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Paul S Bernstein, Rajalekshmy Shyam, Aruna Gorusupudi, Binxing Li, Preejith P Vachali, Kelly Nelson; In vivo Mechanisms of meso-Zeaxanthin Production. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3618.
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© ARVO (1962-2015); The Authors (2016-present)
meso-Zeaxanthin is a carotenoid that is rarely encountered in nature outside of the vertebrate eye. It is not synthesized by plants or micro-organisms, and it is not a constituent of a normal human diet, yet this carotenoid comprises one-third of the primate macular pigment. Feeding studies in monkeys and quails have identified lutein to be the precursor of meso-zeaxanthin, but the biochemical mechanisms responsible for this process remain a mystery. In the current study, we undertook a systematic approach to identify the production of meso-zeaxanthin in an isolated biological system to help elucidate its biochemical mechanisms of production.
Fertilized eggs from White Leghorn chickens were incubated at 37°C under 50% humidity in dark conditions. Eggs were removed during various time points during development. Yolk, brain, eyes, blood, and liver were isolated from the embryo. Tissue homogenization, carotenoid extraction, and ester saponification were conducted. Tissue, yolk, and blood carotenoids were chromatographically separated on chiral or C-30 columns. At least 5 eggs were used for each time point under consideration.
Lutein and zeaxanthin were found in all non-ocular tissues that we examined, but no meso-zeaxanthin was ever detected. Among the ocular tissues, we first observed the presence of meso-zeaxanthin at E17 in the RPE/choroid, two days prior to its detection in the retina. In the following embryonic days, there was progressive increase in the meso-zeaxanthin concentration in the RPE/choroid and retina. Whenever RPE/choroid of an embryo was devoid of meso-zeaxanthin, the corresponding retina was negative as well. On the other hand, we never encountered an embryo in which the retina was positive for meso-zeaxanthin when the corresponding RPE/choroid was negative. Unlike the retina, RPE/choroid carotenoids are not esterified which makes them more accessible to potential enzymes that may mediate lutein to meso-zeaxanthin conversion.
This is the first report of developmentally regulated synthesis of meso-zeaxanthin in a vertebrate model system, and our observations suggest that the RPE/choroid is the primary site of meso-zeaxanthin synthesis. Identification of the meso-zeaxanthin isomerase enzyme in the developing chicken embryo will facilitate our ability to determine the mechanisms of production of this unique carotenoid in humans and to determine its possible role in ocular health and disease.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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