Explain the Transportation of Vitamin A?
The chylomicra and retinal-binding protein play an important role in the transport of retiuol. This chylomicra complex enters the lymphatic system via the thoracic duct and into systemic circulation. chylomicra deliver retinyl esters, some unesterified retinol and carotenoids to many extra hepatic tissues such as bone marrow, spleen. Blood cells, lungs, kidney. Chylomicron remnants deliver retinyl ester and a portion of the casotenoids not taken by peripheral tissue to the lives. For carotenoids reaching the liver, a small portion can be cleaved to form retinol, some may be incorporated into the very low-density lipoprotein (VLDLs) synthesized in the liver, and then be released as part of VLDLs for circulation to various tissues in the body and some may be stored in the liver.
As for the retinyl esters, reaching the liver, hydrolysis of retinyl esters occurs. Within the cells, retinol binds with 'a cellular retinol-binding protein (C-P). CRBP is thought to function both to help control concentration of free retinol within the cell cytoplasm and thus prevent its oxidation, and to direct the vitamin to specific enzymes of metabolism. The enzymatic metabolism of retinol includes esterification by enzymes such as lecithin retinol acyl trunsfel.asc (LRAT) or acyl Co A retinol acyl trunsjrm.se (ARAT), oxidation of retinol to retinal by NAD (P)H-dependent retinal dehydrogenase, and phosphoryiation of retinol to retinyl phosphate by ATP for glycoprotein function.
Retinol not metabolized or transported from the liver may be stored in shall cells called stellate cells (along with lipid droplets) following re-esterification, Retinol mobilization from the liver and delivery to target tissue are dependent on the synthesis and secretion of retinol-binding protein (RBP). It would be interesting to note that RBP is a 183 amino acid residue, has a molecular weight of about 21,000 and is present in concentration at 3-4 mg/dl RBP in turn, joins the binding site on a larger protein, transthyretin (TTR). Thus the hepatic parenchymal cell is involved in the uptake and storage of vitamin A in the liver and its release into circulation as the retinol-RBP-TTR complex. The retinol-RBP-TTR complex circulates in the plasma with a half-life of about 11 hours. Some tissues that take up retinol from the RBP-TTR complex include the adipose, skeletal, kidney, white blood cells and bone marrow.