ABSTRACT
Xanthophylls in the Retina? ..........................................................................206 11.4 Why Are Macular Xanthophylls So Stably Stored in the Retina? ...............207 11.5 Can Xanthophyll-Membrane Interactions Enhance a Protective
Activity of Macular Xanthophylls Against AMD? ......................................208 11.5.1 High Incorporation Yield ..................................................................208 11.5.2 Transmembrane Orientation Enhancing Antioxidant Properties
of Carotenoids ...................................................................................208 11.5.3 Two Orientations of Lutein in Retina Membranes ...........................209 11.5.4 Indirect Antioxidant Action of Macular Xanthophylls
by Changing Membrane Properties .................................................. 210 11.5.5 Cis Isomers in Retina Membranes .................................................... 211 11.5.6 Macular Xanthophylls Located in the Most Vulnerable Regions
of Photoreceptor Outer-Segment Membranes .................................. 212 11.5.7 Colocalization of Lutein in Membrane Domains Together with
Unsaturated and Polyunsaturated Phospholipids Significantly Enhancing Antioxidant Activity ....................................................... 214
11.6 Concluding Remarks .................................................................................... 215 Acknowledgment ................................................................................................... 216 References .............................................................................................................. 216
Only two carotenoids (namely, lutein and zeaxanthin) are selectively accumulated in the retina of the human eye from blood plasma, where more than 20 other carotenoids are available (Khachik et al. 1997). A third carotenoid, mesozeaxanthin, is formed directly in the human retina (where it is found) from lutein (Landrum and Bone 2001). All of these dipolar, terminally dihydroxylated carotenoids (named “macular xanthophylls”) are accumulated in nerve fibers and photoreceptor outer segments (POSs) and impede the onset of age-related macular degeneration (AMD) (Mares-Perlman et al. 2002; Gale et al. 2003; Cho et al. 2004; Mares 2004). The distribution of macular xanthophylls in the retina is not uniform. Their concentration in the fovea can reach values as high as 0.1 to 1 mM, which is about 100 times greater than the values in peripheral retina and about 1000 times greater than in other organs and tissues (Bone et al. 1985; Landrum et al. 1999). Additionally, the ratio of total lutein to total zeaxanthin differs when comparing the central fovea to the more peripheral regions of the retina; zeaxanthin is predominant in the former and lutein in the latter (Bone et al. 1988; Handelman et al. 1988).
