1pp Vitamin A 2011

1pp Vitamin A 2011 - VITAMIN VITAMIN A Spring 2011 2011...

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ITAMIN A VITAMIN A pring 2011 Spring 2011
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itamin A Discovery Vitamin A Discovery iscovered in early 1900s by McCollum (Univ • Discovered in early 1900s by McCollum (Univ Wisconsin) and Osborne and & Mendel (Yale) •B o t h groups studied effects of diets with purified protein and carbohydrate on growth and survival of young rats rowth ceased and rats died unless supplemented • Growth ceased and rats died unless supplemented with butter, fish oils etc • Unknown substance called “fat-soluble A”
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Vitamin A Discovery • Steenbock: –Factor in yellow vegetables with milar functions as fat- luble similar functions as fat soluble factor A
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Vitamin A: Chemical Forms •Re t ino l • Retinol retinal • Retinal retinoic acid • 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
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Vitamin A: Structure • Chemical Forms –Retinol usually in all-trans configuration –Usually present in tissues in sterified form (usually with esterified form (usually with palmitate)
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Vitamin A: Structure –Carotenoids •plant origin β -carotene •essentially 2 molecules of tinal attached tail to tail retinal attached tail to tail
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Vitamin A: Food/ Supplement Sources reformed retinol- nimal products Preformed retinol animal products –L iver gg yolk Egg yolk – Margarine (fortified) ilk ost fortified – Milk-most fortified – Fish (tuna, sardines, herring) od liver oil – Cod liver oil – Supplements- usually esters
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Digestion and Absorption Vitamin A •Dietary vitamin A as retinyl ester - retinyl palmitate •Emulsification of fat-micelles taken up •Retinyl esters hydrolyzed in lumen or yy y brush border of cell
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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 rotect from degradation – protect from degradation – protect membranes which unbound A can damage – escort retinoids to enzymes that metabolize them y
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Cellular Retinoid Binding Proteins g • In small intestine CRBP-II is abundant – escorts retinol to lecithin:retinol cyltransferase (LRAT) acyltransferase (LRAT) • esterifies retinol
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rovitamin A Carotenoids to Retinoids Provitamin A Carotenoids to Retinoids • Humans can absorb carotenoids intact leaved within the mucosal cell or can • Cleaved within the mucosal cell or can be transported unmetabolized an also be cleaved in liver • Can also be cleaved in liver • Cleavage enzyme is 15,15’-dioxygenase – Forms 2 molecules of retinal
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Some beta-carotene can be absorbed intact
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Vitamin A Absorption • Retinol must be esterified to leave mucosa • Newly formed retinyl esters incorporated into chylomicrons – Chylomicrons enter lymphatic circulation
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Vitamin A Transport to Liver • Some retinyl esters removed from chylomicrons by tissues – chylomicron remnants remain hylomicron remnants deliver • Chylomicron remnants deliver retinyl esters to liver
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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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1pp Vitamin A 2011 - VITAMIN VITAMIN A Spring 2011 2011...

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