The cyanogen bromide peptides of the apoprotein of low density lipoprotein (ApoB): its molecular weight from a chemical view

Instability of purified 9-kDa porcine intestinal calcium-binding protein in the absence of calcium

Purified 9-kDa porcine intestinal calcium-binding protein (ICaBP, Calbindin D9K) is unstable when stored at 4 degrees C in the absence of Ca(II). Cleavage of the polypeptide occurs producing approximately 5.2- and 3.7-kDa fragments. The former dimerizes giving a species which migrates on sodium dodecylsulfate-urea gels with an Mr 13,700, in contrast to the observed Mr 11,000 for native ICaBP. The fragmentation also results in an irreversible loss of high affinity Ca(II) bound to ICaBP prevents fragmentation. The dimer can be isolated from aged preparations of apo-ICaBP and is stable on further storage with or without Ca(II) present. The observed molecular weights of the fragments along with the amino acid analysis and ultraviolet spectra of the dimer suggest cleavage of the polypeptide chain of ICaBP in the vicinity of residue 49.

Protein sequencing by tandem mass spectrometry

Methodology for determining amino acid sequences of proteins by tandem mass spectrometry is described. The approach involves enzymatic and/or chemical degradation of the protein to a collection of peptides which are then fractionated by high-performance liquid chromatography. Each fraction, containing as many as 10-15 peptides, is then analyzed directly, without further purification, by a combination of liquid secondary-ion/collision-activated dissociation mass spectrometry on a multianalyzer instrument. Interpretation of collision-activated dissociation mass spectra is described, and results are presented from a study of soluble peptides produced by treatment of apolipoprotein B with cyanogen bromide and trypsin.

The biochemistry of epsilon-amino groups of lysine residues from apolipoprotein B of human low density lipoprotein

The availability of epsilon-lysine residues of apolipoprotein B in LDL for chemical or enzymic modification was investigated. Amino acid analyses of detergent-solubilized apolipoprotein B, following cyanoethylation with acrylonitrile, revealed that 10% of the lysine in apolipoprotein B were unreactive. The unreactive residues were associated with the most hydrophobic subfraction of apolipoprotein B. Since apolipoprotein B has a high molecular weight a study was undertaken to determine whether lysine residues were crosslinked to glutamic acid via epsilon-(gamma-glutamyl)lysine as demonstrated for fibrin. Apolipoprotein B was digested exhaustively with proteases. The content of epsilon-(gamma-glutamyl) lysine was determined by chromatography and isotope dilution. In contrast to earlier reports for serum LDL the data showed that less than 0.01 moles of lysine/mole of LDL apolipoprotein B were present as epsilon-(gamma-glutamyl)lysine in plasma LDL. It was determined also that the crosslinks were not found in apolipoprotein B during clotting since LDL was not a substrate for clotting factor XIII which forms the bond in fibrin. Furthermore, the lipoprotein contained no inherent transglutaminase activity. It is concluded that the lysine residues in LDL, which are unreactive to cyanoethylation, can not be detected in the digests as epsilon-(gamma-glutamyl)lysine.

Isolation and characterization of three monoclonal antibodies to human serum low density lipoprotein apoprotein B

Human serum low density lipoprotein (LDL) is a large (Mr = 2-3 X 10(6), complex particle composed of lipid, protein and carbohydrate. We obtained about 40 mouse spleen-myeloma hybrid cell lines which produce antibodies against LDL. Three of them, SC2, SC3 and SC10, have been cloned and subcloned and their antibody products characterized. They recognize three non-overlapping epitopes in native LDL. Two of them, SC3 and SC10, also are capable of recognizing very low density lipoprotein, (VLDL), whereas SC2 reacts only weakly with VLDL. All three antigenic determinants remain intact, and accessible to antibodies on the LDL protein apo B, prepared by delipidation in a 'non-denaturing' detergent, sodium deoxycholate. However, apo B prepared by organic solvent, ether-ethanol, or sodium dodecyl sulfate (SDS) delipidation, while reacting strongly with SC10, is only poorly recognized by SC2 or SC3. Proteolysis of LDL with trypsin, chymotrypsin, Staphylococcus aureus protease, papain or thermolysin gives, in each case, several non-identical protein fragments which are separable by SDS-polyacrylamide gel electrophoresis. Upon immunoblotting, some of these fragments are now recognized by either SC3 or SC10 but not SC2, some are recognized by both SC3 and SC10, and others are immunologically unreactive. The protein bands that are separated by SDS gel electrophoresis are composed of several non-identical fragments and contain the antigenic sites to differing degrees. Some of the immunologically reactive fragments do not appear to contain carbohydrate. Reduction and carboxymethylation do not destroy the immunoreactivity of LDL toward any of the antibodies; however, modification of lysine residues by citraconic anhydride markedly diminishes the reactivity of LDL toward SC3. It is likely that the two antibodies SC3 and SC10 are directed against different linear amino acid sequences or very stable domains, whereas the third, SC2, is directed against a more fragile conformational domain of apo B.

Molecular cloning and expression of partial cDNAs and deduced amino acid sequence of a carboxyl-terminal fragment of human apolipoprotein B-100

Apolipoprotein (apo) B-100 cDNAs were identified in a human liver cDNA library cloned in the expression vector lambda gt11. The beta-galactosidase-apoB-100 fusion protein was detected by two independently produced low density lipoprotein polyclonal antisera and by three apoB-100 monoclonal antibodies that crossreact with apoB-74. It was not recognized by two apoB-100 monoclonal antibodies that crossreact with apoB-26. The longest clone, lambda B8, was completely sequenced. It contains a 2.8-kilobase DNA fragment containing the codons for the carboxyl-terminal 836 amino acid residues of apo-B-100, as well as the 3' untranslated region of apoB-100 mRNA. We have thus mapped apoB-74 to the carboxyl-terminal portion of apoB-100. The deduced amino acid sequence of the cloned DNA matches the sequences of 14 apoB-100 peptides determined in our laboratory. Minor differences in amino acid sequence were noted in three of the peptides, suggesting polymorphism of apoB-100 at the protein and DNA levels. Secondary structure predictions reveal an unusual pattern for apolipoproteins, consisting of beta-structure (24%), alpha-helical content (33%), and random structure (30%). Ten amphipathic helical regions of 10-24 residues were identified. This carboxyl-terminal fragment of apoB-100 is considerably more hydrophobic than other apolipoproteins with known structure. Its lipid binding regions might include stretches of highly hydrophobic beta-sheets as well as amphipathic helices. Our findings on apoB structure might be important for understanding the role of apoB-100-containing lipoproteins in atherosclerosis.

Human apolipoprotein B: structure of carboxyl-terminal domains, sites of gene expression, and chromosomal localization

Apolipoprotein (apo-) B is the ligand responsible for the receptor-mediated catabolism of low density lipoproteins, the principal cholesterol-transporting lipoproteins in plasma. The primary structure of the carboxyl-terminal 30 percent (1455 amino acids) of human apo-B (apo-B100) has been deduced from the nucleotide sequence of complementary DNA. Portions of the protein structure that may relate to its receptor binding function and lipid binding properties have been identified. The apo-B100 messenger RNA is about 19 kilobases in length. The apo-B100 gene is expressed primarily in liver and, to a lesser extent, in small intestine, but in no other tissues. The gene for apo-B100 is located in the p24 region (near the tip of the short arm) of chromosome 2.

Human apolipoprotein B cDNA clone isolation and demonstration that liver apolipoprotein B mRNA is 22 kilobases in length

An expression library made in plasmids pUC8 and pUC9 with mRNA derived from the human hepatoma cell line HepG2 was screened with a rabbit antiserum to human low density lipoprotein (LDL). Approximately 12,000 clones were screened and five positives were identified. The cDNA inserts were all 1500-1600 base pairs in length. The insert from one clone, pB8, was isolated, labeled by nicktranslation, and found to cross-hybridize strongly with the other four cDNA clones. The pB8 clone produces a fusion protein of approximately equal to 37.5 kDa that reacts in electrophoretic transfer blot analysis with rabbit anti-human LDL. This reactivity can be abolished by pretreatment of the antiserum with purified human LDL, p = 1.025 - 1.050 g/ml. A pB8-derived probe was used to demonstrate that apolipoprotein B (apo B) mRNA is present in HepG2 cells and liver extracts but not in HeLa cells or extracts from small intestine, heart, aorta, spleen, brain, skeletal muscle, lung, kidney, or ovary. RNA transfer blot analysis revealed that HepG2 cell apo B mRNA was approximately equal to 22 kilobases in length. These cDNA clones should allow the isolation of the apo B gene and ultimately the elucidation of the primary structure of this protein.

A partial cDNA clone for human apolipoprotein B

A human liver cDNA library was screened for sequences coding for apolipoprotein B (apo B), the major protein of human low density lipoproteins. A mixture of synthetic oligonucleotides (26 bases long) coding for an amino acid sequence known to exist in apo B was used as a hybridization probe. A clone was identified that had a cDNA insert of 593 base pairs and that contained sequences coding for a peptide of 24 residues that had earlier been isolated from apo B by limited proteolysis. The entire nucleotide sequence of the cDNA insert consists of one open reading frame coding for 197 amino acids. Apo B-related RNAs were found in human liver, baboon liver, and the human hepatoma cell line HepG2. None were detected in placenta, simian virus 40 (SV40)-transformed fibroblasts, and a lymphoblastoid cell line. The length of the mature apo B mRNA was estimated to be 18 kb, enough to code for a protein with a molecular weight in the neighborhood of 500,000.

Solubilization of proteins in dimethyl sulfoxide by permethylation: application to structural studies of apolipoprotein B

The delipidated protein moiety, apolipoprotein B, of human low-density lipoproteins was permethylated in potassium butoxide/dimethyl sulfoxide with methyl iodide. The derivatized protein was soluble in dimethyl sulfoxide and, in the presence of sodium dodecyl sulfate, in an aqueous buffer. Analysis of the methylated apolipoprotein B by polyacrylamide gradient gel electrophoresis in the presence of sodium dodecyl sulfate revealed five discrete bands of lower molecular mass than that of the parent 265-kDa protein, which disappeared upon permethylation. The electrophoretic behavior of the methylated apolipoprotein B was distinctly different from that of the other methylated proteins studied, including transferrin, bovine serum albumin, aldolase, beta-lactoglobulin, and apolipoprotein A-I, all of which had a higher apparent molecular weight after permethylation as compared to the corresponding native polypeptide. Calculated on the basis of methylated standard proteins the five polypeptides of apolipoprotein B have apparent molecular masses of 9.0, 16.6, 25.6, 35.7, and 46.7 kDa. The results suggest that the protein moiety of human low-density lipoprotein consists of subunits. In general, the results indicate that the permethylation method can be used to solubilize hydrophobic proteins in organic solvents for structural studies.

Pulse-chase studies of the synthesis of apolipoprotein B in a human hepatoma cell line, Hep G2

We have used pulse-chase methodology to study the synthesis of apolipoprotein B in a human hepatoma-derived cell line, the Hep G2 cells. A 2-min pulse with [35S]methionine was followed by a chase period varying from 5-90 min. A protein of large molecular mass (estimated molecular mass: 312 +/- 41 kDa, mean +/- SD, n = 8) could be immunoprecipitated from the cells at all chase periods between 5 min and 60 min with both monoclonal antibodies to a narrow density cut of the low density lipoprotein LDL-2 (density: 1.030-1.055 g/ml) and polyclonal antibodies to the apolipoprotein B apo B 100 or to a narrow density cut of LDL-2 (density: 1.030-1.055 g/ml). In addition to this large molecular mass protein, nascent polypeptides could be precipitated after 5, 10 and 15 min of chase. The apolipoprotein B molecules that had been labelled during the pulse disappeared from the cells after 60-90 min of chase, while they started to appear in the medium after 30-35 min of chase. The results obtained indicate (a) that apolipoprotein B is synthesized as one polypeptide with a large molecular mass, (b) that newly synthesized apolipoprotein B molecules are secreted after a delay of 30-35 min, (c) that no intracellular accumulation of apolipoprotein B occurs, and (d) that apolipoprotein B is recovered in the density fraction less than 1.21 g/ml of the medium suggesting that it is secreted in lipoprotein form.

Evidence for a structural relationship between apoB75kDa and human plasma apolipoprotein B 100, from translation of human liver mRNA in vitro and immunochemical studies with monoclonal and polyclonal antibodies

We have investigated the relation between an 80-kDa protein synthesized in vitro in protein-synthesizing system programmed with human liver mRNA [Olofsson, S.-O., Elias, P., Boström, K., Lundholm, K., Norfeldt, P.-I., Wiklund, O., Fager, G., and Bondjers, G. (1983) FEBS Lett. 156, 63-66] and a 70-80-kDa protein, apoB75kDa, isolated from the low-density lipoproteins-2 (LDL-2) [Olofson, S.-O., Boström, K., Svanberg, U., and Bondjers, G. (1980) Biochemistry 19, 1059-1064]. Five monoclonal antibodies directed against LDL-2 as well as polyclonal antibodies against a narrow density cut of LDL-2 (d = 1.030 - 1.055) were used to precipitate apoB-related proteins synthesized in vitro in a protein-synthesizing system programmed with human liver mRNA (or total RNA fraction). With all monoclonal antibodies as well as the polyclonal antibodies, a protein with an estimated molecular mass of 80 +/- 1.3 kDa (mean +/- SD, n = 12) could be precipitated. The observation that all monoclonal antibodies used reacted with apoB75kDa indicates a close immunological relation between this 80-kDa protein and apoB75kDa. Limited proteolysis of the 80-kDa protein (synthesized in the presence of [35S]-methionine) with Staphylococcus aureus V8 protease generated six [35S]-methionine-containing bands that could be separated on a polyacrylamide gradient gel (12-20%). All these radioactive bands corresponded to major protein-stained bands obtained after limited proteolysis of apoB75kDa. This observation suggests a structural relation between the two proteins. Taken together, our results indicate that a protein corresponding to apoB75kDa is synthesized in vitro in a protein synthesizing system programmed with human liver mRNA (or total RNA fraction). We have also compared apoB75kDa and the major component of apoLDL-2, apoB100 [Kane, J. P., Hardman, D.A., and Paulus, H.E. (1980) Proc. Natl Acad. Sci USA 77, 2465-2469] by immunochemical methods. We could demonstrate that six monoclonal antibodies directed against four to six different epitopes on LDL-2, as well as polyclonal antibodies to apoB100 and apoB75kDa, all reacted with apoB75kDa and apoB100. These observations indicate a close immunological relation between the two proteins. Taken together our results support the hypothesis that apoB100 has a subunit structure. We therefore suggest that apoB75kDa is a subunit of apoB100 synthesized in human liver.

Human liver RNA-programmed in vitro synthesis of a polypeptide related to human apolipoprotein B

In an in vitro synthesizing system programmed with RNA from human liver a polypeptide with an estimated Mr of 80 000 (80 kDa) +/- 1400 (mean +/- SD, n = 5) was synthesized. This polypeptide could be precipitated with antiserum to a narrow density cut of LDL (d = 1.030-1.055) or antiserum against the high-Mr form of apoB (apoB 100 [4]). The synthesized protein is immunologically related to a 75 kDa protein isolated from LDL. We suggest that the 80 kDa protein represents a primary translation product of apoB synthesized in human liver.

Detection of unique antigenic determinants on human plasma low density lipoprotein and on delipidated apolipoprotein B

To obtain detailed information on the role played by apolipoprotein B (apo B) in determining the structural and functional properties of human plasma low density lipoprotein, we have initiated immunochemical studies of the polypeptide. We report here the establishment of six hybridoma lines that secrete monoclonal antibodies to low density lipoprotein. In addition to recognizing antigenic determinants on low density lipoprotein, all six monoclonal antibodies react with epitope(s) on very low density, but not high density, lipoproteins. The immunoreactivity of these antibodies with low density lipoprotein and with detergent-delipidated apo B was compared in an enzyme-linked immunosorbent assay. Although all six of the antibodies reacted with the apoprotein when it was prepared in a nonionic detergent known to maintain the secondary structure of the protein, three of the six antibodies showed partial or total loss of activity with NaDodSO4- delipidated apo B. The specificity of these antibodies was tested by the ability of affinity-purified biotinylated antibodies to compete with unlabeled antibodies for antigenic sites on low density lipoprotein in a competition enzyme-linked immunosorbent assay developed with avidin-peroxidase. This competition assay allowed us to divide the antibodies into a minimum of two groups (I and II) based on the antigenic determinants on apo B that they recognized. The epitope on apo B recognized by group II antibodies was perturbed in NaDodSO4, whereas the determinant(s) on the protein reactive with group I antibodies was unaffected.

Monoclonal antibodies against human low density lipoprotein. Stoichiometric binding studies using Fab fragments

Because of its physical properties, apolipoprotein B (apo B) has remained poorly characterized. In an attempt to elucidate apo B structure, the Fab fragments of 3 different monoclonal anti-human apo B antibodies were tested in a quantitative assay for their binding to human low density lipoprotein (LDL). In each case the assay gave a linear Scatchard plot with a maximum of 1 Fab fragment bound to a single LDL particle. This result favors an LDL model containing a single large Mr apo B protein, which is not composed of multiple, identical, small Mr subunits.

Degradation of apolipoprotein B of low density lipoprotein by cultured bovine smooth muscle cells. Accumulation of intermediates in the presence of protease inhibitors

The incubation of bovine aortic smooth muscle cells with 125I-labeled low density lipoprotein in the presence of protease inhibitors resulted in the significant intracellular accumulation of intact apolipoprotein B, as well as a number of high molecular weight degradation intermediates. This effect was brought about both by leupeptin, a specific inhibitor of thiol proteases (40% inhibition of degradation), as well as by the more general lysosomotrophic inhibitors, chloroquine and NH4Cl. Qualitatively identical spectra of degradation intermediates were formed in the presence of chloroquine and NH4Cl as determined by autoradiography of SDS-polyacrylamide gel electrophoretic fractions, ranging from the apolipoprotein B band (Mr = 340,000) to bands with molecular weights of less than 14,000. The effect of NH4Cl was reversible and release of inhibition resulted in the sequential loss of intermediates from the cells; those species having higher molecular weights disappearing first. Inhibition by leupeptin was associated with the accumulation of degradation products in the lower molecular weight range only (Mr less than or equal to 72,000). These results provide evidence that apolipoprotein B proteolysis progresses in distinct stages via specific breakdown products and suggest that the thiol cathepsins become more active later in the degradation pathway.

Characterization of apolipoprotein B from human serum low density lipoprotein in n-dodecyl octaethyleneglycol monoether: an electron microscope study

We have studied the structure of the totally delipidated polypeptide (apolipoprotein B [apo B]) present in low-density serum lipoprotein in detergent (n-dodecyl octaethyleneglycol monoether) solution by electron microscopy. The protein-detergent complex appears as a rod-shaped particle, 75-80 nm long and 4.5-5.5 nm wide. The volume of this particle is consistent with the previously published composition reported by Watt and Reynolds (1980, Biochemistry 19:1593-1598) of two copies of apo B and five to six equivalent micelles of detergent. The asymmetric particle possesses a high degree of flexibility and a strong tendency to self-associate in an orderly fashion. The extent of this association is pH dependent.

Cathepsin-D-dependent initiation of the hydrolysis by lysosomal enzymes of apoprotein B from low-density lipoproteins

The degradation of 135I-apoprotein B of human low-density lipoprotein by cell extracts of cultured bovine aortic smooth muscle cells was determined by measuring the formation of acid-soluble products and by analyzing the electrophoretic patterns of digested apoprotein in gels containing sodium dodecyl sulfate. Degradation resulted in an initial rapid accumulation of a limited number of distinct smaller fragments. Two products with apparent molecular weights of 220,000 and 200,000 predominated. Pepstatin inhibited proteolysis almost completely, as measured by either assay. Leupeptin decreased hydrolysis to acid-soluble products by approximately 50%, but had no effect on the initial cleavage of intact apoprotein B. Similar results were found in the case of extracts from cultured human skin fibroblasts and from adult bovine arterial smooth muscle. Leupeptin inhibited intracellular degradation of 125I-apoprotein B in cultured cells by approximately 50%. It is concluded that the intralysosomal degradation of apoprotein B involves an initial limited endoproteolytic attack at susceptible sites by cathepsin D. This and other enzymes, including cathepsin B, then act synergistically to bring degradation to completion.