The carotenoids of macular pigment and bisretinoid lipofuscin precursors in photoreceptor outer segments

The oxygen atom–containing carotenoids (xanthophylls), zeaxanthin and lutein, Figure 16.1, are obtained by humans through the dietary intake of fruits and vegetables and become incorporated in the retina as macroscopically visible macular pigment. These yellow-colored pigments are particularly abundant in the fovea, their concentration declining steeply toward the peripheral retina. Of the two carotenoids, zeaxanthin is more concentrated in the central 10° of the retina while lutein dominates at eccentricities greater than 35° (Bone et al., 1988; Snodderly et al., 1991). The specificity of this distribution indicates the selective uptake of macular pigments by specific binding proteins (Bhosale et al., 2004). Nevertheless, the concentration of lutein and zeaxanthin in the macula varies among individuals (Bone et al., 1997) and it is likely that the extent of oral intake is responsible for these differences (Hammond et al., 1997; Landrum et al., 1997). Indeed, the long-term intake of the dietary supplements of lutein increases the levels of macular pigment (Bhosale et al., 2007). The highest levels of lutein and zeaxanthin are present in photoreceptor cell axonal processes (Henle’s fibers) (Snodderly et al., 1984) but 25% of total retinal carotenoids are present within photoreceptor outer segments (Rapp et al., 2000; Sommerburg et al., 1999). Given their hydrophobicity, lutein and zeaxanthin readily integrate into the lipophilic compartment of cell membranes (Landrum and Bone, 2001).