Xanthophylls cryptoxanthin

The oxidation of carotenes results in the formation of a diverse array of xanthophylls (Fig. 13.7). Zeaxanthin is synthesised from P-carotene by the hydroxylation of C-3 and C-3 of the P-rings via the mono-hydroxylated intermediate P-cryptoxanthin, a process requiring molecular oxygen in a mixed-function oxidase reaction. The gene encoding P-carotene hydroxylase (crtZ) has been cloned from a number of non-photosynthetic prokaryotes (reviewed by Armstrong, 1994) and from Arabidopsis (Sun et al, 1996). Zeaxanthin is converted to violaxanthin by zeaxanthin epoxidase which epoxidises both P-rings of zeaxanthin at the 5,6 positions (Fig. 13.7). The... 

There are basically two types of carotenoids those that contain one or more oxygen atoms are known as xanthophylls those that contain hydrocarbons are known as carotenes. Common oxygen substituents are the hydroxy (as in p-cryptoxanthin), keto (as in canthaxanthin), epoxy (as in violaxanthin), and aldehyde (as in p-citraurin) groups. Both types of carotenoids may be acyclic (no ring, e.g., lycopene), monocyclic (one ring, e.g., y-carotene), or dicyclic (two rings, e.g., a- and p-carotene). In nature, carotenoids exist primarily in the more stable all-trans (or all-E) forms, but small amounts of cis (or Z) isomers do occur. - ... 

Carrots were also the main sonrces of a-carotene, whereas tomatoes and tomato prodncts were the major sources of lycopene. Lutein was mainly provided by peas in the Republic of Ireland and United Kingdom. Spinach was found to serve as the major source in other countries. Lutein and zeaxanthin xanthophylls are found in a wide variety of fruits and vegetables, particularly green leafy vegetables, but also in some animal products such as egg yolks. In all countries, P-cryptoxanthin was obtained primarily from citrus fruits. 

The degree of linkage of a compound may also affect its bioaccessibility in the gut. It is generally admitted that a compound linked with other molecules (e.g., via esterification, glycosylation, etc.) is not absorbed as well as its free form and thus it must be hydrolyzed in the gut in order to be taken up by enterocytes. Due to the presence of hydroxyl or keto groups on their molecules, the xanthophylls (lutein, zeaxanthin, and P-cryptoxanthin) are found in both free and esterified (monoester or diester) forms in nature, but few studies have been conducted to date to assess the bioavailabilities of these esters. 

Fresh peppers are excellent sources of vitamins A and C, as well as neutral and acidic phenolic compounds (Howard and others 2000). Levels of these can vary by genotype and maturity and are influenced by growing conditions and processing (Mejia and others 1988 Howard and others 1994 Lee and others 1995 Daood and others 1996 Simmone and others 1997 Osuna-Garcia and others 1998 Markus and others 1999 Howard and others 2000). Peppers have been reported to be rich in the provitamin A carotenoids (3-carotene, a-carotene, and (3-cryptoxanthin (Minguez-Mosquera and Hornero-Mendez 1994 Markus and others 1999), as well as xanthophylls (Davies and others 1970 Markus and others 1999). Bell peppers have been shown to exert low antioxidant activity (Al-Saikhan and others 1995 Cao and others 1996 Vinson and others 1998) or may even act as pro-oxidants (Gazzani and others 1998). 

Ripe tomato fruits accumulate significant amounts of lycopene, but only trace amounts of xanthophylls. Dharmapuri and others (2002) overexpressed the lycopene (3-cyclase (b-Lcy) and (3-carotene hydroxylase (b-Chy) genes under the control of the fruit-specific Pds promoter, and transgene and protein expression was followed through semiquantitative reverse- transcription PCR, Western blotting, and enzyme assays. Fruits of the transformants showed a significant increase of (3-carotene, (3-cryptoxanthin, and zeaxanthin the carotenoid composition of leaves remained unaltered, and the transgenes and the phenotype were inherited in a dominant Mendelian fashion. 

In the enterocyte, provitamin A carotenoids are immediately converted to vitamin A esters. Carotenoids, vitamin A esters, and other lipophilic compounds are packaged into chylomicrons, which are secreted into lymph and then into the bloodstream. Chylomicrons are attacked by endothelial lipoprotein lipases in the bloodstream, leading to chylomicron remnants, which are taken up by the liver (van den Berg and others 2000). Carotenoids are exported from liver to various tissues by lipoproteins. Carotenes (such as (3-carotene and lycopene) are transported by low-density lipoproteins (LDL) and very low-density lipoproteins (VLDL), whereas xanthophylls (such as lutein, zeax-anthin, and (3-cryptoxanthin) are transported by high-density lipoproteins (HDL) and LDL (Furr and Clark 1997). 

Xanthophyll esters are common in fruits and vegetables. Few data exist regarding the effect of carotenoid esterification on carotenoid bioavailability. Xanthophyll esters are readily broken in the human intestine (West and Castenmiller 1998 Breithaupt and others 2003 Faulks and Southon 2005). Chitchumroonchokchai and Failla (2006) demonstrated that hydrolysis of zeaxanthin esters increases zeaxanthin bioavailability. Wingerath and others (1995) did not find (3-cryptoxanthin esters in chylomicrons from humans fed with tangerine juice. Herbst and others (1997) demonstrated that lutein diesters are more bioavailable than free lutein. However, the question of whether the free or the esterified form is more bioavailable to humans is still an ongoing discussion. 

Structurally, vitamin A (retinol) is essentially one half of the molecule of (3-carotene. Thus, (3-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted (3 ring with a Cn polyene chain is the minimum requirement for vitamin A activity, y -Carotene, a-carotene, (3-cryptoxanthin, a-cryptoxanthin, and (3-carotene 5,6-epoxide, all having one unsubstituted ring, have about half the bioactivity of (3-carotene (Table7.4) On the other hand, the acyclic carotenoids, devoid of (3-rings, and the xanthophylls, in which the (3-rings have hydroxy, epoxy, and carbonyl substituents, are not provitamin A-active for humans. 

Dissolve 1 to 2 mg lutein, zeaxanthin, P-cryptoxanthin, and other xanthophylls directly in 100 ml reagent alcohol containing 30 ppm BHT. Dissolve 1 to 2 mg lycopene, a-carotene, and P-carotene in 10 ml THF stabilized with BHT, then dilute to 100 ml with reagent alcohol. 

Kimura et al. (74) recommended a procedure in which the carotenoids are dissolved in petroleum ether, an equal volume of 10% methanolic KOH is added, and the mixture is left standing overnight (about 16 h) in the dark at room temperature. This treatment caused no loss or isomerization of /3-carotene, while completely hydrolyzing /3-cryptoxanthin ester. Losses of xanthophylls could be reduced to insignificant levels by using an atmosphere of nitrogen or an antioxidant. 

The human body stores a variety of carotenes (lycopene, a- and (3-carotenes), as well as xanthophylls (lutein, zeaxanthin and cryptoxanthin) [38]. Besides the main naturally occurring all-E configuration, there also exist some Z-stereoi-somers of (3-carotene and lycopene in the human serum at remarkable levels, as shown in Table 5.2.2 [39,40]. 

Horwitz, 2006). The carotene fraction elutes first, as it is more nonpolar compared to xanthophylls. Although the method is relatively inexpensive and does not require specialized equipment, it is time consuming and does not always yield the complete separation of carotenoid species. This causes a problem, especially when quantifying provitamin A carotenoids since carotene species (including a- and y-carotene) are all treated as 3-carotene. In addition, all xanthophylls (including provitamin A 3-cryptoxanthin) are ignored (Schwartz, 1998). 

Van Berkel and Zhou first tested (3-carotene with ESI positive in 1994 (van Berkel and Zhou, 1994). In this study, a doubly charged molecular ion of (3-carotene was observed as the primary species when triflur-oacetic acid was present in the solution. Van Breemen was the first to utilize ESI as an interface between HPLC and MS to analyze carotenoids (van Breemen, 1995). In this study, ESI operated in negative mode ionized xanthophylls (astaxanthin, (3-cryptoxanthin, and lutein), but did not ionize hydrocarbon carotenes (lycopene and (3-carotene). In contrast, ESI positive produced only [M" ] for all carotenoids in this study, and the addition of halogenated solvents to the post-column effluent greatly enhanced signal intensity (van Breemen, 1995). A later study by Guarantini et al. demonstrated the ability of ESI positive to produce both [M" "] and [M + H]" " for a number of xanthophylls, and these authors attributed the production of the two species to solvent system... 

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