RETINOL ESTERIFICATION IN BOVINE RETINAL-PIGMENT EPITHELIUM - REVERSIBILITY OF LECITHIN - RETINOL ACYLTRANSFERASE

Esterification of all-trans-retinol is a key reaction of the vertebrat e visual cycle, since it produces an insoluble, relatively non-toxic, form of the vitamin for storage and supplies substrate for the isomeri zation reaction. CoA-dependent and -independent pathways have been des cribed for retinol esterification in retinal pigment epithelium (RPE). The CoA-independent reaction, catalysed by lecithin:retinol acyltrans ferase (LRAT) was examined in more detail in this study. Addition of r etinol to RPE microsomes results in a burst of retinyl ester synthesis , followed by a rapid apparent cessation of the reaction. However, [H- 3]retinol, added when retinyl ester synthesis has apparently ceased, i s rapidly incorporated into retinyl ester without a net increase in th e amount of ester. The specific radioactivities of [H-3]retinol and [H -3]retinyl ester reach the same value. [C-14]Palmitate from palmitoyl- CoA is incorporated into pre-existing retinyl ester in the absence of net ester synthesis, too. These exchange reactions suggest that the re action has reached equilibrium at the plateau of the progress curve an d that only the accumulation of retinyl ester, and not its synthesis, has stopped during this phase of the reaction. Studies with geometrica l isomers of retinol revealed that the rate of exchange of all-transre tinol with all-trans-retinyl esters was about 6 times more rapid than exchange of 11-cis-retinol with 11-cis-retinyl ester. This is the firs t demonstration of the reversibility of LRAT and the first example of stereospecificity of retinyl ester synthesis in the visual system. Rev ersal of the LRAT reaction could contribute to the mobilization of 11- cis-retinol from 11-cis-retinyl ester pools.