Studies of synthetic peptide analogs of the amphipathic helix. Competitive displacement of exchangeable apolipoproteins from native lipoproteins

Improvement of Thermal Stability of Amphipathic Peptide-Phospholipid Nanodiscs via Lateral Association of α-Helices by Disulfide Cross-Linking

Amphipathic α-helical peptides have been reported to form discoidal particles or nanodiscs with phospholipids, in which a lipid bilayer patch is encircled by peptides. Peptide-based nanodiscs have broad applicability because of their ease of preparation, size flexibility, and structural plasticity. We previously revealed that the nanodiscs formed by apolipoprotein-A-I-derived peptide 18A showed temperature-dependent structural destabilization above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayer. It has been suggested that this destabilization is due to the migration of peptides bound to the edge of the discs to the bilayer surface. In this study, we designed a peptide that could stabilize nanodisc structures against the phase transition of lipid bilayers by disulfide cross-linking of peptides. An 18A-dimer cross-linked by a proline residue, 37pA (Ac-18A-P-18A-CONH₂), also showed thermal destabilization of nanodiscs like 18A. However, cross-linking the sides of the two α-helices of the cysteine-substituted analogue 37pA-C2 with disulfide bonds led to the formation of nanodiscs that were more stable to temperature changes. This stabilizing effect was mainly due to the formation of a cyclic 37pA-C2 monomer by intramolecular disulfide cross-linking. These results suggest that the lateral association of two α-helices, which is the basis of the double-belt structure, is an important factor for the implementation of stable nanodiscs. The results of this study will help in development of more stable nanoparticles with membrane proteins in the future.

Spiers Memorial Lecture: Analysis and de novo design of membrane-interactive peptides

Membrane-peptide interactions play critical roles in many cellular and organismic functions, including protection from infection, remodeling of membranes, signaling, and ion transport. Peptides interact with membranes in a variety of ways: some associate with membrane surfaces in either intrinsically disordered conformations or well-defined secondary structures. Peptides with sufficient hydrophobicity can also insert vertically as transmembrane monomers, and many associate further into membrane-spanning helical bundles. Indeed, some peptides progress through each of these stages in the process of forming oligomeric bundles. In each case, the structure of the peptide and the membrane represent a delicate balance between peptide-membrane and peptide-peptide interactions. We will review this literature from the perspective of several biologically important systems, including antimicrobial peptides and their mimics, α-synuclein, receptor tyrosine kinases, and ion channels. We also discuss the use of de novo design to construct models to test our understanding of the underlying principles and to provide useful leads for pharmaceutical intervention of diseases.

Apolipoprotein Mimetic Peptide Inhibits Neutrophil-Driven Inflammatory Damage via Membrane Remodeling and Suppression of Cell Lysis

Neutrophils are crucial for host defense but are notorious for causing sterile inflammatory damage. Activated neutrophils in inflamed tissue can liberate histone H4, which was recently shown to perpetuate inflammation by permeating membranes via the generation of negative Gaussian curvature (NGC), leading to lytic cell death. Here, we show that it is possible to build peptides or proteins that cancel NGC in membranes and thereby suppress pore formation, and demonstrate that they can inhibit H4 membrane remodeling and thereby reduce histone H4-driven lytic cell death and resultant inflammation. As a demonstration of principle, we use apolipoprotein A-I (apoA-I) mimetic peptide apoMP₁. X-ray structural studies and theoretical calculations show that apoMP₁ induces nanoscopic positive Gaussian curvature (PGC), which interacts with the NGC induced by the N-terminus of histone H4 (H4n) to inhibit membrane permeation. Interestingly, we show that induction of PGC can inhibit membrane-permeating activity in general and "turn off" diverse membrane-permeating molecules besides H4n. In vitro experiments show an apoMP₁ dose-dependent rescue of H4 cytotoxicity. Using a mouse model, we show that tissue accumulation of neutrophils, release of neutrophil extracellular traps (NETs), and extracellular H4 all strongly correlate independently with local tissue cell death in multiple organs, but administration of apoMP₁ inhibits histone H4-mediated cytotoxicity and strongly prevents organ tissue damage.

Temperature- and composition-dependent conformational transitions of amphipathic peptide-phospholipid nanodiscs

Nanodiscs are discoidal particles in which a lipid bilayer is encircled by amphipathic molecules such as proteins, peptides, or synthetic polymers. The apolipoprotein-A-I-derived peptide 18A is known to form nanodiscs in the presence of phospholipids, but the detailed mechanism of the formation and deformation of these nanodiscs in response to changes in the surrounding environment is not well understood. Here, we investigated the temperature- and composition-dependent structural changes of 18A-phosphatidylcholine complexes using fluorescence spectroscopy, dynamic light scattering, circular dichroism, static ³¹P NMR, and electron microscopy. We found that the nanodiscs in fast isotropic rotational motion increased in size above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayers, resulting in the formation of enlarged nanodiscs and a lamellar phase. The lamellar phase was found to be oriented along the magnetic field. Further increase in temperature induced the formation of lipid vesicles. These transformations were explained using a transition model based on the migration of the peptide from the rim of the nanodiscs to the liquid-crystalline bilayer phase. The study outcomes provide a basis for understanding the design principles of discoidal nanostructures for structural biology and nanomedicine applications.

The Role of HDL and HDL Mimetic Peptides as Potential Therapeutics for Alzheimer's Disease

The role of high-density lipoproteins (HDL) in the cardiovascular system has been extensively studied and the cardioprotective effects of HDL are well established. As HDL particles are formed both in the systemic circulation and in the central nervous system, the role of HDL and its associated apolipoproteins in the brain has attracted much research interest in recent years. Alzheimer’s disease (AD) is the most prevalent neurodegenerative disorder and the leading cause of dementia worldwide, for which there currently exists no approved disease modifying treatment. Multiple lines of evidence, including a number of large-scale human clinical studies, have shown a robust connection between HDL levels and AD. Low levels of HDL are associated with increased risk and severity of AD, whereas high levels of HDL are correlated with superior cognitive function. Although the mechanisms underlying the protective effects of HDL in the brain are not fully understood, many of the functions of HDL, including reverse lipid/cholesterol transport, anti-inflammation/immune modulation, anti-oxidation, microvessel endothelial protection, and proteopathy modification, are thought to be critical for its beneficial effects. This review describes the current evidence for the role of HDL in AD and the potential of using small peptides mimicking HDL or its associated apolipoproteins (HDL-mimetic peptides) as therapeutics to treat AD.

High-density lipoprotein mimetic peptide 4F mitigates amyloid-β-induced inhibition of apolipoprotein E secretion and lipidation in primary astrocytes and microglia

The apolipoprotein E (apoE) ε4 allele is the primary genetic risk factor for late-onset Alzheimer's disease (AD). ApoE in the brain is produced primarily by astrocytes; once secreted from these cells, apoE binds lipids and forms high-density lipoprotein (HDL)-like particles. Accumulation of amyloid-β protein (Aβ) in the brain is a key hallmark of AD, and is thought to initiate a pathogenic cascade leading to neurodegeneration and dementia. The level and lipidation state of apoE affect Aβ aggregation and clearance pathways. Elevated levels of plasma HDL are associated with lower risk and severity of AD; the underlying mechanisms, however, have not been fully elucidated. This study was designed to investigate the impact of an HDL mimetic peptide, 4F, on the secretion and lipidation of apoE. We found that 4F significantly increases apoE secretion and lipidation in primary human astrocytes as well as in primary mouse astrocytes and microglia. Aggregated Aβ inhibits glial apoE secretion and lipidation, causing accumulation of intracellular apoE, an effect that is counteracted by co-treatment with 4F. Pharmacological and gene editing approaches show that 4F mediates its effects partially through the secretory pathway from the endoplasmic reticulum to the Golgi apparatus and requires the lipid transporter ATP-binding cassette transporter A1. We conclude that the HDL mimetic peptide 4F promotes glial apoE secretion and lipidation and mitigates the detrimental effects of Aβ on proper cellular trafficking and functionality of apoE. These findings suggest that treatment with such an HDL mimetic peptide may provide therapeutic benefit in AD. Read the Editorial Highlight for this article on page 580.

Apolipoprotein A-I and Cancer

High-density lipoprotein (HDL) and apolipoprotein A-I (apoA-I), the predominant protein in plasma HDL, have long been the focus of intense studies in the field of atherosclerosis and cardiovascular disease. ApoA-I, in large part, is responsible for HDL assembly and its main atheroprotective function, that of shuttling excess cholesterol from peripheral tissues to the liver for excretion (reverse cholesterol transport). Recently, a protective role for HDL in cancer was suggested from several large clinical studies where an inverse relationship between plasma HDL-cholesterol (HDL-C) levels and risk of developing cancer was noted. This notion has now been tested and found to be supported in mouse tumor studies, where increasing levels of apoA-I/HDL were discovered to protect against tumor development and provision of human apoA-I was therapeutic against established tumors. This mini-review discusses the emerging role of apoA-I in tumor biology and its potential as cancer therapeutic.

Self-reproduction of nanoparticles through synergistic self-assembly

We describe a self-reproduction mechanism of nanometer-sized particles (i.e., nanodiscs) through chemical ligation of the precursors and self-assembly of the building blocks. The ligation reaction was accelerated on lipid bilayer surfaces, and the products spontaneously assembled into nanodiscs with lipid molecules. With the increase in the number of nanodiscs, a rapid proliferation of the nanodiscs occurred through the spatial rearrangements of the molecules between the pre-existing nanodiscs and the unreacted materials, rather than template- or complex-enhanced ligation of the precursors. The subsequent process of surface-enhanced ligation of integrated precursors matured the nanoparticles into identical copies of the pre-existing assembly. Our study showed that the synergistic self-assembly mechanism probably underlie the self-replication principles for heterogeneous multimolecular systems.

Molecules that mimic apolipoprotein A-I: potential agents for treating atherosclerosis

Certain amphipathic α-helical peptides can functionally mimic many of the properties of full-length apolipoproteins, thereby offering an approach to modulate high-density lipoprotein (HDL) for combating atherosclerosis. In this Perspective, we summarize the key findings and advances over the past 25 years in the development of peptides that mimic apolipoproteins, especially apolipoprotein A-I (apoA-I). This assemblage of information provides a reasonably clear picture of the state of the art in the apolipoprotein mimetic field, an appreciation of the potential for such agents in pharmacotherapy, and a sense of the opportunities for optimizing the functional properties of HDL.

Comparison of the binding of alpha-helical and beta-sheet peptides to a hydrophobic surface

The induction and stabilisation of secondary structure for a series of amphipathic alpha-helical and beta-sheet peptides upon their binding to lipid-like surfaces has been characterised by reversed phase high-performance liquid chromatography (RP-HPLC). In addition, a series of peptides which have been shown to switch from beta-sheet to alpha-helical conformation upon transfer from a polar to a non-polar solution environment also have been studied. Binding parameters related to the hydrophobic contact area and affinity for immobilised C18 chains were determined at temperatures that ranged from 5 to 85 degrees C, allowing conformational transitions for the peptides during surface adsorption to be monitored. The results demonstrated that all peptides which adopt secondary structure in solution also exhibited large changes in their interactive properties. Overall, this study demonstrates that the hydrophobic face of each amphipathic peptide dominates the binding process and that hydrophobic interactions are a major factor controlling the surface induction of secondary structure.

Studies of kinetics and equilibrium membrane binding of class A and class L model amphipathic peptides

We studied the kinetics and equilibrium membrane binding of two amphipathic alpha-helical peptides: the 18L peptide, which belongs to the class L (lytic peptides), and the Ac-18A-NH2 peptide of the class A (apolipoprotein), according to classification of Segrest et al. ((1990) Proteins, 8, 103-117). Both for cationic 18L and zwitterionic Ac-18A-NH2, the presence of acidic lipids increased the membrane binding constants by two orders of magnitude. The free energy of peptide-membrane association was in the range of 8.5-12.8 kcal/mol. Binding isotherms corresponded to monomer partitioning with saturation at high peptide/lipid ratios. This was also supported by stopped flow studies of the kinetics of peptide-membrane association as measured by peptide tryptophan fluorescence or by energy transfer from the peptide to the lipid-anchored anthrylvinyl fluorophor. The apparent time required for peptide-membrane equilibration was in the millisecond range. At low peptide/lipid ratios it depended on lipid concentration and was independent of the peptide concentration. The rate of peptide-membrane association was found to be relatively close to the diffusion limit. Thus peptide-membrane affinity was mostly determined by the peptide dissociation rate, i.e. higher membrane affinity correlated with a decrease in dissociation rate and with a slower peptide exchange. We have shown that the dynamic character of the peptide membrane equilibrium can be used for selective peptide targeting and disruption of membranes with a specific lipid composition.

Structure-activity relationships in the fusion of small unilamellar phosphatidylcholine vesicles induced by a model peptide

Limited proteolysis of fatty acid-free bovine serum albumin by pepsin yields several well characterized peptides, one of which (P9, M(r) 9,000), induces fusion of small unilamellar vesicles (SUV) of phosphatidylcholine at pH 3.6. Circular dichroism (CD) of P9 solutions confirmed that the peptide undergoes a reversible transition between pH 7 and pH 3.6. The spectral changes observed with CD suggest that in the low pH conformation there is a decrease in the alpha-helical contents and an exposure of hydrophobic residues. CD and differential ultraviolet spectroscopy demonstrated that P9 binds to micelles of hexadecylphosphorylcholine and the binding produces changes in the tertiary structure of the peptide. Reduction and carboxymethylation of the two disulfide bridges of P9 produced loss of the ability to induce fusion of SUV, although the reduced peptide binds to vesicles, induces loss of entrapped marker and produces vesicle disruption. In the active form P9 exposes hydrophobic groups, one amphiphilic alpha-helix and requires the integrity of the disulfide bridge-stabilized tertiary structure.

Lipoprotein association of human apolipoprotein E/A-I chimeras. Expression in transfected hepatoma cells

Both apolipoprotein (apo) E and apoA-I are associated with lipoproteins, although with different particle classes. ApoE is associated with very low density lipoprotein (VLDL) and with the larger high density lipoprotein (HDL) subspecies, while apoA-I is found predominantly in association with most HDL subclasses. The genes encoding these proteins have a similar overall structure with the nucleotide sequences of the third and fourth exons coding for the mature protein. In an effort to understand the difference in lipoprotein association patterns of these two apoproteins, we have constructed and expressed chimeric apoproteins using cDNAs in which the third (n) and fourth (c) exons of human apoE and apoA-I are exchanged. McArdle rat hepatoma cells (McA-RH7777), which secrete VLDL- and HDL-like particles, were stably transfected with these cDNAs, and the cDNAs for human apoE and human apoA-I. Single spin NaBr gradient fractions of lipoprotein deficient serum-treated cell medium from transfected McA-RH7777 cells were analyzed. The distributions of transfected human apoE and apoA-I and endogenous rat apoE and apoA-I were compared with those of the chimeras. Among HDL subspecies, human apoE expressed by these cells is associated with particles of density 1.108 g/ml. Similarly, chimera apoA-InEc (exon 3 of apoA-I and exon 4 of apoE) is found in particles of density 1.111 g/ml. Human apoA-I, however, distributes in a broader range of particles with peak densities of 1.111 g/ml and 1.164 g/ml. The distribution of the complementary chimera, apoEnA-Ic, follows this same pattern, with peak particle densities of 1.098 and 1.137 g/ml. This is in contrast to the narrow distributions of endogenous rat apoE and apoA-I, which were found in particles of density 1.099 and 1.089 g/ml, respectively. When metabolically labeled medium was fractionated via gel filtration column chromatography, apoA-InEc was found to associate with the VLDL fractions; apoEnA-Ic was absent from these same fractions. These results suggest that the fourth exon largely determines the distinctive lipoprotein distribution patterns of these two human apoproteins and that the human apoA-I fourth exon sequence may account for the polydisperse HDL pattern as observed by others in transgenic mice expressing human apoA-I.

Synthetic model peptides for apolipoproteins. II. Characterization of the discoidal complexes generated between phospholipids and synthetic model peptides for apolipoproteins

The structure, composition and physico-chemical properties of complexes generated between phospholipids and synthetic model peptides for the amphipathic helices of the plasma apolipoproteins were studied. The sequences of the peptides were derived from that of the 18A peptide and designed to either enhance or decrease ionic interactions between pairs of peptides, as described in the accompanying paper. Complexes were prepared with dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), or with DPPC and cholesterol, and isolated on a Superose 6HR column. Association kinetics for the DMPC-peptides complexes were followed by measuring the turbidity as a function of the temperature. The diameters of the DPPC-peptide complexes, measured by gradient gel electrophoresis (GGE), were about 120 A. Fluorescence polarization measurements after labeling with diphenyl hexatriene (DPH) yielded transition temperatures of, respectively, 40.6, 41.5 and 41.8 degrees C for the DPPC/18AM1-, DPPC/18AM4- and DPPC/18A-peptide complexes. These values were confirmed by differential scanning calorimetry. Circular dichroism and infrared spectroscopy revealed that the peptides adopt an alpha-helical structure in solution and this percentage increased from 30-40% in the free peptides up to 50-60% in the complexes. Attenuated total reflection (ATR) infrared measurements of the complexes indicated that the peptides are oriented parallel to the acyl chains of the phospholipid bilayer. Denaturation of the peptides and of the peptide-lipid complexes was monitored by Trp fluorescence under addition of increasing amounts of GdmCl. The mid-points of the denaturation curves lie at, respectively, 0.05, 0.25 and 0.35 M GdmCl for the 18AM4, 18A and 18AM1 peptide and are shifted towards higher GdmCl concentrations after peptide-lipid binding. GdmCl denaturation decreased the alpha-helical content of the peptides and of the complexes, as monitored by circular dichroism measurement. The helix to random coil structure transition occurred at, respectively, 2.1, 2.2, and 2.0 M GdmCl for 18A, 18AM1 and 18AM4, compared to 5.1, 5.0, and 5.3 M in the corresponding complexes. These data suggest altogether that the structural properties, the mode of lipid-protein association and the stability of the phospholipid-peptide complexes are similar to those of native plasma apolipoproteins. The 18A and 18AM4 peptides which contain charged residues along the edge of the helix, leading to salt bridge formation between peptides were shown to mimic the amphipathic helices of the plasma apolipoproteins.

Synthetic model peptides for apolipoproteins. I. Design and properties of synthetic model peptides for the amphipathic helices of the plasma apolipoproteins

Amphipathic helical peptides are the lipid-binding motives of the plasma apolipoproteins, and synthetic peptide analogs have been used to unravel the mechanism of lipid association within this class of proteins. Hydrophobic interactions between the apolar amino acid residues belonging to the hydrophobic face of the amphipathic helices and the lipids are the major driving forces in the peptide-lipid association to form discoidal complexes. Ionic interactions and salt bridge formation between contiguous peptide chains in the complex can, however, contribute to the overall stability of the lipid-protein particle. This was studied by designing peptide analogs to the helical repeats of the apolipoproteins with variable degrees of salt bridge formation between adjacent peptide chains. The most stable conformation for pairs of synthetic peptides was calculated by energy minimisation together with the energy of interaction between peptides. The sequence of the peptides was derived from that of the 18A peptide synthesized by Segrest et al., and the theoretical calculations confirmed that ionic interactions between residues close to each other, along the edge of two adjacent anti-parallel peptides, can significantly contribute towards the stability of a peptide-phospholipid complex.

Lecithin:cholesterol acyltransferase activation by synthetic amphipathic peptides

The amphipathic helical theory of Segrest and colleagues (FEBS Lett.:38:247-253, 1974) proposes that the lipid-binding segments of serum apolipoproteins are in an alpha helical conformation. Furthermore the helices have a hydrophobic face and a hydrophilic face with a specific distribution of positively and negatively charged residues. The importance of the pattern of the charged residues in the lipid binding and lecithin:cholesterol acyltransferase (LCAT) activation by the segments is still debated. We designed a 30-residue peptide, GALA, which in the alpha helical conformation has a hydrophilic face composed of glutamic acid residues (Sabbarao et al.: Biochemistry 26:2964-2972, 1987). GALA behaves like the serum apolipoproteins in its interaction with dimyristoylphosphatidylcholine (DMPC) at neutral pH; the amino terminal tryptophan of GALA undergoes a blue shift in its fluorescence emission spectrum, and the circular dichroism (CD) spectrum indicates that GALA acquires alpha helical structure in the presence of DMPC. A DMPC-GALA:19/1 (molar ratio) complex can be isolated by gel-permeation chromatography. This complex has a discoidal structure with the approximate dimensions of 44-A edge thickness and a 170- to 350-A diameter. GALA activates LCAT with DMPC but not with unsaturated phospholipids as the substrate. The apparent partition coefficient of GALA into DMPC vesicles is 100-fold larger than into egg phosphatidylcholine vesicles. The interaction of GALA with unsaturated lipids at neutral pH is so weak that no detectable change in the spectroscopic properties of GALA or the structure of the liposomes can be detected under the conditions used here. The sequence of GALA differs from previously studied model Apo A1 peptides by the absence of positively charged residues on the hydrophilic face. This indicates that positive charges in Apo A1-like peptides are not required in order to form discoidal structures with saturated phospholipids or to activate LCAT with such lipid substrates.

Differences in the hydrophobic properties of discrete alpha-helical domains of rat and human apolipoprotein A-IV

The primary structure of human apolipoprotein A-IV has been analyzed to identify amphipathic alpha-helical regions and compare their hydrophobic properties with analogous segments of rat apolipoprotein A-IV. Significant differences were found in the mean hydrophobicity and/or mean helical hydrophobic moment of several discrete alpha-helical domains of rat and human apolipoprotein A-IV, which may determine the differences in their biophysical and biological behavior.

Activation of phosphatidylcholine-sterol acyltransferase by human apolipoprotein E isoforms

The reaction catalysed by phosphatidylcholine-sterol acyltransferase (EC 2.3.1.43) is believed to be the major source of cholesteryl ester in human plasma; the enzyme requires a protein activator. Several human apolipoproteins were found to exhibit an activator function, the major one being apolipoprotein A-I. Human apolipoprotein E exists in the population mainly in three different genetic isoforms; apolipoprotein E-2, E-3 and E-4. These isopeptides were isolated from subjects homozygous for one of the isoforms, incorporated into phospholipid/cholesterol/[14C]cholesterol complexes by the cholate dialysis procedure and used to measure capacity to activate phosphatidylcholine-sterol acyltransferase in comparison to apolipoprotein A-I lipid substrate particles prepared by the same procedure. Acyltransferase activity was measured by the formation of [14C]cholesteryl ester from [14C]cholesterol using purified enzyme. With egg yolk phosphatidylcholine as acyl donor, apo E was 15-19% as efficient as apolipoprotein A-I for activation of the acyltransferase. Apo-E-stimulated cholesteryl ester formation by the enzyme was enhanced when 1-oleoyl-2-palmitoyl-glycerophosphocholine was used as a substrate phospholipid (45% of apo A-I/phosphatidylcholine control) and most pronounced with dimyristoylglycerophosphocholine (75% of apo A-I/phosphatidylcholine control). No significant difference in activation was found between apo E isoforms. It is concluded that apolipoprotein E activates phosphatidylcholine-sterol acyltransferase in vitro and that apolipoprotein E isoforms are similarly effective.

Amphiphilic cationic peptides mediate cell adhesion to plastic surfaces

Four amphiphilic peptides, each with net charges of +2 or more at neutrality and molecular weights under 4 kilodaltons, were found to mediate the adhesion of normal rat kidney fibroblasts to polystyrene surfaces. Two of these peptides, a model for calcitonin (peptide 1, MCT) and melittin (peptide 2, MEL), form amphiphilic alpha-helical structures at aqueous/nonpolar interfaces. The other two, a luteinizing hormone-releasing hormone model (peptide 3, LHM) and a platelet factor model (peptide 4, MPF) form beta-strand structures in amphiphilic environments. Although it contains only 10 residues, LHM mediated adhesion to surfaces coated with solutions containing as little as 10 pmoles/ml of peptide. All four of these peptides were capable of forming monolayers at air-buffer interfaces with collapse pressures greater than 20 dynes/cm. None of these four peptides contains the tetrapeptide sequence Arg-Gly-Asp-Ser, which has been associated with fibronectin-mediated cell adhesion. Ten polypeptides that also lacked the sequence Arg-Gly-Asp-Ser but were nonamphiphilic and/or had net charges less than +2 at neutrality were all incapable of mediating cell adhesion (Pierschbacher and Ruoslahti, 1984). The morphologies of NRK cells spread on polystyrene coated with peptide LHM resemble the morphologies on fibronectin-coated surfaces, whereas cells spread on surfaces coated with MCT or MEL exhibit strikingly different morphologies. The adhesiveness of MCT, MEL, LHM, and MPF implies that many amphiphilic cationic peptides could prove useful as well defined adhesive substrata for cell culture and for studies of the mechanism of cell adhesion.

Synthesis of peptide labelled with p-iodophenylalanine which corresponds to the first amphipathic region of apolipoprotein C-I

The amino terminal fragment (1-15) of apolipoprotein C-1, H-Thr-Pro-Asp-Val-Ser-Ser-Ala-Leu-Asp-Lys-Leu-Lys-Glu-Phe14-Gly was prepared by the solid phase method. However, the phenylalanine residue at position 14 was replaced with p-iodophenylalanine in the chemical synthesis. The preparation of t-butyloxy-carbonyl-p-iodophenylalanine is described. After completion of the synthesis, the product was deprotected and cleaved from the resin with liquid HF. The peptide was purified by gel filtration on Sephadex G-25 and preparative high performance liquid chromatography. The purified material was shown to be homogeneous by amino acid analytical data and by chromatography in three different analytical reversed phase HPLC systems. The peptide was then labelled by the chloroamine-T procedure and good incorporation of 125I was obtained. After purification of the product by gel filtration on Biogel P-2, the labelled pentadecapeptide was tested for ability to bind to Very Low Density Lipoproteins (VLDL) in the following manner: VLDL was isolated from normolipemic serum by ultracentrifugal flotation and 100-microliter samples were incubated with labelled material dissolved in 200 microliter of 0.5 M NaCl, 0.02 M sodium phosphate buffer, pH 7.4 at 37 degrees for 30 min. The VLDL fraction was again isolated by ultracentrifugal flotation and the incorporation of radioactivity into the lipoprotein was measured. Under these conditions, a sample of [3H]-native apolipoprotein C-I was incorporated to an extent of 83% of the total sample, while the [125I]-pentadecapeptide exhibited an incorporation of 8.7%.

Apo E-containing lipoproteins in low or high density lipoprotein deficiency

Apolipoprotein (apo) E-containing subfractions of very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), and high density lipoprotein (HDL) have been described in normolipidemic and hyperlipidemic subjects. These lipoproteins exist, however, in the presence of large amounts of apo A-I- and apo B-containing lipoproteins so that it has been difficult to assess the independence of these apo E-containing subclasses from the major lipoprotein classes. The present study has approached this question by taking advantage of three hypolipidemic states in which one or more of the major apolipoproteins is deficient or absent. After separating lipoproteins from whole plasma by molecular sieve chromatography followed by radioimmunoassay of column fractions, we found that two subjects with abetalipoproteinemia had no apo E-containing lipoproteins the size of VLDL or IDL and all the plasma apo E was in a fraction of large HDL. Two subjects with Tangier disease and two with familial apo A-I/C-III deficiency had extremely low levels of HDL cholesterol and of apo A-I-containing lipoproteins. In spite of the absence of classical HDL, a major fraction of apo E-containing lipoproteins was reproducibly observed at the elution volume characteristic of large HDL and was identical to that found in normal subjects. These data thus suggest the existence of apo E-containing lipoproteins that are the size of HDL and are not dependent upon the presence of either apo B or apo A-I. While studies in normal subjects indicate that apo E is associated with other apolipoproteins in HDL, further investigations will be needed to determine the full composition of these apo E-containing lipoproteins in the lipoprotein-deficient patients described in this report.

Lipoprotein lipase- and phospholipase A2-catalyzed hydrolysis of phospholipid vesicles with an encapsulated fluorescent dye effects of apolipoproteins

The self-quenching dye, 6-carboxyfluorescein, has been encapsulated into sonicated vesicles of egg phosphatidylcholine. Porcine pancreatic phospholipase A2 and bovine milk lipoprotein lipase catalyze the hydrolysis of the phosphatidylcholine resulting in the release of the encapsulated dye and a large increase in 6-carboxyfluorescein fluorescence. The fluorescence increase occurs in parallel with the formation of lysophosphatidylcholine and is strongly dependent on Ca2+ for phospholipase A2 catalysis and on apolipoprotein C-II for hydrolysis by lipoprotein lipase. Other apolipoproteins, including apolipoproteins C-III, C-I, and A-I, do not enhance lipoprotein lipase activity towards this substrate. We conclude that the enhancement of lipoprotein lipase activity by apolipoprotein C-II is a specific property of the activator protein due to its interaction with lipoprotein lipase or an enzyme/lipid interface and not a characteristic of lipid-binding proteins in general.