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The esterification of alcohols with fatty acids is a universal mechanism to form inert storage forms of sterols, di- and triacylglycerols, and retinoids. In ocular tissues, formation of retinyl esters is an essential step in the enzymatic regeneration of the visual chromophore (11-cis-retinal). Acyl-CoA wax alcohol acyltransferase 2 (AWAT2), also known as multifunctional O-acyltransferase (MFAT), is an integral membrane enzyme with a broad substrate specificity that has been shown to preferentially esterify 11-cis-retinol and thus contribute to formation of a readily available pool of cis retinoids in the eye. However, the mechanism by which this promiscuous enzyme can gain substrate specificity is unknown. Here, we provide evidence for an allosteric modulation of the enzymatic activity by 11-cis retinoids. This regulation is independent from cellular retinaldehyde-binding protein (CRALBP), the major cis-retinoid binding protein. This positive-feedback regulation leads to decreased esterification rates for 9-cis, 13-cis, or all-trans retinols and thus enables preferential synthesis of 11-cis-retinyl esters. Finally, electron microscopy analyses of the purified enzyme indicate that this allosteric effect does not result from formation of functional oligomers. Altogether, these data provide the experimental basis for understanding regulation of AWAT2 substrate specificity.

It has been proposed that cis-retinol dehydrogenase (cRDH) acts within the body to catalyze the oxidation of 9-cis-retinol, an oxidative step needed for 9-cis-retinoic acid synthesis, the oxidation of 11-cis-retinol [an oxidative step needed for 11-cis-retinal (visual chromophore) synthesis], and 3 α-hydroxysteroid transformations. To assess in vivo the physiological importance of each of these proposed actions of cRDH, we generated cRDH-deficient (cRDH−/−) mice. The cRDH−/− mice reproduce normally and appear to be normal. However, the mutant mice do have a mild visual phenotype of impaired dark adaptation. This phenotype is evidenced by electroretinagram analysis of the mice and by biochemical measures of eye levels of retinoid intermediates during recovery from an intense photobleach. Although it is thought that cRDH is expressed in the eye almost solely in retinal pigment epithelial cells, we detected cRDH expression in other retinal cells, including ganglion cells, amacrine cells, horizontal cells, and the inner segments of the rod photoreceptor cells. Aside from the eye, there are no marked differences in retinoid levels in other tissues throughout the body for cRDH−/− compared with cRDH+/+ mice. Moreover, we did not detect any nonvisual phenotypic changes for cRDH−/− mice, suggesting that these mice do not have problems in metabolizing 3 α-hydroxysteroids. Thus, cRDH may act essentially in the visual cycle but is redundant for catalyzing 9-cis-retinoic acid formation and 3 α-hydroxysteroid metabolism.—Shang, E., K. Lai, A. I. Packer, J. Paik, W. S. Blaner, M. de Morais Vieira, P. Gouras, and D. J. Wolgemuth. Targeted disruption of the mouse cis-retinol dehydrogenase gene: visual and nonvisual functions. J. Lipid Res. 2002. 43: 590–597.

Retinosomes are intracellular lipid bodies found in the retinal pigment epithelium (RPE). They contain retinyl esters (REs) and are thought to be involved in visual chromophore regeneration during dark adaptation and in case of chromophore depletion. However, key enzymes in chromophore regeneration, retinoid isomerase (RPE65), and lecithin:retinol acyltransferase (LRAT) are located in the endoplasmic reticulum (ER). The mechanism and the enzyme responsible for mobilizing REs from retinosomes remained unknown. Our study demonstrates that patatin-like phospholipase domain containing 2 (PNPLA2) mobilizes all-trans-REs from retinosomes. The absence of PNPLA2 in mouse eyes leads to a significant accumulation of lipid droplets in RPE cells, declined electroretinography (ERG) response, and delayed dark adaptation compared with those of WT control mouse. Our work suggests a function of PNPLA2 as an RE hydrolase in the RPE, mobilizing REs from lipid bodies and functioning as an essential component of the visual cycle., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

The esterification of alcohols with fatty acids is a universal mechanism to form inert storage forms of sterols, di- and triacylglycerols, and retinoids. In ocular tissues, formation of retinyl esters is an essential step in the enzymatic regeneration of the visual chromophore (11- cis -retinal). Acyl-CoA wax alcohol acyltransferase 2 (AWAT2), also known as multifunctional O -acyltransferase (MFAT), is an integral membrane enzyme with a broad substrate specificity that has been shown to preferentially esterify 11- cis -retinol and thus contribute to formation of a readily available pool of cis retinoids in the eye. However, the mechanism by which this promiscuous enzyme can gain substrate specificity is unknown. Here, we provide evidence for an allosteric modulation of the enzymatic activity by 11- cis retinoids. This regulation is independent from cellular retinaldehyde-binding protein (CRALBP), the major cis -retinoid binding protein. This positive-feedback regulation leads to decreased esterification rates for 9- cis , 13- cis , or all- trans retinols and thus enables preferential synthesis of 11- cis -retinyl esters. Finally, electron microscopy analyses of the purified enzyme indicate that this allosteric effect does not result from formation of functional oligomers. Altogether, these data provide the experimental basis for understanding regulation of AWAT2 substrate specificity., (Copyright © 2017 by the American Society for Biochemistry and Molecular Biology, Inc.)