Chylomicron carotenoid transport

Fig. 2. Tissue distribution and metabolism of retinoids in fish. Dietary carotenoids (e.g. /3-carotene (/3C)) and retinyl esters (e.g. retinyl palmitate (RP)) are converted into retinol (Rol) in the lumen of the gut. Retinol is then re-esterified and packaged into chylomicrons and transported to the portal circulation. When required elsewhere, stored retinyl esters (e.g. RP) in the liver are hydrolyzed to retinol and transported in the blood bound to the retinol-binding protein (RBP). Retinol is converted in target tissues to RA, RP or retinal (Ral). RA may exert its effects locally, or be returned to the circulation and transported throughout the body bound to albumin. RA can then be sequestered in other tissues.

This approach can be used only for fat-soluble compounds that follow the same lymphatic route to be transported to the liver as carotenoids. The bioavailability of the compound of interest is determined by monitoring the appearance of the compound and its newly formed intestinal metabolites in the postprandial chylomicron fraction of plasma [also called the density < 1.006 kg/L fraction or triglyceride-rich lipoprotein (TRL) fraction because it is generally a mixture of chylomicrons (CMs) and very low density lipoproteins (VLDLs)] as a function of the time after ingestion. 

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). 

Vitamin A in foods is present in its ester form and, after enzyme-catalyzed hydrolysis, it is absorbed into the gastrointestinal tract. After forming an emulsion in the presence of bile salts and pancreatic juices, it is incorporated into molecules of chylomicrons (Olson, 1996). Other forms of carotenoids are oxidatively converted to retinal in the intestinal mucosa. Then, via lymphatic vessels, it is transported from the bloodstream to the liver (Cortner et al., 1987). Vitamin A is primarily stored in the liver, and is also found in ester form in lipocytes. Transport from the liver depends on the level of retinol binding proteins (RBP) (Wolf and Phil, 1991). Vitamin A is well absorbed... 

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