ABSTRACT
Lutein and zeaxanthin interact with lipids and transmembrane proteins as structural molecules in bioactive phospholipid membranes. The unique hydroxyl moieties of these carotenoids are responsible for their polar properties and affect solubility, aggregation, and reactivity in membranes. Macular xanthophylls are most soluble in nonpolar or dipolar solvents.[6] In primates, xanthophylls are preferentially accreted in lipophilic tissue, but may align within hydrophilic aspects of phospholipid molecules in areas adjacent to the aqueous compartment of cells.[11]
The stereochemical configuration of lutein results in orientation of the e-hydroxyl group with the equatorial plane of its ionone ring; the b-hydroxyl group is oriented in an axial direction relative to its ring plane. For zeaxanthin, both b-hydroxyl groups are oriented in an axial direction relative to the ring structures. The stereochemical configuration of zeaxanthin thus constrains the C5-C6 and C50-C60 double bonds to 40 at the plane of the conjugated polyene chain. The functional implication of this condition is that the ionone ring system can operate independent of the polyene chain.[6,7]
Structural differences in the orientation of the hydroxyl groups in lutein and zeaxanthin impart specific stereochemical properties to each of these xanthophylls and may affect their recognition by transmembrane and binding proteins.[12] In model liposomic membrane systems, lutein has been observed to exist in orientations orthogonal and parallel to the membrane surface.[13] Zeaxanthin exists mainly in an orthogonal orientation.[14,15]
Lutein binds to b-tubulin, a cytoskeletal protein involved in maintenance of cell shape. It has been suggested that this structural complex may stabilize the dynamic volatility of tubulin within the retina.[16,17]
Absorption and Attenuation of Short-Wavelength Radiation
Lutein acts as a filter of short-wavelength light associated with photochemical damage and the generation of ROIs.[18] Photochemical retinal injury induced by short-wavelength light (440 nm) affects retinal photoreceptor outer segments and the retinal pigment epithelium (RPE), a layer of the retina that supports the photoreceptors. Photic damage is maximized at irradiation levels between 400 and 450 nm. At 440 nm, the intensity of light energy required to produce retinal damage is 1=20 that required around 533 nm.[19] Macular pigment has a peak spectral absorbance of 460 nm, a range of absorption from approximately 390 to 515 nm, and may filter 40-90% of incident ‘‘blue’’ light.[6]
The number of conjugated double bonds in the polyene chain and characteristics of the ionone rings
determine the peak spectral absorption of a carotenoid. Lutein contains 10 conjugated double bonds; zeaxanthin contains 11. In both of these compounds, nine of the bonds are fully conjugated.[7] Subtle differences in the interaction of unsaturated bonds within the polyene hydrocarbon chain with those of the ionone rings lead to stereochemical differences in these compounds; such relationships are manifested as differences in the spectral absorption parameters. The wavelength of maximum absorption for lutein is 445 nm. For zeaxanthin, the value is 451 nm (Fig 2).
