Properties of the abnormal human plasma lipoprotein (LP-X) characteristic of cholestasis after chemical modification with succinic anhydride

The abnormal human low-density lipoprotein class characteristic of biliary obstruction (LP-X) was reacted with [¹⁴C]succinic anhydride to an extent of 70–80 moles of succinyl groups incorporated per 10⁵ g of LP-X protein. The modified lipoprotein retained the typical morphology and ultracentrifugal flotation and sedimentation properties of LP-X but failed to react with antiserum to the native lipoprotein. On agar and agarose gel electrophoresis the succinylated lipoprotein had an increased mobility toward the anode relative to LP-X, as a result of the increased negative charge on the protein component.Partial delipidation of succinylated LP-X and ultracentrifugal fractionation of the protein into a fraction containing phospholipids plus at least three relatively small proteins (Apo-X) and an essentially lipid-free protein, chemically similar and immunologically identical with albumin, permitted us to evaluate the extent of reaction of these two protein classes with succinic anhydride in intact LP-X. On the average, the Apo-X fraction had 72 moles of succinyl groups incorporated per 10⁵ g of protein, whereas the albumin fraction incorporated 55 moles per 10⁵ g of protein.Extensive reaction of susceptible amino acid residues (mostly lysines) with succinic anhydride, without disruption of the lipoprotein structure, indicates that these protein groups are accessible to the reagent and are not involved in critical protein-lipid interactions. Elimination of immunoreactivity upon succinylation of LP-X implies that, at least for the Apo-X component, lysine residues participate in the interaction with LP-X antibodies. Also, the present results strongly support the view that albumin is not merely adsorbed to LP-X, and suggest, furthermore, that protein-protein interactions are not directly responsible for the characteristic stacking of LP-X discs as seen in the electron microscope.     

The influence of LP-X and other lipoproteins associated with hepatic dysfunction on the activity of lecithin:cholesterol acyltransferase.

A study was made of the in vitro effects of the abnormal serum lipoproteins associated with liver disease on the activity of the enzyme lecithin:cholesterol acyltransferase. At lipoprotein concentrations equivalent to those found in hepatic disease sera, the results indicate that: (1) LP-X levels greater than 2.5 mg/ml produced total inhibition of enzyme activity. (2) LP-X levels remained constant even up to 36 h incubation, despite active cholesterol esterification in the presence of LP-X concentrations less than 2.5 mg/ml. In addition, the specific activity of radiolabelled LP-X, and its electrophoretic properties remained unchanged after incubation showing that the molecule remained intact. (3) Low density lipoproteins other than LP-X stimulated the enzymes activity, but this effect was overcome by LP-X. (4) Additional concentrations of high density lipoproteins also produced enhancement of enzyme activity, but at higher levels inhibition was seen. LP-X prevented the enhancement of lecithin:cholesterol acyltransferase activity. (5) Small samples of pure LP-X, obtained with the minimum of physical manipulation, showed a complete absence of cholesterol ester and triacylglycerol from the molecule. The implications of these results are discussed, particularly in relation to other reports which have presented evidence that LP-X is a substrate for lecithin:cholesterol acyltransferase.     

Serum amyloid A-containing human high density lipoprotein 3. Density, size, and apolipoprotein composition

Serum amyloid A protein (apo-SAA), an acute phase reactant, is an apolipoprotein of high density lipoproteins (HDL), in particular the denser subpopulation HDL3. The structure of HDL3 isolated from humans affected by a variety of severe disease states was investigated with respect to density, size, and apolipoprotein composition, using density gradient ultracentrifugation, gradient gel electrophoresis, gel filtration, and solid phase immunoadsorption. Apo-SAA was present in HDL particles in increasing amounts as particle density increased. Apo-SAA-containing HDL3 had bigger radii than normal HDL3 of comparable density. Purified apo-SAA associated readily with normal HDL3 in vitro, giving rise to particles containing up to 80% of their apoproteins as apo-SAA. The addition of apo-SAA resulted in a displacement of apo-A-I and an increase in particle size. Acute phase HDL3 represented a mixture of particles, polydisperse with respect to apolipoprotein content; for example, some particles were isolated that contained apo-A-I, apo-A-II, and apo-SAA, whereas others contained apo-A-I and apo-SAA but no apo-A-II. We conclude that apo-SAA probably associates in the circulation of acute phase patients with existing HDL particles, causing the remodeling of the HDL shell to yield particles of bigger size and higher density that are relatively depleted of apo-A-I.     

Apolipoproteins B, C-III and E in two major subpopulations of low-density lipoproteins

To determine the concentration and distribution of apolipoproteins C-III and E in low density lipoproteins (LDL) of d 1.025-1.043 g/ml, fresh human plasma was fractionated by single-spin density gradient ultracentrifugation into five layers. Two major subpopulations including layer 2 (d 1.025-1.029 g/ml) and layer 3 (d 1.032-1.043 g/ml) were isolated and characterized by determination of flotation coefficient, neutral lipids and apolipoproteins B, C-III and E. The apolipoprotein B/C-III/E ratio of layer 2 was 100/(3.3 +/- 2.0)/(5.1 +/- 2.9) (wt/wt) and that of layer 3 was 100/(0.61 +/- 0.32)/(0.58 +/- 0.29) (wt/wt). These weight ratios corresponded to molar ratios of 1.0/(1.90 +/- 1.16)/(0.74 +/- 0.42) and 1.0/(0.34 +/- 0.18)/(0.08 +/- 0.04), respectively. Layer 2 contained 6-23% of the total plasma apolipoprotein B or 7-27% of total LDL2 (d 1.019-1.063 g/ml) apolipoprotein B. Layer 3 contained 41-65% of plasma apolipoprotein B or 62-86% of LDL2 apolipoprotein B. About 5-17% of apolipoprotein C-III and 8-30% of apolipoprotein E in plasma are distributed in layers 2 and 3 with the majority present in layer 2. These results show an evident apolipoprotein heterogeneity of LDL2 isolated from normolipidemic subjects. Moreover, they show that the relatively small amounts of apolipoprotein C-III and apolipoprotein E in lower-density segments of LDL2 take on a greater significance when presented in molar rather than weight concentrations. The existence of different ratios of apolipoprotein C-III/apolipoprotein E in layer 2 and layer 3 suggest the presence in LDL2 of varying amounts of several discrete apolipoprotein B- and/or apolipoprotein C-III- and apolipoprotein E-containing lipoprotein particles.     

Human lymphedema fluid lipoproteins: particle size, cholesterol and apolipoprotein distributions, and electron microscopic structure

The concentration of cholesterol, apolipoproteins A-I, B, and E has been determined in lymphedema fluid from nine patients with chronic primary lymphedema. The concentrations were: 38.14 +/- 21.06 mg/dl for cholesterol, 15.6 +/- 6.17 mg/dl for apolipoprotein A-I, 7.5 +/- 2.8 mg/dl for apolipoprotein B, and 1.87 +/- 0.50 mg/dl for apolipoprotein E. These values represent 23%, 12%, 6%, and 38% of plasma concentrations, respectively. The ratio of esterified to unesterified cholesterol in lymphedema fluid was 1.46 +/- 0.45. Lipoproteins of lymphedema fluid were fractionated according to particle size by gradient gel electrophoresis and by exclusion chromatography. Gradient gel electrophoresis showed that a majority of high density lipoproteins (HDL) of lymphedema fluid were larger than ferritin (mol wt 440,000) and smaller than low density lipoproteins (LDL); several discrete subpopulations could be seen with the large HDL region. Fractionation by exclusion chromatography showed that more than 25% of apolipoprotein A-I and all of apolipoprotein E in lymphedema fluid was associated with particles larger than plasma HDL2. Apolipoprotein A-I also eluted in fractions that contained particles the size of or smaller than albumin. Isolation of lipoproteins by sequential ultracentrifugation showed that less than 25% of lymphedema fluid cholesterol was associated with apolipoprotein B. The majority of apolipoprotein A-containing lipoproteins of lymphedema fluid were less dense than those in plasma. Ultracentrifugally separated fractions of lipoproteins were examined by electron microscopy. The fraction d less than 1.019 g/ml contained little material, while fraction d 1.019-1.063 g/ml contained two types of particles: round particles 17-26 nm in diameter and square-packing particles 13-17 nm on a side. Fractions d 1.063-1.085 g/ml had extensive arrays of square-packing particles 13-14 nm in size. Fractions d 1.085-1.11 g/ml and fractions d 1.11-1.21 g/ml contained round HDL, 12-13 nm diameter and 10 nm diameter, respectively. Discoidal particles were observed infrequently.     

Isolation and characterization of lipoproteins produced by human hepatoma-derived cell lines other than HepG2

A total of six established human hepatoma-derived cell lines, including Hep3B, NPLC/PRF/5 (NPLC), Tong/HCC, Hep 10, huH1, and huH2, were screened for their ability to accumulate significant quantities of lipoproteins in serum-free medium. Only two cell lines, Hep3B and NPLC, secreted quantitatively significant amounts of lipoproteins. In a 24-h period the accumulated mass of apolipoproteins (apo) A-I, A-II, B, and E and albumin for Hep3B cells was 1.96, 1.01, 1.96, 1.90, and 53.2 micrograms/mg cell protein per 24 h, respectively. NPLC cells secreted no detectable albumin but the 24-h accumulated mass for apolipoproteins A-I, A-II, B, and E was 0.45, 0.05, 0.32, and 0.68 micrograms/mg cell protein per 24 h, respectively. Twenty four-hour serum-free medium of Hep3B cells contained lipoproteins corresponding to the three major density classes of plasma; percent protein distribution among the lipoprotein classes was 4%, 41%, and 56% for very low density lipoprotein ("VLDL"), low density lipoprotein ("LDL"), and high density lipoprotein ("HDL"), respectively. NPLC was unusual since most of the lipoprotein mass was in the d 1.063-1.235 g/ml range. Hep3B "LDL", compared with plasma LDL, contained elevated triglyceride, phospholipid, and free cholesterol. Nondenaturing gradient gel electrophoresis revealed that Hep3B "LDL" possessed a major component at 25.5 nm and a minor one at 18.3 nm. Immunoblots showed that the former contained only apoB while the latter possessed only apoE. Like plasma VLDL, Hep3B "VLDL" particles (30.5 nm diameter) isolated from serum-free medium contained apoB, apoC, and apoE. "HDL" harvested from Hep3B and NPLC medium were enriched in phospholipid and free cholesterol and poor cholesteryl ester which is similar to the composition of HepG2 "HDL." "HDL" from Hep3B and NPLC culture medium on gradient gel electrophoresis had peaks at 7.5, 10, and 11.9 nm which were comparable to major components found in HepG2 cell medium. Hep3B cells, in addition, possessed a particle that banded at 8.2 nm which appeared to be an apoA-II without apoA-I particle by Western blot analysis. The cell line also produced a subpopulation of larger-sized "HDL" not found in HepG2 medium. NPLC "HDL" had a distinct peak at 8.3 nm which by Western blot was an apoE-only particle. Electron microscopy revealed that "HDL" harvested from Hep3B and NPLC medium consisted of discoidal and small, spherical particles like those of HepG2. The "HDL" apolipoprotein content of each cell line was distinct from that of HepG2. ApoA-II at 35% of apolipoprotein distinguishes Hep3B "HDL" from HepG2, which contains only 10%.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparison of glycosidase activities in epidermis, palatal epithelium and buccal epithelium.

1. beta-Glucosidase, alpha-glucosidase, beta-galactosidase and alpha-mannosidase were measured in epidermis, palatal and buccal epithelium of the pig (Sus scrofa). 2. All three epithelia contained similar alpha-mannosidase activity (1.7-3.2 nmol mg tissue-1 hr-1 at pH 4), and none contained significant alpha-glucosidase. 3. Specific activity of beta-glucosidase was high (9-13 nmol mg tissue-1 hr-1 at pH 4) in epidermis and palate, but activity was low (less than 2 nmol mg tissue-1 hr-1) in buccal epithelium. 4. Only epidermis contained a high level of beta-galactosidase (5.8 nmol mg tissue-1 hr-1). 5. Differences in glycosidase profiles may underlie differences in permeability barrier properties in these epithelia.     

Properties of cocksfoot streak and cocksfoot cryptic, two viruses infecting cocksfoot (Dactylis glomerate) in Scotland

A Scottish isolate of cocksfoot streak virus (CSV-S) was found to have flexuous filamentous particles which, in sap of infected cocksfoot plants, had a modal length of 712 nm. It was transmitted from infected to healthy cocksfoot plants in a non-persistent manner by Myzus persicae and by mechanical inoculation of infective sap extracts containing an anti-oxidant. Apart from cocksfoot, mechanical inoculation of infective sap succeeded in infecting only four of 22 plant species tested. The infectivity of sap extracts containing 0.2% thioglycerol was lost after heating for 10 min at 55^oC but not 50^oC, storage at room temperature for 48 but not 24 hours, and after diluting 10-2 to 10-3. Highly purified preparations of CSV-S particles sedimented as a single component with a sedimentation coefficient of 139S and had a buoyant density in rubidium bromide of 1.31 g/cm3. Virus particles were composed of one protein and one ssRNA species with estimated Mr of 31 000 and 3.2 times 106 respectively. In ELISA, an antiserum prepared to CSV-S detected the virus in all aerial parts of infected cocksfoot plants and, when present in the ratio of 1 infected leaf: 1000 healthy leaves. Both CSV-S-infected and -uninfected cocksfoot also contained a previously undescribed virus with isometric particles c. 30 nm in diameter. This virus, named cocksfoot cryptic virus (CCV), was seed-borne in two cvs of cocksfoot tested and its particles contained two dsRNA species of estimated Mt of 1.14 times 106 and 1.27 times 106. Despite the fact that particles of CSV-S were largely free from CCV particles following exclusion chromatography on agarose beads prior to immunisation, immunoelectron microscopy (IEM) showed that the antiserum prepared to CSV-S also contained some antibodies to CCV. Evidence from IEM suggested a possible distant serological relationship of CCV to ryegrass and beet (BCV 1 or BCV 2, or both) cryptoviruses, all members of sub-group A of cryptoviruses.     

A density gradient study of the lipoprotein and apolipoprotein distribution in the chicken, Gallus domesticus

Plasma lipoproteins from 5-week old male chickens were separated over the density range 1.006-1.172 g/ml into 22 subfractions by isopycnic density gradient ultracentrifugation, in order to establish the distribution of these particles and their constituent apolipoproteins as a function of density. Lipoprotein subfractions were characterized by electrophorectic, chemical and morphological analyses, and their protein moieties were defined according to net charge at alkaline pH, molecular weight and isoelectric point. These analyses have permitted us to reevaluate the density limits of the major chicken lipoprotein classes and to determine their main characteristics, which are as follows: (1) very-low-density lipoproteins (VLDL), isolated at d less than 1.016 g/ml, were present at low concentrations (less than 0.1 mg/ml) in fasted birds; their mean diameter determined by gradient gel electrophoresis and by electron microscopy was 20.5 and 31.4 nm respectively; (2) as the the density increased from VLDL to intermediate density lipoproteins (IDL), d 1.016-l.020 g/ml) and low-density lipoproteins (LDL, d 1.020-1.046 g/ml), the lipoprotein particles contained progressively less triacylglycerol and more protein, and their Stokes diameter decreased to 20.0 nm; (3) apolipoprotein B-100 was the major apolipoprotein in lipoproteins of d less than 1.046 g/ml, with an Mr of 350000; small amounts of apolipoprotein B-100 were detectable in HDL subfractions of d less than 1.076 g/ml; urea-soluble apolipoproteins were present in this density range as minor components of Mr 38000-39000, 27000-28000 (corresponding to apolipoprotein A-1) and Mr 11000-12000; (4) high density lipoprotein (HDL, d 1.052-1.130 g/ml) was isolated as a single band, whose protein content increased progressively with increase in density; the chemical composition of HDL resembled that of human HDL2, with apolipoprotein A-1 (M 27000-28000) as the major protein component, and a protein of Mr 11000-12000 as a minor component; (5) heterogeneity was observed in the particle size and apolipoprotein distribution of HDL subfractions: two lipoprotein bands which additional apolipoproteins of Mr 13000 and 15000 were detected. These studies illustrate the inadequacy in the chicken of the density limits applied to fractionate the lipoprotein spectrum, and particularly the inappropriateness of the 1.063 g/ml density limit as the cutoff for LDL and HDL particle populations in the species.     

The fusion of abnormal plasma lipoprotein (LP-X) and the erythrocyte membrane in patients with cholestasis studied by electronmicroscopy

Adhesion followed by fusion of LP-X vesicles with the erythrocyte membrane is an important contribution to the erythrocyte enlargement in patients with intra or extra hepatic cholestasis. Adhesion of LP-X vesicles is demonstrated by thin section and freeze-etch electronmicroscopy. Fusion of LP-X with the erythrocyte membrane is deduced from biochemical data and freeze-etch electronmicroscopy in that the uptake of cholesterol and lecithin coincides with the increase in smoooth areas on the fracture faces of the erythrocyte membrane.     

Physical and chemical characteristics of apolipoprotein A-I-lipid complexes produced by Chinese hamster ovary cells transfected with the human apolipoprotein A-I gene

Chinese hamster ovary cells transfected with the human apolipoprotein A-I gene linked to the human metallothionein gene promoter region secrete large quantities of apolipoprotein A-I (7.1 +/- 0.4% total secreted protein) in the presence of zinc. Approx. 16% of the secreted apolipoprotein A-I is complexed with lipid and can be isolated ultracentrifugally at d less than or equal to 1.21 g/ml. The latter complexes are composed of discs and vesicles as judged by electron microscopy and can be further separated by column chromatography into three fractions: fraction I, mostly vesicles (60-260 nm) and large discs (18-20 nm diameter); fraction II, discs 14.2 +/- 2.6 nm diameter; and fraction III, nonresolvable by electron microscopy. The latter fraction is extremely lipid-poor (94% protein, 6% phospholipid); in contrast, the protein, phospholipid and unesterified cholesterol content for the other fractions are 43, 33 and 24%, respectively, for fraction I and 53, 33 and 14%, respectively, for fraction II. Fraction II particles contain three and four apolipoprotein A-Is per particle as determined by protein crosslinking while large structures in fraction I contain primarily six to seven apolipoprotein A-Is per particle. Following incubation with purified lecithin: cholesterol acyltransferase, discoidal particles were transformed into apparent spherical particles 12.9 +/- 3.4 nm diameter; this transformation coincided with 19-21% conversion of unesterified cholesterol to esterified cholesterol. The apolipoprotein A-I-lipid complexes isolated from Chinese hamster ovary cell media are similar to nascent HDL found in plasma of lecithin:cholesterol acyltransferase-deficient patients and those secreted by the human hepatoma line, Hep G2. The ability of the Chinese hamster ovary cell nascent HDL-like particles to undergo transformation in the presence of purified lecithin:cholesterol acyltransferase indicates that they are functional particles.     

Comparison of the cytolytic effects in vitro on Trypanosoma brucei brucei of plasma, high density lipoproteins, and apolipoprotein A-I from hosts both susceptible (cattle and sheep) and resistant (human and baboon) to infection.

The African trypanosome, Trypanosoma brucei brucei causes a fatal wasting disease in livestock but does not ordinarily infect humans, apparently because this unicellular parasite is lysed by high density lipoproteins (HDL) in human serum. To assess whether there is a specific active constituent in trypanolytic HDL, we have systematically compared the cytotoxic action on T.b.brucei in vitro of native and delipidated HDL, and of individual apolipoproteins, from nonpermissive hosts (human and baboon) with their counterparts from susceptible hosts (cattle and sheep). When suspensions of trypanosomes were incubated for 2 h at 37 degrees C with human or baboon plasma most cells were lysed, but not with bovine or sheep plasma. Similarly, HDL isolated from human and baboon plasma were trypanolytic (typically about 95% and 60% lysis, respectively, at 1 mg protein/ml), whereas bovine and sheep HDL were benign (less than 8% lysis). Subfractionation of human HDL by serial isopycnic ultracentrifugation and by heparin-Sepharose affinity chromatography established that the denser and smaller particles had greater trypanolytic activity both in vitro and in vivo. When human HDL was delipidated, the trypanocidal activity was associated with the water-soluble protein (apolipoprotein) fraction and not with the lipid constituents. Bovine apolipoproteins were also weakly trypanolytic in free solution (20-40% lysis), but not when complexed with cholesterol-phospholipid liposomes (less than 10% lysis). The major apolipoprotein of human HDL, apolipoprotein (apo) A-I had full trypanolytic activity (89-95% lysis at 1 mg protein/ml) when purified, whether in solution or incorporated into liposomes, but other apolipoproteins isolated from human HDL, including apoA-II, apoC, and apoE, were nontrypanolytic. Purified baboon apoA-I was also trypanolytic, though less potent than human apoA-I, but apoA-I from permissive hosts (cattle and sheep) was inactive when presented in liposomes. Incubation of bovine or sheep HDL with purified human apoA-I, and subsequent separation of the HDL by ultracentrifugation, produced chimeric HDL containing significant amounts of the human apolipoprotein; these particles showed appreciable trypanolytic activity. By contrast, human HDL particles in which about 70% of the apoA-I had been displaced with apoA-II had markedly reduced lytic properties compared to the native HDL (30% versus 80% lysis at 0.6 mg total protein/ml). We tentatively conclude that the trypanolytic activity of native human or baboon plasma resides in the apoA-I content of the HDL particles and that, conversely, bovine and sheep plasma are inactive because the apoA-I polypeptide present in their HDL lacks trypanocidal activity.     

Identification and partial characterization of discrete apolipoprotein A-containing lipoprotein particles secreted by human hepatoma cell line HepG2

The purpose of this study was to identify the apolipoprotein A-containing lipoprotein particles produced by HepG2 cells. The apolipoprotein A-containing lipoproteins separated from apolipoprotein B-containing lipoproteins by affinity chromatography of culture medium on concanavalin A were fractionated on an immunosorber with monoclonal antibodies to apolipoprotein A-II. The retained fraction contained apolipoproteins A-I, A-II and E, while the unretained fraction contained apolipoproteins A-I and E. Both fractions were characterized by free cholesterol as the major and triglycerides and cholesterol esters as the minor neutral lipids. Further chromatography of both fractions on an immunosorber with monoclonal antibodies to apolipoprotein A-I showed that 1) apolipoprotein A-II only occurs in association with apolipoprotein A-I, 2) apolipoprotein A-IV is only present as part of a separate lipoprotein family (lipoprotein A-IV), and 3) apolipoprotein E-enriched lipoprotein A-I:A-II and lipoprotein A-I are the main apolipoprotein A-containing lipoproteins secreted by HepG2 cells.     

Apolipoprotein composition of HDL in cholesteryl ester transfer protein deficiency

Our purpose was to compare HDL subpopulations, as determined by nondenaturing two-dimensional gel electrophoresis followed by immunoblotting for apolipoprotein A-I (apoA-I), apoA-II, apoA-IV, apoCs, and apoE in heterozygous, compound heterozygous, and homozygous subjects for cholesteryl ester transfer protein (CETP) deficiency and controls. Heterozygotes, compound heterozygotes, and homozygotes had CETP masses that were 30, 63, and more than 90% lower and HDL-cholesterol values that were 64, 168, and 203% higher than those in controls, respectively. Heterozygotes had approximately 50% lower pre-beta-1 and more than 2-fold higher levels of alpha-1 and pre-alpha-1 particles than controls. Three of the five heterozygotes' alpha-1 particles also contained apoA-II, which was not seen in controls. Compound heterozygotes and homozygotes had very large particles not observed in controls and heterozygotes. These particles contained apoA-I, apoA-II, apoCs, and apoE. However, these subjects did not have decreased pre-beta-1 levels. Our data indicate that CETP deficiency results in the formation of very large HDL particles containing all of the major HDL apolipoproteins except for apoA-IV. We hypothesize that the HDL subpopulation profile of heterozygous CETP-deficient patients, especially those with high levels of alpha-1 containing apoA-I but no apoA-II, represent an improved anti-atherogenic state, although this might not be the case for compound heterozygotes and homozygotes with very large, undifferentiated HDL particles.     

Speciated human high-density lipoprotein protein proximity profiles

It is expected that the attendant structural heterogeneity of human high-density lipoprotein (HDL) complexes is a determinant of its varied metabolic functions. To determine the structural heterogeneity of HDL, we determined major apolipoprotein stoichiometry profiles in human HDL. First, HDL was separated into two main populations, with and without apolipoprotein (apo) A-II, LpA-I and LpA-I/A-II, respectively. Each main population was further separated into six individual subfractions using size exclusion chromatography (SEC). Protein proximity profiles (PPPs) of major apolipoproteins in each individual subfraction was determined by optimally cross-linking apolipoproteins within individual particles with bis(sulfosuccinimidyl) suberate (BS(3)), a bifunctional cross-linker, followed by molecular mass determination by MALDI-MS. The PPPs of LpA-I subfractions indicated that the number of apoA-I molecules increased from two to three to four with an increase in the LpA-I particle size. On the other hand, the entire population of LpA-I/A-II demonstrated the presence of only two proximal apoA-I molecules per particle, while the number of apoA-II molecules varied from one dimeric apoA-II to two and then to three. For most of the PPPs described above, an additional population that contained a single molecule of apoC-III in addition to apoA-I and/or apoA-II was detected. Upon composition analyses of individual subpopulations, LpA-I/A-II exhibited comparable proportions for total protein (～58%), phospholipids (～21%), total cholesterol (～16%), triglycerides (～5%), and free cholesterol (～4%) across subfractions. LpA-I components, on the other hand, showed significant variability. This novel information about HDL subfractions will form a basis for an improved understanding of particle-specific functions of HDL.     

The fusion of abnormal plasma lipoprotein (LP-X) and the erythrocyte membrane in patients with cholestasis studied by electronmicroscopy.

Adhesion followed by fusion of LP-X vesicles with the erythrocyte membrane is an important contribution to the erythrocyte enlargement in patients with intra or extra hepatic cholestasis. Adhesion of LP-X vesicles is demonstrated by thin section and freeze-etch electronmicroscopy. Fusion of LP-X with the erythrocyte membrane is deduced from biochemical data and freeze-etch electronmicroscopy in that the uptake of cholesterol and lecithin coincides with the increase in smooth areas on the fracture faces of the erythrocyte membrane.     

Hibiscus latent ringspot virus, a newly recognised virus from Hibiscus rosa-sinensis (Malvaceae) in western Nigeria

Hibiscus latent ringspot virus (HLRV) was prevalent in Hibiscus rosa-sinensis in Ibadan, Nigeria. It was readily transmitted mechanically to 22 of 73 species from seven of 20 families, but was best propagated in Nicotiana clevelandii or Hibiscus cannabinus and assayed in Chenopodium murale. HLRV was readily purified from systemically infected hosts by differential centrifugation of leaf extracts clarified with 8.0% n-butanol, followed by molecular permeation chromatography on controlled-pore glass beads (700 Å, 120–200 mesh). The virus has isometric particles c. 28 nm in diameter which sedimented as three components (T, M and B), with sedimentation coefficients (s°_{20, w}) of 51; 114 and 132 S and buoyant densities in caesium chloride of 1.32, 1.49 and 1.52 g/cm³, respectively. All three components contained a single polypeptide of rnol. wt 53.6 × 10³. T component particles contained only protein but M and B components also contained single-stranded RNA of rnol. wt 2.2 × 10⁶ and 2.5 × 10⁶, respectively. The properties of HLRV suggest affinities with nepoviruses but no serological relationship was detected between HLRV and 15 recognised or possible members of the nepovirus group.     

Cholesterol efflux from macrophages mediated by high-density lipoprotein subfractions, which differ principally in apolipoprotein A-I and apolipoprotein A-II ratios

High-density lipoprotein (HDL) was fractionated by preparative isoelectric focussing into six distinct subpopulations. The major difference between the subfractions was in the molar ratio of apolipoprotein A-I to apolipoprotein A-II, ranging from 2.1 to 0.5. The least acidic particles had little apolipoprotein A-II, were larger and contained the most lipid. The efflux capacity of the HDL subfractions was tested with mouse peritoneal macrophages and a mouse macrophage cell line (P388D1), either fed with acetylated low-density lipoprotein or free cholesterol. All the HDL subfractions were equally able to efflux cholesterol. The efflux was concentration dependant and linear for the first 6 h. The HDL subfractions bound with high affinity (Kd = 6.7-7.9 micrograms/ml) at 4 degrees C to the cell surface of P388D1 cells (211,000-359,000 sites/cell). Ligand blotting showed that all the HDL subfractions bound to membrane polypeptides at 60, 100, and 210 kDa. These HDL binding proteins may represent HDL receptors. In summary HDL particles, which differed principally in ratio of apolipoprotein A-I to apolipoprotein A-II behaved in a similar manner for both cholesterol efflux and cell surface binding.     

Characterization of A-I-containing lipoproteins in subjects with A-I Milano variant

The A-I Milano variant of apolipoprotein A-I (A-IM), by virtue of its Arg-173----Cys substitution, is capable of forming a disulfide bond with the 77-amino-acid apolipoprotein A-II polypeptide (A-IIS) as well as with itself to produce dimers, A-IM/A-IIS and A-IM/A-IM, respectively. A-I-containing lipoproteins (Lp): particles with A-II (Lp(A-I with A-11)) and particles without A-II (Lp(A-I without A-II)) in the plasma of two nonhyperlipidemic A-IM carriers were investigated to determine the effect of A-IM on these lipoproteins. Despite the existence of abnormal apolipoprotein dimers and the unusually low HDL cholesterol (17 and 14 mg/dl), A-I (67 and 75 mg/dl), and A-II (18 and 18 mg/dl) levels in the two carriers, the plasma A-I of the carriers was distributed between Lp(A-I with A-II) and Lp(A-I without A-II) in a proportion comparable to that observed in normals. As expected, A-IM/A-IIS mixed dimer was found in carrier Lp(A-I with A-II). However, A-IM/A-IM dimer was located almost exclusively in carrier Lp(A-I without A-II). Chemical (dimethylsuberimidate) crosslinking of the protein moieties of the major subpopulations of Lp(A-I with A-II) and Lp(A-I without A-II) of normal and A-IM carriers showed that Lp(A-I with A-II), which is located predominantly in the 7.8-9.7 nm interval ((HDL2a + 3a + 3b)gge), had an apparent protein molecular weight equivalent to two molecules of A-I and one to two molecules of A-II per particle. Most of the Lp(A-I without A-II) particles, located predominantly in the size intervals of 9.7-12.9 nm (designated (HDL2b)gge) and 8.2-8.8 nm (HDL3a)gge) had protein moieties exhibiting a molecular weight equivalence predominantly of four and three molecules of A-I, respectively. A small quantity of particles with apparent protein content of two molecules of A-I in the 7.2-8.2 nm interval ((HDL3b + 3c)gge) was also detected. These studies showed that in nonhyperlipidemic A-IM carriers, the occurrence of apolipoprotein dimers had not markedly affected the protein stoichiometry of Lp(A-I with A-II) and Lp(A-I without A-II).