Substrate specificity of retinyl ester hydrolase activity in retinal pigment epithelium

In the eye, hydrolysis of stored retinyl esters is catalyzed by retinyl ester hydrolase (REH) activities in retinal pigment epithelium (RPE) membranes. In the present study, biochemical analyses were conducted to determine the substrate specificity of these activities. Specific activities determined for hydrolysis of various retinol isomers of retinyl palmitate (9-cis-, 11-cis-, 13-cis-, and all-trans-retinyl palmitates) indicated that 11-cis-retinyl palmitate is preferentially hydrolyzed (1.7 nmol/min/mg) compared to the other isomers (0.1-0.3 nmol/min/mg). Examination of the specificity of REH activity for 11-cis-retinyl esters of varied acyl chain length (-myristate, -palmitate, and -stearate) and degree of saturation (-oleate and -linoleate) further demonstrated that palmitate is the preferred fatty acyl moiety. Notably, retinyl esters possessing chain lengths which more closely approximate that of the palmitate ester exhibited higher rates of hydrolysis. Similar results were obtained in retinyl ester-plasma membrane fusion studies in which hydrolysis took place within the membrane domain rather than at the lipid-water interface. REH substrate specificity was further assessed in competition studies in which 11-cis-retinyl palmitate hydrolysis was monitored in the presence of 13-cis-, 9-cis-, or all-trans-retinyl palmitate. Results show that addition of these retinyl palmitate isomers does not affect the rate of hydrolysis of 11-cis-retinyl palmitate. However, the hydrolytic rates associated with other retinyl palmitate isomers were significantly reduced in the presence of 11-cis-retinyl palmitate. Finally, cholesterol ester hydrolase activity was found to be distinct from the observed 11-cis-REH activity and the presence of cholesterol oleate did not affect the rate of 11-cis-retinyl palmitate hydrolysis. Collectively, these data support the hypothesis that a distinct, membrane-associated, 11-cis-retinyl palmitate-specific retinyl ester hydrolase activity exists in the retinal pigment epithelium.