Inhibitors of retinyl ester formation also prevent the biosynthesis of 11-cis-retinol

Abstract

Lecithin retinol acyl transferase (LRAT) from the retinyl pigment epithelium is potently inhibited by all-trans-retinyl .alpha.-bromoacetate in the micromolar range. The inhibition is competitive and reversible. The retinyl pigment epithelium also contains an enzymatic activity capable of converting added all-trans-retinol into 11-cis-retinol. This isomerization is likely to require the intermediate formation of all-trans-retinyl esters, which are themselves produced by LRAT action. Here this possibility is directly tested by studying the effect of all-trans-retinyl .alpha.-bromoacetate on the isomerization reaction. When pigment epithelium membranes are preincubated with all-trans-retinyl .alpha.-bromoacetate, they form neither retinyl esters nor 11-cis-retinol from added all-trans-retinol. However, if the pigment epithelium membranes are first allowed to form all-trans-retinyl esters from all-trans-retinol before the addition of all-trans-retinyl .alpha.-bromoacetate, then 11-cis-retinol formation proceeds at close to the rate found in the absence of inhibitor. In addition, 11-cis-retinyl esters are not formed under these conditions, eliminating the possibility of a direct ester-ester isomerization route. Therefore, all-trans-retinyl esters are obligate intermediates in the biosynthesis of 11-cis-retinol.