SUBSTRATE-SPECIFICITY OF RETINYL ESTER HYDROLASE ACTIVITY IN RETINAL-PIGMENT EPITHELIUM

In the eye, hydrolysis of stored retinyl esters is catalyzed by retiny l 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 a ctivities determined for hydrolysis of various retinol isomers of reti nyl palmitate (9-cis-, 11-cis-, 13-cis-, and all-trans-retinyl palmita tes) indicated that 11-cis-retinyl palmitate is preferentially hydroly zed (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 -stea rate) and degree of saturation (-oleate and -linoleate) further demons trated that palmitate is the preferred fatty acyl moiety. Notably, ret inyl esters possessing chain lengths which more closely approximate th at of the palmitate ester exhibited higher rates of hydrolysis. Simila r results were obtained in retinyl ester-plasma membrane fusion studie s in which hydrolysis took place within the membrane domain rather tha n at the lipid-water interface. REH substrate specificity was further assessed in competition studies in which 11-cis-retinyl palmitate hydr olysis was monitored in the presence of 13-cis-, 9-cis-, or all-trans- retinyl palmitate. Results show that addition of these retinyl palmita te isomers does not affect the rate of hydrolysis of 11-cis-retinyl pa lmitate. However, the hydrolytic rates associated with other retinyl p almitate isomers were significantly reduced in the presence of 11-cis- retinyl palmitate. Finally, cholesterol ester hydrolase activity was f ound to be distinct from the observed 11-cis-REH activity and the pres ence of cholesterol oleate did not affect the rate of 11-cis-retinyl p almitate hydrolysis. Collectively, these data support the hypothesis t hat a distinct, membrane-associated, 11-cis-retinyl palmitate-specific retinyl ester hydrolase activity exists in the retinal pigment epithe lium.