title: Retinyl esters are the substrate for isomerohydrolase.
abstract: Regeneration of 11-cis retinal from all-trans retinol in the retinal pigment epithelium (RPE) is a critical step in the visual cycle. The enzyme(s) involved in this isomerization process has not been identified and both all-trans retinol and all-trans retinyl esters have been proposed as the substrate. This study is to determine the substrate of the isomerase enzyme or enzymatic complex. Incubation of bovine RPE microsomes with all-trans [(3)H]-retinol generated both retinyl esters and 11-cis retinol. Inhibition of lecithin retinol acyltransferase (LRAT) with 10-N-acetamidodecyl chloromethyl ketone (AcDCMK) or cellular retinol-binding protein I (CRBP) diminished the generation of both retinyl esters and 11-cis retinol from all-trans retinol. The 11-cis retinol production correlated with the retinyl ester levels, but not with the all-trans retinol levels in the reaction mixture. When retinyl esters were allowed to form prior to the addition of the LRAT inhibitors, a significant amount of isomerization product was generated. Incubation of all-trans [(3)H]-retinyl palmitate with RPE microsomes generated 11-cis retinol without any detectable production of all-trans retinol. The RPE65 knockout (Rpe65(-/-)) mouse eyecup lacks the isomerase activity, but LRAT activity remains the same as that in the wild-type (WT) mice. Retinyl esters in WT mice plateau at 8 weeks-of-age, but Rpe65(-/-) mice continue to accumulate retinyl esters with age (e.g., at 36 weeks, the levels are 20x that of WT). Our data indicate that the retinyl esters are the substrate of the isomerization reaction.
authors.affiliation: Department of Ophthalmology, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
authors.forename: Gennadiy
authors.initials: G
authors.lastname: Moiseyev
authors.forename: Rosalie K
authors.initials: RK
authors.lastname: Crouch
authors.forename: Patrice
authors.initials: P
authors.lastname: Goletz
authors.forename: John
authors.initials: J
authors.lastname: Oatis
authors.forename: T Michael
authors.initials: TM
authors.lastname: Redmond
authors.forename: Jian-xing
authors.initials: JX
authors.lastname: Ma
mesh_terms: D000217:Acyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism; D000818:Animals; D001059:Apoproteins / Q000737:chemistry; D002352:Carrier Proteins; D002417:Cattle; D003553:Cystine / Q000031:analogs & derivatives* / Q000494:pharmacology; D004224:Diterpenes; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000737:chemistry; D004952:Esters; D005136:Eye Proteins / Q000235:genetics / Q000378:metabolism; D005227:Fatty Acids / Q000032:analysis; D007659:Ketones / Q000494:pharmacology; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008861:Microsomes / Q000201:enzymology / Q000378:metabolism; D010857:Pigment Epithelium of Eye / Q000201:enzymology / Q000378:metabolism; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D012177:Retinol-Binding Proteins / Q000737:chemistry; D054840:Retinol-Binding Proteins, Cellular; D000084562:Retinyl Esters; D013379:Substrate Specificity; D014801:Vitamin A / Q000031:analogs & derivatives* / Q000032:analysis / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism*; D019745:cis-trans-Isomerases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism*; D000099178:Retinoid Isomerohydrolase
chemical_list: D000217:Acyltransferases; D001059:Apoproteins; D002352:Carrier Proteins; D003553:Cystine; D004224:Diterpenes; D004791:Enzyme Inhibitors; D004952:Esters; D005136:Eye Proteins; D005227:Fatty Acids; D007659:Ketones; D011506:Proteins; D012177:Retinol-Binding Proteins; D054840:Retinol-Binding Proteins, Cellular; D000084562:Retinyl Esters; D014801:Vitamin A; D019745:cis-trans-Isomerases; D000099178:Retinoid Isomerohydrolase; C465379:HI-131; C014794:retinol palmitate; C056942:lecithin-retinol acyltransferase