Details

Autor(en) / Beteiligte

• Kaschula, Catherine H.
• Jin, Ming-hao
• Desmond-Smith, Nicholas S.
• Travis, Gabriel H.

Titel

Acyl CoA:retinol acyltransferase (ARAT) activity is present in bovine retinal pigment epithelium

Ist Teil von

• Experimental eye research, 2006, Vol.82 (1), p.111-121

Ort / Verlag

England: Elsevier Ltd

Erscheinungsjahr

2006

Quelle

MEDLINE

Beschreibungen/Notizen

• 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.