Biosynthesis of 11-cis-retinoids and retinyl esters by bovine pigment epithelium membranes

Previously, we have shown that retina/pigment epithelium membranes from the amphibian can synthesize 11-cis-retinoids from added all-trans-retinol Bernstein, P.S., Law, W.C., & Rando, R.R. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1849-1853]. The activity was largely localized to the pigment epithelium. Here it is shown that, in the bovine system, the activity resides exclusively in the membranes of the pigment epithelium. Subcellular fractionation does not reveal a particular organelle where the activity resides. Washed bovine pigment epithelium membranes, which are devoid of retinoid redox activity, convert added all-trans-retinol to a mixture of 11-cis-retinol and its palmitate ester. all-trans-Retinal and all-trans-retinyl palmitate are not converted into 11-cis-retinoids by the membranes. The membranes show substantial ester synthetase activity, producing large amounts of all-trans-retinyl palmitate. Diverse chemical reagents, such as ethanol, hydroxylamine, and p-(hydroxymercuri)benzoate, inhibit both ester synthetase and isomerase activities in a roughly parallel fashion, suggesting a possible functional linkage between the two activities.