Absorption and Transport

Because carotenoids are hydrophobic molecules, they are associated with lipophilic sites in cells, such as bilayer membranes. Polar substituents such as hydroxyl groups decrease their hydrophobicity and their orientation with respect to membranes. Lycopene and 0-carotene are aligned parallel to membrane surfaces to maintain a hydrophobic environment, whereas the more polar xanthophylls lutein and zeaxanthin become oriented perpendicular to membrane surfaces to keep their hydroxyl groups in a more hydrophilic environment. These differences can affect the physical nature of a membrane as well as its function. Carotenoids can form complexes with proteins, which would aid them in moving through an aqueous environment. They can also interact with hydrophobic regions of lipo-proteins. Carotenoproteins have been found mainly in plants and invertebrates, but intracellular 0-carotene-binding proteins have been found in bovine liver and intestine and in livers of the rat and ferret. In addition, a xanthophyll-binding protein has been found in human retina and macula. Carotenoids are also present in nature as crystalline aggregates (lycopene in chromoplasts) or as fine dispersions in aqueous media (0-carotene in oranges).

In the intestinal lumen (Figure 3) where carote-noids are released from the food matrix, cleavage of carotenoproteins and fatty acid esters by carboxylic ester hydrolase, which is secreted by the pancreas, can occur. Carotenoids are then solubilized into lipid micelles. These hydrophobic compounds are thus more efficiently absorbed when accompanied by at least a small amount of fat. The amount of fat for optimal carotenoid absorption seems to differ among carotenoids. For example, lutein esters require more fat for optimal absorption than 0-carotene. These differences have not been quantified for each carotenoid. In addition, the presence of a non-absorbable, fat-soluble component was shown to decrease carotenoid absorption. Sucrose polyester, a nonabsorbable fat replacer decreased carotenoid

Stomach

Intestine

Intestinal mucosal cells

Lymph

Blood

Liver

Food matrix (crt)

Digestive enzymes

Bile salts

HCl crt

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