Determination of hepatic acyl-coenzyme A-cholesterol acyltransferase activity in LPN hamsters: a model for cholesterol gallstone formation

Acyl-coenzyme A-cholesterol acyltransferase (ACAT) catalyses the esterification of cholesterol with long-chain fatty acyl-coenzyme A derivatives and has been implicated in the development of cholesterol gallstones. In this study we have examined several key components of the hepatic ACAT assay in order to develop a reliable and sensitive ACAT assay for LPN hamsters, a breed of golden Syrian hamster which has been characterized recently by this laboratory as a particularly good model for studying the pathogenesis of cholesterol gallstones. The newly developed ACAT assays were subsequently used to examine whether hepatic ACAT activity is altered in this animal model. Important new methodological findings were: (i) ACAT activity displayed two pH optima, one at 7.0 when assayed using endogenous cholesterol as substrate, and the other at about pH 8.5-9.0 when assayed in the presence of exogenous cholesterol; (ii) ACAT activity increased markedly when exogenous cholesterol was delivered to ACAT in Tween 80 (125-fold) or hydroxypropyl-beta-cyclodextrin (200-fold) in contrast to the use of cholesterol/phosphatidylcholine liposomes (9-fold); (iii) the addition of dithiothreitol, but not reduced glutathione, to the assay mixture resulted in a marked decrease in ACAT activity. Using the optimal assay conditions (exogenous cholesterol added), hepatic ACAT activity was shown to be significantly reduced in hamsters fed a high sucrose lithogenic diet compared with controls (587+/-42 vs 737+/-44 pmol/min per mg; P=0.025). In contrast, ACAT activity measured using endogenous cholesterol as a substrate was greater in sucrose-fed hamsters compared with controls (22.3+/-2.5 vs 13.2+/-2.9 pmol/min per mg; P= 0.030). These results highlight the importance of using an ACAT activity assay which has been well characterized and supports the hypothesis that the pathogenesis of cholesterol gallstones in LPN hamsters is related to an altered hepatic cholesterol metabolism.     

Heterocyclic amides: inhibitors of acyl-CoA:cholesterol O-acyl transferase with hypocholesterolemic activity in several species and antiatherosclerotic activity in the rabbit

A series of heterocyclic amides were synthesized and evaluated as inhibitors of acyl-CoA: cholesterol O-acyltransferase (ACAT) in vitro and for cholesterol lowering in cholesterol-fed rats. Compounds were evaluated for cell-based macrophage ACAT inhibition, bioactivity, and adrenal toxicity. Candidates were selected for evaluation in cholesterol-fed dogs and, ultimately, the injured cholesterol-fed rabbit model of atherosclerosis. The heterocyclic amides potently inhibited rabbit liver ACAT (IC50's = 0.014-0.11 microM), and the majority of compounds significantly lowered plasma cholesterol (42-68%) in an acute cholesterol-fed rat model at 3 mg/kg. The most efficacious compounds in the rat were evaluated for bioactivity in vivo and arterial ACAT inhibition in a cell-based macrophage ACAT assay. Two highly bioactive analogs, (+/-)-2-(3-dodecylisoxazol-5-yl)-2-phenyl-N-(2,4,6-trimethoxypheny l) acetamide (13a) and (+/-)-2-(5-dodecylisoxazol-3-yl)-2-phenyl-N-(2,4,6-trimethoxypheny l) acetamide (16a), were selected for further study and were found to be nontoxic in a guinea pig model of adrenal toxicity. Compounds 13a and 16a lowered total cholesterol in the cholesterol-fed rat, rabbit, and dog models of pre-established hypercholesterolemia. Compound 13a in the injured cholesterol-fed rabbit model of atherosclerosis was effective in slowing the development of cholesteryl ester-rich thoracic aortic lesions, reducing lesion coverage by 53% at a dose of 1 mg/kg.     

2-Azetidinone cholesterol absorption inhibitors: structure-activity relationships on the heterocyclic nucleus

A series of azetidinone cholesterol absorption inhibitors related to SCH 48461 ((-)-6) has been prepared, and compounds were evaluated for their ability to inhibit hepatic cholesteryl ester formation in a cholesterol-fed hamster model. Although originally designed as acyl CoA: cholesterol acyltransferase (ACAT) inhibitors, comparison of in vivo potency with in vitro activity in a microsomal ACAT assay indicates no correlation between activity in these two models. The molecular mechanism by which these compounds inhibit cholesterol absorption is unknown. Despite this limitation, examination of the in vivo activity of a range of compounds has revealed clear structure-activity relationships consistent with a well-defined molecular target. The details of these structure-activity relationships and their implications on the nature of the putative pharmacophore are discussed.     

Disruption of the acyl-CoA:cholesterol acyltransferase gene in mice: evidence suggesting multiple cholesterol esterification enzymes in mammals

The microsomal enzyme acyl-CoA:cholesterol acyltransferase (ACAT; EC 2.3.1.26) catalyzes the esterification of cellular cholesterol with fatty acids to form cholesterol esters. ACAT activity is found in many tissues, including macrophages, the adrenal glands, and the liver. In macrophages, ACAT is thought to participate in foam cell formation and thereby to contribute to atherosclerotic lesion development. Disruption of the gene for ACAT (Acact) in mice resulted in decreased cholesterol esterification in ACAT-deficient fibroblasts and adrenal membranes, and markedly reduced cholesterol ester levels in adrenal glands and peritoneal macrophages; the latter finding will be useful in testing the role of ACAT and macrophage foam cell formation in atherosclerosis. In contrast, the livers of ACAT-deficient mice contained substantial amounts of cholesterol esters and exhibited no reduction in cholesterol esterification activity. These tissue-specific reductions in cholesterol esterification provide evidence that in mammals this process involves more than one form of esterification enzyme.     

Potent inhibitors of acyl-CoA:cholesterol acyltransferase. 2. Structure-activity relationships of novel N-(2,2-dimethyl-2,3-dihydrobenzofuran-7-yl)amides

Novel N-(2,2-dimethyl-2,3-dihydrobenzofuran-7-yl)amide derivatives 1 were synthesized and tested for their ability to inhibit rabbit small intestinal ACAT (acyl-CoA:cholesterol acyltransferase) and lower serum total cholesterol in cholesterol-fed rats. Among the synthesized compounds, N-(2,2,4,6-tetramethyl-2,3-dihydrobenzofuran-7-yl)amide derivatives showed potent ACAT inhibitory activity. The synthesis and structure-activity relationships of these compounds are described. A methyl group at position 6 of the 2,3-dihydrobenzofuran moiety was important for potent ACAT inhibitory activity. In the series of N-(2,2,4,6-tetramethyl-2,3-dihydrobenzofuran-7-yl) amides, lipophilicity of the acyl moiety was necessary for the potent ACAT inhibitory activity. The highly lipophilic acid amides N-(2,2,4,6-tetramethyl-2,3-dihydrobenzofuran-7-yl)-2,2- dimethyldodecanamide (10) and 6-(4-chlorophenoxy)-N-(2,2,4,6-tetramethyl-2,3-dihydrobenzofuran-7-y l)-2,2-dimethyloctanamide (50) showed potent activity. Introduction of a dimethylamino group at position 5 of the 2,3-dihydrobenzofuran moiety resulted in highly potent activity. The most potent compound, N-[5-(dimethylamino)-2,2,4,6-tetramethyl-2,3-dihydrobenzofuran-7-yl ]-2,2-dimethyldodecanamide (13, TEI-6620), showed highly potent ACAT inhibitory activity (rabbit small intestine IC50 = 0.020 microM, rabbit liver IC50 = 0.009 microM), foam cell formation inhibitory activity (rat peritoneal macrophage IC50 = 0.030 microM), extremely potent serum cholesterol-lowering activity in cholesterol-fed rats (71% at a dose of 0.3 mg/kg/day po), and good bioavailability in fed dogs (Cmax = 2.68 microg/mL at 1 h, 10 mg/kg po).     

Clinical experience with the holmium:YAG laser for transmyocardial laser revascularization and myocardial denervation as a mechanism

Several novel N-(4,5-diphenylthiazol-2-yl)-N'-aryl or alkyl (thio)ureas and N-(4,5-diphenylthiazol-2-yl)alkanamides were prepared as potential acyl-CoA: cholesterol O-acyltransferase (ACAT) inhibitors. Synthesis was accomplished by reaction of 2-amino-4,5-diphenylthiazole with the suitable isocyanate, isothiocyanate or acyl chloride. Some analogues without the 5-phenyl substituent or both the phenyl groups in 4 and 5 position of the thiazole ring were also prepared. Moreover, some bioisosters of the title compounds in which the thiazole ring was replaced by an imidazole were synthesized starting from the 2-amino-4,5-diphenyl-1H-imidazole. The ability of synthesized compounds to inhibit ACAT was evaluated in vitro by measuring the formation of cholesteryl[14C]oleate from cholesterol and [1-14C]oleoyl-CoA in rat liver microsomes. Among the tested compounds, only some thiazole ureas were able to inhibit ACAT in a reasonable degree. N-(4,5-diphenylthiazol-2-yl)- N'-[2,6-bis(2-methylethyl)phenyl] urea (11) and N-(4,5-diphenylthiazol-2-yl)-N'-n-butyl urea (16) were the most active compounds in the series showing IC50 values in the low micromolar range.     

The hypolipidemic action of the ACAT inhibitor HL-004 in hamsters fed normal chow

1. A novel ACAT (acyl-CoA: cholesterol acyltransferase) inhibitor, HL-004, exhibited a strong inhibitory effect on the hepatic and intestinal ACAT, but was less effective on the adrenal ACAT in vitro. 2. HL-004 selectively decreased serum VLDL cholesterol, and inhibited hepatic ACAT activity in hamsters fed normal chow. 3. These results suggest that the cholesterol-lowering effect of HL-004 can be attributed to a decrease in hepatic VLDL secretion via inhibition of ACAT.     

Effect of trifluoperazine on certain arterial wall lipid-metabolizing enzymes inducing atherosclerosis in rhesus monkeys

The effect of trifluoperazine (TFP) was investigated on arterial wall lipid-metabolizing enzymes like acyl-CoA:cholesterol acyltransferase (ACAT) and cholesterol ester hydrolase (CEH) in rhesus monkeys. The activity was determined in aortic wall homogenates obtained from rhesus monkeys fed an atherogenic diet coupled with intramuscular injections of adrenaline and TFP. Although TFP had no significant effect on serum cholesterol and triglycerides, it decreased significantly the formation of atherosclerotic lesions by decreasing the esterification of cholesterol, by inhibiting ACAT and enhancing its utilization by activating CEH. Hence, the preventive effect of TFP on the development of atherosclerosis in rhesus monkeys is mediated through its ability to influence the activities of arterial wall lipid-metabolizing enzymes like ACAT and CEH.     

Lithogenic diet and gallstone formation in mice: integrated response of activities of regulatory enzymes in hepatic cholesterol metabolism

Supersaturation of bile with cholesterol is a prerequisite of the development of gallstones. With the intention to study the integrated response of enzymes regulating hepatic cholesterol metabolism during gallstone formation we used an established model for the induction of cholesterol gallstone disease in mice. Ten mice were fed on a lithogenic diet containing 10 g cholesterol/kg and 5 g cholic acid/kg for 8 weeks and were compared with ten mice fed on a standard pellet diet. Cholesterol crystals or gallstones developed in 90% of gallbladders in treated mice. The lithogenic diet had an inhibitory effect on the rate-limiting enzyme of cholesterol biosynthesis, hepatic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.88) activity, 39.6 (SEM 2.8) v. 171.0 (SEM 47.3) pmol/min per mg protein. Cholesterol 7 alpha-hydroxylase (EC 1.14.13.17) activity, regulating bile acid synthesis, was decreased by 80%, and this was assumed to be due to cholic acid in the diet. The cholesterol-enriched diet also induced a tenfold increase in cholesterol esterification rate in the liver, i.e. acyl-CoA:cholesterol acyl transferase (ACAT; EC 2.3.1.26) activity. The total, as well as esterified, cholesterol contents of liver homogenates were significantly higher in cholesterol- and cholic acid-treated mice and correlated well with the ACAT activity (rs 0.72 (P < 0.005), and rs 0.68 (P < 0.01) respectively). A significantly higher ACAT activity was obtained in mice given cholesterol and cholic acid even when the enzyme was saturated with exogenous cholesterol, thus indicating an increased amount of the enzyme. The formation of gallstones is dependent on a delicate balance between lithogenic factors (increased absorption of cholesterol and reduced secretion of bile acids) and defence mechanisms (decreased synthesis and increased esterification of cholesterol). In the specific animal model studied here the two defence mechanisms cannot compensate for the increased absorption of cholesterol and the reduced synthesis of bile acids.     

Acyl-CoA : cholesterol acyltransferase inhibitors from Magnolia obovata

In the course of a search for acyl-CoA : cholesterol acyltransferase (ACAT) inhibitors from natural sources, new types of ACAT inhibitors were isolated from the extract of Magnolia obovata leaves, and identified as obovatol, honokiol, and magnolol. The active compounds inhibit rat liver ACAT with IC50 values of 42, 71, and 86 microM, respectively.     

Polyacetylene analogs, isolated from hairy roots of Panax ginseng, inhibit Acyl-CoA : cholesterol acyltransferase

In the course of our screening program for acyl-CoA : cholesterol acyltransferase (ACAT) inhibitors from Korean herbal medicines, ACAT inhibitors were isolated from the hairy roots of Panax ginseng (Araliaceae) and identified as panaxynol, panaxydol, panaxydiol, and panaxytriol. These active compounds inhibit rat liver ACAT with IC50 values of 94, 80, 45 and 79 microM, respectively.     

Effect of estrogen and progesterone on the expression of hepatic and extrahepatic sterol 27-hydroxylase in baboons (Papio sp)

Sterol 27-hydroxylase plays an important role in cholesterol metabolism in hepatic and extrahepatic tissues. To determine whether female sex steroid hormones influence its expression, we measured plasma and hepatic 27-hydroxycholesterol, hepatic mRNA levels, activity of sterol 27-hydroxylase, and adrenal mRNA levels of this enzyme in baboons (n = 6 per group) treated with placebo, estrogen, estrogen + progesterone, and progesterone. We also measured hepatic cholesterol concentration and hepatic acyl coenzyme A:cholesterol acyltransferase (ACAT) activity to determine their relationship with hepatic sterol 27-hydroxylase activity. Plasma 27-hydroxycholesterol concentration was increased by estrogen and estrogen + progesterone and was negatively correlated with plasma (P = .090) and LDL (P = .026) cholesterol concentrations. Similarly, hepatic sterol 27-hydroxylase activity was increased by estrogen and estrogen + progesterone and was negatively correlated with plasma (P = .056) and LDL (P = .052) cholesterol concentrations but was positively correlated with hepatic and plasma 27-hydroxycholesterol concentrations (P < .001). Hepatic ACAT activity was increased by progesterone (P < .004) and was positively correlated with plasma (P = .002) and LDL (P = .009) cholesterol concentrations but was negatively correlated with hepatic sterol 27-hydroxylase activity (P = .035). Hepatic and adrenal gland mRNA levels for sterol 27-hydroxylase were increased by estrogen alone or in combination with progesterone (P < .05). Hepatic sterol 27-hydroxylase activity was positively correlated with hepatic mRNA levels (P < .001), an observation suggesting that estrogen increases the activity of sterol 27-hydroxylase by increasing its synthesis. Hepatic cholesterol concentration was not influenced by the hormone treatment. These observations suggest that estrogen alone or in combination with progesterone increases the synthesis of sterol 27-hydroxylase in hepatic and extrahepatic tissues, and the increased activity of hepatic sterol 27-hydroxylase resulting from the increased synthesis is associated with a hypolipidemic effect on plasma LDL levels. Furthermore, progesterone alone increases the hepatic ACAT activity, but given in combination with estrogen progesterone does not have the same effect on hepatic ACAT activity. The effect of estrogen on hepatic ACAT activity may be mediated by sterol 27-hydroxylase and its effect on cholesterol metabolism (decreased cholesterol synthesis and increased output of cholesterol in the bile) in liver.     

Inhibitors of acyl-CoA:cholesterol O-acyltransferase. 2. Identification and structure-activity relationships of a novel series of N-alkyl-N-(heteroaryl-substituted benzyl)-N'-arylureas

A series of N-alkyl-N-(heteroaryl-substituted benzyl)-N'-arylurea and related derivatives represented by 2 and 3 have been prepared and evaluated for their ability to inhibit acyl-CoA:cholesterol O-acyltransferase in vitro and to lower plasma cholesterol levels in cholesterol-fed rats in vivo. Among these novel compounds, the type 3 series was superior. A pyrazol-3-yl group on the N-benzyl group of this trisubstituted urea (i.e. 3, Ar1 = pyrazol-3-yl) was identified as a heteroaromatic ring providing a good profile of biological activity. As a result of optimization of the combination with the N-alkyl group (R) and N-aryl group (Ar3), compound 3aq (FR186054) was identified as a new, orally efficacious ACAT inhibitor, which exhibited potent in vitro ACAT inhibitory activity (rabbit intestinal microsomes IC50 = 99 nM) and excellent hypocholesterolemic effects in cholesterol-fed rats, irrespective of administration mode (ED50 = 0.046 mg/kg dosed via the diet, ED50 = 0. 44 mg/kg administered by gavage in PEG400 vehicle). Moreover, a toxicological study revealed compound 3aq to be nontoxic to the adrenal glands of dogs when tested at a single dose of 10 mg/kg po.     

Elevation of serum cholesterol levels in mice by the antioxidant butylated hydroxyanisole

The food antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are structurally related to the hypocholesterolemic drug probucol. The purpose of this study was to determine if BHA can lower serum cholesterol levels as is observed with probucol. Treatment of mice with 0.75% BHA in their feed for 10 days resulted in a significant (P < or = 0.01) elevation of serum cholesterol levels. This effect contrasts with the cholesterol-lowering effect of probucol. Further, the degree of cholesterol elevation was comparable to that observed in mice administered 3% cholesterol in their feed for 7 days. The enzyme acyl CoA:cholesterol acyltransferase (ACAT) was decreased significantly (P < or = 0.01) in liver microsomes from BHA-treated mice. In contrast, hepatic microsomal ACAT activity was increased significantly (P < or = 0.01) in cholesterol-fed mice. These results suggested that the increased serum cholesterol observed in BHA-treated mice was not accompanied by an increase in hepatic cholesterol levels. Indeed, hepatic microsomal cholesterol levels were reduced in BHA-treated mice, but were increased significantly (P < or = 0.01) in cholesterol-fed mice. These results demonstrate that the common food additive BHA elevates serum cholesterol levels by a mechanism that apparently involves the decreased uptake of cholesterol by the liver.     

Immunolocalization of acyl-coenzyme A:cholesterol O-acyltransferase in macrophages

Macrophages in atherosclerotic lesions accumulate large amounts of cholesteryl-fatty acyl esters ("foam cell" formation) through the intracellular esterification of cholesterol by acyl-coenzyme A:cholesterol O-acyltransferase (ACAT). In this study, we sought to determine the subcellular localization of ACAT in macrophages. Using mouse peritoneal macrophages and immunofluorescence microscopy, we found that a major portion of ACAT was in a dense reticular cytoplasmic network and in the nuclear membrane that colocalized with the luminal endoplasmic reticulum marker protein-disulfide isomerase (PDI) and that was in a similar distribution as the membrane-bound endoplasmic reticulum marker ribophorin. Remarkably, another portion of the macrophage ACAT pattern did not overlap with PDI or ribophorin, but was found in as yet unidentified cytoplasmic structures that were juxtaposed to the nucleus. Compartments containing labeled beta-very low density lipoprotein, an atherogenic lipoprotein, did not overlap with the ACAT label, but rather were embedded in the dense reticular network of ACAT. Furthermore, cell-surface biotinylation experiments revealed that freshly harvested, non-attached macrophages, but not those attached to tissue culture dishes, contained approximately 10-15% of ACAT on the cell surface. In summary, ACAT was found in several sites in macrophages: a cytoplasmic reticular/nuclear membrane site that overlaps with PDI and ribophorin and has the characteristics of the endoplasmic reticulum, a perinuclear cytoplasmic site that does not overlap with PDI or ribophorin and may be another cytoplasmic structure or possibly a unique subcompartment of the endoplasmic reticulum, and a cell-surface site in non-attached macrophages. Understanding possible physiological differences of ACAT in these locations may reveal an important component of ACAT regulation and macrophage foam cell formation.     

Increased acylCoA-cholesterol ester acyltransferase activity in gallbladder mucosa in patients with gallbladder cholesterolosis

OBJECTIVE: Although cholesterolosis of the human gallbladder is a relatively common disease, its etiology has not been fully understood. The aim of this study was to determine this etiology. METHODS: The lipid composition of the gallbladder mucosa and gallbladder bile and the enzyme activities (acylCoA-cholesterol ester acyltransferase [ACAT] and cholesterol ester hydrolase [CEH]) of the gallbladder mucosa were measured in control subjects, patients with cholesterolosis, and patients with cholesterol gallstone disease. RESULTS: Levels of cholesterol ester in gallbladder mucosa in patients with cholesterolosis (n = 12) were higher than those in control subjects (n = 8). With regard to the lipid content in gallbladder bile, no differences were found in concentrations of cholesterol, phospholipids, and bile acids among control subjects (n = 11), patients with cholesterolosis (n = 13), and those with cholesterol gallstone disease (n = 15). In gallbladder mucosa, ACAT activity was significantly higher in patients with cholesterolosis (n = 10) than in control subjects (n = 8), whereas CEH activity did not differ between the two groups. As a result, the ACAT/CEH activity ratio was higher in patients with cholesterolosis than in control subjects. CONCLUSIONS: It would be suggested that cholesterol ester synthesis of gallbladder mucosa might play an etiological role in the development of cholesterolosis.     

Cholesterol biosynthesis, peroxisomes and peroxisomal disorders: mevalonate kinase is not only deficient in Zellweger syndrome but also in rhizomelic chondrodysplasia punctata

We examined the inhibitory potency of F-1394 ((1S,2S)-2-[3-(2,2-dimethylpropyl)-3-nonylureido]cyclohexane -1-yl 3-[(4R)-N-(2,2,5,5-tetramethyl-1,3-dioxane-4-carbonyl)amino]propionate), an acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on ACAT activity and its hypolipidemic effect. F-1394 inhibited whole-cell ACAT activity in HepG2 cells with an IC50 value of 42 nM. The potency of F-1394 was greater than that of the five other ACAT inhibitors tested (YM-17E, CI-976, 57-118, CL-277,082 and DL-melinamide). In rats made hyperlipidemic by Triton WR-1339, F-1394 caused a reduction in the hepatic secretion rate of cholesterol. These data suggest that inhibition of hepatic ACAT activity helps to reduce very low density lipoprotein secretion from the liver into the circulation.     

Effect of the ACAT inhibitor, HL-004, on cholesterol metabolism in macrophages

The effects of a novel acyl-CoA: cholesterol acyltransferase (ACAT) inhibitor, HL-004, on cholesterol metabolism were examined in mice peritoneal macrophages. Cholesteryl ester-rich foam cells were induced by incubating macrophages with acetylated LDL. HL-004 prevented the accumulation of cholesteryl ester in the presence of the cholesterol acceptor, HDL. In the absence of HDL, HL-004 generated large amounts of free cholesterol in the cell. Moreover, HL-004 stimulated the efflux of cholesterol from preestablished foam cells in the presence of HDL. These results suggest that the inhibition of foam cell formation and the stimulation of foam cell regression by HL-004 are attributed to intracellular ACAT inhibition, and that HL-004 would be expected to exhibit an antiatherosclerotic effect through direct action on arterial wall.     

Immunodepletion experiments suggest that acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) protein plays a major catalytic role in adult human liver, adrenal gland, macrophages, and kidney, but not in intestines

The first acyl-coenzyme A:cholesterol acyltransferase (ACAT) cDNA cloned and expressed in 1993 is designated as ACAT-1. In various human tissue homogenates, ACAT-1 protein is effectively solubilized with retention of enzymatic activity by the detergent CHAPS along with high salt. After using anti-ACAT-1 antibodies to quantitatively remove ACAT-1 protein from the solubilized enzyme, measuring the residual ACAT activity remaining in the immunodepleted supernatants allows us to assess the functional significance of ACAT-1 protein in various human tissues. The results showed that ACAT activity was immunodepleted 90% in liver (83% in hepatocytes), 98% in adrenal gland, 91% in macrophages, 80% in kidney, and 19% in intestines, suggesting that ACAT-1 protein plays a major catalytic role in all of the human tissue/cell homogenates examined except intestines. Intestinal ACAT activity is largely resistant to immunodepletion and is much more sensitive to inhibition by the ACAT inhibitor Dup 128 than liver ACAT activity.