Inhibitors of sterol synthesis. 15-oxygenated steryl ester hydrolase activity of rat liver at neutral and acid pH

5 alpha-Cholest-8(14)-en-3 beta-ol-15-one (15 ketosterol) is a potent inhibitor of cholesterol biosynthesis with significant hypocholesterolemic activity. The results of a recent study (Schroepfer, G.J., Jr., Christophe, A., Chu, A.J., Izumi, A., Kisic, A. and Sherrill, B.C. (1988) Chem. Phys. Lipids 48, 29-58) have indicated that, after intragastric administration of the 15-ketosterol in triolein to rats, most of the compound in intestinal lymph occurs in the form of the oleate ester, which is associated with chylomicrons. Moreover, after intravenous administration of chylomicrons containing the oleate ester of 15-[2,4-3H]ketosterol, rapid and selective uptake of 3H by liver was observed, which was associated with the rapid and substantial appearance of labeled free 15-ketosterol in liver. The present study concerns the capabilities of rat liver fractions to catalyze the hydrolysis of 15-ketosteryl oleate. Efficient hydrolysis was observed at acid pH with a digitonin-solubilized extract of rat liver, with a rate similar to that for the hydrolysis of cholesteryl oleate. The distribution of acid 15-ketosteryl oleate hydrolase of whole liver homogenate on a metrizamide isopycnic density gradient was similar to that of acid cholesteryl oleate hydrolase and acid phosphatase, suggesting that the lysosomal acid lipase is the enzyme responsible for the hydrolysis of the 15-ketosteryl oleate at acid pH. At neutral pH, 15-ketosteryl oleate and cholesteryl oleate was hydrolyzed at similar rates by the microsomal fraction of liver homogenate, whereas the 15-ketosteryl oleate was hydrolyzed at a much lower rate than cholesteryl oleate by the cytosolic fraction. The distribution of neutral 15-ketosteryl oleate hydrolase activity of whole liver homogenate on a metrizamide isopycnic density gradient was most correlated to a microsomal esterase, whereas cholesteryl oleate hydrolase activity was most correlated to a cytosolic enzyme. Both 15-ketosteryl oleate and cholesteryl oleate hydrolase activities were correlated to a mitochondrial marker enzyme.     

Metabolism of doubly-labeled chylomicron cholesteryl esters in the rat

Chylomicrons labeled in vitro with doubly-labeled cholesteryl esters were injected intravenously into fasted rats, and the tissue distribution and chemical form of each isotope were observed for 24 hr. The use of doubly-labeled cholesteryl esters provided information about the metabolism of both the sterol and the fatty acid moieties. Similar results were obtained with doubly-labeled cholesteryl palmitate, oleate, and linoleate. In each instance, most (80-90%) of the chylomicron cholesteryl ester was removed from the plasma by the liver; small amounts were also taken up by all other tissues examined. There was no hydrolysis during uptake. In the liver the newly absorbed cholesteryl esters underwent slow hydrolysis (60% after 1 hr and 85-90% after 3.5 hr); the rate of reesterification of the liberated cholesterol was still slower. After 24 hr only 20-28% of the labeled cholesterol present in the animal was found in the liver. Labeled fatty acid disappeared from the liver, and was redistributed among other tissues, much more rapidly than the labeled cholesterol. Most of the labeled fatty acid apparently underwent oxidation, since only 15-20% of the injected labeled fatty acid was present in the animal after 24 hr. At this time the three fatty acids were differently distributed between and within the tissues. These differences reflected some known differences of fatty acid concentration and lipid composition in the various tissues.     

Rapid transport of fatty acids from rat liver endothelial to parenchymal cells after uptake of cholesteryl ester-labeled acetylated LDL

Acetylated low-density lipoprotein (acetyl-LDL) radiolabeled in the oleate moiety of cholesteryloleate was injected into rats. Isolation of the various liver cell types at different times after acetyl-LDL injection by a low-temperature procedure allowed the intrahepatic metabolism of the oleate moiety to be followed in vivo. The cholesteryloleate radioactivity is rapidly cleared from the circulation and at 5 min after injection recovered into parenchymal and endothelial liver cells, mainly as cholesteryloleate ester. At longer time intervals after injection, the amount of cholesteryl esters associated with the endothelial cells was sharply decreased and the [14C]oleate was redistributed within the liver and mainly recovered in the parenchymal cells. The cholesteryl ester initially directly taken up by the parenchymal cells was also rapidly hydrolysed but, in contrast to the endothelial cells, the [14C]oleate remained inside the cells and was incorporated into triacylglycerols and phospholipids. The 14C radioactivity in parenchymal cells taken up between 5 and 30 min after injection of the cholesteryl [14C]oleate-labeled acetyl-LDL (transported as oleate from endothelial cells), followed a similar metabolic route as the amount which was directly associated to parenchymal cells. The data indicate that the liver and, in particular, the liver endothelial cell has the full capacity to rapidly catabolize modified lipoproteins. In this catabolism, the liver functions as an integrated organ in which fatty acids, formed from cholesteryl esters in endothelial cells, are rapidly transported to parenchymal cells, indicating the concept of metabolic cooperation between the various liver cell types.     

Comparison of the metabolism of chylomicrons and chylomicron remnants by the perfused liver

1. The hepatic metabolism of chylomicrons and chylomicron remnants was compared after adding approximately equal numbers of each lipoprotein particle to the perfusate of isolated livers. 2. At least 40% of the added remnants were metabolized by the liver compared with less than 3% for chylomicrons. 3. There was significantly more net removal of labelled remnants than of chylomicrons by the liver. 4. A greater proportion of labelled cholesterol than of labelled triacylglycerol fatty acids was transferred to the liver from each lipoprotein. 5. Cholesteryl esters of remnants were hydrolysed to triacylglycerol fatty lipoprotein. 5. Cholesteryl esters of remnants were hydrolysed to triacylglycerol fatty acids of remnants were oxidized to CO2 more extensively than those of chylomicrons. 6. There was greater oxidation of remnant glycerolipic [(1(-14)C]oleate than of glycerolipid [1(-14)C]palmitate. 7. A large fraction of the fatty acids of remnants, but not of chylomicrons, was transferred to phospholipids, which were released by the liver in a lipoprotein of relative density less than 1.006. 8. Label from remnants, but not from chylomicrons, was found in lipoproteins of relative density greater than 1.006, which were not released during perfusion but could be flushed out from the liver at the end of perfusion.     

Metabolism of chylomicrons by the isolated rat liver

Isolated livers perfused with washed corn oil chylomicrons labeled in vivo with palmitic acid-1-(14)C removed a large proportion of the chylomicrons. Slices from these livers oxidized chylomicron fatty acid esters to both carbon dioxide and acetoacetate. The liver slices also generated free fatty acids from chylomicron lipids and converted chylomicron triglycerides to phospholipids. Similar activities were observed in rat liver slices prepared shortly after the intravenous administration of chylomicrons to intact rats. The observed chylomicron uptake and lipid conversions were similar in livers from both fed and fasted rats. Fasting increased the oxidation of chylomicron fatty acid esters by livers labeled in vivo and by perfusion. In livers removed from intact rats given labeled chylomicrons, the triglyceride-(14)C to phospholipid-(14)C ratio was high, a finding unexpected if the liver had acquired this (14)C by removal of circulating fatty acids formed by extrahepatic lipolysis. These results demonstrate the ability of the liver to remove and utilize chylomicrons directly and suggest that direct removal accounts for a significant portion of the chylomicron fatty acids utilized by the liver of intact rats.     

5 alpha-Cholest-8(14)-en-3 beta-ol-15-one. In vivo conversion to cholesterol upon oral administration to a nonhuman primate

The metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one (I), a potent inhibitor of cholesterol synthesis with marked hypocholesteremic activity, has been studied in a nonhuman primate. A mixture of [2,4-3H]-I and [4-14C]-cholesterol was administered to a male baboon in the form of a feedball. Blood was samples at 4, 8, 12, 16, and 24 hr. Detailed analyses of the plasma lipids indicated very rapid absorption of I (relative to cholesterol) and metabolism to cholesterol, cholesteryl esters, and esters of I. The labeled cholesterol was characterized by chromatographic techniques and by purification by way of its dibromide derivative. The levels of 3H in plasma associated with I, esters of I, cholesterol, and cholesteryl esters each showed a different time course. By 24 hr after the administration of [2,4-3H]-I, most of the 3H in plasma was associated with cholesterol and cholesteryl esters. The levels of total 3H and 14C in plasma at various times after the administration of the mixture of [2,4-3H]-I and [4-14C]-cholesterol differed markedly with 3H showing a maximum value at 4 hr and 14C showing a maximum value at 24 hr.     

Intravascular metabolism of lipoprotein cholesteryl esters in African green monkeys: differential fate of doubly labeled cholesteryl oleate

High density lipoproteins (HDL), doubly labeled with [3H]cholesteryl oleate and cholesteryl [14C]oleate, were reinjected to study HDL cholesteryl ester metabolism in African green monkeys. The transfer of labeled HDL cholesteryl ester to low density lipoprotein (LDL) was rapid and equilibration of the [3H]cholesteryl oleate and cholesteryl [14C]oleate specific activities in LDL and HDL occurred within 90 min after reinjection. The apparent rates of disappearance from the circulation of the two moieties of the cholesteryl ester were different. In the same four animals, the residence time for the turnover of plasma [3H]cholesterol averaged 6.1 days while the residence time for the removal of cholesteryl [14C]oleate from plasma was approximately 2.1 days. These results suggest that for some lipoprotein cholesteryl esters removed from plasma, the cholesterol moiety subsequently reappeared in plasma. The difference between the rate of decay of the 14C-labeled fatty acid moiety, which represents all of the cholesteryl ester removed from plasma (0.48 pools/day) and the decay of the 3H-labeled cholesterol moiety, which represents the sum of cholesteryl ester removal and cholesterol reappearance (0.16 pools/day), is the fraction of the cholesteryl ester pool recycled per day (0.32 pools/day or 22.5 mg/kg per day). In other words, approximately 68% of the cholesterol moiety that was removed from plasma as cholesteryl oleate reappeared in the plasma cholesterol pool. These studies support the concept that an efficient reutilization cycle for plasma cholesterol occurs, i.e., the cholesteryl ester molecule can exit and the cholesterol moiety can re-enter plasma without effective equilibration of the cholesterol moiety with extravascular cholesterol pools.     

Metabolism of chylomicron arachidonic and linoleic acid in the rat

Chyle and chylomicrons, obtained after feeding thoracic duct cannulated rats [3H]arachidonic (20:4) and [14C]linoleic acid (18:2) in cream, were injected i.v. into recipient animals. 7.5-15 min after injection, the 14C/3H ratio of the triacylglycerols remaining in plasma was about half of that in the injected chylomicrons, indicating that the chylomicron remnants formed retained relatively more [3H]20:4 than [14C]18:2. The 14C/3H ratio of plasma diacylglycerols was about 6-fold lower than that of plasma free fatty acids. The proportion of [3H]20:4 found in plasma cholesteryl esters was several-fold higher than that of [14C]18:2. Inhibition of hepatic lipase by a specific antiserum did not significantly influence the clearance of triacylglycerols, but increased the amount of 3H in plasma diacylglycerols. It also prevented the rapid clearance of phosphatidylethanolamine from plasma. The liver uptake of [3H]20:4 exceeded that of [14C]18:2. Antiserum against hepatic lipase diminished the difference. In contrast, the 14C/3H ratio of adipose tissue was higher than that of the injected chyle lipoproteins.     

Effects of cholesterol content on the metabolism of protein-free emulsion models of lipoproteins

After intravenous injection, emulsions with compositions similar to chylomicrons behaved metabolically as described for chylomicrons, with faster removals of triacylglycerols than cholesteryl esters from the blood after injection into rats, and with greater uptakes of cholesteryl esters than triacylglycerols by the liver. In contrast, emulsions with a high content of free cholesterol showed equal removal rates from the blood of triacylglycerols and cholesteryl esters; and similar uptakes by the liver. This pattern of metabolism was that expected for a chylomicron core remnant particle. Emulsions poor in cholesteryl ester but rich in free cholesterol showed remnant-like behavior, whereas emulsions rich in cholesteryl ester but poor in free cholesterol were metabolized like nascent chylomicron particles. The amount of free cholesterol appeared to regulate metabolism by affecting the binding of apolipoproteins to the particle surface. Emulsions with a high content of free cholesterol bound less A-I, A-IV and C apolipoproteins, and the relative amount of apolipoprotein E was increased. All of these effects are consistent with the metabolic differences between chylomicrons and remnant particles, suggesting that the amount of free cholesterol plays a regulatory role in chylomicron metabolism.     

Absorption, transport, and storage of retinyl-15-14C palmitate-9,10-3H in the rat

Retinyl-15-(14)C palmitate-9,10-(3)H was fed to rats in order to study hydrolysis and reesterification of this ester during digestion, absorption, transportation, and storage. After administration there was a progressive increase in the (14)C/(3)H ratio of the retinyl esters as they moved from intestinal contents to intestinal mucosa, lymph, and liver, which indicates that repeated hydrolysis and reesterification occur during the digestion and assimilation of this ester.     

Metabolism of protein-free lipid emulsion models of chylomicrons in rats

Emulsions were prepared by ultrasonication of mixtures of triolein, cholesteryl oleate, phosphatidylcholine and cholesterol in aqueous dispersions, then purified by ultracentrifugation. After injection into rats, the metabolism of the artificial, protein-free emulsions was comparable to the metabolism of chylomicrons collected from rat intestinal lymph during the absorption of fat. Like chylomicrons, the emulsion triacylglycerol was removed from the plasma more quickly than emulsion cholesteryl ester. Also like chylomicrons, much more emulsion cholesteryl ester than triacylglycerol appeared in the liver 10 min after injection, and only trace amounts appeared in the spleen. Because the artificial emulsions gained apolipoproteins when incubated with plasma, their metabolism was probably facilitated by the recipient rat plasma apolipoproteins and so, in rats made apolipoprotein-deficient by treatment with estrogen, the removal of emulsions from the plasma was slowed. Removal was also slowed in hyperlipidemic rats fed a high-fat, high-cholesterol diet to expand the plasma pools of the triacylglycerol-rich lipoproteins and remnants. The results indicate that the metabolism of lymph chylomicrons can be modeled by artificial, protein-free lipid emulsions not only in the initial partial hydrolysis by lipoprotein lipase, but also in the delivery of a remnant-like particle to the liver.     

Metabolic heterogeneity of post-lipolysis rat mesenteric lymph small chylomicrons produced in vitro

The study was undertaken to investigate the metabolic rat of post-lipolysis mesenteric lymph small chylomicrons produced in vitro. Small chylomicrons doubly labeled with [3H]cholesterol (more than 70% in cholesteryl esters) and [14C]palmitate-labeled triglycerides were collected from rat mesenteric lymph during periods of fasting. Lipolysis was performed in vitro with lipoprotein lipase purified from bovine milk. More than 98% of the chylomicron-triglycerides could be hydrolyzed to fatty acids. Post-lipolysis chylomicrons were separated by zonal ultracentrifugation, characterized, and tested for biological behavior in intact rats. Following lipolysis the lipoproteins lost nearly all their triglycerides, apoA-I, and apoC, and were relatively enriched with cholesteryl esters, unesterified cholesterol, phospholipids, and apoB. Three preparations were tested for biological behavior: pooled (total) post-lipolysis chylomicrons (diameter approximately 250 A); particles at the ascending part of the zonal effluent (diameter approximately 300 A), and at the descending part (diameter approximately 200 A). After intravenous injection to intact rats, [3H]cholesteryl ester decay was very rapid with pooled lipoproteins and the 300-A preparation (t1/2 = 5-10 min). The 200-A preparation in contrast stayed in circulation much longer (t1/2 = 60-90 min). The study thus demonstrated metabolic heterogeneity of post-lipolysis small chylomicrons and indicated that some may form an LDL-like subpopulation with a plasma lifetime slower than "remnants" but faster than LDL.     

Inhibitors of sterol synthesis. Metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one after intravenous administration to bile duct-cannulated rats

The metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one has been studied after intravenous administration to bile duct-cannulated rats. Very rapid and substantial conversion of the 15-ketosterol to polar biliary metabolites was observed in both male and female rats. For example, upon intravenous injection of [4-14C]5 alpha-cholest-8(14)-en-3 beta-ol-15-one to male bile duct-cannulated rats, approximately 86% of the administered 14C was recovered in bile in the first 38 h. Of the total amount of 14C recovered in bile in 38 h, approximately 50% was excreted in bile in the first 70 min and approximately 90% was excreted within 8 h after the injection of the 15-ketosterol. A substantial fraction of the polar biliary metabolites was shown to undergo enterohepatic circulation. Of the radioactivity derived from the labeled 15-ketosterol which was not recovered in bile or other excreta at 48 h after the intravenous administration of the 15-ketosterol, most (approximately 79%) was recovered in the form of cholesterol and cholesteryl esters of blood and the various tissues. The very substantial and rapid biliary excretion of polar metabolites of the 15-ketosterol (or of cholesterol derived from the 15-ketosterol), coupled with inhibition of the intestinal absorption of cholesterol by the 15-ketosterol, may contribute to the overall hypocholesterolemic action of the 15-ketosterol which has been observed in rodents and in nonhuman primates by providing a metabolic pathway(s) wherein a substantial fraction of the absorbed 15-ketosterol is rapidly removed from the body by biliary excretion in the form of polar metabolites.     

Cholesteryl ester hydrolysis in rat liver lysosomes: different response to female sex hormones

The regulation of the hydrolysis of cholesteryl oleate by female sex hormones was studied in the lysosomal fraction of rat liver. Cholesterol ester hydrolase activity was determined at pH 5.0 with an acetone-dissolved cholesteryl [1-14C]oleate substrate preparation. The administration of a single dose of progesterone decreased the enzyme activity during a 3- to 24-hr period following hormone injection. This effect was not correlated to changes in the lysosomal protein synthesis rate. The lysosomal hydrolysis of cholesteryl esters was also inhibited in a noncompetitive manner by the addition of progesterone at concentrations higher than 100 microM. The esterase failed to respond to the estradiol in vivo as well as in vitro. The findings of the present paper suggest that the lysosomal breakdown of cholesteryl esters in rat liver may be under selective hormonal regulation and that the inhibitory effect of progesterone on the enzyme activity might be, at least in part, responsible for the liver cholesterol ester accumulus produced by the administration of the hormone.     

Mitochondrial glycerol-3-phosphate acyltransferase-1 directs the metabolic fate of exogenous fatty acids in hepatocytes

Because excess triacylglycerol (TAG) in nonadipose tissues is closely associated with the development of insulin resistance, interest has increased in the metabolism of long-chain acyl-CoAs toward beta-oxidation or the synthesis and storage of TAG. To learn whether a mitochondrial isoform of glycerol-3-phosphate acyltransferase (mtGPAT1) competes with carnitine palmitoyltransferase I (CPT I) for acyl-CoAs and whether it contributes to the formation of TAG, we overexpressed rat mtGPAT1 13-fold in primary hepatocytes obtained from fasted rats. When 100, 250, or 750 microM oleate was present, both TAG mass and the incorporation of [14C]oleate into TAG increased more than twofold in hepatocytes overexpressing mtGPAT1 compared with vector controls. Although the incorporation of [14C]oleate into CO2 and acid-soluble metabolites increased with increasing amounts of oleate in the media, these metabolites were approximately 40% lower in the Ad-mtGPAT1 infected cells, consistent with competition for acyl-CoAs between CPT I and mtGPAT1. A 50-60% decrease was also observed in [14C]oleate incorporation into cholesteryl ester. With increasing amounts of exogenous oleate, [14C]TAG secretion increased appropriately in vector control-infected hepatocytes, suggesting that the machinery for VLDL-TAG biogenesis and secretion was unaffected. Despite the marked increases in TAG synthesis and storage in the Ad-mtGPAT1 cells, however, the Ad-mtGPAT1 cells secreted the same amount of [14C]TAG as the vector control cells. Thus, in isolated hepatocytes, mtGPAT1 may synthesize a cytosolic pool of TAG that cannot be secreted.     

Metabolism of high density lipoprotein lipids by the rat liver: evidence for participation of hepatic lipase in the uptake of cholesteryl ester.

In order to determine the role of hepatic lipase in the hepatic uptake and metabolism of high density lipoprotein (HDL) triglycerides, cholesteryl esters, and phospholipids, isolated rat livers were perfused with a reconstituted HDL (rHDL) radiolabeled with [3H]triolein and [14C]cholesteryl oleate or palmitoyl-[14C]linoleoyl phosphatidylcholine. A bolus of radiolabeled rHDL was injected into the portal vein and livers were perfused for 5 min using a nonrecirculating perfusion system. Recovery of rHDL triolein in the liver as intact triolein was used to determine the amount of unmetabolized rHDL remaining in the liver. After correcting for the amount of unmetabolized rHDL remaining in the liver, about 30% of the rHDL triolein was hydrolyzed of which 19% was recovered in the liver and 11% in the perfusate. Moreover, about 7% of the rHDL phosphatidylcholine was hydrolyzed to lysophosphatidylcholine, all of which was recovered in the perfusate. Although there was no hydrolysis of rHDL cholesteryl oleate, about 30% of the cholesteryl oleate was taken up by the liver. Preperfusion of the liver with heparin to deplete the liver of hepatic lipase resulted in about a 70% reduction in rHDL triolein hydrolysis and about a 75% reduction in rHDL cholesteryl oleate uptake. Although hepatic lipase hydrolyzes both triglycerides and phosphatidylcholines, elimination of the triolein from rHDL had no effect on the uptake of rHDL cholesteryl oleate, but replacement of the rHDL phosphatidylcholine with a nonhydrolyzable phosphatidylcholine diether resulted in an 87% reduction in cholesteryl oleate uptake. These results indicate that hepatic lipase is necessary for the hepatic uptake of both HDL triglycerides and cholesteryl esters and that the uptake of cholesteryl esters is not dependent on the hydrolysis of HDL triglycerides but is dependent on the hydrolysis of HDL phospholipids.     

Hepatic processing of the cholesteryl ester from low density lipoprotein in the rat

Human low density lipoprotein (LDL), radiolabeled in the cholesteryl ester moiety, was injected into estrogen-treated and -untreated rats. The hepatic and extrahepatic distribution and biliary secretion of [3H]cholesteryl esters were determined at various times after injection. In order to follow the intrahepatic metabolism of the cholesteryl esters of LDL in vivo, the liver was subfractioned into parenchymal and Kupffer cells by a low temperature cell isolation procedure. In control rats, the LDL cholesteryl esters were mainly taken up by the Kupffer cells. After uptake, the [3H]cholesteryl esters are rapidly hydrolyzed, followed by release of [3H]cholesterol from the cells to other sites in the body. Up to 24 h after injection of LDL, only 9% of the radioactivity appeared in the bile, whereas after 72 h, this value was 30%. Hepatic and especially the parenchymal cell uptake of [3H]cholesteryl esters from LDL was strongly increased upon 17 alpha-ethinylestradiol treatment (3 days, 5 mg/kg). After rapid hydrolysis of the esters, [3H]cholesterol was both secreted into bile (28% of the injected dose in the first 24 h) as well as stored inside the cells as re-esterified cholesterol ester. It is concluded that uptake of human LDL by the liver in untreated rats is not efficiently coupled to biliary secretion of cholesterol (derivatives), which might be due to the anatomical localization of the principal uptake site, the Kupffer cells. In contrast, uptake of LDL cholesterol ester by liver hepatocytes is tightly coupled to bile excretion. The Kupffer cell uptake of LDL might be necessary in order to convert LDL cholesterol (esters) into a less toxic form. This activity can be functional in animals with low receptor activity on hepatocytes, as observed in untreated rats, or after diet-induced down-regulation of hepatocyte LDL receptors in other animals.     

Acyl-CoAs are functionally channeled in liver: potential role of acyl-CoA synthetase

Acyl-CoA synthetase (ACS) catalyzes the activation of long-chain fatty acids to acyl-CoAs, which can be metabolized to form CO(2), triacylglycerol (TAG), phospholipids (PL), and cholesteryl esters (CE). To determine whether inhibiting ACS affects these pathways differently, we incubated rat hepatocytes with [(14)C]oleate and the ACS inhibitor triacsin C. Triacsin inhibited TAG synthesis 70% in hepatocytes from fed rats and 40% in starved rats, but it had little effect on oleate incorporation into CE, PL, or beta-oxidation end products. Triacsin blocked [(3)H]glycerol incorporation into TAG and PL 33 and 25% more than it blocked [(14)C]oleate incorporation, suggesting greater inhibition of de novo TAG synthesis than reacylation. Triacsin did not affect oxidation of prelabeled intracellular lipid. ACS1 protein was abundant in liver microsomes but virtually undetectable in mitochondria. Refeeding increased microsomal ACS1 protein 89% but did not affect specific activity. Triacsin inhibited ACS specific activity in microsomes more from fed than from starved rats. These data suggest that ACS isozymes may be functionally linked to specific metabolic pathways and that ACS1 is not associated with beta-oxidation in liver.     

Inhibitors of sterol synthesis. The effects of dietary 5 alpha-cholest-8(14)-en-3 beta-ol-15-one on the fate of [4-14C]cholesterol and [2,4-3H]5 alpha-cholest-8(14)-en-3 beta-ol-15-one after intragastric administration to rats

The effect of dietary administration (0.1% in a rat chow diet) of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one, a potent inhibitor of cholesterol biosynthesis with marked hypocholesterolemic activity, on the fate of [4-14C]cholesterol and [2,4-3H]5 alpha-cholest-8(14)-en-3 beta-ol-15-one has been studied after intragastric administration of the labeled sterols to rats. In general, the distribution of 3H in major tissues paralleled that of 14C with no unusual concentration of 3H in any of the organs. Only trace amounts of 3H and 14C were recovered in urine. Administration of the 15-ketosterol was associated with decreased absorption of the labeled cholesterol as indicated by decreased levels of 14C in the various tissues and organs of the 15-ketosterol-treated rats (relative to ad libitum and pair-fed control animals) and increased levels of 14C in feces and intestinal contents at 12 and 48 h after the administration of the labeled cholesterol. Studies of the distribution of 3H in liver indicated rapid conversion of the 15-ketosterol to cholesterol and cholesteryl esters. The amounts of 3H recovered in the various tissues and organs at both 12 and 48 h after the administration of the labeled sterols were considerably less than the corresponding values for 14C, a finding which suggests a lower absorption of the 15-ketosterol (relative to cholesterol) and/or a more rapid clearance and biliary excretion of the 15-ketosterol and its metabolites.     

Cholesterol feeding alters the metabolism of thoracic-duct lymph lipoprotein cholesterol in rabbits but not in rats

1. Labelled thoracic-duct lymph was collected from rats and rabbits after test meals containing [(14)C]cholesterol and [2-(3)H]glyceryl trioleate. 2. The metabolism of labelled cholesterol and triglyceride was studied in normally fed and cholesterol-fed rats and rabbits injected with radioactive lymph from the same species. 3. In normally fed animals of both species, 10min after intravenous administration, about 80% of lymph cholesteryl ester but only about 10% of triglyceride was recovered in the liver after clearance from the plasma. This distribution is consistent with participation of ;remnant' particles in the metabolism of dietary lymph particles. 4. The metabolism of cleared lymph lipoprotein constituents was unchanged in cholesterol-fed rats, but the recovery of cholesteryl ester in the livers of the cholesterol-fed rabbits was decreased to 30% of the cleared dose. 5. The low recovery in cholesterol-fed rabbits was accounted for mainly by increased hydrolysis of cholesteryl ester. 6. It is proposed that differences between rats and rabbits in metabolism of dietary cholesterol might be partly due to the observed enhancement of hydrolysis of lymph lipoprotein cholesteryl ester in rabbits.