Utilization of exogenous retinol by frog pigment epithelium

High-performance liquid chromatography was used to investigate the utilization of exogenous 11,12-3H2-retinol by frog pigment epithelium (RPE) in vitro or after intraocular injection into the dark-adapted, whole animal. Isolated frog RPE contains an adequate supply of acyl donors and can esterify all-trans, 11-cis and 13-cis isomers of retinol. The esterifying activity is restricted to the particulate fraction. Homogenates of choroid cannot esterify retinol. The ester formed by the RPE is primarily palmitate, and is therefore identical with the endogenous retinyl ester. Frog RPE also formed 13-cis retinyl palmitate from all-trans retinol, probably by esterification of 13-cis retinol formed non-enzymatically from the all-trans isomer. None of the in vitro experiments provided any evidence for the formation of 11-cis retinoid. There was slow appearance of label in 11-cis retinyl palmitate when 3H-all-trans retinol was injected intraocularly into the intact frog. After 15 hr its specific activity was only 20% of that of the all-trans retinyl palmitate. This rate of formation of 11-cis retinoid is inadequate for rhodopsin regeneration. However, it is more than an order of magnitude too fast to be accounted for by phagocytosis of rhodopsin. It is suggested that 11-cis retinoid is generated in the retina and is slowly transferred to the site of esterification in the RPE.