Acyl CoA:retinol acyltransferase (ARAT) activity is present in bovine retinal pigment epithelium.
Visual perception is mediated by a family of G protein-coupled receptors called the opsins. The light-absorbing chromophore in most opsins is 11-cis-retinaldehyde, which is isomerized to all-trans-retinaldehyde upon absorption of a photon. Restoration of light sensitivity to the photobleached opsin requires chemical re-isomerization of the chromophore. This is carried out by an enzymatic pathway called the visual cycle in retinal pigment epithelial cells. The isomerase in this pathway uses fatty-acyl esters of all-trans-retinol as substrate. A retinyl-ester synthase that produces these esters, called lecithin:retinol acyltransferase (LRAT), has been extensively characterized. Based on prior biochemical studies and the phenotype in lrat(-/-) knockout mice, it has been assumed that LRAT is the sole or dominant retinyl-ester synthase in the retinal pigment epithelium. Here we demonstrate the presence of a second ester synthase activity in these cells called acyl CoA:retinol acyltransferase (ARAT). We show that this activity uses palmitoyl coenzyme A as an acyl donor, unlike LRAT which uses phosphatidylcholine. Similar to LRAT, ARAT esterifies both all-trans-retinol and 11-cis-retinol. LRAT and ARAT are both potently inhibited by the retinyl-ester analog, all-trans-retinylbromoacetate, but only ARAT is inhibited by progesterone. Unexpectedly, the maximum turnover rate (V(max)) of ARAT was similar to that of LRAT. However, the Michaelis constant (K(M)) of ARAT was 10-fold higher than the K(M) of LRAT for all-trans-retinol. These observations suggest that ARAT may complement LRAT to provide additional retinyl-ester synthase activity under conditions of high all-trans-retinol. These conditions occur in the retina following exposure to bright light.
Kaschula
Catherine H
CH
Jules Stein Eye Institute, UCLA School of Medicine, Los Angeles, CA 90095, USA.
Jin
Ming-Hao
MH
Desmond-Smith
Nicholas S
NS
Travis
Gabriel H
GH
D000217:Acyltransferases / Q000378:metabolism; D000818:Animals; D002417:Cattle; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry; D003001:Cloning, Molecular; D006801:Humans; D015151:Immunoblotting; D051379:Mice; D008861:Microsomes / Q000737:chemistry; D010171:Palmitoyl Coenzyme A / Q000494:pharmacology; D010857:Pigment Epithelium of Eye / Q000201:enzymology*; D051142:Retinol O-Fatty-Acyltransferase / Q000032:analysis* / Q000235:genetics; D013268:Stimulation, Chemical; D014162:Transfection / Q000379:methods
D010171:Palmitoyl Coenzyme A; D000217:Acyltransferases; C056942:lecithin-retinol acyltransferase; D051142:Retinol O-Fatty-Acyltransferase