Acyl CoA:retinol acyltransferase in rat small intestine: its activity and some properties of the enzymic reaction.

The present study was conducted to examine whether the intestinal esterification of retinol could be due to a microsomal acyl-CoA transferase. When the 'microsomal fraction' of rat mucosa was incubated with [3H]retinol and palmitoyl-CoA or oleoyl-CoA, [3H]retinyl esters were formed as identified by alumina column chromatography and reverse phase high-pressure liquid chromatography (HPLC). Unlabeled retinol and [1-14C]palmitoyl-CoA yielded retinyl[1-14C]palmitate. The esterifying activity was lost when microsomes were heated at 60 degrees C for 30 min. Only negligible activity was observed without exogenous acyl-CoA while 10-20 muM gave optimum activity provided that 2-5 mg/ml of albumin was present. Replacement of acyl-CoA by palmitate gave no esterification, indicating that the activity was not a reversed hydrolase reaction. Optimum pH was 7.1-7.6 and optimal concentration of retinol was 15 muM. With palmitoyl-CoA, the formation of retinyl ester was 1.00 +/- 0.26 nmol . mg protein-1 . min-1 (x +/- SD, n = 4) in rats killed postprandially versus 2.06 +/- 0.66 (n = 5) after 36 hr of fasting. Oleoyl-CoA gave lower activity: 0.52 +/- 0.14 and 1.41 +/- 0.36, respectively. The variation with feeding and fasting was significant (P less than 0.05) and corresponded to that of the intestinal acyl-CoA:cholesterol acryltransferase (ACAT). Inhibition of retinol esterification was observed with taurocholate and the thiol-blocking agent 5,5'-dithiobis (2-nitrobenzoic acid). The data show that rat intestinal microsomes catalyze the formation of retinyl esters by an acyl-CoA:retinol acyltransferase with several properties in common with ACAT located in the same subcellular fraction.