Abstract
Esterification of endogenous cholesterol in human small intestinal mucosa by acyl-CoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26) was studied using [1-14C]oleoyl-CoA as substrate. The reaction was linear for 2 min only. The esterification of cholesterol was stimulated by albumin, but this effect was dependent on the oleoyl-CoA concentration. When the albumin concentration was 5 g/liter, maximal esterification was obtained with 35 microM oleoyl-CoA. The pH optimum was 7.2-7.8. The ACAT specific activity was highest in microsomal preparations from jejunum (0.21 +/- 0.19 (n = 18) nmol cholesteryl oleate . mg microsomal protein-1 . min-1), and lower in proximal duodenum and distal ileum. Whole homogenates of biopsies had about 1/4 of the activity of the corresponding microsomal preparation. Microsomal preparations from jejunum contained acyl-CoA hydrolase (EC 3.1.2.2) which under the prevailing conditions had a maximal activity of 4.4 nmol oleate formed . microsomal protein-1 . min-1. The high activity of intestinal ACAT in man renders it possible that this enzyme plays a role in cholesterol absorption.
Highlights
T h e high activity of intestinal ACAT in manrenders it ate its activity, we have tested first the optimal condipossible thatthisenzyme plays a role in cholesterol ab- tions of the assay using microsomal preparations of tsAioncreyp:lt-iiCtosonaA.c"t:icevlhigoteylreausndted,rosloamcPey.,ltprKaro.npsSfeaerartairesesemion, fahntudhmeKaen. nRszm.ymaNlliocirnutreemsa-.ctionjse.ejgumnuemntasnodfththeentshme all specific intestine
Esterification of intracellularcholesterol in most tissues is catalyzed by acyl-CoA:cholesterol acyltransferase [23], but attemptotsdetect ACAT in the intestine using ATP, CoA, and fatty acids have been unsuccessful [24] and the mechanism of cholesterol esterification in the mucosal cell has been poorly understood [23,25].Using an efficient acyl-CoAgenerating system, we have recently been able to demonstrate CoA-dependent esterification in different species [1, 2, 3]
With preformed [ l-14C]oleoyl-CoAas substrate we have in this report proved the existence of ACAT in human small intestine
Summary
Our data strongly indicate that cholesterol esterification is catalyzed by an acyl-CoA: cholesterol acyltransferase(ACAT)(EC2.3.1.26).Inthesestudies,the enzyme activity was determinedusingassubstrate endogenous cholesterol labeled with trace amountsof [7a-3H]cholesterol[4], and complete equilibration of Chemicals [l-14C]Oleoyl-CoA(ca. 45 mCiimmo1) was from N e w England Nuclear, Boston,MA. Our data strongly indicate that cholesterol esterification is catalyzed by an acyl-CoA: cholesterol acyltransferase(ACAT)(EC2.3.1.26).Inthesestudies,the enzyme activity was determinedusingassubstrate endogenous cholesterol labeled with trace amountsof [7a-3H]cholesterol[4], and complete equilibration of Chemicals [l-14C]Oleoyl-CoA(ca. Unlabeled oleoylCoA and CoA were purchased from Sigma Chemical Co., St. Louis, MO. T h e oleoyl-CoA was dissolved in isotope with endogenous cholesterolpools was assumed in the calculation of total ansdpecific ACAT activities. Box 1046, University of Oslo, Blindern, Oslo 3, Norway Bovine serumalbumin(fraction V; Sigma Chemical Co.)was essentially fatty acid-free. It was dissolved in potassium phosphate buffer (0.2 M, p H 7.4), heat-inactivated at60°C for 30 min, and stored at -20°C. All other chemicals and solvents were standard commercial high purity materials
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