Xanthophylls reactions

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... 

Stndies of the antoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. stndied the antoxidation of lycopene," P-carotene," and phytofluene" " in liposomal snspensions and identified oxidative cleavage compounds. Stabilities to oxidation at room temperature of various carotenoids incorporated in pig liver microsomes have also been studied." The model took into account membrane dynamics. After 3 hr of reactions, P-carotene and lycopene had completely degraded, whereas xanthophylls tested were shown to be more stable. 

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. 

The reaction of xanthophylls such as zeaxanthin (25), isozeaxanthin [j8,/S-carotene-4,4 -diol (78)], and lutein [)8,e-carotene-3,3 -diol (79)] with CHCI3-HCI... 

Demmig-Adams B and Adams, III WW (1996) The role of xanthophyll cycle carotenoids in the protection of photosynthesis. Trends Plant Sci 1 21-26 DiMagno TJ, Laible PD, Reddy NR, Small GJ, Norris JR, Schiffer M and Hanson DK (1998) Protein chromophore interactions spectral shifts report the consequences ofmutations in the bacterial photosynthetic reaction center. Spectrochimica Acta A 54 1247-1267... 

Xanthophyll pigments in the light harvesting complex (LHCII) are primarily bound in the -trans form (Kiihlbrandt et al, 1994). This does not mean that these xanthophyll pigments do not undergo triplet reactions. Triplets have been found in all major xanthophylls in plants, including lutein, neoxanthin and violaxanthin and xanthophyll triplets have been observed in isolated LHC (Peterman et al, 1995). 

Gilmore AM, Hazlett TL, Debrunner PG and Govindjee (1996b) Comparative time-resolved Photosystem II chlorophyll a fluorescence analyses reveal distinctive differences between photoinhibitory reaction center damage and xanthophyll cycle dependent energy dissipation. Photoschem Photobiol 64 552-563... 

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