Purification and Partial Characterization of a Lutein-binding Protein from Human Retina

Dietary intake of lutein and zeaxanthin appears to be advantageous for protecting human retinal and macular tissues from degenerative disorders such as age-related macular degeneration. Selective concentration of just two of the many dietary carotenoids suggests that uptake and transport of these xanthophyll carotenoids into the human foveal region are mediated by specific xanthophyll-binding proteins such as GSTP1 which has previously been identified as the zeaxanthin-binding protein of the primate macula. Here, a membrane-associated human retinal lutein-binding protein (HR-LBP) was purified from human peripheral retina using ion-exchange chromatography followed by size-exclusion chromatography. After attaining 83-fold enrichment of HR-LBP, this protein exhibited a significant bathochromic shift of approximately 90 nm in association with lutein, and equilibrium binding studies demonstrated saturable, specific binding toward lutein with a K(D) of 0.45 muM. Examination for cross-reactivity with antibodies raised against known lutein-binding proteins from other organisms revealed consistent labeling of a major protein band of purified HR-LBP at approximately 29 kDa with an antibody raised against silkworm (Bombyx mori) carotenoid-binding protein (CBP), a member of steroidogenic acute regulatory (StAR) protein family with significant homology to many human StAR proteins. Immunolocalization with antibodies directed against either CBP or GSTP1 showed specific labeling of rod and cone inner segments, especially in the mitochondria-rich ellipsoid region. There was also strong labeling of the outer plexiform (Henle fiber) layer with anti-GSTP1. Such localizations compare favorably with the distribution of macular carotenoids as revealed by resonance Raman microscopy. Our results suggest that HR-LBP may facilitate lutein's localization to a region of the cell subject to considerable oxidative stress.