Isolation and characterization of a dog serum lipoprotein having apolipoprotein A-I as its predominant protein constituent.

The serum high density lipoproteins (HDL) of normolipemic dogs (beagles) were isolated in the density range of p 1.063 to 1.21 g/ml, and characterized in terms of composition and physical properties (flotation and diffusion coefficients, partial specific volume, molecular weight, electrophoretic mobility, ultraviolet absorption, and circular dichroism). The results indicated that canine HDL is a relatively homogeneous class with a molecular weight of about 230 000 and general properties similar to those reported for human HDL. After delipidation, the resulting apolipoprotein, apo-HDL, was fractionated by Sephadex G-200 column chromatography in urea or guanidine hydrochloride solutions. About 90% of the apo-HDL consisted of a protein with a molecular weight of about 28 000, similar in amino acid composition to human apolipoprotein A-I and having the same NH2 terminus (aspartic acid) and COOH terminus (glutamine) and no carbohydrates. Two other proteins were isolated, one having an apparent mol wt of 55 000 and representing, at least in part, an aggregate of apolipoprotein A-I and the other component with a mol wt of 8000, not yet characterized. The results indicate that canine HDL, as an intact complex, has general physical properties that lie between those reported for human HDL2 and HDL3, and that it differs compositionally from the human products mainly in its predominant content of apo-A-I. These findings together with evidence for the relatively homogeneous nature of the canine HDL provide new prospects for unraveling the relationship between polypeptide composition and HDL structure.     

Proteolytic digestion of human serum apolipoproteins by porcine pancreatic elastase

We studied the proteolytic action in vitro of free and alpha 2-macroglobulin-bound porcine pancreatic elastase [EC 3.4.21.11] on the apolipoproteins of plasma: very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL). Polyacrylamide gel electrophoresis, isoelectric focusing and immunodiffusion tests of elastase-treated plasma lipoproteins revealed that apolipoprotein C-II and C-III polypeptides were more susceptible to elastase in free form than plasma apolipoproteins (A-I, A-II, B, and E). Elastase bound to alpha 2-macroglobulin did not show any such activities.     

Human apolipoprotein A-I liberated from high-density lipoprotein without denaturation.

Apolipoprotein A-I (apoA-I) was liberated from human high-density lipoprotein (HDL) without exposure to organic solvents or chaotropic salts by the action of isolated insect hemolymph lipid transfer particle (LTP). LTP-catalyzed lipid redistribution results in transformation of HDL into larger, less dense particles accompanied by an overall decrease in HDL particle surface area:core volume ratio, giving rise to an excess of amphiphilic surface components. Preferential dissociation of apolipoprotein versus phospholipid and unesterified cholesterol from the particle surface results in apolipoprotein recovery in the bottom fraction following ultracentrifugation at a density = 1.23 g/mL. ApoA-I was then isolated from other contaminating HDL apolipoproteins by incubation with additional HDL in the absence of LTP, whereupon apolipoprotein A-II and the C apolipoproteins reassociate with the HDL surface by displacement of apoA-I. After a second density gradient ultracentrifugation, electrophoretically pure apoA-I was obtained. Sedimentation equilibrium experiments revealed that apoA-I isolated via this method exhibits a tendency to self-associate in an aqueous solution while its circular dichroism spectrum was indicative of a significant amount of alpha-helix. Both measurements are consistent with that observed on material prepared by denaturation/renaturation. The ability of apoA-I to activate lecithin:cholesterol acyltransferase was found to be similar to that of apoA-I isolated by conventional methods. The present results illustrate that LTP-mediated alteration in lipoprotein particle surface area leads to dissociation of substantial amounts of surface active apoprotein components, thus providing the opportunity to isolate apoA-I without the denaturation/renaturation steps common to all previous isolation procedures.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contribution of HDL-apolipoproteins to the inhibition of low density lipoprotein oxidation and lipid accumulation in macrophages

High-density lipoprotein (HDL) is known as a protective factor against atherosclerosis. However, whether HDL-apolipoproteins (apo-HDL) contribute to the protection in arterial cells remains unclear. The localization patterns of human apolipoproteins in atherosclerotic arteries were determined using immunohistochemical examination. The results indicate that several apolipoproteins are retained in component cells of the coronary artery walls. To elucidate the possible roles of apo-HDL in the protection of atherosclerotic lesion formation, we investigated the effects of apo-HDL on the formation of conjugated diene (CD) in a cell-free system and thiobarbituric acid-reactive substances (TBARS) in the medium of a macrophage-mediated LDL oxidation system. The results showed that apo-HDL significantly exerted an inhibitory effect on LDL lipid oxidation in vitro. In addition, apo-HDL decreased cholesterol influx but enhanced cholesterol efflux from J774 macrophages in a dose-dependent manner. These results are consistent with the notion that there is reduced intracellular lipid accumulation in apo-HDL treated macrophages. These data provide a direct evidence for apo-HDL in protecting LDL from oxidative modification and in reducing the accumulation of cholesterol and lipid droplets by J774 macrophages.     

Identification of apolipoproteins involved in the interaction of human high density lipoprotein3 with receptors on cultured cells

Human high density lipoprotein (HDL), devoid of apolipoproteins E or B, binds with high affinity and specificity to cultured cells derived from several tissues. In order to investigate the ligand specificity of the putative receptor, we have performed competitive inhibition studies to identify the components of high density lipoprotein that bind to cell surfaces of rat adrenal cortical cells and human skin fibroblasts. Radiolabeled HDL3 was displaced with unlabeled apolipoprotein-dimyristoylphosphatidylcholine recombinant particles containing AI, AII, CIII-1, and E apolipoproteins, but not by dimyristoylphosphatidylcholine complexed to albumin or by low density lipoprotein. Because exchange may readily occur between apolipoproteins in HDL and in recombinants this observation may not be truly representative of ligand competition. Further experiments using Fab fragments prepared from pure IgG to each apolipoprotein showed that binding of radioiodinated HDL to cells was suppressed following preincubation of HDL with Fab fragments raised against apolipoproteins AI or AII but not against apolipoproteins E or CIII-1 or albumin. In additional studies with apolipoprotein recombinants specific saturable binding was demonstrated between apo-AI or -AII recombinants and adrenocortical cells whereas binding of apo-CIII-2 was characterized by a large nonsaturable component which almost equaled the specific binding. The data, therefore, provide evidence for the involvement of the two major apolipoproteins (AI and AII) in HDL recognition by cellular receptors.     

Relation of angiographically defined coronary artery disease to plasma lipoprotein subfractions and apolipoproteins.

The relation of coronary artery disease to plasma lipoproteins was examined in 104 men aged 35-65 years undergoing coronary angiography for suspected myocardial ischaemia. A score reflecting the number, degree, and length of stenoses in seven major coronary arteries was assigned to each angiogram. Lipid concentrations in lipoprotein subfractions were measured after preparative ultracentrifugation; plasma apolipoprotein concentrations were measured by electroimmunoassay. Men with high coronary scores tended to have lower plasma high-density lipoprotein (HDL) cholesterol concentrations and higher low-density lipoprotein (density 1.019-1.063 g/ml) cholesterol concentrations than subjects of similar age with low coronary scores (p approximately equal to 0.1). The strongest relation, however, was with the cholesterol concentration in the HDL2 subfraction (density 1.063-1.125 g/ml) of HDL, which averaged 44% lower in the severely affected patients (p less than 0.005). No associations were found between the coronary score and HDL3 cholesterol, the cholesterol content of lipoproteins of density less than 1.019 g/ml, plasma triglyceride, or the concentrations of apolipoproteins AI, AII, and E. The high coronary scores associated with low HDL2 concentrations reflected an increase in the number of both partial and complete stenoses distributed throughout the coronary tree. In contrast the sizes of the lesions and the proportion producing complete occlusion were unrelated to HDL2.     

Surface exposure of apolipoproteins in high density lipoproteins. I. Reactivities with agarose-immobilized proteases.

The exposure of apolipoproteins at the surface of human plasma high density lipoproteins (HDL) was assessed by their accessibility to agarose-immobilized forms of trypsin and chymotrypsin. Proteolysis of lipid-free apolipoproteins and the lipoprotein subfractions HDL2 (d = 1.08--1.125 g/ml) and HDL3 (d = 1.125--1.195 g/ml) that differ in lipid-to-protein ratio was compared by polyacrylamide gel electrophoresis and isoelectric focusing of the apolipoproteins and peptide fragments and by quantitation of the various carboxyl-terminal groups formed. Gel filtration of the proteolyzed lipoproteins on Sephadex G-150 column indicated that more than 90% of the apolipoproteins and peptides remain associated with lipoprotein complexes. Proteolysis of lipoproteins occurred more slowly and with less fragmentation of the lipoproteins and apolipoproteins than proteolysis of thelipid-free apolipoproteins or the proteolysis of lipoproteins by soluble proteases reported by other investigators. The difference in lipid content of HDL2 and HDL3 made little difference in their proteolysis. Proteolysis of the lipoproteins by agarose-trypsin was more rapid at 37 degrees C than at 22 degrees C, but the proteolytic products were similar and differed from the products from the lipid free proteins. Peptide fragments from lipoproteins were larger than those from lipid-free proteins, which suggests masking of potentially cleavable groups by lipid. The amounts (mol/g protein) of new carboxyl-terminal tyrosine and phenylalanine released by agarose -chymotrypsin were much greater from the lipid-free proteins, but about 3/4 of the tryptophan residues were inacessible in both lipoproteins and lipid-free proteins. In agarose-trypsin digestion, lysine residues were slightly more masked than arginine in the absence of lipids and much more so in the lipoproteins. However, in the lipoproteins apoA-II, which contains lysine but no arginine, was cleaved more rapidly and extensively by agarose-trypsin than apoA-I.     

Mechanism of the HDL2 stimulation of progesterone secretion in cultured placental trophoblast

Lipoprotein cholesterol (C) supports the high rate of progesterone production by the human placenta as endogenous cholesterol synthesis is low. To study underlying mechanisms whereby lipoproteins, including high density lipoprotein-2 (HDL2), stimulate progesterone secretion, trophoblast cells were isolated from human term placentas and maintained in primary tissue culture. Lipoproteins were added at several concentrations and medium progesterone secretion was determined. HDL2 (d 1.063-1.125 g/ml) as well as low density lipoproteins (LDL) (d 1.019-1.063 g/ml) but not HDL3 (d 1.125-1.21 g/ml) stimulated progesterone secretion in a dose-dependent manner, with HDL2 cholesterol entering the cell and serving as substrate for progesterone synthesis. Conversely, LDL and HDL2 produced a significant decrease in [2-14C]acetate incorporation into cell cholesterol. Cholesterol-depleted lipoproteins did not stimulate progesterone secretion. The stimulating effect of LDL was abolished by apolipoprotein modification by cyclohexanedione or reductive methylation and by the addition of anti-LDL receptor antibody or 10 microM chloroquine to the medium. [14C]acetate conversion into cholesterol was accelerated by these procedures. However, HDL2 stimulation of progesterone secretion and reduction of [14C]acetate incorporation into cholesterol was not blocked by chemical modification of apolipoproteins, anti-LDL receptor antibody, or chloroquine. Treatment of HDL2 with tetranitromethane or dimethylsuberimidate also did not block the stimulation of progesterone. To determine whether the capacity of HDL2 to deliver cholesterol to the trophoblast cells was restricted to subfractions differing in apoE content, HDL2 was chromatographed on heparin-Sepharose and three fractions (A, B, and C) were obtained. Fraction A was poorest in apoE and free cholesterol, fraction B contained the majority of cholesterol, and fraction C was the richest in apoE and free cholesterol. When added to trophoblast cells, fraction A stimulated little progesterone secretion, fraction B stimulated moderately, and fraction C did so greatly. Modification of these subfractions with cyclohexanedione or reductive methylation did not inhibit these effects. In conclusion, HDL2 stimulated progesterone secretion in human trophoblast cell culture. Contrary to LDL, the HDL effect was not mediated by apolipoproteins or the LDL receptor pathway. The ability of HDL2 to stimulate progesterone secretion is consistent with the passive transfer of free cholesterol to the cell membrane from a physicochemically specific subfraction of HDL. This mechanism may be an auxiliary source of cholesterol for human steroidogenic cells.     

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles.

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.     

Isolation and identification of HDL particles of low molecular weight

Small particles of high density lipoproteins (HDL) were isolated from fresh, fasting human plasma and from the ultracentrifugally isolated high density lipoprotein fraction by means of ultrafiltration through membranes of molecular weight cutoff of 70,000. These particles were found to contain cholesterol, phospholipids, and apolipoproteins A-I and A-II; moreover, they floated at a density of 1.21 kg/l. They contained 67.5% of their mass as protein and the rest as lipid. Two populations of small HDL particles were identified: one containing apolipoprotein A-I alone [(A-I)HDL] and the other containing both apolipoproteins A-I and A-II [A-I + A-II)HDL]. The molar ratio of apoA-I to apoA-II in the latter subclass isolated from plasma or HDL was 1:1. The molecular weights of these subpopulations were determined by nondenaturing gradient polyacrylamide gel electrophoresis and found to be 70,000; 1.5% of the plasma apoA-I was recovered in the plasma ultrafiltrate.     

SAA-only HDL formed during the acute phase response in apoA-I+/+ and apoA-I-/- mice.

Serum amyloid A (SAA) is an acute phase protein of unknown function that is involved in systemic amyloidosis and may also be involved in atherogenesis. The precise role of SAA in these processes has not been established. SAA circulates in plasma bound to high density lipoprotein-3 (HDL3). The pathway for the production of SAA-containing HDL is not known. To test whether apolipoprotein (apo)A-I-HDL is required in the production of SAA-HDL, we analyzed the lipopolysaccharide (LPS)-induced changes in apoA-I+/+ and apoA-I-/- mice. In apoA-I+/+ mice, after injection of LPS, remodeling of HDL occurred: total cholesterol increased and apoA-I decreased slightly and shifted to lighter density. Dense (density of HDL3) but large (size of HDL2 ) SAA-containing particles were formed. Upon fast phase liquid chromatography fractionation of plasma, >90% of SAA eluted with HDL that was enriched in cholesterol and phospholipid and shifted "leftward" to larger particles. Non-denaturing immunoprecipitation with anti-mouse apoA-I precipitated all of the apoA-I but not all of the SAA, confirming the presence of SAA-HDL devoid of apoA-I. In the apoA-I-/- mice, which normally have very low plasma lipid levels, LPS injection resulted in significantly increased total and HDL cholesterol. Greater than 90% of the SAA was lipid associated and was found on dense but large, spherical HDL particles essentially devoid of other apolipoproteins.We conclude that serum amyloid A (SAA) is able to sequester lipid, forming dense but large HDL particles with or without apoA-I or other apolipoproteins. The capacity to isolate lipoprotein particles containing SAA as the predominant or only apolipoprotein provides an important system to further explore the biological function of SAA.     

Amphipathic helixes and plasma lipoproteins: Thermodynamic and geometric considerations

In this paper analyses are made of the thermodynamic and geometric properties of the predicted association between amphipathic helixes and phospholipid vesicles. From thermodynamic considerations it is proposed that a major driving force for such an association is the negative free energy gained by the transfer of a number of hydrophobic residues (contained within the non-polar faces of amphipathic helixes), from water to the interior of a phospholipid bilayer. The mechanism proposed is that in the aqueous state a potentially amphipathic sequence forms a non-helical hydrophobic patch on the surface of the apolipoprotein. Formation of an amphipathic helix and simultaneous burial of the hydrophobic residues in the surface of a phospholipid bilayer provides the driving force for lipid association. From this model an estimate of the upperlimit for the hydrophobically driven free energy of lipid association (?40?65 kcal/mol) is calculated for the 4 apolipoproteins with known sequences.On the basis of geometrical considerations a model for an intermediate state of high density lipoprotein (HDL) synthesis is proposed. This model consists of a cholesterol-containing phospholipid bilayer disc whose ‘naked’ hydrophobic edges are shielded from the aqueous phase by amphipathic helixes of the apolipoproteins. Exposure of these ‘bicycle tire’ micelles to the enzyme lecithin: cholesterol acyl transferase (LCAT) is postulated to result in the formation of mature spherical HDL particles with cholesteryl ester forming a neutral lipid core.     

Dissociation of apolipoprotein A-I from porcine and bovine high density lipoproteins by guanidine hydrochloride.

Dissociation of apolipoprotein A-I from pig and steer high density lipoproteins (HDL) deficient in apoA-II was determined by exposing native HDL fractions to 6 M guanidine hydrochloride (Gdn-HCl) at 37 degrees C for periods from 5 min to 18 h. Bovine high density lipoprotein (HDL-B) was isolated at d 1.063--1.100 g/ml while porcine high density lipoprotein (HDL-P) was isolated at d 1.125--1.21 g/ml. Incubation for 5 min with Gdn-HCl resulted in a 45 and 3% loss of apo-A-I from HDL-P and HDL-B, respectively. Exposure to the denaturant for 3 h resulted in a 75% loss of apoA-I from HDL-P and a 30% loss from HDL-B. Analytic ultracentrifugation, patterns paralleled the degree of apoA-I dissociation from each HDL species. The initial flotation peak for HDL-P shifted from F degrees 1.20 2.68 to F degrees 1.20 10.75 after 3 h exposure while HDL-B showed only a small shift from F degrees 1.20 8.30 to F degrees 1.20 8.96 after 3 h exposure. HDL-P particle diameter increased 25% after 5 min of Gdn-HCl treatment and large, flattened structures predominated after 3 h. There was no changes in the size of HDL-B after 5 min exposure and only 16% increase in particle diameter after 3 h. The difference in behavior of HDL-B and HDL-P to Gdn-HCl exposure is discussed in terms of differences in apolipoprotein A-I amino acid composition, interaction of apolipoprotein A-I with phospholipids and the possible involvement of the cholesteryl ester core.     

A 70-kDa apolipoprotein designated ApoJ is a marker for subclasses of human plasma high density lipoproteins

A new apolipoprotein, termed apolipoprotein J (apoJ), was purified from human plasma by immunoaffinity chromatography. ApoJ is a glycoprotein consisting of disulfide-linked subunits of 34-36 and 36-39 kDa. Each subunit is glycosylated and has a pI range of 4.9-5.4. ApoJ exists in the plasma associated with high density lipoproteins (HDL) and specifically with subclasses of HDL which also contain apoAI and cholesteryl ester transfer protein activity. Immunoaffinity purified apoJ-HDL subclasses have apparent molecular masses of 80, 160, 240, 340, and 520 kDa, as determined by gradient gel electrophoresis. By negative staining electron microscopy, apoJ-HDL range in diameter from 5 to 16 nm. Fractionation of plasma by vertical gradient density centrifugation revealed apoJ-HDL in HDL2 (d 1.063-1.125 g/ml) with the majority overlapping HDL3 (d 1.125-1.21 g/ml) and very high density lipoprotein (d 1.21-1.25 g/ml). The bimodal density distribution of apoJ-HDL suggests that these subclasses have a unique metabolic relationship and may play a role in the transport of cholesterol from peripheral tissues to the liver.     

On the structural similarity of ApoA-I and ApoA-II of human high density lipoproteins.

The possible evolutionary origin of apolipoproteins was studied by comparing the primary structures of different plasma apolipoproteins and other phospholipid-binding proteins. Apolipoprotein A-I (ApoA-I) and apolipoprotein A-II (ApoA-II) of human high density lipoprotein (HDL) are related. The resemblance of these two HDL apolipoproteins are apparently restricted to the carboxyl terminal regions suggesting that these portions of the molecules are derived from the same ancestor. The homologous carboxyl terminal segments may be involved in the regulation of HDL metabolism or in the interaction with phospholipids.     

Lipid profile of body builders with and without self-administration of anabolic steroids

Twenty-four top-level body builders [13 anabolic steroid users (A); 11 non-users (N)] and 11 performance-matched controls (C) were examined to determine the effect on lipids, lipoproteins and apolipoproteins of many years of body building with and without simultaneous intake of anabolic steroids and testosterone. After an overnight fast, triglycerides (TG), total cholesterol (TOTC), high density lipoprotein cholesterol (HDLC), low density lipoprotein cholesterol (LDLC), the HDLC subfractions HDL2C and HDL3C, as well as apolipoprotein A-I (Apo A-I), apolipoprotein A-II (Apo A-II) and apolipoprotein B (Apo B) were determined. Both A and N, compared to C, showed significantly lower HDLC and higher LDLC concentrations, with the differences between A and C clearly pronounced. In a subgroup of 6 body builders taking anabolic steroids at the time of the study, HDLC, HDL2C, HDL3C, Apo A-I and Apo A-II were all significantly lower and LDLC was significantly higher than in a second subgroup of 7 body builders who had discontinued their intake of anabolic steroids at least 4 weeks prior to the study. In some single cases HDLC was barely detectable (2-7 mg.dl-1). The TG and TOTC remained unchanged. The present findings suggest that many years of body building among top-level athletes have no beneficial effect on lipoproteins and apolipoproteins. Simultaneous use of anabolic steroids results in part in extreme alterations in lipoproteins and apolipoproteins, representing an atherogenic profile. After discontinuing the use of anabolic steroids, the changes in lipid metabolism appear to be reversible.     

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.     

Quantitative determination of human apolipoprotein D by electroimmunoassay and radial immunodiffusion.

1. An electroimmunoassay and a radial immunodiffusion procedure are described for the quantitative determination of human serum apolipoprotein D. Purified apolipoprotein D and antisera to both lipoprotein D and apolipoprotein D were used to standardize the assays. The assays are applicable to measurement of apolipoprotein D in serum and density classes. The electroimmunoassay is more sensitive (50 ng apolipoprotein D quantitatively detectable), rapid (time required for completion of assay is 5 h) and precise (the within- and between-assay coefficients of variation are 4 and 7%, respectively) than radial immunodiffusion. However, comparable results were obtained by both methods (r = 0.85). 2. Serum apolipoprotein D levels of normal subjects and hyperlipoproteinemic phenotypes IIa, IIb, III, IV and V were in the same range (10 to 12 mg/dl). In contrast, patients with hyperchylomicronemia (type I) had decreased apolipoprotein D levels (5 mg/dl; P less than 0.001). The apolipoprotein D in serum of normolipidemic subjects was detectable in all density classes but measurable only in HDL2 (21%), HDL3 (43%) and VHDL (36%). 3. Rocket electrophoresis is also a valuable tool for assessing the structural relationships among apolipoproteins or their constituent polypeptides. Interaction between serum and a mixture of antibodies to A-I, A-II and apolipoprotein D resulted in the formation of separate lipoprotein A and lipoprotein D rockets indicating that apolipoprotein D is not a constituent polypeptide of apolipoprotein A. This observation confirms the existence of lipoproteins A and D as separate lipoprotein families.     

Thermal transitions in human very-low-density lipoprotein: fusion, rupture, and dissociation of HDL-like particles

Very-low-density lipoproteins (VLDL) are metabolic precursors of low-density lipoproteins (LDL) and a risk factor for atherosclerosis. Human VLDL are heterogeneous complexes containing a triacylglycerol-rich apolar lipid core and polar surface composed of phospholipids, a nonexchangeable apolipoprotein B, and exchangeable apolipoproteins E and Cs. We report the first stability study of VLDL. Circular dichroism and turbidity data reveal an irreversible heat-induced VLDL transition that involves formation of larger particles and repacking of apolar lipids but no global protein unfolding. Heating rate effect on the melting temperature indicates a kinetically controlled reaction with high activation energy, Ea. Arrhenius analysis of the turbidity data reveals two kinetic phases with Ea = 53 +/- 7 kcal/mol that correspond to distinct morphological transitions observed by electron microscopy. One transition involves VLDL fusion, partial rupture, and dissociation of small spherical particles (d = 7-15 nm), and another involves complete lipoprotein disintegration and lipid coalescence into droplets accompanied by dissociation of apolipoprotein B. The small particles, which are unique to VLDL denaturation, are comparable in size and density to high-density lipoproteins (HDL); they have an apolar lipid core and polar surface composed of exchangeable apolipoproteins (E and possibly Cs) and phospholipids. We conclude that, similar to HDL and LDL, VLDL are stabilized by kinetic barriers that prevent particle fusion and rupture and decelerate spontaneous interconversion among lipoprotein classes and subclasses. In addition to fusion, VLDL disruption involves transient formation of HDL-like particles that may mimic protein exchange among VLDL and HDL pools in plasma.     

Mass spectrometric measurements of the apolipoproteins of bovine (Bos taurus) HDL

As is the case in most mammals, high density lipoproteins (HDL) also comprise the major group of lipid carriers that circulate in bovine (Bos taurus) blood. As a continuation of our proteogenomic studies of mammalian apolipoproteins, we have obtained molecular masses for several of the apolipoproteins associated with bovine HDL. The major apolipoprotein on the HDL surface is apoA-I, but other apolipoproteins were also detected. Using electrospray-ionization mass spectrometry (ESI-MS), we report on values for apolipoproteins, A-I, proA-I and A-II, as well as post-translationally modified apoA-I. Analyses of tryptic fragments did reveal the presence of apoA-IV and apoC-III. However, in contrast to our previous studies of other mammalian HDL, we did not detect apoC-I. Interestingly, examination of the current assembly for the bovine genome does not show any evidence for an apoC-I gene.