COMPARISON OF RETINYL ESTER HYDROLASE ACTIVITIES IN BOVINE LIVER AND RETINAL-PIGMENT EPITHELIUM

Various properties of retinyl ester hydrolysis in the liver and the re tinal pigment epithelium (RPE) have been studied, yet the relationship between the retinyl ester hydrolase (REH) activities in these tissues of the same species is not known. In the present study, REH activitie s in bovine liver and RPE microsomes were compared to explore potentia l biochemical relationships of retinyl ester metabolism in these tissu es. Rates of [H-3]all-trans retinyl palmitate hydrolysis by liver and RPE were comparable (i.e., V-max(app) approximate to 300 pmol/min per mg; K-m(app) approximate to 30 mu M), while hydrolysis of [H-3]11-cis retinyl palmitate by RPE was significantly higher (V-max(app) = 1,667 pmol/min per mg). When equimolar amounts (10 mu M) Of either [C-14]tri olein or unlabeled 11-cis retinyl palmitate were added to [H-3]all-tra ns REH assays, all-trans REH activities in liver and RPE demonstrated similar rime-dependent inhibition profiles. In contrast, hydrolysis of [H-3]11-cis retinyl palmitate by RPE was relatively unaffected by add ition of either [C-14]triolein or unlabeled all-trans retinyl palmitat e. Additionally, modification of the microsomal proteins with N-ethylm aleimide produced profound, dose-dependent alterations in K-m(app) val ues for all-trans retinyl ester hydrolysis, whereas K-m(app) for 11-ci s REH in the RPE was not significantly altered. These results have elu cidated common biochemical features of all-trans retinyl ester hydroly sis in liver and RPE. In contrast, hydrolysis of 11-cis retinyl ester in RPE is characterized by a distinctive substrate preference and uniq ue biochemical properties.