The Functional Role of Xanthophylls in the Primate Retina - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 262

Topography of the MP

0

mm
Eccentricity

-2.75 -1.25 +1.25 +2.75

9.4 ° 4.3 ° 4.3 ° 9.4 °

degrees

2.5 mm

5.5 mm

D

C
A
a

a: Foveola
A: Fovea
C: Parafovea
D: Perifovea

1.5 mm

0.35 mm

0.4 μm

-0.75 -0.175 +0.175 +0.75 mm

Eccentricity
degrees

2.6 ° 0.6 ° 0.6 ° 2.6 °

0

FIGURE 13.4 (See color insert following page 336.) Topography of the MP. This i gure schematically
shows the distribution of the yellow MP across the retina: horizontally (top) and vertically (bottom). (From
Gass, J.D., Stereoscopic Atlas of Macular Diseases Diagnosis and Treatment , Vol. 1, Mosby - Year Book Inc.,
3, 1997. With permission.)

As already mentioned, macular zeaxanthin comprises two stereoisomers, the normal dietary
(3 R ,3 S )-zeaxanthin(=( meso )-zeaxanthin), of which the latter is not normally

a dietary component (Bone et al. 1993) and is not found in any other compartment of the body except in
the retina. The concentration of ( meso )-zeaxanthin in the retina decreases from a maximum within the
central fovea to a minimum in the peripheral retina, similar to the situation with (3 R ,3

′ R )-zeaxanthin and (3 R ,3 ′

R )-zeaxanthin.
This distribution inversely rel ects the relative concentration of lutein in the retina and gave rise to
a hypothesis (Bone et al. 1997) that ( meso )-zeaxanthin is formed in the retina from lutein. This was
coni rmed by an experiment in which xanthophyll-depleted monkeys had been supplemented with
chemically pure lutein or (3 R ,3

′

R )-zeaxanthin (Johnson et al. 2005). ( Meso )-Zeaxanthin was exclu-
sively detected in the retina of lutein-fed monkeys but not in retinas of zeaxanthin-fed animals, dem-
onstrating that it is a retina-specii c metabolite of lutein only. The mechanism of its formation has not
been established but may involve oxidation-reduction reactions that are mediated photochemically,
enzymatically, or both. Thus, ( meso )-zeaxanthin is a metabolite unique to the primate macula.
The observation that lutein in the retina is converted to ( meso )-zeaxanthin appears to be physio-
logically plausible: due to the presence of, in comparison to lutein, one additional conjugated double
bond, it has a stronger singlet oxygen quenching capability (Cantrell et al. 2003). Furthermore, it has
been reported that ( meso )-zeaxanthin provides a somewhat better protection against the oxidation

′