Effects of intravenous infusions of commercial fat emulsions (Intralipid 10 or 20%) on rat plasma lipoproteins: phospholipids in excess are the main precursors of lipoprotein-X-like particles

Like most commercial parenteral emulsions, Intralipid contains the same amount of phospholipids (12 mg/ml) to stabilize 100 or 200 mg of soybean oil (10 or 20% formula, respectively). By centrifugation, 10 or 20% Intralipid was separated into a supernatant, fat particles containing the bulk of triacylglycerols stabilized by a fraction of phospholipids and an infranatant--called mesophase--consisting mainly of phospholipids used in excess as emulsifier. We observed that the initial triacylglycerol/phospholipid ratio of the emulsion (100/12 and 200/12, respectively) determines the size of the triacylglycerol-rich particles (260 and 350 nm) as well as the phospholipid content of the mesophase (6.02 and 4.67 mg/ml). To understand the mechanism of the lipoprotein-X (LPX) accumulation generally reported after intravenous fat infusions, plasma lipid levels and lipoprotein profiles were first compared in the rats after infusion (at a constant rate of 0.5 or 1 ml/h for 43 h) of Intralipid 10 or 20%. For the same intravenous triacylglycerol load (100 mg/h), rats infused with Intralipid 10% at 1 ml/h displayed higher triacylglycerol levels than rats infused with the 20% emulsion at 0.5 ml/h, suggesting that the size of exogenous fat particles modulated the catabolic rate of their triacylglycerols. The plasma levels of LPX varied according to the infusion rate of phospholipids not associated with triacylglycerol-rich particles of the emulsion. Moreover, an apo E and apo B enrichment of plasma and an elevation of the apo B48/apo B100 ratio was always observed after Intralipid infusions. In order to confirm that phospholipids of the mesophase are the main LPX precursors, lipoprotein profiles were then compared in the rats after intravenous infusion, at a constant rate of 1 ml/h, of either the mesophase or a suspension of triacylglycerol-rich particles isolated from Intralipid 20%. As expected, significant LPX amounts were only detected in rats infused with the pure mesophase of the emulsion. It was concluded that products of the lipolysis of exogenous fat particles play only a minor role in the formation of LPX. In fact these abnormal lipoproteins are generated by phospholipids of the mesophase which, like infused liposomes, actively mobilize endogenous free cholesterol. Consequently, in order to be considered as true chylomicron models for safe fat delivery in parenteral nutrition and in order to prevent some detrimental effects on cholesterol metabolism, commercial emulsions should be cleared of phospholipid excess.     

Role of lipid transfers in the formation of a subpopulation of small high density lipoproteins

The effect of lipid transfers on the structure and composition of high density lipoproteins (HDL) has been studied in vitro in incubations that contained the lipoprotein-free fraction of human plasma as a source of lipid transfer protein. These incubations did not contain lecithin:cholesterol acyltransferase activity and were not supplemented with lipoprotein lipase. Incubations were performed at 37 degrees C for 6 hr in both the presence and absence of either added very low density lipoproteins (VLDL) or the artificial triglyceride emulsion, Intralipid. Incubation in the absence of added VLDL or Intralipid had little or no effect on the HDL. By contrast, incubation in the presence of either VLDL or Intralipid resulted in marked changes in the HDL. The effect of incubation with VLDL was qualitatively similar to that of Intralipid; both resulted in obvious transfers of lipid and changes in the density, particle size, and composition of HDL. Incubation of the plasma fraction of density 1.006-1.21 g/ml, total HDL, or HDL3 with either VLDL or Intralipid resulted in the following: 1) a depletion of the cholesteryl ester and free cholesterol content and an increase in the triglyceride content of both HDL2 and HDL3; 2) a decrease in density and an increase in particle size of the HDL3 to form a population of HDL2-like particles; and 3) the formation of a discrete population of very small lipoproteins with a density greater than that of the parent HDL3. The newly formed lipoproteins had a mean particle radius of 3.7-3.8 nm and consisted mainly of protein, predominantly apolipoprotein A-I and phospholipid.     

Effects of several common long chain fatty acids on the properties and lipid composition of the very low density lipoprotein secreted by the perfused rat liver.

1. Livers from normal fed male rats were perfused in vitro with a bloodless medium which contained intially 3% bovine serum albumin and 100 mg% glucose. Albumin alone, or myristate (14 : 0), palmitate (16 : 0), palmitoleate (16 : 1), stearate (18 : 0), oleate (18 : 1), or linoleate (18:2) was infused at a constant rate (496 mumol/4 h), as a complex with albumin, during the experiment. 2. The very low density lipoprotein secreted by the liver after infusion of unsaturated fatty acids (16 : 1, 18 :1, 18 : 2) has a faster rate-zonal mobility in the ultracentrifuge and is, therefore, probably a larger particle with fewer moles of phospholipid and cholesterol relative to triacyglycerol (triacyglycerol/phospholipids/cholesterol = 100/25.1/16.4) than the very low density lipoproteins produced after infusion of saturated (14 : 0, 16 : 0, 18 : 0) fatty acids (triacyglycerol/phospholipids/cholesterol = 100/30.1/19.1). The molar ratio of phosphoipids/cholesterol of the very low density lipoprotein was similar regardless of which fatty acid was infused. The predominant fatty acid of the very low density lipoprotein or hepatic triacyglycerol, in all cases, was the infused acid. 3. We conclude that free fatty acid regulates the quantity and proportions of triacyglycerol, phospholipids, and cholesterol secreted by the liver in the very low density lipoprotein, and therefore, may secondarily influence concentrations of lipids in the very low density lipoprotein and other plasma lipoproteins circulating in vivo.     

Plasma lipoproteins,postheparin plasma lipoprotein lipase activity intralipid half-life,triglyceride secretion rate and liver lipids in the mouse fed different cholesterol diets

Mice (SC), fed a semipurified diet containing cholesterol, cholic acid and sucrose, exhibited, in comparison to control animals (S), an increase in cholesterol, phospholipid and protein of VLDL, LDL₁ and LDL₂, but triglyceride of the same lipoproteins decreased, as did total plasma triglycerides. Postheparin plasma lipoprotein lipase activity of SC animals was 1.72 times that of S mice. At the same time Intralipid half-life in SC mice was decreased by 52%. Triglyceride secretion rate, after Triton WR 1339 treatment, and liver triglyceride content were reduced in SC animals. HDL mass was decreased in SC mice. Mice (AC) fed a standard diet containing cholesterol showed, in comparison to normal fed animals (A), an increase in cholesterol of VLDL, LDL₁ and LDL₂ but triglyceride of the same lipoproteins decreased as did total plasma triglycerides. Postheparin plasma lipoprotein lipase activity of AC animals was unmodified as was Intralipid half-life. In AC animals triglyceride secretion rate, after Triton WR 1339 treatment, was reduced but in a less extent than in SC mice. Liver triglyceride was unmodified. HDL mass was decreased in AC mice.     

Stimulation of cholesterol esterification in hepatic microsomes by lipoproteins from normal and hypercholesterolemic rabbit serum.

Incubation of plasma lipoproteins with rabbit hepatic microsomes enriched the microsomes with free cholesterol and stimulated cholesterol esterification. The rate of cholesterol esterification correlated well (r = 0.96) with the concentration of microsomal free cholesterol. Lipoproteins from normal and hypercholesterolemic serum varied in their propensity to stimulate cholesterol esterification. Among the normal lipoproteins, low density lipoproteins was more stimulatory than either high density lipoproteins or intermediate density lipoproteins. However, the intermediate density lipoproteins fraction from hypercholesterolemic serum was consistently more stimulatory than any of the normal lipoproteins. The augmentation of cholesterol content, when microsomes were exposed to mixed hyperlipidemic lipoproteins, was proportionately much greater than augementation of phospholipid or protein concentration.     

A comparative study of serum lipoproteins in rabbits fed a natural ingredient diet or low-fat, cholesterol-free, semipurified diets containing casein or isolated soy protein

In rabbits fed a cholesterol-free, semipurified diet containing isolated soy protein, the average total serum cholesterol level was similar to that of rabbits fed a natural ingredient (chow) diet. However, the cholesterol and protein levels in very low density (VLDL) and low density lipoproteins (LDL) tended to increase, while the levels in high density lipoproteins (HDL) were reduced to about half of those on the chow diet, with little change in the cholesterol to protein ratio. Substitution of casein for soy protein in the semipurified diet caused a four- to five-fold increase in total serum cholesterol and a doubling of lipoprotein protein, with an increase of 1.4- to 3.0-fold in the cholesterol to protein ratio of the different lipoprotein fractions. Analysis of the apoproteins (apo) of the plasma lipoproteins indicated that apo B, E, and C all tended to increase in the VLDL and LDL of rabbits fed the soy protein diet compared with those fed chow diet. The levels of each of the apoproteins were increased further by substituting casein for soy protein in the semipurified diet. In this case, apo E showed the greatest relative increase (2.7-fold) in VLDL, while apo B and E were increased to a similar extent (about 4-fold) in LDL. Apo C was approximately doubled in each of these fractions. The apo A content in HDL of rabbits fed the semipurified diets was about half that of rabbits fed chow diet. No marked changes were noted in the apo E or C content of HDL. Separation of isoforms of the soluble apoproteins showed variations between individual animals, but these variations seemed largely unrelated to diet. The results of these studies indicate that semipurified diets produce changes in the serum lipoprotein patterns of rabbits that are only partly due to the protein component of these diets.     

Hypolipidemic effect of pregnancy in the rabbit

New Zealand white rabbits showed large decreases in plasma cholesterol and phospholipid concentrations during the second half of pregnancy. All lipoproteins (very low density, low density, and high density) participated in the decrease. Very large decreases in plasma cholesterol concentrations were observed even when the animals were maintained on high cholesterol diets. Increases in plasma cholesterol concentrations, after the intravenous administration of Triton WR 1339, were at least as great in pregnant as in nonpregnant animals. It is concluded that the decrease in plasma cholesterol concentrations is not the result of impaired plasma lipoprotein production.     

Effect of infusion of "tris-galactosyl-cholesterol" on plasma cholesterol, clearance of lipoprotein cholesteryl esters, and biliary secretion in the rat

As shown by us previously (van Berkel et al. 1985. J. Biol. Chem. 260: 2694-2699 and van Berkel et al. 1985. J. Biol. Chem. 260: 12203-12207) the clearance of both low density lipoproteins (LDL) and high density lipoproteins (HDL) from the blood can be greatly enhanced by pretreatment of these lipoproteins with a tris-galactosylated cholesterol derivative, which makes these particles recognizable by hepatic galactosyl-receptors. Here we report that intravenous infusion of the (water-soluble) tris-galactosyl-cholesterol in rats caused a dose-dependent decrease of the plasma cholesterol level. This fall was sustained long after termination of the infusion. It was not observed upon infusion of tris-glucosyl-cholesterol. The fall in plasma cholesterol was accompanied by an increase in hepatic cholesterol. Upon injection of rat HDL and LDL labeled in their cholesteryl ester moieties, plasma clearance of label in both lipoproteins was enhanced in rats infused with tris-galactosyl-cholesterol, the stimulation being more pronounced when the label was in HDL. The appearance of label in bile was also enhanced in the rats receiving the compound, again more markedly when the label was given as HDL. Ninety four percent or more of the radioactivity excreted in the bile was in the form of bile salts, with conjugated cholate being the major species in both control and treated rats; 6% or less of the radioactivity in the bile was as free cholesterol. Infusion of tris-galactosyl-cholesterol constitutes a new and defined method of lowering plasma lipoprotein levels by enhancing their uptake in the liver.     

Characterization of monoclonal anti-rabbit apolipoprotein E antibodies and chemical composition of lipoproteins separated by anti-apolipoprotein E immuno-affinity chromatography

Six mouse monoclonal antibodies against rabbit apolipoprotein E (apo E) have been developed. Of these monoclonal antibodies, clone 5 revealed a high affinity for purified apo E, very low density lipoprotein (VLDL) and beta-VLDL. This monoclonal antibody was used to prepare an immunoaffinity column. Coupled to Sepharose 4B, this antibody allowed complete removal of lipoproteins containing apo E from plasma of New Zealand white (NZW) rabbits; 62, 46, 14, and 3% of VLDL-, IDL-, LDL-, and HDL-protein, respectively, were bound to the anti-apo E affinity column. The bound VLDL was significantly rich in free cholesterol (FC) and cholesteryl esters (CE) relative to the unbound VLDL, whereas bound IDL, LDL and HDL were significantly rich in FC only. All of the bound fractions were characterized by significantly increased ratios of FC/phospholipids (PL). These results indicate that the two lipoprotein populations with and without apo E have different lipid compositions. The relatively high content of cholesterol in lipoproteins containing apo E suggests a contribution of apo E to plasma cholesterol transport.     

The interaction of lipolysis products of very low density lipoprotein with plasma high density lipoprotein (HDL): perfusate HDL with plasma HDL subfractions

The nature of the interaction of high density lipoproteins (HDL), formed during lipolysis of human very low density lipoprotein (VLDL) by perfused rat heart, with subfractions of human plasma HDL was investigated. Perfusate HDL, containing apoliproproteins (apo) E, C-II, and C-III but no apo A-I or A-II, was incubated with a subfraction of HDL (HDL-A) containing apo A-I and A-II, but devoid of apo C-II, C-III, and E. The products of the incubation were resolved by heparin-Sepharose or hydroxylapatite chromatography under conditions which allowed the resolution of the initial HDL-A and perfusate HDL. The fractions were analyzed for apolipoprotein content and lipid composition and assessed for particle size by electron microscopy. Following the incubation, the apo-E-containing lipoproteins were distinct from perfusate HDL since they contained apo A-I as a major component and apo C-II and C-III in reduced proportions. However, the HDL-A fraction contained apo C-II and C-III as major constituents. Associated with these changes in apolipoprotein composition, the apo-E-rich lipoproteins acquired cholesteryl ester from the HDL-A fraction and lost phospholipid to the HDL-A fraction. The HDL-A fraction maintained a low unesterified cholesterol/phospholipid molar ratio (0.23), while the apo-E-containing lipoproteins possessed a high ratio (0.75) characteristic of the perfusate HDL.(ABSTRACT TRUNCATED AT 250 WORDS)     

Core modification of human low-density lipoprotein by artificial triacylglycerol emulsion

To determine whether an apolipoprotein-free artificial triacylglycerol emulsion can substitute for VLDL in studying cholesterol ester-triacylglycerol exchange processes between triacylglycerol-rich lipoproteins and cholesterol ester-rich lipoproteins, we used Intralipid to modify human plasma LDL. Intralipid was incubated with LDL in the presence of lipoprotein-poor plasma (d greater than 1.21 g/ml) at 37 degrees C. Intralipid served as an acceptor for cholesterol ester and as a donor of triacylglycerol, modifying the low-density lipoproteins so that triacylglycerol became the major core lipid in the particle - the contribution of cholesterol ester to LDL mass decreased from 38% to 18%, while that of triacylglycerol increased from 4.9% to 26%. On lipolysis most added LDL triacylglycerol (59-72%) was hydrolyzed, resulting in a smaller particle than the "native' LDL particle with net loss of cholesterol ester. Incubation of LDL with the original Intralipid emulsion resulted in modified LDL with a high relative weight of phospholipid (27.7%). On removal of excess phospholipid from Intralipid and incubation of the resultant "washed' Intralipid with LDL, the relative weight of phospholipid in modified LDL decreased to 20%, which was similar to that observed after incubation of LDL with VLDL. We demonstrate that artificial triacylglycerol emulsion can indeed substitute for VLDL in neutral lipid exchange processes, and further confirm that transfer of core cholesterol ester and triacylglycerol occurs independently of the apolipoproteins present in triacylglycerol-rich lipoproteins and LDL.     

Investigations on the mechanism of hypercholesterolemia observed in copper deficiency in rats

The mechanism of hypercholesterolemia effect of Cu²⁺ deficiency was studied in rats. There was increased activity of hepatic hydroxymethylglutaryl-coenzyme A reductase and increased incorporation of labelled acetate into free cholesterol of liver in the Cu²⁺ deficient rats. Incorporation of label into ester cholesterol was however decreased in the liver. Concentration of bile acids in the liver was not significantly altered. Increase in the incorporation of labelled acetate into serum cholesterol and increase in the concentration of cholesterol and apo B in the low density lipoproteins + very low density lipoproteins fractions were observed. Activity of lipoprotein lipase of the extrahepatic tissues decreased in the Cu²⁺ deficient rats.     

Effect of dietary cholesterol level on the composition of thoracic duct lymph lipoproteins isolated from nonhuman primates

The effect of two different levels of dietary cholesterol (0.16 mg/Kcal and 0.79 mg/cal) on the composition of thoracic lymph duct lipoproteins was studied in two species of nonhuman primates, Ceropithecus aethiops (African green monkey) and Macaca fascicularis (cynomolgus monkey). Diet was infused intraduodenally at a constant rate to facilitate comparisons among animals. The higher level of dietary cholesterol resulted in an increase in the amount of cholesteryl ester in lymph chylomicrons and VLDL. Cholesteryl oleate was the predominant cholesteryl ester present in lymph d less than 1.006 g/ml lipoproteins and it was the predominant cholesteryl ester formed from exogenous radiolabeled cholesterol. The percentage of saturated and monounsaturated cholesteryl esters in lymph chylomicrons and VLDL significantly increased with the higher dietary cholesterol level. The apoprotein distribution of chylomicrons and VLDL was qualitatively similar during infusions of both diets. The apoprotein B of intestinal chylomicrons and VLDL, termed apoprotein B2, was qualitatively similar during low and high cholesterol diet infusion and was significantly smaller than that of plasma LDL apoB, termed apoprotein B1, as indicated by its electrophoretic mobility in SDS-polyacrylamide gels. The major phospholipid present in lymph chylomicrons and VLDL was phosphatidylcholine and the phospholipid composition of the particles was not affected by diet. Lymph d greater than 1.006 g/ml lipoproteins were separated and the cholesterol mass distribution among lipoprotein fractions was found to be similar during both diet infusions. With an increase in the level of dietary cholesterol, the percentage esterification of cholesterol mass and of exogenous cholesterol radioactivity increased in LDL and HDL from lymph. Lymph LDL and HDL contained less free and esterified cholesterol when their composition was compared to that for these lipoproteins in plasma. We conclude that the primary effect of increased dietary cholesterol level was to increase the cholesteryl ester content of all lymph lipoproteins; cholesterol distribution among lymph lipoproteins was unaffected.     

Effect of human high density lipoproteins, anti-apolipoproteins CII and CIII, and hydrolysis of very low density lipoprotein (VLDL) cholesterol ester on VLDL catabolism in vitro

Lipolysis of human very low density lipoproteins (VLDL) by lipoprotein lipase (LPL) was inhibited in the presence of high density lipoproteins (HDL), anti-apolipoprotein (apo) CII, and by increasing the VLDL free cholesterol content but not with anti-apo CIII or lipoprotein-free plasma. The experiments lend direct evidence that the composition of VLDL and their milieu are important determinants of lipolysis by LPL. Apo CIII may not be critical in LPL mediated VLDL catabolism.     

Effect of Intralipid-infusion on erythrocyte membrane in rabbits

Through Intralipid infusion in rabbits, the phospholipids derived from Intralipid were incorporated into erythrocytes, although Intralipid is mainly composed of triglycerides. This is supported by the increase in oleic acid and the compensatory decrease in linoleic acid of the phospholipids in the erythrocyte membrane, corresponding to the content of linoleic acid in the phospholipids from Intralipid. The excess phospholipid rendered the membrane more fluid, probably by overwhelming the rigidifying effect of the increased cholesterol content. Furthermore, the shape of erythrocytes was changed from biconcave to spur, dose dependently. The morphological alterations in erythrocyte membranes could not be completely elucidated by the changes in lipid. These results suggested that the alteration in lipid metabolism in Intralipid-infused rabbits caused various effects on the erythrocyte membrane, through the elevation of triglyceride, cholesterol, and phospholipid contents in plasma.     

Influence of probucol on cholesterol and lipoprotein metabolism in man

The mechanisms for the hypocholesterolemic action of probucol were examined in 17 patients with various levels of plasma cholesterol and triglycerides (TG). All the patients were studied on a metabolic ward. The first period of 6 weeks was for control. Thereafter, probucol was started, and after 2-6 months of drug treatment, the patients were readmitted for another 6-week period for a repeat study. During treatment with probucol, the cholesterol decreased in total plasma by an average of 12%, in low density lipoproteins (LDL) by 11%, and in high density lipoproteins (HDL) by 9%. The TG in total plasma and in very low density lipoproteins (VLDL) remained unchanged during probucol treatment. Turnover of low density lipoprotein apoprotein (apoLDL) was estimated following injection of 125I-labeled apoLDL. Probucol increased the fractional catabolic rate (FCR) for apoLDL by an average of 23%, but did not change apoLDL synthesis. The drug produced no consistent changes in fecal excretion of cholesterol (neutral steroids) and bile acids, in cholesterol absorption, in lipid composition of gallbladder bile, in biliary secretion of cholesterol and bile acids, or in the activities of lipoprotein lipase and hepatic lipase. These data show that probucol lowers LDL by increasing its catabolism. This effect appears to be independent of any changes in metabolism of cholesterol or bile acids.     

Synthesis and secretion of lipoproteins by human hepatocytes in culture

Summary Confluent monolayers of normal human hepatocytes obtained by collagenase perfusion of liver pragments were incubated in a serum-free medium. Intracellular apolipoproteins apo AI, apo C, apo B, and apo E were detected between Day 1 and Day 6 of the culture by immunoenzymatic staining using polyclonal antibodies directed against these apoproteins and monoclonal antibodies directed against both forms of apo B (B100 and B48). Translation of mRNA isolated from these hepatocytes in an acellular system revealed that apo AI and apo E were synthesized as the precusor forms of mature plasma apo AI and apo E. Three lipoprotein fractions corresponding to the density of very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) were isolated from the medium at Day 5 of culture and examined by electron microscopy after negative staining. VLDL and LDL particles are similar in size and shape to plasma lipoproteins; spherical HDL are larger than normal plasma particles isolated at the same density. Their protein represented 44, 19.5, and 36.5% respectively, of the total lipoprotein protein. The secretion rate of VLDL protein corresponded to that measured in primary cultures of rat hepatocytes. After incorporation of [³H]glycerol, more than 92% of the [³H]triglyceride secreted into the medium was recovered in the VLDL fraction. These results demonstrate that primary cultures of normal human hepatocytes are able to synthesize and secrete lipoproteins and thus could be a useful model to study lipoprotein metabolism in human liver.     

In vitro regulation of cholesterol metabolism by low density lipoproteins in skin fibroblasts from hypo-and hyperresponding squirrel monkeys.

When squirrel monkeys (Saimiri sciureus) are fed diets containing cholesterol, some individuals (hyperresponders) become hypercholesterolemic, while others (hyporesponders) are able to maintain nearly normal plasma cholesterol concentrations. Skin fibroblasts were grown from three hyperresponder and threehyporesponder squirrel monkeys, previously characterized on the basis of their plasma cholesterol response to two cholesterol-containing diets and the pheno-type of their parents. The rates of cholesterol synthesis and esterification were determined in the cultured fibroblasts incubated with low density lipoproteins isolated from normocholesterolemic squirrel monkeys or hypercholesterolemic rhesus monkeys. Both lipoprotein preparations influenced the metabolic parameters measured in a similar manner in cells from both hypo- and hyperresponder animals. Exposure of skin fibroblasts to low density lipoproteins resultd in a stimulation of cholesterol esterification and a suppression of cholesterol synthesis in cells from both hypo- and hyperresponder animals. When incubated with increasing concentrations of low density lipoprotein cholesterol, up to 50 microgram/ml, fibroblasts from both hypo-and hyperresponding animals responded with a similar maximum percentage suppression of sterol synthesis. Thus, hyperresponsiveness to dietary cholesterol in squirrel monkeys, although a heritable characteristic, is not associated with an inability of low density lipoprotein to suppress cholesterol synthesis or stimulate cholesterol esterification as occurs in familial hypercholesterolemia in man.     

Remodeling of human plasma lipoproteins by detergent perturbation

Detergent perturbation, the treatment of total human plasma lipoproteins (TLP) with sodium cholate and its subsequent removal, has been used to study lipoprotein dynamics and stability. At physiological TLP concentrations, detergent perturbation converts low-density lipoproteins (LDL) and high-density lipoproteins (HDL) to higher-particle weight species with the concomitant release of apo A-I but not apo A-II as a lipid-poor species. Detergent perturbation of isolated HDL also releases lipid-poor apo A-I and forms larger HDL species, whereas detergent perturbation of an isolated LDL has no effect on its size. A model is presented in which detergent perturbation induces transfer of PC from metastable HDL and LDL to mixed micelles with sodium cholate. The remaining LDL and HDL are unstable because of the loss of their surface components, phospholipid and/or apo A-I, and fuse to give larger LDL and HDL particles. These effects on HDL, i.e., PC transfer, apo A-I dissociation, and particle fusion, emulate the activity of human plasma phospholipid transfer protein. Thus, detergent perturbation is a new and potentially powerful method for determining lipoprotein stability, studying the mechanisms for remodeling of plasma lipoproteins, and preparing new forms of HDL and LDL with unique interactions with lipoprotein transporters and receptors.     

Lipoprotein affinity of human apolipoprotein A-IV during cholesterol esterification

We have studied the lipoprotein distribution of human apo A-IV during cholesterol esterification by the action of endogenous lecithin-cholesterol acyltransferase. Using immunologic and radiotracer techniques at 4 degrees C, apo A-IV was found in two discrete monomeric and dimeric populations, unassociated with plasma lipoproteins. With incubation at 37 degrees C, apo A-IV initially associated with the high density lipoprotein-3 fraction, but thereafter dissociated from its surface, and reappeared as unbound protein and in association with a complex in the low density lipoprotein size range. Inclusion of LCAT inhibitors in the incubations abolished these changes. We conclude that the changes in lipoprotein distribution of human apo A-IV closely parallel the formation and exchange of plasma cholesteryl esters.