Structural and functional properties of natural and chemical variants of apolipoprotein A-I

Human apoA-I[Lys107del] mutation affects lipid surface behavior of apoA-I and its ability to form large nascent HDL

Population studies have found that a natural human apoA-I variant, apoA-I[K107del], is strongly associated with low HDL-C but normal plasma apoA-I levels. We aimed to reveal properties of this variant that contribute to its unusual phenotype associated with atherosclerosis. Our oil-drop tensiometry studies revealed that compared to WT, recombinant apoA-I[K107del] adsorbed to surfaces of POPC-coated triolein drops at faster rates, remodeled the surfaces to a greater extent, and was ejected from the surfaces at higher surface pressures on compression of the lipid drops. These properties may drive increased binding of apoA-I[K107del] to and its better retention on large triglyceride-rich lipoproteins, thereby increasing the variant's content on these lipoproteins. While K107del did not affect apoA-I capacity to promote ABCA1-mediated cholesterol efflux from J774 cells, it impaired the biogenesis of large nascent HDL particles resulting in the formation of predominantly smaller nascent HDL. Size-exclusion chromatography of spontaneously reconstituted 1,2-dimyristoylphosphatidylcholine-apoA-I complexes showed that apoA-I[K107del] had a hampered ability to form larger complexes but formed efficiently smaller-sized complexes. CD analysis revealed a reduced ability of apoA-I[K107del] to increase α-helical structure on binding to 1,2-dimyristoylphosphatidylcholine or in the presence of trifluoroethanol. This property may hinder the formation of large apoA-I[K107del]-containing discoidal and spherical HDL but not smaller HDL. Both factors, the increased content of apoA-I[K107del] on triglyceride-rich lipoproteins and the impaired ability of the variant to stabilize large HDL particles resulting in reduced lipid:protein ratios in HDL, may contribute to normal plasma apoA-I levels along with low HDL-C and increased risk for CVD.

Understanding the role of apolipoproteinA-I in atherosclerosis. Post-translational modifications synergize dysfunction?

BACKGROUND

The identification of dysfunctional human apolipoprotein A-I (apoA-I) in atherosclerotic plaques suggests that protein structure and function may be hampered under a chronic pro inflammatory scenario. Moreover, the fact that natural mutants of this protein elicit severe cardiovascular diseases (CVD) strongly indicates that the native folding could shift due to the mutation, yielding a structure more prone to misfold or misfunction. To understand the events that determine the failure of apoA-I structural flexibility to fulfill its protective role, we took advantage of the study of a natural variant with a deletion of the residue lysine 107 (K107del) associated with atherosclerosis.

METHODS

Biophysical approaches, such as electrophoresis, fluorescence and spectroscopy were used to characterize proteins structure and function, either in native conformation or under oxidation or intramolecular crosslinking.

RESULTS

K107del structure was more flexible than the protein with the native sequence (Wt) but interactions with artificial membranes were preserved. Instead, structural restrictions by intramolecular crosslinking impaired the Wt and K107del lipid solubilization function. In addition, controlled oxidation decreased the yield of the native dimer conformation for both variants.

CONCLUSIONS

We conclude that even though mutations may alter protein structure and spatial arrangement, the highly flexible conformation compensates the mild shift from the native folding. Instead, post translational apoA-I modifications (probably chronic and progressive) are required to raise a protein conformation with significant loss of function and increased aggregation tendency.

GENERAL SIGNIFICANCE

The results learnt from this variant strength a close association between amyloidosis and atherosclerosis.

Fibrillar conformation of an apolipoprotein A-I variant involved in amyloidosis and atherosclerosis

BACKGROUND

Different protein conformations may be involved in the development of clinical manifestations associated with human amyloidosis. Although a fibrillar conformation is usually the signature of damage in the tissues of patients, it is not clear whether this species is per se the cause or the consequence of the disease. Hereditary amyloidosis due to variants of apolipoprotein A-I (apoA-I) with a substitution of a single amino acid is characterized by the presence of fibrillar protein within the lesions. Thus mutations result in increased protein aggregation. Here we set up to characterize the folding of a natural variant with a mutation leading to a deletion at position 107 (apoA-I Lys107-0). Patients carrying this variant show amyloidosis and severe atherosclerosis.

METHODS

We oxidized this variant under controlled concentrations of hydrogen peroxide and analyzed the structure obtained after 30-day incubation by fluorescence, circular dichroism and microscopy approaches. Neutrophils activation was characterized by confocal microscopy.

RESULTS

We obtained a high yield of well-defined stable fibrillar structures of apoA-I Lys107-0. In an in vitro neutrophils system, we were able to detect the induction of Neutrophils Extracellular Traps (NETs) when we incubated with oxidized apoA-I variants. This effect was exacerbated by the fibrillar structure of oxidized Lys 107-0.

CONCLUSIONS

We conclude that a pro-inflammatory microenvironment could result in the formation of aggregation-prone species, which, in addition may induce a positive feed-back in the activation of an inflammatory response.

GENERAL SIGNIFICANCE

These events may explain a close association between amyloidosis due to apoA-I Lys107-0 and atherosclerosis.

Binding of human apoA-I[K107del] variant to TG-rich particles: implications for mechanisms underlying hypertriglyceridemia

We found earlier that apoA-I variants that induced hypertriglyceridemia (HTG) in mice had increased affinity to TG-rich lipoproteins and thereby impaired their catabolism. Here, we tested whether a naturally occurring human apoA-I mutation, Lys107del, associated with HTG also promotes apoA-I binding to TG-rich particles. We expressed apoA-I[Lys107del] variant in Escherichia coli, studied its binding to TG-rich emulsion particles, and performed a physicochemical characterization of the protein. Compared with WT apoA-I, apoA-I[Lys107del] showed enhanced binding to TG-rich particles, lower stability, and greater exposure of hydrophobic surfaces. The crystal structure of truncated, Δ(185-243), apoA-I suggests that deletion of Lys107 disrupts helix registration and disturbs a stabilizing salt bridge network in the N-terminal helical bundle. To elucidate the structural changes responsible for the altered function of apoA-I[Lys107del], we studied another mutant, apoA-I [Lys107Ala]. Our findings suggest that the registry shift and ensuing disruption of the inter-helical salt bridges in apoA-I[Lys107del] result in destabilization of the helical bundle structure and greater exposure of hydrophobic surfaces. We conclude that the structural changes in the apoA-I[Lys107del] variant facilitate its binding to TG-rich lipoproteins and thus, may reduce their lipolysis and contribute to the development of HTG in carriers of the mutation.

The crystal structure of the C-terminal truncated apolipoprotein A-I sheds new light on amyloid formation by the N-terminal fragment

Apolipoprotein A-I (apoA-I) is the main protein of plasma high-density lipoproteins (HDL, or good cholesterol) that remove excess cell cholesterol and protect against atherosclerosis. In hereditary amyloidosis, mutations in apoA-I promote its proteolysis and the deposition of the 9-11 kDa N-terminal fragments as fibrils in vital organs such as kidney, liver, and heart, causing organ damage. All known amyloidogenic mutations in human apoA-I are clustered in two residue segments, 26-107 and 154-178. The X-ray crystal structure of the C-terminal truncated human protein, Δ(185-243)apoA-I, determined to 2.2 Å resolution by Mei and Atkinson, provides the structural basis for understanding apoA-I destabilization in amyloidosis. The sites of amyloidogenic mutations correspond to key positions within the largely helical four-segment bundle comprised of residues 1-120 and 144-184. Mutations in these positions disrupt the bundle structure and destabilize lipid-free apoA-I, thereby promoting its proteolysis. Moreover, many mutations place a hydrophilic or Pro group in the middle of the hydrophobic lipid-binding face of the amphipathic α-helices, which will likely shift the population distribution from HDL-bound to lipid-poor/free apoA-I that is relatively unstable and labile to proteolysis. Notably, the crystal structure shows segment L44-S55 in an extended conformation consistent with the β-strand-like geometry. Exposure of this segment upon destabilization of the four-segment bundle probably initiates the α-helix to β-sheet conversion in amyloidosis. In summary, we propose that the amyloidogenic mutations promote apoA-I proteolysis by destabilizing the protein structure not only in the lipid-free but also in the HDL-bound form, with segment L44-S55 providing a likely template for the cross-β-sheet conformation.

Formation of methionine sulfoxide-containing specific forms of oxidized high-density lipoproteins

Atherosclerosis is characterized by the accumulation of both lipoprotein-derived lipids and inflammatory cells in the affected vascular wall that results in a state of heightened oxidative stress and that is reflected by the accumulation of oxidized lipoproteins. Circulating oxidized low-density lipoprotein (oxLDL) is used as a surrogate marker for coronary artery disease, although the 'escape' of oxLDL from the vessel wall is hindered by the large size of this lipoprotein and its specific retention by the extracellular matrix. Also, the oxidation of lipoproteins in human atherosclerotic lesions is not limited to LDL. In fact, the lipids of all classes of lipoproteins are oxidized to a comparable extent. Examining the fate of lipid hydroperoxides, the primary lipid peroxidation products, in high-density lipoproteins (HDL) undergoing oxidation, revealed that they become reduced to the corresponding alcohols by specific Met residues of apolipoprotein A-I (apoA-I) and apoA-II. As a consequence, Met residues in apoA-I and apoA-II become selectively and consecutively oxidized to their respective Met sulfoxide (MetO) forms that can be separated by HPLC. This review describes the characterization of specifically oxidized HDL with an emphasis on MetO formation, the structural and functional consequences of such oxidation, and the potential utility of specifically oxidized HDL as a surrogate marker of atherosclerosis.

Characterization of specifically oxidized apolipoproteins in mildly oxidized high density lipoprotein

Atherosclerosis is a state of heightened oxidative stress. Oxidized LDL is present in atherosclerotic lesions and used as marker for coronary artery disease, although in human lesions lipids associated with HDL are as oxidized as those of LDL. Here we investigated specific changes occurring to apolipoprotein A-I (apoA-I) and apoA-II, as isolated HDL and human plasma undergo mild, chemically induced oxidation, or autoxidation. During such oxidation, Met residues in apoA-I and apoA-II become selectively and consecutively oxidized to their respective Met sulfoxide (MetO) forms that can be separated by HPLC. Placing plasma at -20 degrees C prevents autoxidation, whereas metal chelators and butylated hydroxytoluene offer partial protection. Independent of the oxidation conditions, apoA-I and apoA-II (dimer) with two MetO residues accumulate as relatively stable oxidation products. Compared to controls, serum samples from subjects with the endothelial cell nitric oxide synthase a/b genotype that is associated with increased coronary artery disease contain increased concentrations of apoA-I with two MetO residues. Our results show that during the early stages, oxidation of HDL gives rise to specifically oxidized forms of apoA-I and apoA-II, some of which may be useful markers of in vivo HDL oxidation, and hence potentially atherosclerosis.

A novel localized amyloidosis associated with lactoferrin in the cornea

We report a novel localized amyloidosis associated with lactoferrin. To elucidate the precursor protein of corneal amyloidosis associated with trichiasis, we analyzed amyloid deposits from three patients by histopathology and biochemistry. Amyloid deposits showed immunoreactivity, confirmed by electron microscopy, for only anti-human lactoferrin antibody. Electrophoresis of amyloid fibrils revealed lactoferrin with and without sugar chains; N-terminal sequence analysis revealed full-length lactoferrin and a truncated tripeptide of N-terminal amino acids, Gly-Arg-Arg. Carboxymethylated wild-type lactoferrin formed amyloid fibrils in vitro. Lactoferrin gene analysis in the three patients revealed a Glu561Asp mutation in all of the patients and a compound heterozygote of Ala11Thr and Glu561Asp mutations in one patient. A heterozygotic Glu561Asp mutation appeared in 44.8% of healthy Japanese volunteers, suggesting that the mutation may not be an essential mutation for amyloid formation (p = 0.104). Results thus suggest that lactoferrin is this precursor protein.

HDL and arteriosclerosis: beyond reverse cholesterol transport

The inverse correlation between serum levels of high density lipoprotein (HDL) cholesterol and the risk of coronary heart disease, the protection of susceptible animals from atherosclerosis by transgenic manipulation of HDL metabolism, and several potentially anti-atherogenic in vitro-properties have made HDL metabolism an interesting target for pharmacological intervention in atheroslcerosis. We have previously reviewed the concept of reverse cholesterol transport, which describes both the metabolism and the classic anti-atherogenic function of HDL (Arterioscler. Thromb. Vasc. Biol. 20 2001 13). We here summarize the current understanding of additional biological, potentially anti-atherogenic properties of HDL. HDL inhibits the chemotaxis of monocytes, the adhesion of leukocytes to the endothelium, endothelial dysfunction and apoptosis, LDL oxidation, complement activation, platelet activation and factor X activation but also stimulates the proliferation of endothelial cells and smooth muscle cells, the synthesis of prostacyclin and natriuretic peptide C in endothelial cells, and the activation of proteins C and S. These anti-inflammatory, anti-oxidative, anti-aggregatory, anti-coagulant, and pro-fibrinolytic activities are exerted by different components of HDL, namley apolipoproteins, enzymes, and even specific phospholipids. This complexity further emphasizes that changes in the functionality of HDL rather than changes of plasma HDL-cholesterol levels determine the anti-atherogenicity of therapeutic alterations of HDL metabolism.

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

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

Recombinant proapoA-I(Lys107del) shows impaired lipid binding associated with reduced binding to plasma high density lipoprotein

In the present study apoA-I (Lys 107del), a naturally occurring human apoA-I variant with a deletion of Lys 107, was expressed in E. coli to examine the effect of this mutation on lipid binding, cholesterol efflux and lecithin:cholesterol acyltranferase (LCAT) activation. Dimyristoyl phosphatidylcholine (DMPC) binding studies revealed slow interaction of proapoA-I(Lys107del) with DMPC relative to normal proapoA-I. After preincubation with human plasma lipoprotein (d<1.225 g/ml) for 1 h at 37 degrees C, 125I-labeled normal proapoA-I chromatographed as a single peak with the high density lipoprotein (HDL) fraction, whereas 125I-labeled proapoA-I(Lys107del) chromatographed with both HDL and free proapoA-I (26% of the radioactivity). Circular dichroism measurements showed that the alpha-helical content of lipid-bound proapoA-I (Lys107del) was reduced to 64 versus 73% of normal proapoA-I. Non-denaturing gradient gel electrophoresis of reconstituted HDL assembled with either proapoA-I(Lys107del) or normal proapoA-I showed that the mutation led to the formation of a second population of smaller rHDL particles. DMPC/proapoA-I(Lys107del) and normal DMPC/proapoA-I complexes exhibited a similar capacity to promote cholesterol efflux from fibroblasts. ProapoA-I (Lys107del) also activated LCAT similar to wild type proapoA-I and human plasma apoA-I. We conclude that deletion of Lys 107 substantially alters the lipid binding properties of the protein, which correlated with reduced binding to plasma HDL in vitro, but did not affect the capacity of the mutant/lipid complex to promote cholesterol efflux or activate LCAT.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.

Oxidation of methionine residues to methionine sulfoxides does not decrease potential antiatherogenic properties of apolipoprotein A-I

The initial stage of oxidation of high density lipoproteins (HDL) is accompanied by the lipid hydroperoxide-dependent, selective oxidation of two of the three Met residues of apolipoprotein A-I (apoA-I) to Met sulfoxides (Met(O)). Formation of such selectively oxidized apoA-I (i.e. apoA-I(+32)) may affect the antiatherogenic properties of HDL, because it has been suggested that Met(86) and Met(112) are important for cholesterol efflux and Met(148) is involved in the activation of lecithin:cholesterol acyl transferase (LCAT). We therefore determined which Met residues were oxidized in apoA-I(+32) and how such oxidation of apoA-I affects its secondary structure, the affinity for lipids, and its ability to remove lipids from human macrophages. We also assessed the capacity of discoidal reconstituted HDL containing apoA-I(+32) to act as substrate for LCAT, and the dissociation of apoA-I and apoA-I(+32) from reconstituted HDL. Met(86) and Met(112) were present as Met(O), as determined by amino acid sequencing and mass spectrometry of isolated peptides derived from apoA-I(+32). Selective oxidation did not alter the alpha-helicity of lipid-free and lipid-associated apoA-I as assessed by circular dichroism, and the affinity for LCAT was comparable for reconstituted HDL containing apoA-I or apoA-I(+32). Cholesteryl ester transfer protein mediated the dissociation of apoA-I more readily from reconstituted HDL containing apoA-I(+32) than unoxidized apoA-I. Also, compared with native apoA-I, apoA-I(+32) had a 2- to 3-fold greater affinity for lipid (as determined by the rate of clearance of multilamellar phospholipid vesicles) and its ability to cause efflux of [(3)H]cholesterol, [(3)H]phospholipid, and [(14)C]alpha-tocopherol from lipid-laden human monocyte-derived macrophages was significantly enhanced. By contrast, no difference was observed for cholesterol and alpha-tocopherol efflux to lipid-associated apolipoproteins. Together, these results suggest that selective oxidation of Met residues enhances rather than diminishes known antiatherogenic activities of apoA-I, consistent with the overall hypothesis that detoxification of lipid hydroperoxides by HDL is potentially antiatherogenic.

In vivo kinetics of human apolipoprotein A-I variants in rabbits

BACKGROUND

Genetic variants of human apolipoprotein (apo) A-I, the major protein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plasma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apolipoprotein is a straightforward technique for determining its metabolism in vivo. In this study, we investigated the in vivo kinetics of several human apo A-I variants, which we had identified previously, in rabbits.

MATERIALS AND METHODS

Apo A-I variants from heterozygous carriers of Lys-107-->0, Lys-107-->Met, Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys and the corresponding normal apo A-I were purified and then radioiodinated with 131I and 125I. A kinetic study of apo A-I variants was performed in normolipidaemic rabbits after simultaneous injection of the two isotopes that had been incorporated into HDL. The fractional catabolic rate (FCR) was calculated from the radioactive decay curve.

RESULTS

Acidic mature (negatively charged) apo A-I variants caused by a single amino acid substitution (Lys-107-->0, and Lys-107-->Met) were catabolized faster (FCR, 1.931 +/- 0.539 per day vs. 1.636 +/- 0.460 per day, P </= 0.01 using the Wilcoxon signed-rank test) and basic mature (positively charged) apo A-I variants (Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys) were catabolized more slowly (FCR 1.470 +/- 0.380 per day vs. 1.654 +/- 0.430 per day, P </= 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an inverse linear relationship was observed between the deviation in the FCR of variant human apo A-I from that of normal human apo A-I and the number of electric charges that the apo A-I variant carried (r = -0. 90, k = -0.188, P = 0.0003), as assessed by a linear regression analysis, suggesting that the electric charge of apo A-I variants may determine, at least in part, its in vivo kinetics in rabbits.

CONCLUSIONS

Genetic variants of apo A-I with a single amino acid substitution show abnormal kinetics, and the electric charge of a apo A-I variant could contribute to determining its kinetics in vivo in this xenologous model.

The carboxyl-terminal hydrophobic residues of apolipoprotein A-I affect its rate of phospholipid binding and its association with high density lipoprotein

We performed a series of mutations in the human apolipoprotein A-I (apoA-I) gene designed to alter specific amino acid residues and domains implicated in lecithin:cholesterol acyltransferase (LCAT) activation or lipid binding. We used the mutant apoA-I forms to establish nine stable cell lines, and developed strategies for the large scale production and purification of the mutated apoA-I proteins from conditioned media. HDL and dimyristoyl phosphatidylcholine binding assays using the variant apoA-I forms have shown that replacement of specific carboxyl-terminal hydrophobic residues Leu222, Phe225, and Phe229 with lysines, as well as replacement of Leu211, Leu214, Leu218, and Leu219 with valines, diminished the ability of apoA-I to bind to HDL and to lyse dimyristoyl phosphatidylcholine liposomes. The findings indicate that Leu222, and Phe225, Phe229 located in the putative random coil region, and Leu211, Leu214, Leu218, and Leu219 located in the putative helix 8, are important for lipid binding. In contrast, substitutions of alanines for specific charged residues in putative helices 7, 8, or 9 as well as various point mutations in other regions of apoA-I, did not affect the ability of the variant apoA-I forms to bind to HDL or to lyse dimyristoyl phosphatidylcholine liposomes. Cross-linking experiments confirmed that the carboxyl-terminal domain of apoA-I participates in the self-association of the protein, as demonstrated by the inability of the carboxyl-terminal deletion mutants delta185-243 and delta209-243 to form higher order aggregates in solution. Lecithin:cholesterol acyltransferase analysis, using reconstituted HDL particles prepared by the sodium cholate dialysis method, has shown that mutants (Pro165-->Ala,Gln172-->Glu) (Leu211-->Val,Leu214-->Val, Leu218-->Val,Leu219-->Val), Leu222-->Lys,Phe225-->Lys, Phe229-->Lys) and delta209-243 reduced LCAT activation (38-68%). Mutant (Glu191-->Ala,His193-->Ala,Lys195-->Ala) enhanced LCAT activation (131%), and mutant (Ala152-->Leu, Leu159-->Trp) exhibited normal LCAT activation as compared with the wild type proapoA-I and plasma apoA-I forms [corrected]. The apparent catalytic efficiency (Vmax(app)/Km(app)) of the apoA-I mutants ranged from 17.8 to 107.2% of the control and was the result of variations in both the Km and the Vmax in the different mutants. These findings indicate that putative helices 6 and 7, and the carboxyl-terminal helices 8 and 9 contribute to the optimum activation of lecithin:cholesterol acyltransferase. In addition to their use in the present study, the variant apoA-I forms generated will serve as valuable reagents for the identification of the domains and residues of apoA-I involved in binding the scavenger receptor BI, and facilitating cholesterol efflux from cells as well as aid in the structural analysis of apoA-I.

Sphingomyelin inhibits the lecithin-cholesterol acyltransferase reaction with reconstituted high density lipoproteins by decreasing enzyme binding

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the formation of cholesterol esters on high density lipoproteins (HDL) and plays a critical role in reverse cholesterol transport. Sphingomyelin, an important constituent of HDL, may regulate the activity of LCAT at any of the key steps of the enzymatic reaction: binding of LCAT to the interface, activation by apo A-I, or inhibition at the catalytic site. In order to clarify the role of sphingomyelin in the regulation of the LCAT reaction and its effects on the structure of apolipoprotein A-I, we prepared reconstituted HDL (rHDL) containing egg phosphatidylcholine, cholesterol, apolipoprotein A-I, and up to 22 mol % sphingomyelin. Because the interfacial properties of substrate particles can dramatically affect LCAT binding and kinetics, we also prepared and analyzed proteoliposome substrates having the same components as the rHDL, except for a 4-fold higher ratio of phospholipid to apolipoprotein A-I. The reaction kinetics of LCAT with the rHDL particles revealed no significant change in the apparent Vmax but showed a concentration-dependent increase in slope of the reciprocal plots and in the apparent Km values with sphingomyelin content. The dissociation constant (Kd) for LCAT with these particles increased linearly with sphingomyelin content up to 22 mol %, changing in parallel with the apparent Km values. No structural changes of apolipoprotein A-I were detected in the particles with increasing content of sphingomyelin, but fluorescence results with lipophilic probes revealed that significant changes in the acyl chain, backbone, and head group regions of the lipid bilayer of the particles are introduced by the addition of sphingomyelin. On the other hand, the proteoliposome substrates also had increased Kdvalues for LCAT at high sphingomyelin contents but compared with the rHDL particles had a 6-10-fold lower affinity for LCAT binding and exhibited kinetics consistent with competitive inhibition by sphingomyelin at the active site. These results show conclusively that the dominant mechanism for the inhibition of LCAT activity with rHDL particles by sphingomyelin is the impaired binding of the enzyme to the interface. The results also underscore the significant differences in the enzyme reaction kinetics with different substrate particles.

Only the two end helixes of eight tandem amphipathic helical domains of human apo A-I have significant lipid affinity. Implications for HDL assembly

Human apolipoprotein A-I (apo A-I) possesses multiple tandem repeating 22-mer amphipathic alpha-helixes. Computer analysis and studies of model synthetic peptides and recombinant protein-lipid complexes of phospholipids have suggested that apo A-I interacts with HDL surface lipids through cooperation among its individual amphipathic helical domains. To delineate the overall lipid-associating properties of apo A-I, the first step is to understand the lipid-associating properties of individual amphipathic helical domains. To this end, we synthesized and studied each of the eight tandem repeating 22-mer domains of apo A-I: residues 44-65, 66-87, 99-120, 121-142, 143-164, 165-186, 187-208, and 220-241. Among the 22-mers, only the N- and C-terminal peptides (44-65 and 220-241) were effective in clarifying multilamellar vesicles (MLVs) of dimyristoylphosphatidylcholine (DMPC). These two peptides also exhibited the highest partition coefficient into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine liposomes, the highest exclusion pressure for penetration into an egg yolk phosphatidylcholine monolayer, and the greatest reduction in the enthalpy of the gel-to-liquid crystalline phase transition of DMPC MLVs. These results suggest that the strong, lipid-associating properties of apo A-I are localized to the N- and C-terminal amphipathic domains. Although each of the eight peptides studied has an amphipathic structure, models based on changes in residual effective amino acid hydrophobicity resulting from differing depths of helix penetration into the lipid are best able to explain the high lipid affinity possessed by the two terminal domains. Differential scanning calorimetry (DSC) studies showed that on a molar basis, apo A-I is about 10 times more effective than the most effective peptide analyzed in reducing the enthalpy of the gel-to-liquid crystalline phase transition of DMPC MLVs. Because previous proteolysis experiments coupled with the present DSC results suggest that the lipid-associating domains of apo A-I are distributed throughout the length of the 243 amino acid residues, we propose that the terminal amphipathic helical domains are involved in the initial binding of apo A-I to the lipid surface to form HDL particles, followed by cooperative binding of the middle six amphipathic helical domains, perhaps aided by salt-bridge formation between adjacent helixes arranged in an antiparallel orientation.

Apolipoprotein A-I conformation in reconstituted discoidal lipoproteins varying in phospholipid and cholesterol content

The effects of the size and cholesterol content on the conformation of apolipoprotein A-I (apoA-I) have been studied in reconstituted discoidal lipoproteins containing two apoA-I per particle (Lp2A-I). The immunoreactivity of a series of 13 epitopes distributed along the apoA-I sequence has been evaluated in Lp2A-I with a phospholipid/apoA-I molar ratio ranging from 31 to 156 and in Lp2A-I with constant phospholipids but varying in cholesterol content from 0 to 22 molecules. The results are compatible with a three domain structure in apoA-I in which the central domain is located between residues 99 and 143 and postulated to be a hinged domain that responds differentially to changes in phospholipid and cholesterol contents. Increasing the phospholipid content results in significant changes of epitope immunoreactivity throughout the N-terminal and central domains of apoA-I with fewer modifications in the C-terminal domain. In contrast, increasing Lp2A-I of two central epitopes, A11 (residues 99-132) and 5F6 (residues 118-148), and an extreme N-terminal epitope, 4H1 (residues 2-8). Interestingly, the effects of increasing cholesterol or phospholipids on these epitopes are opposite. This suggests a specific effect of cholesterol on the central domain tertiary structure between residues 99 and 143. Competition binding assays among pairs of antibodies binding to apoA-I on Lp2A-I are best explained by invoking inter- as well as intramolecular competitions. The specificity of the intermolecular competitions suggests an N to C termini arrangement of the two apoA-I molecules around the disc. Increasing the phospholipid content of Lp2A-I mainly increases the competitions between 3G10 and antibodies binding to most adjacent epitopes. Simultaneously as Lp2A-I enlarges, several of these antibodies also enhance the binding of 3G10. This has been interpreted as evidence of a structural rearrangement of apoA-I as a result of the size increase where the alpha-helix (residues 99-121) that contains the 3G10 epitope is increasingly interacting with lipids resulting in the enhanced expression of this epitope. The increasing interactions of apoA-I helices with lipids in the enlarging disc are compatible with previous reports of a greater apoA-I stability in the large discs. By contrast, cholesterol has limited but specific effects on antibody competitions and decreases the interaction of the N-terminal domain with the domain containing 3G10, either by direct cholesterol protein interaction or by modification of the lipid phase packing.

Screening for naturally occurring apolipoprotein A-I variants: apo A-I(delta K107) is associated with low HDL-cholesterol levels in men but not in women

Isoelectric focussing (IEF) in carrier ampholyte-generated pH gradients and hybrid isoelectric focussing (HIEF) in immobilized pH gradients under nondenaturing conditions were used in parallel to screen 5,500 plasma samples for naturally occurring variants of apolipoprotein A-I (apo A-I). The following defects were identified in four unrelated subjects heterozygous for apo A-I variants: apo A-I(delta K107)(2 x), apo A-I(K107M)(1 x), and apo A-I(E41R)(1 x). The later variant is a novel finding. Family studies did not reveal any association of apo A-I(K107M) and apo A-I(E41R) with dyslipidemia, but identified several heterozygotes for apo A-I(delta K107) who had low levels of high density lipoprotein (HDL)-cholesterol. Therefore, and since the apo A-I(delta K107) is the most frequent apo A-I variant in Germany (1: 5,000) we evaluated our data and that reported from 11 families with 32 heterozygous carriers and 30 unaffected controls. This analysis revealed that apo A-I(delta K107) is associated with lower HDL-cholesterol (-30%) and higher triglycerides (+48%) in men but not in women as compared with unaffected family members as well as with controls from the Prospective Cardiovascular Münster (PROCAM) study. Moreover, 11 of 15 male apo A-I(delta K107) heterozygotes but only 2 of 17 female apo A-I(delta K107) heterozygotes had HDL-cholesterol levels below the 20th percentile of sex-matched controls from the PROCAM study. We conclude that heterozygosity for apo A-I(delta K107) decreases HDL-cholesterol and increases triglycerides in men but not in women.

Mannitol prevents methionine sulphoxidation mediated electrophoretic heterogeneity of apolipoprotein A-I

Hybrid isoelectric focusing of apolipoprotein A-I in polyacrylamide gels with immobilized pH-gradients under non-denaturing conditions resulted in the occurrence of additional bands which could prevent the specific and sensitive detection of genetic variants. Hybrid isoelectric focusing of two chromatographically distinguishable apolipoprotein A-I isoforms that differ by sulphoxidation of methionine residues, apo A-I(Met) and apo A-I(MetSO), revealed that the additional bands were caused by this post-translational modification. Several antioxidative additives and conditions were compared for their ability to prevent methionine sulphoxidation in apolipoprotein A-I. In the presence of 200 g/L mannitol in the gel, apolipoprotein A/I focused as a single band. Since methionine sulphoxidation in proteins is a general phenomenon either taking place in vivo or in vitro by isoelectric focusing, we conclude that isoelectric focusing in the presence of mannitol will improve the quality of resolution of many proteins in gels with immobilized pH-gradients.