetables, such as collard greens, spinach, and kale, are also good sources of the carotenes. The most important of the carotenes is β-carotene (beta-carotene), C40H56. The oxidation of carotenes in animal bodies converts them to retinol.

The chemical structure of retinol was determined in 1931 by Swiss chemist Paul Karrer (1889–1971), and the compound was first prepared synthetically shortly thereafter by Austrian-German chemist Richard Kuhn (1900–1967). The first successful process for producing retinol commercially was developed in the mid-1940s by German chemist Otto Isler (1920–1992), then employed at the pharmaceutical company Roche, located in Sissein, Germany. Isler's process involved a complex series of reactions that begins with the combination of a fourteen carbon hydrocarbon and a six carbon hydrocarbon to create the fundamental backbone from which the retinol molecule is constructed. Regular production of vitamin A began in 1948 with a projected output of 10 kilograms per month, which before long was raised to 50 kilograms per month. The Roche plant at Sissein continues to produce retinol today.

COMMON USES AND POTENTIAL HAZARDS

Vitamin A is probably best known for its role in maintaining normal vision. Deficiencies of the compound are likely to manifest themselves earliest in a variety of eye problems, most commonly night blindness. Night blindness is a condition in which one loses the ability to distinguish objects in reduced light. If left untreated, vitamin A deficiencies may lead to decreased ability to see in normal light and, eventually, to complete blindness.

Interesting Facts

  • Animals that live in very cold climates have high concentrations of retinol in their livers. A polar bear liver contains enough retinol to kill a human who eats it. Vitamin A deficiency is the leading cause of childhood blindness in the world today.

But vitamin A has been shown to have a number of other functions in the body. It is essential for the maintenance of growth, bone formation, reproduction, proper immune system function, and healing of wounds. A number of additional claims have been made for the compound, although evidence is not as strong as it is for the above functions. For example, it may be effective in preventing or treating a variety of conditions such as measles, intestinal parasites, osteoporosis, inflammatory bowel disease, bone marrow disorders, certain types of cancer, tuberculosis, peritonitis, osteoarthritis, food poisoning, Alzheimer's disease, miscarriage, and HIV/AIDS. In each of these cases, evidence is not yet strong enough to show a clear-cut connection between retinol and disease, but research is being conducted to determine how strong the association may be.

Retinol is available commercially in a variety of formulations, including tablets, capsules, and creams. Such products usually contain a modified form of retinol that is more easily absorbed by the body. For example, a product known as tretinoin is a synthetic form of retinol known as all-trans retinoic acid. The term all trans means that all of the double bonds in retinoic acid are located on the same side of the molecule. Products containing tretinoin are used to treat acne, pimples, wrinkles, blackheads, freckles, sun-spots, and even pre-cancerous lesions. They work by increasing the rate with which the skin sheds old cells and replaces them with new cells.

Vitamin A supplements in pill or capsule form are available in two formulations, those that contain retinol and those that contain beta carotene. It is not possible to take too much of the latter type of vitamin A. The body will not convert excess amounts of carotene into retinol but will, instead, excrete the excess in the urine or stool. An excess of retinol-based vitamin A, by contrast, may result in certain medical problems. Since the vitamin is fat soluble, in may be stored in body fat and reach relatively high concentrations if too much is ingested. An excess of retinol in the body may be associated with liver damage, osteoporosis, rash, fatigue, bone and joint pain, nausea, insomnia, and personality changes.

FOR FURTHER INFORMATION

Powers, Jennifer I. "Acne Medication." In Chemistry: Foundations and Applications. Volume 1. Edited by J. J. Lagowski. New York: Macmillan Reference USA, 2004, 12-15.

"Vitamin A." Bristol University School of Chemistry. http://www.chm.bris.ac.uk/webprojects2002/schnepp/vitamina.html (accessed on November 3, 2005).

"Vitamin A (Retinol)." Hypertexts for Biomedical Sciences. http://arbl.cvmbs.colostate.edu/hbooks/pathphys/misc_topics/vitamina.html (accessed on November 3, 2005).

"Vitamin A (Retinol)." University of Maryland Medical Center. http://www.umm.edu/altmed/ConsSupplements/VitaminARetinolcs.html (accessed on November 3, 2005).

See AlsoBeta-Carotene

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