Vitamin A is absorbed in the intestine via the same pathway used for dietary lipids, requiring bile salts, dietary fat, and pancreatic juice. Retinyl esters are hydrolyzed in the intestinal lumen to retinol by pancreatic triacylglycerol lipase (EC 220.127.116.11) and the brush border enzyme lysophospholipase (EC 18.104.22.168). The retinol is absorbed into the enterocytes by facilitated diffusion at normal concentrations. At pharmacological doses, however, retinol can be absorbed by passive diffu-sion.23,24 Bioavailability for most forms of preformed vitamin A is 70% to 90%. Upon cellular entry, retinol associates with a retinol binding protein (RBP), most likely RBP2. RBPs represent a group of proteins involved in intracellular and intercellular binding and transport of vitamin A. These intra-cellular proteins function in the transport and metabolism of retinol and retinoic acid by solubilizing them in aqueous media and presenting them to the appropriate enzymes while protecting them from catabolizing enzymes.25 These proteins also limit the concentration of free retinoids within the cell.
Provitamin A carotenoids are absorbed by passive diffusion and also depend on absorbable fats and bile. It is currently assumed that one twelfth of normal dietary j-carotene but only one twenty-fourth of the other provitamin A carotenoids is absorbed and bioconverted. This explains the low activity of the provitamin A carotenoids in relation to retinol as shown in Table 28.5. The enterocytes are the main site of j-carotene transformation to retinal,26 but the enzymes that catalyze the transformation also occur in hepatic and other tissue. Up to 20% to 30% of provitamin A carotenoids are absorbed unchanged. Following absorption by the ente-rocytes, j-carotene is symmetrically cleaved into retinal by S-carotene 15,15'-monooxygenase (EC 22.214.171.124), which requires molecular oxygen.27 j-Carotene can give rise to two molecules of retinal; whereas with the other three carotenoids, only one molecule is possible by this transformation. This is a result of the composition of j-carotene, which has a structure identical to retinal in both halves of the molecule, whereas the other provitamin As have a structure identical to retinal in one half of the molecule (see Fig. 28.2). Asymmetric cleavage of the carotenoids by j-carotene-9,10-monooxygen-ase has also been reported,26,28,29 but its role in humans is not fully resolved. The retinal formed from the provitamin As is then reduced to retinol by retinol dehydrogenase (EC 126.96.36.199) or alcohol dehydrogenase (EC 188.8.131.52).
Within the enterocytes, the retinol is esterified by three enzymes, diacylglycerol O-acyltransferase (DGAT, EC 184.108.40.206), retinol O-fatty-acyltransferase (EC 220.127.116.11), also known as acyl-coenzyme A (CoA):retinol acyltransferase (ARAT), and phosphatidylcholine-retinol O-acyltransferase (EC 18.104.22.168), also known as lecithin:retinal acyltransferase (LRAT). LRAT esterifies retinol bound to RBP2, whereas ARAT can esterify unbound retinol. It has been proposed that LRAT esterifies retinol at normal doses, while ARAT esterifies excess retinol.30 It has also been suggested that high doses of retinol may increase retinoic acid formation in the enterocytes through a retinoic acid responsive element (RARE) that promotes RBP2 expression.31
The retinyl esters are incorporated into chylomicrons, which in turn enter the lymph. Once in the general circula
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