Xanthophyll metabolism

DHARMAPURI S, ROSATI C, PALLARA P, AQUILANI R, BOUVIER F, CAMARA B and GIULIANO G (2002) Metabolic engineering of xanthophyll content in tomato fruits , FEBSLett, 519, 30-34. 

However, the metabolic pathways of lutein and zeaxanthin are only beginning to be discovered. Several derivatives of dietary xanthophylls have been identified in the retina, such as 3 -epilutein, meso-zeaxanthin, 3 -oxolutein, and 3-methoxyzeaxanthin, and it has been suggested that they may be formed as a result of nonenzymatic oxidative modifications (Bernstein et al., 2001,2002b Bhosale et al., 2007b Khachik et al., 1997). The macula lutea contains predominantly meso-zeaxanthin (Figure 15.1), which is believed to originate from either oxidative modification or double bond isomerization of dietary lutein (Khachik et al., 1997, 2002). 

Interestingly, it has been shown that supplementation of greenfinches with lutein and zeaxanthin at a ratio of 20 1 increases plasma levels of triglycerides and bird body mass (Horak et al., 2006). These data suggest that xanthophylls may affect lipid metabolism. 

The incorporation of label from mevalonate into ABA, a sesquiterpenoid, has been demonstrated in different parts of plants ( . . 41). This indicates that ABA can be synthesized throughout the plant. In addition to the direct incorporation of three isoprene units, derived from mevalonate, into ABA, an indirect biosynthetic pathway via carotenoids has been proposed. This idea stems from the finding that xanthophylls, in particular violaxanthin, can either photochemically or enzymatically be converted to the neutral inhibitor xanthoxin (42) (Figure 3). When labeled xanthoxin was fed in the transpiration stream to bean or tomato shoots, ca. 10% was converted to ABA over an 8-hr period (43). However, the importance of the biosynthetic route to ABA via xanthophylls and xanthoxin in normal metabolism remains to be established, and most of the evidence favors the direct synthesis route via a precursor (see 2). 

Retinoid-A term undergoing nearly continual modification in order to accurately describe the characteristics of a large family derived from the carotenes and xanthophylls. The family exhibits specific properties required in vision, metabolism and reproduction. Many of these properties are dependent on the stereo-chemistry of the materials. Only some of the stereo-chemical features may be required in a given application. The definition of Spom, et. al. in 1994 is the most widely used. However, it does not stress the visual properties required to form a chromophore in vision. 

Triglycerides are the principal ingredients of fats and oils from vegetable and animal sources. The crude oils contain minor quantities of free fatty acids and other substances. Jamieson and Baughman14 report that other substances present in crude cottonseed oil are raffinose, pentosans, resins, proteoses, peptones, xanthophyll, and chlorophyll. Most of these so-called impurities perform necessary functions in the metabolism of the animal or vegetable from which they are derived, and certain impurities which act as antioxidants are of value in industrial and domestic applications of the oils. An ideal method of purification would remove the undesirable components without disturbing those that have value. 

Despite the existing evidence attesting to the safety of dietary astaxanthin, little is known about the bioavailability and metabolism of this carotenoid in humans. Several steps are involved in the assimilation of carotenoids by mammals, including transfer from the food matrix, transfer to lipid micelles in the small intestine, uptake by intestinal mucosal cells, transport to the lymph system, and eventually, deposition of the carotenoid or its metabolites in specific tissues. " A number of factors can influence the progression of these steps, including the nature of the food matrix, " the structure of the carotenoid (including potential esterification and the nature of its isomeric composition), the presence of other carotenoids, " and the amount and types of lipids in the diet. Overall, human metabolism of astaxanthin should be somewhat similar to that of the other xanthophylls, but subtle differences are expected. 

Dharmapuri S, Rosati C, Pallara P, Aquilani R, Bouvier F, Camara B, GiuUano G (2002) Metabolic engmetaing of xanthophyll ctmtent in tomato fruits. F BS Lett 519 30-34... 

The metabolic engineering of xanthophyll contents in tomato has also been tried. The overexpression of lycopene p-cyclase (LCY-B) and a Capsicum p-carotene hydroxylase (B-CHY) using the fruit-specific pds promoter resulted in increased... 

The presence of oxygenated functional groups also modifies the bioavailability of these compounds. It has been demonstrated recently that some ketocarotenoids are more rapidly absorbed and metabolized than other carotenes such as, for instance, lycopene. These xanthophylls do not present provitamin A activity, but their antioxidant action is more effective than that of p-carotene. The incorporation of the carotenoid pigments into cell structures is affected by the pigment stmcture and the presence of functional groups that may modify the interaction with other molecules. Such stmcture, as mentioned above, determines the effectiveness of the pigment s action. 

Upon presenting the Nobel Prize to Karrer, Wilhelm Palmaer, chair of the prize committee at the time, described him as a scientist with the ability to visualize great and important problems as well as their smaller parts and one who in his own unique way approached problems and pursued new ideas by using his own methods [61] . Karrer s methodology has borne much fruit over the decades. The spinoffs from his work on the carotenoids and xanthophylls is still evolving today intense research on vision, vitamins, hormones, metabolic pathways, and enzymes. 

The carotenogenic enzymes from the substrate IPP to B-ca-rotene are shown in Fig. 3. The further metabolism of carotenes to xanthophylls and other oxygenated carotenoids is scarcely understood on the enzyme level and will not be treated here. 

GGPP is a key branch point metabolite of polyisoprenoid metabolism. Apart from the cyclic diterpenes, it is the precursor of the tetraterpenes (carotenes, xanthophylls), and acyclic diterpenes (phytol). Cyclic diterpenes compete with compounds of established importance for either GGPP or its precursors (e.g., famesyl pyrophosphate). The spotty nature of resin diterpene distribution may in part be explained by the obvious demand for carotenoids and phytyls and the relatively unestablished requirement for these nongibberellin diterpenes. 

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