Evidence that cholesteryl ester hydrolase and triglyceride lipase are different enzymes in rat liver

Studies on intracellular cholesteryl ester hydrolase (CEH) and triglyceride lipase (TGL) from rat adipose tissue and adrenal cortex have suggested that a single protein is responsible for both activities. To determine whether one hepatic protein catalyzes both reactions, we studied several properties of CEH and TGL in rat liver. During liver perfusion with heparin, perfusate peaks of TGL and CEH did not consistently coincide, and TGL activity was considerably higher and less heat-stable than that of CEH. Significant TGL, but not CEH, activity was released during incubation of isolated hepatocytes. Although microsomes isolated from hepatocytes contained both activities, the specific activities of CEH and TGL in cytosol from hepatocytes were 95% and 3%, respectively, of those found in cytosol from whole liver. Preincubation of liver cytosol with 5 mM Mg²⁺ decreased CEH, but not TGL, activity. Intracellular CEH and TGL activities were completely separated by prep-disc gel electrophoresis. Finally, both cytosolic and microsomal TGL, but not CEH, activities were inhibited by antiserum against rat hepatic TGL. We conclude that extracellular TGL does not have CEH activity and intracellular CEH differs from TGL.     

Hepatic zonation of the formation and hydrolysis of cholestery esters in periportal and perivenous parenchymal cells

The periportal (PP) and perivenous (PV) zones of the liver acinus differ in enzyme complements and capacities for cholesterol and bile acid synthesis and other metabolic processes. The aim of this investigation was to determine the acinar distribution of the catalytic activity of the enzymes governing the formation and hydrolysis of cholesteryl esters using PP and PV hepatocytes from normal or cholestyramine-fed rats. The hepatocyte subpopulations were isolated by centrifugal elutriation, characterized according to the distribution pattern of a number of cell parameters and marker enzymes, and assayed for acyl-CoA: cholesterol acyltransferase (ACAT) and lysosomal, cytosolic and microsomal cholesteryl ester hydrolase (CEH). In normally fed rats, no zonation was found in the activity of lysosomal CEH and ACAT, and the activity of both cytosolic and microsomal CEH zonated toward the PV zone of the acinus. Concentrations of free and esterified cholesterol in homogenates, cytosol, and microsomes of PP and PV cells were, however, similar. Cholestyramine raised significantly the PV/PP ratio of ACAT because of an exclusive PP reduction of activity and abolished the heterogeneity in microsomal CEH because of a greater inhibitory PV response, whereas the PV dominance of cytosolic CEH and the homogeneous distribution of lysosomal CEH were unaffected. These results demonstrated homogeneity within the liver acinus for the enzymatic degradation of endocyted lipoprotein-derived cholesteryl esters, a structural zonation of the cytosolic CEH and a dynamic zonation of ACAT and the microsomal CEH, with a PV dominance of the enzymatic capacity for the degradation of stored cholesteryl esters in normal livers.     

The esterification of cholesterol in the yolk sac membrane of the chick embryo

The uptake of lipid from the yolk by the yolk sac membrane of the chick embryo is accompanied by the rapid esterification of a large proportion of the yolk cholesterol. This could arise from enhanced acyl-CoA:cholesterol acyltransferase (ACAT) activity and/or inhibition of cholesteryl ester hydrolase (CEH) activity. The activity of ACAT was therefore measured in microsomes obtained from yolk sac membranes at various stages of development. A high level of activity (up to 929 pmol of cholesteryl oleate formed per min per mg protein) was found during the second half of this period. Supplementation with exogenous cholesterol stimulated ACAT activity in microsomes obtained from the tissue at the earlier, but not at the later, stages of development suggesting that the enzyme became saturated with microsomal cholesterol as development proceeded. Correlating with this, the concentration of cholesterol in the microsomes increased 4-fold between 9 and 20 d of development. The activity of CEH was very low in the microsomes and could not be detected in the cytosolic fraction. The activity of a protein, which has been shown to function as an inhibitor of CEH, was found to be present at all stages of development. The high activity of ACAT, together with the low activity of CEH and an active CEH inhibitor protein is a combination well suited to promote an essentially unidirectional conversion of cholesterol to cholesteryl ester. This process may be a major determinant of the rate of lipid transfer from the yolk to the embryo.     

The influence of vitamin E and selenium on lipid peroxidation and aldehyde dehydrogenase activity in rat liver and tissue

Malondialdehyde (MDA) production and cytosolic aldehyde dehydrogenase (ALDH) response were examined in rat liver tissues after feeding different levels of dietary vitamin E and/or selenium and polyunsaturated fat for 12–38 wk. MDA production was significantly increased by vitamin E deficiency or by high levels of polyunsaturated fat intake, but not by selenium deficiency. The activity of cytosolic ALDH increased upon increased production of MDA after 12–16 wk of feeding the lipid peroxidation-inducing diets. However, ALDH activity was suppressed after 38 wk of feeding the vitamin E-deficient diet. The results indicate that the hepatic cytosolic ALDH may be involved in the metabolism of MDA during a relatively short-term increase inin vivo lipid peroxidation, but that ALDH activity becomes suppressed after more severein vivo lipid peroxidation has been produced. Hepatic and plasma α-tocopherol levels and lipid peroxidation products were measured for the various dietary groups.     

The effects of simvastatin and cholestyramine,alone and in combination,on hepatic cholesterol metabolism in the male rat

The influence of dietary simvastatin, cholestyramine, and the combination of simvastatin plus cholestyramine on hepatic cholesterol metabolism has been investigated in male rats. Recovery from the effects of the drugs was also investigated by refeeding normal chow for 24 h. Both drugs, alone and in combination, increased 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase activityin vitro, but activity returned toward control values, after drug withdrawal. Acyl-CoA:cholesterol acyltransferase (ACAT) was significantly reduced (P<0.001) by simvastatin (−75%), cholestyramine (−71%), and by the drug combination (−81%), due both to a decrease in microsomal cholesterol and to nonsubstrate-dependent modulation of enzyme activity. Refeeding control diet increased ACAT activity but not to control levels. The enhanced activity arose partly from higher microsomal cholesterol and partly from increases in total enzyme activity. Cytosolic neutral cholesteryl ester hydrolase (CEH) activity was substantially elevated by simvastatin (3-fold) and by the drug combination (6-fold), whereas the effect of cholestyramine was smaller (1.5-fold). Normal chow for 24 h only partially returned cytosolic CEH activity to control values. Microsomal CEH activity was increased by simvastatin, alone and in combination with cholestyramine (1.4 to 1.7-fold), and was also enhanced, in the cholestyramine-treated animals, following drug withdrawal. Removal of simvastatin did not allow recovery of this enzyme activity, while withdrawal of the drug combination led to values 29% below controls. The results indicate that in the rat, simvastatin and cholestyramine alter both ACAT and CEH activity, as well as inhibiting HMG-CoA reductase activity.     

Characterization of a cytosolic protein in rat liver inhibiting neutral cholesteryl ester hydrolase

Neutral cholesteryl ester hydrolase activity (EC 3.1.1.13) present in microsomes isolated from lactating rat mammary glands was found to be inhibited by a factor (or factors) occurring in the cytosolic fraction of male rat liver. The inhibitor was heat-labile, non-dialyzable, destroyed by proteolysis, and was stable following preparation of an acetone/diethyl ether powder of the cytosolic fraction. The protein also inhibited the activity of hormone-sensitive lipase (HSL) (from bovine adipose tissue) and esterase fromCandida cylindracea, but seemed to be more active against the neutral hydrolase found in rat liver microsomes. For the mammary gland microsomal cholesteryl ester hydrolase, the extent of the inhibitory effect was dependent on the concentration of the cytosolic protein, 50% inhibition being achieved by about 100 μg of cytosolic protein, and on the method of initiating the enzyme assay. Kinetic analysis indicated that, under circumstances where the reaction was initiated by the addition of substrate, the inhibition was characterized as “uncompetitive”. When an inhibitor/substrate complex was allowed to form in the absence of enzyme, an element of “competitive” inhibition was introduced into the reaction. Food withdrawal reducted the activity of the inhibitor in live by 56%, but activity was fully restored by short-term re-feeding. In contrast, feeding a diet high in fat led to a 34% increase in activity. The present findings suggest that the inhibitory factor(s) may be involved in the regulation of the hydrolysis of cholesteryl esters in the liver and also in other cell types.     

Temperature lability and cAMP-dependent protein kinase activation of cholesteryl ester hydrolase as a function of age in developing rat testis

Cholesteryl ester hydrolase (CEH) was measured at 32°C and 37°C, and with and without cofactors for stimulation of cyclic AMP-dependent protein kinase, in 104,000×g supernatants from rats aged 14–365 days. Activity at the two temperatures was also partially resolved by cation exchange FPLC. Total specific activity of CEH was relatively constant, with or without addition of cofactors, from 14 to 47 days, during which time temperature labile CEH was a very small fraction of total CEH activity. At later times, 51–150 days, activity was increased as much as two-fold, both with and without cofactors, with most of the increase occurring in the temperature labile fraction. Activation of temperature stable and temperature labile activities, where present, by protein kinase cofactors could be demonstrated in all age groups, but was highly variable as a function of age and protein concentration used in the assay. Apparent induction of temperature labile activity over the interval 47–51 days coincides with reported increases in testosterone synthesis and first appearance of spermatozoa in the testis. This and other lines of evidence suggest unique roles for these enzymes in regulation of availability of free cholesterol for testosterone and membrane synthesis, respectively.     

Characterization of a cytosolic protein inhibiting lysosomal acid cholesteryl ester hydrolase

An inhibitor of lysosomal acid cholesteryl ester hydrolase (Acid CEH), (EC 3.1.1.13) was found in the cytosolic fraction of rat liver and various other tissues. The extent of the inhibitory effect was dependent on the concentration of the cytosolic protein. The Acid CEH inhibitor was heat-labile, nondialyzable, and its inhibitory activity significantly decreased by trypsin or chymotrypsin digestion, but not by lipase digestion. The inhibitor had no effect on the activity of cathepsin D, β-glucuronidase and acid phosphatase, which are other enzymes found in lysosomes. The present findings suggest that the inhibitor may be involved in the regulation of the hydrolysis of cholesteryl esters in lipoproteins that have been transferred into the liver.     

Effects of aurothioglucose on iron-induced rat liver microsomal lipid peroxidation

Aurothioglucose (ATG), an inhibitor of selenium-dependent glutathione peroxidase activity, at a concentration of 100 μM, strongly increases lipid peroxidation of rat liver microsomes exposed to either ferrous ion (10 μM) or the combination of ferric ion (10 μM) and ascorbic acid (500 μM), in the presence of reduced glutathione (GSH, 800 μM). This effect was not achieved using heat-inactivated microsomes and was dependent on the presence of GSH. ATG did not affect the lag period associated with ascorbic acid/ferric ion-induced microsomal lipid peroxidation (previously attributed to an undefined GSH-dependent microsomal agent), but did increase the rate of peroxidation subsequent to the lag period. The potent GSH-dependent inhibition of microsomal lipid peroxidation by cytosol (10% of total volume) was completely reversed by ATG (100 μM). ATG similarly reversed an inhibition of phosphatidyl-choline hydroperoxide-dependent liposomal peroxidation that has been attributed to phospholipid hydroperoxide glutathione peroxidase (PHGPX), an enzyme distinct from the classical glutathione that cannot utilize intact phospholipids. ATG inhibited, in addition to the classifical selenium-dependent glutathione peroxidase, both cytosolic and microsomal (basal and N-ethyl maleimide-stimulated) glutathione S-transferase activities with greater than 80% inhibition achieved at 100 μM ATG. ATG, at concentrations up to 250 μM, did not inhibit PHGPX activity measured by the coupled-enzyme method in the presence of Triton X-100 (0.1%). These data demonstrate the potential of ATG to increase toxicity of lipid peroxidative stimuli by inhibition of microsomal and cytosolic defense mechanisms. Although ATG did not inhibit Triton-enhanced PHGPX activity, overall evidence points toward inhibition of this enzyme as the mechanism for ATG-augmented lipid peroxidation and supports the conclusion that PHGPX plays a major role in the cellular defense mechanism.     

Reversible ATP-dependent inactivation of adipose diacylglycerol acyltransferase

Diacylglycerol acyltransferase from rat adipose tissue is shown to be inactivated by 30 to 40% upon incubation with adenosine 5′-triphosphate (ATP) and Mg²⁺. The activity responsible for this inactivation is associated with the cytosolic fraction, specific for ATP, prevented when ATP is substituted by β,γ-methylene-ATP, and partially blocked by 1 mM ethylenediaminetetraacetate or 40 mM NaF, but not by inhibitors of adenosine 3′,5′-cyclic-monophosphate (cAMP)-dependent protein kinase and/or protein kinase C (PKC). The cytosolic activity cannot be mimicked by (cAMP)-protein kinase nor by PKC. Inactivated diacylglycerol acyltransferase from ATP/cytosol-treated microsomes can be reactivated by incubation with partially purified protein phosphate from rat liver, and can be inactivated again by further addition of ATP in the presence of cytosol. The results suggest the existence in adipose tissue of a protein kinase other than cAMP-protein kinase or PKC, which may be involved in the regulation of triacylglycerol synthesis.     

Rapid three-step purification of a hepatic neutral cholesteryl ester hydrolase which is not the pancreatic enzyme

A rat liver cytosolic cholesteryl ester hydrolase (CEH) was purified 12,600-fold by ammonium sulfate precipitation, cation exchange chromatography and gel permeation high-performance liquid chromatography, with an overall yield of 20%. Its properties are compared to those of pancreatic CEH, with which it has sometimes been identified. Liver CEH exhibited a single silver stained band following SDS-polyacrylamide gel electrophoresis (M_r=66 kDa), was activated by 0.5–10 mM taurocholate but was strongly inhibited by higher levels of taurocholate, which activate pancreatic CEH. Whereas bile salts are known to induce formation of a hexamer of pancreatic CEH, in the current study, 0.5 mM taurocholate dissociated a multimeric form of liver CEH to monomer. Liver CEH did not coelute with pancreatic CEH from cation exchange and chromatofocusing columns, exhibited no immunoreactivity with anti-rat pancreatic CEH IgG in Western blots, was not inhibited by anti-rat pancreatic CEH IgG and had a different amino acid composition from pancreatic CEH. In contrast to liver CEH, which is known to be activated by protein kinases A and C, pancreatic CEH was unaffected by cofactors for protein kinase A and was inhibited by cofactors for protein kinase C.     

Calcium deficiency modifies polyunsaturated fatty acid metabolism in growing rats

Fatty acid desaturase activities were determined in liver microsomes from calcium-deficient rats and compared to calcium-sufficient ones. The calcium-deprived diet (0.5 g/kg) administered for 60 d caused a 30% inhibition in the Δ5 desaturase activity and a 45–55% decrease in Δ6 and Δ9, respectively, facts that cannot be attributed to a reduction in food intake. In vitro addition of calcium, ethyleneglycol-bis(β-aminoethyl ether)N,N-tetraacetic acid, and/or cytosol fractions from control or calcium-deficient rats to microsomes from both groups of animals indicates that the reduced desaturase capacities would be the consequence of an indirect effect of calcium deprivation. The present work shows that the reduced unsaturated fatty acid biosynthesis might be the result of modifications in the physicochemical properties of microsomal membranes. Such changes could also be derived from the inhibition of phospholipase A₂ activity induced by calcium deficiency.     

The activity and properties of a hepatic acid cholesteryl ester hydrolase obtained from rats of different age groups

The activity of lysosomal acid cholesteryl ester hydrolase (acid CEH, EC 3.1.1.13) in rat liver was determined at 3, 5, 7, 10 and 20 wk following birth. The levels of acid CEH activity showed a marked decrease as rats grew older, whereas those of other lysosomal marker enzymes, such as acid phosphatase, β-glucuronidase and cathepsin B and D, showed only a slight decrease. On the other hand, acid CEH activity was detected in all subcellular fractions obtained from rat liver, but the enzyme activity in these fractions did not show the age-related decrease observed in the lysosomal fraction. The results presented here suggest that the marked alteration of lysosomal acid CEH activity that accompanies aging may be related to its possible involvement in the regulation of cholesterol concentration in rat liver.     

Effect of environmental temperature changes on rat liver fatty acid desaturases

Female rats warm-adapted at 30–32 C for 20–25 days and then shifted to 13–15 C for 12, 24, 48, 72 and 120 hr showed that Δ9 desaturase and fatty acid synthetase activity decay after 24 hr of cold exposure, while Δ6 and Δ5 desaturases were increased after this period of time. These results were confirmed by an increase of arachidonic acid of heart and liver microsomes phosphatidylcholine and a decrease of oleic acid. Neither NADH-cyt b₅ reductase nor NADH-cyt c reductase activity of liver microsomes were significantly affected. Male rats warm-adapted under the same conditions and then shifted to 13–15 C for 120 hr did not show significant changes in fatty acid synthetase, Δ9 and Δ6 desaturases and enzymes of the microsomal electron transport chain. Therefore, the desaturase response to environmental temperature changes could be plausibly linked to female hormones.     

Effects of dietary proteins on linoleic acid desaturation and membrane fluidity in rat liver microsomes

The effect of dietary protein, casein (CAS) and soybean protein (SOY), on linoleic acid desaturation in liver microsomes was studied in rats. The activity of Δ6 desaturase in total and rough endoplasmic reticula (ER and RER) was significantly higher in the CAS group than in the SOY group. In ER and smooth endoplasmic reticulum, the steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene, when incorporated into the membrane, was decreased in the SOY group and accompanied by a reduction in the cholesterol/phospholipid (CHOL/PL) ratio, consistent with an increase in membrane fluidity. In a separate study, the effect of varying dietary proteins, CAS, milk whey protein, egg albumin, SOY, potato protein and wheat gluten, on the relationship between the Δ6 desaturase activity and microsomal membrane fluidity was also examined. The results indicated that the dietary protein-dependent change in the liver microsomal CHOL/PL ratio affected membrane fluidity, and subsequently the activity of Δ6 desaturase in liver microsomes. However, since dietary protein influenced the Δ6 desaturase activity in RER without influencing membrane fluidity, it is possible that some regulation might have taken place at the level of enzyme synthesis.     

Studies on liver phosphatidyl cholines: II. Effect of sex,age and species differences on phosphatidyl cholines from liver mitochondria and microsomes

Liver mitochondrial and microsomal phosphatidyl cholines differing in the degree of unsaturation of their fatty acids have been separated into four fractions by silver ion silica gel TLC. The levels of the four phosphatidyl choline fractions were determined for male and female rats and mice, fetal and young rabbits, and female hamsters and guinea pigs. The sum of phosphatidyl choline fractions 1, 2, and 3 of mitochondria and microsomes was greater in the female rat than in the male rat with the difference being a reflection of a higher level of fraction 3 which contains arachidonic acid. The female rat has greater concentration of phosphatidyl choline fractions 1 and 3 of mitochondria. Similar results were seen in mouse liver microsomes but not in mitochondria. The levels of the individual four fractions varied from species to species. No change occurred in the levels of the phosphatidyl choline fractions of fetal (−9 and −3 days) rabbits, but an increase was seen in the level of fraction 4 between day 3 and day 35 in both the mitochondria and microsomal fractions of liver. The concentration of mitochondrial and microsomal protein, total phospholipid and total lecithin phosphorus were determined in rat, mouse, hamster and guinea pig. The total phospholipid phosphorus/protein (μg/mg) of microsomes was greater in all species than that observed in mitochondria. Liver microsomes contain 45–50% of total phospholipid phosphorus as lecithin whereas mitochondria contains 32–37%. The fatty acid patterns of mitochondria and microsomal phosphatidyl cholines were determined and the ratio of palmitate to stearate was greater than two for mice and hamsters and approximately 0.5 for rat and guinea pigs.     

Hydroxylation of fatty acids and alcohols by hepatic microsomal cytochrome P-450 system from the Mongolian gerbil

The liver microsomes of the Mongolian gerbilMeriones unguiculatus catalyzed the hydroxylation of various saturated fatty acids (C₈−C₁₈), alcohols (C₁₂ and C₁₆) and hydrocarbon (C₁₂) to the corresponding ω- and (ω-1)-hydroxy derivatives. Lauric acid was hydroxylated most effectively among saturated fatty acids and the order of activity as hydroxylation substrates was C₁₂>C₁₄>C₁₃>C₁₆>C₁₀>C₁₈>C₈. The specific activity of laurate hydroxylation (5.99 nmol/mg microsomal protein/min) in gerbil liver microsomes was higher than that observed in other species. 1-Dodecanol was also hydroxylated very effectively (4.58 nmol/mg microsomal protein/min) by gerbil liver microsomes, but in general the hydroxylation rates for fatty alcohols were much lower than those for the corresponding acids. It was found from both inhibitor and cofactor studies that the enzyme catalyzing the hydroxylation of fatty acids and alcohols in the liver microsomes of the Mongolian gerbil was a typical cytochrome P-450-linked monooxygenase, and at least two different cytochrome P-450 species were involved in the hydroxylation. Presented in part at the AOCS annual meeting (a joint meeting with the Japan Oil Chemists' Society), Honolulu, Hawaii, May 1986.     

The existence of a soluble plasmalogenase in guinea pig tissues

The distribution of plasmalogenase for the hydrolysis of 1-alkenyl-2-acyl-sn-glycero-3-phosphoethanolamine (plasmenylethanolamine) in the subcellular fractions of guinea pig tissues was examined. Plasmalogenase activity was found in high abundance in the cytosolic fractions of the brain and the heart. Assessment of microsomal marker enzyme activities in the cytosolic fraction revealed that plasmalogenase activity in the cytosol was not due to microsomal contaminations. The characteristics of the cytosolic plasmalogenase were very similar to the microsomal enzyme with respect to the pH profile of the reaction, the presence of divalent cations and K_m values for plasmenylethanolamine. However, the cytosolic enzyme was slightly less stable at 55°C than the microsomal enzyme. Cytosolic enzyme activity was eluted as a broad peak in Sepharose 6B chromatography with an average molecular weight of 250,000. Our results demonstrate that most of brain plasmalogenase activity is soluble which makes the brain cytosol an excellent source to initiate the purification of this enzyme.     

The effect of embryological development on phosphatidylethanolamine methyltransferase,phosphatidyldimethylethanolamine methyltransferase and choline phosphotransferase of rabbit liver microsomes

The effect of embryological development on the two biosynthetic enzymes involved in phosphatidylcholine biosynthesis in liver microsomes of −12, −9, 0, +4, +14, +36 day old rabbits has been determined. The specific activity (pmol phosphatidylcholine formed/min/mg microsomal protein) of the phosphatidyletanolamine methyltransferase in the liver microsomes is very low before birth and a 33% increase at birth occurs when compared to the-12 day old fetal livers. The pmol of phosphatidylcholine formed/min/mg protein by the choline phosphotransferase pathway in fetal liver microsomes is 5, 10, 73, 199, 107 and 307 times greater than by the phosphatidylethanolamine methyltransferase pathway for −12, −9, 0, +4, +14, +36 day old rabbits, respectively. The specific activities of the choline phosphotransferase in the liver microsomes increased from the −12 day old fetal livers to 1.6, 19, 73, 39, 27 times for the −9, 0, +4, +14 and +36 day old animals, respectively. The choline phosphotransferase pathway in comparison to the phosphatidylethanolamine methyltransferase pathway is providing the major phosphatidylcholines in the membranes of the endoplasmic reticulum before birth and early fetal development of the rabbit.     

Relationships between cholesterogenesis,microsomal sterols and HMG-CoA reductase in the perfused rat liver

The relationships between cholesterogenesis and the activity of HMG-CoA reductase of microsomes prepared with or without sodium fluoride, and between changes of cholesterogenesis and microsomal sterols were studied in the isolated rat liver perfused with or without oleic acid in the presence of AY-9944. AY-9944 inhibits the conversion of 7-dehydrocholesterol, measured colorimetrically as “fast-acting” sterols, to cholesterol, measured colorimetrically as “slow-acting” sterols. The level of “fast-acting” sterols is used to estimate cholesterogenesis and changes in microsomal sterols. It was observed that the activity of HMG-CoA reductase of microsomes prepared with or without fluoride reflects the relative changes in cholesterogenesis of the perfused livers. In addition, the amount of “fast-acting” and “slow-acting” sterols in microsomes correlates with increases in the activity of HMG-CoA reductase and cholesterogenesis.