Xanthophyll General

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. 

Chemical properties of carotenoids play an important role in carotenoid micellarization and, therefore, bioavailability. Apolar carotenoids (carotenes) are generally incorporated in the central region, which is highly hydrophobic, of the oil droplets, whereas polar carotenoids (xanthophylls) are localized on the surface, and therefore xanthophylls are more easily micellarized and absorbed than carotenes (Borel and others 1996). van het Hof and others (2000) found in humans that lutein is five times more bioavailable than (3-carotene. 

Vegetables are the major sources of provitamin A carotenoids, followed by fruits. Milk products, egg yolk, shellfish, and Crustacea also contain active carotenoids, which are derived from the animal s diet. The carotenoid distribution in green leafy vegetables is generally constant, whereas fruits contain a greater variety of carotenoids in varying concentrations. In ripening fruits, the decrease in chlorophylls is frequently accompanied by an increase in the concentration of carotenoids and an increase in the ratio of carotenes to xanthophylls. 

In general, RP-HPLC has been applied successfully to the qualitative and quantitative estimation of carotenes (64), xanthophylls, cis- and trans-carotenoids (54,60), and carotenoid fatty acid esters (53,65). Figure 5 shows an HPLC separation of (A) paprika extract, containing carotenoid esters, and (B) saponified paprika extract, containing the corresponding carotenoids (74a). 

While esterified xanthophylls can be extracted using solvent mixtures similar to those used for hydrocarbon carotenes, nonesterified xanthophylls are generally extracted using more polar solvents. For example, xanthophylls have been extracted from spinach using a mixture of methanol and tetrahydroftiran (Kopas-Lane and Warthesen, 1995). Acetone alone has also been used to extract lutein and zeaxanthin from a variety of fresh and processed vegetables (Updike and Schwartz, 2003). However, for the extraction of a wide range of carotenoids, a mixture of polar and nonpolar solvents works best. 

Carotenoids. Carotenoids are fat-soluble pigments comprised of isoprene units and, in plants, are generally associated with membranes (30). Carotenoids are generally classified as either carotenes, which are structurally related to hydrocarbons, or xanthophylls which are hydroxy, epoxy and oxy derivatives and are frequently esterified. The carotenoids most important in Imparting color to fruits and vegetables are derivatives of a- and -carotenes and lycopene. Carotenoids vary in their stability but, due to their unsaturated nature, they are generally susceptible to oxidation. Carotenes are important to... 

Low O2 generally delays or Inhibits the synthesis of lycopene, 11-carotene, and xanthophylls In tomato fruit (31.321. In sweet pepper, high COg delayed development of red color equally whether combined with 21% or 3% 02 (33). C2H4 Is known to accelerate the biosynthesis of carotenoids (34). 

The remainder of the fibre is composed of waxes, which are generally found on the fibre surface pigments, primarily chlorophyll, xanthophyll and carotene, along with other coloured material and residual protein. 

Since increased 02 generation appeared to correlate with increased carotenoid biosynthesis, we examined the effect of DQ exposure on carotenoid levels and composition. Exposure of cultures to DQ increased the levels of carotenoid produced by about 40%, and also increased the relative proportion of xanthophylls and diminished the levels of carotene precursors (31). In addition to producing more carotenoids, the increased proportion of xanthophylls would provide greater resistance to oxidative stress since xanthophylls are generally more effective antioxidants than carotenes (2). This pattern of increased levels of carotenoids and a higher proportion of xanthophylls also takes place as cultures age. The astaxanthin pathway may function in part to prevent aging of yeast and possibly to supply antioxidant capacity to their progeny. Microscopic examination of autofluorescence supported that carotenoids are... 

In order to define the carotenoid structures necessary for LHCII assembly and stabilization, a number of different carotenoids have been used in reconstitution assays with only one carotenoid component present. Not only the xanthophyll cycle carotenoids zeaxanthin and antheraxanthin turned out to promote reconstitution but also heterologous carotenoids as diverse structurally as astaxanthin, okenone, and fucoxanthin. In general, a hydroxyl group in position 3 of at least one of the cyclohexane ring seems to be important for complex formation (D. Phillip, S. Hobe, A. Young, and H. Paulsen, unpublished). Similarly, the major LHCII from... 

The comproportionation constant K m can be measured by simultaneous electrochemistry andEPR (SEEPR) and has been determined not only for the three carotenoids mentioned above (Khaled et al, 1991), but also for the xanthophylls echinenone, isozeaxanthin, and rhodoxanthin (Jeevarajan et al, 1994b). The EMR spectra of the monocations of these pigments are generally found dX g 2.0026 with a Gaussian half width ofthe order of 13 to 16 G. 

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