Effects of 1,2-cyclohexanedione modification on the metabolism of very low density lipoprotein apolipoprotein B: potential role of receptors in intermediate density lipoprotein catabolism

The conversion of very low density (VLDL) to low density lipoproteins (LDL) is a two-step process. The first step is mediated by lipoprotein lipase, but the mechanism responsible for the second is obscure. In this study we examined the possible involvement of receptors at this stage. Apolipoprotein B (apoB)-containing lipoproteins were separated into three fractions, VLDL (Sf 100-400), an intermediate fraction IDL (Sf 12-100), and LDL (Sf 0-12). Autologous 125I-labeled VLDL and 131I-labeled 1,2-cyclohexanedione-modified VLDL were injected into the plasma of four normal subjects and the rate of transfer of apoB radioactivity was followed through IDL to LDL. Modification did not affect VLDL to IDL conversion. Thereafter, however, the catabolism of modified apoB in IDL was retarded and its appearance in LDL was delayed. Hence, functional arginine residues (and by implication, receptors) are required in this process. Confirmation of this was obtained by injecting 125I-labeled IDL and 131I-labeled cyclohexanedione-treated IDL into two additional subjects. Again, IDL metabolism was delayed by approximately 50% as a result of the modification. These data are consistent with the view that receptors are involved in the metabolism of intermediate density lipoprotein.     

Effects of bezafibrate on apolipoprotein B metabolism in type III hyperlipoproteinemic subjects

This study was designed to investigate the response of Type III hyperlipoproteinemic subjects to bezafibrate therapy. The metabolism of apolipoprotein B was examined in four lipoprotein subclasses of Sf 60-400 (large very low density lipoprotein (VLDL)), Sf 20-60 (small VLDL), Sf 12-20 (intermediate density lipoprotein (IDL)), and Sf 0-12 (low density lipoprotein (LDL)) before and during bezafibrate therapy. Treatment reduced the plasma concentration of VLDL and raised high density lipoprotein (HDL) cholesterol. There was no net change in LDL cholesterol or its associated apolipoprotein B. The decrease in plasma VLDL derived mainly from an inhibition of synthesis of both large and small subfractions which reduced the number of particles in the circulation without normalizing their lipid composition. Catabolism of the larger VLDL also increased, presumably as a result of lipoprotein lipase activation. Although the plasma concentration of LDL was unchanged, both its synthesis and catabolism were perturbed. Its fractional catabolic rate fell by 50%, but the impact that this would have had on its steady state level in the circulation was apparently blunted by a decrease in its synthesis from Sf 12-20 IDL. In the control phase of the study, most IDL apolipoprotein B was converted to LDL. Bezafibrate therapy channelled this material towards direct catabolism.     

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.     

Metabolic heterogeneity in the formation of low density lipoprotein from very low density lipoprotein in the rat: evidence for the independent production of a low density lipoprotein subfraction.

The formation of low density lipoprotein (LDL) from very low density lipoprotein (VLDL) was studied after injecting 14C-radiomethylated or 125I-radioiodinated VLDL into rats. VLDL and LDL B apoprotein specific radioactivity time curves were obtained after tetramethylurea extraction of the lipoproteins. In all experiments, the specific activity of LDL B apoprotein did not intercept the VLDL curve at maximal heights, suggesting that not all LDL B apoprotein is derived from VLDL B apoprotein. Further subfractionation of LDL into the Sf 12-20, 5-12, and 0-5 ranges showed that most (65%) LDL B apoprotein was present in the Sf 0-5 fraction and that only a small proportion (6-15%) of this fraction was derived from VLDL. However, the curves obtained for the Sf 12-20 and 5-12 subfractions were consistent with a precursor-product relationship in which all of these fractions were derived entirely from VLDL catabolism. These results contrasted strikingly with similar data obtained for normal humans in which all LDL is derived from VLDL. In the rat, it appears that most of the B apoprotein in the Sf 0-5 range, which contains 65% of the total LDL B apoprotein, enters the plasma independently of VLDL secretion.     

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.     

Isolation of serum chylomicrons prior to density gradient ultracentrifugation of other serum lipoprotein classes

A method for the removal of serum chylomicrons before density gradient ultracentrifugation of the other serum lipoproteins using an SW 41 swinging bucket rotor is presented. In a preliminary spin, the chylomicrons with an Sf greater than 400 X 10(-13) s float to the top of the gradient, whereas the other lipoproteins are retained in the infranatant fraction. After removal of the chylomicrons, the other serum lipoproteins are subsequently fractionated by isopycnic density gradient ultracentrifugation. Analysis of the separated lipoprotein fractions suggested that this procedure permits isolation of a chylomicron fraction consisting solely of chylomicrons but that the very low density lipoprotein fraction subsequently isolated also contains chylomicrons or chylomicron remnants with an Sf less than 400 X 10(-13) s, and that there is considerable overlap in flotation rate and particle size of very low density lipoproteins and chylomicrons.     

A new large chylomicron remnant from cholesterol-fed rabbits

The lipoproteins of density less than 1.063 g/ml of cholesterol-fed rabbits were subjected to analytical ultracentrifugation. In many rabbits two peaks were found in the very low density (Sf greater than 20) portion of the lipoprotein spectrum. They were isolated by preparative ultracentrifugation and analysed. The smaller particles (remnant chylomicrons) had a peak Sf of 37, mean diameter of 36 nm, mean density of 1.00 g/ml, and their chemical composition agreed closely with previous reports. The larger particles had a peak Sf of 270, mean diameter of 80 nm, mean density of 0.97 g/ml and a high (80%) cholesterol ester and low (4%) triglyceride content. The fatty acid composition of the cholesterol esters, phospholipids and triglycerides was similar in both fractions. It is proposed that these large lipoprotein particles are also remnant chylomicrons. Possible reasons are presented to explain the presence of this second peak in the very low density lipoprotein spectrum.     

Apolipoprotein B metabolism in homozygous familial hypercholesterolemia

This report describes the metabolism of apolipoprotein B-containing lipoproteins in seven familial hypercholesterolemic (FH) homozygotes and compares the results to the values obtained from five healthy control subjects. The concentration, composition, and metabolism of large, triglyceride-rich very low density lipoproteins (VLDL1, Sf 60-400) were the same in the control and FH groups, indicating that this component of the VLDL delipidation cascade ws unaffected by the absence of receptors. In contrast, familial hypercholesterolemic small VLDL2 (Sf 20-60) was enriched with cholesterol and depleted in triglyceride. Moreover, its plasma concentration was elevated as a result of an increase in its synthesis and a defect in the removal of a remnant population within this density interval. The latter accounted for up to 50% of the total mass of the fraction. Onward transfer of apolipoprotein B (apoB) from small VLDL through intermediate density lipoprotein (IDL) to low density lipoprotein (LDL) was retarded, suggesting that receptors were involved in this supposedly lipase-mediated event. IDL and LDL concentrations increased up to fourfold above normal in the plasma of the FH patients due partly to the delay in maturation and partly to defective direct catabolism. We conclude that the LDL receptor plays multiple and important roles in the metabolism and transformation of apoB-containing particles in the Sf 0-400 flotation interval.     

Lipoprotein lipase. Isolation and characterization of a second enzyme species from postheparin plasma.

A lipoprotein lipase species (mol wt 69 250) has been isolated from rat postheparin plasma, which differs from the low-molecular-weight species previously characterized in its amino acid composition and hexosamine content, and in its lower affinity for triglyceride-rich lipoprotein substrates. However, both enzymes are activated by the same coprotein (C-terminal glutamic acid, apo-C-2) from human very low density lipoprotein and have a similar specificity for lipid esters. Neither purified enzyme is activated by heparin. Both are inhibited by molar sodium chloride. Both enzyme species can be recovered from the same plasma samples. The possible relationship of these proteins to the different functional lipoprotein lipase activities of muscle and adipose tissues is discussed.     

Mechanism of salt-mediated inhibition of lipoprotein lipase.

The activity of lipoprotein lipase isolated from rat postheparin plasma has been determined with synthetic lipids, in the presence and absence of apoprotein of the natural substrate very low density lipoprotein, as a function of medium ion-pair concentration of a number of different inorganic salts. The several kinetic effects of lipoprotein protein on lipase activity were specifically and quantitatively reversed in the presence of molar sodium chloride or solutions of equivalent effective ion concentrations of other salts. Salt-mediated inhibition was fully reversible by silution and was independent of substrate concentration. Inhibition was a function of the identity of the salt anion within a Hofmeister (lyotropic) series: I- greater than SCN- greater than NO3- greater than Cl- greater than F-, and, in these terms, was not significantly different for a series of inorganic chlorides (Li+, Na+, K+, Cs+). The effects of salts on the natural lipoprotein substrates, chylomicrons, and very low density lipoproteins were similar to those obtained with a synthetic lipid-protein substrate complex. These findings are discussed in the light of recent ideas on the activation of lipoprotein lipase.     

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.     

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.     

Comparison of the phospholipase activity of bovine milk lipoprotein lipase against rat plasma very low density and high density lipoprotein.

The hydrolytic activity of a lipoprotein lipase from bovine milk against triacylglycerol and phosphatidylcholine of rat plasma very low density lipoprotein was determined and compared to that against phosphatidylcholine of high density lipoprotein. 85--90% of the triacylglycerol in very low density lipoprotein were hydrolyzed to fatty acids and 25--35% of the phosphatidylcholine to lysophosphatidylcholine. High density lipoprotein phosphatidylcholine was only minimally susceptible to the enzyme. Even with high amounts of enzyme and prolonged incubation periods, lysophosphatidylcholine generation did not exceed 2--4% of the original amounts of labeled phosphatidylcholine in the high density lipoprotein. We conclude that phospholipids in high density lipoprotein are not substrates for the phospholipase activity of this lipoprotein lipase. These observations suggest that factors other than the presence of apolipoprotein C-II and of glycerophosphatides are of importance for the activity of lipoprotein lipases.     

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.     

Phospholipid removal during degradation of rat plasma very low density lipoprotein in vitro.

The hydrolysis of glycerophospholipids in very low density lipoprotein by enzyme(s) released into circulation after the injection of heparin to rats was studied. [32P]Lysolecithin was formed rapidly from [32P]lecithin when very low density lipoprotein, labeled biosynthetically with 32P, was incubated with postheparin plasma. The [32P]lysolecithin was associated with the plasma protein fraction of density greater than 1.21 g/ml, whereas [32P]lecithin exchanged between very low and high density lipoproteins. Inhibition of the plasma lecithin: cholesterol acyl transferase activity did not change the excess [32P]lysolecithin formation in postheparin plasma, and only a negligible amount of radioactivity was associated with blood cells when the incubation was repeated in whole blood. Analysis of the results has demonstrated that phospholipids are removed from VLDL by two pathways: hydrolysis of glycerophospholipids by the heparin-releasable phospholipase activity (greater than50%) and transfer to high density lipoproteins (less than50%). The tissue origin of the postheparin phospholipase was studied in plasma obtained from intact rats and supradiaphragmatic rats using specific inhibitors of the extrahepatic lipase system (protamine sulfate and 0.5 M NaCl). The phospholipase activity could be ascribed to both the hepatic and extrahepatic lipase systems. It is concluded that hydrolysis of glycerophospholipids is the major mechanism responsible for the removal of phospholipids from very low density lipoprotein during the degradation of the lipoprotein. It is suggested that phospholipid hydrolysis occurs concomitantly with triglyceride hydrolysis, predominantly in extrahepatic tissues.     

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)     

Lipoprotein lipase activity in the flight muscle of Locusta migratoria and its specificity for haemolymph lipoproteins

Lipoprotein lipase activity in flight muscle homogenates of Locusta migratoria was measured, using natural radiolabelled lipoproteins as substrates. The flight specific lipoprotein A⁺ (or low density lipophorin) stimulated lipoprotein lipase activity several-fold compared to the resting lipoprotein A_y (or high density lipophorin). However, with the high mol. wt lipoprotein fraction O_AKH as a substrate, lipase activity was even doubled compared to lipoprotein A⁺. Lipase activity was not increased in flight muscle homogenates of insects which had flown. Neither adipokinetic hormone, nor octopamine had any direct effect on lipoprotein lipase activity. Aspects of hormonal regulation and apoprotein activation of the locust flight muscle lipoprotein lipase are discussed and compared with the model for vertebrate lipoprotein lipase.     

On the metabolic function of heparin-releasable liver lipase

Intravenous administration of specific antibody against heparin-releasable liver lipase (liver lipase) induced a 75% inhibition of the enzyme activity $\text{in situ}$. Administration of the antibody resulted in an increase of high density lipoprotein (density range 1.050–1.13 g/ml; HDL₂) phospholipid levels (20% after 1 h; 54% after 4 h). Short-term (1 h) treatment with antibody had no significant effect on any of the other lipoprotein components. After long-term (4 h) treatment the free cholesterol level of HDL₂ and all components in the very low density lipoprotein (VLDL) + intermediate density lipoprotein (IDL) fraction were elevated (1.5–2.0 fold). In the low density lipoprotein (LDL) fraction only the phospholipid level was affected (increased by 72%). All lipid components in the HDL₃ fraction were decreased by the antibody treatment, but this decrease was only statistically significant for the cholesterolesters. The rate of removal of iodine-labeled high density lipoprotein (HDL) and LDL from serum was not affected by the antibody treatment.These results suggest that liver lipase may promote phospholipid removal $\text{in vivo}$ and show that a lowering of liver lipase $\text{in situ}$ has profound consequences for serum lipoprotein metabolism.     

Size of lipoproteins in intestinal lympho of sheep and suckling lambs.

The relative importance of chylomicrons (Sf greater than 400) and very low density lipoproteins (Sf 20--400) in transporting lipids in lymph was investigated in surgically prepared adult sheep and pre-ruminant lambs fed low fat diets or infused intraduodenally with corn oil. The concentration of triacylglycerol in the intestinal lymph of sheep and lambs was increased from 520 and 925 mg/100 ml to 2326 and 2367 mg/100 ml respectively when corn oil was infused into the duodenum and the ratio of triacylglycerol to phospholipid changed from 3.7 and 5.5 to 9.5 and 9.7 respectively. The flow of lymph also increased. Electron microscopy and analytical and preparative ultracentrifugation showed that lymph lipoproteins from sheep and lambs fed low fat diets consisted mainly of lipoproteins 50 nm in diameter and that very low density lipoproteins (Sf 20--400) contirbuted up to 75% of the Sf greater than 20 lipoproteins. There were no lipoproteins with diameters above 150 nm. Infusion of corn oil into the duodenum of sheep and lambs increased the diameters of lymph lipoproteins. Most were 80--100 nm in diameter but substantial numbers above 150 and up to 400 mn were observed. The maximum contribution of very low density lipoproteins (Sf 20--400) to lipoproteins of Sf greater than 20 was 27--30%. The above findings demonstrate that the size of intestinal lymph lipoprotein particles increases with the amount of lipid absorbed from the small intestines and that the transport of lymph lipids, in ruminants, is similar to that previously found in rats, rabbits and man.     

Isolation and characterization of an apoA-II-containing lipoprotein (LP-A-II:B complex) from plasma very low density lipoproteins of patients with Tangier disease and type V hyperlipoproteinemia

Previous studies have shown that very low density lipoproteins (VLDL) from patients with Tangier disease are less effective as a substrate for human milk lipoprotein lipase (LPL) than VLDL from normal controls as assessed by measuring the first order rate constant (k1) of triglyceride hydrolysis. Tangier VLDL also has a higher content of apolipoprotein (apo) A-II than normal VLDL. To explore the possible relationship between the relatively high concentration of apoA-II in VLDL and low k1 values, Tangier VLDL were fractionated on an anti-apoA-II immunosorber. The retained fraction contained a newly identified triglyceride-rich lipoprotein characterized by the presence of apolipoproteins A-II, B, C-I, C-II, C-III, D, and E (LP-A-II:B:C:D:E or LP-A-II:B complex), whereas the unretained fraction consisted of previously identified triglyceride-rich apoB-containing lipoproteins free of apoA-II. In VLDL from patients with Tangier disease or type V hyperlipoproteinemia, the LP-A-II:B complex accounted for 70-90% and 25-70% of the total apoB content, respectively. The LP-A-II:B complexes had similar lipid and apolipoprotein composition; they were poor substrates for LPL as indicated by their low k1 values (0.014-0.016 min-1). In contrast, the apoA-II-free lipoproteins present in unretained fractions were effective substrates for LPL with k1 values equal to or greater than 0.0313 min-1. These results indicate that triglyceride-rich lipoproteins consist of several apoB-containing lipoproteins, including the LP-A-II:B complex, and that lipoprotein particles of similar size and density but distinct apolipoprotein composition also possess distinct metabolic properties.