Retinoid isomerohydrolase
isomerase activity
RPE65 has an additional function as the lutein to <i>meso</i>-zeaxanthin isomerase in the vertebrate eye.
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. <i>meso</i>-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 <i>meso</i>-zeaxanthin by a double-bond shift mechanism, but its identity has been elusive. We previously found that <i>meso</i>-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-<i>trans-</i>retinyl esters to 11-<i>cis</i>-retinol, is also the isomerase enzyme responsible for the production of <i>meso</i>-zeaxanthin in vertebrates. Its RNA is up-regulated 23-fold at the time of <i>meso</i>-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 <i>meso</i>-zeaxanthin from lutein. Pharmacologic inhibition of RPE65 function resulted in significant inhibition of <i>meso</i>-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.