1Chapter 3 The Functional Role of Xanthophylls in the Primate Retina
In autumn, as chlorophyll gradually disappears from leaves, the colorful world of carotenoids unfolds with colors ranging from deep red to light yellow. The prime reason for the presence of carotenoids in plants, however, is not to generate the beauty of the autumnal colors; it is their capability to drive nonphotochemical quenching reactions and to dissipate the energy of excess absorbed light as heat to protect the photosynthetic reaction centers from damage that makes carotenoids so important (Nayak et al. 2001). For millennia, photosynthetic organisms have been using these important properties of carotenoids (Frank and Cogdell 1996). As evolution proceeded, at least one other system that is simultaneously exposed to light and oxygen has adopted these principles as well: the eye, where xanthophylls have been used not only as active antioxidants but also as passive intraocular (blue)-light lters. The occurrence of such intraocular color lters in the vertebrate kingdom was comprehensively reviewed in 1933 by Walls and Judd (1933). The yellow corneae of some sh species or the oil droplets in the retina of reptiles and birds are examples of these intraocular color lters based on the presence of carotenoids. An alternative carotenoid system, not based on oil droplets, is present in humans and nonhuman primates. This is the “macula lutea,” also called the yellow spot at the location of highest visual acuity within the retina. Two xanthophylls, lutein and zeaxanthin, are responsible for its distinctive yellow color. These molecules in the macula lutea are concentrated up to a combined concentration of about 1 mM, the highest concentration of carotenoids found anywhere in the primate body (Landrum et al. 1999b).