Xanthophyll formation

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... 

Douillard R, Burghoffer C, and Costes C. 1983. Conditions de formation et proprietes des complexes excito-niques formes par les xanthophylles en presence d eau. Physiologie Vegetale 21 375-383. 

As already mentioned, macular zeaxanthin comprises two stereoisomers, the normal dietary (3/(,37()-/caxanthin and (3f ,3 S)-zeaxanthin(=(meyo)-zeaxanthin), of which the latter is not normally a dietary component (Bone et al. 1993) and is not found in any other compartment of the body except in the retina. The concentration of (tneso)-zeaxanthin in the retina decreases from a maximum within the central fovea to a minimum in the peripheral retina, similar to the situation with (3/ ,37 )-zeaxanthin. This distribution inversely reflects the relative concentration of lutein in the retina and gave rise to a hypothesis (Bone et al. 1997) that (meso)-zeaxanthin is formed in the retina from lutein. This was confirmed by an experiment in which xanthophyll-depleted monkeys had been supplemented with chemically pure lutein or (3/ ,37 )-zeaxanthin (Johnson et al. 2005). (Meyo)-Zeaxanthin was exclusively detected in the retina of lutein-fed monkeys but not in retinas of zeaxanthin-fed animals, demonstrating that it is a retina-specific metabolite of lutein only. The mechanism of its formation has not been established but may involve oxidation-reduction reactions that are mediated photochemically, enzymatically, or both. Thus, (meso)-zeaxanthin is a metabolite unique to the primate macula. 

Saponification causes a significant loss of xanthophylls, even when carried out under relatively mild conditions (ambient temperature for 3 h) (21). In addition, several different saponification procedures have been shown to promote the formation of cis isomers of /3-carotene (74). Since saponification prolongs the analysis and is error prone, it should be carried out only when needed, as in high-fat samples or those containing carotenol esters. 

Mein, J.R. Dolnikowski, G.G. Ernst, H. Russell, R.M. Wang, X.D. 2011. Enzymatic formation of apo-carotenoids from the xanthophyll carotenoids lutein, zeaxanthin and P-cyryptoxanthin by ferret carotene-9, lO -monooxygenase. Arch. Biochem. Biophys. 506 109-121. 

Carotene is the major dietary precursor of vitamin A and therefore represents a fundamental component in our diet. The later steps of carotenoid biosynthesis in plants involve the formation of xanthophylls, which are oxygenated derivatives. Among these, capsanthin results from the activity of a bifunctional enzyme, the capsanthin-capsorabin synthase (CCS), that catalyses the conversion of the ubiquitous antheraxanthin and violaxanthin, into capsanthin and capsorubin (Fig. 11.3). 

Pathways. Studies of carotenoid transformations that take place when a mutant strain, PGl, of the green alga Scenedesmus obliquus is transferred from dark to light conditions have indicated that the transformations 15-cw-phytoene (180) 15-c/5-phytofluene (181) - 15-cis- -carotene (182) -> trans-C-caro-tene (183) (Scheme 7) take place in the biosynthesis of the normal cyclic carotenoids. The results were also in agreement with the formation of the xanthophylls lutein (16) and zeaxanthin (174) from the corresponding carotenes. 

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