Comparison of retinyl ester hydrolase activities in bovine liver and retinal pigment epithelium

Abstract

Various properties of retinyl ester hydrolysis in the liver and the retinal 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 activities in bovine liver and RPE microsomes were compared to explore potential biochemical relationships of retinyl ester metabolism in these tissues. Rates of [3H]all-trans retinyl palmitate hydrolysis by liver and RPE were comparable (i.e., Vmaxapp approximately 300 pmol/min per mg; K(m)app approximately 30 microM), while hydrolysis of [3H]11-cis retinyl palmitate by RPE was significantly higher (Vmaxapp = 1,667 pmol/min per mg). When equimolar amounts (10 microM) of either [14C]triolein or unlabeled 11-cis retinyl palmitate were added to [3H]all-trans REH assays, all-trans REH activities in liver and RPE demonstrated similar time-dependent inhibition profiles. In contrast, hydrolysis of [3H]11-cis retinyl palmitate by RPE was relatively unaffected by addition of either [14C]triolein or unlabeled all-trans retinyl palmitate. Additionally, modification of the microsomal proteins with N-ethylmaleimide produced profound, dose-dependent alterations in K(m)app values for all-trans retinyl ester hydrolysis, whereas K(m)app for 11-cis REH in the RPE was not significantly altered. These results have elucidated common biochemical features of all-trans retinyl ester hydrolysis in liver and RPE. In contrast, hydrolysis of 11-cis retinyl ester in RPE is characterized by a distinctive substrate preference and unique biochemical properties.