Chemistry Reference
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virtually identical for the isolated carotenoid molecule, the molecule in solution, or the molecule
in a cell environment. However, the applicability of the method can be expected to depend heavily
on potentially confounding tissue properties such as a saturation of the carotenoid Raman response
at high concentrations, and the existence of other molecules with potentially interfering scattering,
absorption, and/or l uorescence contributions. A crucial task therefore is the validation of the RRS
detection method for the particular tissue environment. If successful, RRS could be used as a novel
optical diagnostic method for the measurement of tissue carotenoid levels, potentially allowing one
to measure large populations in clinical and i eld settings, and to track their changes occurring over
time as a consequence of developing pathology and/or tissue uptake.
A tissue site that appears to be particularly interesting for the application of the Raman method
is the macula lutea. It is located in the human retina and contains the highest concentration of caro-
tenoids in the human body. Of the about ten carotenoid species found in human serum, only two
carotenoids, lutein and zeaxanthin, are selectively taken up at this tissue site. Their concentrations
can be as high as several 10 ng per gram of tissue, however, in the healthy human retina. Due to their
strong absorption in the blue-green spectral range, the macular carotenoids, also termed macular
pigment, MP, impart a yellow coloration to the macula, which contains a high density of photore-
ceptors, enabling high-acuity color vision. When viewed in cross section, MP is located anterior
to the photoreceptor outer segments and the retinal pigment epithelium (Snodderly et al. 1984a,b)
and therefore is thought to shield these vulnerable tissues from light-induced oxidative damage by
blocking phototoxic short-wavelength visible light. Also, MP may directly protect the cells in this
area, since lutein and zeaxanthin are efi cient antioxidants and scavengers of reactive oxygen spe-
cies. There is increasing evidence that MP may help mediate protection against visual loss from
age-related macular degeneration, AMD (Seddon et al. 1994, Landrum and Bone 2001, Krinsky
et al. 2003, Krinsky and Johnson 2005, AREDS 2007), the leading cause of irreversible blindness
affecting a large portion of the elderly population. Since the MP compounds are taken up through
the diet, there is a chance that early age screening of MP concentrations to identify individuals with
low levels of MP, accompanied with dietary interventions such as nutritional supplementation, will
help prevent or delay the onset of the disease.
MP concentrations in the healthy human retina are usually assumed to be highest in the very
center of the macula, the foveola, and to drop off rapidly with increasing eccentricity, especially
when using low-spatial resolution techniques such as heterochromatic l icker photometry (Snodderly
et al. 2004). However, recently emerging high-resolution optical imaging techniques based on lipo-
fuscin l uorescence (autol uorescence) excitation and rel ection methods have already demonstrated
a much more complex pattern of MP distributions in the living human retina, such as those with
depletions and ring-shaped concentration distributions (Robson et al. 2003, Trieschmann et al.
2003, Delori 2004, Berendschot and van Norren 2006). It would be important to coni rm these
interesting features with an imaging Raman method, which by comparison would be a more direct,
carotenoid specii c method, and to track the MP distributions and any potential changes occurring
in them upon dietary modii cations or supplementation.
Aside from the human retina, RRS spectroscopy also appears to be interesting for the detection
of carotenoids in human skin. In this tissue, which constitutes the largest organ of the human body,
the carotenoid species lycopene and beta-carotene are thought to play an important protective role
as antioxidants, like in the protection of skin from ultraviolet and short-wavelength visible radiation.
The carotenoids lutein and lycopene may also have protective functions for cardiovascular health,
and lycopene may play a role in the prevention of prostate cancer. It is conceivable that skin levels
of these species are correlated with corresponding levels in internal tissues. Objective measure-
ments of carotenoid levels are also of interest in improving dietary data collected in epidemiological
studies, which in turn are used in developing public health guidelines that promote healthier diets.
The protective effects of diets rich in fruits and vegetables have been observed for many disease
outcomes, including various cancers (Kolonel et al. 2000, Michaud et al. 2000) and cardiovascular
disease (Liu et al. 2000). Since carotenoids are a good biomarker for fruit and vegetable intake,
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