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. 2019 Mar 12;10:440. doi: 10.3389/fmicb.2019.00440

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Copyright © 2019 Liu, Cheng, Tang, Zhong, Ming and Dong.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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The metabolism of retinoids. Retinal can be originally formed by the cleavage of proretinoid carotenoids such as beta-carotene by the enzyme BCMO1 (not shown here). Retinol is formed by the reversible reduction of retinal by one of the retinal reductase family members. The enzyme lecithin:retinol acyl-transferase (LRAT) [with an ortholog gene called diacylglycerol O-acyltransferase (DGAT) in Schistosoma] synthesizes retinyl esters by transferring a fatty acyl moiety from the sn-1 position of membrane phosphatidyl choline such as lecithin [PC(22:6/20:1)] to retinol. Unesterified retinol is liberated from retinyl ester stores by the catalysis of a retinyl ester hydrolase (REH). Retinol is oxidized by catalysis of one retinol dehydrogenase (RDH) to retinal, which is then irreversibly oxidized by one retinal dehydrogenase (RALDH) to form transcriptionally active retinoic acid. Retinoic acid is finally oxidized/catabolized to more water-soluble hydroxy- and oxo- forms by one of several cytochrome P450 enzyme family members.