Methyl butyrate-hydrolyzing activity of hepatic triglyceride lipase from rat post-heparin plasma

Hepatic triglyceride lipase was obtained from post-heparin plasma of rats in an electrophoretically homogeneous form. The enzyme had an isoelectric point at pH 4.9 and molecular weight of 65,000. The relation between the "lipase" and "esterase" activities of the enzyme was studied using emulsified triolein and water-soluble methyl butyrate (80 mM) as substrates. The same enzyme protein catalyzed the hydrolyses of both emulsified triolein and water-soluble methyl butyrate. The relation of activity to the methyl butyrate concentration differed from those for pancreatic lipase and liver esterase. During purification, the ratio of methyl butyrate to triolein-hydrolyzing activity of the enzyme increased. On digestion of the enzyme with trypsin, the "lipase" activity was retained. However, the trypsin-treated enzyme was adsorbed to a heparin-Sepharose column and eluted with 0.75 M NaCl, like the untreated enzyme. These results suggest that rat hepatic triglyceride lipase contains a so-called "hydrophobic recognition site" that is destroyed by trypsin treatment and is distinct from the heparin-binding and catalytic sites.

Lipoprotein lipase- and phospholipase A2-catalyzed hydrolysis of phospholipid vesicles with an encapsulated fluorescent dye effects of apolipoproteins

The self-quenching dye, 6-carboxyfluorescein, has been encapsulated into sonicated vesicles of egg phosphatidylcholine. Porcine pancreatic phospholipase A2 and bovine milk lipoprotein lipase catalyze the hydrolysis of the phosphatidylcholine resulting in the release of the encapsulated dye and a large increase in 6-carboxyfluorescein fluorescence. The fluorescence increase occurs in parallel with the formation of lysophosphatidylcholine and is strongly dependent on Ca2+ for phospholipase A2 catalysis and on apolipoprotein C-II for hydrolysis by lipoprotein lipase. Other apolipoproteins, including apolipoproteins C-III, C-I, and A-I, do not enhance lipoprotein lipase activity towards this substrate. We conclude that the enhancement of lipoprotein lipase activity by apolipoprotein C-II is a specific property of the activator protein due to its interaction with lipoprotein lipase or an enzyme/lipid interface and not a characteristic of lipid-binding proteins in general.

Post-heparin plasma hepatic triacylglycerol lipase-catalyzed hydrolysis of tributyrin. Effect of lipid interface

The mechanism of action of hepatic triacylglycerol lipase (EC 3.1.1.3) was examined by comparing the hydrolysis of a water-soluble substrate, tributyrin, with that of triolein by hepatic triacylglycerol lipase purified from human post-heparin plasma. The hydrolyzing activities toward tributyrin and triolein were coeluted from heparin-Sepharose at an NaCl concentration of 0.7 M. The maximal velocity of hepatic triacylglycerol lipase (Vmax) for tributyrin was 17.9 mumol/mg protein per h and the Michaelis constant (Km) value was 0.12 mM, whereas the Vmax for triolein was 76 mumol/mg per h and the Km value was 2.5 mM. The hydrolyses of tributyrin and triolein by hepatic triacylglycerol lipase were inhibited to similar extends by procainamide, NaF, Zn2+, Cu2+, Mn2+, SDS and sodium deoxycholate. Triolein hydrolysis was inhibited by the addition of tributyrin. Triolein hydrolysis was also inhibited by the addition of dipalmitoylphosphaidylcholine vesicles. In contrast, the additions of triolein emulsified with Triton X-100 and dipalmitoylphosphatidylcholine vesicles enhanced the rate of tributyrin hydrolysis by hepatic triacylglycerol lipase. In the presence of dipalmitoylphosphatidylcholine, the Vmax and Km values of hepatic triacylglycerol lipase for tributyrin were 41 mumol/mg protein per h and 0.12 mM, respectively, indicating that the enhancement of hepatic triacylglycerol lipase activity for tributyrin by dipalmitoylphosphatidycholine vesicles was mainly due to increase in the Vmax. The enhancement of hepatic triacylglycerol lipase activity for tributyrin by phospholipid was not correlated with the amount of tributyrin associated with the phospholipid vesicles. On Bio-Gel A5m column chromatography, glycerol tri[1-14C]butyrate was not coeluted with triolein emulsion, and hepatic triacylglycerol lipase activity was associated with triolein emulsion even in the presence of 2 mM tributyrin. These results suggest that hepatic triacylglycerol lipase has a catalytic site for esterase activity and a separate site for lipid interface recognition, and that on binding to a lipid interface the conformation of the enzyme changes, resulting in enhancement of the esterase activity.

Post-heparin plasma hepatic triacylglycerol lipase-catalyzed tributyrin hydrolysis. Effect of trypsin treatment

Hepatic triacylglycerol lipase (EC 3.1.1.3) hydrolyzes water-insoluble fatty acid esters, e.g., trioleoylglycerol (lipase activity) and water-soluble fatty acid esters, e.g., tributyrin (esterase activity). Esterase activity of hepatic triacylglycerol lipase is enhanced by triolein emulsion and phospholipid vesicles [1]. The catalytic mechanism and structure of human hepatic triacylglycerol lipase isolated from human post-heparin plasma and the effect of trypsin treatment on the lipase and esterase activities of the enzyme were examined. Treatment of hepatic triacylglycerol lipase with trypsin resulted in loss of its lipase activity, but had no effect on its esterase activity. Chromatography of hepatic triacylglycerol lipase on Bio-Gel A5m showed that hepatic triacylglycerol lipase binds to dipalmitoylphosphatidylcholine vesicles. However, on chromatography of the trypsin-treated enzyme after incubation with dipalmitoylphosphatidylcholine vesicles, a part of hepatic triacylglycerol lipase that retained esterase activity was eluted separately from the dipalmitoylphosphatidylcholine vesicles. Addition of vesicles of dipalmitoylphosphatidylcholine to the trypsin-treated enzyme did not enhance its esterase activity. These results are consistent with the hypothesis that hepatic triacylglycerol lipase has a catalytic site that hydrolyzes tributyrin and a lipid interface recognition site, and that these sites are different: trypsin modified the lipid interface recognition site of the hepatic triacylglycerol lipase but not the catalytic site.

Hydrolysis of cholesterol ester in artificial lipid mixtures by cholesterol esterase released from particulate fractions of rat arterial wall

Acid and neutral cholesterol esterase activities in rat arterial wall were released from the lysosomal fraction and microsomal fraction respectively into the 105,000 X g supernatant fraction by treatment with Triton X-100, heparin and dextran sulfate. The percentage releases of acid cholesterol esterase by Triton X-100 (0.1%), heparin (50 micrograms/ml) and dextran sulfate (1 mg/ml) were 21%, 18% and 4%, respectively, while those of neutral cholesterol esterase were 66%, 56% and 39%, respectively. The cholesterol esterase released by dextran sulfate, especially that from the microsomal fraction, hydrolyzed cholesterol ester in artificial lipid mixtures with similar lipid compositions to those of the deposits in fatty streaks and fibrous plaques of atheromatous lesions.

Effect of niceritrol on lipid metabolism of aorta in atherosclerotic rats

Atherosclerotic lesions were formed in the aorta of rats given a high cholesterol diet containing propylthiouracil (PTU) and vitamin D2 (atherogenic diet) for 8 weeks. The effects of niceritrol (pentaerythritol tetranicotinate), which lower the plasma lipid level, on lipid metabolism in the arterial wall of the atherosclerotic rats were studied. Niceritrol significantly decreased the plasma cholesterol level of atherosclerotic rats, which was 823 mg/100 ml, or about ten times that of control rats. On treatment with niceritrol, the cholesterol level was reduced most in the very low density lipoprotein (VLDL) fraction (d less than 1.006). Heparin-releasable lipoprotein lipase activity in epididymal adipose tissue, lipoprotein lipase activity in post-heparin plasma, and VLDL-triolein hydrolyzing activity in adipose tissue stromal vessels were all higher in niceritrol-treated atherosclerotic rats. Of the enzymes in the arterial wall concerned with cholesterol ester metabolism, acid cholesterol esterase activity was decreased in atherosclerotic rats, while niceritrol treatment increased this activity. The ratio of acyl-CoA cholesterol acyltransferase activity (ACAT) to neutral cholesterol esterase activity was higher in atherosclerotic rats than in control rats, but was lower in niceritrol-treated rats than in atherosclerotic rats. From these results, it is concluded that niceritrol modifies enzyme activities in such a way as to reduce the cholesterol ester content of the arterial wall and lower plasma VLDL and LDL cholesterol levels.

Hydrolysis of a fluorescent phospholipid substrate by phospholipase A2 and lipoprotein lipase

The fluorescent phospholipid 1-acyl-2-[6-[(7-nitro-2,1,3benzoxadiazol-4 -yl) amino]-caproyl] phosphatidylcholine (C6-NBD-PC) was used as a substrate for porcine pancreatic phospholipase A2 (PA2) and bovine milk lipoprotein lipase (LpL). Hydrolysis of C6-NBD-PC by either enzyme resulted in a greater than 50-fold fluorescence enhancement with no shift in the emission maximum at 540 nm; Ca++ was required for PA2 catalysis. Identification of the products of hydrolysis showed cleavage at the sn-1 and sn-2 positions for LpL and PA2, respectively. For PA2, but not for LpL, there was a marked enhancement of enzyme catalysis at lipid concentrations above the critical micellar concentration of the lipid. Furthermore, apolipoprotein C-II, the activator protein of LpL for long-chain fatty acyl substrates, did not enhance the rate of catalysis of the water-soluble fluorescent phospholipid for either enzyme.

Effect of pantethine on fatty acid oxidation in microvessels of rat brain

Fatty acid oxidation in brain microvessels decreased greatly when persistent hypertension developed in spontaneously hypertensive rats (SHR). Treatment of SHR with pantethine [D-bis-(N-pantothenyl-beta-aminoethyl) disulfide] in vivo for 4 weeks restored their fatty acid oxidation activity to the control level. The mechanism of the activating effect of pantethine on fatty acid oxidation was investigated in brain microvessels. Pantethine and its metabolites (pantetheine and 4'-phosphopantetheine) activated three steps of fatty acid oxidation, i.e., acyl-CoA synthetase, carnitine acyltransferase and intramitochondrial oxidation. The relation between changes in fatty acid oxidation activities and injuries of brain microvessels and the protective effect of pantethine against such injuries is discussed.

Lipoprotein lipase-catalyzed hydrolysis of phosphatidylcholine of guinea pig very low density lipoproteins and discoidal complexes of phospholipid and apolipoprotein: effect of apolipoprotein C-II on the catalytic mechanism

To elucidate the mechanism by which apolipoprotein C-II (apoC-II) enhances the activity of lipoprotein lipase (LpL), discoidal phospholipid complexes were prepared with apoC-III and di[(14)C]palmitoyl phosphatidylcholine (DPPC) and containing various amounts of apoC-II. The rate of DPPC hydrolysis catalyzed by purified bovine milk LpL was determined on the isolated complexes. The rate of hydrolysis was optimal at pH 8.0. Analysis of enzyme kinetic data over a range of phospholipid concentrations revealed that the major effect of apoC-II was to increase the maximal velocity (V(max)) some 50-fold with a limited effect on the Michaelis constant (K(m)). V(max) of the apoC-III complex containing no apoC-II was 9.2 nmol/min per mg LpL vs. 482 nmol/min per mg LpL for the complex containing only apoC-II. The effect of apoC-II on enzyme kinetic parameters for LpL-catalyzed hydrolysis of DPPC complexes was compared to that on the parameters for hydrolysis of DPPC and trioleoylglycerol incorporated into guinea pig very low density lipoproteins (VLDL(p)) which lack the equivalent of human apoC-II. Tri[(3)H]oleoylglycerol-labeled VLDL(p) were obtained by perfusion of guinea pig liver with [(3)H]oleic acid. Di[(14)C]palmitoyl phosphatidylcholine was incorporated into the VLDL(p) by incubation of VLDL(p) with sonicated vesicles of di[(14)C]palmitoyl phosphatidylcholine and purified bovine liver phosphatidylcholine exchange protein. The rates of LpL-catalyzed hydrolysis of trioleoylglycerol and DPPC were determined at pH 7.4 and 8.5 in the presence and absence of apoC-II. In the presence of apoC-II, the V(max) for DPPC hydrolysis in guinea pig VLDL(p) increased at both pH 7.4 and pH 8.5 (2.4- and 3.2-fold, respectively); the value of K(m) did not change at either pH (0.23 mm). On the other hand, the kinetic value of K(m) for triacylglycerol hydrolysis in the presence of apoC-II decreased at both pH 7.4 (3.05 vs. 0.54 mm) and pH 8.5 (2.73 vs. 0.62 mm). These kinetic studies suggest that apoC-II enhances phospholipid hydrolysis by LpL in apoC-III-DPPC discoidal complexes and VLDL(p) mainly by increasing the V(max) of the enzyme for the substrates, whereas the activator protein primarily causes a decrease in the apparent K(m) for triacylglycerol hydrolysis.-Shirai, K., T. J. Fitzharris, M. Shinomiya, H. G. Muntz, J. A. K. Harmony, R. L. Jackson and D. M. Quinn. Lipoprotein lipase-catalyzed hydrolysis of phosphatidylcholine of guinea pig very low density lipoproteins and discoidal complexes of phospholipid and apolipoprotein: effect of apolipoprotein C-II on the catalytic mechanism.

Lipoprotein lipase catalyzed hydrolysis of water-soluble p-nitrophenyl esters. Inhibition by apolipoprotein C-II

Bovine milk lipoprotein lipase (LpL) catalyzes the hydrolysis of the water-soluble esters p-nitrophenyl acetate (PNPA) and p-nitrophenyl butyrate (PNPB). The same protein and same active site are involved in hydrolysis of water-soluble p-nitrophenyl esters and emulsified trioleoylglycerol since (a) trioleoylglycerol hydrolysis and PNPB hydrolysis activities coelute from the heparin-Sepharose affinity column used to purify LpL and (b) LpL-catalyzed hydrolyses of trioleoylglycerol and PNPB are inhibited to equal extents by phenylmethanesulfonyl fluoride. The effect of apolipoprotein C-II (apoC-II) on the LpL-catalyzed hydrolysis of PNPA and PNPB has been determined. ApoC-II inhibits hydrolysis of both esters, with a maximum extent of inhibition of 70-90%. Inhibition of the LpL-catalyzed hydrolysis of PNPB is specific for apoC-II, since apolipoproteins A-I, C-I, and C-III-2 have little effect on this reaction, and is partial noncompetitive in form. KI values for apoC-II inhibition of the LpL-catalyzed hydrolysis of PNPA and PNPB are in the range 0.26-0.83 microM. The effect of apoC-II on the temperature dependences of LpL-catalyzed hydrolysis of both esters and on NaCl inhibition of LpL-catalyzed PNPB hydrolysis is consistent with a change in rate-determining step with LpL and apoC-II interact. These results indicate not only that there is an interaction between apoC-II and LpL in aqueous solution in the absence of a lipid interface but also that this interaction conformationally modulates the active site of the enzyme.

Lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol in a phospholipid interface. A monolayer study

The lipoprotein lipase-catalyzed hydrolysis of triacylglycerol was determined in a lipid monolayer containing egg phosphatidylcholine and tri[14C]oleoylglycerol. In the presence of purified bovine milk lipoprotein lipase and fatty acid-free albumin, the rate of hydrolysis of tri[14C]oleoylglycerol, as determined by the decrease in surface activity, was dependent upon enzyme concentration and was enhanced by the addition of apolipoprotein C-II, the activator protein for the enzyme. Increasing the triacylglycerol content of the phospholipid monolayer from 1 to 6 mol% (relative to phospholipid) enhanced the rate of catalysis in the presence and absence of apolipoprotein C-II. However, at low substrate concentrations (less than 4 mol% tri[14C]oleoylglycerol), the activation factor for apolipoprotein C-II was greater than at high (4-6 mol%) triacylglycerol concentrations. The addition of sphingomyelin to the phosphatidylcholine monolayer decreased lipoprotein lipase activity. Based on these monolayer studies, we conclude that lipoprotein lipase catalyzes the hydrolysis of triacylglycerol at a phospholipid interface and that the rate of catalysis is dependent on the lipid composition of the monolayer.

Effect of apolipoproteins on the hepatic lipase-catalyzed hydrolysis of human plasma high density lipoprotein2-triacylglycerols

The effect of apolipoproteins on the hepatic lipase-catalyzed hydrolysis of high density lipoprotein (HDL) triacylglycerols was studied in an in vitro system consisting of purified human post-heparin hepatic lipase, HDL2 and albumin. The apparent values of the Michaelis constant (Km) and maximal velocity (Vmax) for the hepatic lipase-catalyzed hydrolysis of HDL2-triacylglycerols were 0.18 mM and 86 nmol free fatty acids released/mg hepatic lipase per min, respectively. The addition of purified human plasma apolipoprotein A-I, A-II, E, C-I or C-III2 (containing 2 mol of sialic acid) to HDL2 caused inhibition of hepatic lipase activity. At a 1:1 weight ratio of added apolipoprotein to HDL2-protein, inhibition was 50% for apolipoprotein E and over 75% for the other apolipoproteins tested. Inhibition of enzyme activity occurred with both the unfractionated HDL2 and the HDL which were reisolated by ultracentrifugation. The major alteration in the composition of the reisolated HDL was an increase in the protein to phospholipid ratio. Based on these results, we speculate on the possible role of the apolipoproteins in the metabolism of HDL2 by hepatic lipase.

Effect of phospholipids on lipase activity in rat arterial wall homogenate

Triolein hydrolyzing activity in homogenates of the rat arterial wall was studied. Two optimal pH values were observed around 5.0 (acid lipase) and 7.0 (neutral lipase). Neutral lipase activity was enhanced by serum and was inhibited by 1 M NaCl but was not inhibited by protamine. Heparin decreased the acid lipase activity. A 1-mM solution of Cu2+ ion and Zn+2 ion decreased both lipase activities. The activities of both acid and neutral lipases were increased by the addition of rat serum (2 micrograms protein serum/ml) and were decreased by the addition of more than 5 micrograms protein serum/ml. The activities of neutral and acid lipases were increased by the addition of 0.25 mM phosphatidylcholine and phosphatidylserine. Both lipase activities were decreased by the addition of 0.25 mM phosphatidylethanolamine. The addition of 0.25 mM sphingomyelin increased neutral lipase activity and decreased acid lipase activity. At concentrations higher than 2 mM, these phospholipids decreased the activities of both lipases. These results suggest that at least acid and neutral lipases exist in the arterial wall cells, and their activities might be modified by phospholipids.

Reciprocal effect of apolipoprotein C-II on the lipoprotein lipase-catalyzed hydrolysis of p-nitrophenyl butyrate and trioleoylglycerol

Interaction of purified bovine milk lipoprotein lipase (LpL) with sonicated vesicles of dipalmitoyl phosphatidylcholine in the gel phase is associated with an increase in the rate of the LpL-catalyzed hydrolysis of p-nitrophenyl butyrate. There is a 6-fold increase in Vmax. Apolipoprotein C-II, the activator protein for LpL, inhibits the LpL-catalyzed hydrolysis of p-nitrophenyl butyrate. With 0.5 mol % tri[14C]oleoylglycerol present in the dipalmitoyl phosphatidylcholine vesicles and in the presence of 20 mM Ca2+, the rate of p-nitrophenyl butyrate hydrolysis is decreased reciprocally compared to trioleoylglycerol hydrolysis and is dependent on apolipoprotein C-II. These results suggest that apolipoprotein C-II enhances the activity of LpL by increasing the affinity of the active site of LpL for triacylglycerol.

Effect of dextran sulfate on the interaction between very low density lipoprotein and lipoprotein lipase

The effect of dextran sulfate on the interaction between very low density lipoprotein (VLDL) and purified bovine milk lipoprotein was studied. Dextran sulfate increased VLDL-triacylglycerol hydrolysis by lipoprotein lipase about 2-fold, but did not alter the Km value for triacylglycerol in VLDL. Strong association of dextran sulfate with the VLDL-lipoprotein lipase complex was demonstrated by gel filtration on BioGel A-5m, although dextran sulfate did not bind to VLDL and only very slightly to lipoprotein lipase. These findings suggest that dextran sulfate increases triacylglycerol hydrolysis in VLDL by binding to the VLDL-lipoprotein lipase complex.

Immunological studies on bovine milk lipoprotein lipase. Effects of Fab fragments on enzyme activity

Rabbit antiserum was prepared against purified bovine mild lipoprotein lipase. Immunoelectrophoresis of lipoprotein lipase gave a single precipitin line against the antibody which was coincident with enzyme activity. The gamma-globulin fraction inhibited heparin-releasable lipoprotein lipase activity of bovine arterial intima, heart muscle and adipose tissue. The antibody also inhibited the lipoprotein lipase activity from adipose tissue of human and pig, but not that of rat and dog. Fab fragments were prepared by papain digestion of the gamma-globulin fraction. Fab fragments inhibited the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine vesicles and trioleoylglycerol emulsions to the same extent. The Fab fragments also inhibited the lipolysis of human plasma very low density lipoproteins. The change of the kinetic parameters for the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol by the Fab fragments was accompanied with a 3-fold increase in Km and a 10-fold decrease in Vmax. Preincubation of lipoprotein lipase with apolipoprotein C-II, the activator protein for lipoprotein lipase, did not prevent inhibition of enzyme activity by the Fab fragments. However, preincubation with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol or Triton X-100-emulsified trioleoylglycerol had a protective effect (remaining activity 7.0 or 25.8%, respectively, compared to 1.0 or 0.4% with no preincubation). The addition of both apolipoprotein C-II and substrate prior to the incubation with the Fab fragments was associated with an increased protective effect against inhibition of enzyme activity; remaining activity with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol was 40.6% and with Triton X-100-emulsified trioleoylglycerol, 45.4%. Human plasma very low density lipoproteins also protected against the inhibition of enzyme activity by the Fab fragments. These immunological studies suggest that the interaction of lipoprotein lipase with apolipoprotein C-II in the presence of lipids is associated with a conformational change in the structure of the enzyme such that the Fab fragments are less inhibitory. The consequence of a conformational change in lipoprotein lipase may be to facilitate the formation of an enzyme-triacylglycerol complex so as to enhance the rate of the lipoprotein lipase-catalyzed turnover of substrate to products.

Studies on CDP-choline:1,2-diacylglycerol cholinephosphotransferase activity in rat arterial wall

The properties of CDP-choline:1,2-diacylglycerol cholinephosphotransferase (CPT) (EC 2.7.8.2.), which catalyzes de novo synthesis of phosphatidylcholine, were studied in rat arterial wall. The optimal pH of CPT of the arterial wall was about 8.5. On subcellular fractionation of the arterial wall, the highest activity was found in the microsome-rich fraction; the cytosolic fraction showed only a trace of activity. The Michaelis constant (KM) for CDP-choline was 0.019 mM. The CPT activity of a homogenate of arterial wall increased linearly with increase in concentration of diolein up to 3.2 mM. 20 mM magnesium and 0.2 mM manganese ions caused marked activation respectively and essential for the activity. Calcium, barium, cobalt, copper, and ferrous ions were inhibitory. 0.5 mM ethylenediaminetetraacetic acid (EDTA) and 0.5 mM glycoletherdiamine-N,N,N'N'-tetraacetic acid (GEDTA) increased the activity in the presence of 10 mM magnesium ion. Sonication of the enzyme solution and addition of high concentration of detergent, such as Triton X-100 and Tween 20, markedly decreased the activity. Porcine liver phosphatidylcholine, phosphatidylethanolamine, and especially polyenephosphatidylcholine increased CPT activity of the arterial wall, while lysophosphatidylcholine was strongly inhibitory. The properties of arterial CPT activity under various conditions are discussed.

Lipid metabolism in arteriosclerotic arterial wall of rats

Arteriosclerotic lesions were formed in rat aorta by the administration of vitamin D2, a high-fat diet and a thyroid suppressing agent. This treatment increased the serum total cholesterol level to 12 times the control level. In the arteriosclerotic lesions that were induced the activities of lysosomal enzymes, such as acid phosphatase and acid lipase, were higher than in controls, that of acid cholesterol esterase was decreased, those of microsomal lipid-synthesizing enzymes--such as acyl-CoA synthetase and cholesterol ester synthesizing activity--were increased and that of neutral cholesterol esterase was decreased. These data suggest that lipid metabolism in arteriosclerotic lesions was changed, resulting in the accumulation of cholesterol esters in the aorta. Administration of high-fat diet and thyroid suppressing agent also increased the serum cholesterol levels to 12-fold the control level, but did not induce arteriosclerotic lesions. After this treatment the activities of hydrolyzing enzymes, such as acid and neutral cholesterol esterase and lipase, in the aorta increased, but the activities of lipid synthesizing enzymes also increased. These data suggest that lipid metabolism in the aorta in this condition changed to compensate for the large influx of serum lipids and to prevent arteriosclerosis. The roles of the serum lipid level, cell injury and lipid metabolism in the aorta in forming arteriosclerotic lesions are discussed on the basis of these results.

Effect of tocopherol deficiency on lipid metabolism in arterial wall of spontaneously hypertensive rats on normal and high cholesterol diets

Lipid metabolism in the arterial wall of spontaneously hypertensive rats (SHR) fed on tocopherol-deficient diet, high-cholesterol diet or both was studied. Serum tocopherol was greatly decreased in tocopherol-deficient SHR. Lipoperoxide, determined as thiobarbituric acid-reactive substances, was higher in tocopherol-deficient SHR than in normal SHR. Tocopherol-deficient SHR showed a decrease in acid cholesterol esterase activity, but no change in neutral cholesterol esterase, acid and neutral lipase, acyl CoA synthetase, cytidine-diphosphate choline-1-2-diacyl glycerol choline phosphotransferase (CPT) or triglyceride synthesizing activity.

Interaction of lipoprotein lipase with phospholipid vesicles. Role of apolipoprotein C-II and heparin

Lipoprotein lipase is bound to heparin-like molecules at the surface of capillary endothelial cells. For maximal activity, the enzyme requires apolipoprotein C-II, a protein constituent of triacylglycerol-rich lipoproteins. In this report, the interactions of apolipoprotein C-II, heparin and sonicated vesicles of dipalmitoylphosphatidylcholine with purified bovine milk lipoprotein lipase were studied by gel filtration on Bio-Gel A5m. In the presence of vesicles of dipalmitoylphosphatidylcholine (1 mg), lipoprotein lipase (25 micrograms) associated with phospholipids even in the absence of apolipoprotein C-II. With limited phospholipid (40 micrograms), the amount of enzyme which associated with lipid decreased in the presence of apolipoprotein C-II (20 micrograms). Human plasma apolipoprotein C-III, another protein constituent of triacylglycerol-rich lipoproteins, also caused a decrease in the amount of enzyme associated with phospholipid. These results suggest that apolipoprotein C-II does not increase the activity of the enzyme by facilitating its interaction with a lipid interface. In the absence of lipid, lipoprotein lipase and apolipoprotein C-II (molar ratio, 1 : 1) eluted from Bio-Gel A5m as two separate components. The interaction of heparin with lipoprotein lipase was studied using a specific [3H]heparin, which was isolated by affinity chromatography on immobilized lipoprotein lipase; the [3H]heparin eluted with 0.6 M NaCl. Specific [3H]heparin coeluted with lipoprotein lipase when the enzyme was associated with phospholipid; the [3H]heparin was released from the enzyme by 0.75 M NaCl.

Regulation of lipase activities in rat brain in vitro

Regulation of acid and neutral lipase activities in rat brain was examined in vitro. Both activities were decreased by SDS, CuCl2 and ZnCl2 and by delipidation. The neutral lipase activity was also markedly reduced by N-ethylmaleimide and PbCl2. The activity of delipidated preparation was increased by addition of phosphatidyl choline at both pH 5.5 and 7.5 and by phosphatidyl serine at acidic pH value. Pretreatments of the enzyme preparation with phospholipase A and trypsin reduced the lipase activities at both pH values. It is suggested that phospholipids play an important role on lipase activity in brain.

Catabolism of human very low density lipoproteins in vitro: a fluorescent phospholipid method for monitoring lipolysis

The catabolism of human plasma very low density lipoproteins (VLDL) by purified bovine milk lipoprotein lipase has been measured in vitro using a fluorescent phospholipid as a method to monitor lipolysis. Dansyl phosphatidylethanolamine (DPE) was incorporated into VLDL to form DPE-VLDL, and the rate of catabolism was followed by measuring the increase in fluorescence at 490 nm after the addition of the enzyme. The studies were performed with VLDL isolated from 20 normal individuals. In addition, the VLDL from 8 mildly obese subjects with primary hypertriglyceridemia (Type IV phenotype) was studied. With this in vitro system and with a constant amount of lipoprotein lipase, the rate of lipolysis did not differ in normal and in these hypertriglyceridemic subjects. Furthermore, there was no correlation between the rates of hydrolysis and the plasma levels of triglyceride or high density lipoprotein cholesterol.

Studies on cholesterol ester hydrolysis in artificial lipid mixtures

Cholesterol ester is present in lipid deposits in atherosclerotic lesions, such as fatty streaks, fibrous plaques and complicated lesions. The possibility of hydrolysis of cholesterol ester in lipid deposits and its mechanism were examined by studying the effects of the various components of lipid deposits on cholesterol ester hydrolysis. Studies were carried out using artificial lipid samples prepared by sonication of mixtures of the components of lipid deposits. Results suggested that phospholipids, especially phosphatidylcholine, play an important role in the hydrolysis and that alteration of lipid components, other than cholesterol ester, influences cholesterol ester hydrolysis in lipid deposits.

Studies on lipase in rat brain

Lipase activity was measured in homogenates of rat cerebral hemispheres using radioactive glycerol trioleate emulsified with Triton X-100 as substrate. The labeled oleic acid was separated from the ester with a methanol-chloroform-heptane mixture. Under these assay conditions, the activity showed pH optima at about 5.5 and 7.5. The final products of these lipase activities were suggested to be free fatty acid and glycerol.

Preparation and properties of immobilized lipoprotein lipase

Purified bovine milk lipoprotein lipase has been covalently attached to CH-Sepharose with water-soluble carbodiimide. The immobilized enzyme retained enzymic activity and was stimulated 7-fold by the addition of human apolipoprotein C-II. Both [3H]heparin and 125I-labeled apolipoprotein C-II bound to the immobilized enzyme; unlabeled heparin and apolipoprotein C-II competed for binding of their respective labeled compounds. Apolipoprotein C-II did not compete for binding of [3H]heparin and vice versa. Human apolipoprotein C-III did not bind to the immobilized enzyme nor did it compete for apolipoprotein C-II binding. We conclude from these studies that both apolipoprotein C-II and heparin interact with immobilized lipoprotein lipase and that they have different binding sites.

Clinical study of niceritrol on serum lipids in the treatment of hyperlipidemia

The effects of niceritrol, a nicotinic acid derivative, on the levels of HDL-cholesterol (HDL-Ch) and a mixture of VLDL- and LDL-Ch (VLDL- + LDL-Ch) were studied in hyperlipidemic patients. Serum total cholesterol (sTC) and serum triglyceride (sTG) were significantly reduced during niceritrol administration. Lipoprotein electrophoresis showed that niceritrol increased the alpha:beta ratio. HDL-Ch showed a significant increase of 12.5% by the 16th week of therapy. This increase was more marked in patients with lower pre-treatment HDL-Ch levels and significant in patients whose pre-treatment sTG levels were in excess of 200 mg/dl. Females displayed higher pre-treatment HDL-Ch levels (38.5 mg/dl) than males (30.6 mg/dl). However, niceritrol increased HDL-Ch significantly in both groups. At 16 weeks, the VLDL- + LDL-Ch level showed a significant decrease of 9.2%; the HDL-Ch:VLDL + LDL-Ch and HDL-CH:sTC ratios were significantly increased throughout niceritrol administration. Niceritrol is thought to be effective in preventing the development and progression of atherosclerosis because it raises the level of anti-atherogenic HDL-Ch and lowers the level of atherogenic VLDL- + LDL-Ch.

Effect of pantethine on cholesterol ester metabolism in rat arterial wall

The total serum cholesterol level in rats fed on a high cholesterol diet (HCD) for 16 weeks was markedly higher than that in rats fed on a normal diet (ND), but pantethine reduced the increased level in rats fed on HCD (P less than 0.05). Acid cholesterol esterase activity (acid CEase) of arterial wall homogenates from rats fed on HCD was significantly lower than that of rats fed on ND (P less than 0.005). Acid CEase activity in the arterial wall of rats fed on HCD for 8 weeks and then ND for 8 weeks was less than that of rats fed on ND for 16 weeks. Acid CEase activity in the arterial wall was increased in rats fed on pantethine-containing diet. The ratio of cholesterol ester synthesizing activity to neutral cholesterol esterase (neutral CEase) activity was higher in rats fed on NCD than in those fed on ND. The ratio was lower in rats on the pantethine-containing diet than in those on NCD. The relationship between hypercholesterolemia and lipid metabolism in the arterial wall and effects of pantethine are discussed on the basis of these results.

Effects of pantetheine on cholesteryl ester synthesis in the arterial wall of rats on high cholesterol diet

Increase of acyl-CoA synthesis was observed when extracts of rat arterial wall were incubated with pantetheine [D-bis-(N-pantothenyl-beta-aminoethyl)-disulfide]. Cholesteryl ester synthesis from palmitate in the arterial wall extract in vitro was higher with arteries from rats on high cholesterol diet than with those from rats on normal diet, but the synthesis was reduced in the arteries of rats on high cholesterol diet with pantetheine. Triglyceride synthesis was higher with arterial wall extracts of rats on high cholesterol diet than with preparations from rats on normal diet and was not reduced with those of rats on high cholesterol diet plus pantetheine. The value of the effects of pantetheine on lipid metabolism in the prevention of atherosclerosis is pointed out.

Studies of cholesterol esterase in rat arterial wall

Cholesterol esterase activity was estimated in homogenates of rat arterial wall using radioactive cholesteryl oleate incorporated into phospholipid vesicles as a substrate. The labeled oleic acid was separated from the ester by addition of benzene-chloroform-methanol mixture. Under these conditions, two pH optima were found at about 4.5 and 7.5. Most of the activities at pH 4.5 and 7.5 were found in the lysosomal and microsomal fraction, respectively. No enzyme activity was detected when the substrate vesicles were prepared with phosphatidylethanolamine or sphingomyelin, but the activity was higher when the substrate vesicles were prepared with phosphatidylserine and highest when they were prepared with phosphatidylcholine. The relationship between enzyme regulation and lipid deposition in the arterial wall is discussed.