Endothelial lipase releases saturated and unsaturated fatty acids of high density lipoprotein phosphatidylcholine

We assessed the ability of endothelial lipase (EL) to hydrolyze the sn-1 and sn-2 fatty acids (FAs) from HDL phosphatidylcholine. For this purpose, reconstituted discoidal HDLs (rHDLs) that contained free cholesterol, apolipoprotein A-I, and either 1-palmitoyl-2-oleoylphosphatidylcholine, 1-palmitoyl-2-linoleoylphosphatidylcholine, or 1-palmitoyl-2-arachidonylphosphatidylcholine were incubated with EL- and control (LacZ)-conditioned media. Gas chromatography analysis of the reaction mixtures revealed that both the sn-1 (16:0) and sn-2 (18:1, 18:2, and 20:4) FAs were liberated by EL. The higher rate of sn-1 FA cleavage compared with sn-2 FA release generated corresponding sn-2 acyl lyso-species as determined by MS analysis. EL failed to release sn-2 FA from rHDLs containing 1-O-1'-hexadecenyl-2-arachidonoylphosphatidylcholine, whose sn-1 position contained a nonhydrolyzable alkyl ether linkage. The lack of phospholipase A(2) activity of EL and its ability to liberate [(14)C]FA from [(14)C]lysophosphatidylcholine (lyso-PC) led us to conclude that EL-mediated deacylation of phosphatidylcholine (PC) is initiated at the sn-1 position, followed by the release of the remaining FA from the lyso-PC intermediate. Thin-layer chromatography analysis of cellular lipids obtained from EL-overexpressing cells revealed a pronounced accumulation of [(14)C]phospholipid and [(14)C]triglyceride upon incubation with 1-palmitoyl-2-[1-(14)C]linoleoyl-PC-labeled HDL(3), indicating the ability of EL to supply cells with unsaturated FAs.

Intracellular distribution and mobilization of unesterified cholesterol in adipocytes: triglyceride droplets are surrounded by cholesterol-rich ER-like surface layer structures

In addition to their central role in triglyceride storage, fat cells are a primary depot of unesterified cholesterol (FC) in the body. In comparison, peripheral cells contain very little FC. This difference in adipocytes versus peripheral tissues is inconsistent with the current theory of cholesterol homeostasis. Attempting to resolve this discrepancy, we examined intracellular storage sites of FC in murine 3T3-F442A adipocytes. Using the cholesterol-binding antibiotic, filipin, in combination with high resolution fluorescence microscopy, intense fluorescent staining characteristically decorated the periphery of triglyceride droplets (TGD) as well as the plasma membrane (PM) of fat cells. Filipin-staining was not visible inside the lipid droplets. Purification of TGD by subcellular fractionation demonstrated that the rise in total FC content of adipocytes upon differentiation was attributable to an increase in TGD-FC, which contributed up to one third of the total cellular FC. The protein component of purified TGD from cultured adipocytes as well as from murine adipocytes obtained from fresh tissues contained the lumenal endoplasmic reticulum (ER) immunoglobulin binding protein (BiP) and the integral ER membrane protein calnexin. Efflux experiments using the extracellular FC acceptors (β)-cyclodextrin or apolipoprotein A-I demonstrated that TGD-associated FC was releasable from TGD. Whereas FC efflux from adipocytes was unaffected in the presence of brefeldin A or monensin, the secretion of a control protein, lipoprotein lipase, was effectively reduced. In summary, our findings identify the TGD surface layer as primary intracellular storage site for FC within adipocytes. We suggest that the structural role of ER-resident proteins in this adipocyte TGD envelope has been previously neglected. Our findings support the suggestion that an ER-like structure, albeit of modified lipid composition, constitutes the lipid droplets' surface layer. Finally, the efflux process of FC from adipocytes upon extracellular stimulation with (beta)-cyclodextrin provides evidence for an energy-dependent intracellular trafficking route between the TGD-FC pool and the PM-FC sites which is distinct from the secretory pathway of proteins.

Origin and function of epoxyeicosatrienoic acids in vascular endothelial cells: more than just endothelium-derived hyperpolarizing factor?

1. In addition to their contribution to endothelium-derived hyperpolarization, our understanding of the physiological function of epoxyeicosatrienoic acids (EET) within the vascular wall and the actual enzymes involved in the formation of the EET in endothelial cells is very limited. In the present study, the expression of potential cytochrome P450 (CYP) mono/epoxygenases was assessed in endothelial cells isolated from porcine and bovine aortas as well as in the human umbilical vein-derived cell lines EA.hy926 and ECV304. 2. Expression of CYP2B1, CYP2E1 and CYP3A could be found. The latter were inducible by dexamethasone/clofibrate for 72 h, a procedure that also enhanced CYP epoxygenase activity in endothelial cells. 3. Enzyme induction yielded increases in capacitative Ca2+ entry and membrane hyperpolarization in response to autacoids, such as bradykinin and thapsigargin. Thiopentone sodium, an inhibitor of endothelial CYP mono/epoxygenase(s), diminished autacoid-induced capacitative Ca2+ entry and membrane hyperpolarization, while the effect of EET remained unchanged. 4. Epoxyeicosatrienoic acids activated endothelial tyrosine kinase activity in a concentration-dependent manner. Arachidonic acid, at 20-fold higher concentrations, also increased tyrosine kinase activity. Because only the effect of arachidonic acid was inhibited by thiopentone sodium, an inhibitor of CYP mono/epoxygenases, these data suggest that arachidonic acid needs to be converted to the EET in order to stimulate tyrosine kinase. 5. All these data provide clear evidence that the CYP epoxygenase-derived arachidonic acid metabolites (EET) not only serve as potential endothelium-derived hyperpolarizing factors but also constitute highly active intracellular messengers with a physiological role including the control of Ca2+ signalling, membrane potential and tyrosine kinase activity.

LDL-mediated interaction of Lp[a] with HepG2 cells: a novel fluorescence microscopy approach

We studied the topography of Lp[a]-LDL-cell interactions by means of fluorescence microscopy, using fluorescence-labeled lipoproteins. In contrast to known methods which are based on noncovalent labeling of lipoproteins by positively charged amphiphiles, the protein moiety of LDL and Lp[a] was covalently labeled with either BODIP-succinimide-ester (green) or rhodamine X iodoacetamide (red). The interaction of the fluorescent lipoproteins with cultured HepG2 cells was studied using a confocal laser scanning fluorescence microscope. LDL and Lp[a], each labeled with a different dye, could be examined separately within a mixture of both lipoproteins during their interaction with HepG2 cells. At 4 degrees C, the majority of both fluorescent particles co-localized and only a few separate LDL- or Lp[a]-binding domains could be observed. Quantification of the amount of fluorescent lipoprotein associated with the cell surface at 4 degrees C showed that binding of Lp[a] was increased in the presence of LDL under these conditions, probably via formation of an Lp[a]-LDL complex. At 37 degrees C, LDL and Lp[a] were taken up by the cells within 10 min. Again the majority of LDL and Lp[a] particles co-localized intracellularly. Only minor amounts of LDL and Lp[a] could be observed separately. As the entire fluorescence of labeled Lp[a] co-localized with excess of LDL in cells, and taking into account the high tendency of LDL-Lp[a] association in solution and on cell surfaces, it is concluded that a significant portion of the internalized Lp[a] is taken up into the cells by the LDL receptor via LDL by a hitchhiking-like process.

Metabolism of Lp(a): assembly and excretion

Lp(a) is one of the most atherogenic lipoproteins, and we know much more about the pathophysiology of Lp(a) than about its physiological function and metabolism. From our previous investigations and the new results reported here, we propose the following model of Lp(a) metabolism: apo(a) is biosynthesized in liver cells and the size of the isoform determines its rate of synthesis and excretion. Specific kringle-4 domains in apo(a), mainly T-6 and T-7, bind in a first step to circulating LDL, followed by the stabilization of the newly formed Lp(a) complex by a disulfide bridge. Circulating Lp(a) interacts specifically with kidney cells, or possibly other tissues, causing cleavage of 2/3-3/4 of the N-terminal part of apo(a) by a collagenase-type protease. Part of the apo(a) fragments is found in the urine, but there are indications that they in fact represent the biologically active form of apo(a). The core portion of Lp(a) in turn is cleared by the LDL-receptor or another specific binding system of the liver. Strategies for reducing plasma Lp(a) levels with medication should aim at interfering with the assembly of Lp(a) on one hand and the stimulation of apo(a) fragmentation on the other hand.

Urinary excretion of apo(a) fragments. Role in apo(a) catabolism

The biosynthesis and assembly of lipoprotein(a) [Lp(a)], a marker for atherosclerotic disease, appears to be well understood. However, information is lacking concerning the mode and site of Lp(a) catabolism. Apo(a) is reported to be excreted into the urine. To study the effect of this pathway on the overall catabolism of Lp(a), urinary apo(a) was characterized by immunoblotting. More than 10 distinct apo(a) bands with molecular masses between 30 and 160 kD were observed. Apo(a) fragments were not complexed to apoB. In more than 30 individuals the size of apo(a) bands was comparable irrespective of their apo(a) phenotype, although marked differences in the relative intensities of the bands were observed. Eight batches of 24-hour urine collections collected from one proband at 2-week intervals exhibited a significant correlation between creatinine and apo(a) concentrations as measured by DELFIA (r = .93; P < .01). In 193 healthy volunteers a highly significant correlation was found between urinary apo(a) concentrations normalized to creatinine levels and plasma Lp(a) values (p = 0.659; P < .0001). Of the total plasma apo(a), 0.073%, i.e., 121 micrograms apo(a), was excreted in the form of apo(a) fragments in 24-hour urine samples from 12 healthy volunteers. We conclude that the catabolism of Lp(a) via excretion of apo(a) fragments accounts for < 1% of the daily Lp(a) catabolism.

Molecular characterization of quail apolipoprotein very-low-density lipoprotein II: disulphide-bond-mediated dimerization is not essential for inhibition of lipoprotein lipase

As part of the avian reproductive effort, large quantities of triglyceride-rich very-low-density lipoprotein (VLDL) particles are transported by receptor-mediated endocytosis into the female germ cells. Although the oocytes are surrounded by a layer of granulosa cells harbouring high levels of active lipoprotein lipase, non-lipolysed VLDL is transported into the yolk. This is because VLDL particles from laying chickens are protected from lipolysis by apolipoprotein (apo)-VLDL-II, a potent dimeric lipoprotein lipase inhibitor [Schneider, Carroll, Severson and Nimpf (1990) J. Lipid Res. 31, 507-513]. To determine whether this protection depends on dimer formation and constitutes a general mechanism to ensure high levels of yolk triglycerides for embryonic utilization in birds, we have now molecularly characterized apo-VLDL-II in the Japanese quail, a frequently used avian species. Quail apo-VLDL-II shows 72% amino acid identity with the chicken protein, with most replacements being in the C-terminal region. Importantly, quail apo-VLDL-II lacks the single cysteine residue present eight residues from the C-terminus of chicken apo-VLDL-II, which is responsible for dimerization of the chicken lipoprotein lipase inhibitor. Nevertheless, monomeric quail and dimeric chicken apo-VLDL-II display, on a molar basis, identical inhibitory effects on lipoprotein lipase, underscoring the biological importance of their function. Furthermore secondary structure prediction of the 3'-untranslated region of the quail message supports a role for loop structures in the strictly oestrogen-dependent production of the lipoprotein lipase inhibitors. Our findings shed new light on the essential role of this small, hormonally regulated, protein in avian reproduction.

A single G to A nucleotide transition in exon IV of the lecithin: cholesterol acyltransferase (LCAT) gene results in an Arg140 to His substitution and causes LCAT-deficiency

We have characterized the molecular defect causing lecithin:cholesterol acyltransferase (LCAT)-deficiency (LCAT-D) in the LCAT gene in three siblings of Austrian descent. The patients presented with typical symptoms including corneal opacity, hemolytic anemia, and kidney dysfunction. LCAT activities in the plasma of these three patients were undetectable. DNA sequence analysis of polymerase chain reaction (PCR)-amplified DNA of all six LCAT exons revealed a new point mutation in exon IV of the LCAT gene, i.e., a G to A substitution in codon 140 converting Arg to His. This mutation caused the loss of a cutting site for the restriction endonuclease HhaI within exon IV: Upon digestion of a 629-bp exon IV PCR product with HhaI, the patients were found to be homozygous for the mutation. Eight of 11 family members were identified as heterozygotes. Transfection studies of COS-7 cells with plasmids containing a wild-type or a mutant LCAT cDNA revealed that, in contrast to the cell medium containing wild-type enzyme, no enzyme activity was detectable upon expression of the mutant protein. This represents strong evidence for the causative nature of the observed mutation for LCAT deficiency in affected individuals and supports the conclusion that Arg140 is crucial for the structure of an enzymatically active LCAT protein.

A double labeling procedure for lipoproteins: independent visualization of dual ligand-receptor interaction with colloidal gold- and 125I-labeled ligands

The analysis of multiple ligand binding to a single receptor molecule poses a methodological challenge. The chicken oocyte 95-kDa receptor for the uptake of the two major yolk lipoprotein precursors very low-density lipoprotein (VLDL) and vitellogenin (VTG) is such a multipotent transport receptor. Here we describe methods for rapid independent and simultaneous analysis of VLDL and VTG binding to this receptor, termed VLDL/VTG receptor. First, further development of a one-step labeling protocol for chicken lipoproteins with colloidal gold (Au) to visualize independently the binding of VLDL and VTG to the chicken VLDL/VTG receptor is reported. The advantage of this protocol is that the preparation of the Au-lipoprotein conjugates is rapid, and utilization of Au-lipoprotein complexes in ligand blots does not require their further purification, while signal enhancement by silver staining is still applicable. Second, the simultaneous use of 125I- and Au-labeled ligands in a one-step ligand blotting procedure facilitates the direct demonstration of the competitive interaction of VLDL and VTG with the receptor. Following sequential processing of the nitrocellulose strips, binding of differently labeled ligands can be studied independently. In summary, we describe a procedure for differential labeling of lipoproteins and its application toward the analysis of receptors that bind more than one ligand.

Chicken yolk contains bona fide high density lipoprotein particles

Lipoproteins, the major nutrient source for developing embryos in egg-laying species, are thought to be transported from the circulation of the hen to the yolk of growing oocytes. In order to fully understand the contribution of the different lipoprotein species to oocyte growth, yolk formation, and embryo development, we have started to elucidate the relationships between the high density lipoproteins (HDL) in serum with the hitherto uncharacterized yolk HDL fraction. Immunoblotting with antibodies against apolipoprotein (apo) A-I, the major protein moiety of circulating HDL, revealed, for the first time, significant amounts of this protein in yolk. Importantly, yolk apoA-I was an integral component of bona fide lipoprotein particles: i) the apoA-I-containing particles could be purified by ultracentrifugal flotation and immunoaffinity chromatography on immobilized anti-apoA-I IgG; ii) the particles resembled serum HDL in ultrastructural, chemical, and biochemical aspects; and iii) in particular, these particles contained another major apolipoprotein, apo II. To date, apo II has been assumed to be unique to the very low density lipoprotein (VLDL) and HDL fractions of laying hen serum. Its residence on yolk HDL particles, together with the other results, strongly implies that yolk HDL, at least to a large part, is derived from serum. This implication is supported by the presence of apoA-I in oocytic coated vesicles. However, an oocyte plasma membrane receptor for the transport of HDL could not be identified; furthermore, immunoelectron microscopy demonstrated that yolk HDL particles do not colocalize with VLDL, known to be endocytosed via a specific receptor. Thus, these studies have revealed that HDL particles are taken up into the oocyte from the serum of the laying hen, and are deposited into the yolk by a mechanism distinct from that involved in the uptake of other yolk lipoproteins.

The role of lecithin: cholesterol acyltransferase for lipoprotein (a) assembly. Structural integrity of low density lipoproteins is a prerequisite for Lp(a) formation in human plasma

The composition of lipoproteins in the plasma of patients with LCAT deficiency (LCAT-D) is grossly altered due to the lack of cholesteryl esters which form the core of normal lipoproteins. When plasma from LCAT-D patients and their relatives was examined we found that nine heterozygotes had plasma Lp(a) levels of 2-13 mg/dl whereas none of 11 affected homozygous individuals from different families contained detectable amounts of Lp(a) in their plasma. Therefore, the binding of apo(a) to LDL density particles was studied in vitro using LDL density fractions prepared from patients, and recombinant apo(a) [r-apo(a)], which was expressed and secreted by transfected COS-7 cells. The LDL from heterozygotes were chemically indistinguishable from normal LDL and homogeneous with regard to morphology, whereas the crude LDL floating fraction from homozygotes consisted of a heterogeneous mixture of large vesicles, and small spheres resembling normal LDL. The LDL density fraction from the LCAT-D patient lacked almost completely cholesteryl esters. Incubation of LCAT-D plasma with active LCAT caused a substantial augmentation of the original subfraction which morphologically resembled normal LDL. Using r-apo(a) and normal LDL or LDL of heterozygous individuals, apoB:r-apo(a) complexes were formed when incubated at 37 degrees C in vitro for 20 h. In contrast, the total LDL floating fraction from a homozygous LCAT-D patient failed to form apoB:r-apo(a) complexes. After treatment with active LCAT, a significant apoB:r-apo(a) association was observed with LCAT-D LDL-density particles. Our data emphasize the importance of the integrity of LDL structure and composition for the formation of Lp(a). In addition, we demonstrate that the absence of LCAT activity has a fundamental impact on the regulation of plasma Lp(a) levels.

Chicken oocyte growth: receptor-mediated yolk deposition

During the rapid final stage of growth, chicken oocytes take up massive amounts of plasma components and convert them to yolk. The oocyte expresses a receptor that binds both major yolk lipoprotein precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL). In the present study, in vivo transport tracing methodology, isolation of coated vesicles, ligand- and immuno-blotting, and ultrastructural immunocytochemistry were used for the analysis of receptor-mediated yolk formation. The VTG/VLDL receptor was identified in coated profiles in the oocyte periphery, in isolated coated vesicles, and within vesicular compartments both outside and inside membrane-bounded yolk storage organelles (yolk spheres). VLDL particles colocalized with the receptor, as demonstrated by ultrastructural visualization of VLDL-gold following intravenous administration, as well as by immunocytochemical analysis with antibodies to VLDL. Lipoprotein particles were shown to reach the oocyte surface by passage across the basement membrane, which possibly plays an active and selective role in yolk precursor accessibility to the oocyte surface, and through gaps between the follicular granulosa cells. Following delivery of ligands from the plasma membrane into yolk spheres, proteolytic processing of VTG and VLDL by cathepsin D appears to correlate with segregation of receptors and ligands which enter disparate sub-compartments within the yolk spheres. In small, quiescent oocytes, the VTG/VLDL receptor was localized to the central portion of the cell. At onset of the rapid growth phase, it appears that this pre-existing pool of receptors redistributes to the peripheral region, thereby initiating yolk formation. Such a redistribution mechanism would obliterate the need for de novo synthesis of receptors when the oocyte's energy expenditure is to be utilized for plasma membrane synthesis, establishment and maintenance of intracellular topography and yolk formation, and preparation for ovulation.

Molecular cloning and functional characterization of chicken cathepsin D, a key enzyme for yolk formation

Upon receptor-mediated endocytosis of very-low-density lipoprotein (VLDL) and vitellogenin into growing chicken oocytes, the protein moieties of these lipoproteins are proteolytically cleaved. Unlike the complete lysosomal degradation in somatic cells, enzymatic ligand breakdown in oocytes generates a characteristic set of polypeptides, which enter yolk storage compartments for subsequent utilization by the embryo. Here, we demonstrate directly that the catalyst for the intraoocytic processing of both apolipoprotein B and vitellogenin is cathepsin D. The enzyme was purified from oocytic yolk, preovulatory follicle homogenates, and liver by affinity chromatography. When plasma VLDL and vitellogenin were incubated with the purified enzyme, fragments indistinguishable from those found in yolk were generated from both precursors under identical, mildly acidic conditions. Amino-terminal sequencing of the pure enzyme demonstrated 88% identity with mammalian cathepsin Ds over 34 residues. On the basis of this information, a full-length clone specifying chicken preprocathepsin D was isolated from a chicken follicle cDNA library by screening with a human cathepsin D probe. Whereas previous studies have demonstrated that the receptors for lipoproteins in somatic cells and oocytes, respectively, of the chicken are the products of different genes, Southern and Northern blot hybridization experiments showed that the enzymes expressed in oocytes and liver are the product of a single gene, giving rise to a 3.3-kb transcript. The primary structure of the 335-residue mature protein suggests a high degree of conservation of known crucial features of aspartyl proteases; however, the absence of the so-called processing region and of an aromatic residue in a region thought to partake in catalysis raise questions with possible evolutionary implications.

The laying hen expresses two different low density lipoprotein receptor-related proteins

We have identified, by a combination of ligand, 45Ca2+, and immunoblotting, two large membrane proteins akin to the mammalian so-called low density lipoprotein (LDL) receptor-related protein (LRP) in chicken tissues. LRP has thus far been demonstrated only in mammalian species where it is thought to act as a receptor for proteinase-alpha 2-macroglobulin complexes and/or chylomicron remnants, lipoproteins not produced in birds. One of the chicken LRPs was demonstrated in liver, and has the same apparent Mr and hallmark biochemical properties as rat liver LRP. The other chicken LRP is smaller (approximately 380 kDa) and is expressed in ovarian follicles, but is undetectable in liver. Immunological analysis demonstrated a lack of cross-reactivity between the two LRPs, as well as between them and the previously identified chicken oocyte-specific 95-kDa receptor for the yolk precursors, very low density lipoprotein, and vitellogenin (Stifani, S., Barber, D. L., Nimpf, J., and Schneider, W. J. (1989) Proc. Natl. Acad. Sci. U.S.A. 87, 1955-1959). As shown by ligand blotting, both chicken LRPs have the ability to interact with vitellogenin, a property they share not only with rat LRP, but also with mammalian LDL receptors. To obtain independent confirmation of the ligand blotting results, the smaller (follicular) LRP was purified and high-affinity binding of vitellogenin to it was demonstrated by a solid-phase filtration binding assay. Amino acid sequences of tryptic fragments of the smaller LRP were obtained, and its homology with human LRP demonstrated through unambiguous alignment of three fragments. Both chicken LRPs, the chicken oocyte 95-kDa receptor, as well as rat LRP, could be shown by ligand blotting to interact specifically with chicken serum alpha 2-macroglobulin. In addition, human apolipoprotein E, a ligand implicated in receptor-mediated metabolism of chylomicron remnants, also binds to the smaller chicken LRP, further emphasizing the similarities between LDL receptors and related proteins from a variety of species. In analogy to the known dichotomy of chicken LDL receptors, which is characterized by the production of the 95-kDa oocyte-specific receptor on one hand and a 130-kDa LDL receptor that is exclusively expressed in somatic cells (Hayashi, K., Nimpf, J., and Schneider, W. J. (1989) J. Biol. Chem. 264, 3131-3139), it appears that the smaller and larger chicken LRPs also may be restricted to the oocyte and somatic cells, respectively.

Evolution of lipoprotein receptors. The chicken oocyte receptor for very low density lipoprotein and vitellogenin binds the mammalian ligand apolipoprotein E

The laying hen expresses two different lipoprotein transport receptors in cell-specific fashion. On the one hand, a 95-kDa oocyte membrane protein mediates the uptake of the major yolk precursors, very low density lipoprotein, and vitellogenin; on the other hand, somatic cells synthesize a 130-kDa receptor that is involved in the regulation of cellular cholesterol homeostasis (Hayashi, K., Nimpf, J., and Schneider, W. J. (1989) J. Biol. Chem. 264, 3131-3139). Here we show that the oocyte-specific receptor binds, in addition to the yolk precursor proteins, an apolipoprotein of mammalian origin, apolipoprotein E. Ligand blotting, a solid-phase binding assay, and antireceptor antibodies were employed to demonstrate that binding of vitellogenin, very low density lipoprotein (via apolipoprotein B), and apolipoprotein E occurs to closely related, if not identical, sites on the 95-kDa oocyte receptor. The binding properties of lipovitellin, which harbors the receptor recognition site of vitellogenin, are analogous to those of apolipoprotein E: both require association with lipid for expression of functional receptor binding. The ligand specificity of the avian oocyte lipoprotein receptor supports the hypothesis that vitellogenin, which has evolved in oviparous species, represents a counterpart to mammalian apolipoprotein E.

Interaction of lipoprotein Lp[a] with the B/E-receptor: a study using isolated bovine adrenal cortex and human fibroblast receptors

The ability of different lipoprotein Lp[a] preparations to compete with LDL-binding to the B/E-receptor was investigated by ligand blot and filter assays. Lp[a] was purified from donors with various genetic polymorphic forms by affinity chromatography using lysine-Sepharose or specific immunoadsorbers. These preparations were free of "LDL-like" material. Part of Lp[a] was reduced and freed from specific apo[a] antigen yielding "Lpa-." 125I-labeled low density lipoproteins (LDL) were incubated with B/E-receptor preparations from bovine adrenal cortex or from human skin fibroblasts, and the competition with unlabeled LDL, Lp[a], Lpa-, apo[a], and apoE-free HDL was studied by a ligand blot or filter assay technique. The following results were obtained. 1) LDL and Lpa- were equally potent in displacing 125I-labeled from B/E-receptor in the ligand blot and the filter assay. Lpa + ( = Lp[a]) also displaced LDL but to a much lesser degree: 50% displacement was observed with LDL and Lpa- at a 1-fold excess, whereas a 7.5-fold excess was required of Lpa +. 2) Apo[a], as well as apoE-free HDL, did not compete with LDL binding. 3) Competition experiments using B/E-receptors from bovine adrenal cortex or from human skin fibroblasts were comparable. 4) There was no difference in the behavior of Lp[a] isolated from the two affinity chromatography methods. 5) Lp[a] of different genetic variants behaved virtually identically. The results are discussed from the point of view of the in vivo metabolism of Lp[a].

Activation of lecithin-cholesterol acyltransferase by apolipoprotein D: comparison of proteoliposomes containing apolipoprotein D, A-I or C-I

To study the activation of lecithin-cholesterol acyl transferase (LCAT) (phosphatidylcholine:sterol O-acyltransferase, EC 2.3.1.43) by apolipoprotein D in comparison to apolipoproteins A-I and C-I, proteoliposomes with a phosphatidylcholine/free cholesterol molar ratio of 24:1, containing 10-300 micrograms/ml of apolipoproteins were used. The proteoliposomes were prepared by the cholate dialysis technique. In all proteoliposome preparations we found rouleaux structures and stacked discs. The particles formed with apolipoprotein A-I were the most homogeneous, followed by apolipoprotein D- and apolipoprotein C-I-containing particles. Apolipoprotein A-I was the most potent LCAT activator in our system followed by apolipoproteins C-I and D. The fractional esterification rate observed with apolipoprotein D-containing substrates amounted to 15-48% that of apolipoprotein A-I-containing ones. Neither apolipoprotein A-I- nor C-I-containing proteoliposomes gave linear reaction kinetics with LCAT. Even during the first 15-30 min of incubation, the kinetics deviated strikingly from linearity at all apolipoprotein concentrations. In contrast, proteoliposomes containing apolipoprotein D exhibited linear reaction kinetics up to 60-90 min. At low apolipoprotein A-I concentrations (5 micrograms/ml), the addition of apolipoprotein D to the incubates resulted in significantly higher esterification rates as compared to substrates containing apolipoprotein A-I only. This was not the case using substrates with high apolipoprotein A-I concentrations (50 micrograms/ml). From our results we speculate that apolipoprotein D may have some stabilizing effect on the enzyme LCAT.

Action of lecithin-cholesterol acyltransferase on low-density lipoproteins in native pig plasma

The action of lecithin-cholesterol acyltransferase (LCAT, EC 2.3.1.43) on the different pig lipoprotein classes was investigated with emphasis on low-density lipoproteins (LDL). It was demonstrated previously that LDL can serve as substrate for LCAT, probably because they contain sufficient amounts of apoA-I and other non-apoB proteins, known as LCAT activators. Upon a 24-h incubation of pig plasma in vitro in the presence of active LCAT, both pig LDL subclasses, LDL-1 and LDL-2, fused together, forming one fraction, as revealed by analytical ultracentrifugation. This fusion was time dependent, becoming visible after 3 h and complete after 18 h of incubation. Concomitantly, free cholesterol and phospholipids decreased and cholesteryl esters increased. When isolated LDL-1 and LDL-2 were incubated with purified pig LCAT for 24 h, LDL-1 floated toward higher densities and LDL-2 toward lower densities, although this effect was not as pronounced as in incubations of whole serum. In further experiments, pig serum was incubated for various periods of time in the presence and absence of the LCAT inhibitor sodium iodoacetate. The individual lipoproteins then were separated by density gradient ultracentrifugation or by specific immunoprecipitation and chemically analyzed. Both methods revealed that in the absence of active LCAT there was a transfer of free cholesterol from LDL to high-density lipoproteins (HDL) and a small transfer of cholesteryl esters in the opposite direction. In the presence of LCAT the loss of free cholesterol started immediately in all three lipoprotein classes, was most prominent in LDL, and was proportional to the newly synthesized cholesteryl esters incorporated in each fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro formation of HDL-2 from HDL-3 and triacylglycerol-rich lipoproteins by the action of lecithin:cholesterol acyltransferase and cholesterol ester transfer protein

In order to study the factors responsible for the formation of high-density lipoprotein subfraction-2 (HDL-2), very-low-density lipoproteins (VLDL) and HDL-3 were mixed and incubated with purified bovine milk lipoprotein lipase, human serum lecithin:cholesterol acyltransferase, cholesteryl ester transfer protein and mixtures thereof. The results can be summarized as follows: Incubation of HDL-3 and VLDL for 24 h at 37 degrees C without enzymes did not cause any significant change in the protein:lipid ratio or in the flotation constant of the HDL. Cholesteryl ester transfer protein treatment caused only an exchange of part of the HDL cholesteryl esters with VLDL triacylglycerols. Lipoprotein lipase caused a slight shift of HDL-hydrated density to lower values; HDL-2b, however, was not formed. Incubation of HDL-3 and VLDL with lecithin:cholesterol acyltransferase or mixtures of lecithin:cholesterol acyltransferase and lipoprotein lipase reduced the HDL-protein:lipid ratio and increased the HDL-flotation rate. The newly formed HDL resembled that of native HDL-2a. The incubation of HDL-3 and VLDL with lecithin:cholesterol acyltransferase and cholesteryl ester transfer protein caused a shift of the HDL-3 into an HDL-2b-like fraction. Particles resembling HDL-2b in the analytical ultracentrifuge were also formed if VLDL + HDL-3 were incubated with lipoprotein lipase or lipoprotein lipase + cholesteryl ester transfer protein in a medium containing low amounts of albumin, insufficient for binding all liberated fatty acids during hydrolysis. The incubation of mixtures of HDL-3 and chylomicrons enriched with apoAI in the presence of lecithin:cholesterol acyltransferase and cholesteryl ester transfer protein caused the formation of HDL-2-like particles which resembled those of native HDL-2 also with respect to the apoAI/AII ratio.

Studies on the substrate specificity of human and pig lecithin: cholesterol acyltransferase: role of low-density lipoproteins

The substrate properties of low-density lipoprotein (LDL) fractions from human and pig plasma and of lipoprotein a [Lp(a)] upon incubation with either pig or human lecithin:cholesterol acyltransferase (LCAT, EC 2.3.1.43) were investigated and compared with those of pig high-density lipoproteins (HDL) or human HDL-3. The cholesterol esterification using purified native pig LDL-1, human LDL, or Lp(a) as a substrate was approximately 36-42% that of pig HDL or human HDL-3, while cholesteryl ester formation with pig LDL-2 was 41-47%. No significant difference was found in the substrate activity between pig HDL and human HDL-3, and between human LDL and Lp(a), respectively. After depletion of pig LDL-1, pig LDL-2, and human LDL from apolipoprotein A-I (apoA-I), cholesteryl ester formation decreased to about 22-28% of the value found with pig HDL. Depletion of human LDL from apolipoprotein E (apoE) did not result in significantly different esterification rates in comparison to native LDL. Total removal of non-apoB proteins from human LDL resulted in esterification rates of approximately 10-15% that of HDL. Readdition of apoA-I to all these LDL fractions produced solely in apoA-I-depleted LDL fractions an increase of cholesteryl ester formation, whereas in those LDL fractions that were additionally depleted from apoE and/or from apoC polypeptides, a further decrease in the esterification rate occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromatofocusing of apolipoproteins from human serum high density lipoprotein

Human HDL was delipidated and the apolipoproteins were fractionated by chromatofocusing. Chromatofocusing, which separates proteins due to their differing isoelectric points, resulted in 8 peaks with corresponding pI values of 7.40, 6.92, 6.64, 5.48, 5.30, 5.18, 4.92 and 4.63. By one single chromatofocusing run four apolipoproteins were obtained in pure form. Two additional polypeptides could be purified during the desalting step using phenyl-Sepharose.

Isolation and characterization of polymorphic forms of porcine apoC-II by chromatofocusing

Chromatofocusing, which separates proteins on the basis of their different isoelectric points, was used to isolate isoforms of apoC-II from porcine very low density lipoproteins. This method was found to be time-saving and the yield of protein recovery was high. With chromatofocusing, three polypeptides were obtained which were characterized by amino acid analysis, double immunodiffusion, and by their ability to activate bovine milk lipoprotein lipase. The three polypeptides had the same amino acid composition, gave a reaction of identity against a monospecific antiserum to porcine apoC-II, but had different isoelectric points between pH 4.8 and 4.4. They all enhanced the activity of lipoprotein lipase, but to a lesser degree than native porcine serum. There was no indication of the existence of apolipoproteins that correspond to human apoC-III polypeptides.