Retinyl esters absorption

Intestinal absorption of dietary cholesteryl ester is decreased but retinyl ester absorption is normal in carboxyl ester lipase knockout mice. Biochemistry 38, 4143-4149. 

VAN VLIET T, SCHREURS w H and VAN DEN BERG H (1995) Intestinal beta-carotene absorption and cleavage in men response to beta-carotene and retinyl esters in the triglyceride-rich lipoprotein fi action after a single oral dose of beta-carotene. Am J Clin Nutr 62(1) 110-16. 

Vitamin A absorption from the small intestine requires dietary fat and pancreatic lipase to break down retinyl esters and bile salts to promote the uptake of retinol and carotene. Drugs, such as mineral oil, neomycin and cholestyramine, that can modify lipid absorption from the gastrointestinal tract can impair vitamin A absorption. The use of oral contraceptives can signihcantly increase plasma vitamin A levels. 

Fig. 1. The structures of key retinoids and their precursors. Fish convert retinyl esters (e.g. retinyl palmitate (RP)) and carotenoids (e.g. /3-carotene) to retinol in the gut lumen prior to intestinal absorption. Retinyl esters (e.g. RP) stored in the liver are synthesized from retinol by lecithin retinol acyltransferase (LRAT) and acyl CoAiretinol acyltransferase (ARAT). The retinyl esters are mobilized through their conversion to retinol by retinyl ester hydrolase (REH), which is then transported in the circulation to various sites in the body. Retinol is further metabolized within specific tissues to retinal by alcohol dehydrogenases (ADH) or short-chain dehydrogenase/reductase. Retinal is converted to the two major biologically active forms of retinoic acid (RA) (all-trans and 9-cis RA). Retinaldehyde dehydrogenase 2 (Raldh2) synthesizes all-trans RA from all-trans precursors and 9-cis RA form 9-cis precursors.

During absorption some /3-carotene is also converted to retinoid (Dimitrov et al, 1988 Olson, 1989 van Vliet et al, 1992 Scita et al, 1993) and transferred via a plasma chylomicron (renmant) retinyl ester compartment to a liver retinyl ester compartment. From here it is released in a plasma retinol-binding protein-retinol (RBP-ROH) compartment for transfer to target tissues. Eventually it is lost irreversibly from the RBP-ROH compart-... 

SD) in order to correspond with the known half-life of chylomicron retinyl esters (IS 10 min) in healthy adult men (Cortner et aL, 1987). Also, the model intestinal absorption of j8-carotene was constrained to be inside the range of two statistical deviations of 15 4.5% based on a j8-carotene balance study (Bowen et aL, 1993) in which 4.3 0.8 punol of a 28-jiunol dose of /3-carotene was absorbed in healthy subjects. Each of these constraints was achieved by including additional data points in the model. Finally, the irreversible loss of retinol from the model system was constrained to a minimum value of 0.7 pimol/day based on the rate of vitamin A depletion in humans (Sauberlich et aL, 1974). These additions to the model provided good statistical certainty on all model parameters, as the FSDs of the FTCs were <25% (see Table I). 

At present the only experimental studies concerning efficiency of absorption and intestinal bioconversion of /SC to vitamin A in humans stems from two reports from the mid 1960s using radiolabeled /SC. These studies were conducted in elderly hospitalized cancer patients with cannulated thoracic lymph ducts using C- or H-/SC and reported that the amount of intact /SC absorbed varied considerably (9 to 30% of dose) and that retinyl esters were the major lymphatic product of /SC metabolism, representing 61 to 88% of recovered H or (Goodman etal, 1966 Blomstrand and Werner,... 

Silicic acid absorption chromatography has also been used fairly extensively in the past for the separation of retinoids. Heat-activated silicic acid will separate retinol, retinaldehyde, and retinoic acid and its more polar metabolites, with recoveries reported to range from 60 to 100% (Dunagin et al., 1966 Zile and DeLuca, 1965, 1968 Garry et al., 1970 Pollack et al., 1973 J. A. Olson, personal communication). In contrast, however, retinyl esters show only a 10-15% recovery after chromatography on silicic acid with the production of various oxidation products (Zile and DeLuca, 1968 Pollack et al., 1973). The... 

Retinol formed by retinyl ester hydrolysis (or originating as such in the diet) and dietary -carotene are solubilized in mixed micelles as discussed above, thus enabling these molecules to reach the microvillus membrane. In studies with everted rat gut sacs in vitro, El-Gorab et al. (1975) reported that micellar solutions significantly enhance uptake of both retinol and p-carotene over emulsions. Maximal uptake occurred at the critical micellar concentration of the bile salt mixture. At higher detergent concentrations, 3-carotene uptake declined whereas retinol absorption remained high. 

Retinyl esters are secreted from absorptive cells mainly in the hydrophobic... 

Retinol, retinyl esters, and retinyl ethers and the carotene phytofluene display bright yellow-green fluorescence on TLC plates or open colunms when illuminated with a long-wavelength (366-nm) mercury lamp, and anhydroretinol appears red. Other retinoids and carotenoids are not fluorescent under these conditions, and must be identified by other methods such as absorption of iodine vapor (formation of brown spots when placed in a chamber containing iodine... 

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