HEALTH AND MEDICAL PRODUCTS:Vitamin A.

Vitamin A

In 1913, T. Osborne and L. Mendel, while conducting experiments using rats, discovered that butter contained a growth-promoting, fat- soluble nutrient necessary for development. Soon known as vitamin A, its chemical character was established in 1933, and it was first synthesized in 1947. Vitamin A consists of three biologically active molecules: retinol [3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4,6,8-non- atetraen-1-ol; C20H30O], retinal (retinaldehyde), and retinoic acid. Each of these compounds is derived from the plant precursor molecule, �-car- otene (a member of a family of molecules known as carotenoids). �-Car- otene, which consists of two molecules of retinal linked at their aldehyde ends, is also referred to as the provitamin form of vitamin A. Ultraviolet light inactivates vitamin A, so it is often destroyed via oxidation upon ex- posure to heat, light, or air. Within the intestine, ingested �-carotene is cleaved via enzymes to initially yield retinal and then subsequently re- duced via enzymes to retinol. Retinol is esterified, delivered to the blood, and then delivered to the liver for storage as a lipid ester. Thus, nearly 90 percent of vitamin A within the body is contained within the liver. Di- etary sources of vitamin A include the provitamin precursor carotene, found in carrots, deep-yellow and deep-green leafy vegetables, and vita- min A, found in fish liver oils, egg yolk, liver, whole milk, butter, and cheese. Vitamin A has three major roles within the body: gene regulation, proper visual functioning, and general body organ health and maintenance. Retinol and retinoic acid are considered hormones of the steroid/ thyroid hormone superfamily of proteins. Within cells, both retinol and retinoic acid bind to specific receptor proteins, and this receptor-vitamin complex then interacts with several genes involved in growth and differentiation to affect their expression. For example, gene expression pat- terns involved with early processes of embryological development, including organogenesis and limb development, are affected by retinoic acid. Vita- min A has a direct role in the process of vision, as the photosensitive compound of most mammalian eyes is a protein called opsin (present in the photoreceptor rod cells within the retina of the eye), which is covalently coupled to an aldehyde form of vitamin A (cis-retinal). Expo- sure of the eye to ultraviolet light results in a series of reactions within the photoreceptor cells that are mediated by photon absorption by cisretinal, which eventually leads to propagation of nerve impulses from the optic nerve of the eye to the brain, where vision is processed. Retinol also functions in the synthesis of certain glycoproteins and polysaccha- rides necessary for mucus production and normal growth regulation within most body organ systems. For example, vitamin A is required to maintain the integrity of the skin and mucus membranes, normal bone and tooth development, and normal reproductive capabilities and additionally acts as an antioxidant to provide anticancer and antiatherosclerosis effects.