Effects of retinol and retinol palmitate on glycolipid and glycoprotein galactosyltransferase activities of rat liver plasma membranes

Abstract

Summary Purified plasma membranes isolated from rat liver exhibited low levels of galactosyltransferase activity toward endogenous glycolipid and glycoprotein acceptors. The addition of retinol (0.005 mM) stimulated the transfer of [ 14 C]galactose from UDP-[ 14 C]galactose to endogenous glycolipid and glycoprotein acceptors of plasma membranes while higher or lower concentrations of retinol were either inhibitory or had no effect. Retinol palmitate (0.005 mM) stimulated the transfer of [ 14 C]galactose from UDP-[ 14 C]galactose into glycolipid but not glycoprotein acceptors of isolated plasma membranes and was inhibitory at concentrations above 0.05 mM. The addition of ATP (0.5 mM) to incubations in the absence or presence of retinol (0.025 or 0.005 mM) inhibited incorporation of [ 14 C]galactose into glycolipid and glycoprotein acceptors. Plasma membranes did not catalyze the synthesis of [ 14 C]galactosyl retinyl phosphate in incubations containing UDP-[ 14 C]galactose and retinyl phosphate. The results of this study indicate that retinol stimulates incorporation of [ 14 C]galactose into glycolipid and glycoprotein acceptors of isolated plasma membranes. The mechanism of stimulation is unknown but does not appear to be a simple detergent effect or the result of retinol acting as a galactosyl retinyl phosphate intermediate.