Bile salt independent retinyl ester hydrolases in the bovine eye

Homogenates of bovine neuroretina and retinal pigment epithelium (RPE) were incubated with 11-cis and all-trans retinyl palmitate to study retinyl ester hydrolysis. The highest activity was found in RPE when 11-cis retinyl palmitate served as substrate (Km = 7.8 microM and Vmax = 44.8 pmol/min/mg). This retinyl ester hydrolase (REH) had an optimum activity at acidic pH (pH 5), which is in contrast to the neutral hydrolase (pH 8) found in the neuroretina. Similar to REH in the liver, REH activities in the bovine eye are not stimulated by bile salts because sodium cholate, taurocholate and deoxycholate did not enhance retinyl ester hydrolysis. Most REH activities in retinal homogenate were soluble, whereas in the RPE, these activities were membrane-bound. Divalent cations such as zinc and cadmium completely inhibited REH activities in the neuroretina. Our results show that bovine ocular tissues contain several retinyl ester hydrolases with distinct biochemical properties.