UPTAKE IN EXTRAHEPATIC TISSUES, USAGE AND BREAKDOWN

Retinol is taken up by various cells in the body. It has been shown that extrahepatic tissues such as the lung, kidney and intestine, contain stellate cells and can therefore store vitamin A.

Inside the cells, retinol is oxidized to retinal by cytosolic medium-chain alcohol dehydrogenases (ADH) 1, 3, and 4, and membrane-bound short chain dehydrogenase/reductases (SDRs) like retinol dehydrogenase (RDH) 1, 5, and 11, CRAD1, 2, and 3, and ret SDR1. Oxidation of retinal to retinoic acid is mediated by the retinal degydrogenase (RALDH) 1-4.

Catabolism of retinoic acid is thought to go through conversion of retinoic acid into more oxidized metabolites such as 4-hydroxy and 4-oxo retinoic acid. This is important in turning off the retinoic acid signal and is mediated by the CYP26 enzymes, of which there are four described so far: Cyp26A1, B1, C1 and D1. CYP26A1 is rapidly induced by retinoic acid, and down-regulated during vitamin A deficiency in rats and mice.