Retinyl ester hydrolase and the visual cycle in the chicken eye

The ability of chicken retina and retinal pigment epithelium (RPE) membrane to hydrolyze vitamin A esters ([9,10-³H]all-trans- and 11-cis- retinyl palmitate) was studied. Hydrolytic activity within the retina was optimal at acidic pH of 5.0, whereas in the RPE significant hydrolytic activity was exhibited over a broad range of hydrogen ion concentrations. The highest rate of hydrolysis was associated with the all-trans-isomer and located within retina and RPE membranes [the apparent maximal velocity (V(max)) and Michaelis-Menten constant (K(m)) were 770 pmol · min · ^-1 · mg^-1 and 45 μM and 300 pmol · min · ^-1 · mg^-1 and 3.6 μM, respectively]. Retinyl ester hydrolase activities for 11-cis-retinyl palmitate in the retina and RPE were correspondingly lower (apparent V(max) of 204 pmol · min · ^-1 · mg^-1 and K(m) of 18.5 μM in the retina; apparent V(max) of 131 pmol · min · ^-1 · mg^-1 and K(m) of 4 μM in the RPE). Together with results from other laboratories, results from the present study suggest that chicken retina contains important enzymes to complete the visual cycle.