Fatty acyl CoA-dependent and -independent retinol esterification by rat liver and lactating mammary gland microsomes

Abstract

Retinol esterification was examined in microsomes from rat liver and lactating mammary gland as a function of the form of retinol substrate, dependence on fatty acyl CoA, and sensitivity to phenylmethylsulfonyl fluoride (PMSF). Retinol bound to cellular retinol-binding protein (CRBP) or dispersed in solvent was esterified in a fatty acyl CoA-independent, PMSF-sensitive reaction, consistent with lecithin:retinol acyltransferase (LRAT) activity. LRAT activity exhibited the same K m (2 μ m retinol) between tissues but a higher V max in liver as compared to that in mammary gland (47 vs 8 pmol/min/mg microsome protein, respectively). Solvent-dispersed retinol was also esterifled in a fatty acyl CoA-dependent, PMSF-resistant reaction, consistent with acyl CoA:retinol acyltransferase (ARAT) activity. Retinol bound to CRBP was not a good substrate for this reaction. ARAT activity displayed a similar V max (300 pmol/min/mg microsome protein) between tissues but K m values of 15 and 5 μ m for retinol and fatty acyl CoA in mammary gland as compared to 30 and 25 μ m, respectively, in the liver. Thus, when substrate was near or below K m , retinol esterification occurred predominantly by LRAT in the liver and ARAT in the mammary gland, respectively. The concentration of CRBP in the cytosol, determined by Western blotting, was approximately 2 μ m in the liver but was almost nondetectable in the mammary gland. These data suggest that retinol esteriflcation is regulated via different mechanisms in liver and mammary gland and support a specific role for CRBP in the liver.