Interconversion of high density lipoproteins during incubation of human plasma

Human apolipoprotein A-II inhibits the formation of pre-beta high density lipoproteins

The role of human apolipoprotein A-II (apoA-II) in the remodeling of human high density lipoproteins (HDL) was investigated during incubation of native and reduced-carboxamidomethylated (RCM) HDL3 with a lipoprotein-depleted plasma fraction (LPDP) in the presence of triglyceride-rich particles (TGRP) isolated from Intralipid. Reduction-carboxamidomethylation of HDL3 entirely converts the disulfide-linked apoA-II dimers into monomers, without affecting the structure, composition and particle size distribution of HDL3. Following incubation with LPDP and TGRP, unmodified HDL3 are mainly converted into large, HDL2 particles (diameter: 9.90 +/- 0.07 nm), enriched in triglycerides and depleted of cholesteryl esters. RCM-HDL3 are converted into both large HDL2 (9.86 +/- 0.07 nm) and small (7.53 +/- 0.06 nm) HDL3. The small products are protein-rich and cholesterol-poor, and consist of two different particles: a component with pre-beta mobility, containing only apoA-I, and a component with alpha mobility, containing both apoA-I and apoA-II. Kinetic studies suggest that a two-step process is involved in the formation of small, pre beta-HDL3, by which changes in lipid composition cause alterations in lipoprotein structure/stability, favoring the dissociation of apolipoproteins and reduction of particle size. These findings indicate that apolipoprotein structure is a major determinant of HDL remodeling, apoA-II potentially counteracting the anti-atherogenic properties of apoA-I by inhibiting the formation of small, pre-beta-migrating HDL.

Triglycerides are major determinants of cholesterol esterification/transfer and HDL remodeling in human plasma

Lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) are responsible for the esterification of cell-derived cholesterol and for the transfer of newly synthesized cholesteryl esters (CE) from HDL to apoB-containing lipoproteins in human plasma. LCAT and CETP are also crucial factors in HDL remodeling, a process by which HDL particles with a high capacity for cell cholesterol uptake are generated in plasma. In the present study, cholesterol esterification and transfer were evaluated in 60 patients with isolated hypercholesterolemia (HC, n = 20) and isolated (HTG, n = 20) or mixed hypertriglyceridemia (MHTG, n = 20) and in 20 normolipidemic healthy individuals (NL). Cholesterol esterification rate (CER) and net CE transfer rate (CETR) were measured in whole plasma. LCAT and CETP concentrations were determined by specific immunoassays. HDL remodeling was analyzed by monitoring changes in HDL particle size distribution during incubation of whole plasma at 37 degrees C. Mean CER and CETR were 48% and 73% higher, respectively, in hypertriglyceridemic (HTG + MHTG) versus normotriglyceridemic individuals. HDL remodeling was also significantly accelerated in plasma from hypertriglyceridemic patients. Strong positive correlations were found in the total sample between plasma and VLDL triglyceride levels and CER (r = .722 and r = .642, respectively), CETR (r = .510 and r = .491, respectively), and HDL remodeling (r = .625 and r = .620, respectively). No differences in plasma LCAT and CETP concentrations were found among the various groups except for a tendency toward higher CETP levels in hypercholesterolemic patients (+51% in MHTG and +20% in HC) versus control subjects (NL). By stepwise regression analysis, VLDL triglyceride level was the sole significant predictor of CER and CETR and contributed significantly together with baseline HDL particle distribution to HDL remodeling. These results indicate that plasma triglyceride level is a major factor in the regulation of cholesterol esterification/transfer and HDL remodeling in human plasma, whereas LCAT/CETP concentrations play a minor role in the modulation of reverse cholesterol transport.

Effects of low-fat diet, calorie restriction, and running on lipoprotein subfraction concentrations in moderately overweight men

We studied the effects of exercise (primarily running), calorie restriction (dieting), and a low-fat, high-carbohydrate diet on changes in lipoprotein subfractions in moderately overweight men in a randomized controlled clinical trial. After 1 year, complete data were obtained for 39 men assigned to lose weight through dieting without exercise, 37 men assigned to lose weight through dieting with exercise (primarily running), and 40 nondieting sedentary controls. We instructed both diet groups to consume no more than 30% total fat, 10% saturated fat, and 300 mg/d of cholesterol, and at least 55% carbohydrates, and the controls were instructed to maintain their usual food choices. Analytic ultracentrifugation was used to measure changes in plasma lipoprotein mass concentrations. In addition, the absorbance of protein-stained polyacrylamide gradient gels was used as an index of concentrations for five high-density lipoprotein (HDL) subclasses that have been identified by their particle sizes, ie, HDL3c (7.2 to 7.8 nm), HDL3b (7.8 to 8.2 nm), HDL3a (8.2 to 8.8 nm), HDL2a (8.8 to 9.7 nm), and HDL2b (9.7 to 12 nm). Relative to controls, weight decreased significantly in men who dieted with exercise (net difference +/- SE, -3.3 +/- 0.4 kg/m2) and in men who dieted without exercise (-2.0 +/- 0.4 kg/m2). Dieting with exercise significantly decreased very-low-density lipoprotein (VLDL)-mass concentrations and significantly increased plasma HDL2-mass, HDL3a, HDL2a, and HDL2b relative to both control and dieting without exercise. There were no significant changes in lipoprotein mass and HDL protein for dieters who did not run.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of plasma cholesteryl ester transfer activity on the LDL and HDL distribution profiles in normolipidemic subjects

The relations of cholesteryl ester transfer protein (CETP) activity to the distribution of low density lipoproteins (LDLs) and high density lipoproteins (HDLs) were investigated in fasting plasma samples from 27 normolipidemic subjects. LDL and HDL subfractions were separated by electrophoresis on 20-160 g/L and 40-300 g/L polyacrylamide gradient gels, respectively. Subjects were subdivided into two groups according to their LDL pattern. Monodisperse patterns were characterized by the presence of a single LDL band, whereas polydisperse patterns were characterized by the presence of several LDL bands of different sizes. To investigate the influence of lipid transfers on LDL patterns, total plasma was incubated at 37 degrees C in the absence of lecithin:cholesterol acyltransferase (LCAT) activity. The incubation induced a progressive transformation of polydisperse patterns into monodisperse patterns. Under the same conditions, initially monodisperse patterns remained unchanged. Measurements of the rate of radiolabeled cholesteryl esters transferred from HDL3s to very low density lipoproteins (VLDLs) and LDLs revealed that subjects with a monodisperse LDL pattern presented a significantly higher plasma CETP activity than subjects with a polydisperse LDL pattern (301 +/- 85%/hr per milliliter versus 216 +/- 47%/hr per milliliter, respectively; p < 0.02). In addition, when total plasma was incubated for 24 hours at 37 degrees C in the absence of LCAT activity, the relative mass of cholesteryl esters transferred from HDLs to apolipoprotein B-containing lipoproteins was greater in plasma with monodisperse LDL than in plasma with polydisperse LDL (0.23 +/- 0.06 versus 0.17 +/- 0.06, respectively; p < 0.02). These results indicated that in normolipidemic plasma, CETP could play an important role in determining the size distribution of LDL particles. The analysis of lipoprotein cholesterol distribution in the two groups of subjects sustained this hypothesis. Indeed, HDL cholesterol levels, the HDL:VLDL+LDL cholesterol ratio, and the esterified cholesterol:triglyceride ratio in HDL were significantly lower in plasma with the monodisperse LDL pattern than in plasma with the polydisperse LDL pattern (p < 0.01, p < 0.01, and p < 0.02, respectively). Plasma LCAT activity did not differ in the two groups. Plasma CETP activity correlated positively with the level of HDL3b (r = 0.542, p < 0.01) in the entire study population. Whereas plasma LCAT activity correlated negatively with the level of HDL2b (r = -0.455, p < 0.05) and positively with the levels of HDL2a (r = 0.475, p < 0.05) and HDL3a (r = 0.485, p < 0.05), no significant relation was observed with the level of HDL3b.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of phosphatidylethanolamine-containing phospholipid-apolipoprotein complexes modified by lecithin-cholesterol acyltransferase

The effect of the inclusion of phosphatidylethanolamine (PE), a phospholipid with unusual packing properties, on the substrate properties of protein-lipid complexes toward lecithin-cholesterol acyltransferase (LCAT) has been studied. Recombinant particles of apolipoprotein A-I with dimyristoylphosphatidylcholine (DMPC), dilauroylphosphatidylethanolamine (DLPE) and cholesterol were prepared at a molar ratio of 1:140:14 (A-I/DMPC/cholesterol) or 1:70:70:14 (A-I/DMPC/DLPE/cholesterol); the efficiency of cholesterol incorporation into complexes containing phosphatidylethanolamine was found to be very pH-dependent, with enhanced cholesterol incorporation at elevated pH values. By incubating the complexes with either purified human LCAT or the d greater than 1.21 g/ml fraction of rat serum as a source of LCAT activity, it was found that a high degree of cholesterol esterification could be achieved with either complex; however, the DLPE-containing complex possessed a much smaller Stokes' diameter than the DMPC-only particle despite compositional similarities between these complexes. With respect to particle diameter the DLPE-containing particles behaved more like complexes prepared with egg yolk lecithin than did complexes prepared with DMPC alone. When human LDL was added to the incubations to provide a source of additional cholesterol, the products were markedly different. Concomitant with an increased cholesteryl ester core was an increase in the protein stoichiometry in both types of particles, from 2 to 3 or 4 apo A-I per particle. The proportion of DLPE to DMPC in the products was reduced from 1:1 to 0.3:1, reflecting a preferential hydrolysis of PE by LCAT, and the Stokes' diameters of the DMPC-only and the DLPE-containing complexes were closely similar. We conclude that the presence of elevated proportions of certain phospholipid species may significantly alter both the physical properties of the particles and their substrate properties with regard to reactions with enzymes of lipid metabolism.

Synergistic effects of lipid transfers and hepatic lipase in the formation of very small high-density lipoproteins during incubation of human plasma

Studies have been performed to determine the involvement of very-low-density lipoproteins (VLDL), cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) in the formation of very small HDL particles. Human whole plasma has been incubated for 6 h at 37 degrees C in the absence and in the presence of various additions. There was minimal formation of very small HDL in incubations of non-supplemented plasma or in plasma supplemented with either VLDL, CETP or HL alone; nor were small HDL prominent after incubating plasma supplemented with mixtures of VLDL plus CETP, VLDL plus HL or CETP plus HL. By contrast, when plasma was supplemented with a mixture containing all three of VLDL, CETP and HL, incubation resulted in an almost total conversion of the HDL fraction into very small particles of radius 3.7 nm. The appearance of these very small HDL was independent of activity of lecithin: cholesterol acyltransferase. It was, however, dependent on both duration of incubation and on the concentrations of the added VLDL, CETP and HL. The effects of these incubations was also assessed in terms of changes to the concentration and distribution of lipid constituents across the lipoprotein spectrum. It was found that not only did lipid transfers and HL exhibit a marked synergism in promoting a reduction in HDL particle size but also that HL, although deficient in intrinsic transfer activity, enhanced the CETP-mediated transfers of cholesteryl esters from HDL to other lipoprotein fractions.

Effects of heparin-induced lipolytic activity on the structure of rat high-density lipoprotein

Following its secretion into the plasma compartment, the high-density lipoprotein (HDL) is presumed to be acted upon by both soluble enzymes, such as lecithin:cholesterol acyltransferase (LCAT), and membrane-associated enzymes, such as lipoprotein lipase and hepatic lipase. Rats were injected intravenously with heparin to release membrane-associated lipolytic activities into the circulation and the collected plasma was incubated overnight at 37 degrees C in the presence or absence of an LCAT inhibitor or an inhibitor of lipoprotein lipase (1 M NaCl). It was observed that lipoprotein lipase accounted for most of the triglyceride hydrolase activity in the heparin-treated plasma, and that the heparin-releasable activities caused an increase in HDL density but no measurable change in particle size when LCAT was inhibited. Heparin treatment caused about a 60% decrease in plasma triacylglycerol during the interval between injection of heparin and blood collection. Although this caused marked compositional changes in the d less than 1.063 g/ml lipoproteins, no changes were observed in the lipid composition or apoprotein distribution in the HDL. Subsequent incubation for 18 h at 37 degrees C produced marked increases in the apoE content of HDL from heparin-treated plasma even when LCAT was inhibited. Time-course studies showed that in the presence of an LCAT inhibitor there was considerable conversion of phosphatidylcholine to lysophosphatidylcholine in heparin-treated plasma, and that this activity was diminished by 1 M NaCl, but that no phospholipolysis was observed in control plasma. By contrast, both heparin-treated and control plasma possessed substantial triglyceride hydrolase activity. The concurrent action of lipases and LCAT was observed to reduce the maximum level of cholesterol esterification which could be achieved in the absence of lipase activity. It is concluded that changes in HDL particle size are mainly attributable to LCAT, but that lipase activities, which are either free in rat plasma or releasable by heparin, play a role in restructuring the phospholipid moiety and altering the protein composition of the HDL, especially with respect to apoE, a potential ligand to cellular receptors.

Effect of lipid transfer protein on plasma lipids, apolipoproteins and metabolism of high-density lipoprotein cholesteryl ester in the rat

The role of human plasma lipid transfer protein (LTP) in lipoprotein metabolism was studied in the rat, a species without endogenous cholesteryl ester and triacylglycerol transfer activity. Partially purified human LTP was injected intravenously into rats. The plasma activity was between 1.5- and 4-fold that of human plasma during the experiments. 6 h after the injection of LTP, a significant increase in serum apoB, and no significant changes in serum total cholesterol, free cholesterol, triacylglycerols, apoA-I, apoE, or apoA-IV were noted. Cholesterol was increased in very-low density and low-density lipoproteins (VLDL and LDL) and decreased in large-sized apoE-rich HDL. ApoA-I-containing particles with a size smaller than in normal rats were present in serum of LTP-treated rats. The mean diameter of HDL particles decreased and apoE, normally present on large-sized HDL, was present on smaller sized particles. The metabolic fate of cholesteryl ester, originally associated with HDL, was studied by injection of [3H]cholesteryl linoleyl ether-labelled apoA-I-rich HDL in the absence and in the presence of LTP. The disappearance of [3H]cholesteryl linoleyl ether, injected as part of apoA-I-rich HDL, from serum was increased in the LTP-treated rats; the t1/2 changed from 3.9 to 2.2 h, resulting in an increased accumulation of [3H]cholesteryl linoleyl ether in the liver. This can be explained by the redistribution of HDL [3H]cholesteryl linoleyl ether to VLDL and LDL in the presence of LTP, leading to the combined contribution of VLDL, LDL and HDL to the hepatic uptake. The present findings show profound effects of LTP on the chemical composition of HDL subspecies, the size of HDL and on the plasma turnover and hepatic uptake of cholesteryl esters originally present in apo A-I-rich HDL.

Conversion of apolipoprotein-specific high-density lipoprotein populations during incubation of human plasma

Incubation studies were performed on plasma obtained from subjects selected for relatively low levels of high-density lipoprotein cholesterol (HDL-C) (no greater than 30 mg/dl) and particle size distributions enriched in the HDL3 subclass. Incubation (12 h, 37 degrees C) of plasma in the presence or absence of lecithin: cholesterol acyltransferase activity produces marked alteration in size profiles of both major apolipoprotein-specific HDL3 populations (HDL3(AI w AII), HDL3 species containing both apolipoprotein A-I and apolipoprotein A-II, and HDL3(AI w/o AII), HDL3 species containing apolipoprotein A-I) as isolated by immunoaffinity chromatography. In the presence or absence of lecithin: cholesterol acyltransferase activity, plasma incubation results in a shift of HDL3(AI w AII) species (initial mean sizes of major components, approx. 8.8 and 8.0 nm) predominantly to larger particles (mean size, 9.8 nm). A less prominent shift to smaller particles (mean size, 7.8 nm) accompanies the conversion to larger particles only when the enzyme is active. Combined shifts to larger (mean size, 9.8 nm) and smaller (mean size, 7.4 nm) particles are observed for HDL3(AI w/o AII) particles (mean size, 8.3 nm) also only in the presence of enzyme activity. However, in the absence of enzyme activity, HDL3(AI w/o AII) species, unlike the HDL3(AI w AII) species, are converted to smaller (mean size 7.4 nm) rather than to larger particles. Like native HDL2b(AI w/o AII) particles, the larger HDL3(AI w/o AII) conversion products exhibit a protein moiety with molecular weight equivalent to four apolipoprotein A-I molecules per particle; small HDL3(AI w/o AII) products are comprised predominantly of particles with two apolipoprotein A-I per particle. Incubation-induced conversion of HDL3 particles in the presence of lecithin: cholesterol acyltransferase activity is associated with increased binding of both apolipoprotein-specific HDL populations to low-density lipoproteins (LDL). The present studies indicate that, in the absence of lecithin: cholesterol acyltransferase activity, the two HDL3 populations follow different conversion pathways, possibly due to apolipoprotein-specific activities of lipid transfer protein or conversion protein in plasma. Our studies also suggest that lecithin: cholesterol acyltransferase activity may play a role in the origins of large HDL2b(AI w/o AII) species in human plasma by participating in the conversion of HDL3(AI w/o AII) particles, initially with three apolipoprotein A-I, to larger particles with four apolipoprotein A-I per particle.

Isolation of a high-density-lipoprotein conversion factor from human plasma. A possible role of apolipoprotein A-IV as its activator

1. A high-density-lipoprotein (HDL) conversion factor was partially purified from human plasma by precipitation with (NH4)2SO4, ultracentrifugation, cation-exchange chromatography, anion-exchange chromatography and chromatography on a column of hydroxyapatite. 2. This factor modulates the particle size of HDL by converting a homogeneous population into new populations of particles, some of which are smaller and others larger than those in the original population. 3. The isolated HDL conversion factor appeared as one major band and at least three minor bands on SDS/polyacrylamide-gel electrophoresis; attempts to purify this factor further resulted in loss of conversion activity. 4. Preparations of the HDL conversion factor were stable after heating to 58 degrees C for 1 h, and were shown not to possess proteolytic activity. 5. The conversion factor was distinct from the known apolipoproteins, none of which had HDL conversion activity. 6. Addition of apolipoprotein A-IV had a dose-dependent potentiating effect on the process promoted by the HDL conversion factor.

Alterations in the HDL system after rapid plasma cholesterol reduction by LDL-apheresis

Changes in high density lipoprotein (HDL) subfraction structure and composition were analyzed during and after extracorporeal removal of apo B containing lipoproteins in seven familial hypercholesterolemic (FH) patients. After the apheretic procedure, carried out with dextran-sulfate-cellulose columns, the plasma levels of very low density lipoproteins (VLDL), low density lipoproteins (LDL), and HDL decreased by 72%, 50%, and 19%, respectively. The free cholesterol to esterified cholesterol ratio in plasma increased, with a 26% drop in the lecithin:cholesterol acyl transferase (LCAT) activity. In the ensuing 24 hours, VLDL, HDL, and LCAT activity approached the pretreatment levels. During this phase, possibly as a consequence of increased cholesterol esterification and exchange of cholesteryl esters for triglycerides between HDL and VLDL, HDL2a particles were detected in plasma. However, these metabolic changes did not result in clearcut modifications in the HDL2-HDL3 subfraction distribution. These findings clearly demonstrate that rapid changes in the plasma VLDL-LDL levels affect several processes involved in the HDL metabolism, but confirm that the HDL system, in spite of a considerable plasticity, displays a marked stability of the HDL2-HDL3 subfraction distribution.

Changes in high-density lipoprotein subfraction distribution and increased cholesteryl ester transfer after probucol

The effects of probucol (500 mg twice daily) on high-density lipoprotein (HDL) subfractions and cholesteryl ester transfer from HDL to lower density lipoproteins were tested in a series of patients with Type II hypercholesterolemia. In this placebo-controlled crossover trial, patients received probucol or placebo for 8 weeks, then switched to the other agent for another 8 weeks. Probucol significantly lowered total, low-density lipoprotein and HDL cholesterol levels. HDL subfractions, separated by rate zonal ultracentrifugation, showed a dramatic reduction in HDL2, whereas changes in HDL3 were not significant. Both subfractions eluted at a characteristically lower volume, indicating a reduced flotation rate. These findings were confirmed by gradient gel electrophoretic separation, which showed a typical reduction or disappearance of HDL2b particles and the prevalence of particles in the HDL3a-HDL3b electrophoretic range in almost all patients. After treatment, cholesteryl ester transfer from HDL to lower density lipoproteins was significantly increased in all patients. These data suggest that probucol may accelerate HDL particle conversion, leading to improvement in reverse cholesterol transport from the periphery to the liver, through HDL and very low density lipoprotein.

Normalization of high density lipoprotein in fish eye disease plasma by purified normal human lecithin: cholesterol acyltransferase

Plasma from a patient with fish eye disease has been enriched with autologous high density lipoproteins (HDL) and supplemented with highly purified normal human plasma lecithin:cholesterol acyltransferase (LCAT). Incubation of such plasma at 37 C in vitro resulted in normalization of its low HDL cholesteryl ester percentage, from 23% to 79%, associated with a two-fold increase in both the cholesteryl ester and triglyceride contents of the HDL fraction, as compared to incubation experiments with absent or heat-inactivated purified normal LCAT. The normalization of the HDL cholesteryl ester percentage induced by incubation with purified normal LCAT also was accompanied by an increase in the size of the original fish eye disease HDL particles, which had a mean mass of 115 kd, to HDL particle populations with mean particle masses ranging from 130-220 kd, depending on the concentration of purified LCAT in the incubate. Both HDL cholesterol esterification and particle enlargement were abolished completely by the LCAT inhibitor DTNB and by heat inactivation of the purified normal LCAT. The results give further evidence that fish eye disease is an alpha-LCAT deficiency.

Structural modifications of rat serum high density lipoprotein by pancreatic phospholipase A2

An important and unusual aspect of the high density lipoprotein (HDL) in the rat is its tendency to undergo marked alterations in structure in response to physiological perturbations. In this study, the role of the surface lipids for maintenance of HDL integrity were investigated. Hydrolysis by pancreatic phospholipase A2 of the phospholipids of rat HDL in the presence of the d greater than 1.21 g/ml fraction of rat serum results in an increase in the particle diameter and an uptake of apo-E and apo A-IV from the lipoprotein-free fraction; augmentation of the albumin concentration in the incubation mixture intensified the observed changes, probably due to enhancement of the compositional changes brought about by phospholipase treatment. Phospholipase A2, treatment of the d less than 1.21 g/ml fraction of rat serum produces only minor changes in the properties of the isolated HDL. These data suggest that changes in apoprotein content reflect an uptake of A-IV and E by the rat HDL, rather than a net loss of apo A-I. Likewise, titration of the action of pancreatic phospholipase A2 on HDL apoprotein composition showed that initially a modest increase in apo A-IV content occurred, but with more extensive phospholipolysis there was a considerably greater increase in the apo-E content of the particle. The data suggest that hydrolysis of phospholipids such as occurs physiologically through the action of lecithin:cholesterol acyl transferase and hepatic lipase may alter the HDL structure independently from changes effected in the neutral lipid core.

Lipoproteins and atherosclerosis

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extrahepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL. These modifications of plasma LDL may be brought about following perturbation of one of several aspects of LDL metabolism. These include decreased LDL receptor activity, increased VLDL production and cholesterol enrichment of the liver-derived VLDL. The events in the arterial wall that make some LDL particles apparently atherogenic are not well understood. In the case of nonhuman primates, large-size LDL are associated with an increased risk of CHD. One characteristic of these LDL is that their core lipids are rich in saturated cholesteryl esters and their transition temperatures are frequently above body temperature. The liquid crystalline cholesteryl ester cores of such LDL may modulate the conformation of apo B on the surface and thereby affect the interaction of these LDL with cellular receptors or connective tissue matrix proteoglycans. It is likely, though, that changes in LDL particle number, LDL particle size and LDL particle composition may each contribute to progression of atherosclerosis. The presumed metabolic events that make HDL protective against atherosclerosis have been termed reverse cholesterol transport, and suggest that small HDL that are deficient in free cholesterol acquire this lipid from cell membranes. The HDL cholesterol is esterified by LCAT in the circulation, forming large HDL that can then deliver the cholesteryl ester to the liver by both direct and indirect means. In most circumstances, it is assumed that an increase in plasma HDL cholesterol concentration reflects an increase in the rate at which HDL is removing cholesterol from tissues and, consequently, a decrease in atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Apolipoprotein A-IMilano. Correlation between high density lipoprotein subclass distribution and triglyceridemia

Carriers of the apolipoprotein A-IMilano (apo A-IM) variant represent a selected group of subjects showing low levels of high density lipoprotein (HDL), variable hypertriglyceridemia, and low prevalence of atherosclerotic vascular disease. The distribution of HDL subfractions and the correlation with abnormalities in triglyceride transport were determined in these subjects. Sera from 24 apo A-IM carriers (A-IM+ and from age- and sex-matched normolipidemic controls (A-IM-) were analyzed by rate zonal ultracentrifugation. The A-IM+ subjects showed a marked decrease of HDL3 mass with reduced flotation rates and major compositional alterations; the HDL2 were nearly absent. The HDL subclasses from 10 A-IM+ subjects were resolved according to particle size by gradient gel electrophoresis (GGE). The HDL patterns detected in the carriers were unique in exhibiting a distinct peak in the (HDL3b)gge interval, undetectable in the controls. Three patterns reflecting the relative contributions of smaller (HDL3b)gge and larger (HDL3a)gge particles could be distinguished in the carriers, and these were clearly related to different triglyceride and HDL cholesterol levels in plasma. These findings in a highly selected group of subjects with generally low HDL levels and quite variable triglyceridemia confirmed the existence of relationships between alterations in triglyceride transport and abnormalities in the HDL subclass distribution, possibly reflecting the variable atherosclerotic risk in hypertriglyceridemic subjects.

Net lipid transfer between lipoproteins in fish-eye disease plasma supplemented with normal high density lipoproteins

Native fish-eye disease plasma, which is deficient of both high density lipoproteins (HDL) and lecithin-cholesterol acyltransferase activity (alpha-LCAT), processing the free cholesterol of these lipoproteins, has been supplemented with normal isolated HDL2 or HDL3 and incubated in vitro at 37 C. After incubation for 0, 7.5 and 24 hr the very low density (VLDL) and low density (LDL) lipoproteins as well as HDL were isolated, and their contents of triglycerides, phospholipids and free, esterified and total cholesterol were quantified. The resulting net mass transfer of the different lipids revealed a functioning transfer of cholesteryl esters and all other analyzed lipids between the lipoproteins, although no de novo esterification of the HDL cholesterol by LCAT in this plasma occurred. In accordance with previous findings there was a functioning esterification process of the free cholesterol of the combined VLDL and LDL of fish-eye disease plasma. The present reports make it reasonable to conclude that the lack of HDL cholesterol esterification in this disease is not a result of a deficiency of cholesteryl ester transfer or lipid transfer activities.

Human plasma lipid transfer protein catalyzes the speciation of high density lipoproteins

The role of purified plasma lipid transfer protein complexes in determining the particle size distribution of human plasma high density lipoproteins (HDL) was examined in vitro. Incubation of HDL2 or HDL3, isolated from normolipemic subjects with very low density lipoproteins (VLDL) or VLDL-remnants and lipid transfer protein complex had little or no effect on HDL particle size. In contrast, HDL isolated from patients with hypertriglyceridemia, designated HDL3D, showed speciation of particle size distribution when incubated with VLDL-remnants and the transfer protein. Incubation of HDL3D with VLDL-remnants and lipid transfer complex resulted in the production of two particles of radius 4.3 and 3.7 nm; incubation with VLDL or in the absence of the transfer protein did not result in a redistribution of particle size. We suggest that the action of lipid transfer protein complex on triacylglycerol-rich lipoprotein remnants and HDL accounts for the low levels of HDL-cholesterol observed in subjects with severe hypertriglyceridemia.

In vitro formation of HDL-2 from HDL-3 and triacylglycerol-rich lipoproteins by the action of lecithin:cholesterol acyltransferase and cholesterol ester transfer protein

In order to study the factors responsible for the formation of high-density lipoprotein subfraction-2 (HDL-2), very-low-density lipoproteins (VLDL) and HDL-3 were mixed and incubated with purified bovine milk lipoprotein lipase, human serum lecithin:cholesterol acyltransferase, cholesteryl ester transfer protein and mixtures thereof. The results can be summarized as follows: Incubation of HDL-3 and VLDL for 24 h at 37 degrees C without enzymes did not cause any significant change in the protein:lipid ratio or in the flotation constant of the HDL. Cholesteryl ester transfer protein treatment caused only an exchange of part of the HDL cholesteryl esters with VLDL triacylglycerols. Lipoprotein lipase caused a slight shift of HDL-hydrated density to lower values; HDL-2b, however, was not formed. Incubation of HDL-3 and VLDL with lecithin:cholesterol acyltransferase or mixtures of lecithin:cholesterol acyltransferase and lipoprotein lipase reduced the HDL-protein:lipid ratio and increased the HDL-flotation rate. The newly formed HDL resembled that of native HDL-2a. The incubation of HDL-3 and VLDL with lecithin:cholesterol acyltransferase and cholesteryl ester transfer protein caused a shift of the HDL-3 into an HDL-2b-like fraction. Particles resembling HDL-2b in the analytical ultracentrifuge were also formed if VLDL + HDL-3 were incubated with lipoprotein lipase or lipoprotein lipase + cholesteryl ester transfer protein in a medium containing low amounts of albumin, insufficient for binding all liberated fatty acids during hydrolysis. The incubation of mixtures of HDL-3 and chylomicrons enriched with apoAI in the presence of lecithin:cholesterol acyltransferase and cholesteryl ester transfer protein caused the formation of HDL-2-like particles which resembled those of native HDL-2 also with respect to the apoAI/AII ratio.

The role of LCAT and cholesteryl ester transfer proteins for the HDL and LDL structure and metabolism

Since the early detection of LCAT by Glomset, a great deal of research has been conducted for establishing the role of this enzyme in the Lp metabolism. It became apparent that LCAT produces more than 3/4 of the CE found in plasma. LCAT acts primarily on Lp with high PC/FC contents and high PC/FC ratios. For expression of the full activity, apo-Lp cofactors such as apo-AI, -CI, -AIV and -E are necessary. Although it was believed for a long time that HDL are the only substrate for LCAT we could demonstrate that LDL and even apoA/C/E free LpB is utilized by LCAT. The CE formed in PC/FC rich Lp are transferred to VLDL and LDL by specific proteins, which also promote the exchange of CE against TG. TG in these particles are hydrolyzed by liver lipase providing new space in the core for further cholesterol esterification. Thus LCAT exerts its physiological role in concert with other enzymes e.g. LPL, hepatic lipase and possibly phospho-lipases as well as with exchange and transfer processes partly catalyzed by specific exchange/transfer proteins. The main function of LCAT without doubt is the reverse cholesterol transport from periphery to liver counteracting the accumulation of CE in reticulo endothelial cells by the scavenger pathway. Metabolic studies revealed that the cholesterol clearance from the circulation proceeds in the same order of magnitude as the esterification by LCAT takes place. This possibly implies that LCAT might be the rate limiting enzyme for cholesterol catabolism from blood.

Increases in the particle size of high-density lipoproteins induced by purified lecithin: cholesterol acyltransferase: effect of low-density lipoproteins

Homogeneous subpopulations of human high-density lipoproteins subfraction-3 (HDL3) have been incubated at 37 degrees C with purified lecithin: cholesterol acyltransferase, human serum albumin and varying concentrations of human low-density lipoproteins (LDL). Changes in HDL particle size and composition during these incubations were monitored. Incubation of HDL3a (particle radius 4.3 nm) in the absence of LDL resulted in an esterification of more than 70% of the HDL free cholesterol after 24 h of incubation. This, however, was sufficient to increase the HDL cholesteryl ester by less than 10% and was not accompanied by any change in particle size. When this mixture was incubated in the presence of progressively increasing concentrations of LDL, which donated free cholesterol to the HDL, the molar rate of production of cholesteryl ester was much greater; at the highest LDL concentration HDL cholesteryl ester content was almost doubled after 24 h and there was an increase in the HDL particle size up to the HDL2 range. In the case of HDL3b (radius 3.9 nm), there were again only minimal changes in particle size in incubations not containing LDL. In the presence of the highest concentration of LDL tested, however, the particles were again enlarged into the HDL2 size range after 24 h incubation. These HDL2-like particles were markedly enriched with cholesteryl ester but depleted of phospholipid and free cholesterol when compared with native HDL2. Furthermore, the ratio of apolipoprotein A-I to apolipoprotein A-II resembled that in the parent-HDL3 and was very much lower than that in native HDL2. It has been concluded that purified lecithin: cholesterol acyltransferase is capable of increasing the size of HDL3 towards that of HDL2 but that other factors must operate in vivo to modulate the chemical composition of the enlarged particles.

Changes in the size and density of human high-density lipoproteins promoted by a plasma-conversion factor

Incubation of human high-density lipoprotein subfraction-3 (HDL3) with rabbit lipoprotein-depleted plasma resulted in marked changes in the density and size of the HDL. After 24 h of incubation at 37 degrees C, the original HDL3 were converted into populations of larger (less dense) and smaller (more dense) particles. The degree of conversion increased with increasing concentrations of lipoprotein-depleted plasma and increasing incubation time. Furthermore, lecithin:cholesterol acyltransferase, lipoprotein lipase and lipid-transfer protein were shown not to be involved in the process. It was therefore proposed that a separate factor, the HDL-conversion factor, was responsible for the observed changes. Conversion-factor activity was assessed in the lipoprotein-depleted plasma of several species and found to be greater in rabbits and rats than in pigs and human subjects. It was also established that the conversion factor was able to be precipitated from rabbit lipoprotein-depleted plasma between 40 and 50% saturation of (NH4)2SO4. This information was used to partially purify the factor from human plasma. The proteins of human plasma which precipitated between 35 and 55% saturation of (NH4)2SO4 were recovered and subjected to ultracentrifugation to isolate the fraction of density 1.21-1.25 g/ml. This fraction, which was rich in HDL-conversion activity, was further purified by cation-exchange chromatography. In conclusion, a factor which promotes the conversion of HDL to populations of larger and smaller particles has been found to exist at various levels of activity in the plasma of several species. Partial purification of the factor from human plasma has been achieved.

Nonpharmacologic and pharmacologic alteration of high-density lipoprotein cholesterol: therapeutic approaches to prevention of atherosclerosis

High-density lipoprotein (HDL) cholesterol, an independent coronary heart disease (CHD) risk factor, is inversely associated with CHD. Whether interventions to increase concentrations of HDL--particularly the HDL2, HDL3, and apolipoprotein A1 subfractions--will reduce the incidence of CHD in high-risk patients is thus an area of intense speculation. Both nonpharmacologic and pharmacologic regimens will raise HDL concentrations. Nonpharmacologic approaches include habitual high-level aerobic exercise and weight loss--both of these somewhat more effective in men than in women--cessation of cigarette smoking, and changing of dietary habits. A number of drugs have been found to elevate HDL cholesterol. These include the bile acid-binding resin cholestyramine, nicotinic acid, gemfibrozil, phenytoin, exogenous estrogens, and alcohol. Terbutaline has also been reported to raise HDL cholesterol. It is not yet known whether, and to what degree, pharmacologic and nonpharmacologic elevation of HDL cholesterol will retard or reverse the progression of atherosclerosis. Conversely, HDL cholesterol is lowered by a broad variety of drugs, including anabolic--androgenic steroids, exogenous progestins, and probucol, which are used therapeutically to reduce low-density lipoprotein (LDL) cholesterol. Some agents used to treat hypertension also reduce HDL cholesterol, especially thiazide diuretics and the beta blockers, with the possible exception of pindolol. In the antiadrenergic class of antihypertensive agents, reserpine and methyldopa lower HDL cholesterol, but the alpha blocker prazosin does not appear to affect HDL cholesterol. The alpha agonist guanabenz has no effect on HDL cholesterol, and the vasodilator carprazidil has been reported to raise HDL cholesterol. In light of these facts, investigations should be undertaken to determine whether the metabolic effects of antihypertensive agents blunt their beneficial effects on CHD.(ABSTRACT TRUNCATED AT 250 WORDS)

Subfractionation and characterization of native and incubation enlarged human plasma high density lipoprotein particles by high performance gel filtration

Incubation of human plasma in vitro at 37 C results in an increase of the mean particle size of the high density lipoproteins (HDL) accompanied by an almost complete disappearance of the original particles present prior to incubation. A rapid high performance gel filtration technique has been developed in order to study the chemical composition of subfractions of native and incubation enlarged HDL particles as a function of particle size. Subfractionation of HDL isolated by preparative ultracentrifugation from 3 normal human plasmas incubated in vitro at 0 and 37 C for 24 hr have been performed using a 150 cm long TSK-G 3000 SW column. The separation time was less than 65 min. The curves obtained at high performance gel filtration of HDL, by monitoring the effluents from the column at 280 nm, agreed well both in positions of peak maxima and relative peak intensities with the particle distribution patterns observed at polyacrylamide gradient gel electrophoresis of the corresponding HDL preparations run in parallel. The different HDL particle subfractions of the effluents from the gel filtration column have been characterized by quantification of free and esterified cholesterol, total phospholipids and apolipoprotein A-I and A-II. The incubation enlarged HDL particles, subfractionated by the high performance gel filtration technique, were found to have a composition which differed from that of native HDL particles of corresponding size. Incubation enlarged HDL had a generally higher and almost constant relative cholesteryl ester content over the whole particle range compared to native HDL in which a continuous increase in relative cholesteryl ester content could be observed when going from large to small particles. The molar ratio of phospholipids to free cholesterol was higher in small native HDL particles than in the corresponding large ones. The relation between apolipoprotein A-I and A-II remained nearly constant between small and large HDL particles in each subfractionation experiment. The results demonstrate that the high performance gel filtration technique is a rapid and reproducible means for studying the composition of subfractions of HDL particle populations.

The low-density-lipoprotein pathway of native and chemically modified low-density lipoproteins isolated from plasma incubated in vitro

Normal fasting human plasma was incubated for 24 h at 37 degrees C in the presence or absence of lecithin:cholesterol acyltransferase (LCAT) inhibitors. The low-density lipoprotein (LDL) fractions of incubated plasma (control LDL and LCAT-modified LDL) were studied with respect to their chemical and functional properties. LCAT-modified LDL differed from control LDL by a decreased phospholipid and free-cholesterol content, but increased cholesteryl esters. Furthermore, an increase of the relative protein content in LDL by 16-20% was found. Apolipoproteins of LCAT-modified LDL exhibited a 10-fold increase of apo AI, a 4-5-fold increase of apo E, and a 2-fold increase of apo C. All these apolipoproteins resided together with apo B on the same particles. LCAT-modified LDL displayed a higher electrophoretic mobility, a higher hydrated density, a decreased flotation constant and a smaller diameter. Cultured human fibroblasts bound and internalized LCAT-modified LDL to a lower extent than control LDL. The degradation, however, was faster. Modified LDL suppressed 3-hydroxy-3-methylglutaryl-CoA reductase activity to a lower extent than did control LDL. Our results demonstrate that LCAT action, together with lipid transfer and exchange processes, markedly alters the chemical and physiochemical properties of LDL. This in turn significantly influences LDL catabolism in vitro.

Evidence of the existence of a high-density lipoprotein transformation factor in pig and rabbit plasma

Experiments have been conducted to elucidate the incubation conditions necessary for the transformation of pig high-density lipoproteins (HDL) from particles resembling the HDL3 subfraction of human plasma into particles resembling human HDL2. All incubations were performed at 37 degrees C for 24 h in the presence of 0.002 M p-chloromercuriphenylsulphonic acid, a chemical inhibitor of lecithin-cholesterol acyltransferase. There was no transformation when pig HDL (1.09 less than d less than 1.21 g/ml) was incubated either in isolation or in the presence of bovine serum albumin. In addition, there was no transformation when the 1.25 g/ml infranatant of pig plasma was added to the incubation. In the presence of the 1.25 g/ml infranatant of rabbit plasma, by contrast, a significant proportion of the pig HDL was transformed into HDL2. The possibility that this result may have been the consequence of the lipid-transfer activity in rabbit plasma was excluded by the observation that a partially purified preparation of rabbit lipid-transfer protein was ineffective in promoting the HDL transformation. Experiments were also performed with the 1.09 g/ml infranatant of pig plasma. This fraction contained HDL and the plasma proteins but no other lipoprotein. In contrast to the absence of change in incubations of a mixture containing isolated pig HDL (1.09 less than d less than 1.21 g/ml) and the 1.25 g/ml infranatant of pig plasma, during incubation of the 1.09 g/ml infranatant there was an obvious formation of HDL2. It has been concluded that there exists in plasma an HDL-transformation factor which is distinct from both lecithin-cholesterol acyltransferase and the lipid-transfer protein. This factor was detectable in the 1.25 g/ml infranatant of rabbit plasma and in the 1.09 g/ml infranatant of pig plasma. As the factor was not detected in isolated pig HDL or in the 1.25 g/ml infranatant of pig plasma, the 1.21-1.25 g/ml fraction of plasma may be a useful starting point for further investigation of this factor.

Electron microscopic structure of serum lipoproteins from patients with fish eye disease

The structure and composition of lipoprotein fractions from two patients with fish eye disease were examined. The composition of very low density lipoproteins (VLDL) was normal, although total mass was greatly elevated. The mean particle sizes of the VLDL were 44.6 +/- 22.2 and 42.8 +/- 19.8 nm for Patients 1 and 2, respectively. Intermediate density lipoprotein (IDL) concentrations in patients were elevated and contained increased triglyceride content; particle sizes for Patients 1 and 2 were 29.4 +/- 3.5 nm and 28.0 +/- 4.1 nm, respectively. In both patients, the triglyceride/cholesteryl ester ratio in LDL was approximately tenfold higher than in normal individuals; however, the LDL particles were somewhat smaller in diameter (23.5 +/- 3.0 nm for Patient 1 and 23.3 +/- 3.8 nm for Patient 2) than those of controls (25.8 +/- 3.0 nm and 24.9 +/- 3.4 nm). In both patients, large vesicular structures were occasionally encountered in the LDL region. The high density lipoprotein (HDL) fraction of fish eye disease patients showed the greatest abnormalities. Not only was the total HDL concentration extremely low (approximately 10% of control levels), but unesterified cholesterol was increased relative to cholesteryl ester. Particle morphology was heterogeneous; the major HDL species was a small spherical particle with a diameter of 7.6 nm. Discoidal particles with a thickness of 4.4 nm and diameters between 17.4 and 20.8 nm were also present, together with large (40-90 nm) vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor binding activity of high-density lipoproteins containing apoprotein E from abetalipoproteinemic and normal neonate plasma

The receptor binding properties of lipoproteins derived from neonates and abetalipoproteinemic patients were examined. Compared to normal adults, the neonate plasma contained reduced cholesterol levels, with only 40% of the total cholesterol transported in the low-density lipoproteins (LDL). When compared at equal cholesterol concentrations, however, the total neonate lipoproteins (d less than 1.21) were as effective as adult d less than 1.21 lipoproteins in stimulating cholesteryl ester formation in cultured human fibroblasts. Analysis of the neonate lipoproteins explained their enhanced ability to deliver cholesterol to the cells via LDL (apoprotein B,E) receptors: the neonate d = 1.02-1.063 fraction contained, in addition to LDL, alpha 2-migrating, apoprotein E-rich high-density lipoproteins (HDL1), which were isolated by Geon-Pevikon electrophoresis. In binding studies performed with human fibroblasts at 4 degrees C, the neonate HDL1 were 14-fold more effective than either neonate or adult human LDL in displacing 125I-LDL from apo-B,E receptors. The neonate HDL (d = 1.063-1.21) contained a subfraction rich in apo-E and apo(E-A-II), which was isolated by heparin-Sepharose chromatography. This fraction was also active in displacing 125I-LDL from the receptors on cultured fibroblasts. Apoprotein E-containing HDL subclasses, similar to those described in the blood of neonates, were present in the d less than 1.063 and d = 1.063-1.21 lipoprotein fractions of patients with abetalipoproteinemia. These HDL with apo-E were enriched in cholesterol and were as effective as normal LDL in competing with 125I-LDL for apo-B,E receptor-mediated binding, internalization, and degradation. When incubated with cultured human fibroblasts, the HDL with apo-E from the abetalipoproteinemic subjects increased the cholesteryl ester mass three- to fourfold. These studies suggest that neonates and abetalipoproteinemic subjects may depend (at least in part) upon lipoproteins containing apo-E to deliver cholesterol to various tissues via the LDL (apo-B,E) receptor.

Incubation of human lipid transfer protein with plasma from normal-fed and cholesterol-fed rats: effect on the distribution of constituents between lipoprotein fractions

Samples of rat plasma, in which activity of lecithin-cholesterol acyltransferase was inhibited, were incubated in vitro at 37 degrees C for 6 hr in the presence and absence of a partially purified preparation of human lipid transfer protein (LTP). After the incubation gel filtration chromatography was used to separate plasma lipoproteins into multiple fractions which were individually assayed to obtain a complete profile of all constituents across the whole lipoprotein spectrum. When plasma from both normal and cholesterol-fed rats was incubated in the absence of LTP there were no changes in the distribution of any of the lipoprotein constituents between different fractions. When, however, normal plasma was incubated in the presence of LTP there was an obvious transfer of cholesteryl esters from high density lipoproteins (HDL) to very low density lipoproteins (VLDL) and of triglyceride from VLDL to HDL. The presence of LTP in incubations of plasma from cholesterol-fed rats, by contrast, resulted in only minimal redistributions of constituents.

Separation of rat plasma HDL subfractions by density gradient centrifugation and the effect of incubation on these fractions

The conditions for the separation of rat high density lipoproteins (HDL) in a single ultracentrifuge run are described. By this method six serum samples can be processed simultaneously. HDL is separated into two main fractions, one with apolipoprotein E and the other with apolipoprotein A-I as the major protein component. The apolipoprotein E-rich HDL contains a relatively high amount of phospholipid and unesterified cholesterol and therefore resembles HDL-1 or apolipoprotein E HDL as isolated by other methods. The other HDL fraction resembles HDL-2. The two HDL fractions appeared to be heterogeneous with respect to apolipoprotein composition. The HDL-1 consisted of particles with and without a low percentage of apolipoprotein A-I. The HDL-2 consisted of particles with a variable amount of apolipoprotein E and A-IV. During incubation of rat serum for 5 h at 37 degrees C in the presence of dithiobis(2-nitrobenzoic acid) (DTNB) a small shift of the HDL-2 peak to lower densities occurred. Incubation of the serum without DTNB led to a loss of cholesterol from the 'light' HDL-1 fractions and an increase in cholesterol ester in fractions at densities intermediate between those of HDL-1 and HDL-2 and in fractions at the densest part of the gradient.

Changes in the density distribution of pig high density lipoproteins during incubation in vitro. Influence of esterified cholesterol transfer activity

Pig plasma, depleted of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) by ultracentrifugation, was incubated in vitro at 37 degrees C in the presence of 0.002 M parachlormercuriphenyl sulfonate, a chemical inhibitor of lecithin: cholesterol acyltransferase. Incubation resulted in a consistent and significant shift in the density distribution of esterified cholesterol within the high density lipoprotein (HDL) fraction. After 24 h of incubation the esterified cholesterol in HDL of density greater than 1.125 g/ml was reduced by a mean of 28%, while that in HDL of density less than 1.125 g/ml was increased by a mean of 77%. Both the rate and the magnitude of this redistribution were markedly enhanced when the incubation mixture contained rabbit lipoprotein-free plasma, a rich source of the esterified cholesterol transfer protein. When pig plasma, depleted of VLDL and LDL, was subjected to rate zonal ultracentrifugation, the HDL in non-incubated samples eluted as a single symmetrical peak in a position comparable to that of human HDL3. After 24 h of incubation at 37 degrees C the density of the HDL was reduced, with a peak midway between the positions of human HDL3 and HDL2. When VLDL- and LDL-depleted pig plasma were mixed with human lipoprotein-free plasma (also a source of esterified cholesterol transfer activity) and incubated for 24 h at 37 degrees C, there was an even greater reduction in the density of pig HDL, which now eluted in a position comparable to that of human HDL2. It was concluded that the capacity of the esterified transfer protein to redistribute esterified cholesterol between different lipoprotein particles may be of importance in the process of HDL interconversion.