Ionone rings

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. 

In order to exhibit provitamin A activity, the carotenoid molecule must have at least one unsubstituted p-ionone ring and the correct number and position of methyl groups in the polyene chain. Compared to aU-trans P-carotene (100% provitamin A activity), a-carotene, P-cryptoxanthin, and y-carotene show 30 to 50% activity and cis isomers of P-carotene less than 10%. Vitamin A equivalence values of carotenoids from foods have been recently revised to higher ratio numbers (see Table 3.2.2) due to poorer bioavailability of provitamin A carotenoids from foods than previously thought when assessed with more recent and appropriate methods. 

The substrate specificity of these enzymes is not stringent for example, CCD1 from tomato was also shown to cleave at the 9,10- and 9, 10 -positions of (1-carotene, zeaxanthin, lutein, violaxan-thin and neoxanthin all of which have different ionone ring modifications. Unlike NCEDs, CCD1 enzymes have no plastid-targeting sequences and are localized in the cytosol. It is postulated that they access the carotenoids in the plastids through a monotopic membrane association (Kloer et al. 

Carotenoids are a class of lipophilic compounds with a polyisoprenoid structure. Most carotenoids contain a series of conjugated double bonds, which are sensitive to oxidative modification and cis-trans isomerization. There are six major carotenoids (ji-carotenc, a-carotene, lycopene, P-cryptoxanthin, lutein, and zeaxanthin) that can be routinely found in human plasma and tissues. Among them, p-carotene has been the most extensively studied. More recently, lycopene has attracted considerable attention due to its association with a decreased risk of certain chronic diseases, including cancers. Considerable efforts have been expended in order to identify its biological and physiochemical properties. Relative to P-carotene, lycopene has the same molecular mass and chemical formula, yet lycopene is an open-polyene chain lacking the P-ionone ring structure. While the metabolism of P-carotene has been extensively studied, the metabolism of lycopene remains poorly understood. 

Lutein has a stucture similar to beta-carotene with a hydroxyl group on the ionone ring at each end of the molecule. It is somewhat less sensitive to oxidation and heat degradation than beta-carotene. It contributes a yellow color. 

Figure 3 Time series for the torsion connecting the ionone ring to the chain of rhodopsin s retinal ligand. All three panels show the same trajectory, cut at 50, 150, and 1,600 ns, respectively.

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 /3-ionone ring (e.g. /1-carotene) can serve as vitamin A precursors. /3-Carotene (6.6)... 

Retinol A primary alcohol containing a p-ionone ring with an unsaturated side chain, retinol is found in animal tissues as a retinyl ester with long-chain fatty acids. 

The parent vitamin A compound, retinol, has the empirical formula C2oH3oO and a molecular weight (MW) of 286.44. The molecule comprises a cyclohexenyl (/3-ionone) ring attached at the carbon-6 (C-6) position to a polyene side chain whose four double bonds give rise to cis-trans (geometric) isomerism. The predominant isomer, all-trans-retinol (Fig. 1), possesses maximal (100%) vitamin A activity and is frequently accompanied in natural foodstuffs by smaller amounts of 13-ds-retinol, which exhibits 75% relative activity in the rat (6). Other cis isomers of retinol also occur in nature, but they are of low potency, and their contribution to the total vita-... 

From a nutritional viewpoint, the carotenoids are classified as provitamins and inactive carotenoids. To have vitamin A activity, the carotenoid molecule must incorporate a molecule of retinol, i.e., an unsubstituted /3-ionone ring with an 11-carbon polyene chain. /3-carotene (C40H56, MW = 536.88), the most ubiquitous provitamin A carotenoid, is composed of two molecules of retinol joined tail to tail thus the compound possesses maximal (100%) vitamin A activity. The structures of all other provitamin A carotenoids incorporate one molecule of retinol and hence theoretically contribute 50% of the biological activity of /3-carotene. Among the 600 or so carotenoids that exist in nature, only about 50 possess vitamin A activity in varying degrees of potency. 

Other studies have shown biological actions from these kinds of structure and indicate interrelationships between the chemicals in terms of their potencies. Tomita [229] found that the substances, vitamin A, vitamin K, vitamin E, /1-carotene, ubiquinone (15), phytol and squalene (16), from green-yellow vegetables could suppress the growth of tumour cells and enhance T-cell cytotoxicity, but /1-carotene, which does have both ends of the chain substituted with a bulky / -ionone ring on each end-group did not. Hydrophobic chain... 

Carotene-A hydrocarbon consisting of a conjugated carbon chain attached to an ionone ring. Most frequently, a conjugated carbon chain terminated at each end with an ionone ring. 

The retinoids of interest in vision all contain an ionone ring attached to a conjugated carbon side chain and two oxygen atoms with conjugation maintained between the two oxygens. 

Vitamin A retinol (P-ionone ring) marine based and marine derived animals Rhodonine,()... 

Vogt suggested in 1989 that the family of forms of Vitamin A based on modifications to the ionone ring was now complete. However, two papers by one team describing a fourth potential form of Vitamin A have appeared44 These... 

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