Human liver apolipoprotein B-100 cDNA: complete nucleic acid and derived amino acid sequence

Identification of Receptor Ligands in Apo B100 Reveals Potential Functional Domains

LDL, VLDL and other members of the low-density lipoparticles (LLPs) enter cells through a large family of receptors. The actual receptor ligand(s) in apolipoprotein B100, one of the main proteins of LLP, remain(s) unknown. The objective of this study was to identify true receptor ligand(s) in apo B100, a molecule of 4563 residues. Apo B100 contains 33 analogues of Cardin-Weintraub arginine/lysine-based receptor ligand motifs and shares key lysine motifs and sequence similarity with the LDL receptor-associated protein, MESD, and heat shock proteins. Eleven FITC-labeled synthetic peptides of 21-42 residues, with at least one ligand, were tested for binding and internalization using HeLa cells. All peptides bind but display different binding capacities and patterns. Peptides B0013, B0582, B2366, and B2932 mediate endocytosis and appear in distinct sites in the cytoplasm. B0708 and B3181 bind and remain on the cell surface as aggregates/clusters. Peptides B3119 (Site A) and B3347 (Site B), the putative ligands, showed low binding and no cell entry capacity. Apo B100 regions in this study share similarities with related proteins of known function including chaperone proteins and Apo BEC stimulating protein, and not directly related proteins, e.g., the DNA-binding domain of interferon regulatory factors, MSX2-interacting protein, and snake venom Zinc metalloproteinase-disintegrin-like proteins.

Generating a detailed protein profile of Fasciola hepatica during the chronic stage of infection in cattle

Fasciola hepatica is a trematode helminth causing a damaging disease, fasciolosis, in ruminants and humans. Comprehensive proteomic studies broaden our knowledge of the parasite's protein profile, and provide new insights into the development of more effective strategies to deal with fasciolosis. The objective of this study was to generate a comprehensive profile of F. hepatica proteins expressed during the chronic stage of infection in cattle by building on previous efforts in this area. The approach included an improved sample preparation procedure for surface and internal layers of the parasite, the application of nano-UPLC-ESI-qTOF-MS (nano-ultra-performance LC and ESI quadrupole TOF MS) integrated with different acquisition methods and in silico database search against various protein databases and a transcript database including a new assembly of publically available EST. Of a total of 776 identified proteins, 206 and 332 were specific to the surface and internal layers of the parasite, respectively. Furthermore, 238 proteins were common to both layers, with comparative differences of 172 proteins detected. Specific proteins not previously identified in F. hepatica, but shown to be immunomodulatory or potential drug targets for other parasites, are discussed.

Low-density lipoprotein modified by myeloperoxidase in inflammatory pathways and clinical studies

Oxidation of low-density lipoprotein (LDL) has a key role in atherogenesis. Among the different models of oxidation that have been studied, the one using myeloperoxidase (MPO) is thought to be more physiopathologically relevant. Apolipoprotein B-100 is the unique protein of LDL and is the major target of MPO. Furthermore, MPO rapidly adsorbs at the surface of LDL, promoting oxidation of amino acid residues and formation of oxidized lipoproteins that are commonly named Mox-LDL. The latter is not recognized by the LDL receptor and is accumulated by macrophages. In the context of atherogenesis, Mox-LDL accumulates in macrophages leading to foam cell formation. Furthermore, Mox-LDL seems to have specific effects and triggers inflammation. Indeed, those oxidized lipoproteins activate endothelial cells and monocytes/macrophages and induce proinflammatory molecules such as TNFα and IL-8. Mox-LDL may also inhibit fibrinolysis mediated via endothelial cells and consecutively increase the risk of thrombus formation. Finally, Mox-LDL has been involved in the physiopathology of several diseases linked to atherosclerosis such as kidney failure and consequent hemodialysis therapy, erectile dysfunction, and sleep restriction. All these issues show that the investigations of MPO-dependent LDL oxidation are of importance to better understand the inflammatory context of atherosclerosis.

Conformation and stability properties of B17: II. Analytical investigations using differential scanning calorimetry

Thermal and stability properties of B17, the 17% N-terminal domain of apo B, were carried out using differential scanning calorimetry spectroscopy, where the thermal characteristics of the polypeptide were studied and analyzed. The heat capacity data of B17 showed that the protein undergoes two transitions between 50 and 90 °C, with T m's at 65.9 and 74.8 °C. While the first transition showed immediate reversibility, the second one-with the higher T m-necessitated a longer cooling (several days) period for its reversibility to be observed and both transitions could be seen in the heat capacity profile of B17. Moreover, the van't Hoff enthalpies determined via calorimetric measurements agreed with the values calculated from the CD analysis reported previously.

Fatty acid and very low density lipoprotein metabolism in obese African American and Caucasian women with type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with increased plasma triglyceride (TG) concentrations, but African Americans (AA) have lower plasma TG than Caucasians (CC). We evaluated the hypothesis that obese AA women have lower plasma TG than obese CC women do because of differences in lipid kinetics. Eleven AA and 11 CC obese women with T2DM, matched on body mass index (BMI) (AA = 37 ± 1, CC = 37 ± 1 kg/m(2)), age, duration of diabetes, percentage body fat, and insulin sensitivity (S(I), determined by an intravenous glucose tolerance test), were studied. Plasma TG concentration (AA = 1.14 ± 0.11, CC = 1.88 ± 0.18 mmol/l), FFA rate of appearance (R(a)) into plasma (AA = 419 ± 27, CC = 503 ± 31 µmol·min(-1)), and total VLDL-TG secretion rate (AA = 18 ± 2, CC = 29 ± 4 µmol·min(-1)) were lower in AA than CC women (all P < 0.05). In contrast, plasma total apolipoprotein (apo)B-100 concentration (AA = 1,542 ± 179, CC = 1,620 ± 118 nmol/l) and VLDL-apoB-100 secretion rate (AA = 1.3 ± 0.1, CC = 1.3 ± 0.1 nmol·min(-1)) were similar in both groups, so the molar ratio of VLDL-TG secretion rate to VLDL-apoB-100 secretion rate was lower in AA women than in CC women. VLDL-TG concentration was lower in AA women due to lower total VLDL-TG secretion rate. However, the VLDL-apoB-100 secretion rate was the same in both groups, demonstrating that AA women secrete smaller VLDL particles containing less TG than do CC women.

Conformation and stability properties of B17: I. Analytical investigations using circular dichroism

Structural characterization of B17, the 17% N-terminal domain of apo B, was carried out using circular dichroic (CD) spectroscopy, where secondary and tertiary structures were studied as a function of temperature and pH. Mild acidic conditions that correlate with histidine protonation invoked a change in the α-helix and random coil contents of the protein, with no apparent change in the β-sheet structural content. Specific changes in the structure of the protein that occur in response to temperature were also investigated to understand the stability and conformational changes of B17. Far- and near-UV CDs were used to probe the thermal changes in the protein. The protonation of some histidine residues was attributed to underlie the increase in the helical content of the protein.

Negatively cooperative binding of high-density lipoprotein to the HDL receptor SR-BI

Scavenger receptor class B, type I (SR-BI), is a high-density lipoprotein (HDL) receptor, which also binds low-density lipoprotein (LDL), and mediates the cellular selective uptake of cholesteryl esters from lipoproteins. SR-BI also is a coreceptor for hepatitis C virus and a signaling receptor that regulates cell metabolism. Many investigators have reported that lipoproteins bind to SR-BI via a single class of independent (not interacting), high-affinity binding sites (one site model). We have reinvestigated the ligand concentration dependence of (125)I-HDL binding to SR-BI and SR-BI-mediated specific uptake of [(3)H]CE from [(3)H]CE-HDL using an expanded range of ligand concentrations (<1 μg of protein/mL, lower than previously reported). Scatchard and nonlinear least-squares model fitting analyses of the binding and uptake data were both inconsistent with a single class of independent binding sites binding univalent lipoprotein ligands. The data are best fit by models in which SR-BI has either two independent classes of binding sites or one class of sites exhibiting negative cooperativity due to either classic allostery or ensemble effects ("lattice model"). Similar results were observed for LDL. Application of the "infinite dilution" dissociation rate method established that the binding of (125)I-HDL to SR-BI at 4 °C exhibits negative cooperativity. The unexpected complexity of the interactions of lipoproteins with SR-BI should be taken into account when interpreting the results of experiments that explore the mechanism(s) by which SR-BI mediates ligand binding, lipid transport, and cell signaling.

Negatively cooperative binding of high-density lipoprotein to the HDL receptor SR-BI

Scavenger receptor class B, type I (SR-BI), is a high-density lipoprotein (HDL) receptor, which also binds low-density lipoprotein (LDL), and mediates the cellular selective uptake of cholesteryl esters from lipoproteins. SR-BI also is a coreceptor for hepatitis C virus and a signaling receptor that regulates cell metabolism. Many investigators have reported that lipoproteins bind to SR-BI via a single class of independent (not interacting), high-affinity binding sites (one site model). We have reinvestigated the ligand concentration dependence of (125)I-HDL binding to SR-BI and SR-BI-mediated specific uptake of [(3)H]CE from [(3)H]CE-HDL using an expanded range of ligand concentrations (<1 μg of protein/mL, lower than previously reported). Scatchard and nonlinear least-squares model fitting analyses of the binding and uptake data were both inconsistent with a single class of independent binding sites binding univalent lipoprotein ligands. The data are best fit by models in which SR-BI has either two independent classes of binding sites or one class of sites exhibiting negative cooperativity due to either classic allostery or ensemble effects ("lattice model"). Similar results were observed for LDL. Application of the "infinite dilution" dissociation rate method established that the binding of (125)I-HDL to SR-BI at 4 °C exhibits negative cooperativity. The unexpected complexity of the interactions of lipoproteins with SR-BI should be taken into account when interpreting the results of experiments that explore the mechanism(s) by which SR-BI mediates ligand binding, lipid transport, and cell signaling.

Increased whole-body adiposity without a concomitant increase in liver fat is not associated with augmented metabolic dysfunction

Aim of this study was to determine whether an increase in adiposity, without a concomitant increase in intrahepatic triglyceride (IHTG) content, is associated with a deterioration in metabolic function. To this end, multiorgan insulin sensitivity, assessed by using a two-stage hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusion, and very low-density lipoprotein (VLDL) kinetics, assessed by using stable isotopically labeled tracer infusion and mathematical modeling, were determined in 10 subjects with class I obesity (BMI: 31.6 +/- 0.3 kg/m(2); 37 +/- 2% body fat; visceral adipose tissue (VAT): 1,225 +/- 144 cm(3)) and 10 subjects with class III obesity (BMI: 41.5 +/- 0.5 kg/m(2); 43 +/- 2% body fat; VAT: 2,121 +/- 378 cm(3)), matched on age, sex, and IHTG content (14 +/- 4 and 14 +/- 3%, respectively). No differences between class I and class III obese groups were detected in insulin-mediated suppression of palmitate (67 +/- 3 and 65 +/- 3%, respectively; P = 0.635) and glucose (67 +/- 3 and 73 +/- 5%, respectively; P = 0.348) rates of appearance in plasma, and the insulin-mediated increase in glucose disposal (218 +/- 18 and 193 +/- 30%, respectively; P = 0.489). In addition, no differences between class I and class III obese groups were detected in secretion rates of VLDL-triglyceride (6.5 +/- 1.0 and 6.0 +/- 1.4 micromol/l x min, respectively; P = 0.787) and VLDL-apolipoprotein B-100 (0.40 +/- 0.05 and 0.41 +/- 0.04 nmol/l x min, respectively; P = 0.866), and plasma clearance rates of VLDL-triglyceride (31 (16-59) and 29 (18-46) ml/min, respectively; P = 0.888) and VLDL-apolipoprotein B-100 (15 (11-19) and 17 (11-25) ml/min, respectively; P = 0.608). We conclude that increased adiposity without a concomitant increase in IHTG content does not cause additional abnormalities in adipose tissue, skeletal muscle, and hepatic insulin sensitivity, or VLDL metabolism.

Mapping oxidations of apolipoprotein B-100 in human low-density lipoprotein by liquid chromatography-tandem mass spectrometry

Human low-density lipoprotein (LDL) is a major cholesterol carrier in blood. Elevated concentration of low-density lipoprotein, especially when oxidized, is a risk factor for atherosclerosis and other cardiac inflammatory diseases. Past research has connected free radical initiated oxidations of LDL with the formation of atherosclerotic lesions and plaque in the arterial wall. The role of LDL protein in the associated diseases is still poorly understood, partially due to a lack of structural information. In this study, LDL was oxidized by hydroxyl radical. The oxidized protein was then delipidated and subjected to trypsin digestion. Peptides derived from trypsin digestion were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Identification of modified peptide sequences was achieved by a database search against apo B-100 protein sequences using the SEQUEST algorithm. At different hydroxyl radical concentrations, oxidation products of tyrosine, tryptophan, phenylalanine, proline, and lysine were identified. Oxidized amino acid residues are likely located on the exterior of the LDL particle in contact with the aqueous environment or directly bound to the free radical permeable lipid layer. These modifications provided insight for understanding the native conformation of apo B-100 in LDL particles. The presence of some natural variants at the protein level was also confirmed in our study.

Apolipoprotein B levels, APOB alleles, and risk of ischemic cardiovascular disease in the general population, a review

Apolipoprotein B is a key component in lipid metabolism. Subendothelial retention of apolipoprotein B containing lipoproteins is a necessary initiating event in atherogenesis, and high plasma levels of apolipoprotein B is a risk factor for atherosclerosis, whereas low levels may provide protection. The present review examines, with focus on general population studies, apolipoprotein B levels as a predictor of ischemic cardiovascular disease, as well as the association of mutations and polymorphisms in APOB with plasma apolipoprotein B levels, and risk of ischemic cardiovascular disease. The studies can be summarized as follows: (1) apolipoprotein B predicts ischemic cardiovascular events in both genders, and is better than LDL cholesterol in this respect; (2) linkage disequilibrium structure in APOB is more complex than expected from HapMap data, because a minimal set of tag single nucleotide polymorphisms capturing the entire variation in APOB cannot be identified, and thus most polymorphisms must be evaluated separately in association studies; (3) APOB mutations and polymorphisms are associated with a range of apolipoprotein B and LDL cholesterol levels, although the magnitude of effect sizes of common polymorphisms are modest; (4) both mutations and polymorphisms are associated with LDL metabolism in vivo; (5) association of APOB mutations and polymorphisms with lipid and disease phenotype cannot be predicted in silico using evolutionary conservation or existing prediction programs; and finally, (6) except for the E4154K polymorphism that possibly predicts a reduction in risk of ischemic cerebrovascular disease and ischemic stroke, common APOB polymorphisms with modest effect sizes on lipid levels do not predict risk of ischemic heart disease, myocardial infarction, ischemic cerebrovascular disease, or ischemic stroke in the general population.

Use of DNA polymorphisms of the apolipoprotein genes to study the role of genetic variation in the determination of serum lipid levels

Cloned DNA probes for the apolipoprotein B (apoB) gene and the gene cluster for apoA-I/C-III/A-IV were used to detect restriction fragment length polymorphisms (RFLPs) at these two loci. Samples have been obtained from clinically well individuals, and the RFLP genotypes of each individual have been determined. The data show that at the locus for apoB, genetic variation associated with an RFLP detected by the enzyme XbaI (but not that associated with RFLPs detected by MspI or EcoRI) is involved in determining the normal levels of serum total cholesterol and low density lipoprotein (LDL) cholesterol. In our study, genetic variation associated with the XbaI RFLP accounts for 14% of the total phenotypic variance in cholesterol levels. Information from all three RFLPs can be used in conjunction to give a better definition of the underlying genetic variation. Data from a second study show that genetic variation in the apoA-I/C-III/A-IV gene cluster, associated with the PstI RFLP, is involved in determining the level of apoA-I and, to a lesser extent, the levels of high density lipoprotein (HDL). When genotypes from three RFLPs were used in conjunction as a haplotype, genetic variation in this gene cluster was shown to account for 16% of the phenotypic variance in apoA-I concentration and for 8% of the phenotypic variance in HDL concentration in our sample. These associations suggest that the isolation and sequencing of the apoB and the apoA-I/C-III/A-IV genes from different individuals will give useful information about how changes in the DNA sequence of these genes may lead to alterations in the levels of their respective apolipoproteins, in the level of the lipoproteins with which they are associated and, possibly, in the levels of lipids in the serum.

Women produce fewer but triglyceride-richer very low-density lipoproteins than men

CONTEXT

Very low-density lipoproteins (VLDL) are a major risk factor for cardiovascular disease. The concentrations of VLDL particles and VLDL-triglyceride (TG) in plasma are lower in women than men, but the mechanisms responsible for these differences are unclear.

OBJECTIVE

The objective of the study was to investigate the effects of sex on VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 kinetics by using stable isotope labeled tracers.

SETTING/PARTICIPANTS

Twenty-six healthy, lean subjects (13 men, aged 29+/-5 yr; 13 women, aged 28+/-6 yr) were studied in the General Clinical Research Center at Washington University School of Medicine.

RESULTS

VLDL-TG and VLDL-apoB-100 concentrations were less in women than men (P<0.05). The secretion rate of VLDL-TG was approximately 70% greater (P<0.05), whereas the secretion rate of VLDL-apoB-100 (i.e. VLDL particles) was approximately 20% less (P<0.05) in women than men. The molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates was therefore more than double (P<0.05) in women than men. VLDL-TG plasma clearance rate was approximately 70% greater in women than men (P<0.05), whereas VLDL-apoB-100 plasma clearance rate was not different between sexes. However, VLDL-TG and VLDL-apoB-100 mean residence times in plasma were both shorter (by 45 and 25%, respectively; P<0.05) in women than men.

CONCLUSIONS

Increased VLDL-TG plasma clearance is responsible for lower VLDL-TG concentration, whereas decreased VLDL-apoB-100 secretion rate, combined with shorter VLDL-apoB-100 residence time in plasma, is responsible for lower VLDL-apoB-100 concentration in women than men. The greater molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates suggests that the liver in women secretes fewer but TG-richer VLDL particles than the liver in men.

Apolipoprotein B binds ferritin by hemin-mediated binding: evidence of direct binding of apolipoprotein B and ferritin to hemin

Apolipoprotein B (apoB) is known to be a ferritin-binding protein. Here we show that apoB binds to ferritin through hemin-mediated binding. Human apoB bound to bovine spleen, horse spleen, and canine liver ferritins, but did not bind to bovine apoferritin, even after incorporation of iron into it. Incubation of apoferritin with hemin resulted in apoB binding with apoferritin at the same level as with holoferritin. In contrast, hemin inhibited binding of apoB to ferritin. Bovine spleen apoferritin bound biotinylated hemin, and hemin inhibited the binding between the apoferritin and biotinylated hemin, suggesting that ferritin binds hemin directly. ApoB and LDL containing apoB bound biotinylated hemin, and their bindings were also inhibited by hemin, but not protoporphyrin IX. These data demonstrate that binding of apoB to ferritin is mediated through ferritin's binding to hemin, and also that apoB binds hemin directly.

Reproducibility of stable isotope-labeled tracer measures of VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics

To gain insight into the mechanisms regulating plasma lipid homeostasis, FFA, VLDL-triglyceride (TG), and VLDL-apolipoprotein B-100 (apoB-100) kinetics are commonly assessed using stable isotope-labeled tracer methods. The reproducibility of these measurements, which is critical for the experimental design, is unknown. Therefore, we investigated the repeatability of plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in eight healthy men using stable isotope-labeled tracer techniques. There were no systematic differences in plasma FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics between the two studies. Intraindividual day-to-day variability for various outcome variables ranged from 15% to 25%, and almost all of this was of biological origin. The most robust outcome variables were FFA rate of appearance and hepatic VLDL-TG and VLDL-apoB-100 secretion rates; the least robust were VLDL-TG and VLDL-apoB-100 plasma clearance rates and mean residence times. Overall, physiologically meaningful differences in mean values (i.e., 25-30% in magnitude) can be obtained with a sample size of 6-10 subjects for paired studies and 12-20 subjects per group for cross-sectional studies, assuming a type I error rate of 0.05 and a type II error rate of 0.20 (i.e., 80% power). These findings will be useful for future studies investigating FFA, VLDL-TG, and VLDL-apoB-100 kinetics with the methods described.

Site-specific glycosylation analysis of human apolipoprotein B100 using LC/ESI MS/MS

Human apolipoprotein B100 (apoB100) has 19 potential N-glycosylation sites, and 16 asparagine residues were reported to be occupied by high-mannose type, hybrid type, and monoantennary and biantennary complex type oligosaccharides. In the present study, a site-specific glycosylation analysis of apoB100 was carried out using reversed-phase high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC/ESI MS/MS). ApoB100 was reduced, carboxymethylated, and then digested by trypsin or chymotrypsin. The complex mixture of peptides and glycopeptides was subjected to LC/ESI MS/MS, where product ion spectra of the molecular ions were acquired data-dependently. The glycopeptide ions were extracted and confirmed by the presence of carbohydrate-specific fragment ions, such as m/z 204 (HexNAc) and 366 (HexHexNAc), in the product ion spectra. The peptide moiety of glycopeptide was determined by the presence of the b- and y-series ions derived from its amino acid sequence in the product ion spectrum, and the oligosaccharide moiety was deduced from the calculated molecular mass of the oligosaccharide. The heterogeneity of carbohydrate structures at 17 glycosylation sites was determined using this methodology. Our data showed that Asn2212, not previously identified as a site of glycosylation, could be glycosylated. It was also revealed that Asn158, 1341, 1350, 3309, and 3331 were occupied by high-mannose type oligosaccharides, and Asn 956, 1496, 2212, 2752, 2955, 3074, 3197, 3438, 3868, 4210, and 4404 were predominantly occupied by mono- or disialylated oligosaccharides. Asn3384, the nearest N-glycosylation site to the LDL-receptor binding site (amino acids 3359-3369), was occupied by a variety of oligosaccharides, including high-mannose, hybrid, and complex types. These results are useful for understanding the structure of LDL particles and oligosaccharide function in LDL-receptor ligand binding.

Direct Evidence for apo B-100-Mediated Copper Reduction: Studies with Purified apo B-100 and Detection of Tryptophanyl Radicals

Copper binding to apolipoprotein B-100 (apo B-100) and its reduction by endogenous components of low-density lipoprotein (LDL) represent critical steps in copper-mediated LDL oxidation, where cuprous ion (Cu(I)) generated from cupric ion (Cu(II)) reduction is the real trigger for lipid peroxidation. Although the copper-reducing capacity of the lipid components of LDL has been studied extensively, we developed a model to specifically analyze the potential copper reducing activity of its protein moiety (apo B-100). Apo B-100 was isolated after solubilization and extraction from size exclusion-HPLC purified LDL. We obtained, for the first time, direct evidence for apo B-100-mediated copper reduction in a process that involves protein-derived radical formation. Kinetics of copper reduction by isolated apo B-100 was different from that of LDL, mainly because apo B-100 showed a single phase-exponential kinetic, instead of the already described biphasic kinetics for LDL (namely alpha-tocopherol-dependent and independent phases). While at early time points, the LDL copper reducing activity was higher due to the presence of alpha-tocopherol, at longer time points kinetics of copper reduction was similar in both LDL and apo B-100 samples. Electron paramagnetic resonance studies of either LDL or apo B-100 incubated with Cu(II), in the presence of the spin trap 2-methyl-2-nitroso propane (MNP), indicated the formation of protein-tryptophanyl radicals. Our results supports that apo B-100 plays a critical role in copper-dependent LDL oxidation, due to its lipid-independent-copper reductive ability.

Significance of acidic sugar chains of apolipoprotein B-100 in cellular metabolism of low-density lipoproteins

We have elucidated the carbohydrate structures of the N-linked sugar chains of human and rabbit apolipoprotein B-100 (apo B-100), which is similar in composition to oligosaccharides (Arch Biochem Biophys 1989;273:197-205, Arteriosclerosis 1990; 10:386-93). We have also shown the negative correlation of the ratio of acidic sugar chains of apo B-100 to the serum cholesterol levels in Watanabe heritable hyperlipidemic rabbits (Atherosclerosis 1992;93:229-35). The acidity of sugar chains is determined by the existence of sialic acid residues at the terminal of oligosaccharides. In the present study we investigated N-linked sugar chains of apo B-100 from patients with coronary artery disease (CAD) who had moderate hypercholesterolemia (less than 400 mg/dL). There was no difference in the structure of their oligosaccharides and the ratio of acidic sugar chains of apo B-100 from CAD patients as compared with that from healthy individuals reported previously. To clarify the role of sialic acid residues in apo B-100 for lipoprotein metabolism, we studied cellular uptake of low-density lipoproteins (LDLs) treated with sialidase (desialylated LDL). Desialylated LDLs were taken up and degraded to a 2-fold greater degree than control LDL by human monocyte-derived macrophages and stimulated cholesterol esterification in these cells. These results indicate that sialic acid residues of apo B- 100 play an important role in cellular uptake and degradation of LDL.

Nucleic acid-binding properties of low-density lipoproteins: LDL as a natural gene vector

The role of apo B-100 as a transcription factor is indicated by the presence of regions in its primary structure that are similar to the DNA-binding domains of the transcription factors ISGF3gamma, STATs, IRFs, and SREBPs as well as by the presence of 11 RNA-binding KH domains. The Apo B-100 sequence also contains numerous bipartite nuclear localization sequences (NLS). A modified gel shift assay was used to show binding of highly purified preparations of human LDLs to fragmented genomic DNA, plasmid DNA, synthetic oligonucleotides (ISRE, 5'-GGGAAACCGAAACTG and E/C, E-box motif and CCAAT, adipocyte-specific genes promoter site), and total RNA from human liver. LDL was observed to bind preferentially to plasmid DNA containing the hCMV IE2 promoter region. In experiments using human liver total RNA, RNA for five different genes was recovered from LDL and VLDL bands. Gene transfection experiments using human skin fibroblast cells were used to study the gene transfer capacity of LDL. Cells transfected with a pEGFP-N1 plasmid DNA and LDL expressed functional GFP, as indicated by fluorescence, at approximately 3 hrs after transfection. Our results strongly support an alternative role for apo B-100, in toto or perhaps as functional fragments, in the control of gene expression and as gene transfer vector.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Bacterial expression and characterization of human recombinant apolipoprotein(a) kringle IV type 9

Elevated plasma lipoprotein(a) [Lp(a)] is an independent risk factor for several vascular diseases. Lp(a) particles are generated through the formation of a disulfide bond between Cys4057 of kringle IV type 9, (KIVt9), of the multikringle apolipoprotein(a) [apo(a)] and a cysteine in apoB-100 low-density lipoprotein (LDL). To better understand this interaction, we have expressed and purified KIVt9 from Escherichia coli as a His-Tag fusionprotein. Dithiothreitol (DTT)-treated purified KIVt9 migrated as a single approximately 17. 3-kDa band on SDS-PAGE gels. Without DTT, an additional band twice the molecular weight of KIVt9 was observed. The double-size band presumably resulted from dimerization of individual kringles, through their unpaired cysteine residues, since a mutation Cys4057 --> Ser ([Ser4057]KIVt9) abolished dimer formation. Using a gel-shift assay, we showed that KIVt9 could couple to 14-amino-acid apoB-100 synthetic peptides (apoB3732-3745 and apoB4319-4332) containing Cys3734 or Cys4326. Both of these apoB-100 cysteines have been reported to associate with apo(a) to generate Lp(a). In the presence of either apoB-100 peptide, KIVt9 was shifted to a higher molecular weight that was consistent with the covalent addition of a 1.2-kDa apoB-100 peptide. Identical apoB-100 peptides in which the cysteine residues were replaced by alanine ([Ala3734]apoB3732-3745 and [Ala4326]apoB4319-4332) had no effect in the gel-shift assay. Furthermore, [Ser4057]KIVt9 did not covalently interact with apoB3732-3745 or apoB4319-4332. These results indicated that KIVt9 couples to the Cys-apoB-100 peptides through a disulfide linkage. This system may be suitable for further investigating the apo(a)/apoB-100 coupling reaction and the structure of KIVt9 through X-ray crystallographic studies.

Human mucin genes MUC2, MUC3, MUC4, MUC5AC, MUC5B, and MUC6 express stable and extremely large mRNAs and exhibit a variable length polymorphism. An improved method to analyze large mRNAs

Of the nine mucin genes that have been characterized, only MUC1 and MUC7 have been fully sequenced, and their transcripts can be detected as distinct bands of predicted size by Northern blot analysis. In contrast, the RNA patterns observed for each of the other MUC genes have usually shown a very high degree of polydispersity. This polydispersity has been believed to be one of the typical features of the mucin mRNAs, but until now, its origin has remained unexplained. In the work described in the present paper, we investigated two possible kinds of explanation for this phenomenon: namely that the extensive polydispersity results from a biological mechanism or that it is artifactual in origin. The data obtained, as a result of improving the purification and blotting methods, allowed us to show that in all of the tissues analyzed, each of the genes, MUC2-6, expresses mRNAs that are stable and are of an unusually large size to be found in eukaryotes (14-24 kilobases). Moreover, allelic variations in length of these mucin transcripts were observed. We demonstrate that these variations are directly related to the variable number of tandem repeat polymorphisms seen at the DNA level.

Human apolipoprotein B transgenic mice generated with 207- and 145-kilobase pair bacterial artificial chromosomes. Evidence that a distant 5'-element confers appropriate transgene expression in the intestine

We reported previously that approximately 80-kilobase pair (kb) P1 bacteriophage clones spanning either the human or mouse apoB gene (clones p158 and p649, respectively) confer apoB expression in the liver of transgenic mice, but not in the intestine. We hypothesized that the absence of intestinal expression was due to the fact that these clones lacked a distant DNA element controlling intestinal expression. To test this possibility, transgenic mice were generated with 145- and 207-kb bacterial artificial chromosomes (BACs) that contained the human apoB gene and more extensive 5'- and 3'-flanking sequences. RNase protection, in situ hybridization, immunohistochemical, and genetic complementation studies revealed that the BAC transgenic mice manifested appropriate apoB gene expression in both the intestine and the liver, indicating that both BACs contained the distant intestinal element. To determine whether the regulatory element was located 5' or 3' to the apoB gene, transgenic mice were generated by co-microinjecting embryos with p158 and either the 5'- or 3'-sequences from the 145-kb BAC. Analysis of these mice indicated that the apoB gene's intestinal element is located 5' to the structural gene. Cumulatively, the transgenic mouse studies suggest that the intestinal element is located between -33 and -70 kb 5' to the apoB gene.

Use of cyclodextrin to deliver lipids and to modulate apolipoprotein B-100 production in HepG2 cells

2-Hydroxypropyl-beta-cyclodextrin (cyclodextrin), cyclodextrin-solubilized oleate, and cyclodextrin-solubilized cholesterol were used to modulate proteolysis and secretion of newly-synthesized apolipoprotein B-100 (apoB) in HepG2 cells. Following cyclodextrin and lipid treatments, cells were pulse-labeled with [3H] leucine, and quantitative immunoprecipitation was used to measure apoB synthesis, apoB secreted into the medium, and the cellular content of undegraded apoB that was not secreted. Three-hour treatment with cyclodextrin-solubilized oleate (0.2 mM) increased secreted apoB from 4% (control cells) to 32% and cellular undegraded apoB from 15% (control cells to 64% of apoB synthesized, which is consistent with earlier studies using bovine serum albumin to complex exogenous oleate. Prolonged daily (4 d or more) administration of 0.5% (3.5 mM) cyclodextrin with medium containing 10% fetal bovine serum increased the secretion of nascent apoB from 5-10% (control) to 17-28% and cellular undegraded apoB from 15-20% (control) to 25-31% of apoB synthesized, respectively. Subsequent administration of cyclodextrin solubilized cholesterol (10-40 micrograms) for only 3 h reversed the cyclodextrin-mediated increase in apoB secretion. The application of 0.5% cyclodextrin to HepG2 cells can rapidly (within minutes) stimulate cholesterol efflux, and transiently (over a 1-2 d period) increase cholesterol synthesis. In the current studies, the cyclodextrin-mediated increase in cholesterol synthesis was not concurrent with the increase in apoB secretion. However, prolonged (15 d) administration of cyclodextrin was shown to increase the cellular free cholesterol concentration by 25-41%, reduce the cellular triglyceride concentration by 59%, and increase apoB secretion 3- to 4-fold, without affecting the cellular cholesteryl ester concentration. In comparison, 14-d treatment with cyclodextrin-solubilized cholesterol (20 micrograms/mL) followed by 1-d equilibration without cholesterol was shown to increase the cellular free cholesterol and cholesteryl ester concentrations by 76% and 10-fold, respectively, although apoB secretion was not affected. It is hypothesized that chronic daily administration of 0.5% cyclodextrin increased the cellular cholesterol concentration and flux in discrete putative regulatory compartments, which "shielded" nascent apoB from rapid proteolysis and facilitated apoB secretion. In conclusion, cyclodextrin was used independently and in combination with cholesterol or oleate to modulate apoB proteolysis and secretion. We speculate that subcellular changes in cholesterol concentration and flux may modulate apoB production in HepG2 cells.

Microsomal triacylglycerol transfer protein prevents presecretory degradation of apolipoprotein B-100. A dithiothreitol-sensitive protease is involved

The role of microsomal triacylglycerol transfer protein (MTP) in the secretion of apolipoprotein B-100 (apoB-100) has been studied using an inhibitor of MTP: 4'-bromo-3'-methylmetaqualone. In vitro, this compound inhibits trioleoylglycerol transfer between lipid vesicles mediated by MTP with an IC50 of 0.9 microM whereas it does not inhibit the lipid transfer mediated by the cholesteryl ester transfer protein. In HepG2 cells, 4'-bromo-3'-methylmetaqualone inhibits the secretion of apoB-100 with an IC50 of 0.3 microM, without affecting the secretion of several other proteins like apoA-I or albumin. Moreover, there is no accumulation of apoB-100 in treated cells. Oleic acid, which increases apoB-100 secretion, only slightly modifies the IC50 of 4'-bromo-3'-methylmetaqualone (0.5 microM). The latter has no effect on the synthesis of major lipids within the cell, but decreases the secretion of triacylglycerol into apoB-100-containing lipoproteins. Pulse/chase experiments reveal that 4'-bromo-3'-methylmetaqualone acts on apoB-100 production either at the co-translational or post-translational level. The cysteine protease inhibitor N-acetyl-leucyl-leucyl-norleucinal does not protect apoB-100 from the 4'-bromo-3'-methylmetaqualone effect but seems to be involved in a later step of apoB-100 intracellular degradation. By contrast, dithiothreitol can totally reverse the effect of the MTP inhibitor on apoB-100 production. The mechanism of MTP-mediated lipid assembly with apoB-100 is discussed.

Increased catabolic rate of low density lipoproteins in humans with cholesteryl ester transfer protein deficiency

The cholesteryl ester transfer protein (CETP) transfers lipids among lipoprotein particles and plays a central role in lipoprotein metabolism. Humans with genetic deficiency of CETP have both elevated HDL cholesterol and apolipoprotein A-I concentrations as well as decreased LDL cholesterol and apolipoprotein B levels. The present study was undertaken to elucidate the metabolic basis for the decreased LDL cholesterol and apo B levels in CETP deficiency. We conducted a series of in vivo apo B kinetic studies in tow unrelated homozygotes with CETP deficiency and in control subjects. A primed constant infusion of stable isotopically labeled phenylalanine was administered to the two CETP deficient subjects and control subjects and apo B kinetic parameters in VLDL, intermediate density lipoproteins, and LDL were obtained by using a multicompartmental model. The fractional catabolic rates (FCR) of LDL apo B were significantly increased in the CETP-deficient subjects (0.56 and 0.75/d) compared with the controls (mean FCR of 0.39/d). Furthermore, the production rates of apo B in VLDL and intermediate density lipoprotein were decreased by 55% and 81%, respectively, in CETP deficiency compared with the controls. In conclusion, CETP-deficient subjects were demonstrated to have substantially increased catabolic rates of LDL apo B as the primary metabolic basis for the low plasma levels of LDL apo B. This result indicates that the LDL receptor pathway may be up-regulated in CETP deficiency.

Presence of an extended duplication in the putative low-density-lipoprotein receptor-binding domain of apolipoprotein B. Cloning and characterization of the domain in salmon

The sequence of the C-terminal 1058 amino acids of atlantic salmon (Salmo salar) apolipoprotein (apo) B was deduced from the nucleotide sequence of cloned cDNA. In comparison with chicken or mammals apoB-100, salmon apoB is C-terminally truncated and extended gaps are found. The two clusters of positively charged residues, previously identified as part of the putative low-density-lipoprotein (LDL) receptor-binding domain of apoB, are brought into close proximity in salmon apoB. This is achieved by the absence between the two clusters of the proline-rich area with the potential to form an amphipathic beta sheet, present in higher vertebrates. In addition, analysis of apoB amino acid sequences currently available in vertebrates revealed the presence of an extended internal duplication in the putative LDL receptor-binding domain. Thus, the two basic clusters would have been duplicated resulting in the presence, except for salmon apoB, of two homologous sites in the C-terminal part of the molecule. The results described here together with earlier biochemical and genetic evidence support the view that Arg3500, a residue mutated in familial defective apoB-100, could be included in a folded critical region of the putative LDL receptor-binding domain of human apoB-100. This region possibly brings the two sub-domains that arise from the duplication close to each other.

Tandem mass spectrometric characterization of a specific cysteic acid residue in oxidized human apoprotein B-100

The oxidation of low density lipoprotein (LDL) in vivo may result in its unregulated uptake by macrophages, with the consequent accumulation of cholesterol that is characteristic of the development of atherosclerosis. This paper describes initial experiments to elucidate structural changes that occur in an in vitro model of LDL oxidation. LDL was isolated from human blood and oxidized in the presence of copper ion. Lipid was removed and the isolated apoprotein was subjected to tryptic hydrolysis. The hydrolysate was separated by high performance liquid chromatography and individual fractions were screened by amino acid analysis to detect cysteic acid residues. Appropriate fractions were analyzed by fast atom bombardment mass spectrometry and hybrid tandem mass spectrometry. In this manner a tryptic fragment was identified that corresponded to residues 4187-4195 (EELCTMFIR), in which the cysteine and methionine residues were oxidized to cysteic acid and methionine sulfoxide, respectively. Identical analysis of LDL not subjected to in vitro oxidation revealed no evidence for this oxidized peptide. Earlier work established a surface location for this cysteine residue (Cys24) on the LDL particle, which suggested that its modification may significantly affect the properties of LDL, such as the propensity to intermolecular interaction via disulfide bridges. The analytical protocol developed here (involving proteolysis, screening of peptide fragments, and tandem mass spectrometry analysis) constitutes a strategy of general applicability to the characterization of targeted modifications of large proteins via mass spectrometry.

Murine mammary-derived cells secrete the N-terminal 41% of human apolipoprotein B on high density lipoprotein-sized lipoproteins containing a triacylglycerol-rich core

The cDNA encoding the N-terminal 41% of human apolipoprotein B (apoB), apoB-41, was transfected into nonhepatic, nonintestinal, mammary-derived mouse cells (C127) to generate stably transfected cells expressing human apoB-41 (C127B-41). As determined by centrifugation, apoB-41 is secreted exclusively on lipoproteins (LPs) having a peak density of 1.13 g/ml. Electron microscopy of apoB-41-containing LPs purified by immunoaffinity chromatography showed round particles about 12 nm in diameter. No discoidal particles were observed. Characterization of apoB-41-associated lipids after labeling C127B-41 cells with [3H]oleate and immunoprecipitating the secreted LPs with antibodies to apoB showed that 3H-labeled triacylglycerols were a major lipid class and accounted for about 54% of the total labeled lipids. Cholesterol esters and phospholipids accounted for about 6% and 22%, respectively. Incubation of cells with 0.4 mM oleate resulted in an increased incorporation of the added oleate into lipids associated with secreted apoB-41, along with a 2- to 3-fold increased secretion of apoB-41. The newly formed LPs appear to be transported through the Golgi complex, as brefeldin A (1 microgram/ml) and monensin (1 microM) greatly reduced (> 90%) the secretion of labeled apoB-41 and the amount of triacylglycerol and phospholipid associated with it. Microsomal triacylglycerol transfer protein (MTP) was not detected in these cells. Taken together, the data presented demonstrate that apoB-41 can direct the assembly and secretion of LPs that contain a triacylglycerol-rich core in nonhepatic cells that apparently lack MTP. These cells, therefore, represent an important model for studying LP assembly and may offer some advantages over cultured hepatic or intestinal cells that express their endogenous apoB gene.

Apolipoprotein B gene DNA polymorphisms (EcoRI and MspI) and serum lipid levels in the Serbian healthy population: interaction of rare alleles and smoking and cholesterol levels

The frequency of restriction fragment length polymorphisms (RFLPs) of the apolipoprotein B (apo B) gene, detected by EcoRI and MspI, and their influence on serum lipids were studied in a total of 239 healthy subjects from the Belgrade area. The influence of interaction between different genotypes and smoking was also studied. The relative frequency of both rare R2 and M2 alleles (lacking the cutting site) was similar to that reported in other groups of Caucasians (0.16 and 0.11, respectively). No association was observed between the apo B genotypes and serum lipid levels adjusted for age, body mass index, and blood pressure either in the whole sample or in either women or men. When smokers and non-smokers were considered separately, smoking had a significant impact on total cholesterol variability in all individuals with genotype M1M2 and high density lipoprotein (HDL) cholesterol variability in women with genotype R1R2. The presence of the rare alleles of these two polymorphisms in smokers was associated with lower lipid levels in the whole sample and in both women and men analyzed separately, except for an increase of HDL cholesterol level in male smokers, heterozygous for EcoRI polymorphism (R1R2).

The effect of variation in the apolipoprotein B gene on plasmid lipid and apolipoprotein B levels. I. A likelihood-based approach to cladistic analysis

A new method is described for employing family data to test for significant haplotype effects on continuously distributed variables, using likelihood-ratio tests of linear models in which haplotype effects are parameterized and familial correlations taken into account. The method is applied to the apolipoprotein B (Apo B) gene, using 5 polymorphisms (Insertion/deletion, Bsp1286I, XbaI, MspI, EcoRI) to define haplotypes in 121 French nuclear families. Eleven haplotypes were found, five of which, combined, account for over 95% of the sample. A haplotype phylogeny is proposed, and is used to define a nested set of models for testing the effects of Apo B variation on total-, low-density-lipoprotein (LDL)-, and high-density-lipoprotein (HDL)-cholesterol, triglyceride, and Apo B levels. Apo B haplotype effects account for about 10% of the genetic variance and 5% of the total variance in HDL-cholesterol and triglyceride levels. Clusters of evolutionarily-related haplotypes with similar phenotypic effects are identified for HDL-cholesterol and triglycerides. Single haplotypes with statistically significant effects are identified for cholesterol, LDL-cholesterol, and Apo B levels.

Apolipoprotein B and low-density lipoprotein structure: implications for biosynthesis of triglyceride-rich lipoproteins

ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine, and is the ligand recognized by the LDL receptor during receptor-mediated endocytosis. ApoB100 is encoded by a single gene on chromosome 2, and the message undergoes a unique processing event to form apoB48 message in the human intestine, and, in some species, in liver as well. The primary sequence is relatively unique and appears unrelated to the sequences of other serum apolipoproteins, except for some possible homology with the receptor recognition sequence of apolipoprotein E. From its sequence, structure prediction shows the presence of both sheet and helix scattered along its length, but no transmembrane domains apart from the signal sequence. The multiple carbohydrate attachment sites have been identified, as well as the locations of most of its disulfides. ApoB is the single protein found on LDL. These lipoproteins are emulsion particles, containing a core of nonpolar cholesteryl ester and triglyceride oil, surrounded by an emulsifying agent, a monolayer of phospholipid, cholesterol, and a single molecule of apoB100. An emulsion particle model is developed to predict accurately the physical and compositional properties of an LDL of any given size. A variety of techniques have been employed to map apoB100 on the surface of the LDL, and all yield a model in which apoB surrounds the LDL like a belt. Moreover, it is concluded that apoB100 folds into a long, flexible structure with a cross-section of about 20 x 54 A2 and a length of about 585 A. This structure is embedded in the surface coat of the LDL and makes contact with the core. During lipoprotein biosynthesis in tissue culture, truncated fragments of apoB100 are secreted on lipoproteins. Here, it was found that the lipoprotein core circumference was directly proportional to the apoB fragment size. A cotranslational model has been porposed for the lipoprotein assembly, which includes these structural features, and it is concluded that in permanent hepatocyte cell lines, apoB size determines lipoprotein core circumference.

Accumulation of "small dense" low density lipoproteins (LDL) in a homozygous patients with familial defective apolipoprotein B-100 results from heterogenous interaction of LDL subfractions with the LDL receptor

The interaction of LDL and LDL subfractions from a patient homozygous for familial defective apoB-100 (FDB) has been studied. His LDL cholesterol ranged from 2.65 to 3.34 g/liter. In cultured fibroblasts, binding, internalization, and degradation of the patient's LDL was diminished, but not completely abolished. The patient's apolipoprotein E concentration was low, and the amount of apolipoprotein E associated with LDL was not elevated over normal. LDL were separated into six subfractions: LDL-1 (1.019-1.031 kg/liter), LDL-2 (1.031-1.034 kg/liter), LDL-3 (1.034-1.037 kg/liter), LDL-4 (1.037-1.040 kg/liter), LDL-5 (1.040-1.044 kg/liter), and LDL-6 (> 1.044 kg/liter). LDL-5 and LDL-6 selectively accumulated in the patient's plasma. Concentrations of LDL-1 to 3 were normal. The LDL receptor-mediated uptake of LDL-1 and LDL-2 could not be distinguished from normal LDL. LDL-3 and LDL-4 displayed reduced uptake; LDL-5 and LDL-6 were completely defective in binding. When apolipoprotein E-containing particles were removed by immunoabsorption before preparing subfractions, LDL-3 and LDL-4, but not LDL-1 and LDL-2, retained some receptor binding activity. We conclude that in FDB, LDL-1 and LDL-2 contain sufficient apolipoprotein E to warrant normal cellular uptake. In LDL-3 and LDL-4, the defective apoB-100 itself displays some receptor binding; LDL-5 and LDL-6 are inable to interact with LDL receptors and accumulate in plasma.

Heterogeneity of molecular weight and apolipoproteins in low density lipoproteins of healthy human males

The molecular weights of five low density lipoprotein (LDL) subfractions from four normal healthy males were determined by analytic ultracentrifuge sedimentation equilibria. Protein content of each subfraction was determined by elemental CHN analysis, and weights of apoprotein peptides were calculated. Molecular weights in subfractions of increasing density were 2.92 +/- 0.26, 2.94 +/- 0.12, 2.68 +/- 0.09, 2.68 +/- 0.28 and 2.23 +/- 0.22 million Da, and protein weight percentages were 21.05, 21.04, 22.05, 23.10 and 29.10, in subfractions 1, 2, 3, 4 and 5, respectively. Total mean apoprotein weights for respective subfractions were 614 +/- 53, 621 +/- 45, 588 +/- 9, 637 +/- 83 and 645 +/- 62 KDa. In addition to a single apoprotein B-100 (apo B-100) peptide with a mean carbohydrate content of 7.1% and a molecular weight of 550 KDa per LDL particle, there may be one or more apoprotein E peptides of 34 KDa and/or apoprotein C-III of 9 KDa. In addition, subfractions 4 and 5 may contain 3-7% apolipoprotein (a). There is considerable heterogeneity among LDL subfractions as well as within the same fraction from different individuals. This heterogeneity may relate to differences in origin, metabolism and/or atherogenicity as a result of their content of apoproteins other than apo B-100.