Radiation inactivation analysis of acyl-CoA:retinol acyltransferase and lecithin:retinol acyltransferase in rat liver.

Microsomes from liver and several other tissues esterify retinol through both fatty acyl-CoA-dependent and -independent reactions. Two activities, acyl-CoA:retinol acyltransferase (ARAT) and lecithin:retinol acyltransferase (LRAT) activities, have been characterized enzymatically but neither has yet been purified and characterized biochemically. We have used the method of radiation inactivation to determine the target sizes of ARAT and LRAT in intact microsomal membranes from rat liver. After exposure of frozen liver microsomes to ionizing radiation, the activity of ARAT decayed exponentially yielding a target size of 73 +/- 18 kDa (mean +/- SD, n = 6). The activity of LRAT was assayed both by monitoring the esterification of retinol bound to the cellular retinol-binding protein (CRBP) and of solvent-dispersed retinol. With both assays a single exponential was observed with radiation doses of 9 to 150 Mrads. The slopes obtained with both LRAT assays were similar, yielding target sizes of 52 +/- 10 kDa (n = 10) for the LRAT assay with CRBP-retinol and 56 +/- 7 kDa (n = 6) for the LRAT assay with dispersed retinol. These target sizes did not differ from each other but were significantly smaller than that of ARAT. These data provide the first physical evidence of the independent entities which catalyze the ARAT and LRAT reactions of liver microsomes.