Evidence for a lecithin-retinol acyltransferase activity in the rat small intestine

Cellular retinol-binding protein, type II (CRBP (II] is an abundant protein of the mature enterocytes of the small intestine. It has been shown to direct retinol to an acyl-CoA-independent esterifying activity that utilizes an endogenous acyl donor (Ong, D.E., Kakkad, B., and MacDonald, P.N. (1987) J. Biol. Chem. 262, 2729-2736). Here we report that this activity in intestinal microsomes will catalyze the transfer of acyl moieties from exogenous phosphatidylcholine (PC) to retinol-CRBP(II) to produce retinyl esters. The microsomal activity displayed positional selectivity as only the sn-1-acyl moiety of PC was transferred to retinol-CRBP(II). The retinyl ester synthase was selective for PC substrates as acyl transfer from phosphatidylethanolamine, phosphatidic acid, or free fatty acid to retinol-CRBP(II) was not observed. Some formation of retinyl esters was observed with exogenous acyl-CoA, but the amount produced was considerably lower than ester formation from exogenous PC and could be shown to be due to a different enzyme activity. Inhibitor studies clearly distinguished between the enzyme activities responsible for the acyl-CoA-dependent esterification and the phosphatidylcholine-dependent esterification of retinol. The results provide strong evidence that retinol-CRBP(II) esterification in the intestine proceeds via a phosphatidylcholine-dependent transacylase mechanism similar to that established for the esterification of cholesterol by lecithin-cholesterol acyltransferase.