A cytochrome P450 monooxygenase that catalyzes the 3,4 desaturation of all-trans-retinol (also called vitamin A1) to all-trans-3,4-didehydroretinol (also called vitamin A2) in the skin. Desaturates with lower efficiency all-trans retinal and all-trans retinoic acid. Forms minor amounts of 3-hydroxy and 4-hydroxy all-trans-retinol derivatives. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate and reducing the second into a water molecule. Two electrons are provided by NADPH via a two-protein mitochondrial transfer system comprising flavoprotein FDXR (adrenodoxin/ferredoxin reductase) and nonheme iron-sulfur protein FDX1 or FDX2 (adrenodoxin/ferredoxin). all-trans-retinol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = all-trans-3,4-didehydroretinol + 2 H2O + 2 oxidized [adrenodoxin] all-trans-retinol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = all-trans-4-hydroxyretinol + H2O + 2 oxidized [adrenodoxin] all-trans-retinol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = all-trans-3-hydroxyretinol + H2O + 2 oxidized [adrenodoxin] Cofactor metabolism; retinol metabolism. Widely expressed, with highest levels in the liver, kidney and pancreas. Expressed in the skin (at protein level). Belongs to the cytochrome P450 family. Extended N-terminus. Seeing through the murk - Issue 192 of June 2017