Metabolism of lipoprotein acylglycerols by liver parenchymal cells.

We investigated the metabolism by hepatocyte suspensions of the acylglycerols in lipoprotein remnants as well as those associated with albumin and low or high density lipoproteins. Remnants, albumin and plasma lipoproteins, rich in monoacylglycerol were prepared by short-term incubations of radio-labeled chylomicra or very low density lipoproteins with extrahepatic lipoprotein lipase in the presence of albumin and low and high density lipoproteins. We demonstrated that liver parenchymal cells contain an active monoacylglycerol acyltransferase that is located on the extracellular surface of the cell plasma membrane. Further, the enzyme is capable of degrading the monoacylglycerol in all the above forms. Triacylglycerol in intact chylomicra and very low density lipoproteins were not metabolized by the cells to any appreciable degree. The degradation of the remnant triacylglycerol appeared to depend solely on the activity of the lipoprotein lipase bound to the lipoprotein remnants. Little uptake of intact lipoprotein acylglycerols by the hepatocytes was observed; instead, hydrolysis of the substrates in the medium always preceded the uptake of the products. The products were then utilized for the synthesis of triacylglycerol and phospholipid within the cells.     

Cell triacylglycerol accumulation from very low density lipoproteins isolated from normal and hypertriglyceridemic human sera.

Human fibroblast cells in culture increased their intracellular triacylglycerol levels when exposed to very low density lipoproteins (VLDL) isolated from human plasma. This response was dependent on the amount of VLDL added. VLDL from normal, type IV or type V sera gave similar results. Lipoprotein lipase enhanced this intracellular triacylglycerol accumulation. It was concluded that human fibroblast cells in culture have at least two mechanisms for triacylglycerol uptake from VLDL: (1) uptake from intact lipoprotein either by surface transfer of lipoprotein lipid or internalization of the entire lipoprotein particle, and (2) re-esterification of lower glyceride and fatty acids released by lipoprotein lipase degradation of VLDL.     

Monoacylglycerol accumulation in low and high density lipoproteins during the hydrolysis of very low density lipoprotein triacylglycerol by lipoprotein lipase.

We have demonstrated that low and high density lipoproteins from monkey plasma are capable of accepting and accumulating monoacylglycerol that is formed by the action of lipoprotein lipase on monkey lymph very low density lipoproteins. Furthermore, the monoacylglycerol that accumulates in both low and high density lipoproteins is not susceptible to further hydrolysis by lipoprotein lipase but is readily degraded by the monoacylglycerol acyltransferase of monkey liver plasma membranes. These observations suggest a new mechanism for monoacylglycerol transfer from triacylglycerol rich lipoproteins to other lipoproteins. In addition, the finding that monoacylglycerol bound to low and high density lipoprotein is degraded by the liver enzyme but not lipoprotein lipase lends support to the hypothesis that there are distinct and consecutive extrahepatic and hepatic stages in the metabolism of triacylglycerol in plasma lipoproteins.     

A selective deficiency of hepatic triacylglycerol lipase in guinea pigs

The properties of postheparin plasma triacylglycerol-hydrolyzing enzymes were investigated in guinea pig and rat. In rat, lipoprotein lipase and hepatic triacylglycerol lipase were separated on a heparin-Sepharose affinity chromatography. In postheparin plasma of guinea pig, however, hepatic triacylglycerol lipase was almost completely absent, while lipoprotein lipase was present. Hepatic triacylglycerol lipase was also deficient in the liver tissue extract of guinea pig. Plasma lipoprotein compositions of high-fat fed and control guinea pigs were analyzed. One of the outstanding changes found in high-fat fed animals was the presence of chylomicronemia. One guinea pig showed gross hyperlipemia with triacylglycerol concentrations of 2715 mg/100 ml. Plasma triacylglycerol concentrations of each lipoprotein fraction of very low density, intermediate density, low density and high density lipoproteins from high-fat fed animals were almost the same as those of the corresponding lipoprotein fractions from controls. Discussion was focused on the development of chylomicronemia in relation to the defects of triacylglycerol-hydrolyzing enzyme systems in this animal.     

Mechanisms of enhancement of cholesteryl ester transfer protein activity by lipolysis

Lipoprotein lipase enhances the cholesteryl ester transfer protein (CETP)-mediated transfer of cholesteryl esters from plasma high density lipoproteins (HDL) to very low density lipoproteins (VLDL). In time course studies the stimulation of cholesteryl ester transfer by bovine milk lipase was correlated with accumulation of fatty acids in VLDL remnants. As the amount of fatty acid-poor albumin in the incubations was increased, there was decreased accumulation of fatty acids in VLDL remnants and a parallel decrease in the stimulation of cholesteryl ester transfer by lipolysis. Addition of sodium oleate to VLDL and albumin resulted in stimulation of the CETP-mediated transfer of cholesteryl esters from HDL to VLDL. The stimulation of transfer of cholesteryl esters into previously lipolyzed VLDL was abolished by lowering the pH from 7.5 to 6.0, consistent with a role of lipoprotein ionized fatty acids. CETP-mediated cholesteryl ester transfer from HDL to VLDL was also augmented by phosholipase A2 and by a bacterial lipase which lacked phospholipase activity. When VLDL and HDL were re-isolated after a lipolysis experiment, both lipoproteins stimulated CETP activity. Postlipolysis VLDL and HDL bound much more CETP than native VLDL or HDL. Lipolysis of apoprotein-free phospholipid/triglyceride emulsions also resulted in enhanced binding of CETP to the emulsion particles. Incubation conditions which abolished the enhanced cholesteryl ester transfer into VLDL remnants reduced binding of CETP to remnants, emulsions, and HDL. In conclusion, the enhanced CETP-mediated transfer of cholesteryl esters from HDL to VLDL during lipolysis is related to the accumulation of products of lipolysis, especially fatty acids, in the lipoproteins. Lipids accumulating in VLDL remnants and HDL as a result of lipolysis may augment binding of CETP to these lipoproteins, leading to more efficient transfer of cholesteryl esters from HDL to VLDL.     

The catabolism of human and rat very low density lipoproteins by perfused rat hearts.

The catabolism of human and rat 125I-labelled very low density lipoproteins (VLDL) was compared by perfusing the lipoproteins through beating rat hearts. Triacylglycerol was removed from the VLDL to a greater extent than the protein moiety, leaving remnants containing relatively more apo-B and less apo-C. The change in apo-C content of the remnants correlated with the loss of triacylglycerol. The extent of removal of triacylglycerol from the rat and human VLDL was similar and in most cases appeared to saturate the heart lipoprotein lipase. The remnants were slightly smaller in size than the VLDL, and included particles which appeared to be partially emptied. In addition to remnants of d less than 1.019 g/ml, iodinated lipoproteins derived from rat and human VLDL were recovered at d 1.019-1.063 and 1.063-1.21 g/ml. The former contained largely cholesterol and cholesteryl esters, while phospholipids were the dominant lipid in the latter. An average of 40% of the 125I-labelled apoprotein lost from the VLDL was associated with the perfused hearts. Very little d 1.019-1.063 g/ml lipoprotein was produced from low (physiological) concentrations of rat VLDL, most of the lipoprotein being removed by the heart. However, lipoproteins of density 1.019-1.063 g/ml were formed from human VLDL at all concentrations in the perfusate, as well as from higher concentrations of the rat VLDL. Agarose gel filtration of lipoproteins following heart perfusion with human VLDL revealed large aggregates containing particles which resemble low density lipoproteins (LDL) in electron microscopic appearance and apoprotein composition, since they contain largely apo-B. These data suggest that at normal concentrations rat VLDL are almost completely catabolised and taken up by the heart without the formation of LDL, while LDL is produced from human VLDL at all concentrations.     

Hypertriglyceridemia is exacerbated by slow lipolysis of triacylglycerol-rich lipoproteins in fed but not fasted streptozotocin diabetic rats.

Hydrolysis by endothelial lipases of triacylglycerol-rich lipoproteins of diabetic origin were compared to lipoproteins of non-diabetic origin. The plasma lipoprotein fraction of density < 1.006 g/ml, including chylomicrons and VLDL, were incubated in vitro with post-heparin plasma (PHP) lipases. The lipoproteins of diabetic origin were hydrolysed at a significantly slower rate than lipoproteins from normal rats by the lipoprotein lipase component of PHP. However, if rats were fasted for 16 h prior to lipoprotein recovery, no differences in rates of VLDL hydrolysis were observed. Slower hydrolysis of lipoproteins of diabetic origin reflected a decrease in the apolipoprotein CII/CIII ratio and other changes in the apolipoprotein profile. To assess whether diabetic rats were less able to clear triacylglycerol independent of changes in the nature of the lipoproteins, we monitored the clearance of chylomicron-like lipid emulsions in hepatectomized rats. In vivo, emulsion triacylglycerol hydrolysis was not slowed due to diabetes. However, control and diabetic rats, which had been fasted for 16 h, cleared triacylglycerol at about twice the rate of fed rats. Triacylglycerol secretion rates in diabetic and control rats were similar, whether fed or fasted. We conclude that in streptozocin diabetic rats, hypertriglyceridemia was not due to overproduction of chylomicron- or VLDL-triacylglycerol, nor to decreased endothelial lipase activities. Rather, in fed diabetic rats, the triacylglycerol-rich lipoproteins are poorer substrates for lipoprotein lipase. This may lead to slower formation of remnants which would exacerbate slow remnant removal. VLDL of diabetic origin were hydrolysed as efficiently as VLDL from control donors, suggesting that in the fed state the lipolytic defect may be specific for chylomicrons.     

Triacylglycerol and very low density lipoprotein secretion into plasma of squirrel monkeys.

We determined the effects of varying the types and level of dietary fat and cholesterol on the increase in plasma total triacylglycerol concentrations after injection of Triton WR-1339, an inhibitor of lipoprotein lipase, into monkeys that had been subjected to an overnight fast. The monkeys that had been treated with Triton WR-1339 were then given a test meal by intragastric intubation. Dietary cholesterol, high levels of fat and saturated fat in the habitual diet reduced the rate of release of triacylglycerol to plasma in the fasted monkey. We also determined the changes in protein and lipid concentrations of the different lipoprotein fractions. The injection of Triton WR-1339 resulted in a linear increase with time in the concentration of protein and triacylglycerol in the very low density (chylomicron-free and d less than 1.006) lipoproteins, but there was an increase in the ratio of traicylglycerol to protein in that fraction. Most of the increase (96%) in very low density protein was in the B protein. Regardless of the habitual diet, a test meal accentuated the rate of triacylglycerol appearance in whole plasma and in the very low density lipoproteins of Triton WR-1339-treated monkeys, and the rate of increase of the protein component after feeding was slightly higher. Thus the administration of a meal to the fasted Triton WR-1339-treated squirrel monkey further increased the proportion of triacylglycerol in very low density lipoproteins. Although dietary cholesterol and saturated fat in the habitual diet depressed the rate of increase in very low density triacylglycerol during fasting, the rate of protein synthesis was not significantly affected. After administration of a test meal the rates of increase in triacylglycerol and protein in the very low density lipoproteins were similar for monkeys from the different diet groups. Triton WR-1339 administration caused a slight and progressive increase in the intermediate density (d 1.006-1.019) lipoproteins and a marked and progressive decrease in the low density (d 1.019-1.063) lipoproteins. There was an immediate (by 5 min) drop of 70% or more in high density (d 1.063-1.21) lipoprotein protein, but the lipids except triacylglycerol remained unchanged. There was a decrease in both the A (the major fraction) and C proteins. The rates of very low density B protein secretion were comparable to the rates of low density lipoprotein catabolism that had been previously demonstrated for this species.     

Comparative metabolism of free and esterified fatty acids by the perfused rat heart and rat cardiac myocytes

Studies have been conducted on the uptake and metabolism of unesterified oleic acid and lipoprotein triacylglycerol by the perfused rat heart, and of oleic acid, free glycerol and lipoprotein triacylglycerol by rat cardiac myocytes. The perfused heart efficiently extracted and metabolized unesterified fatty acid and the fatty acid released during lipolysis of the recirculating triacylglycerol. The released glyceride glycerol, however, was largely accumulated in the perfusion media. Cardiac myocytes also extracted and rapidly metabolized unesterified fatty acid. As with the intact heart, free glycerol was poorly utilized by cardiac myocytes. Although the cells appeared to extract a small amount of available extracellular triacylglycerol presented as very low density lipoprotein, this was shown to be unmetabolized, suggesting adsorption rather than surface lipolysis and uptake of the released fatty acid. The data suggest that myocytes are unable to metabolize triacylglycerol fatty acids without prior lipolysis by extracellular (capillary endothelial) lipoprotein lipase.     

The rabbit as an animal model of hepatic lipase deficiency

A natural deficiency of hepatic lipase in rabbits has been exploited to gain insights into the physiological role of this enzyme in the metabolism of plasma lipoproteins. A comparison of human and rabbit lipoproteins revealed obvious species differences in both low-density lipoproteins (LDL) and high-density lipoproteins (HDL), with the rabbit lipoproteins being relatively enlarged, enriched in triacylglycerol and depleted of cholesteryl ester. To test whether these differences related to the low level of hepatic lipase in rabbits, whole plasma or the total lipoprotein fraction from rabbits was either kept at 4 degrees C or incubated at 37 degrees C for 7 h in (i) the absence of lipase, (ii) the presence of hepatic lipase and (iii) the presence of lipoprotein lipase. Following incubation, the lipoproteins were recovered and subjected to gel permeation chromatography to determine the distribution of lipoprotein components across the entire lipoprotein spectrum. An aliquot of the lipoproteins was subjected also to gradient gel electrophoresis to determine the particle size distribution of the LDL and HDL. Both hepatic lipase and lipoprotein lipase hydrolysed lipoprotein triacylglycerol and to a much lesser extent, also phospholipid. There were, however, obvious differences between the enzymes in terms of substrate specificity. In incubations containing hepatic lipase, there was a preferential hydrolysis of HDL triacylglycerol and a lesser hydrolysis of VLDL triacylglycerol. By contrast, lipoprotein lipase acted primarily on VLDL triacylglycerol. When more enzyme was added, both lipases also acted on LDL triacylglycerol, but in no experiment did lipoprotein lipase hydrolyse the triacylglycerol in HDL. Coincident with the hepatic lipase-induced hydrolysis of LDL and HDL triacylglycerol, there were marked reductions in the particle size of both lipoprotein fractions, which were now comparable to those of human LDL and HDL3, respectively.     

Utilization of free fatty acids complexed to human plasma lipoproteins by mammalian cell suspensions

The purpose of this study was to determine whether lipoprotein-bound free fatty acid could be utilized by isolated mammalian cells. Ehrlich ascites tumor cells were incubated in vitro with radioactive free fatty acids that were bound to human plasma lipoproteins. Under these conditions, lipoprotein-bound free fatty acids were readily taken up by the cells. After 2 min of incubation with free fatty acids bound to low density lipoproteins, most of the radioactivity that was associated with the cells was in the form of free fatty acids. As the incubation continued, increasing amounts of radioactivity were incorporated into CO(2) and cell lipids, particularly phospholipids. Most of the free fatty acid uptake was the result of fatty acid transfer from low density lipoproteins to the cell, not from irreversible incorporation of the intact free fatty acid-low density lipoprotein complex. Fatty acid uptake increased as the ratio of free fatty acid to low density lipoprotein was raised. When albumin was added to the medium, free fatty acid uptake decreased. A large percentage of the newly incorporated cellular radioactivity was released into the medium if the cells were exposed subsequently to a solution containing albumin. Most of the released radioactivity was in the form of free fatty acid. The results with this experimental model suggest that lipoprotein-bound free fatty acid, like albumin-bound free fatty acid, is readily available for uptake by isolated cells. The mechanism of free fatty acid utilization by the Ehrlich cell is similar when either low density lipoprotein or serum albumin serves as the fatty acid carrier.     

Contraceptive steroids increase hepatic uptake of chylomicron remnants in healthy young women

In an investigation of alterations in cholesterol metabolism during contraceptive steroid use, we studied plasma clearance of chylomicron remnants. Six healthy women were studied on and off contraceptive steroid therapy. Remnant clearance was measured from the disappearance of retinyl palmitate administered intravenously in plasma endogenously labeled with retinyl palmitate. We also measured cholesterol in HDL and its subfractions and postheparin lipoprotein lipase and hepatic triglyceride lipase activities. Plasma decay of retinyl palmitate was biexponential. The rapid component, reflecting chylomicron remnant removal, accounted for about 90% of the total clearance in all studies. During contraceptive steroid intake, both rapid and slow decay constants and the calculated plasma clearance rates were significantly increased (mean values: rapid decay constant, control 0.048 versus treated 0.101 min-1, P less than 0.05; slow decay constant, 0.004 versus 0.014 min-1, P less than 0.01; plasma clearance 74 versus 115 ml/min, P less than 0.025) indicating enhanced hepatic uptake of chylomicron remnants and probably an increased hepatic uptake of higher density lipoproteins (d greater than 1.006 g/ml). Total postheparin lipolytic activity and lipoprotein lipase activity were depressed in all six women (P less than 0.05) and hepatic triglyceride lipase activity was increased in four of five subjects. Contraceptive steroids also caused a decrease in the HDL2/HDL3 cholesterol ratio (P less than 0.05), implying impaired peripheral lipoprotein triglyceride hydrolysis and/or increased HDL2 clearance by hepatic triglyceride lipase. In conclusion, during intake of contraceptive steroids, the plasma clearance of chylomicron remnants and higher density lipoproteins was increased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Receptor-mediated uptake of remnant lipoproteins by cholesterol-loaded human monocyte-macrophages

Normal human monocyte-macrophages were cholesterol-loaded, and the rates of uptake and degradation of several lipoproteins were measured and compared to rates in control cells. Receptor activities for 125I-rabbit beta-very low density lipoproteins (beta-VLDL), 125I-human low density lipoprotein, and 125I-human chylomicrons were down-regulated in cholesterol-loaded cells; however, the rate of uptake and degradation of 125I-human chylomicron remnants was unchanged from control cells. Cholesterol-loaded alveolar macrophages from a Watanabe heritable hyperlipidemic rabbit, which lack low density lipoprotein receptors, showed receptor down-regulation for 125I-beta-VLDL but not for 125I-human chylomicron remnants. In addition to chylomicron remnants, apo-E-phospholipid complexes competed for 125I-chylomicron remnant uptake, but apo-A-I-phospholipid complexes did not. Chylomicrons competed for lipoprotein uptake in control cells but were not recognized under conditions of cholesterol loading. Chylomicron remnants and beta-VLDL were equally effective in competing for 125I-beta-VLDL and 125I-chylomicron remnant uptake in cholesterol-loaded macrophages. When normal human monocyte-macrophages were incubated in serum supplemented with chylomicron remnants, the cholesteryl ester content increased 4-fold over cells incubated in serum with low density lipoprotein added. We conclude: 1) specific lipoprotein receptor activity persists in cholesterol-loaded cells; 2) this receptor activity recognizes lipo-proteins (at least in part) by their apo-E content; and 3) cholesteryl ester accumulation can occur in monocyte-macrophages incubated with chylomicron remnants.     

Lipoprotein metabolism in hepatic lipase deficiency: studies on the turnover of apolipoprotein B and on the effect of hepatic lipase on high density lipoprotein

Hepatic lipase deficiency produces significant distortion in the plasma lipoprotein profile. Particles with reduced electrophoretic mobility appear in very low density lipoprotein (VLDL). Intermediate density lipoprotein (IDL) increases markedly in the circulation and plasma low density lipoprotein (LDL) levels fall. At the same time there is a mass redistribution within the high density lipoprotein (HDL) spectrum leading to dominance in the less dense HDL2 subfraction. The present study examines apolipoprotein B turnover in a patient with hepatic lipase deficiency. The metabolism of large and small very low density lipoproteins was determined in four control subjects and compared to the pattern seen in the patient. Absence of the enzyme did not affect the rate at which large very low density lipoproteins were converted to smaller particles within this density interval (i.e., of VLDL). However, subsequent transfer of small very low density lipoproteins to intermediate density particles was retarded by 50%, explaining the abnormal accumulation of VLDL in the patient's plasma. Despite this, intermediate density particles accumulated to a level 2.4-times normal because their subsequent conversion to low density lipoprotein has been almost totally inhibited. Consequently, the plasma concentration of low density lipoprotein was only 10% of normal. On the basis of these observations, hepatic lipase appears to be essential for the conversion of small very low density and intermediate density particles to low density lipoproteins. The pathways of direct plasma catabolism of these species were not affected by the enzyme defect. In vitro studies were performed by adding purified hepatic lipase to the patient's plasma.(ABSTRACT TRUNCATED AT 250 WORDS)     

Analysis and lipoprotein lipase activation capacity of plasma lipoproteins isolated from atherosclerosis-susceptible White Carneau pigeon and atherosclerosis-resistant Show Racer pigeon

The lipoprotein composition and apoprotein composition of the major lipoprotein fraction (high density lipoprotein) were compared in White Carneau and Show Racer plasma. The capacity of the plasma and lipoproteins to activate the triacylglycerol hydrolyzing activity of lipoprotein lipase in vitro was compared in the two strains of birds and found to be identical in each case. It appears unlikely that differences in lipoprotein composition or tissue lipoprotein lipase activity will be reflected in the flux rates of lipoproteins in the two strains which have different susceptibilities to atherosclerosis.     

Lipolysis products promote the formation of complexes of very-low-density and low-density lipoproteins

In the course of lipolysis, surface lipid products may accumulate on very-low-density lipoproteins (VLDL). To investigate potential lipoprotein interactions mediated by such products, radiolabeled low-density lipoproteins (LDL) were incubated with VLDL and bovine milk lipoprotein lipase in the presence of limited free fatty acid acceptor. With partial VLDL degradation, association of radiolabeled LDL with VLDL remnants or larger aggregates of VLDL density was demonstrated by gradient gel electrophoresis, agarose chromatography, and density gradient ultracentrifugation. VLDL-LDL complex formation was also observed in incubations with lipid extracts from lipolyzed VLDL or with purified palmitic acid in the absence of lipolysis. Complex formation was inhibited by addition of increasing amounts of albumin as free fatty acid acceptor, but could be detected at molar ratios of free fatty acids/albumin that occur in vivo. Composition analysis of LDL reisolated following incubation with VLDL and lipase under conditions favoring partial complex formation revealed enrichment in glycerides and depletion of cholesterol. We conclude that lipolysis products can promote the formation of stable complexes of LDL and VLDL, and that physical interactions of this nature may play a role in the transfer of lipids and apolipoproteins between lipoprotein particles.     

The effects of oral agent or insulin treatments on the plasma lipoproteins and the plasma lipoprotein lipase activator in diabetic patients

The structure and the metabolism of plasma lipoproteins are altered in diabetes mellitus. Insulin or oral agent treatments affect the lipoprotein metabolism in addition to improving hyperglycemia. However, it is not clear whether the alterations seen in lipoproteins during treatment are related to the degree of diabetic control or to the mode of diabetic treatment. The effects of insulin or oral agent treatments on the plasma lipoproteins and lipoprotein lipase activator were compared in a strictly defined non-obese, non-insulin dependent diabetic patient. Both treatment groups had similar plasma triglyceride, total cholesterol, low and high density lipoprotein cholesterol, and lipoprotein lipase activator levels. Lipoprotein lipase activator contents of the very low density lipoproteins correlated positively with their triglyceride (r = 0.803 in insulin, r = 0.828 in oral agent treated patients) and protein (r = 0.713 in insulin, r = 0.862 in oral agent treated patients) contents. The findings of this study indicated that plasma lipid levels, very low density lipoprotein compositions, and lipoprotein lipase activator contents were not significantly different in non-obese, non-insulin dependent diabetic patients treated with either oral hypoglycemic agents or insulin.     

Cholesterol is necessary for triacylglycerol-phospholipid emulsions to mimic the metabolism of lipoproteins

Emulsions with lipid compositions similar to the triacylglycerol-rich lipoproteins were metabolized similarly to natural chylomicrons or very-low-density lipoproteins when injected intravenously in rats. Radioactive labels tracing the emulsion triacylglycerols and cholesteryl esters were both removed rapidly from the blood stream, but the removal rate of triacylglycerols was faster than that of cholesteryl ester. Most of the removed cholesteryl ester label was found in the liver, but only a small fraction of the triacylglycerol label was found in this organ, consistent with hepatic uptake of the remnants of the injected emulsion. Emulsions otherwise identical but excluding unesterified cholesterol were metabolized differently. The plasma removal of triacylglycerols remained fast, but the cholesteryl esters were removed very slowly. Heparin stimulated lipolysis, but failed to increase the rate of removal of cholesteryl esters from emulsions lacking cholesterol. Evidently, emulsions lacking cholesterol were acted on by the enzyme lipoprotein lipase, but the resultant triacylglycerol-depleted remnant particle remained in the plasma instead of being rapidly taken up by the liver. Therefore, the presence of emulsion cholesterol is a critical determinant of early metabolic events, and the findings point to a similar role for cholesterol in the natural triacylglycerol-rich lipoproteins.     

Apolipoprotein and lipid distribution between vesicles and HDL-like particles formed during lipolysis of human very low density lipoproteins by perfused rat heart

A study was undertaken to determine the relative association of lipid and apolipoproteins among lipoproteins produced during lipolysis of very low density lipoproteins (VLDL) in perfused rat heart. Human VLDL was perfused through beating rat hearts along with various combinations of albumin (0.5%), HDL2, the infranatant of d greater than 1.08 g/ml of serum, and labeled sucrose. The products were resolved by gel filtration, ultracentrifugation, and hydroxylapatite chromatography. The composition of the lipoprotein products was assessed by analysis of total lipid profiles by gas-liquid chromatography and immunoassay of apolipoproteins. A vesicle particle, which trapped and retained 1-2% of medium sucrose, co-isolated with VLDL and VLDL remnants by gel filtration chromatography but primarily with the low density lipoprotein (LDL) fraction when isolated by ultracentrifugation. The vesicle was resolved from apoB-containing LDL lipolysis products by hydroxylapatite chromatography of the lipoproteins. The vesicle lipoprotein contained unesterified cholesterol (34%), phosphatidylcholine and sphingomyelin (50%), cholesteryl ester (6%), triacylglycerol (5%), and apolipoprotein (5%). The apolipoprotein consisted of apoC-II (7%), apoC-III (93%), and trace amounts of apoE (1%). When viewed by electron microscopy the vesicles appeared as rouleaux structures with a diameter of 453 A, and a periodicity of 51.7 A. The mass represented by the vesicle particle in terms of the initial amount in VLDL was: cholesterol (5%), phosphatidylcholine and sphingomyelin (3%), apoC-II (0.5%), apoC-III (2.2%). The majority of the apoC and E released from apoB-containing lipoproteins was associated with neutral-lipid core lipoproteins proteins which possessed size characteristics of HDL. The vesicles were also formed in the presence of HDL and serum and were not disrupted by serum HDL. It is concluded that lipolysis of VLDL in vitro results in the production of VLDL remnants and LDL apoB-containing lipoproteins, as well as HDL-like lipoproteins. A vesicular lipoprotein which has many characteristics of lipoprotein X found in cholestasis, lecithin: cholesterol acyltransferase deficiency, and during Intralipid infusion is also formed. The majority of apolipoprotein C and E released from apoB-containing lipoproteins is associated with the HDL-like lipoprotein. It is suggested that the formation and stability of the vesicle lipoprotein may be related to the high ratio of cholesterol/phospholipid in this particle.     

Studies on the mechanism of hypertriglyceridemia in Tangier disease. Determination of plasma lipolytic activities, k1 values and apolipoprotein composition of the major lipoprotein density classes

Mechanisms responsible for hypertriglyceridemia in Tangier disease were elucidated by an analysis of the plasma post-heparin lipolytic activities and the structural and metabolic properties of very low (VLDL) and low (LDL) density lipoproteins. The levels of lipoprotein lipase activity in six Tangier patients were significantly lower (P less than 0.001) than in 40 control subjects (8.1 +/- 3.3 (+/- S.D.) vs. 14.1 +/- 3.7 units/ml). In contrast, the levels of hepatic triacylglycerol lipase were higher (P less than 0.01) than in normal controls (14.4 +/- 3.9 vs. 9.3 +/- 4.0 units/ml). Because kinetic parameters such as Km or Vmax cannot be obtained with naturally occurring triacylglycerol-rich lipoproteins, the pseudo-first-order rate constant (k1) of triacylglycerol hydrolysis was used to assess the effectiveness of triacylglycerol-rich lipoproteins as substrates for lipoprotein lipase. The k1 values for Tangier VLDL (k1 = 0.017 +/- 0.002 min-1) were significantly lower (P less than 0.001) than the k1 values (0.036 +/- 0.008 min-1) for control VLDL. Both the Tangier and control LDL2 are similar in their resistance to the action of lipoprotein lipase, as shown by their low k1 values (0.002 +/- 0.001 and 0.001 +/- 0.001 min-1, respectively). The major compositional difference between the lipoproteins of Tangier disease and normal subjects was a significant increase in the percent content of apolipoprotein A-II in all lipoprotein particles with d less than 1.063 g/ml, with the greatest increase occurring in VLDL and the lowest in LDL2. These results were interpreted as indicating that, in Tangier disease, there is a lower reactivity of VLDL with lipoprotein lipase which may in part be attributed to the abnormal apolipoprotein composition. This finding, in conjunction with the reduced levels of lipoprotein lipase activity, may explain the hypertriglyceridemia in Tangier disease.