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
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