Structural Domains of Human Apolipoprotein B-100

Structural changes induced by acidic pH in human apolipoprotein B-100

Acidification in the endosome causes lipoprotein release by promoting a conformational change in the LDLR allowing its recycling and degradation of LDL. Notwithstanding conformational changes occurring in the LDLR have expanded considerably, structural changes occurring in LDL particles have not been fully explored yet. The objectives of the present work were to study structural changes occurring in apoB100 by infrared spectroscopy (IR) and also LDL size and morphology by dynamic light scattering (DLS) and electron microscopy (EM) at both pH 7.4 and 5.0. We determined by IR that pH acidification from 7.4 to 5.0, resembling that occurring within endosomal environment, induces a huge reversible structural rearrangement of apoB100 that is characterized by a reduction of beta-sheet content in favor of alpha-helix structures. Data obtained from DLS and EM showed no appreciable differences in size and morphology of LDL. These structural changes observed in apoB100, which are likely implied in particle release from lipoprotein receptor, also compromise the apoprotein stability what would facilitate LDL degradation. In conclusion, the obtained results reveal a more dynamic picture of the LDL/LDLR dissociation process than previously perceived and provide new structural insights into LDL/LDLR interactions than can occur at endosomal low-pH milieu.

Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity

Familial hypercholesterolaemia (FH) is an inherited autosomal dominant disorder resulting from defects in the low-density lipoprotein receptor (LDLR), in the apolipoprotein B (APOB) or in the proprotein convertase subtilisin/kexin type 9 (PCSK9) genes. In the majority of the cases FH is caused by mutations occurring within LDLR, while only few mutations in APOB and PCSK9 have been proved to cause disease. p.(Arg3527Gln) was the first mutation in APOB being identified and characterized. Recently two novel pathogenic APOB variants have been described: p.(Arg1164Thr) and p.(Gln4494del) showing impaired LDLR binding capacity, and diminished LDL uptake. The objective of this work was to analyse the structure of p.(Arg1164Thr) and p.(Gln4494del) variants to gain insight into their pathogenicity. Secondary structure of the human ApoB100 has been investigated by infrared spectroscopy (IR) and LDL particle size both by dynamic light scattering (DLS) and electron microscopy. The results show differences in secondary structure and/or in particle size of p.(Arg1164Thr) and p.(Gln4494del) variants compared with wild type. We conclude that these changes underlie the defective binding and uptake of p.(Arg1164Thr) and p.(Gln4494del) variants. Our study reveals that structural studies on pathogenic variants of APOB may provide very useful information to understand their role in FH disease.

Apo B100 similarities to viral proteins suggest basis for LDL-DNA binding and transfection capacity

LDL mediates transfection with plasmid DNA in a variety of cell types in vitro and in several tissues in vivo in the rat. The transfection capacity of LDL is based on apo B100, as arginine/lysine clusters, suggestive of nucleic acid-binding domains and nuclear localization signal sequences, are present throughout the molecule. Apo E may also contribute to this capacity because of its similarity to the Dengue virus capsid proteins and its ability to bind DNA. Synthetic peptides representing two apo B100 regions with prominent Arg/Lys clusters were shown to bind DNA. Region 1 (0014Lys-Ser0160) shares sequence motifs present in DNA binding domains of Interferon Regulatory Factors and Flaviviridae capsid/core proteins. It also contains a close analog of the B/E receptor ligand of apo E. Region 1 peptides, B1-1 (0014Lys-Glu0054) and B1-2 (0055Leu-Ala0096), mediate transfection of HeLa cells but are cytotoxic. Region 2 (3313Asp-Thr3431), containing the known B/E receptor ligand, shares analog motifs with the human herpesvirus 5 immediate-early transcriptional regulator (UL122) and Flaviviridae NS3 helicases. Region 2 peptides, B2-1 (3313Asp-Glu3355), and B2-2 (3356Gly-Thr3431) are ineffective in cell transfection and are noncytotoxic. These results confirm the role of LDL as a natural transfection vector in vivo, a capacity imparted by the apo B100, and suggest a basis for Flaviviridae cell entry.

Absence of hyperlipidemia in LDL receptor-deficient mice having apolipoprotein B100 without the putative receptor-binding sequences

OBJECTIVE

To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, on the production of LDL and development of atherosclerosis in vivo.

METHODS AND RESULTS

Ldlr(-/-)Apobec1(-/-) mice lacking the LDLR and apoB editing enzyme accumulated LDL in plasma and developed severe atherosclerosis when they had wild-type apoB100. In marked contrast, in Ldlr(-/-)Apobec1(-/-) mice carrying the Apob100-beta mutation, in the 2 putative LDLR-binding domains of apoB prevented both LDL accumulation and atherosclerosis. Intestinal absorption of lipids and triglyceride secretion from the liver were not affected. However, the VLDL particles with apoB100-beta were larger in volume by about 70%, and carried approximately four times as much apoE per particle. ApoB100-beta synthesis rate in the primary hepatocytes was normal, but its intracellular degradation was enhanced. Additionally, mutant apoB100 VLDL cleared from the circulation more quickly in vivo through apoE-LRP-mediated mechanism than VLDL with wild-type apoB100. In contrast, uptake of the 2 VLDL by macrophages were not different.

CONCLUSIONS

While conformational change to apoB100 during conversion of VLDL to LDL exposes LDLR binding domains and facilitates LDLR-mediated lipoprotein clearance, it may also inhibit LRP-mediated VLDL uptake and contribute to LDL accumulation in familial hypercholesterolemia.

Low density lipoprotein aged in plasma forms clusters resembling subendothelial droplets: aggregation via surface sites

In early phases of atherogenesis, droplets and vesicles accumulate in the subendothelial extracellular space of arterial intima. There is much evidence to suggest that these droplets, ranging between 100 and 400 nm, derive from modified low-density lipoprotein (LDL). In investigations of the formation mechanism of these droplets, LDL fusion was previously induced in vitro by proteolysis, lipolysis, oxidation, and vigorous shaking, but all treatments failed to reproduce the size distribution range of in vivo droplets, mostly resulting, instead, in particles with a diameter intermediate between that of one and two LDL. Our approach was meant to mimic LDL aging in plasma. LDL isolated from plasma that was incubated overnight at 37 degrees C is slightly modified in the secondary structure of its protein component and is primed to form very large aggregates according to a reaction-limited mechanism. This mechanism requires interactions between selected surface sites, whereas massive fusion is ruled out. In the frame of the general theory for colloids, the aggregation of LDL aged in plasma fulfills all the requirements of the reaction-limited mechanism, encompassing 1), exponential growth; 2), fractal structure, with the dimension of elementary constituent still consistent with a single LDL; and 3), extreme polydispersity of aggregates, with shape and dimension very close to that of droplets observed in vivo.

Atherosclerosis: another protein misfolding disease?

The secondary structure and conformation of apo-B 100 in low-density lipoproteins (LDL) are imposed by lipid-protein interactions and dynamics, and affected by the introduction or removal of lipids during the course of lipoprotein metabolism. Following an alteration of the water-lipid interface as a result of, for example, oxidation of lipids, the supramolecular structure becomes destabilized and apoB can misfold. These events have been observed in LDL(-), a fraction of oxidatively modified LDL isolated in vivo. This modified lipoprotein possesses several atherogenic properties and represents an in vivo counterpart of in vitro modified LDL that is implicated in atherosclerosis. The misfolding of apoB, its aggregation, resistance to proteolysis, and cytotoxicity are common motifs shared by LDL(-) and amyloidogenic proteins. Based on these analogies, we propose that atherogenesis could be considered as a disease produced by the accumulation of cytotoxic and pro-inflammatory misfolded lipoproteins.

Translocational status of ApoB in the presence of an inhibitor of microsomal triglyceride transfer protein

Despite numerous studies demonstrating that microsomal triglyceride transfer protein (MTP) activity is critical to apoB secretion, there is still controversy as to whether MTP directly facilitates the translocation of apoB across the membrane of the endoplasmic reticulum (ER) through either the recruitment of lipids and/or chaperone activity. In the present study, a specific inhibitor of MTP (BMS 197636) was utilized in HepG2 cells to investigate whether a direct relationship exists between the translocation of apoB across the ER membrane and the lipid-transferring activity of MTP. Inhibition of MTP (with 10 and 50 nmol/L of the inhibitor) did not significantly affect the translocation of newly synthesized apoB (P = 0.77) or the translocational efficiency of the steady-state apoB mass (P = 0.45), despite a 49% decrease in apoB secretion and increased proteosomal degradation. These results compared well with subcellular fractionation experiments which showed no significant change in the fraction of apoB accumulated in the lumen of isolated microsomes in MTP-treated cells (P = 0.35). In summary, MTP lipid transfer activity does not appear to influence translocational status of apoB, but its inhibition is associated with an increased susceptibility to proteasome-mediated degradation and reduced assembly and secretion of apoB lipoprotein particles.

Well-defined regions of apolipoprotein B-100 undergo conformational change during its intravascular metabolism

Apolipoprotein B (apoB)-100-containing lipoproteins are secreted from the liver as large triglyceride-rich very low density lipoproteins (VLDLs) into the circulation, where they are transformed, through the action of lipases and plasma lipid transfer proteins, into smaller, less buoyant, cholesteryl ester-rich low density lipoproteins (LDLs). As a consequence of this intravascular metabolism, apoB-containing lipoproteins are heterogeneous in size, in hydrated density, in surface charge, and in lipid and apolipoprotein composition. To identify specific regions of apoB that may undergo conformational changes during the intravascular transformation of VLDLs into LDLs, we have used a panel of 29 well-characterized anti-apoB monoclonal antibodies to determine whether individual apoB epitopes are differentially expressed in VLDL, intermediate density lipoprotein (IDL), and LDL subfractions isolated from 6 normolipidemic subjects. When analyzed in a solid-phase radioimmunoassay, the expression of most epitopes was remarkably similar in VLDLs, IDLs, and LDLs. Two epitopes that are close to the apoB LDL receptor-binding site show an increased expression in large (1.019 to 1.028 g/mL), medium (1.028 to 1.041 g/mL), and small (1.041 to 1.063 g/mL) LDLs compared with VLDLs and IDLs, and 2 epitopes situated between apoB residues 4342 and 4536 are significantly more immunoreactive in small and medium-sized LDLs compared with VLDLs, IDLs, and large LDLs. Therefore, as VLDL is converted to LDL, conformational changes identified by monoclonal antibodies occur at precise points in the metabolic cascade and are limited to well-defined regions of apoB structure. These conformational changes may correspond to alterations in apoB functional activities.

The NH2-terminal region of apolipoprotein B is sufficient for lipoprotein association with glycosaminoglycans

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. The co-localization of apolipoprotein (apo) B and proteoglycans within lesions has suggested that retention is due to lipoprotein interaction with these highly electronegative glycoconjugates. Both apoB100- and apoB48-containing lipoproteins, i.e. low density lipoproteins (LDLs) and chylomicron remnants, are atherogenic. This suggests that retention is due to determinants in the initial 48% of apoB. To test this, the interaction of an apoB fragment (apoB17), and apoB48- and apoB100- containing lipoproteins with heparin, subendothelial matrix, and artery wall purified proteoglycans was studied. ApoB100-containing LDL from humans and human apoB transgenic mice and apoB48-containing LDLs from apoE knockout mice were used. Despite the lack of the carboxyl-terminal 52% of apoB, the apoB48-LDL bound to heparin-affinity gel as well as did apoB100-LDL. An NH2-terminal fragment containing 17% of full-length apoB was made using a recombinant adenovirus; apoB17 bound to heparin as well as did LDL. Monoclonal antibodies against the NH2-terminal region of apoB decreased apoB100 LDL binding to heparin, whereas antibodies against the LDL receptor-binding region did not alter LDL-heparin interaction. The role of the NH2-terminal region of apoB in LDL interaction with matrix molecules was also assessed. Media containing apoB17 decreased LDL binding to subendothelial matrix by 42%. Moreover, removal of the apoB17 by immunoprecipitation abrogated the inhibitory effect of these media. Antibodies to the NH2-terminal region decreased LDL binding to matrix and dermatan sulfate proteoglycans. Purified apoB17 effectively competed for binding of LDL to artery derived decorin and to subendothelial matrix. Thus, despite the presence of multiple basic amino acids near the LDL receptor-binding domain of LDL, the NH2-terminal region of apoB is sufficient for the interaction of lipoproteins with glycoconjugates produced by endothelial and smooth muscle cells. The presence of a proteoglycan-binding site in the NH2-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.

Conformational changes in apolipoprotein B modulate intracellular assembly and degradation of ApoB-containing lipoprotein particles in HepG2 cells

The linkage between the conformation of apolipoprotein B100 (apoB) and the intracellular assembly and degradation of apoB-containing lipoproteins was investigated in the present study. Disruption of disulfide bond formation in newly synthesized apoB molecules through the use of the reducing agent DTT resulted in a decrease in the secretion of apoB-containing lipoproteins from HepG2 cells compared with control cells. The synthesis of total apoB (apoB100 plus nascent chains), as well as a number of control proteins, such as albumin and alpha 1-antitrypsin, was decreased significantly in DTT-treated cells. However, the intracellular accumulation of full-length apoB100 molecules was not inhibited in the presence of DTT. Subcellular fractionation indicated that apoB molecules isolated from the microsomes of DTT-treated cells had an increased association with the microsomal membrane compared with apoB isolated from untreated cells. Analysis of the distribution of apoB-containing lipoproteins from the lumen of isolated microsomes demonstrated that in the presence of DTT, there was a shift in the distribution, such that there was a decrease in the formation of HDL-sized (lipid-poor) apoB-containing lipoproteins and a decrease in the formation of LDL/VLDL apoB particles. Alterations in apoB conformation and their impact on degradation were also investigated by using DTT and by inhibiting N-linked glycosylation with tunicamycin. DTT appeared to change the rate and pattern of apoB degradation. Degradation was accelerated in both intact and permeabilized HepG2 cells. ApoB degradation occurred in DTT-treated permeabilized cells without the usual generation of the 70-kD and 335-kD fragments and was largely N-acetyl-leucyl-leucyl-norleucinal (ALLN) insensitive. In tunicamycin-treated cells, DTT further accelerated the degradation of unglycosylated apoB. Overall, the data suggest that the misfolding of apoB may prevent the proper association of apoB with lipids, resulting in impairment of the assembly of mature apoB-containing lipoproteins. Alteration in the conformation of apoB also appears to alter the degradation pathway of apoB, such that the protein is degraded through a pathway that is at least in part ALLN insensitive.

Studies on intracellular translocation of apolipoprotein B in a permeabilized HepG2 system

Recent evidence suggests that the rate of apolipoprotein B-100 (apoB) translocation may be a key regulatory point in the production of apoB-containing lipoproteins. We have developed an in vitro system to measure the translocation rate of apoB in HepG2 cells. Intact cells were initially pretreated with oleate and N-acetyl-Leu-Leu-norleucinal to maximize the translocation rate while minimizing degradation. Cells were pulsed with [35S]methionine, chased (5-30 min), and then permeabilized with digitonin (75 microg/ml). Permeabilized cells were incubated with or without trypsin (200 microg/ml) for 10 min, and digestion was halted with soybean trypsin inhibitor (2 mg/ml). The rate of translocation was determined by comparing the amount of immunoprecipitable intact apoB in trypsin-treated cells with that in control cells at each time point. Under these conditions, two control proteins, alpha1-antitrypsin and transferrin, were fully protected from trypsin digestion, confirming the integrity of the secretory pathway in permeabilized cells. The percentage of apoB translocated steadily increased from 36% after 5 min to 71% after a 30-min chase (mean percentage, n = 3). A characteristic apoB fragmentation pattern resulted from trypsin digestion, and protected fragments of various size including N-terminal 60-70-kDa fragments were identified. Subcellular fractionation of the cells confirmed that the apoB pool protected from trypsin digestion was luminal in nature, confirming its translocation. ApoB translocation was significantly increased in oleate-treated cells compared with untreated cells. Inhibition of peptidylprolyl isomerase through the use of cyclosporin A and disruption of disulfide bond formation using dithiothreitol reduced the percentage of translocated apoB by 37 and 63%, respectively. Dithiothreitol induced specific changes in the pattern of protected apoB fragments, suggesting a conformational change in apoB that may hinder its translocation. Inhibition of N-linked glycosylation with tunicamycin did not significantly alter the rate of apoB translocation but appeared to stimulate its degradation. Together, the data suggest that the rate of apoB translocation across the membrane of the ER is determined by both lipid availability as well as the correct conformation of nascent apoB molecules.

Reconstituted high density lipoprotein reduces the capacity of oxidatively modified low density lipoprotein to accumulate cholesteryl esters in mouse peritoneal macrophages

Oxidized low density lipoprotein (ox-LDL) was incubated with discoidal complexes of apolipoprotein A-I (apo A-I) and dimyristoylphosphatidylcholine (DMPC) (DMPC/apo A-I) in a cell-free system and re-isolated on Sephacryl S-400 gel filtration chromatography. Analyses of re-isolated ox-LDL showed that apo A-I was transferred from DMPC/apo A-I to ox-LDL, which accounted for 10% of the total protein of ox-LDL. Re-isolated ox-LDL also showed a 2.2-fold increase in phospholipid and a 14% decrease in cholesterol content on an apo B basis. The electrophoretic mobility of re-isolated ox-LDL was markedly reduced almost to that of native LDL. Moreover, the amounts of re-isolated ox-LDL to be degraded by mouse peritoneal macrophages as well as the capacity of re-isolated ox-LDL to accumulate cholesteryl esters (CE) in these cells were markedly reduced (60% and 80% reduction, respectively), suggesting that the ligand activity of ox-LDL for the scavenger receptor was significantly reduced upon treatment with DMPC/apo A-I. Parallel incubation of ox-LDL with free apo A-I led to a similar incorporation of apo A-I into ox-LDL. However, it had no effects on the ligand activity of ox-LDL. Thus, it is likely that the reduction in the ligand activity of ox-LDL by DMPC/apo A-I is explained by the change in the lipid moiety (mainly phospholipid) of ox-LDL. Since discoidal high density lipoprotein (HDL) is known to occur in vivo, this phenomenon might explain one of the anti-atherogenic functions of HDL.

The effect of lipid peroxidation and lipolysis on the ability of lipoproteins to influence thromboplastin activity

High, low and very low density lipoproteins and lipoprotein (a) were prepared from porcine serum. The apolipoprotein components of the lipoproteins were then isolated and resuspended in soybean lecithin. Apolipoprotein B was also resuspended in lipids more representative of those found in LDL and VLDL. Lipid peroxidation was induced in samples of all the lipoproteins and reconstituted apolipoproteins by incubation with either Cu2+ ions or hedgehog 15-lipoxygenase. Furthermore, aliquots of the samples were incubated with a mixture of lipases. The effect of native preparations and the treated samples on the procoagulant activity of thromboplastin was examined. Native HDL, apo A-II, native LDL, reconstituted LDL and apo B inhibited thromboplastin activity, whereas native VLDL and reconstituted VLDL enhanced this activity. While the ability of HDL and apolipoprotein A-II to inhibit thromboplastin was unaltered by either Cu2+ oxidation, lipoxygenase oxidation or lipolysis, VLDL and particles resembling VLDL, which acted cooperatively with thromboplastin lost their activating potential. On the other hand, LDL and particles resembling LDL changed from being inhibitory to enhancing the thromboplastin activity following oxidation, but not after lipolysis. Apolipoprotein B fragments obtained by mild digestion of this protein, expressed an inhibitory effect towards thromboplastin, while extensive degradation of the protein reduced its inhibitory potential. It is suggested that modifications of lipoproteins in vivo can lead to a hypercoagulable state by modulation of the cofactor activity of thromboplastin to factor VII.

Decreased affinity of low density lipoprotein (LDL) particles for LDL receptors in patients with cholesteryl ester transfer protein deficiency

We have reported that the disorder of lipoprotein metabolism in hyperalphalipoproteinaemic patients with a deficiency of cholesteryl ester transfer protein (CETP) is characterized by the polydisperse low density lipoprotein (LDL) particles and the accumulation of cholesteryl ester (CE) in high density lipoprotein (HDL) particles, forming cholesterol-induced HDL (HDLc)-like particles. In the present study we have investigated the interaction of these abnormal LDL with LDL receptors of normal human fibroblasts. Since the ultracentrifugally separated LDL fraction (1.019 < d < 1.063 g mL-1) from the CETP-deficient patients contained HDLc-like particles, these particles were removed by anti-apolipoprotein (apo) A-I immunoaffinity column chromatography. The lipoproteins eluted in the unbound fraction of this column did not contain apo A-I, so this fraction was considered to be authentic LDL. The authentic LDL of the patients were deficient in CE and rich in triglycerides and apo B. The authentic LDL itself showed polydispersity, ranging in size from 23 nm to 30 nm. The affinity of these abnormal LDL particles for LDL receptors was analysed by a competitive assay in which cold LDL from the patients or control compete with 125I-labelled LDL for fibroblast LDL receptors. The concentration of LDL particles at which 50% of 125I-labelled normal LDL was replaced was two to three times higher for the patients than for the normal control. Therefore, the affinity of patient LDL was thought to be reduced compared to that of control LDL. These results demonstrate that CETP may play an important role in making LDL particles homogeneous and rich in CE.(ABSTRACT TRUNCATED AT 250 WORDS)

Re-evaluation of the structure of low density lipoproteins

Low density lipoprotein (LDL) is an established atherogenic factor. Much effort has therefore been devoted to elucidation of its structure, yielding the generally accepted model according to which the neutral lipids (cholesterol ester and triglycerides) form a lipid core emulsified by phospholipids, cholesterol and the amphipathic Apolipoprotein B. Yet, the detailed structure of LDL is not clear. The present work was carried out with the aim of re-evaluating the LDL structure using the minimal number of assumptions: in view of the previously noted surface deficit (lack of sufficient PL and cholesterol to cover the surface of the lipid core) we have assumed that polar head groups are not covered by apo B. Other than that, we have 'allowed' Apo B to penetrate into the PL monolayers and the lipidic core and to pertrude into the solution (be elevated above the PL head group level). We have also 'allowed' neutral lipid penetration into the monolayer and variation of the thickness of the phospholipid monolayers within reasonable boundaries. Based on the established values of relevant constants (molecular weights and volumes, densities and surface areas) we have computed the radius of the particle, the penetration of Apo B into lipidic milieus and the fraction of the surface area covered by Apo B as functions of the LDL composition, the monolayer thickness and the 'elevation' of Apo B above this monolayer. These computations show that at least 40% of the LDL surface must be covered by protein and that the protein penetrates, on the average, only about a half of the PL monolayer. Thus it is not very likely to penetrate into the lipid core. These general features are preserved in the smaller LDL particles of hypertriglyceridemic patients. Assuming that no PL head group is covered by Apo B, the previously described immobilization of 20% of the phospholipids is likely to result from the interaction of Apo B with neighboring PL. According to our computations this can be regarded consistent with the previously proposed arrangement of the apo B as a '3-4 domain structure' or a long string configuration but inconsistent with 'one domain' or 'twenty domain' structures.

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.

The inhibition of thromboplastin by apolipoprotein-B and the effect of various associated lipids

Human apolipoprotein B was purified by barium sulphate adsorption, subsequent delipidation and gel filtration. The protein was then reconstituted in soya bean lecithin and its inhibitory effect towards thromboplastin was assayed by incubation with rabbit brain thromboplastin. The use of an antibody against human apolipoprotein B diminished this inhibition. The level of inhibition of thromboplastin by the reconstituted apolipoprotein was found to be dependent on the concentration of the phospholipids with which it was reconstituted, reaching a maximum inhibition at a lipid: protein ratio of 1:1 (w/w). However, higher phospholipid concentrations or inclusion of cholesterol esters or triglycerides diminished and at certain concentrations reversed the inhibitory effect of the apolipoprotein. These results point towards apolipoprotein B as an inhibitor whose activity towards thromboplastin could be dependent on the complexes it forms with the surrounding lipids.

Modifications of low-density lipoprotein induced by arterial proteoglycans and chondroitin-6-sulfate

Association of low-density lipoproteins (LDL) with arterial chondroitin sulfate proteoglycans (CSPG) appears to contribute to their deposition in the extracellular intimal compartment and to its internalization by macrophages. CSPG and LDL interact by ionic bridges with formation of soluble and insoluble complexes. We studied the alterations on LDL structure induced by its association with arterial CSPG and other glycosaminoglycans (GAG). In soluble complexes, at low and at physiological ionic strength, arterial CSPG and sulfated GAG modify the kinetics of apoB-100 proteolysis by trypsin. However, less marked alterations in the peptide patterns were observed with proteinase V8 and almost none with thermolysin. This is indirect evidence that the presence of CSPG and GAG modified the exposure of polar regions of apoB-100 in LDL. Competitive binding experiments with agarose-bound heparin and soluble GAG also suggest that after formation of insoluble complexes with arterial CSPG and resolubilization the exposure of Lys, Arg-rich segments of apoB-100 is increased. Results from differential scanning calorimetry and differential thermal spectrophotometry showed that the CSPG and GAG-induced modifications reduced the thermal stability of the surface and core in LDL. If present in vivo, the structural alterations of polar segments of the LDL protein moiety may influence the outcome of its interaction with the arterial mesenchyma.