Macular pigment and its measurement

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Interest has been raised into the protective role of the oxygenated xanthophylls group of carotenoids in the eye, particularly the retina. This group includes lutein, zeaxanthin, and meso-zeaxanthin, and they are the only carotenoids present in the lens1 and retina.2-5 Within the retina they are also known as macular pigment (MP), and the proposed specific function of xanthophylls at the macula is supported by the fact that macular levels are several thousand times higher than serum levels.6

Macular pigment structure

In the central macula, lutein, zeaxanthin, and meso-zeaxanthin are found in equal quantities, but the ratio of meso-zeaxanthin to zeaxanthin decreases with increasing eccentricity.7 Meso-zeaxanthin has been found in the human macula, retina and retinal pigment epithelium (RPE), but not in the plasma or liver.8 This forms the basis for the assumption that meso-zeaxanthin is formed via isomerisation of lutein,7 and is not obtained directly through the diet. The conversion mechanism is thought to be concentrated at the macula.7 A putative lutein-binding protein has been found in the retinae of human eyes,9 which binds with high affinity and specificity to lutein and other xanthophylls. It has been suggested that people who are less responsive to xanthophyll supplementation may be so because of genetic differences that result in reduced or less efficient binding proteins.10 This protein may also act as an enzyme for the conversion of lutein to meso-zeaxanthin.