We discuss vitamin A with emphasis on its absorption, transport, cellular uptake, storage, and intracellular metabolism. Dietary retinyl esters are hydrolyzed to retinol in the intestinal lumen before absorption by enterocytes. Carotenoids are absorbed and then partially converted to retinol in the enterocytes. In enterocytes retinol is esterified before incorporation into chylomicrons together with triacylglycerols. Chylomicrons reach the general circulation by way of the intestinal lymph, and chylomicron remnants are formed in the blood capillaries. The remnants, which contain almost all the absorbed retinol, are cleared by the liver parenchymal cells, and to some extent also by cells in blood, bone marrow, adipose tissue, and spleen. The uptake is most probably mediated via surface receptors for low-density lipoproteins or a low-density lipoprotein-receptor-related protein. In the liver parenchymal cells the retinyl esters are rapidly hydrolyzed to retinol, which binds to retinol-binding protein. Normally, most of the absorbed retinol coming into the liver parenchymal cell is transferred on retinol-binding protein to stellate cells, which store retinol as retinyl esters in lipid droplets.
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