Retinol esterification by microsomes from the mucosa of human small intestine. Evidence for acyl-Coenzyme A retinol acyltransferase activity.
The mechanism of the intestinal esterification of retinol has been obscure. Recently, an acyl-Coenzyme A (CoA):retinol acyltransferase (ARAT) was found in rat intestinal microsomes, and experiments were therefore conducted to determine whether a corresponding enzyme exists in human small intestine. When microsomes were incubated with [3H]retinol and palmitoyl-CoA, or retinol and [1-14C]palmitoyl-CoA, radioactive retinyl palmitate was formed as identified by alumina column chromatography and reverse-phase high-pressure liquid chromatography. Heating the microsomes for 30 min at 60 degrees C resulted in loss of activity. The esterification was negligible without exogenous acyl-CoA and markedly stimulated by palmitoyl-, oleoyl-, and stearoyl-CoA in concentrations up to 20 microM. The acyl-CoA was successfully replaced by an acyl-CoA generating system, but not by unactivated palmitate (2.5-200 microM). The assay was dependent on the presence of albumin with optimum activity at 2-10 mg/ml. The optimal retinol concentration was 20-30 microM and pH approximately 7.4. The esterifying activity was completely inhibited by 8 mM of taurocholate and to 90% by 1 mM of 5,5'-dithiobis(2-nitrobenzoic acid). Activity was found throughout the small intestine. In jejunum the rate of retinol esterification was: 3.44 +/- 2.24 nmol [3H]retinyl ester formed . mg microsomal protein-1 . min-1 (mean +/- SD, n = 12). The corresponding activity in whole homogenates of biopsies were 1.17 +/- 0.28 (n = 8). It is concluded that human small intestine contains a microsomal acyl-CoA:retinol acyltransferase. Due to its high activity in vitro this enzyme is likely to be responsible for the intestinal esterification of retinol.
D000217:Acyltransferases / Q000037:antagonists & inhibitors / Q000502:physiology*; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D004951:Esterification; D005227:Fatty Acids / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007413:Intestinal Mucosa / Q000201:enzymology*; D008861:Microsomes / Q000201:enzymology*; D051142:Retinol O-Fatty-Acyltransferase; D013656:Taurocholic Acid / Q000494:pharmacology; D014801:Vitamin A / Q000378:metabolism*
D005227:Fatty Acids; D014801:Vitamin A; D013656:Taurocholic Acid; D004228:Dithionitrobenzoic Acid; D000217:Acyltransferases; D051142:Retinol O-Fatty-Acyltransferase