RETINAL TUBULIN BINDS MACULAR CAROTENOIDS

Purpose. To investigate the biochemical mechanisms responsible for the specific uptake, concentration, and stabilization of the carotenoids lutein and zeaxanthin in the macula. Methods. Soluble extracts of bovi ne retina mixed with radioactive carotenoids were purified by hydropho bic interaction, ion exchange, and gel filtration chromatography. Caro tenoid-associated proteins in these purified preparations were identif ied through photoaffinity labeling and protein microsequencing. Simila r purifications on human macular tissue without the addition of exogen ous carotenoids also were performed. Results. Experiments on bovine re tinal tissue demonstrated that tubulin is the major soluble carotenoid -binding protein. When soluble extracts of human macular protein were examined, the endogenous carotenoids lutein and zeaxanthin were found to copurify with tubulin. Conclusions. Tubulin is found in abundance i n the receptor axon layer of the fovea, where it can serve as a locus for the deposition of the high concentrations of macular carotenoids f ound there. The binding interaction of carotenoids and tubulin in the Henle's fiber layer could play an important role in the photoprotectiv e effects of the macular carotenoids against the progression of age-re lated macular degeneration. The association of carotenoids with tubuli n, a protein that can form highly ordered linear arrays, may provide a n explanation for the dichroic phenomenon of Haidinger's brushes.