Department of Ophthalmology and Visual Sciences, Moran Eye Center, University of Utah, Salt Lake City, UT 84132.
Department of Neurobiology and Anatomy, University of Utah, Salt Lake City, UT 84112.
Proc Natl Acad Sci U S A. 2017 Oct 10;114(41):10882-10887. doi: 10.1073/pnas.1706332114. Epub 2017 Sep 5.
Carotenoids are plant-derived pigment molecules that vertebrates cannot synthesize de novo that protect the fovea of the primate retina from oxidative stress and light damage. -Zeaxanthin is an ocular-specific carotenoid for which there are no common dietary sources. It is one of the three major carotenoids present at the foveal center, but the mechanism by which it is produced in the eye is unknown. An isomerase enzyme is thought to be responsible for the transformation of lutein to -zeaxanthin by a double-bond shift mechanism, but its identity has been elusive. We previously found that -zeaxanthin is produced in a developmentally regulated manner in chicken embryonic retinal pigment epithelium (RPE)/choroid in the absence of light. In the present study, we show that RPE65, the isomerohydrolase enzyme of the vertebrate visual cycle that catalyzes the isomerization of all-retinyl esters to 11--retinol, is also the isomerase enzyme responsible for the production of -zeaxanthin in vertebrates. Its RNA is up-regulated 23-fold at the time of -zeaxanthin production during chicken eye development, and we present evidence that overexpression of either chicken or human RPE65 in cell culture leads to the production of -zeaxanthin from lutein. Pharmacologic inhibition of RPE65 function resulted in significant inhibition of -zeaxanthin biosynthesis during chicken eye development. Structural docking experiments revealed that the epsilon ring of lutein fits into the active site of RPE65 close to the nonheme iron center. This report describes a previously unrecognized additional activity of RPE65 in ocular carotenoid metabolism.
类胡萝卜素是脊椎动物无法从头合成的植物衍生色素分子，可保护灵长类动物视网膜黄斑免受氧化应激和光损伤。-玉米黄质是一种眼部特异性类胡萝卜素，没有常见的饮食来源。它是黄斑中心存在的三种主要类胡萝卜素之一，但它在眼睛中产生的机制尚不清楚。一种异构酶酶被认为负责通过双键移位机制将叶黄素转化为-玉米黄质，但它的身份一直难以捉摸。我们之前发现，在没有光照的情况下，-玉米黄质在鸡胚胎视网膜色素上皮（RPE）/脉络膜中以发育调控的方式产生。在本研究中，我们表明，脊椎动物视觉循环的异构水解酶 RPE65 也是负责脊椎动物 -玉米黄质产生的异构酶。它的 RNA 在鸡眼发育过程中 -玉米黄质产生时上调 23 倍，我们提供的证据表明，在细胞培养中过表达鸡或人 RPE65 会导致 -玉米黄质从叶黄素产生。RPE65 功能的药理抑制导致鸡眼发育过程中 -玉米黄质生物合成的显著抑制。结构对接实验表明，叶黄素的ε环与 RPE65 的活性位点靠近非血红素铁中心。本报告描述了 RPE65 在眼部类胡萝卜素代谢中的一个以前未被认识到的额外活性。
