Determination of cysteine on low-density lipoproteins using the fluorescent probe, 5-iodoacetamidofluoresceine

Simple bioconjugate chemistry serves great clinical advances: albumin as a versatile platform for diagnosis and precision therapy

Albumin is the most abundant circulating protein in plasma and has recently emerged as a versatile protein carrier for drug targeting and for improving the pharmacokinetic profile of peptide or protein based drugs. Three drug delivery technologies related to albumin have been developed, which include the coupling of low-molecular weight drugs to exogenous or endogenous albumin, conjugating bioactive proteins by albumin fusion technology (AFT), and encapsulation of drugs into albumin nanoparticles. This review article starts with a brief introduction of human serum albumin (HSA), and then summarizes the mainstream chemical strategies of developing HSA binding molecules for coupling with drug molecules. Moreover, we also concisely condense the recent progress of the most important clinical applications of HSA-binding platforms, and specify the current challenges that need to be met for a bright future of HSA-binding.

Addition of a cysteine to glucagon-like peptide-1 (GLP-1) conjugates GLP-1 to albumin in serum and prolongs GLP-1 action in vivo

Glucagon-like peptide-1 (GLP-1) is a promising new therapeutic agent for the treatment of type 2 diabetes. However, GLP-1 has a short half-life (t(1/)(2)<2min) due to rapid degradation by dipeptidyl peptidase IV in vivo. To circumvent this problem, a recombinant mGLP-1 with a cysteine at the C-terminus of GLP-1 was expressed in Escherichia coli and purified by affinity and reverse-phase chromatography. This addition of a cysteine facilitates mGLP-1 binding to serum albumin both in vitro and in vivo, thus protecting mGLP-1 from protease degradation. Similar to GLP-1, mGLP-1 stimulated cAMP production in PC12 cells and exhibited insulinotropic activity in MIN6 cells under in vitro culture conditions. Importantly, in glucose tolerance tests mice treated with mGLP-1 exhibited much lower glucose levels and much higher insulin levels versus that in mice treated with unmodified GLP-1. Furthermore, the effects of mGLP-1 on reduction of blood glucose levels lasted for 6-7h, while the effects of unmodified GLP-1 only lasted for 0.5-1h after injection. These results demonstrate that mGLP-1 is biologically active and its pharmaceutical efficacy is largely enhanced by the cysteine-mediated covalent conjugation with albumin in the serum after injection. Therefore, the mGLP-1 with a cysteine may be a better potential therapeutic drug than the unmodified GLP-1 for treating type 2 diabetes.

Effect of gangliosides on the copper-induced oxidation of human low-density lipoproteins

The role of gangliosides in the copper-induced oxidative modification of human low-density lipoprotein (LDL) was studied focusing on the early stage of LDL oxidation in which the concentration of conjugated dienes increases only weakly. The changes in the protein and lipid component were followed using fluorescence spectroscopy. The results indicate that binding of gangliosides to LDL causes slower destruction of tryptophan fluorescence and suppresses cross-linking between the reactive groups of the protein and the products of lipid peroxidation. The protective role of gangliosides could be assigned to their interference with the lipid-protein interaction in the LDL particle, which might be important for the maintenance of the native plasma antioxidant status in vivo.

The study of structural accessibility of free thiol groups in human low-density lipoproteins

The experimental evidence for the apolipoprotein B100 (apoB) domain structuring in low-density lipoprotein (LDL) was investigated focusing on the accessibility of free thiol groups. Three different spectroscopic methods were combined with the biochemical perturbations of LDL particle. The spectrophotometric method was adapted for LDL and the exposure of free thiols was analyzed in the native LDL and LDL exposed to sequential denaturation. The results indicate that 24-h denaturation does not expose all free thiols in LDL. Using thiol-specific spin labeling and electron paramagnetic resonance spectroscopy (EPR), different populations of labeled thiols were resolved. The comparison of the EPR spectra of native LDL and LDL with selectively blocked thiol groups revealed significant difference in the respective hyperfine splittings. The phenomenon can arise due to different polarity and/or mobility of the nitroxides in the microenvironments of spin label binding sites of these two LDL samples. The results indicate that nine thiol groups in apoB are distributed in different domains of LDL: two are more exposed, two are buried deeply in the lipid matrix of the particle and the rest are located in hydrophobic parts of this extremely complex protein-lipid assembly. These observations provide experimental support for the emerging theoretical models of apoB.

Probing the cysteine-34 position of endogenous serum albumin with thiol-binding doxorubicin derivatives. Improved efficacy of an acid-sensitive doxorubicin derivative with specific albumin-binding properties compared to that of the parent compound

We have recently proposed a macromolecular prodrug strategy for improved cancer chemotherapy based on two features (Kratz, F.; et al. J. Med. Chem 2000, 43, 1253-1256.): (a) rapid and selective binding of thiol-reactive prodrugs to the cysteine-34 position of endogenous albumin after intravenous administration and (b) release of the albumin-bound drug in the acidic environment at the tumor site due to the incorporation of an acid-sensitive bond between the drug and the carrier. To investigate this therapeutic strategy in greater depth, four (maleinimidoalkanoyl)hydrazone derivatives of doxorubicin were synthesized differing in the length of the aliphatic spacer (1, -(CH(2))(2)-; 2, -(CH(2))(3)-; 3, -(CH(2))(5)-; 4, -(CH(2))(7)-). The albumin-binding doxorubicin prodrugs, especially the (6-maleimidocaproyl)hydrazone derivative of doxorubicin (3), are rapidly and selectively bound to the cysteine-34 position of endogenous albumin. 3 was distinctly superior to the parent compound doxorubicin in three animal tumor models (RENCA, MDA-MB 435, and MCF-7) with respect to antitumor efficacy and toxicity.

Structural aspects of thiol-specific spin labeling of human plasma low density lipoprotein

A novel thiol-specific spin labeling procedure for the protein component (apoprotein B, apoB) of low density lipoproteins (LDLs) is presented. A methanethiosulfonate spin label was used to probe the free cysteine residues of apoB with electron paramagnetic resonance (EPR) spectroscopy. The results indicated that the spin labeled sites are predominantly buried in the LDL particle in two distinct environments that differ in their mobility restrictions. The suitability of thiol-specific labeling for the study of the stability and conformation of apoB was demonstrated in experiments with denaturing agents. The results presented in this work offer a new approach for the matching of EPR data with the primary structure of apoB.

The seventh myth of lipoprotein(a): where and how is it assembled?

Our understanding of the genetics, metabolism and pathophysiology of the atherogenic plasma lipoprotein Lp(a) has considerably increased over past years. Nevertheless, the precise mechanisms regulating the biosynthesis and assembly of Lp(a) are poorly understood and controversially discussed. Lp(a) plasma concentrations are determined by synthesis and not by degradation. Transcriptional and post-translational mechanisms have been identified as regulating Lp(a) production in primary hepatocytes and transfected cell lines. Assembly of Lp(a) occurs extracellularly from newly synthesized apolipoprotein(a) and circulating LDL. This view has recently been challenged by in-vivo kinetic studies in humans which are compatible with an intracellular assembly event. Lp(a) assembly is a complex two-step process of multiple non-covalent interactions between apolipoprotein(a) and apolipoprotein B-100 of LDL followed by covalent disulfide linkage of two free cysteine residues on both proteins.

Expression of large genomic clones in transgenic mice: new insights into apolipoprotein B structure, function and regulation

Extensive manipulation of the apolipoprotein B gene in yeast and bacterial artificial chromosome clones and subsequent expression of these clones in transgenic mice have provided fresh insights into several aspects of apolipoprotein B biology, including the identification of sequences important for lipoprotein (a) assembly, the demonstration that intestinal expression of apolipoprotein B is controlled by DNA sequences > 50 kb from the gene, and the extraordinary finding that apolipoprotein B is expressed in the heart.

Interaction of apolipoprotein[a] with apolipoproteinB-100 Cys3734 region in lipoprotein[a] is confirmed immunochemically

Monospecific polyclonal antibodies (MPAbs) to apoB-100 regions Cys3734 and Cys4190 were isolated by affinity chromatography using the synthetic polypeptides, Q3730VPSSKLDFREIQIYKK3746 and G4182IYTREELSTMFIREVG4198, respectively, coupled to a hydrophilic resin. Molecular modeling and fluroescence labeling studies have suggested that Cys67 located in kringle type 9 (LPaK9, located between residues 3991 and 4068 of the apo[a] sequence inferred by cDNA) of the apo[a] molecule is disulfide linked to Cys3734 of apoB-100 in human lipoprotein[a] (Lp[a]). This possibility has been further explored with MPAbs. Four species of MPAbs directed to a Cys3734 region of apoB-100 (3730-3746) were isolated from goat anti-human LDL serum by a combination of synthetic peptide (Q3730VPSSKLDFREIQIYKK3746) affinity chromatography and preparative electrophoresis (electrochromatography). MPAbs to the Cys4190 region of apoB-100, a second or alternative disulfide link-site between apo[a] and apoB-100, were also isolated using a synthetic peptide (G4182IYTREELSTMFIREVG4198) affinity resin. Results of immunoassays showed that binding of these four MPAbs to Lp[a] was significantly lower than to LDL. In contrast, MPAbs to the apoB-100 region 4182-4198 which contains Cys4190, a second or alternative disulfide link-site between apo[a] and apoB-100, displayed a less significant difference in binding to Lp[a] and LDL. These results provide additional evidence that the residues 3730-3746 of apoB-100 interact significantly with apo-a- in Lp-a-, and that Cys3734 is a likely site for the disulfide bond connecting apo[a] and apoB-100.

Mutagenesis of the human apolipoprotein B gene in a yeast artificial chromosome reveals the site of attachment for apolipoprotein(a)

Lipoprotein(a) [Lp(a)] is a lipoprotein formed by the disulfide linkage of apolipoprotein (apo) B100 of a low density lipoprotein particle to apolipoprotein(a). Prior studies have suggested that one of the C-terminal Cys residues of apo-B100 is involved in the disulfide linkage of apo-B100 to apo(a). To identify the apo-B100 Cys residue involved in the formation of Lp(a), we constructed a yeast artificial chromosome (YAC) spanning the human apo-B gene and used gene-targeting techniques to change Cys-4326 to Gly. The mutated YAC DNA was used to generate transgenic mice expressing the mutant human apo-B100 (Cys4326Gly). Unlike the wild-type human apo-B100, the mutant human apo-B100 completely lacked the ability to bind to apo(a) and form Lp(a). This study demonstrates that apo-B100 Cys-4326 is required for the assembly of Lp(a) and shows that gene targeting in YACs, followed by the generation of transgenic mice, is a useful approach for analyzing the structure of large proteins coded for by large genes.

Site-specific mutagenesis demonstrates that cysteine 4326 of apolipoprotein B is required for covalent linkage with apolipoprotein (a) in vivo

In the formation of the lipoprotein(a) (Lp(a)) particle, apolipoprotein(a) (apo(a)) and apolipoprotein B (apoB) are covalently linked via a disulfide bond in both humans and human-apo(a)/apoB transgenic mice. Studies based upon fluorescent labeling of free cysteine residues have suggested that cysteine 3734 of the 4 carboxyl-terminal cysteines of apoB (Cys-3734, Cys-3890, Cys-4190, and Cys-4326) is the most likely candidate to form a disulfide bond with apo(a). However, other recent studies using truncated apoB molecules suggest that Cys-4326, the terminal cysteine of apoB, may be implicated in the binding to apo(a). In order to definitively show which of apoB's carboxyl-terminal cysteines is essential in interacting with apo(a) we have used RecA-assisted restriction enzyme digestion coupled with site-specific mutagenesis to convert Cys-3734 and Cys-4326 to serine within separate 90-kilobase pair apoB P1 phagemid clones. Transgenic mice containing the normal or mutated apoB transgenes were created, and the covalent association of mutated apoB with apo(a) was assessed in mice transgenic for both apoB and apo(a). Analysis by ultracentrifugation and immunoblotting revealed that Cys-4326, but not Cys-3734, was essential in the formation of the covalent bond between apo(a) and apoB in vivo.

The binding of animal low-density lipoproteins to human apolipoprotein(a)

Lipoprotein(a) [Lp(a)] is a risk factor for coronary artery disease. It is composed of lipids and apolipoprotein(a) [apo(a)] linked to apolipoprotein B (apoB) by a disulphide bond between Cys-4057 of apo(a)'s kringle 36 and possibly Cys-3734 of apoB. We call this the covalent apo(a): apoB-Lp interaction, to distinguish it from the non-covalent apo(a)/Lp(a): apoB-Lp interaction, which is probably mediated by apo(a)'s kringle 33 and residues 3304-3317 of apoB. The non-covalent interaction could be the initial interaction which brings apo(a) and apoB together prior to covalent linkage and Lp(a) formation. The non-covalent apo(a)/Lp(a)-binding site on apoB is evolutionarily more ancient than the covalent apo(a)-binding site on apoB. Both human and non-human low-density lipoproteins (LDLs) bind non-covalently to human apo(a)/Lp(a); however, only rabbit and human LDLs bind covalently to human apo(a). The non-covalent interaction between mouse LDL and human apo(a)/Lp(a) has a Kd of (1.7 +/- 1.33) x 10(-7) M (n = 3). This explains the co-localization of human apo(a) and mouse apoB in the atherosclerotic lesions of human apo(a) transgenic mice and supports our hypothesis that the non-covalent interaction is a contributing factor to apo(a) atherogenicity.

A two-step model for lipoprotein(a) formation

Lipoprotein(a) (Lp(a)), a risk factor for coronary artery disease, is a LDL-like particle with apolipoprotein(a) (apo(a)) covalently linked to apolipoprotein B (apoB), the main protein component of LDL. Apo(a) is highly homologous to plasminogen and its gene probably arose by duplication of the plasminogen gene. It has many repeats of kringle-4-like domain, classified as type 1 through type 10 (T1-T10). T9 is responsible for the covalent linkage between apo(a) and LDL. However, we found that T9 has no affinity for LDL. Therefore, an initial noncovalent interaction between apo(a) and LDL is necessary to bring T9 and LDL together. T6 and possibly T7 of apo(a) were identified as the kringles which mediate this initial interaction. With these findings, a two-step model for Lp(a) formation is proposed. This model should be useful in the design of Lp(a) formation inhibitors. These inhibitors are potential antihyperlipoprotein(a) drugs.

Alternative mRNA splicing and differential promoter utilization determine tissue-specific expression of the apolipoprotein B mRNA-editing protein (Apobec1) gene in mice. Structure and evolution of Apobec1 and related nucleoside/nucleotide deaminases

Apolipoprotein (apo) B mRNA editing consists of a C-->U conversion involving the first base of the codon CAA, encoding Gln 2153, to UAA, a stop codon. Editing occurs in the intestine only in most mammals, and in both the liver and intestine in a few mammalian species including mouse. We have cloned the cDNA for the mouse apoB mRNA editing protein, apobec1. Expression of mouse apobec1 cDNA in HepG2 cells results in the editing of the intracellular apoB mRNA. The cDNA predicts a 229-amino acid protein showing 92, 66, and 70% identity to the rat, rabbit, and human proteins, respectively. Based on the estimated values of divergence of apobec1 sequences in terms of the numbers of synonymous and non-synonymous suhstitutions per site, we found that apobec1 is a fairly rapidly evolving protein. Sequence comparison among mammalian apobec1 sequences has permitted the identification of seven conserved regions that may be functionally important for editing activity. We present a phylogenetic tree relating apobec1 sequences to double-stranded RNA adenosine deaminase and other nucleotide/nucleoside deaminases. Northern blot analysis indicates that apobec1 mRNA exists in two different sizes, a approximately 2.2-kilobase (kb) form in small intestine and a approximately 2.4-kb form in liver, spleen, kidney, lung, muscle, and heart. To study the molecular basis for the different sized apobec1 mRNAs, we cloned the apobec1 gene and characterized its exon-intron organization together with the sequences expressed in the hepatic and intestinal mRNA. The mouse apobec1 gene contains 8 exons and spans approximately 25 kb, and is located in chromosome 6. The major hepatic mRNA contains all 8 exons, whereas the major small intestinal mRNA misses the first 3 exons and its transcription is initiated in exon 4. The intestinal mRNA also contains at its 5' end a unique 102-nucleotide piece that is absent in the liver mRNA. We also identified two alternatively spliced hepatic apobec1 mRNAs with different acceptor sites in exon 4. Transient expression studies using promoter-reporter gene constructs in HeLa, Hepa, and Caco-2 cells indicate that the 5'-flanking sequences of the liver mRNA (i.e. upstream of exon 1) have predominantly hepatic promoter activity and the 5'-flanking sequences of the major small intestine mRNA (i.e. upstream of exon 4) have preferential intestinal promoter activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Copper-induced LDL peroxidation investigated by 1H-NMR spectroscopy

Oxidatively modified LDL (oLDL) is thought to play a key role in the pathogenesis of atherosclerosis. We have studied Cu(2+)-induced peroxidation reactions of LDL and have elucidated the sequence of events which subsequently occur within LDL particles by 1H-NMR spectroscopy. Studies of chloroform/methanol extracts show that LDL arachidonate is oxidised by Cu2+ at a higher rate and to a greater extent than linoleate, giving isomeric hydroperoxides with predominantly trans,trans double-bonds, whilst only cis,trans isomers were detected as intrinsic hydroperoxides in control LDL samples. These intrinsic hydroperoxides were not degraded during peroxidation, suggesting that they are not involved in the initiation of Cu(2+)-induced peroxidation. Aldehydes arising from the decomposition of hydroperoxides were also detected, as well as saturated fatty acids which were released into the external aqueous medium. Decomposition pathways of the two major isomeric hydroperoxides are discussed. Cu(2+)-induced oxidation of LDL cholesterol appears to occur only after hydroperoxide breakdown, with esterified cholesterol being oxidised to a greater extent than free cholesterol. Phospholipid hydrolysis appeared to parallel the peroxidation of arachidonic acid, and the released lysophosphatidylcholine may become associated with apoB. These results suggest that hydroperoxide breakdown (probably in phospholipids) may be a key event in the peroxidation process, leading to the oxidation of cholesterol and propagation into the core of LDL.

Cryoelectron microscopy of low density lipoprotein in vitreous ice

In this report, images of low density lipoprotein (LDL) in vitreous ice at approximately 30 A resolution are presented. These images show that LDL is a quasi-spherical particle, approximately 220-240 A in diameter, with a region of low density (lipid) surrounded by a ring (in projection) of high density believed to represent apolipoprotein B-100. This ring is seen to be composed of four or five (depending on view) large regions of high density material that may represent protein superdomains. Analysis of LDL images obtained at slightly higher magnification reveals that areas of somewhat lower density connect these regions, in some cases crossing the projectional interiors of the LDL particles. Preliminary image analysis of LDL covalently labeled at Cys3734 and Cys4190 with 1.4-nm Nanogold clusters demonstrates that this methodology will provide an important site-specific marker in studies designed to map the organization of apoB at the surface of LDL.

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.

Subunit composition of lipoprotein(a) protein

We determined the molecular weight of four different apo(a) polymorphs by sedimentation equilibrium in 6 M guanidine hydrochloride in order to estimate the molar ratio of apo(a) to apoB in Lp(a). They had molecular weights of 289,000, 310,000, 341,000, and 488,000 and 15, 16, 18, and 27 kringle 4 domains, respectively. Their carbohydrate content was similar (23.2 wt %), as was their partial specific volume (0.682 mL/g). Knowing the mass of apo(a), we estimated the molar ratio of apo(a) to apoB from (1) the molecular weight of the protein moiety of the four respective parent Lp(a) particles as calculated from their mass and percentage composition and the mass of apoB, (2) the mass of apo(a) lost from Lp(a) upon its reduction and carboxymethylation, by determining the difference in mass between Lp(a) and Lp(a-), and (3) from the mass (measured by sedimentation equilibrium in 6 M guanidine hydrochloride) of the lipid-free apoB-apo(a) complex (1.06 x 10(6) daltons) of the Lp(a) particle with the smallest apo(a) polymorph by subtracting the mass of apoB. Our results obtained with each of the three different physicochemical methods indicated that the protein moiety of each of the four Lp(a) particles that was investigated consisted of a complex of two molecules of apo(a) and one molecule of apoB.

Carboxyl-terminal truncation of apolipoproteinB-100 inhibits lipoprotein(a) particle formation

Recombinant expression systems for both apo(a) and apoB were used to identify sequences in apoB which are required for Lp(a) formation. Incubation of a [35S]Cys-labelled 17-kringle form of apo(a) with supernatants from rat hepatoma (McA-RH7777) cells expressing apoB-88, apoB-94 and apoB-100 resulted in covalent r-Lp(a) formation only with apoB-100. Additionally, apoB-86 present in the LDL of a hypobetalipoproteinemic subject did not associate with a 12-kringle form of recombinant apo(a) to form r-Lp(a) complexes. Our data suggest that sequences within the C-terminal 6% of apoB-100 are essential for Lp(a) assembly.

Primary structure of apoB-100

Apolipoprotein B-100 (apoB-100) is the major protein in low-density lipoprotein (LDL) and contains the ligand for binding LDL to its cell surface receptor. Lipoprotein [a] (Lp[a]) is a lipoprotein that consists of LDL and apolipoprotein [a] (apo[a]). The primary structure of apoB-100 has been determined by a combination of recombinant DNA and protein sequencing methods. Using high-performance liquid chromatographic techniques, we have identified sulfhydryl and disulfide groups of apoB-100 from LDL. Sixteen of the 25 cysteine residues in apoB-100 exist in disulfide form. All 14 cysteine residues within the N terminal end of apoB-100 are linked in disulfide bridges. Using the fluorescent sulfhydryl probe, 5-iodoacetoamidofluoresceine, two free sulfhydryls of apoB-100 on LDL were identified at positions 3734 and 4190. Based on its differential susceptibility to trypsin, apoB-100 can be divided into five domains: domain 1 (residues 1-1000), largely trypsin-releasable (TR); domain 2 (residues 1001-1700), alternating TR and trypsin non-releasable (TN); domain 3 (residues 1701-3070), largely TN; domain 4 (residues 3071-4100), mainly TR and mixed; and domain 5 (residues 4101-4536), almost exclusively TN. Based on our data, we propose that the structure of apoB-100 in LDL is probably an elongated form that wraps around the LDL particle, and that Cys3734 of apoB-100 may be the cysteine residue linked to a cysteine of apo[a].

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.

Cys4057 of apolipoprotein(a) is essential for lipoprotein(a) assembly

Lipoprotein(a) contains one copy each of apolipoprotein B-100 and apolipoprotein(a). It has been hypothesized that a disulfide bond might exist between Cys4057 of apolipoprotein(a) and Cys3734 in apolipoprotein B-100. To investigate the role of Cys4057 for lipoprotein(a) assembly, wild-type and in vitro mutagenized apolipoprotein(a) cDNA plasmids were expressed in the human hepatocarcinoma line HepG2. The mutant plasmids encoded apolipoprotein(a) species with Cys4057 exchanged to either serine or glycine. Untransfected HepG2 cells, although able to secrete apolipoprotein B-100-containing lipoproteins, do not synthesize detectable amounts of apolipoprotein(a). After transfection of wild-type plasmid, almost all apolipoprotein(a) in the culture supernatant was present in lipoprotein(a)-like particles as demonstrated by immunoblotting, density-gradient centrifugation, and ELISA. The same analysis performed with supernatants of cells transfected with plasmids mutated in codon 4057 revealed free apolipoprotein(a) glycoprotein without detectable amounts of lipoprotein-associated apolipoprotein(a). Our results strongly suggest the existence of a disulfide bridge between Cys4057 of apolipoprotein(a) and apolipoprotein B-100 within recombinant lipoprotein(a) particles. Furthermore, they indicate that disulfide bridge formation is essential for assembly of the lipoprotein(a)-like complex produced by HepG2 cells and suggest a similar role of Cys4057 during lipoprotein(a) assembly in vivo.

Transgenic mice expressing high plasma concentrations of human apolipoprotein B100 and lipoprotein(a)

The B apolipoproteins, apo-B48 and apo-B100, are key structural proteins in those classes of lipoproteins considered to be atherogenic [e.g., chylomicron remnants, beta-VLDL, LDL, oxidized LDL, and Lp(a)]. Here we describe the development of transgenic mice expressing high levels of human apo-B48 and apo-B100. A 79.5-kb human genomic DNA fragment containing the entire human apo-B gene was isolated from a P1 bacteriophage library and microinjected into fertilized mouse eggs. 16 transgenic founders expressing human apo-B were generated, and the animals with the highest expression had plasma apo-B100 levels nearly as high as those of normolipidemic humans (approximately 50 mg/dl). The human apo-B100 in transgenic mouse plasma was present largely in lipoproteins of the LDL class as shown by agarose gel electrophoresis, chromatography on a Superose 6 column, and density gradient ultracentrifugation. When the human apo-B transgenic founders were crossed with transgenic mice expressing human apo(a), the offspring that expressed both transgenes had high plasma levels of human Lp(a). Both the human apo-B and Lp(a) transgenic mice will be valuable resources for studying apo-B metabolism and the role of apo-B and Lp(a) in atherosclerosis.

Proposed mechanisms for binding of apo[a] kringle type 9 to apo B-100 in human lipoprotein[a]

The protein component of human lipoprotein[a] consists primarily of two apolipoproteins, apo[a] and apo B-100, linked through a cystine disulfide(s). In the amino acid sequence of apo bd, Cys4057 located within a plasminogen kringle 4-like repeat sequence (3991-4068) is believed to form a disulfide bond with a specific cysteine residue in apo B-100. Our fluorescence-labeling experiments and molecular modeling studies have provided evidence for possible interactions between this apo[a] kringle type and apo B-100. The fluorescent probe, fluorescein-5-maleimide, was used in parallel experiments to label free sulfhydryl moieties in lipoprotein[a] and low-density lipoprotein (LDL). In apo B-100 of LDL, Cys3734 was labeled with the probe, but this site was not labeled in autologous lipoprotein[a]. The result strongly implicates Cys3734 of apo B-100 as the residue forming the disulfide linkage with Cys4057 of apo[a]. To explore possible noncovalent interactions between apo B-100 and apo[a], the crystallographic coordinates for plasminogen kringle 4 were used to generate molecular models of the apo[a] kringle-repeat sequence (3991-4068, LPaK9), the only plasminogen kringle 4 type repeat in apo[a] having an extra cysteine residue not involved in an intramolecular disulfide bond. The Cys4057 residue (henceforth designated as Cys67 in the LPaK9 sequence) is believed to form an intermolecular disulfide bond with a cysteine of apo B-100. In computer graphics molecular models of LPaK9, Cys67 is located on the surface of the kringle near the lysine ligand binding site. Selected segments of the LDL apo B-100 sequence that contain free sulfhydryl cysteines were subjected to energy minimization and docking with the ligand binding site and adjacent regions of the LPaK9 model. In the docking experiments, apo B-100 segment 3732-3745 (PSCKLDFREIQIYK) displayed the best fit and the largest number of van der Waals contacts with models of LPaK9. Other apo B-100 peptides with sulfhydryl cysteine were found to be less compatible when minimized with this kringle. These results support and extend previously suggested mechanisms for a complex interaction between apo[a] and apo B-100 that involve more than a simple covalent disulfide bond.

Structural and functional properties of apolipoprotein B in chemically modified low density lipoproteins

The structural and compositional changes occurring during in vitro chemical modification of apolipoprotein B-100 (apo B), the apolipoprotein component of low density lipoproteins (LDL), were investigated in this study. The functional properties of chemically modified apo B and especially its potential to induce accumulation of cholesterol esters in macrophages were related to the structural changes of apo B. Acetylation, maleylation or malondialdehyde conjugation did not significantly affect the lipid composition of LDL. However, the unsaturated cholesteryl esters content, especially that of cholesteryl arachidonate was significantly decreased through Cu-oxidation. The number of reactive lysine residues in apo B was decreased by Cu-catalyzed LDL oxidation, acetylation, maleylation and by malondialdehyde conjugation. The number of free cysteines decreased from six in native apo B-100 to three in Cu-oxidized LDL. The tryptophan fluorescence intensity decreased most in malondialdehyde-conjugated LDL and in Cu-oxidized LDL, compared with acetylated and maleylated LDL. The secondary structure of native and chemically modified LDL was measured by attenuated total reflection infrared spectroscopy and by circular dichroism. No significant changes were observed in the secondary structure of any of the modified LDL. These data suggest that neither acetylation, malondialdehyde treatment or even Cu-oxidation substantially altered the secondary structure of apo B, in spite of significant modifications in the primary structure. Incubation of chemically modified LDL with J774 macrophages induced an accumulation of cellular cholesteryl esters and foam cell formation. The highest cholesterol accumulation was induced after malondialdehyde treatment of LDL. These data suggest that the cellular uptake and accumulation of modified LDL is not modulated by changes in the apo B structure. Rather it seems dependent upon the net charge of the apo B protein and probably involves the modification of critical lysine residues.

Lp(a) as a marker for coronary heart disease risk

Lipoprotein(a) or Lp(a) represents a class of lipoprotein particles having lipid composition similar to low-density lipoprotein (LDL) and a protein moiety, apoB100, covalently linked to apo(a), a glycoprotein with striking structural similarities to plasminogen. High plasma levels of Lp(a) are associated with an increased risk for atherosclerotic cardiovascular disease (ASCVD) by mechanisms yet to be determined. From in vitro and ex vivo observations it is apparent that because of its structural properties, Lp(a) can have both atherogenic and thrombogenic potentials. Means for correcting the high plasma levels of Lp(a) are still limited in effectiveness. The emerging notion that the pathogenicity of Lp(a) is increased in the presence of other risk factors, invites development of approaches for their correction, particularly in subjects with personal and/or family history of ASCVD.

Isolation and characterization of sulfhydryl and disulfide peptides of human apolipoprotein B-100

Twenty-three of the 25 cysteine residues in apolipoprotein B-100 have been isolated directly from tryptic or peptic peptide mixtures. Sixteen cysteine residues exist in disulfide forms: Cys-1-Cys-3, Cys-2-Cys-4, Cys-5-Cys-6, Cys-7-Cys-8, Cys-9-Cys-10, Cys-11-Cys-12, Cys-13-Cys-14, and Cys-20-Cys-21. All of these except Cys-20-Cys-21 are recently discovered disulfide linkages. In addition to Cys-22 and Cys-24, which have been described as sulfhydryls on low density lipoprotein, Cys-15 to Cys-18 and Cys-23 are in the reduced form. Cys-19 and Cys-25 are not yet confirmed. Our results revealed that all identified disulfide linkages are located in the trypsin-releasable regions and that all except Cys-1-Cys-3 and Cys-2-Cys-4 are linked to the neighboring cysteine. We propose a linear model of apolipoprotein B-100 in low density lipoprotein that wraps around the low density lipoprotein molecule.