Though this xanthophyll concentration in the retina has been observed and associated with a lower risk for the disease, the cause of macular degeneration and the precise role these compounds play in protecting against vision loss remain a mystery.
But Harrison and colleagues had observed in their previous work that SR-B1 was involved when intestinal cells absorb these nutrients from the diet, and believed that the same transporter would be needed to help the nutrients travel to cells in the eye as well.
Lutein and zeaxanthin typically represent about 80 percent of the total carotenoid content of the retina, while beta-carotene, a major dietary carotenoid, is found in only trace amounts. That high concentration of one type of carotenoid over another also suggested that a specific binding protein would be involved in the absorption process, Harrison said.
The scientists worked with a line of human retinal pigment epithelial cells from the lining of the retina, which served as a model for how macula cells function. The researchers introduced to these cells three types of carotenoids typically found in eye cells the xanthophylls lutein and zeaxanthin, as well as beta carotene.
As expected, the retinal pigment epithelial cells absorbed much more of the xanthophylls than the beta carotene. To test the role of the SR-B1 transporter, the researchers used two different methods to block the protein's action. Under both experimental circumstances, blocking the SR-B1 protein also blocked the cells' absorption of the two xanthophylls by between 41 percent and 87 percent compared
|Contact: Earl Harrison|
Ohio State University