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 opsi...
Gespeichert in:
Veröffentlicht in: | Experimental eye research 2006, Vol.82 (1), p.111-121 |
---|---|
Hauptverfasser: | , , , |
Format: | Artikel |
Sprache: | eng |
Schlagworte: | |
Online-Zugang: | Volltext |
Tags: |
Tag hinzufügen
Keine Tags, Fügen Sie den ersten Tag hinzu!
|
Zusammenfassung: | 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. |
---|---|
ISSN: | 0014-4835 1096-0007 |
DOI: | 10.1016/j.exer.2005.05.010 |