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Query: UMLS:C0027960 (mole)
 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Apolipoprotein J (apoJ) defines a heterogeneous subclass of human plasma high-density lipoproteins (HDL) having a bimodal distribution of molecular mass of 70-90 kDa (approximately 50%) and 200 kDa or larger (approximately 50%). ApoJ-HDL are unstable in stored plasma, and must be evaluated within 24 h. All apoJ-HDL in freshly obtained plasma have alpha 2 electrophoretic mobility and are distinct from a minor subpopulation of apoAI-HDL which electrophorese in the pre beta region. Although apoAI is not associated with the majority of plasma apoJ-HDL, a small fraction of these particles also containing apoAI. There is little variation in the apoJ/apoAI mole ratio of apoJ-HDL immunoaffinity purified from the same individual on different days. In addition, there is a constant ratio among individuals, assessed for five volunteers, of 4.9 +/- 0.6. Purified apoJ added directly to apoJ-depleted plasma can interact with apoAI or with apoAI-containing lipoproteins, as evidenced by the association of apoAI with apoJ that is reisolated by immunoaffinity chromatography. The amount of apoAI associated with apoJ increases linearly with increasing amount of apoJ added, over the range of apoJ concentrations tested. No other known apolipoprotein is associated with apoJ. By two-dimensional electrophoretic analysis, the lipoproteins containing both apoJ and apoAI have approximate molecular masses of 350-400 kDa. Taken together, the results suggest that the interaction between apoJ and apoAI is physiologically important and that lipoproteins which contain both apoJ and apoAI can be produced in the plasma. ApoJ-HDL and apoJ/apoAI-HDL may have different functions and metabolic fates or may represent different stages of apoJ catabolism.
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PMID:Structure and stability of apolipoprotein J-containing high-density lipoproteins. 139 Jun 41

The apparent pressures in the surface monolayer of emulsion particles can be estimated by comparing the absorption of an apolipoprotein to planar lipid monolayers and to emulsions. Lipids are spread at an air-water interface in a Pockels/Langmuir surface balance and the adsorption of [14C]-labeled apolipoproteins placed in the subphase is studied as a function of surface pressure using the surface radioactivity method. An apoprotein surface concentration/initial lipid surface pressure curve (gamma/pi i) is constructed. The maximum apolipoprotein surface concentration gamma e of emulsions is derived from standard emulsion/apolipoprotein binding isotherms. The apparent emulsion surface pressure is then estimated by comparing gamma e to the gamma/pi i curve. Apolipoprotein A-I has been used as an example of a probe to estimate the effective surface pressure in approximately 1000 A diameter egg yolk phosphatidylcholine/cholesterol/triolein emulsion particles. When the cholesterol content of emulsions is low, the surface pressure of the emulsion is about 17 dyne cm-1. At high cholesterol concentrations (0.49 cholesterol/phospholipid mole ratio) the surface pressure is increased to 25 dyne cm-1. The addition of the maximum amounts of apoA-I to these particles raises the effective surface pressure of the emulsion to about 30 dyne cm-1 and stabilizes the particles.
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PMID:A technique to estimate the apparent surface pressure of emulsion particles using apolipoproteins as probes. 141 72

Apolipoprotein C-III (apo C-III) is a 79 amino acid glycoprotein. The sugar moiety of apoC-III is attached to amino acid residue 74 and is thought to consist of 1 mole of galactose, 1 mole of N-acetyl-galactosamine, and either 0, 1, or 2 moles of sialic acid. This results in three isoproteins called C-III0, C-III1, and C-III2 designated by the number of sialic acid residues. It has been assumed, although not experimentally tested, that apoC-III0 lacks sialic acid residues but possesses the D-galactosyl-(1-3)-N-acetyl-D-galactosamine sugar backbone. To verify the structure of the three apoC-III isoproteins, we applied the method of 252Cf plasma desorption mass spectrometry to measure the exact molecular weight (Mr) of each of the isoproteins. Our data confirmed the proposed structure of apoC-III1 and apoC-III2. However, the difference in mass between apoC-III1 (9420.6, 9420.0, 9422.2 daltons) and apoC-III0 (8763.9, 8764.9, 8765.5 daltons, respectively, in three subjects) suggests that the latter is missing not just sialic acid but the entire sugar moiety. This finding may have important implications for the metabolism of apoC-III. The accuracy and reproducibility of Mr measurements described in this paper suggest that this technique holds promise for the detection of apolipoprotein amino acid substitutions or modifications undetected by conventional techniques such as isoelectric focusing.
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PMID:Apolipoprotein C-III0 lacks carbohydrate residues: use of mass spectrometry to study apolipoprotein structure. 261 77

Confluent monolayers of the human hepatoblastoma-derived cell line, Hep G2, were incubated in serum-free medium. Conditioned medium was ultracentrifugally separated into d less than 1.063 g/ml and d 1.063-1.20 g/ml fractions since very little VLDL was observed. The d less than 1.063 g/ml fraction was examined by electron microscopy; it contained particles of 24.5 +/- 2.3 nm diameter, similar in size to plasma LDL; a similar size was demonstrated by nondenaturing gradient gel electrophoresis. These particles possessed apoB-100 only. The d less than 1.063 g/ml fraction had a lipid composition unlike that of plasma LDL; unesterified cholesterol was elevated, there was relatively little cholesteryl ester, and triglyceride was the major core lipid. The d 1.063-1.20 g/ml fraction was heterogeneous in size and morphology. Electron microscopy revealed discoidal particles (14.9 +/- 3.2 nm long axis and 4.5 +/- 0.2 nm short axis) as well as small spherical ones (7.6 +/- 1.4 nm diameter). Nondenaturing gradient gel electrophoresis consistently showed the presence of peaks at 13.4 11.9, 9.7, and 7.4 nm. The latter peak was conspicuous and probably corresponded to the small spherical structures seen by electron microscopy. Unlike plasma HDL, Hep G2 d 1.063-1.20 g/ml lipoproteins contained little or no stainable material in the (HDL3a)gge region by gradient gel electrophoresis. Hep G2 d 1.063-1.20 g/ml lipoproteins differed significantly in composition from their plasma counterparts; unesterified cholesterol and phospholipid were elevated and the mole ratio of unesterified cholesterol to phospholipid was 0.8. Cholesteryl ester content was extremely low. ApoA-I was the major apolipoprotein, while apoE was the next most abundant protein; small quantities of apoA-II and apoCs were also present. Immunoblot analysis of the d 1.063-1.20 g/ml fraction after gradient gel electrophoresis showed that apoE was localized in the larger pore region of the gel (apparent diameter greater than 12.2 nm); the apoA-I distribution in this fraction was very broad (7.1-12.2 nm), and included a distinct band at 7.4 nm. Immunoblotting after gradient gel electrophoresis of concentrated medium revealed that a significant fraction of apoA-I in the uncentrifuged medium was in a lipid-poor or lipid-free form. This cell line may be a useful model for investigating the metabolism of newly formed HDL.
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PMID:Characterization of lipoproteins produced by the human liver cell line, Hep G2, under defined conditions. 301 29

The two major apolipoproteins of marmoset serum have been isolated and characterized, and on the basis of physicochemical and immunological criteria are homologous with the human AI and B-100 proteins. Marmoset apolipoprotein AI was the principal protein of high-density lipoproteins (HDL) and was purified by gel filtration chromatography and electrophoresis in alkaline-urea polyacrylamide gel followed by electrophoretic elution. Purified marmoset apolipoprotein AI displayed an Mr of approx. 27000, was polymorphic (five forms) on isoelectric focussing, with pI values in the range 4.8-5.0, and migrated similarly to human apolipoprotein AI in alkaline-urea gels. An overall resemblance was seen in the amino acid composition of marmoset apolipoprotein AI and that of its human counterpart with the notable exception that marmoset AI contained 1 isoleucine residue/mole. An immunological reaction of partial identity between the human and monkey proteins was seen upon immunodiffusion of their HDLs against antiserum to human apolipoprotein AI. Marmoset B-100 was the predominant apoprotein of VLDL and LDL, resembling the human protein in its elution profile on gel filtration chromatography in anionic detergent, and in its high apparent Mr (approx. 520000). The marmoset and human B-100 proteins were alike in amino acid composition and carbohydrate content. Moreover, their immunological behaviour with an antiserum to marmoset apolipoprotein B showed them to share certain antigenic determinant(s). We conclude that the physicochemical properties of the principle apolipoproteins of Callithrix jacchus, a New World primate, markedly resemble those of the human AI and B-100 proteins, suggesting therefore that they may function similarly in lipid transport and metabolism. Counterparts to human apolipoproteins AII, E, CII and CIII have also been tentatively identified.
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PMID:Isolation, characterization and comparative aspects of the major serum apolipoproteins, B-100 and AI, in the common marmoset, Callithrix jacchus. 641 12

Human and bovine A-I apolipoproteins were incubated with multibilayer liposomes of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) and several mixtures of these two lipids. The reactions were carried out at temperatures around the transition temperature of the lipids, and the formation of small, micellar complexes of protein with lipid was followed as a function of time. Micellar complexes were isolated by ultracentrifugation and were characterized in terms of stoichiometry, lipid composition by gas chromatography, approximate size by gel filtration, and phase transition behavior by fluorescence polarization measurements. The results indicate a decrease in reaction rates with increasing DPPC contents of the mixtures, consistent with the higher stability of DPPC bilayers. Reactions have optimal rates at the transition temperature and are limited to the temperature range where gel and liquid-crystalline phases coexist. The isolated complexes with DMPC and DPPC have similar molecular weights in the range from 2 X 10(5) to 2.5 X 10(5), but lipid/protein mole ratios differ by about 40%. The lower lipid/protein ratio of DPPC complexes (100:1 mol/mol) is compensated by the longer acyl chains of this lipid, such that the acyl chain area of both complexes stabilized by apolipoprotein is essentially identical.
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PMID:Interactions of dipalmitoyl- and dimyristoylphosphatidylcholines and their mixtures with apolipoprotein A-I. 679 87

Studies were carried out to determine whether apolipoprotein (apo) A-II, like apoA-I, can recruit phospholipid and cholesterol from cell membranes, thereby forming nascent apoA-II-specific HDL. ApoA-II and apoA-I were purified from plasma and each was incubated with CHO cells at a concentration of 10 micrograms/ml. Lipid-containing complexes were isolated from the medium in both cases; the composition of the apoA-II- and apoA-I-specific complexes were similar where percent protein, phospholipid, and cholesterol were 35 +/- 3, 38 +/- 2, and 25 +/- 1 for apoA-II, respectively, and 40 +/- 2, 35 +/- 1, and 24 +/- 2 for apoA-I, respectively. On a per mole of apolipoprotein basis, apoA-I recruited significantly more phospholipid and cholesterol than dimeric apoA-II suggesting that apoA-I with its greater number of alpha helices binds more lipid. By electron microscopy, nascent apoA-II- and apoA-I-specific particles were predominantly discoidal in morphology. ApoA-II complexes were unique in their nondenaturing polyacrylamide gradient gel size distribution as six distinct populations of particles with diameters of 8.1, 9.3, 10.4, 11.8, 13.1, and 14.6 nm were routinely noted, compared with apoA-I which formed only three major populations with diameters of 7.3, 9.2, and 11.0 nm. Nascent apoA-I complexes incubated with purified lecithin:cholesterol acyltransferase (LCAT) were transformed into predominantly 8.4 nm particles. The latter is similar in size to plasma HDL3a, LpA-I particles, suggesting that extracellularly assembled apoA-I-lipid complexes can directly give rise to a major plasma LpA-I subpopulation upon interaction with LCAT. Unlike apoA-I, apoA-II-lipid complexes could not serve as substrates for LCAT and did not undergo transformation. This study also demonstrates, for the first time, that apoA-II and apoA-I show a preference in phospholipid recruitment from membranes. Although phosphatidylcholine is the major phospholipid removed by both apolipoproteins, apoA-II preferentially recruits phosphatidylethanolamine (PE) as its second most abundant phospholipid while apoA-I recruits sphingomyelin. As PE is usually associated with the inner leaflet of the membrane, it is likely that dimeric apoA-II, compared with apoA-I, can penetrate farther into the membrane and extract PE. This ability of apoA-II to insert more deeply into the lipid milieu may explain the known ability of apoA-II to resist dissociation from the mature HDL particle.
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PMID:Recruitment of cell phospholipids and cholesterol by apolipoproteins A-II and A-I: formation of nascent apolipoprotein-specific HDL that differ in size, phospholipid composition, and reactivity with LCAT. 770 40

The role of HDL and its major protein constituent, apolipoprotein (apo) A-I, in promoting the removal of excess cholesterol from cultured cells has been well established; however, the mechanisms by which this occurs are not completely understood. To address the effects of apoA-I modification on cellular unesterified (free) cholesterol (FC) efflux, three recombinant human apoA-I deletion mutants and plasma apoA-I were combined with 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) and FC to make reconstituted high density lipoprotein (rHDL) discoidal complexes. These particles were characterized structurally and for their efficiency as acceptors of mouse L-cell fibroblast cholesterol. The deletion mutant proteins lacked NH2-terminal (apoA-I (Delta44-126)), central (apoA-I (Delta139-170)), or COOH-terminal (apoA-I (Delta190-243)) domains of apoA-I. The three deletion mutants all displayed lipid-binding abilities and formed discoidal complexes that were similar in major diameter (13.2 +/- 1.5 nm) to those formed by human apoA-I when reconstituted at a 100:5:1 (POPC:FC:protein) mole ratio. Gel filtration profiles indicated unreacted protein in the preparation made with apoA-I (Delta190-243), which is consistent with the COOH terminus portion of apoA-I being an important determinant of lipid binding. Measurements of the percent alpha-helix content of the proteins, as well as the number of protein molecules per rHDL particle, gave an indication of the arrangement of the deletion mutant proteins in the discoidal complexes. The rHDL particles containing the deletion mutants had more molecules of protein present than particles containing intact apoA-I, to the extent that a similar number of helical segments was incorporated into each of the discoidal species. Comparison of the experimentally determined number of helical segments with an estimate of the available space indicated that the deletion mutant proteins are probably more loosely arranged than apoA-I around the edge of the rHDL. The abilities of the complexes to remove radiolabeled FC were compared in experiments using cultured mouse L-cell fibroblasts. All four discoidal complexes displayed similar abilities to remove FC from the plasma membrane of L-cells when compared at an acceptor concentration of 50 microg of phospholipid/ml. Thus, none of the deletions imposed in this study notably altered the ability of the rHDL particles to participate in cellular FC efflux. These results suggest that efficient apoA-I-mediated FC efflux requires the presence of amphipathic alpha-helical segments but is not dependent on specific helical segments.
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PMID:Apolipoprotein A-I structural modification and the functionality of reconstituted high density lipoprotein particles in cellular cholesterol efflux. 879 7

To model the common structural unit in the system of reverse cholesterol transport, we studied the composition, structure, and physicochemical properties of complexes generated between dipalmitoylphosphatidylcholine (DPPC) or palmitoyllinoleoylphosphatidylcholine (PLPC) and apoE3 in the absence and in the presence of cholesterol (Chol); the data were compared with similar experiments using apoA-I, the major proteins of high-density lipoproteins. The conformation and organization of lipid-binding domains of apoE3 within the complexes were calculated by computer modeling. The transition temperatures of DPPC within discoidal complexes with mean diameters of 116 A (GGE) or 148 A (EM) were higher for complexes versus liposomes both in the absence and in the presence of Chol. Association of apoE3 with DPPC resulted in a more structured state of the apolipoprotein molecule versus the soluble apolipoprotein; this state was characterized by parallel orientation of alpha-helixes of apoE3 and DPPC acyl chains. Substrate efficiency of the apoE3-PLPC-Chol complexes in the lecithin-cholesterol acyltransferase (LCAT) reaction expressed as Vmax/Km was 0.5 mole cholesteryl esters/h per 1 microM. The transformation of discoidal apoE3-DPPC-Chol complexes into spherical particles was induced by LCAT and accumulation of cholesteryl esters was approximately 62% of the total cholesterol. Parallel orientation of phospholipid acyl chains with helical segments disappeared in these particles. Discoidal apoE3-DPPC complexes incorporated unesterified cholesterol released from Chol-loaded J774 macrophages. The data support the concept that association of apoE3 and apoA-I with phospholipids is qualitatively similar due to similar orientation of helical repeats in the C-terminal domains of apoE3 and apoA-I.
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PMID:Composition and structural and functional properties of discoidal and spherical phospholipid-apoE3 complexes. 927 98

Human carriers of apolipoprotein (apo) A-IMilano are heterozygous for an Arg173-->Cys substitution in the apoA-I primary sequence; despite severe reductions in HDL cholesterol concentrations, affected individuals do not develop coronary heart disease, suggesting that apoA-IMilano may possess antiatherogenic properties. As the beneficial effects of wild-type apoA-I are linked to its role in HDL cholesterol transport, we examined the capacity of apoA-IMilano to recruit cell cholesterol and activate lecithin:cholesterol acyltransferase (LCAT) (two key events in the antiatherogenic reverse cholesterol transport pathway). ApoA-IMilano and wild-type apoA-I were expressed in Chinese hamster ovary cells, and their ability to recruit membrane phospholipid and cholesterol for the assembly of nascent HDL was compared. Both clonal cell lines exhibited similar levels of apolipoprotein accumulation in serum-free medium (approximately 2 micrograms/mg cell protein per 24 hours), and 15% of each apolipoprotein was associated with membrane lipids to form nascent HDL (d = 1.063 to 1.21 g/mL). SDS-PAGE showed that a majority (66 +/- 12%) of the lipidated apoA-IMilano was in the homodimer form. Compositional analyses revealed that apoA-IMilano nascent HDL had a significantly lower (P < .001) unesterified cholesterol/phospholipid mole ratio (0.47 +/- 0.10) than wild-type apoA-I complexes (1.29 +/- 0.14), indicating that apoA-IMilano had a reduced capacity to recruit cell cholesterol. In addition to the reduced unesterified cholesterol/phospholipid ratio, apoA-IMilano nascent HDL consisted mostly of small 7.4-nm particles compared with wild-type apoA-I, in which 11- and 9-nm particles predominated. Despite these changes in nascent HDL particle size and composition, apoA-IMilano activated LCAT normally. We conclude that, even though apoA-IMilano is a normal activator of LCAT, it is less efficient that wild-type apoA-I in recruiting cell cholesterol, suggesting that the putative antiatherogenic properties attributed to apoA-IMilano may be unrelated to the initial stages of reverse cholesterol transport.
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PMID:Evidence that apolipoprotein A-IMilano has reduced capacity, compared with wild-type apolipoprotein A-I, to recruit membrane cholesterol. 932 56


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