Xanthophylls transport

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. 

So far, we have focused mainly on the potential pathways of carotenoid uptake by the RPE from the choroidal blood supply. As mentioned earlier, POS contain lutein and zeaxanthin, and their distal tips are phagocytosed by the RPE. Therefore POS is another source of xanthophylls in the RPE and this pathway of carotenoid delivery and their further fate can be easily tested in cultured RPE. It has been shown that exposure of RPE cells in vitro to HDL stimulates efflux of phospholipids from phagocytosed POS out of the cell (Ishida et al., 2006). Thus it is of interest to determine whether that transport may potentially include xanthophylls and whether other types of lipoproteins may... 

The yellow coloration in the Monarch as well as the larva of three other species of butterfly from South Florida is exclusively due to the specific accumulation of exceptionally high levels of lutein producing a pigmented epidermis. This active accumulation, reminiscent of the specific accumulation that occurs in the primate macula, indicates that butterfly larva is an excellent animal model for the study of carotenoid transport and binding. As such, elucidation of the mechanism of transport and binding of lutein in the epidermis and other tissues of these butterfly larvae may provide insight into xanthophyll uptake within the human eye (Bhosale et al. 2004). 

In the enterocyte, provitamin A carotenoids are immediately converted to vitamin A esters. Carotenoids, vitamin A esters, and other lipophilic compounds are packaged into chylomicrons, which are secreted into lymph and then into the bloodstream. Chylomicrons are attacked by endothelial lipoprotein lipases in the bloodstream, leading to chylomicron remnants, which are taken up by the liver (van den Berg and others 2000). Carotenoids are exported from liver to various tissues by lipoproteins. Carotenes (such as (3-carotene and lycopene) are transported by low-density lipoproteins (LDL) and very low-density lipoproteins (VLDL), whereas xanthophylls (such as lutein, zeax-anthin, and (3-cryptoxanthin) are transported by high-density lipoproteins (HDL) and LDL (Furr and Clark 1997). 

Despite the existing evidence attesting to the safety of dietary astaxanthin, little is known about the bioavailability and metabolism of this carotenoid in humans. Several steps are involved in the assimilation of carotenoids by mammals, including transfer from the food matrix, transfer to lipid micelles in the small intestine, uptake by intestinal mucosal cells, transport to the lymph system, and eventually, deposition of the carotenoid or its metabolites in specific tissues. " A number of factors can influence the progression of these steps, including the nature of the food matrix, " the structure of the carotenoid (including potential esterification and the nature of its isomeric composition), the presence of other carotenoids, " and the amount and types of lipids in the diet. Overall, human metabolism of astaxanthin should be somewhat similar to that of the other xanthophylls, but subtle differences are expected. 

The mechanism of the putative xanthophyll-mediated quenching is as yet unexplained. Perhaps the ascorbate-enhanced develops as a consequence of zeaxanthin forming a pigment-pigment or pigment-protein complex which interacts with PSll. Accordingly, antimycin s inhibitory effect on ascorbate-enhanced q may be explained as a direct or indirect block on zeaxanthin complex formation. Zeaxanthin formation and q development could also be completely independent phenomena that coincidentally respond to the redox state of electron transport components. Indeed, the availability of violaxanthin for deepoxidation has been related to a redox component near plastoquinone (4)... 

The possibility that epoxide carotenoids may function in oxygen evolution and transport has been suggested. In higher plants, three xanthophylls— violaxanthin, zeaxanthin, and antheraxanthin—undergo a series of photoin-duced interconversions (violaxanthin cycle) (Fig. 15), as reviewed by Hager (1975). Evidence as to the significance of this cycle is not conclusive, however. Two Russian workers, Sapozhnikov and Saakov, support the conclusion that the violaxanthin cycle is involved in oxygen transport (for review, see Sapozhnikov, 1973). However, other workers have concluded that ca-... 

Table 1 Comparison of Agglutination Reactions of /Xntisera to Lipids, Xanthophylls and Proteins with Inside-out Vesicles and with Stroma-freed Chioroplasts of. V. tabacwn var. J.W.B. and Comparison of the Inhibitory Effect of these. ntisera on Electron Transport Reactions of Photosystem II.

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