Schematic diagram showing the metabolic pathways of vitamin A metabolism. This illustration lists the major steps of RA metabolism in cell, please refer to the text for details. Abbreviation: NAD, nicotinamide adenine dinucleotide; NADH, The reduced form of NAD; NADP, nicotinamide adenine dinucleotide phosphate; NADPH, The reduced form of NADP; CRBP: cellular retinol-binding protein; LRAT: lecithin retinol acyltransferase; REH: retinyl ester hydrolase; ADH: alcohol dehydrogenase; RDH: retinol dehydrogenase; SDR: short-chain dehydrogenase/reductases; ALDH: aldehyde dehydrogenase; RALDH: retinaldehyde dehydrogenase. CRAD: cis- retinoid/androgen dehydrogenase. Modified from [18, 25].