tion, chylomicrons are converted into chylomicron remnants, which are cleared primarily by the liver but possibly other tissues.30 As the esters enter the hepatocytes, they are hydrolyzed to retinol which is then bound to RBP1 or RBP4. These are released into the blood or transferred to liver stellate cells for storage. Within the stellate cells, the retinol bound to RBP is esterified for storage by LRAT and ARAT. Stellate cells contain up to 80% of the bodies vitamin A stores.32 The RBP-retinol complex released into the general circulation from hepatocytes or stellate cells is extensively bound to transthyretin (TTR), which protects RBP-retinol from glomerular filtration and subsequent renal excretion.33 Retinol alone is taken up by target cells, probably by passive diffusion, from circulating RBP-retinol and once inside, is bound to RBP1 and possibly RBP2. Other retinoids may also circulate in plasma in low amounts, such as free retinol and 13-cis retinoic acid, but these are most likely bound to albumin.
Elimination of vitamin A generally involves the glu-curonidation of retinol or retinoic acid followed by renal or biliary excretion that may result in enterohepatic cycling. Other pathways for oxidizing retinoids to more polar substances also exist. Normally, no unchanged retinol is excreted. Retinal, retinoic acid, and other metabolites are, however, found in the urine and feces.
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