Retinol vitamin carotenoids

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. 

Zeegers, M.P. et al.. Are retinol, vitamin C, vitamin E, folate and carotenoids intake associated with bladder cancer risk Results from the Netherlands Cohort Study, Br. J. Cancer, 85, 977, 2001. 

Vitamin Ai (retinol) is derived in mammals by oxidative metabolism of plant-derived dietary carotenoids in the liver, especially -carotene. Green vegetables and rich plant sources such as carrots help to provide us with adequate levels. Oxidative cleavage of the central double bond of -carotene provides two molecules of the aldehyde retinal, which is subsequently reduced to the alcohol retinol. Vitamin Ai is also found in a number of foodstuffs of animal origin, especially eggs and dairy products. Some structurally related compounds, including retinal, are also included in the A group of vitamins. 

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). 

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... 

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. 

Retinoids are essential to fish and other vertebrates. Predators acquire them in the diet, either as retinol or retinyl esters they may also be acquired through the conversion of dietary carotenoids produced by plants (Fig. 2). In fish, carotenoids may be metabolized to retinol (vitamin Aj) or didehydroretinol (vitamin A2) (Fig. 1). For example, fish can convert canthaxanthin to /3-carotene47, which is a dimer of retinal that is readily converted to retinol (Figs. 1 and 2). Didehydroretinol (Fig. 1) is formed... 

Schuurman, A.G., Goldbohm, R.A., Brants, H.A., and van den Brandt, P.A., A prospective cohort study on intake of retinol, vitamins C and E, and carotenoids and prostate cancer risk (Netherlands), Cancer Causes Control, 13, 573,2002. 

A. Specific levels. Serum vitamin A (retinol) or carotenoid assays may assist in the diagnosis of hypervitaminosis A. Levels of 25-hydroxy vitamin D are useful in assessing excessive intake. Other serum vitamin concentration measurements are not useful. 

The fat-soluble vitamins can be divided into four different groups, based on biological activity vitamin A (retinol and carotenoids), vitamin D (chole-calciferol and ergocalciferol), vitamin E (tocopherols and tocotrienols), and vitamin K (phylloquinone and menaquinone), all of which may be present in a munber of closely related forms. 

The highest sensitivity and selectivity in vitamin E LC assays are obtained by using fluorescence or electrochemical detection. In the former, excitation at the low wavelength (205 nm) leads to improved detection limits but at the expense of selectivity, compared with the use of 295 nm. Electrochemical detection in the oxidation mode (amperometry or coulometry) is another factor 20 times more sensitive. In routine practice, however, most vitamin E assays employ the less sensitive absorbance detection at 292-295 nm (variable wavelength instrument) or 280 nm (fixed wavelength detectors). If retinol and carotenoids are included, a programmable multichannel detector, preferably a diode array instrument, is needed. As noted previously, combined LC assays for vitamins A, E, and carotenoids are now in common use for clinical chemistry and can measure about a dozen components within a 10 min run. The NIST and UK EQAS external quality assurance schemes permit interlaboratory comparisons of performance for these assays. 

Physiological forms of vitamin A include retinol (vitamin Ai) and its esters, 3-dehydroretinol (vitamin A2) and its esters, retinal (retinene, vitamin A aldehyde), 3-dehydroretinal (retine-2), retinoic acid, neovitamin A, and neo-b-vitamin Aj. Active analogues and related compounds, known as vitamins A are a-, (3-, and y-carotene, neo-(3-carotene B, cryptoxanthine, myxoxanthine, torular-hodin, aphanicin, and echinenone. Foods of plant origin do not contain vitamin A, but are rich sources of previtamin A. About 50 of the 500 known carotenoids can be converted to vitamin A. (3-Carotene is the most important of all the carotenoids that occur in nature. (3-Carotene (provitamin A), precursor of vitamin A, is found in plants. 

Among these carotenoids, there are around 50 carotenoids of provitamin A which could be converted to retinol (vitamin A, 18) by carotenoid 15,15 -oxygenases (CO) such as a P-carotene 15,15 -monooxygenase in their mucosal epithelial cells of the small intestine in human digestive tract (Figure 4) [9, 10]. 

From above results, it suggests that retinoids such as retinal (16), retinol (vitamin A, 18) and retinoic acid (20), and carotenoids such as P-carotene (2), P-cryptoxanthin (5) and lycopene (3) could regulate the expression of P,P-carotene 15,15 -monooxygenase using a transcriptional feedback mechanism through their interaction with the members of the retinoic acid (20) receptor (RAR) family (Figure 5) [11]. 

Among carotenoids in daily diet derived from mainly vegetables and fruits in plant kingdom, the most common and abundant carotenoids are provitamin A which could be converted into vitamin A or retinol (vitamin A, 18) of the active form of vitamin A such as yellow P-carotene (2), piuple a-carotene (1), Y-carotene (41), P-cryptoxanthin (5) and P-apo-8 -carotenal (25), and nonprovitamin A of no convertion to retinol (vitamin A, 18) such as lycopene (3), lutein (6), canthaxanthin (45) and zeaxanthin (9) (Figure 9) [9, 19, 20],... 

The daily requirement of vitamin A (Table 6.3) is provided to an extent of 75% by retinol intake (as fatty acid esters, primarily retinyl palmitate), while the remaining 25% is through P-carotene and other provitaminactive carotenoids. Due to the limited extent of carotenoid cleavage, at least 6 g of P-carotene are required to yield 1 g retinol. Vitamin A absorption and its storage in the fiver occur essentially in the form of fatty acid esters. Its content in fiver is 250pg/g fresh tissue, i.e. a total of about 240-540 mg is stored. The fiver supplies the blood with free retinol, which then binds to proteins in blood. The plasma concentration of retinol averages 1.78 pmol/1 in women and 2.04 lamol/l in men. 

Milk. Simultaneous analysis of retinol and carotenoids present in human milk was carried out on a YMC RP 5-p.m ODS column (0.46 X 25 cm) by isocratic elution with tetrahydrofuran/methanol (90 10) (216). Thirty-four carotenoids, including 13 geometric isomers and eight metabolites, along with vitamins A and E in milk of lactating mothers, were separated by a combination of normal and RP-HPLC (217). 

Food sample pretreatment may consist of either (a) saponification to quantify the free forms (retinol or xanthophylls may occur free or ester-ified in foods) [95,96] or (b) direct extraction to determine the unaltered A vitamers [84,88]. Alkaline hydrolysis is also an expedient to simplify the vitamin A analysis, since retinol is the only form to be quantified nevertheless, due to its sensitivity to light and oxygen, it is important to prevent photo-oxidation by inclusion of a antioxidant (ascorbic acid, hydroquinone, or pyrogallol). A drawback of hot saponification is the generation of artifacts, such as geometric isomers of retinol and carotenoids [97]. 

Humans do not have the ability to synthesize retinol (vitamin A) and have thus evolved to derive this compound from the diet, either directly as preformed retinol from foods of animal origin or from the metabohsm of carotenoids primarily derived from plant tissues. Several hundred carotenoids have been isolated, named and their structures elucidated, but only a few have a known biochemical role in human metabolism or are present in the diet in sufficient quantities to be detected in plasma (Khachik et al. 1992). The chemical... 

Carotenoids absorb visible light (Section 13 21) and dissipate its energy as heat thereby protecting the organism from any potentially harmful effects associated with sunlight induced photochemistry They are also indirectly involved m the chemistry of vision owing to the fact that p carotene is the biosynthetic precursor of vitamin A also known as retinol a key substance m the visual process... 

The stmcture of vitamin A [11103-57-4] and some of the important derivatives are shown in Figure 1. The parent stmcture is aH-Zra/ j -retinol [68-26-8] and its lUPAC name is (all-E)-3,7-dimethyl-9-(2,6,6-trimethyl-l-cyclohexen-l-yl)-2,4,6,8-nonatetraen-l-ol (1). The numbering system for vitamin A derivatives parallels the system used for the carotenoids. In older Hterature, 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 stmctures of vitamin A and -carotene were elucidated by Karrer in 1930 and several derivatives of the vitamin were prepared by this group (5,6). 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 [116-31-4] (5) (7). 

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