Abstract
Human retinal macular pigment (MP) is formed by the carotenoids lutein and zeaxanthin (including the isomer meso-zeaxanthin). MP has several functions in improving visual performance and protecting against the damaging effects of light, and MP levels are used as a proxy for macular health–specifically, to predict the likelihood of developing age-related macular degeneration. While the roles of these carotenoids in retinal health have been the object of intense study in recent years, precise mechanistic details of their protective action remain elusive. We have measured the Raman signals originating from MP carotenoids in ex vivo human retinal tissue, in order to assess their structure and conformation. We show that it is possible to distinguish between lutein and zeaxanthin, by their excitation profile (related to their absorption spectra) and the position of their ν1 Raman mode. In addition, analysis of the ν4 Raman band indicates that these carotenoids are present in a specific, constrained conformation in situ, consistent with their binding to specific proteins as postulated in the literature. We discuss how these conclusions relate to the function of these pigments in macular protection. We also address the possibilities for a more accurate, consistent measurement of MP levels by Raman spectroscopy.
Highlights
The macula lutea is an oval-shaped, highly-pigmented region near the centre of the retina of humans and other primates, the “yellow spot” first described in the 18th Century [1]
We show that it is possible to discriminate between the different carotenoids constituting macular pigment (MP) using resonance Raman excited at different wavelengths
As the position of the (0,0) absorption transition of carotenoids is directly related to their conjugation length, this transition for zeaxanthin should be red-shifted relative to that of lutein by about 5 nm, for the pigments in the same conditions [17]
Summary
The macula lutea is an oval-shaped, highly-pigmented region (diameter 5–6 mm) near the centre of the retina of humans and other primates, the “yellow spot” first described in the 18th Century [1]. It is responsible for sharp, clear central vision and the ability to perceive colour. The yellow pigmentation of the macula is due to the presence of the dietary carotenoids lutein and zeaxanthin (including the isomer meso-zeaxanthin). Their functions are multiple–reduction of “haze” effects; reduction of low-light glare; and protection against age-related macular degeneration (AMD, the principal cause of blindness in old age).
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