Esterification by rat liver microsomes of retinol bound to cellular retinol-binding protein

We have investigated the esterification by liver membranes of retinol bound to cellular retinol-binding proteins (CRBP). When CRBP carrying [³H]retinol as its ligand was purified from rat liver cytosol and incubated with rat liver microsomes, a significantly fraction of the [³H]retinol was converted to [³H]retinyl ester. Esterification of the CRBP-bound [³H]retinol, which was maximal at pH 6-7, did not require the addition of an exogenous fatty acyl group. Indeed, when additional palmitoyl-CoA or coenzyme A was provided, the rate of esterification increased either very slightly or not at all. The esterification reaction had a K(m) for [³H]retinol-CRBP of 4 ± 0.6 μM and a maximum velocity of 145 ± 52 pmol/min/mg of microsomal protein (n = 4). The major products were retinyl palmitate/oleate and retinyl stearate in a ratio of approximately 2 to 1 over a range of [³H]retinol-CRBP concentrations from 1 to 8 μM. The addition of progesterone, a known inhibitor of the acyl-CoA:retinol acyltransferase reaction, consistently increased the rate of retinyl ester formation when [³H]retinol was delivered bound to CRBP. These experiments indicate that retinol presented to liver microsomal membranes by CRBP can be converted to retinyl ester and that this process, in contrast to the esterification of dispersed retinol, is independent of the addition of an activated fatty acid and produces a pattern of retinyl ester species similar to that observed in intact liver. A possible role of phospholipids as endogenous acyl donors in the esterification of retinol bound to CRBP is supported by our observations that depletion of microsomal phospholipid with phospholipase A₂ prior to addition of retinol-CRBP decreased the retinol-esterifying activity almost 50%. Conversely, incubating microsomes with a lipid-generating system containing choline, CDP-choline, glycerol 3-phosphate, and an acyl-CoA-generating system prior to addition of retinol-CRBP increased retinol esterification significantly as compared to buffer-treated controls.