Apolipoprotein A-II: chemical synthesis and biophysical properties of three peptides corresponding to fragments in the amino-terminal half

Three peptide fragments of apolipoprotein A-II corresponding to residues 17--31, 12--31, and 7--31 have been synthesized by solid-phase techniques and purified to apparent homogeneity. Each of these fragments contains residues 18--30, a region previously proposed to possess potential amphipathic helical properties. Secondary structural changes of these synthetic fragments accompanying their interaction with phospholipid have been studied by circular dichroism. The magnitude of this interaction has been evaluated from the yields and stoichiometry of lipid-protein complexes isolated by density gradient ultracentrifugation. Fragment 17--31, the smallest peptide containing the proposed amphipathic helix, did not interact with dimyristoylphosphatidylcholine (DMPC) single bilayer vesicles at 24 degrees C; upon addition of DMPC, no ellipticity change could be detected nor could a stable lipid-peptide complex be isolated. However, fragments 12--31 and 7--31 did interact with phosphilipid; in the absence of lipid, both fragments had primarily disordered structures, but when isolated as DMPC-peptide complexes, both fragments possessed increased helical structure. The phospholipid:peptide molar ratio was 14:1 for fragment 12--31 and 27:1 for fragment 7--31. Studies of space-filling models of these fragments suggest that hydrophobicity and/or length are important properties of phospholipid binding apoproteins.

A review of the unique features of HDL apoproteins

The human plasma high density lipoproteins (HDL) are a heterogeneous ensemble of five proteins associated with both neutral and polar lipids. The sequence of all five proteins are known. ApoA-I and apoA-II are the major protein components; apoC-I, apoC-II and apoC-III are the minor protein components. All these apoproteins spontaneously recombine with phospholipids to give stable lipid-protein complexes and freely exchange between the two major HDL subclasses, HDL2 and HDL3. In addition, ApoC-I, apoC-II, and apoC-III exchange between HDL and very low density lipoproteins. Furthermore, certain HDL apoproteins are activators for plasma enzymes that are important in lipid metabolism. ApoA-I and apoC-I activate lecithin/cholesterol acyltransferase; apoC-II is an activator of lipoprotein lipase. The regions of apoC-I and apoC-II that are involved in the activation of these enzymes have been localized with synthetic peptides. Studies of synthetic and native fragments of apoA-II, apoC-I, apoC-II, and apoC-III as well as model lipid-binding peptides have identified specific regions with structural features common to lipid-binding proteins. These special properties, which include helical potential, sequences with a critical amphipathic length, and high hydrophobicity of the nonpolar side of the amphipathic helix, are the determinants of HDL structure and metabolism.

Structure of the dimyristoylphosphatidylcholine vesicle and the complex formed by its interaction with apolipoprotein C-III: X-ray small-angle scattering studies

Single bilayer vesicles of dimyristoylphosphatidylcholine have been investigated by small-angle X-ray scattering at 28 degrees C. The results indicate that these vesicles are hollow spherical shell structures with an outer radius of approximately 12 nm and a molecular weight of (3.2 +/- 0.5) X 10(6). The shell was found to be 4.4 +/- 0.2 nm thick with a cross-sectional electron-density profile characteristic for a single phospholipid bilayer. Upon interaction of these vesicles with apolipoprotein C-III from human very low density lipoproteins at a protein/lipid ratio greater than 0.08 (g/g), a complex containing 0.25 g of protein/g of lipid, with molecular weight of (3.9 +/- 0.4) X 10(5), is formed. The shape analysis indicates a highly asymmetric particle with an internal partition of low and high electron density resembling that produced by a bilayer structure. Model calculations and curve-fitting procedures show good agreement between the experimental scattering curve and that computed for an oblate ellipsoidal structure with dimensions of 17 X 17 X 5 nm and a 1 nm thick shell of high electron density surrounding the core of low electron density.

Lipid-protein interactions in the plasma lipoproteins

The purpose of this review has been to discuss new information about the mechanism of lipid and apoprotein interaction in the plasma lipoproteins. A special form of the amphipathic helix has been identified as a major structural element of the apolipoproteins sequenced to date. Evidence is reviewed concerning the role of the amphipathic helix in the binding to phospholipids. Several different models for the organization of the components of HDL, LDL and LP-X have evolved from extensive structural studies. Resolution of the differences among these models will require additional experimental testing. Verification of models based on the study of reconstituted HDL will require rigorous proof of native structure in these particles. A detailed description of the molecular organization of the lipid and protein constituents of the plasma lipoproteins is still lacking. Further structural and sequence studies with apoB and the "arginine-rich" protein are needed. Crystallization of an apoprotein or lipoprotein and determination of the three-dimensional structure would be a major achievement. With such further detailed structural information, it may then be possible to correlate changes in structure with determinants of metabolism.

Lipoprotein-X: a substrate for lecithin: cholesterol acyltransferase

The action of lecithin:cholesterol acyltransferase (LCAT) was studied on an abnormal lipoprotein (LP-X) rich in phosphatidylcholine and cholesterol from the plasma of patients with obstructive liver disease. 60 mg LP-X isolated free of other lipoproteins and subsequently labelled with 3H-cholesterol were incubated with 1 mg highly purified enzyme in the presence of albumin. After 45 h at 37 degrees C, the incubation mixture was subjected to zonal ultracentrifugation. 3H-cholesterol and 3H-cholesteryl esters were quantified in each fraction of the zonal gradient. More than 95% of the lipoproteins in this mixture banded in the density range of LP-X with no change in size distribution, but did contain 593 nmoles of newly formed cholesteryl esters. Agarose electrophoresis revealed an alpha-migrating band in addition to the original beta-band. Also on agar, the typically cathode migrating LP-X was changed to anode moving material. These studies indicate that LP-X can serve as a substrate for LCAT.

Physical properties of the dimyristoylphosphatidylcholine vesicle and of complexes formed by its interaction with apolipoprotein C-III

The structure of a single bilayer vesicle of dimyristoylphosphatidylcholine has been characterized by sedimentation, densimetry, and light-scattering measurements. The molecular weight, partial specific volume, Stokes radius, and degree by hydration were found to be 2.68 X 10(6), 0.972 cm3/g, 125 A, and 0.86 g/g, respectively. From these quantities, a spherically symmetrical model has been derived that features a phospholipid bilayer 35.5 A thick and a hydration shell 9.3 A thick. This particle was shown to bind apolipoprotein C-III (apoC-III) up to 0.08 g/g without loss of its original vesicular structure. At protein-lipid ratios in excess of 0.08 g/g, sedimentation, gel chromatography, and light-scattering measurement indicated a dramatic decrease in Stokes radius and molecular weight. The sedimentation data showed these parameters to become constant at protein-lipid ratios in excess of 0.25 g/g. In this region, the Stokes radius and molecular weight were found to be approximately 80 A and 442 000, respectively. Within the constraints of these values and other data, several models for this complex are discussed.

Isolation, chemical characterization, and biophysical properties of three different abnormal lipoproteins: LP-X1, LP-X2, and LP-X3

Three different but related abnormal lipoprotein species, LP-X1, LP-X2, and LP-X3, have been isolated from cholestatic plasma by ethanol precipitation and zonal ultracentrifugation. All three populations are rich in phospholipids (64.9 to 67.5%) and cholesterol (23.0 to 26.8%) but poor in cholesteryl esters (0.4 to 1.9%), triglycerides (1.8 to 3.2%), and protein (3.2 to 6.7%) with differences in chemical composition which result in buoyant densities (1.038, 1.049, and 1.058, respectively) to allow their separation. LP-X1, LP-X2, and LP-X3 exhibited apparent flotation rates of 17.3, 9.7, and 3.2 Svedbergs and Stokes radii of 339, 343, and 294 A, respectively. As determined from circular dichroic measurements, the protein constituents of all three particles possessed a high degree of alpha helical structure (41 to 65%). Each LP-X particle exhibited abnormally low fluidity as evaluated by electron paramagnetic resonance. All of the particles contained human serum albumin and the C-proteins as major protein constituents, but only LP-X2 and LP-X3 contained apolipoprotein A-I and apolipoprotein E.

Influence of dietary trans-fatty acids on swine lipoprotein composition and structure

Four groups of 20 weanling swine each were fed either (a) basal diet, (b) basal plus hydrogenated fat (13% trans), (c) basal plus hydrogenated fat (13% trans) and 0.4% cholesterol, or (d) basal plus beef tallow (all cis). After six months of feeding, the animals were killed and the blood and aortas were removed. Very low density, low density, and high density lipoproteins were then isolated from the plasma by ultracentrifugal flotation. Although the fatty acid composition of the basal diet was different from the diets supplemented with either hydrogenated fat containing trans-fatty acid or beef tallow containing all cis, the lipid and fatty acid compositions of each of the isolated lipoprotein classes for the four groups of animals were remarkably similar. Elaidate was clearly incorporated into the lipoproteins of animals fed hydrogenated fat, but the level of incorporation was generally less than 5%. In a direct comparison of the structure of the lipoproteins from the different groups, we did not find any significant differences in their physical properties as determined by pyrene fluorescence and electron paramagnetic resonance methods. Grossly visible fatty streaks and fibrous plaques were not found in any of the swine aorta. However, light and electron microscopy indicated the presence of atherosclerotic lesions in the distal abdominal aorta and bifurcation. These studies demonstrate that a diet containing a substantial amount of trans-fatty acid leads to a small but definite incorporation into the swine lipoproteins. However, such changes had relatively little effect on lipoprotein structure or the presence of atherosclerotic lesions in these 6-month-old swine.

Composition-structure-function correlations in the binding of an apolipoprotein to phosphatidylcholine bilayer mixtures

We have studied the lipid binding of apoC-III with two types of mixed vesicles of DMPC (dimyristoyl phosphatidylcholine) and DPPC (dipalmitoyl phosphatidyl-choline). DMPC vesicles mixed with those of DPPC produce a macroscopic mixture in which the DMPC and DPPC vesicles remain intact. The circular dichroism and fluorescence spectra of apoC-III in the presence of this macroscopic mixture exhibit major changes near the transition temperature of each of the pure lipids, confirming the independent existence of the two PC's. Combining DMPC:DPPC macroscopic mixtures with apoC-III above the transition temperature, Tc 23 degrees C, of DMPC produces an isolatable complex consisting of 4:1 DMPC:DPPC. If the DMPC and DPPC are within the same vesicle, this microscopic lipid mixture has properties that are functions of the temperature and lipid composition. Spectral analysis of apoC-III in the presence of the micromixtures reveals a single transition, which occurs between the respective thermal transitions of DMPC (23 degrees C) and DPPC (41 degrees C). The lipid: protein complexes isolated from the microscopic mixtures have a DMPC: DPPC ratio identical to that of the vesicle with which the apoprotein was mixed; the formation of these complexes is most efficient above the temperature range where these microscopic mixtures induce major structural changes in the apoprotein. The absence of the preferential binding to DMPC observed in the macroscopic mixtures suggests that apoC-III interacts with large lipid domains within a vesicle and does not selectively bind the low melting DMPC. The temperature dependence of the binding of apoC-III to the microscopic mixtures may be due to phase separation or to cocrystallization; our data support the latter process.

Structure of an apolipoprotein-phospholipid complex: apoC-III induced changes in the physical properties of dimyristoylphosphatidylcholine

The effect of ApoC-III, a major apoprotein constituent of human very low density lipoproteins, on the physical properties of dimyristoylphosphatidylcholine (DMPC) vesicles has been studied by magnetic resonance and fluorescence techniques. The sharp gel-liquid crystalline transition usually observed at 23 C in DMPC is both broadened and elevated when ApoC-III is bound as determined (a) from measurements of microscopic viscosity by pyrene excimer fluorescence, (b) from the distribution of di-tert-butyl nitroxide between the bulk aqueous phase and the fluid lipid phase, and (c) from the motion of fatty acyl chains of spin-labeled phosphatdylcholine. Experiments involving the translocation of ascorbate and charged nitroxide ions and the movement of paramagnetic Eu 3+ ions indicate that when ApoC-III binds to DMPC vesicles, it increases their permeability or destroys their original bilayer structure. These two possibilities were distinguishable by gel filtration of the DMPC-ApoC-III complex (approximately 34 mol mol) that indicated that the product particles were significantly smaller than the original vesicles. Taken together, the data indicate that ApoC-III binding to DMPC not only decreases the acyl chain motion of individual lipid molecules, but also induces break-down of bilamellar vesicular structure to give significantly smaller complexes.

Interaction of apoliprotein C-III with phosphatidylcholine vesicles. Dependence of aproprotein-phospholipid complex formation on vesicle structure

We have studied the interaction of an apolipoprotein from human very low density lipoproteins (apoC-III) with egg yolk phosphatidylcholine in the form of single- and multi-bilayer vesicles. The reactivity of single-bilayer vesicles with apoC-III appears to be greater than that of the multi-bilayer vesicles according to several thermodynamic and spectrosconic criteria. In the complexes formed by the association of apoC-III with single-bilayer vesicles, the alpha-helical content of the peptide backbone and the apolarity of the environment around the tryptophan residues are greater than that observed in the complexes formed with the multibilayer vesicles. A higher yield and more homogeneous density distribution of lipid-apoprotein complexes results from the interaction of apoC-III with the single-bilayer vesicles relative to those obtained with the multi-bilayer vesicles. The enthalpy of association of apoC-III with phospholipid was greater for the single-shelled vesicles (25 kcal/mol apoC-III) than for the multi-shelled ones (18 kcal/mol apoC-III). The difference in reactivity of these two types of liposomes is not due to a difference in their fluidities since their fatty acid compositions are identical, but may be due to a difference in their areas of sterically accessible phospholipid, their permeabilities to the apoprotein, their radii of curvation, or a combination of these factors.

Multiple thermotropic phase transitions in Escherichia coli membranes and membrane lipids. A comparison of results obtained by nitroxyl stearate paramagnetic resonance, pyrene excimer fluorescence, and enzyme activity measurements

At characteristic temperatures, membranes from Escherichia coli cells enriched in exogenous elaidic acid exhibit two abrupt changes in the slope of Arrhenius plots of two enzyme activities. For NADH oxidase, these changes occur at 27 degrees and 32 degrees, whereas for D-lactate oxidase, these changes occur at 31 degrees and 36 degrees. Pyrene excimer fluorescence and spin-labeled fatty acid paramagnetic resonance results indicate that the beginning, midpoint, and end of a single structural change(order leads to disorder transition) occurs at 25.5-29.0 degrees, 30.0-31.0 degrees, and 33.0-35.5 degrees, respectively. These data suggest that for NADH oxidase, the observed activity changes correspond to the beginning and midpoint of a single membrane lipid structural change, whereas for D-lactate, the activity changes correspond to the midpoint and end of that structural change. In addition to the membrane structural change spanning the range of 25.5-35.5 degrees, a second change (9.5-21.0 degrees) was also observed. This transition was detected by 5- and 16-2,2-dimethyloxazolidinyl-1-oxyl (doxyl) stearates, but not by 12-doxyl stearate or pyrene. Structural changes in the extracted lipids were observed in the temperature ranges 4.0-9.0 degrees, 14.0-20.0 degrees, and 25.0-35.5 degrees. The two higher ranges correlate well with the ranges for structural changes observed in the intact membrane. Observations of these multiple transitions in both intact membranes and extracted lipids strongly suggest that these lipids segregate into domains of different fluidity and composition.

Bovine serum albumin. Study of the fatty acid and steroid binding sites using spin-labeled lipids

Three spin-labeled derivatives of stearic acid and two derivatives of palmitic acid have been used to study the structure of the strong fatty acid binding site of bovine serum albumin. The steroid and indole binding sites have been studied using spin-labeled derivatives of androstol and indole, respectively. Paramagnetic resonance and fluorescence quenching data suggest that the fatty acid, steroid, and indole binding sites may be identical. The mobility of the nitroxyl group at C-8 of palmitic acid bound to albumin at a 1:1 molar ratio is unaffected when the carboxyl group is esterified. When the nitroxyl group is located at C-5 on this acid its motion is detectably increased by esterification of the carboxyl group but the magnitude of this change is small. This result suggests that the carboxyl group may play a minor role in the binding of fatty acids to the strongest fatty acid binding site of albumin. When stearic acid derivatives bearing the nitroxide at C-5, C-12, and C-16 are bound to albumin at a ligand to albumin ratio of 1, the order of mobility at 0-30 degrees is C-16 greater than C-12 congruent to C-5. Although motion at the methyl terminus is always greater than at the COOH terminus in the range 0-60 degrees, a simple monotonic increase in chain motion between the two termini is not observed. Arrhenius plots of the motion parameters for these bound fatty acids show two abrupt changes in slope. The temperature ranges for these changes are 15-23 degrees and 38-45 degrees. These results suggest that when one mole of spin-labeled fatty acid is bound to albumin, the protein undergoes a conformational change in each of these temperature ranges.

The mechanism of lipid-binding by plasma lipoproteins

In this article, we review the current state of knowledge about the structure of human plasma high density lipoproteins. Recent advances in this field, including amino acid sequencing and lipid-binding studies, suggest that the plasma lipoproteins have specific sites with special structural features that account for their ability to bind and transport lipid. A hypothesis which describes the structure of these lipid-binding sites and their interaction with lipid is presented.