Radioimmunoassay of human high density lipoprotein apo-protein A-1.

A double antibody radioimmunoassay technique was developed for the measurement of apolipoprotein A-I, the major apoprotein of human high density lipoproteins. Apolipoprotein A-I was prepared from human delipidated high density lipoprotein (d equal to 1.085-1.210) by gel filtration and ion-exchange chromatography. Purified apolipoprotein A-I antibodies were obtained by means of apolipoprotein A-I immunoadsorbent. Apolipoprotein A-I was radiolabeled with 125-I by the iodine monochloride technique. 65-80% of 125 I-labeled apolipoprotein A-I could be bound by the different apolipoprotein A-I antibodies, and more than 95% of the 125-I-labeled apolipoprotein A-I was displaced by unlabeled apolipoprotein A-I. The immunoassay was found to be sensitive for the detection of about 10 ng of apolipoprotein A-I in the incubation mixture, and accurate with a variability of only 3-5% (S.E.M.). This technique enables the quantitation of apolipoprotein A-I in whole plasma or high density lipoprotein without the need of delipidation. The quantitation of apolipoprotein A-I in high density lipoprotein was found similar to that obtained by gel filtration technique. The displacement capacity of the different lipoproteins and apoproteins in comparison to unlabeled apolipoprotein A-I was: very low density lipoprotein, 1.8%; low density lipoprotein, 2.6%; high density lipoprotein, 68%; apolipoprotein B, non-detectable; apolipoprotein C, 0.5%; and apolipoprotein A-II, 4%. The distribution of immunoassayable apolipoprotein A-I among the different plasma lipoproteins was as follows: smaller than 1% in very low density lipoprotein and low density lipoprotein; 50% in high density lipoprotein, and 50% in lipoprotein fraction of density greater than 1.21 g/ml. The amount of apolipoprotein A-I in the latter fraction was found to be related to the number of centrifugations.     

Lipoprotein secretion by human monocyte derived macrophages

Cholesterol-loaded human monocyte derived macrophages secrete distinct class of lipoprotein. Following macrophages incubation in serum-free medium containing [14C]-oleic acid the cells secrete lipoprotein associated radioactivity that was found in triglycerides, phospholipids and cholesteryl ester. Macrophage lipoprotein secretion was analyzed by non-denatured gradient gel electrophoresis, agarose lipoprotein electrophoresis and discontinuous density gradient ultracentrifugation. The lipoprotein secreted by human macrophages was shown to be triglyceride-enriched and contain a protein resembling apolipoprotein E.     

Highly polymorphic apolipoprotein A-IV locus in the baboon

Apolipoprotein A-IV is found in mesenteric lymph chylomicrons, very low density lipoprotein particles, high density lipoprotein particles, and in the lipoprotein-free fraction of plasma. Apolipoprotein A-IV is polymorphic in a variety of species including human, dog, and horse. Efforts to estimate the impact of apolipoprotein A-IV structural variation on quantitative lipid levels in humans have been limited by the low frequency of the less common alleles. In the baboon, Papio hamadryas anubis, we have found apolipoprotein A-IV to be highly variable at the protein level with five alleles appearing at polymorphic frequency. We have confirmed the autosomal codominant inheritance of these five alleles in pedigreed baboons. The baboon has been shown to be a suitable animal model for the study of atherosclerosis, and the existence of a common, multi-allele apolipoprotein A-IV polymorphism in the baboon may be useful in elucidating the role of apolipoprotein A-IV in lipid metabolism.     

A density gradient study of the lipoprotein and apolipoprotein distribution in the chicken, Gallus domesticus

Plasma lipoproteins from 5-week old male chickens were separated over the density range 1.006-1.172 g/ml into 22 subfractions by isopycnic density gradient ultracentrifugation, in order to establish the distribution of these particles and their constituent apolipoproteins as a function of density. Lipoprotein subfractions were characterized by electrophorectic, chemical and morphological analyses, and their protein moieties were defined according to net charge at alkaline pH, molecular weight and isoelectric point. These analyses have permitted us to reevaluate the density limits of the major chicken lipoprotein classes and to determine their main characteristics, which are as follows: (1) very-low-density lipoproteins (VLDL), isolated at d less than 1.016 g/ml, were present at low concentrations (less than 0.1 mg/ml) in fasted birds; their mean diameter determined by gradient gel electrophoresis and by electron microscopy was 20.5 and 31.4 nm respectively; (2) as the the density increased from VLDL to intermediate density lipoproteins (IDL), d 1.016-l.020 g/ml) and low-density lipoproteins (LDL, d 1.020-1.046 g/ml), the lipoprotein particles contained progressively less triacylglycerol and more protein, and their Stokes diameter decreased to 20.0 nm; (3) apolipoprotein B-100 was the major apolipoprotein in lipoproteins of d less than 1.046 g/ml, with an Mr of 350000; small amounts of apolipoprotein B-100 were detectable in HDL subfractions of d less than 1.076 g/ml; urea-soluble apolipoproteins were present in this density range as minor components of Mr 38000-39000, 27000-28000 (corresponding to apolipoprotein A-1) and Mr 11000-12000; (4) high density lipoprotein (HDL, d 1.052-1.130 g/ml) was isolated as a single band, whose protein content increased progressively with increase in density; the chemical composition of HDL resembled that of human HDL2, with apolipoprotein A-1 (M 27000-28000) as the major protein component, and a protein of Mr 11000-12000 as a minor component; (5) heterogeneity was observed in the particle size and apolipoprotein distribution of HDL subfractions: two lipoprotein bands which additional apolipoproteins of Mr 13000 and 15000 were detected. These studies illustrate the inadequacy in the chicken of the density limits applied to fractionate the lipoprotein spectrum, and particularly the inappropriateness of the 1.063 g/ml density limit as the cutoff for LDL and HDL particle populations in the species.     

Chimpanzee lipoprotein(A): Relationship between apolipoprotein(A) isoform size and the density profile of lipoprotein(A) in animals with different heterozygous apo(A) phenotypes

In a previous study [C. Doucet et al., J. Lipid Res 35:263–270, 1994], we have shown that plasma lipoprotein (a) [Lp(a)] levels were significantly elevated in a population of unrelated chimpanzees as compared to those in normolipidemic human subjects. Nonetheless, the inverse correlation between Lp(a) levels and apolipoprotein (a) [apo(a)] isoforms typical of man was maintained in the chimpanzee. In the present study, we describe the density profiles of apo B- and apo A1-containing lipoproteins and of Lp(a) in chimpanzee plasmas heterozygous for apo(a) isoforms after fractionation by single spin ultracentrifugation in an isopycnic gradient. The distribution of apo(a) isoforms in the density gradient was also examined by SDS-agarose gel electrophoresis and immunoblotting using chemiluminescence detection. In all double-band phenotypes examined, the smallest isoform was present along the entire length of the density gradient. The density distribution of the second isoform varied according to the size difference between the respective isoforms. Two isoforms close in size (difference in apparent molecular mass ? 60 kDa) were present together in every gradient subfraction. On the contrary, when the two isoforms displayed distinct molecular mass (maximal difference in apparent molecular mass = 340 kDa), then the largest was principally present in the densest fractions of the gradient (d > 1.1 mg/ml). These observations suggest that Lp(a) particles with small apo(a) isoforms are more susceptible to interact with other lipoproteins than are Lp(a) particles with large isoforms.     

Human apolipoprotein A-I liberated from high-density lipoprotein without denaturation.

Apolipoprotein A-I (apoA-I) was liberated from human high-density lipoprotein (HDL) without exposure to organic solvents or chaotropic salts by the action of isolated insect hemolymph lipid transfer particle (LTP). LTP-catalyzed lipid redistribution results in transformation of HDL into larger, less dense particles accompanied by an overall decrease in HDL particle surface area:core volume ratio, giving rise to an excess of amphiphilic surface components. Preferential dissociation of apolipoprotein versus phospholipid and unesterified cholesterol from the particle surface results in apolipoprotein recovery in the bottom fraction following ultracentrifugation at a density = 1.23 g/mL. ApoA-I was then isolated from other contaminating HDL apolipoproteins by incubation with additional HDL in the absence of LTP, whereupon apolipoprotein A-II and the C apolipoproteins reassociate with the HDL surface by displacement of apoA-I. After a second density gradient ultracentrifugation, electrophoretically pure apoA-I was obtained. Sedimentation equilibrium experiments revealed that apoA-I isolated via this method exhibits a tendency to self-associate in an aqueous solution while its circular dichroism spectrum was indicative of a significant amount of alpha-helix. Both measurements are consistent with that observed on material prepared by denaturation/renaturation. The ability of apoA-I to activate lecithin:cholesterol acyltransferase was found to be similar to that of apoA-I isolated by conventional methods. The present results illustrate that LTP-mediated alteration in lipoprotein particle surface area leads to dissociation of substantial amounts of surface active apoprotein components, thus providing the opportunity to isolate apoA-I without the denaturation/renaturation steps common to all previous isolation procedures.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gradient acrylamide/agarose gels for electrophoretic separation of intact human very low density lipoproteins, intermediate density lipoproteins, lipoprotein a, and low density lipoproteins

An exponential gradient gel with 0-10% acrylamide and 0.5% agarose was developed for electrophoresis of intact high molecular weight lipoproteins. This system resolves very low density lipoproteins, intermediate density lipoproteins, lipoprotein a, and low density lipoproteins in a size-dependent fashion. The characteristic relative mobility of these species can be determined in relation to protein and colloidal gold reference materials. Electron microscopy of selected lipoprotein fractions confirmed that relative mobility was related to apparent lipoprotein diameter. The composite gel medium can be used with prestained lipoproteins and permits immunoelectroblotting for qualitative analysis of apolipoprotein constituents.     

Lipid and apolipoprotein distribution as a function of density in equine plasma lipoprotein

1. Equine lipoproteins were isolated from plasma by density gradient ultracentrifugation and apolipoprotein composition determined by SDS-polyacrylamide gel electrophoresis. 2. VLDL and IDL were present at low concentration (0.2 mg/ml). Two apoB components of Mr corresponding to human apoB-100 and one apoB-48-like component were represented in VLDL fraction. 3. LDL-1 and LDL-2 subfractions have displayed an almost equal concentration (0.4 mg/ml). Two apoB-100-like components were the major apolipoproteins in each fraction. Small amounts of apoB-48-like component were detectable in LDL-1 and LDL-2. 4. HDL-2 represented a major class of equine lipoproteins (1.8 mg/ml). ApoA-1-like component was the dominant protein in HDL-1, HDL-2 and HDL-3. Dimeric apoA-II-like components were slightly represented in HDL subfractions. 5. HDL-3 displayed the same apolipoprotein pattern as HDL-1 and HDL-2, but two further minor proteins of Mr 20,000 and 14,000 were detected. 6. VHDL represented a minor class of lipoprotein (0.2 mg/ml). ApoA-I-like component was the major apolipoprotein of VHDL. Small amounts of apoA-IV-like, apoE-like, and Mr 55,000 protein were detectable. 7. ApoC-like of Mr lower than 10,000 was represented in all equine lipoprotein classes.     

Isolation and partial characterization of apolipoprotein (a) from human lipoprotein (a)

A procedure was developed for the dissociation of apolipoprotein (a) (apo (a)) from pure human lipoprotein (a) (Lp(a)) prepared by density gradient ultracentrifugation and gel filtration. Lp(a) was ultracentrifuged through a layer of saline which was adjusted to a density of 1.182 g/mL and contained 30 mM dithiothreitol (50 mM) and phenylmethylsulfonyl fluoride (1.25 mM). Following centrifugation, the lipid and apolipoprotein B (apo B) were recovered as a lipoprotein (Lp(a) B) in the supernatant fraction, while the apo (a) was recovered as a lipid-poor protein pellet. An investigation of the supernatant lipoprotein by electron microscopy and compositional analysis revealed that it was similar in size and composition to low density lipoprotein (LDL) isolated from the same density range and contained apo B100 with an amino acid and carbohydrate composition which was similar to apo B from LDL. Estimates of the apparent molecular weight of the apo (a) varied amongst individuals but was always greater than apo B100 (congruent to 450,000). The amino acid composition of apo (a), which was very distinct from apo B, was characterized by a higher content of serine, threonine, proline, and tyrosine, but lower amounts of isoleucine, phenylalanine, and lysine when compared with apo B of Lp(a) or LDL. The apo (a) contained a much higher proportion of carbohydrate, in particular N-acetylgalactosamine, galactose, and N-acetylneuraminic acid (which were three- to six-fold higher) than the apo B of Lp(a). It is concluded that apo (a) is distinct from other apolipoproteins owing to its low avidity for lipid and the nature of the interaction with apo B. Lp(a) consists of an LDL-like particle with a carbohydrate-rich apo (a) attached to the surface of apo B.     

Dissociation of the lipid-enzyme complex of hormone-sensitive lipase using high density lipoprotein or apolipoprotein A-I

Hormone-sensitive lipase in homogenates of adipose tissue occurs as a large, lipid-rich complex including several acylhydrolase activities that emerge quantitatively in the void volume on gel filtration chromatography (2% agarose). Incubation with intact human plasma high density lipoprotein or with lipid-free apolipoprotein A-I, however, disrupted the lipid-rich complex almost completely and most of the enzyme activity eluted from a 2% agarose column at about Ve = 2.3 x Vo. This use of the detergent-like properties of apolipoprotein A-I may be of value for dissociation of other lipid-associated or membrane-bound enzymes.     

Screening for lipoprotein[a] elevations in plasma and assessment of size heterogeneity using gradient gel electrophoresis

Plasma was screened for the presence of lipoprotein[a] using 2-16% nondenaturing, polyacrylamide gradient gel electrophoresis. Gels were scanned with a densitometer after staining with Sudan black B. Bands that migrated above low density lipoprotein bands were identified as lipoprotein[a] by immunoblotting with polyclonal and monoclonal antibodies to apolipoprotein[a]. Lipoprotein[a] was measured by gradient gel electrophoresis and by radioimmunoassay in 115 male patients with premature coronary artery disease and 132 control subjects. Lipoprotein[a] bands were detected in 96.7% of subjects with lipoprotein[a] values above 40 mg/dl; in 31.3% with values between 21 and 40 mg/dl, and in 6.5% with values below 20 mg/dl. This gel methodology is a simple and effective procedure for detecting elevated plasma lipoprotein[a] levels and for investigating size heterogeneity, but does not replace immunoassay for quantitation.     

The baboon gene for apolipoprotein A-I: characterization of a cDNA clone and identification of DNA polymorphisms for genetic studies of cholesterol metabolism

We have isolated and sequenced a baboon apolipoprotein A-I (ApoA-I) clone from a liver cDNA library using a human cDNA hybridization probe. This baboon cDNA contains the entire ApoA-I coding region (801 bp, 267 aa), a 3' untranslated region, and a poly(A)tail. Among comparisons with apoAI sequences from other species, the baboon cDNA is most similar to that of the cynomolgus macaque (99.2% homologous) and least similar to the rat sequence (72.6% homologous). A high frequency of nonsynonymous substitutions are observed by alignment of baboon and human apoAI cDNAs, but comparisons of hydrophilicity profiles show that protein structure is conserved by substitutions of aa with similar properties. A polymorphic PstI cleavage site was identified by Southern blot analysis and subsequently mapped to the 5' end of the baboon apoAI gene. To identify effects of apoAI allelic variation on cholesterol metabolism, we used immunoblotting to compare the distributions of ApoA-I among lipoprotein size classes in baboons from each genotype under basal and atherogenic diets. We observed an increase of ApoA-I in high density lipoprotein (size class 1) particles after atherogenic diets in homozygotes for one allele, as compared to slight decreases in the other genotypes.     

Provision of cholesterol to lymphocytes by high density and low density lipoproteins. Requirement for low density lipoprotein receptors

The capacity of lipoprotein fractions to provide cholesterol necessary for human lymphocyte proliferation was examined. When endogenous synthesis of cholesterol was blocked, proliferation of mitogen-stimulated normal human lymphocytes was markedly inhibited unless an exogenous source of sterol was supplied. All lipoprotein fractions with the exception of high density lipoprotein subclass 3 were able to provide cholesterol for lymphocyte proliferation. Each of the lipoprotein subfractions capable of providing cholesterol was also able to regulate endogenous sterol synthesis in cultured human lymphocytes. Provision of cholesterol by lipoproteins required the interaction of apolipoprotein B or apolipoprotein E with specific receptors on normal lymphocytes. Apolipoprotein modification by acetylation or methylation, which markedly reduced the ability to regulate sterol biosynthesis, also diminished the capacity of lipoproteins to provide cholesterol. In addition, depletion of apolipoprotein B- and apolipoprotein E-containing particles from high density lipoprotein decreased its ability to suppress cholesterol synthesis and prevented it from providing cholesterol to proliferating lymphocytes. Monoclonal antibodies directed against the receptor-recognition sites on apolipoprotein B and apolipoprotein E were used to define the specific apolipoproteins required for the provision of cholesterol to lymphocytes by the various lipoprotein fractions. The antibody to apolipoprotein B inhibited cholesterol provision by both low density lipoprotein (LDL) and other lipoprotein fractions. The antibody to apolipoprotein E did not decrease provision of cholesterol by LDL but did inhibit the capacity of other fractions to provide cholesterol. In addition, a monoclonal antibody against the ligand binding site on the LDL receptor inhibited provision of cholesterol to normal lymphocytes by all lipoproteins. Finally, lymphocytes lacking LDL receptors were unable to obtain cholesterol from any lipoprotein fraction. These studies demonstrate that LDL receptor-mediated interaction with apolipoprotein B or apolipoprotein E is essential for the provision of cholesterol to normal human lymphocytes from all lipoprotein sources.     

Isolation and partial characterization of the lipoprotein families A and A-I from high-density lipoproteins of human serum

Procedures for the isolation of two lipoprotein fractions from plasma high-density lipoproteins (HDL), characterized by apolipoprotein A-I and apolipoprotein A-I together with apolipoprotein A-II, have been elaborated. Apolipoprotein A-I was identified as the protein moiety of one of these fractions (lipoprotein A-I) with polyacrylamide gel electrophoresis (at basic and acidic pH, as well as in the presence of sodium dodecyl sulphate), immuno-double-diffusion, and amino acid analysis. Apolipoproteins A-I and A-II were identified as the protein moiety of the other fraction (lipoprotein A) with polyacrylamide gel electrophoresis (basic and acidic pH) and immuno-double-diffusion. Lipoprotein A-I consisted of spherical particles with a diameter similar to that of HDL as judged from negative strains in the transmission electron microscope. The diameter was estimated to be 8.7 nm from gel chromatography. Lipoprotein A-I migrated in the HDL position on crossed immunoelectrophoresis. On iso-electric focusing lipoprotein A-I appeared as multiple bands in the pH range 5.05-5.55. Lipoprotein A-I had the density of an HDL-2 fraction (rho: 1.063-1.105). Lipoprotein A consisted of spherical particles with a diameter similar to that of HDL, as judged from negative strains in the transmission electron microscope. The diameter was estimated to be 7.9 nm from gel chromatography. The molar ratio between the A-I and A-II polypeptides was estimated to 1.3:1 with electroimmunoassay and calculations from the amino acid compositions. Lipoprotein A migrated in the position of HDL on crossed immuno-electrophoresis. On iso-electric focusing lipoprotein A appeared as one major and two minor bands in the pH range 5.10-5.30. Lipoprotein A had the hydrated density of an HDL-2 fraction.     

Characterization of the apolipoprotein B polypeptide of human plasma low density lipoprotein in detergent and denaturation solutions.

Apolipoprotein B, the polypeptide moiety of human serum low density lipoprotein, is subject to degradation (as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) both in the intact particle and after delipidation. Protease inhibitors, sodium azide, and nitrogen saturation did not influence the rate or degree of degradation. Lipid-free apolipoprotein B prepared by gel exclusion chromatography in sodium dodecyl sulfate bound a limited number of detergent molecules (up to 300) in monomeric sodium dodecyl sulfate solutions; circular dichroic spectra of this complex were similar to spectra of the intact lipoprotein. Near the critical micelle concentrations, a large, cooperative increase in detergent binding occurred, accompanied by circular dichroic changes indicating increased alpha helicity. By sucrose density centrifugation, lysopalmitoyl phosphatidylcholine could be substituted for the anionic detergent; about 300 mol of lysolipid were bound to the polypeptide. Replacement of detergent with guanidine hydrochloride by dialysis produced a soluble polypeptide with no ordered structure at denaturant concentrations above 7 M. At lower guanidine hydrochloride concentrations, structural elements were regained in a broad, reversible transition. It appears that apolipoprotein B is an easily degraded polypeptide with regions resembling water-soluble proteins but other regions which interact with lipid (or synthetic amphiphiles) and produce an overall insolubility in aqueous solution in the absence of amphiphilic ligands.     

Lipid transfer particle-induced transformation of human high density lipoprotein into apolipoprotein A-I-deficient low density particles

Incubation of human high density lipoprotein (HDL) particles (density = 1.063-1.21 g/ml) with catalytic amounts of Manduca sexta lipid transfer particle (LTP) resulted in alteration of the density distribution of HDL protein such that the original HDL particles were transformed into new particles with an equilibrium density = 1.05 g/ml. Concomitantly, substantial amounts of protein were recovered in the bottom fraction of the density gradient. The LTP-induced alteration in HDL protein density distribution was dependent on the LTP concentration and incubation time. Electrophoretic analysis revealed that the lower density fraction contained apolipoprotein A-II (apoA-II) as the major apoprotein component while nearly all of the apoA-I was recovered in the bottom fraction. Lipid analysis of the HDL substrate and product fractions revealed that the apoA-I-rich fraction was nearly devoid of lipid (less than 1%, w/w). The lipid originally associated with HDL was recovered in the low density, apoA-II-rich, lipoprotein fraction, and the ratios of individual lipid classes were the same as in control HDL. Electron microscopy and gel permeation chromatography experiments revealed that the LTP-induced product lipoprotein population comprised particles of larger size (19.7 +/- 1.4-nm diameter) than control HDL (10.6 +/- 1.4-nm diameter). The results suggest that facilitated net lipid transfer between HDL particles altered the distribution of lipid such that apoprotein migration occurred and donor particles disintegrated. Similar results were obtained when human HDL3 or HDL2 density subclasses were employed as substrates for LTP. The lower surface area to core volume ratio of the larger, product lipoprotein particles compared with the substrate HDL requires that there be a decrease in the total exposed lipid/water interface which requires stabilization by apolipoprotein. Selective displacement of apoA-I by apoA-II or apoC, due to their greater surface binding affinity, dictates that apoA-I is preferentially lost from the lipoprotein surface and is therefore recovered as lipid-free apoprotein. Thus, it is conceivable that the structural arrangement of HDL particle lipid and apoprotein components isolated from human plasma may not represent the most thermodynamically stable arrangement of lipid and protein.     

Human plasma lipoprotein [a]. Structural properties

When lipoprotein [a] was isolated in the presence of the proteolytic inhibitor Trasylol, its apoprotein exhibited one dominant band corresponding to a molecular weight of about 1.2 million when analyzed by electrophoresis on 3.25% sodium dodecyl sulfate-polyacrylamide gels. After chemical reduction, this band was missing but was replaced by two bands, one corresponding to a molecular weight of about 490,000 and the other to a molecular weight of about 645,000. Before treatment with reducing agents, the apolipoprotein [a] and apolipoprotein B immunoreactivities were detectable in the same electrophoretic band, but after reduction the apolipoprotein [a] was demonstrated to be separate from the apolipoprotein B. These results suggest that the apoprotein of lipoprotein [a] is composed of two subunits which are similar in molecular weight and are held together by one or more disulfide bonds. One subunit possesses apolipoprotein [a] and the other apolipoprotein B immunoreactivity. The secondary structure of the apoprotein components within lipoprotein [a] has been studied by circular dichroism and found to differ significantly from the secondary structure of the apoproteins in low density lipoproteins and high density lipoproteins. About 30% alpha-helical structure was measured in lipoprotein [a] compared to 48% in low density lipoproteins and 70% in high density lipoproteins. Lipoprotein [a] exhibited a much higher percentage of disordered structure than either of the other two lipoproteins.     

Structural heterogeneity of apoB-containing serum lipoproteins visualized using cryo-electron microscopy.

Cryo-electron microscopy was used to analyze the structure of lipoprotein particles in density gradient subfractions of human very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), and low density lipoprotein (LDL). Lipoproteins from a normolipidemic subject with relatively large and buoyant LDL (pattern A) and from a subject with a predominance of small dense LDL (pattern B) were compared. Projections of VLDL in vitreous ice were heterogeneous in size, but all were circular with a relatively even distribution of contrast. Selected projections of LDL, on the other hand, were circular with a high density ring or rectangular with two high density bands. Both circular and rectangular LDL projections decreased in average size with increasing subfraction density, but were found in all of 10 density gradient subfractions, both in pattern A and in pattern B profiles. Preparations of total IDL contained particles with the structural features of VLDL as well as particles resembling LDL. IDL particles resembling LDL were observed in specific density gradient subfractions in the denser region of the VLDL;-IDL density range. Within the group of IDL particles resembling LDL considerable heterogeneity was observed, but no structural features specific for the pattern A or pattern B lipoprotein profile were recognized.The observed structural heterogeneity of the apolipoprotein B-containing serum lipoproteins may reflect differences in the composition of these particles that may also influence their metabolic and pathologic properties.     

Distribution of lecithin-cholesterol acyltransferase (LCAT) in human plasma lipoprotein fractions. Evidence for the association of active LCAT with low density lipoproteins

The distribution of lecithin-cholesterol acyltransferase (LCAT) in human plasma was assessed by measuring both LCAT mass and activity in plasma fractions separated by sequential flotation ultracentrifugation, single-spin gradient ultracentrifugation, dextran sulfate-Mg²⁺ precipitation or agarose gel filtration. Although most of the LCAT was found to be associated with the high density lipoprotein fraction, a small amount of active LCAT (approximately 1% of the plasma LCAT mass and activity) was consistently associated with the low density lipoprotein fraction. LCAT was not found in the very low density lipoprotein fraction. The LDL-associated LCAT may play an important role in the acylation of lysolecithin by lysolecithin acyltransferase activity of LCAT.     

Apolipoprotein A-V interaction with members of the low density lipoprotein receptor gene family

Apolipoprotein A-V is a potent modulator of plasma triacylglycerol levels. To investigate the molecular basis for this phenomenon we explored the ability of apolipoprotein A-V, in most experiments complexed to disks of dimyristoylphosphatidylcholine, to interact with two members of the low density lipoprotein receptor family, the low density lipoprotein receptor-related protein and the mosaic type-1 receptor, SorLA. Experiments using surface plasmon resonance showed specific binding of both free and lipid-bound apolipoprotein A-V to both receptors. The binding was calcium dependent and was inhibited by the receptor associated protein, a known ligand for members of the low density lipoprotein receptor family. Preincubation with heparin decreased the receptor binding of apolipoprotein A-V, indicating that overlap exists between the recognition sites for these receptors and for heparin. A double mutant, apolipoprotein A-V (Arg210Glu/Lys211Gln), showed decreased binding to heparin and decreased ability to bind the low density lipoprotein receptor-related protein. Association of apolipoprotein A-V with the low density lipoprotein receptor-related protein or SorLA resulted in enhanced binding of human chylomicrons to receptor-covered sensor chips. Our results indicate that apolipoprotein A-V may influence plasma lipid homeostasis by enhancing receptor-mediated endocytosis of triacylglycerol-rich lipoproteins.