The selectivity of acyl coenzyme A: Retinol O-acyltransferase toward its fatty-acyl-CoA substrate

Abstract

Although vitamin A is known to be stored in the liver primarily as retinyl palmitate, the selectivity of one of the microsomal enzymes thought to esterify retinol in vivo, acyl CoA: retinol O-acyltransferase (ARAT), has not been defined with respect to the acyl-CoA substrate. Therefore, retinol and liver microsomes were incubated with several exogenous CoA-thioester derivatives, present together and at equimolar amounts in the reaction mixture, and the observed formation rates were corrected for acyl-donor endogenous to the microsomes. CoA thioesters of oleic and C 12−C 20 saturated fatty acids were effective substrates for rat-liver ARAT in vitro, whereas polyunsaturated derivatives were virtually ineffective. ARAT from polar-bear liver displayed a similar specificity. ARAT selectivity observed in vitro fails to account for the fatty-acid composition of retinyl esters in vivo. Thus, either the concentration of palmitoyl CoA at the intracellular site of the ARAT-catalyzed transacylation is uniquely high, or the esters of retinol that occur in vivo are synthesized by another enzyme.