Vitamin A

Vitamin A is important for growth and development, for the maintenance of the immune system and good vision. Vitamin A is needed by the retina of the eye in the form of retinal[1].

Retinol acetate: pale-yellow crystals (mp: about 60 °C). Once melted retinol acetate tends to yield a supercooled melt. Retinol propionate: reddish-brown oily liquid. Retinol palmitate: a fat-like, light yellow solid or a yellow oily liquid, if melted (mp: about 26 °C).

The structure of vitamin A and some of the important derivatives are shown in Figure 1. The parent structure is all-trans-retinol [CAS: 68-26-8] and its IUPAC name is (all-E)-3,7-dimethyl-9-(2,6,6- trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraen-1-ol. The numbering system for vitamin A derivatives parallels the system used for the carotenoids. In older literature, vitamin A compounds are named as derivatives of trimethyl cyclohexene and the side chain is named as a substituent. For retinoic acid derivatives, the carboxyl group is denoted as C-1 and the trimethyl cyclohexane ring as a substituent on C-9. The structures of vitamin A and β-carotene were elucidated by Karrer in 1930 and several derivatives of the vitamin were prepared by this group. In 1935, Wald isolated a substance found in the visual pigments of the eye and was able to show that this material was identical with Karrer’s retinaldehyde.

ChEBI: All-trans-retinol is a retinol in which all four exocyclic double bonds have E- (trans-) geometry. It has a role as a human metabolite, a mouse metabolite and a plant metabolite. It is a retinol and a vitamin A.

The provitamin A carotenoids are obtained from plant sources and comprise some of the best sources of this vitamin. The provitamin As are found in deep green, yellow,and orange fruits and vegetables, such as carrots, spinach, broccoli, kale, collard and turnip greens, mangoes, apricots, nectarines, pumpkins, and sweet potatoes.Vitamin A belongs to a class of compounds referred to as the retinoids, which consist of four isoprenoid units joined in a head-to-tail manner. All double bonds in the isoprenoid units are implied to be in the E (trans) configuration unless stated otherwise. Currently, the term retinoid is applied to retinol and its naturally occurring derivatives plus synthetic analogs, which need not have vitamin A activity. Vitamin A is currently used as a generic descriptor for all retinoids that exhibit the biological activity of retinol, the original substance identified as vitamin A. The term vitamin A1 and all-trans retinol have been used to refer to retinol whereas vitamin A2 was used to refer to 3,4-dehydroretinol.

vitamin A can act as a keratinization regulator, helping to improve the skin’s texture, firmness, and smoothness. Vitamin A esters, once in the skin, convert to retinoic acid and provide anti-aging benefits. Vitamin A is believed to be essential for the generation and function of skin cells. Continued vitamin A deficiency shows a degeneration of dermal tissue, and the skin becomes thick and dry. Surface application of vitamin A helps prevent skin dryness and scaliness, keeping the skin healthy, clear, and infection resistant. Its skin regeneration properties appear enhanced when combined with vitamin e. Vitamin A is a major constituent of such oils as cod liver and shark, and many fish and vegetable oils. See also retinol; retinoic acid; retinylpalmitate.