Carotenoids photoreceptors

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

Song PS and Moore TA. 1974. On the photoreceptor pigment for phototropism and phototaxis Is a carotenoid the most likely candidate Photochemistry and Photobiology 19 435 441. 

The recognition of the importance of MP in maintaining the health of the retina has led to the development of a number of methods for determining its concentration in situ. These methods, necessarily noninvasive, are routinely employed in dietary supplementation studies with lutein or zeaxanthin to monitor the uptake of the carotenoids into the retina. Every method exploits the optical properties of lutein and zeaxanthin, specifically their absorbance at visible wavelengths. The detection of a light signal, modified by the carotenoids, is accomplished either by the retinal photoreceptors themselves (psychophysical methods) or by a physical detector such as a photomultiplier,... 

Kirschfeld, K. (1982). Carotenoid pigments Their possible role in protecting against photooxidation in eyes and photoreceptor cells. Proc. R. Soc. Lond. B Biol. Sci. 216(1202) 71-85. 

Carotenoids accumulating in the human body are obtained exclusively from our diet. Out of almost 50 carotenoids present in a typical human diet, about 14 are absorbed into the blood (Khachik et al., 1997), and only two of them—lutein and zeaxanthin (Figure 15.1)—accumulate in the retina (Bernstein et al., 2001 Bone and Landrum, 1992 Bone et al., 1988, 1997 Davies and Morland, 2004 Khachik et al., 1997, 2002). Lutein and zeaxanthin are particularly concentrated in photoreceptor axons and inner plexiform layer in the area including and surrounding... 

The greatest concentration of the macular pigment is present in the avascular part of the retina. This suggests that the RPE may play the predominant role in uptake and transport of xanthophylls to the photoreceptors. Moreover, about 25% of the total retinal xanthophylls are present in the POS (Rapp et al., 2000 Sommerburg et al., 1999), which, under normal conditions, are intimately associated with the RPE. This proximity lends further support to the hypothesis of a role for the RPE in the selective uptake of carotenoids into the retina. 

Once internalized within the RPE, there must be a mechanism for carotenoid transport to photoreceptors. The RPE metabolizes lipids from phagocytosed POS and provides a constant supply of lipids to photoreceptors for the synthesis of new discs and molecular renewal of lipids within existing discs (Strauss, 2005). Thus there is a constant transfer of lipids from the RPE to photoreceptors. It has been shown in the rabbit and monkey that intraveneous administration of lipophilic benzopor-phyrin bound to LDLs results in an efficient delivery of the fluorescent photosensitizer not only to the RPE but also to photoreceptors this occurs within 20 min following injection (Haimovici et al., 1997 Miller et al., 1995). 

In addition to its presence in the RPE, ABCA1 has been found to be localized in the neural retina, particularly in the ganglion cell layer and rod photoreceptor inner segments (Tserentsoodol et al., 2006a), suggesting it may be involved in carotenoid transport throughout the retina. 

It has been shown in many studies that protective effects of carotenoids can be observed only at small carotenoid concentrations, whereas at high concentrations carotenoids exert pro-oxidant effects via propagation of free radical damage (Chucair et al., 2007 Lowe et al., 1999 Palozza, 1998, 2001 Young and Lowe, 2001). For example, supplementation of rat retinal photoreceptors with small concentrations of lutein and zeaxanthin reduces apoptosis in photoreceptors, preserves mitochondrial potential, and prevents cytochrome c release from mitochondria subjected to oxidative stress induced by paraquat or hydrogen peroxide (Chucair et al., 2007). However, this protective effect has been observed only at low concentrations of xanthophylls, of 0.14 and 0.17 pM for lutein and zeaxanthin, respectively. Higher concentrations of carotenoids have led to deleterious effects (Chucair et al., 2007). 

As the RPE plays an important role in lipid metabolism and regulation of dynamic transport between the choriocapillaris and photoreceptors, it is important to determine whether carotenoids affect these pathways in the RPE. 

The Carotenoids of Macular Pigment and Bisretinoid Lipofuscin Precursors in Photoreceptor Outer Segments... 

The basic structures of the two commonly suggested bluelight photoreceptor candidates, riboflavin and carotenoid, are shown in Fig. 4. 

Nevertheless, the avena coleoptile exhibits a curvature to unilateral UV-illumina-tion with a satisfactory log-linear response/time relationship38) (the bending mode is similar to that observed for the second positive curvature which develops from the coleoptile base cf. 2.2). Fig. 5 338) shows that the double-peaked action spectrum does not match neither flavin (Fig. 5 5,16S)) nor carotenoid absorption (Fig. 5 4,183)), most likely excluding both as photoreceptors. The growth hormone auxin (cf. 2.4 and Scheme 1) has been discussed to be a possible photoreceptor. However, in this case, this is not supported by the action spectrum either. 

It has been suggested that the photoreceptor might be the 15—15-cis-isomer of j3-carotenoid, which — in contrast to the trans-isomer — shows a UV-peak, as demonstrated in Fig.6 2 and 3,183,194). However, its position exhibits ahypsochromic shift of 30 to 40 nm compared to known action spectra. Moreover, in the only case investigated, that of Phycomyces, no cis-0-carotenoid has been found138). 

Composite action spectra characteristics of carotenoid (Fig. 8 2,169)) and flavin (Fig. 8 1,49)), imitated by the low temperature absorption spectra, are compared with the avena action spectrum (Fig. 8 3). Song and Moore pointed out on this basis, that the carotenoid is a rather unlikely photoreceptor, whereas the flavin is169). 

And only for that (extrapolated) case, the near UV-peak of the action spectrum vanishes, from which the authors conclude that the active photoreceptor is probably carotenoid in nature (cf., Fig. 6 3). 

Chlorophylls occur very frequently in the plant kingdom, they are responsible for the colour of vegetables and some fruits. They also occur in algae and several bacteria. Chlorophylls in plants are photoreceptors and in photosynthesis the presence of a closed circuit of conjugated double bonds allows them to absorb light. Because of their predominant importance as photoreceptors a considerable number of analytical methods have been developed for the separation and quantitative determination. The analytical methods applied for the measurement of chlorophylls and carotenoids in food products have been reviewed previously [273],... 

Foster In view of the concentration of chromophore in the visual system, a 500-fold reduction of chromophore in the eye may have very httle effect on the inner retinal photoreceptors. It is striking that Drosophila carotenoid-depletion experiments only reduce the visual ERG but did not abolish it. 

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