P-ionone ring

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. 

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. 

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. 

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

In contrast to [3-carotene, a-carotene, or P-cryptoxanthin, lycopene is not a provitamin A carotenoid the absence of the P-ionone rings at the two ends of the molecule prevents its acceptance as a substrate by the 15,15 central cleavage enzyme to generate retinal. A mammalian enzyme that catalyzes asymmetrical oxidative cleavage of P-carotene at the 9, 10 position has recently been identified this cleaved lycopene to form apolycopenales... 

Large down-field or up-field changes have been observed at the C16, C17 of the p-ionone ring, and on the retinal polyene chain around C9-C10-C11-C12 and... 

Spooner PJR, Sharpies JM, Goodall SC et al (2003) Conformational similarities in the P-ionone ring region of the rhodopsin chromophore in its ground state and after photoactivation to the metarhodopsin-I intermediate. Biochemistry (Mosc) 42 13371-13378... 

HOMO were primarily distributed on the Schiff base side (N side) and on the P-ionone ring side (C5 side), respectively, the HOMO-LUMO transition had an intramolecular charge-transfer (ICT) character. The HOMO-LUMO gap increased to 8.27, 7.69, 7.45, and 7.70 eV for HB, HG, HR, and Rh, respectively, because of the polarized ESP distribution, which increased the excitation energy. 

Provitamin A. Carotenoids are a large family of coloured compounds that are abundant in plants. About 10% of carotenoids have the P-ionone ring, which is needed for vitamin A activity, e.g. p-carotene found in carrots. Carotenoids have numerous double bonds that ensme they are efficient free radical scavengers and they can neutralise singlet oxygen. 

Carotenes isomeric unsaturated hydrocarbons with 9 conjugated trans double bonds, 4 branch methyl groups and usually a -ionone ring at one end. Isomers differ in the arrangement at the other end of the chain (Hg.). C. with P-ionone rings are oxidatively cleaved to vitamin A in the wall of the small intestine, and arc therefore important as provitamins. In this... 

P-C., M, 536.85, m.p. 183 °C (evacuated tube), X ,x (CHQj) 497 and 466 nm, has 2 tenninal P-ionone rings and is optically inactive. Its intense yellow color is due to 11 trans conjugated double bonds. P-C. is the most common C. in plants and is also found in bacteria, fungi and animals (milk, fat, blood, serum, etc). The C. mixture of the carrot contains about 85 % P-C. It is now produced synthetically on a large scale and used as a food, pharmaceutical and cosmetic colorant. 

Tomes, M. L. Competitive effect of the p- and 5-carotene genes on a- or P-ionone ring formation in the tomato. Genetics 1967, 227. 

All photosynthetic tissues, as well as some nonphotosynthetic tissue and Protista, contain carotenoids. It is said that over 400 carotenoids have been identified, but that most are xanthophylls. As noted above, xanthophylls are carotenoid derivatives with oxygen substitutions in the ring or side chain and have reduced or no vitamin A activity (Simpson and Chichester, 1981). Vitamin A activity for vertebrates requires that a carotenoid have at least the unaltered p-ionone ring with an attached polyene side chain containing 11 carbon atoms. Although some compounds with chemical modifications in the P-ionone ring still... 

This theoretical study [174] revealed that the SET mechanism would operate only in polar media while the HAT mechanism would operate in all the solvents though it would also be more favoured in polar media. The most favoured site of (3-carotene for hydrogen abstraction was found to be C4 of the p-ionone ring (Fig. 3.7a). 

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