Retinol vitamin A

Vitamin A (retinol, 6.1) is the parent of a range of compounds known as retinoids, which possess the biological activity of vitamin A. In general, animal foods provide preformed vitamin A as retinyl esters (e.g. 6.5, which are easily hydrolysed in the gastrointestinal tract) while plant foods provide precursors of vitamin A, i.e. carotenoids. Only carotenoids with a -ionone ring (e.g. j -carotene) can serve as vitamin A precursors. -Carotene (6.6) 

Retinol can be oxidized to retinal (6.2) and further to retinoic acid (6.3). Cis-trans isomerization can also occur, e.g. the conversion of all trans-retinal to 11-ds-retinal (6.4), which is important for vision. 

The major dietary sources of retinol are dairy products, eggs and liver, while important sources of jS-carotene are spinach and other dark-green leafy vegetables, deep orange fruits (apricots, cantaloupe) and vegetables (squash, carrots, sweet potatoes, pumpkin). The richest natural sources of vitamin A are fish liver oils, particularly halibut and shark. 

Vitamin A activity is present in milk as retinol, retinyl esters and as carotenes. Whole cows milk contains an average of 52jug retinol and 21 ng carotene per 100 g. The concentration of retinol in raw sheep s and pasteurized goats milks is 83 and 44 g per 100 g, respectively, although milks of these species are reported (Holland et ai, 1991) to contain only trace amounts of carotenes. Human milk and colostrum contain an average of 58 and 155, ug retinol per lOOg, respectively. In addition to their role as provitamin A, the carotenoids in milk are reponsible for the colour of milk fat (Chapter 11). 

The concentration of vitamin A and carotenoids in milk is strongly influenced by the carotenoid content of the feed. Milk from animals fed on pasture contains higher levels of carotenes than that from animals fed on concentrate feeds. There is also a large seasonal variation in vitamin A concentration summer milk contains an average of 62 fig retinol and 31 fig carotene per 100 g while the values for winter milk are 41 and 11 g per 

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Retinoic acid (vitamin A acid). Retinol (vitamin A... 

Fat-Soluble Retinol (vitamin A) Ergocalciferol (vitamin Dg) Cholecalciferol (vitamin Dg) n-Tocopherol (vitamin E) Vitamin K ... 

Transthyretin (TTR) A protein complex found in blood that binds both retinol (vitamin A) and thyroxine. 

A small but variable proportion of the carotenoids with one or two P-ionone rings (mainly P-carotene) are cleaved in the enterocytes to produce retinol (vitamin A). This process is very tightly controlled, so that too much vitamin A is not produced, although the control mechanism is not clear. Some cleavage of P-carotene can also occur in the liver, but this does not account for the turnover of P-carotene in the body. Small amounts of carotenoids are subject to enterohepatic circulation, but this does not account for losses. 

Four deuteriated retinols, 26-29, with 3 to 5 deuterium atoms have been synthesized29 for metabolism of vitamin A studies in humans30. Deuterium has been introduced into appropriate intermediates, used in the reaction scheme shown in equation 12, by base-catalysed exchange with 2H20 or perdeuterioacetone. The numbering system for retinol (vitamin A alcohol) is shown in equation 12. 

While the human body can remove an excess of any water-soluble vitamin, excesses of fat-soluble vitamins are more serious. Early arctic explorers discovered that the Inuit regarded seal liver and polar bear liver as taboo and must not be eaten. Those explorers who ignored this advice risked retinol poisoning as the livers of both these species are rich in retinol (vitamin A) that can not be excreted. The effects of retinol poisoning are extremely unpleasant. It is for this reason that fortification with fat-soluble vitamins is not likely to be undertaken. 

The answer is d. (Hardman, p 1575.) Isotretinoin is actually a form of high-dose vitamin A therapy Vitamin A itself or retinol (vitamin A could be used, but they have less advantageous pharmacokinetic properties. Antibiotics such as tetracyclines are used in acne, but they have little effect on the nodulocystic form... 

Figure 12.8 Vitamin Ar All-tranj-retinol (Vitamin A alcohol) is usually referred to as Vitamin A,.

J. Boehnlein, A. Sakr, J. L. Lichtin, R. L. Bronaugh, Characterization of Esterase and Alcohol Dehydrogenase Activity in Skin. Metabolism of Retinyl Palmitate to Retinol (Vitamin A) During Percutaneous Absorption , Pharm. Res. 1994, 11, 1155-1159. 

Zachman, R. D. (1989). Retinol (vitamin A) and the neonate Special problems of the human premature infant. Am. J. Clin. Nutr. 50, 413-424. 

Retinol (vitamin A) is found in foods of mammalian origin in the form of retinyl ester, or in fruits and vegetables as carotenoids with provitamin A activity, especially P-carotene (provitamin A). In enterocytes, retinol binds to cellular retinol-binding protein type II (CRBPII), which directs the esterification by the enzyme lecithin retinol acyltransferase (LRAT). 

The stability of some vitamins is influenced by aw. In general, the stability of retinol (vitamin A), thiamin (vitamin Bj) and riboflavin (vitamin B2) decreases with increasing aw. At low av (below 0.40), metal ions do not have a catalytic effect on the destruction of ascorbic acid. The rate of loss of ascorbic acid increases exponentially as aw increases. The photodegradation of riboflavin (Chapter 6) is also accelerated by increasing aw. 

The retinoids, a family of molecules that are related to retinol (vitamin A), are essential for vision, reproduction, growth, and maintenance of epithelial tissues. Retinoic acid, derived from oxidation of dietary retinol, mediates most of the actions of the retinoids, except for vision, which depends on retinal, the aldehyde derivative of retinol. 

VITAMIN A. This substance also has been referred to as retinol, axerophthol, biosterol, vitamin Ai, anti-xerophthalmic vitamin, and anti-infective vitamin. The physiological forms of the vitamin include Retinol (vitamin A ) and esters 3-dehydroretinol (vitamin A2) and esters 3-dehydroretinal (retinme-2) retinoic acid neovitamin A neo-b-vitamin Ai. The vitamin is required by numerous animal species. All vertebrates and some invertebrates convert plant dietary carotenoids in gut to vitamin Ai. which is absorbed. Most animal species store appreciable amounts... 

B Stancher, F Zonta. High-performance liquid chromatography of the unsaponifiable from samples of marine and freshwater fish fractionation and identification of retinol (vitamin A)) and dehydroretinol (vitamin A2) isomers. J Chromat 287 353-364, 1984. 

Other antioxidant species are synthesized by cells like uric acid, ubiquinol or thiols (cystein, homocystein, etc.). In addition, many compounds found in food display antioxidant properties retinol (vitamin A) and its precursor /(-carotene, and polyphenols (flavonoids, etc.). Figure 8.2 shows the apparent standard potential of some LMWA and ROS explaining the spontaneous oxido-reduction reactions at the origin of the antioxidant protection system. 

Rather surprisingly, the average person knows the names of several cofactors and coenzyme precursors, because many of these cannot be synthesized in vivo, and must be supplied in one s diet. This is the reason why a balanced diet should include retinol (vitamin A), ascorbic add (vitamin C), tocopherol (vitamin E), thiamine (vitamin Eh), folic acid etc., as well as zinc and iron. 

Axerophthol Axerophthylium Gadol Oleovitamin A Retinol Vitamin A Vitamin At Xerophthol... 

Vitamin A is found in nature and is available in several forms. Retinol (vitamin A) is an unsaturated alcohol containing an ionone ring and can be obtained from fish liver oil (Table 13-2), egg yolk, milk, and butter. Vitamin A2 (dehydroretinol) is present in freshwater fishes. /3-carotene, a carotenoid, is the most important precursor of this vitamin. 

Vitamin A is an essential fat-soluble compound acquired from the diet. The parent form of vitamin A is all-iram-retinol. Vitamin A is needed to maintain normal vision, normal reproduction (including spermatogenesis, conception, and placenta formation), and normal cell differentiation (including bone remodeling, maintenance of differentiated epithelial linings and skin, em-... 

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