Publications
245 results found
Scott J, 1993, Apolipoprotein E and Alzheimer's disease., Lancet, Vol: 342, ISSN: 0140-6736
Venkatesan S, Cullen P, Pacy P, et al., 1993, Stable isotopes show a direct relation between VLDL apoB overproduction and serum triglyceride levels and indicate a metabolically and biochemically coherent basis for familial combined hyperlipidemia., Arterioscler Thromb, Vol: 13, Pages: 1110-1118, ISSN: 1049-8834
Familial combined hyperlipidemia (FCHL) may be genetically and metabolically more heterogeneous than previously thought. A consistent feature is an increase in circulating very-low-density lipoprotein (VLDL) apolipoprotein (apo) B, which could be due to either an increase in apoB production or a decrease in its catabolism. Therefore, we directly measured VLDL apoB production in the postabsorptive state in seven FCHL subjects (four male, three female) and seven normal control subjects (three male, four female) by using L-[1-13C]leucine as an endogenous label. Mean age and body mass index did not differ significantly between the two groups. The mean total cholesterol levels were 4.7 +/- 0.8 and 8.8 +/- 1.6 mmol/L (+/- SD, P < .01) and the mean triglyceride levels were 0.84 +/- 0.14 and 3.30 +/- 1.10 mmol/L (+/- SD, P < .01) in the control and FCHL groups, respectively. Although the fractional production rate of VLDL apoB was 38% lower in the FCHL group than in the control subjects (0.11 +/- 0.03 versus 0.18 +/- 0.02 pool/h; mean +/- SD, P < .01), its absolute production rate was 2.7 times greater (534 +/- 193 micrograms/kg per hour in FCHL versus 196 +/- 71 micrograms/kg per hour in control subjects; mean +/- SD, P < .01). There was a linear relation (r = 0.8, P = .03) between triglyceride levels and the VLDL apoB production rate in FCHL, the slope of which indicated a similar VLDL triglyceride-to-apoB ratio in the FCHL and control groups. We conclude that FCHL is a metabolically coherent disorder and that the increase in circulating apoB and triglyceride levels in FCHL is due to secretion of an increased number of VLDL particles, each containing, on average, a normal amount of triglyceride and one molecule of apoB.(ABSTRACT TRUNCATED AT 250 WORDS)
Scott J, 1993, Nature, nurture, and hypercholesterolaemia., Lancet, Vol: 341, Pages: 1312-1313, ISSN: 0140-6736
Shoulders CC, Narcisi TM, Jarmuz A, et al., 1993, Characterization of genetic markers in the 5'flanking region of the apo A1 gene., Hum Genet, Vol: 91, Pages: 197-198, ISSN: 0340-6717
Genetic variation of apo A1/C3/A4 is associated with hyperlipidaemia and coronary heart disease. We report the polymerase chain reaction (PCR) conditions for determining three polymorphic sites in the 5'flanking region of apoA1 using DNA prepared from small aliquots of whole blood. These polymorphisms identify six haplotypes that will be of value in genetic studies.
Navaratnam N, Shah R, Patel D, et al., 1993, Apolipoprotein B mRNA editing is associated with UV crosslinking of proteins to the editing site., Proc Natl Acad Sci U S A, Vol: 90, Pages: 222-226, ISSN: 0027-8424
Apolipoprotein (apo) B100 mRNA undergoes editing of C-6666 to a U residue, which generates a stop-translation codon and defines the carboxyl terminus of apoB48. To aid purification of the editing enzyme we have undertaken UV crosslinking of a 32P-labeled substrate for apoB mRNA editing in vitro to proteins in an enterocyte editing extract. Proteins of 60 (p60) and 43 (p43) kDa, prominent among crosslinking bands, were competed for by unlabeled substrate, but not by nonspecific RNA, and did not crosslink to antisense RNA. Editing in vitro and UV crosslinking were inhibited by NaCl and vanadyl ribonucleoside complexes and by chemical modification of sulfhydryl, imidazolium, and guanidinium groups on the protein. The editing activity copurified predominantly with p60. To define the binding site for p60 on the substrate RNA, a series of scanning and point mutant RNAs, previously used to define nucleotides 6671-6681 as essential for editing, were used in competition studies with wild-type substrate. Results demonstrated that p60 binding is centered on nucleotides 6671-6674. We suggest that p60 contains the RNA-recognition component of the apoB mRNA-editing enzyme.
Hodges P, Scott J, 1993, Editing of Mammalian apolipoprotein B mRNA by site-specific RNA deamination, RNA editing, Editors: Benne, Publisher: Ellis Horwood Ltd, Pages: 126-143
SCOTT J, 1992, MEDICAL GENETICS - ARTERIAL HARDENING IN MICE, NATURE, Vol: 360, Pages: 631-632, ISSN: 0028-0836
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PATEL SB, PESSAH M, BEUCLER I, et al., 1992, DETECTION OF NORMALLY EDITED APOLIPOPROTEIN-B MESSENGER-RNA IN ANDERSON DISEASE - A DEFECT OF LIPOPROTEIN ASSEMBLY, CIRCULATION, Vol: 86, Pages: 208-208, ISSN: 0009-7322
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White AL, Graham DL, LeGros J, et al., 1992, Oleate-mediated stimulation of apolipoprotein B secretion from rat hepatoma cells. A function of the ability of apolipoprotein B to direct lipoprotein assembly and escape presecretory degradation., J Biol Chem, Vol: 267, Pages: 15657-15664, ISSN: 0021-9258
Physiological concentrations of oleate stimulate apolipoprotein (apo) B-containing lipoprotein secretion from HepG2 cells without increasing apoB mRNA levels. The purpose of this study was to determine whether oleate acts by increasing translation of apoB mRNA or through posttranslational effects on the apoB protein. To address the mechanism of oleate-stimulated secretion of apoB, a series of carboxyl terminally truncated apoB constructs was made. Each contained the SV40 early promoter, the apoB 5'-untranslated region, and SV40 polyadenylation signals. Any difference in the response to oleate between endogenous apoB and the proteins encoded by the constructs or between the constructs themselves should thus depend on the protein sequence. Stable transformants were established for each of the constructs in the rat hepatoma cell line McArdle-RH7777. The effect of oleate on secretion of the apoB protein products was determined by labeling with [35S]methionine, immunoprecipitation, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Carboxyl-terminal truncation of apoB41 resulted in a loss of the ability of apoB secretion to respond to oleate. Ultracentrifugation of secreted proteins on continuous CsCl gradients from 1.0-1.4 g/ml revealed that this correlated with a decrease in the ability of apoB to be recovered as a buoyant lipoprotein particle. Addition of oleate decreased the densities at which the short forms of apoB secreted as lipoproteins were recovered. Pulse-chase analysis of the secretion of apoB100 and of the truncated proteins revealed that they all underwent rapid posttranslational intracellular degradation. We conclude that oleate has no effect on the translation of apoB mRNA but promotes the secretion of apoB-containing lipoproteins by reducing presecretory degradation of those forms of apoB that can produce buoyant lipoproteins.
Borén J, Graham L, Wettesten M, et al., 1992, The assembly and secretion of ApoB 100-containing lipoproteins in Hep G2 cells. ApoB 100 is cotranslationally integrated into lipoproteins., J Biol Chem, Vol: 267, Pages: 9858-9867, ISSN: 0021-9258
The possibility that apoB 100 is cotranslationally translocated to the endoplasmic reticulum lumen and integrated into lipoproteins has been investigated. ApoB 100 nascent polypeptides were shown to be secreted from pulse-labeled Hep G2 cells after treatment with puromycin and chase for 1 or 2 h in the presence of puromycin and cycloheximide. These nascent polypeptides banded during sucrose gradient ultracentrifugation between the position of the high (HDL) and the low (LDL) density lipoproteins, revealing an inverse relationship between the length of the polypeptide and the density of the fraction. ApoB 100 occurred in the position of LDL and very low density lipoproteins (VLDL). Electronmicroscopy studies of the apoB-containing particles from the gradient indicated an increase in size with increasing length of the polypeptide. Furthermore, labeling studies indicated that the triglyceride load increased with the length of the polypeptide. An inverse relationship between the size of C-terminally truncated apoB polypeptides and the density of the assembled lipoproteins was also observed in experiments with transfected minigenes coding for apoB 41, apoB 29, and apoB 23. These proteins appeared on HDL particles. Pulse-chase experiments indicated that 80-200-kDa apoB nascent polypeptides on particles with HDL density, with time, were converted into larger polypeptides on lighter particles, to be fully replaced by apoB 100 on LDL-VLDL particles. The formation of these LDL-VLDL particles could be blocked by cycloheximide. Sixty-five percent of pulse-labeled apoB nascent polypeptides present in the microsomal fraction was released by sodium carbonate treatment, and 77% of these polypeptides could be recovered on the immature particles (banding between HDL and LDL) after sucrose gradient ultracentrifugation. Pulse-chase experiments indicated that these nascent polypeptides, on the immature lipoproteins, had the capacity to be precursors for all the apoB 100-containing LDL a
PEASE R, HARRISON G, LEIPER J, et al., 1992, APOLIPOPROTEIN-B INTERMEDIATES - REPLY, NATURE, Vol: 356, Pages: 115-116, ISSN: 0028-0836
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Hodges P, Scott J, 1992, Apolipoprotein B mRNA editing: a new tier for the control of gene expression., Trends Biochem Sci, Vol: 17, Pages: 77-81, ISSN: 0968-0004
Two forms of apolipoprotein (apo) B are found in mammals. The shorter form is translated from an edited mRNA in which a specific cytidine base is deaminated to a uridine, creating a new stop codon. Apo B mRNA editing is mediated by a site-specific cytidine deaminase that recognizes a downstream target sequence in the RNA. The enzyme has no energy or cofactor requirements and no RNA component, and thus bears no obvious relationship to RNA processing events such as splicing or polyadenylation. While apo B mRNA editing activity may have arrived late in evolution to target dietary lipid to the liver in mammals, the discovery of the editing activity in tissues and cells that do not express apo B suggests a more widespread role in the generation of RNA and protein diversity.
Scott J, 1992, Molecular and cellular biology of apolipoprotein B, Genetics of coronary heart disease, Editors: Bearn, Oslo, Publisher: Institute of Medical Genetics, Pages: 99-113
Scott J, 1992, The molecular biology of apolipoprotein B, Cellular and molecular biology of atherosclerosis, Editors: Gotto, London, Publisher: Springer, Pages: 83-92, ISBN: 9783540197041
Pease RJ, Harrison GB, Scott J, 1991, Cotranslocational insertion of apolipoprotein B into the inner leaflet of the endoplasmic reticulum., Nature, Vol: 353, Pages: 448-450, ISSN: 0028-0836
Apolipoprotein (apo) B100 is required for the distribution of hepatic triglyceride to peripheral tissues as very-low-density lipoproteins. The translocation of apo B100 into the endoplasmic reticulum (ER) and its subsequent assembly into lipoprotein particles is of particular interest as the protein is both very large (relative molecular mass 512,000) and insoluble in water. It has been proposed that apo B translocation occurs in discrete stages and is completed post-translationally. Several sites of arrest of translocation were reported to be present in apo B15 (the N-terminal 15% of the protein). We have re-examined this question by in vitro translation coupled with translocation into microsomes, and find no evidence for transmembrane segments in truncated apo B proteins. Translocated apo B17 is strongly associated with the membrane of the ER, being only partially releasable with alkaline carbonate, and remaining bound to the microsomes following disruption with saponin. The efficient binding of short segments of apo B, despite the absence of transmembrane domains, suggests that apo B is cotranslationally inserted into the inner leaflet of the ER. This will obviate problems caused by the size and insolubility of apo B100, because the growing hydrophobic protein chains will never exist in a lipid-free form during translocation. From the inner leaflet, apo B in association with membrane-derived lipid can bud into the lumen of the ER to form nascent lipoprotein particles.
Bhattacharya S, Ameis D, Cullen P, et al., 1991, VNTR polymorphism in the hepatic lipase gene (LIPC)., Nucleic Acids Res, Vol: 19, ISSN: 0305-1048
Bhattacharya S, Wilson TM, Wojciechowski AP, et al., 1991, Hypervariable polymorphism in the APOC3 gene., Nucleic Acids Res, Vol: 19, ISSN: 0305-1048
Shah RR, Knott TJ, Legros JE, et al., 1991, Sequence requirements for the editing of apolipoprotein B mRNA., J Biol Chem, Vol: 266, Pages: 16301-16304, ISSN: 0021-9258
Apolipoprotein (apo) B48 is produced in the mammalian intestine by a tissue-specific RNA-editing mechanism, which mediates a C to U conversion at position 6666 in apoB mRNA. This generates an inframe translation stop codon (UAA) in place of glutamine (CAA) at position 2153. To establish the sequences required for editing we have used an in vitro conversion assay to monitor the editing of synthetic RNAs by rat intestinal extracts. Transcripts containing 55 nucleotides (positions 6649-6703) or more of human apoB mRNA sequence were edited in vitro. Transcripts containing 42 nucleotides (positions 6648-6689) and 26 nucleotides (positions 6662-6687) were edited at 62 and 24% efficiency, respectively, of the 55-nucleotide sequence. To delineate the precise sequence requirements for editing, mutants were generated where 6-nucleotide sections of the 55-base region were changed to anti-sense sequence. Mutation of the 12-nucleotide region immediately downstream of C-6666 abolished editing, and mutation of 6-base sequences immediately 3' and 5' of this 12-nucleotide region significantly reduced editing. Having identified the key region of interest, a panel of 46 mutant RNAs carrying single base substitutions or deletions between nucleotide positions 6657 and 6685 was constructed. Mutagenesis in the sequence 5'-TGATCAGTATA-3' (positions 6671-6681) downstream of C-6666 had the most dramatic effect, since almost all mutations abolished or greatly reduced conversion in vitro. These results suggest that editing is a highly sequence-specific process. We propose that this downstream region is a recognition and/or binding site for the editing enzyme. A search for this sequence in other genes may help to reveal other RNAs that undergo editing.
Greeve J, Navaratnam N, Scott J, 1991, Characterization of the apolipoprotein B mRNA editing enzyme: no similarity to the proposed mechanism of RNA editing in kinetoplastid protozoa., Nucleic Acids Res, Vol: 19, Pages: 3569-3576, ISSN: 0305-1048
Intestinal apolipoprotein B mRNA is edited at nucleotide 6666 by a C to U transition resulting in a translational stop codon. The enzymatic properties of the editing activity were characterised in vitro using rat enterocyte cytosolic extract. The editing activity has no nucleotide or ion cofactor requirement. It shows substrate saturation with an apparent Km for the RNA substrate of 2.2 nM. The editing enzyme requires no lag period prior to catalysis, and does not assemble into a higher order complex on the RNA substrate. In crude cytosolic extract editing activity is completely abolished by treatment with micrococcal nuclease or RNAse A. Partially purified editing enzyme is no longer sensitive to nucleases, but is inhibited in a dose dependent manner by nuclease inactivated crude extract. The buoyant density of partially purified editing enzyme is 1.3 g/ml, that of pure protein. Therefore, the apolipoprotein B mRNA editing activity consists of a well defined enzyme with no RNA component. The nuclease sensitivity in crude cytosolic extract is explained by the generation of inhibitors for the editing enzyme. The editing of apo B mRNA has little similarity to complex mRNA processing events such as splicing and unlike editing in kinetoplastid protozoa does not utilise guide RNAs.
Graham DL, Knott TJ, Jones TC, et al., 1991, Carboxyl-terminal truncation of apolipoprotein B results in gradual loss of the ability to form buoyant lipoproteins in cultured human and rat liver cell lines., Biochemistry, Vol: 30, Pages: 5616-5621, ISSN: 0006-2960
Apolipoprotein B has an obligatory role in the production of chylomicrons, VLDL, and LDL. Familial hypobetalipoproteinemia is a codominant disorder characterized by reduced levels of apo B containing lipoproteins in plasma. We have previously described mutations of the apo B gene in persons with hypobetalipoproteinemia that predict truncated forms of apo B designated apo B29 (1305 amino acid residues) and apo B39 (1799 residues). Apo B39 was present in the VLDL and LDL fractions of plasma, but apo B29 was not detected in the lipoprotein or infranatant fractions of plasma. Here we have investigated the regions of apo B necessary for apo B containing lipoprotein secretion by expression of constructs designed to express truncated forms of apo B. Apo B13 (583 residues), apo B17 (784 residues), apo B23 (1084 residues), apo B29 (1306 residues), and apo B41 (1880 residues) were transiently expressed in HepG2 cells, and apo B23 and apo B41 were stably expressed in McArdle 7777 cells. Lipoprotein (d less than 1.25 g/mL) and infranatant (d greater than 1.25 g/mL) fractions of conditioned medium were analyzed by immunoprecipitation and SDS-PAGE. The distribution between lipoprotein and infranatant fractions varied: apo B41 was found solely in the lipoprotein fraction; apo B29, apo B23, and apo B17 were present in both fractions, but with stepwise truncation, progressively more apo B was recovered in the infranatant; apo B13 was only in the infranatant. These results demonstrate that deletion from the carboxyl terminal of apo B41 results in a gradual loss of the ability of the truncated proteins to form buoyant lipoprotein particles.
Navaratnam N, Patel D, Shah RR, et al., 1991, An additional editing site is present in apolipoprotein B mRNA., Nucleic Acids Res, Vol: 19, Pages: 1741-1744, ISSN: 0305-1048
Human intestinal apolipoprotein (apo) B mRNA undergoes a C to U RNA editing at nucleotide 6666 to generate a translation stop at codon 2153, which defines the carboxy-terminal of apo B48. Here we show that two of eleven human intestinal cDNAs spanning residue 6666 were edited from a genomically-encoded C to a T at residue 6802 as well as at residue 6666. This additional editing converts Thr (ACA) codon 2198 to Ile (AUA). Synthetic RNA including the nucleotide 6802 was edited in vitro by intestinal extracts at 10-15% of the editing efficiency of nucleotide 6666. A sequence is identified as important for recognition by the editing activity. No secondary structural homology was identified between the two edited sites. No other sequence in the region between 6411 and 6893 nucleotides of apo B mRNA was found to be edited in vivo or in vitro. Apo B RNA editing extracts from intestine did not edit maize cytochrome oxidase II mRNA.
Hodges PE, Navaratnam N, Greeve JC, et al., 1991, Site-specific creation of uridine from cytidine in apolipoprotein B mRNA editing., Nucleic Acids Res, Vol: 19, Pages: 1197-1201, ISSN: 0305-1048
Human apolipoprotein (apo) B mRNA is edited in a tissue specific reaction, to convert glutamine codon 2153 (CAA) to a stop translation codon. The RNA editing product templates and hybridises as uridine, but the chemical nature of this reaction and the physical identity of the product are unknown. After editing in vitro of [32P] labelled RNA, we are able to demonstrate the production of uridine from cytidine; [alpha 32P] cytidine triphosphate incorporated into RNA gave rise to [32P] uridine monophosphate after editing in vitro, hydrolysis with nuclease P1 and thin layer chromatography using two separation systems. By cleaving the RNA into ribonuclease T1 fragments, we show that uridine is produced only at the authentic editing site and is produced in quantities that parallel an independent primer extension assay for editing. We conclude that apo B mRNA editing specifically creates a uridine from a cytidine. These observations are inconsistent with the incorporation of a uridine nucleotide by any polymerase, which would replace the alpha-phosphate and so rule out a model of endonucleolytic excision and repair as the mechanism for the production of uridine. Although transamination and transglycosylation remain to be formally excluded as reaction mechanisms our results argue strongly in favour of the apo B mRNA editing enzyme as a site-specific cytidine deaminase.
Wojciechowski AP, Farrall M, Cullen P, et al., 1991, Familial combined hyperlipidaemia linked to the apolipoprotein AI-CII-AIV gene cluster on chromosome 11q23-q24., Nature, Vol: 349, Pages: 161-164, ISSN: 0028-0836
Familial combined hyperlipidaemia (FCHL) is a common inherited disorder of lipid metabolism with a prevalence of 0.5-2.0% (refs 1, 2). It is estimated to cause 10% of premature coronary heart disease. The underlying metabolic and genetic defects in FCHL have not been identified, but a population study has suggested an association between FCHL and an XmnI restriction fragment length polymorphism (RFLP) within the apolipoprotein AI-CIII-AIV gene cluster. Here we confirm this association and show that it results from linkage disequilibrium between FCHL and the 6.6-kilobase (kb) allele of the XmnI RFLP. Subsequent analysis in seven FCHL families, ascertained through a proband carrying the 6.6 kb XmnI allele, demonstrated linkage to the AI-CIII-AIV cluster on 11q23-q24, zeta = 6.86 with no recombinants. This assignment will facilitate the identification of the mutation that causes hyperlipidaemia in these families.
Borén J, White A, Wettesten M, et al., 1991, The molecular mechanism for the assembly and secretion of ApoB-100-containing lipoproteins., Prog Lipid Res, Vol: 30, Pages: 205-218, ISSN: 0163-7827
We have reviewed the literature on the intracellular transport of ApoB-100 and the assembly of the ApoB-100-containing lipoproteins. ApoB-100 is a large molecule (4536 aa) that requires some 15 min to be completed. During the synthesis, the protein could take one of two pathways: a degradational pathway and a pathway that leads to secretion of the protein on mature lipoproteins. The degradational pathway starts with a cotranslational incorporation of ApoB-100 into the membrane of the endoplasmic reticulum in such a way that a relatively large portion of the sequence is exposed on the cytoplasmic surface of this membrane. The membrane bound ApoB-100 is retained in the ER and will eventually undergo intracellular degradation. To enter the pathway that leads to lipoprotein formation, ApoB-100 has to be cotranslationally translocated to the lumen of the ER. ApoB-100 will interact with the lipids during this translation-translocation process and the mature lipoprotein is released into the lumen of the secretory pathway when ApoB-100 is completed and leaves the ribosome. In addition to the mature lipoproteins, the secretory pathway contains an ApoB-100-containing lipoprotein with the density of a HDL particle. This particle is not secreted from the cells but is retained and eventually degraded. Of importance for the retention are sequences present in the C-terminal half of the protein. The mature lipoproteins rapidly leave the ER lumen and are transported to the Golgi apparatus, through which transfer takes considerably longer. The assembly process is a potential site for the regulation of the secretion of the ApoB-100-containing lipoproteins. This process is dependent on active synthesis of phosphatidylcholine and it is also highly dependent on the rate of triacylglycerol synthesis. On the other hand, ApoB-100 appears to be constitutively expressed. An increase in the rate of lipoprotein assembly induced by an increased triacylglycerol synthesis gives rise to an increase
Kunitake ST, Young SG, Chen GC, et al., 1990, Conformation of apolipoprotein B-100 in the low density lipoproteins of tangier disease. Identification of localized conformational response to triglyceride content., J Biol Chem, Vol: 265, Pages: 20739-20746, ISSN: 0021-9258
The low density lipoproteins (LDL) from patients with Tangier disease are enriched in triglycerides, 27% of LDL mass versus 7% for normal LDL. To study whether this unique LDL core lipid composition affects the surface disposition of apolipoprotein (apo) B-100, we analyzed the LDL by protease digestion and in competitive radioimmunoassays. Limited proteolytic digestion of Tangier LDL by Staphylococcus aureus V8 protease generated a prominent fragment of 120 kDa (cleavage site at residue 1076), which was not visible in similarly digested normal LDL. In competitive radioimmunoassay, Tangier LDL bound weakly to the apoB-specific monoclonal antibody MB20, compared with control LDL. We localized the MB20 epitope between residues 1031 and 1084 of apoB-100, probably very near residue 1076. DNA sequencing of exon 21 of apoB genomic clones (coding for residues 1014-1084) from a Tangier patient revealed no difference from the normal DNA sequence, thus eliminating a protein polymorphism as a basis for the altered protease sensitivity and antibody binding. When the triglyceride contents of Tangier LDL were reduced to 10% of mass by incubation with normal high density lipoproteins, production of the 120-kDa fragment by proteolysis decreased and MB20 binding increased in affinity, implying a change toward normal conformation of apoB-100. Thus, using two independent techniques, proteolytic digestion and binding of monoclonal antibodies, we have demonstrated an alternative conformation of apoB-100 in the vicinity of residue 1076, which reflects the content of triglycerides in the LDL particle.
Scott J, Wallis SC, Davies MS, et al., 1990, RNA editing: a novel mechanism for regulating lipid transport from the intestine, From phenotype to gene in common disorders, Editors: Berg, Retterstøl, Refsum, Copenhagen, Publisher: Mosby Elsevier Health Science, Pages: 92-106, ISBN: 9788716099532
Law A, Scott J, 1990, A cross-species comparison of the apolipoprotein B domain that binds to the LDL receptor., J Lipid Res, Vol: 31, Pages: 1109-1120, ISSN: 0022-2275
Apolipoprotein (apo)-B-100 is the ligand that mediates the clearance of low density lipoprotein (LDL) from the circulation by the apoB,E (LDL) receptor pathway. Clearance is mediated by the interaction of a domain enriched in basic amino acid residues on apoB-100 with clusters of acidic residues on the apoB,E (LDL) receptor. A model has been proposed for the LDL receptor binding domain of apoB-100 based on the primary amino acid sequence (Knott, T. J., et al. 1986. Nature. 323: 734-738). Two clusters of basic residues (A: 3147-3157 and B: 3359-3367) are apposed on the surface of the LDL particle by a disulfide bridge between Cys 3167 and 3297. Support for this single domain model has been obtained from the mapping of epitopes for anti-apoB monoclonal antibodies that block the binding of apoB to the LDL receptor. Here we test this model by comparing the nucleotide (from 9623 to 10,442) and amino acid sequence (from 3139 to 3411) of apoB-100 in seven species (human, pig, rabbit, rat, Syrian hamster, mouse, and chicken). Overall, this region is highly conserved. Cluster B maintains a strong net positive charge and is homologous across species in both primary and secondary structure. However, the net positive charge of region A is not conserved across these species, but the region remains strongly hydrophilic. The secondary structure of the region between clusters A and B is preserved, but the disulfide bond is unique to the human sequence. This study suggests that the basic region B is primarily involved in the binding of apoB-100 to the apoB,E (LDL) receptor.
Scott J, 1990, RNA editing: a novel mechanism for the regulation of dietary cholesterol absorption. The Humphry Davy Rolleston lecture 1989., J R Coll Physicians Lond, Vol: 24, Pages: 101-106, ISSN: 0035-8819
Jessup W, Rankin SM, De Whalley CV, et al., 1990, Alpha-tocopherol consumption during low-density-lipoprotein oxidation., Biochem J, Vol: 265, Pages: 399-405, ISSN: 0264-6021
1. The kinetics of the depletion of alpha-tocopherol in human low-density lipoprotein (LDL) were measured during macrophage-mediated and cell-free oxidation. The formation of oxidatively modified, high-uptake species of LDL in these systems was not detectable until all of the endogenous alpha-tocopherol had been consumed. 2. Supplementation of the alpha-tocopherol content of LDL by loading in vivo extended the duration of the lag period during which no detectable oxidative modification occurred. 3. The addition of a flavonoid (morin) prevented both alpha-tocopherol consumption and oxidative modification of LDL. 4. The alpha-tocopherol contents of LDLs from a range of individual donors could not be used to predict their relative resistance to oxidation, indicating that other endogenous antioxidants may also be present, and quantitatively significant, in human LDL.
Pease RJ, Milne RW, Jessup WK, et al., 1990, Use of bacterial expression cloning to localize the epitopes for a series of monoclonal antibodies against apolipoprotein B100., J Biol Chem, Vol: 265, Pages: 553-568, ISSN: 0021-9258
Bacterial expression of apolipoprotein (apo) B cDNA constructs has been used to map a series of monoclonal antibodies (mAbs) to apoB by immunoblotting. In some cases assignments have been confirmed and refined by (i) semipurification of expressed protein, CNBr digestion, and assignment of the immunoreactive fragments; (ii) controlled digestion of the cDNA with the exonuclease Bal31 and bacterial expression of the truncated proteins that result; or (iii) expression of specific segments of cDNA amplified by the polymerase chain reaction. Forty mAbs were mapped to a minimum of 17 separate determinants on apoB. Tryptic fragments have been used to confirm the epitope assignments. In addition, this approach in conjunction with immunoassay, enables some deductions to be made about the trypsin-accessible regions in low density lipoprotein (LDL). The cleavage pattern obtained predicts retention of structure in the cysteine-rich domain of the amino terminus and also in the LDL receptor binding region. Trypsinized LDL was shown to bind to the LDL receptor by an authentic process, using monoclonal antibodies as competing ligands. In conjunction with the previous paper (Milne, R. W., Theolis, R., Maurice, R., Pease, R. J., Weech, P. K., Rassart, E., Fruchart, J.-C., Scott, J., and Marcel, Y. L. (1989) J. Biol. Chem. 265, 19754-19760) the mapped mAbs have been used to define the receptor-binding domain of apoB100 in LDL.
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