Vit A 3 pp - VITAMIN A 2011 Vitamin A Discovery •...

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Unformatted text preview: VITAMIN A 2011 Vitamin A Discovery • Discovered in early 1900s by McCollum (Univ Wisconsin) and Osborne and & Mendel (Yale) • Both groups studied effects of diets with purified protein and carbohydrate on growth and survival of young rats • Growth ceased and rats died unless supplemented with butter, fish oils etc • Unknown substance called “fat-soluble A” • Steenbock: –Factor in yellow vegetables with Vitamin A Discovery similar functions as fat-soluble factor A Vitamin A: Chemical Forms • Retinol • Retinol retinal • Retinal retinoic acid • Retinol can be oxidized reversibly to retina • Retinol can be oxidized reversibly to retinal – same biological properties • Retinal can be irreversibly oxidized to retinoic acid – active in growth but not vision or reproduction Vitamin A: Structure • Chemical Forms –Retinol usually in all-trans configuration –Usually present in tissues in esterified form (usually with palmitate) –Carotenoids •plant origin Vitamin A: Structure • - carotene •essentially 2 molecules of retinal attached tail to tail Vitamin A: Food/ Supplement Sources • Preformed retinol-animal products – Liver – Egg yolk – Margarine (fortified Margarine (fortified) – Milk-most fortified – Fish (tuna, sardines, herring) – Cod liver oil – Supplements- usually esters •Dietary vitamin A as retinyl ester- retinyl palmitate •Emulsification of fat-micelles taken up Digestion and Absorption Vitamin A •Retinyl esters hydrolyzed in lumen or brush border of cell Intracellular Retinoid-Binding Proteins • Four cellular retinol-binding proteins (CRBP-I-IV) and 2 cellular retinoic acid binding proteins (CRABP-I & II) – expressed in different cells and tissues solubilize retinoids – solubilize retinoids – protect from degradation – protect membranes which unbound A can damage – escort retinoids to enzymes that metabolize them • In small intestine CRBP-II is abundant – escorts retinol to lecithin:retinol Cellular Retinoid Binding Proteins escorts retinol to lecithin:retinol acyltransferase (LRAT) • esterifies retinol Provitamin A Carotenoids to Retinoids • Humans can absorb carotenoids intact • Cleaved within the mucosal cell or can be transported unmetabolized be transported unmetabolized • Can also be cleaved in liver • Cleavage enzyme is 15,15’-dioxygenase – Forms 2 molecules of retinal Some beta-carotene can be absorbed intact Vitamin A Absorption • Retinol must be esterified to leave mucosa • Newly formed retinyl esters incorporated into chylomicrons – Chylomicrons enter lymphatic circulation Vitamin A Transport to Liver • Some retinyl esters removed from chylomicrons by tissues – chylomicron remnants remain chylomicron remnants remain • Chylomicron remnants deliver retinyl esters to liver • Retinyl esters taken up by liver parenchymal cells – Esters are hydrolyzed within the Vitamin A Liver Uptake & Metabolism Esters are hydrolyzed within the...
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This note was uploaded on 12/09/2011 for the course HUN 4445 taught by Professor Sitren during the Spring '08 term at University of Florida.

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Vit A 3 pp - VITAMIN A 2011 Vitamin A Discovery •...

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