Nascent high density lipoproteins formed by ABCA1 resemble lipid rafts and are structurally organized by three apoA-I monomers

This report details the lipid composition of nascent HDL (nHDL) particles formed by the action of the ATP binding cassette transporter A1 (ABCA1) on apolipoprotein A-I (apoA-I). nHDL particles of different size (average diameters of ～ 12, 10, 7.5, and <6 nm) and composition were purified by size-exclusion chromatography. Electron microscopy suggested that the nHDL were mostly spheroidal. The proportions of the principal nHDL lipids, free cholesterol, glycerophosphocholine, and sphingomyelin were similar to that of lipid rafts, suggesting that the lipid originated from a raft-like region of the cell. Smaller amounts of glucosylceramides, cholesteryl esters, and other glycerophospholipid classes were also present. The largest particles, ～ 12 nm and 10 nm diameter, contained ～ 43% free cholesterol, 2-3% cholesteryl ester, and three apoA-I molecules. Using chemical cross-linking chemistry combined with mass spectrometry, we found that three molecules of apoA-I in the ～ 9-14 nm nHDL adopted a belt-like conformation. The smaller (7.5 nm diameter) spheroidal nHDL particles carried 30% free cholesterol and two molecules of apoA-I in a twisted, antiparallel, double-belt conformation. Overall, these new data offer fresh insights into the biogenesis and structural constraints involved in forming nascent HDL from ABCA1.     

Ratio determination of plasma wild-type and L159R apoA-I using mass spectrometry: tools for studying apoA-IFin

In this report, methods are described to isolate milligram quantities of a mutant apolipoprotein A-I (apoA-I) protein for use in structure-function studies. Expression of the L159R apoA-I mutation in humans reduces the concentration of plasma wild-type apoA-I, thus displaying a dominant negative phenotype in vivo. Earlier attempts to express and isolate this mutant protein resulted in extensive degradation and protein misfolding. Using an Escherichia coli expression system used predominantly for the isolation of soluble apoA-I mutant proteins, we describe the expression and purification of L159R apoA-I (apoA-I(Fin)) from inclusion bodies. In addition, we describe a mass spectrometric method for measuring the L159R-to-wild-type apoA-I ratio in a 1 microl plasma sample. These new methods will facilitate further studies into the mechanism behind the dominant negative phenotype associated with the expression of the L159R apoA-I protein in humans.     

Control of cholesteryl ester transfer protein activity by sequestration of lipid transfer inhibitor protein in an inactive complex

Lipid transfer inhibitor protein (LTIP) is a physiologic regulator of cholesteryl ester transfer protein (CETP) function. We previously reported that LTIP activity is localized to LDL, consistent with its greater inhibitory activity on this lipoprotein. With a recently described immunoassay for LTIP, we investigated whether LTIP mass is similarly distributed. Plasma fractionated by gel filtration chromatography revealed two LTIP protein peaks, one coeluting with LDL, and another of approximately 470 kDa. The 470 kDa LTIP complex had a density of 1.134 g/ml, indicating approximately 50% lipid content, and contained apolipoprotein A-I. By mass spectrometry, partially purified 470 kDa LTIP also contains apolipoproteins C-II, D, E, J, and paraoxonase 1. Unlike LDL-associated LTIP, the 470 kDa LTIP complex does not inhibit CETP activity. In normolipidemic subjects, approximately 25% of LTIP is in the LDL-associated, active form. In hypercholesterolemia,this increases to 50%, suggesting that lipoprotein composition may influence the status of LTIP activity. Incubation (37 degrees C) of normolipidemic plasma increased active, LDL-associated LTIP up to 3-fold at the expense of the inactive pool. Paraoxon inhibited this shift by 50%. Overall, these studies show that LTIP activity is controlled by its reversible incorporation into an inactive complex. This may provide for short-term fine-tuning of lipoprotein remodeling mediated by CETP.     

The formation of phosphatidylcholine oxidation products by stimulated phagocytes

Phagocytic cells produce a variety of oxidants as part of the immune defence, which react readily both with proteins and lipids, and could contribute to the oxidation of low density lipoprotein in atherosclerosis. We have investigated the oxidation of phospholipid vesicles by neutrophils and mononuclear cells, to provide a model of lipid oxidation in the absence of competing protein. Phorbol 12-myristate 13-acetate-stimulated neutrophils were incubated with phospholipid vesicles containing dipalmitoyl phosphatidylcholine, palmitoyl-arachidonoyl phosphatidylcholine (PAPC) and stearoyl-oleoyl phosphatidylcholine, before extraction of the lipids for analysis by HPLC coupled to electrospray mass spectrometry. The formation of monohydroperoxides (814 m/z) and bishydroperoxides (846 m/z) of PAPC was observed. However, the major oxidized product occurred at 828 m/z, and was identified as 1-palmitoyl-2-(5,6-epoxyisoprostane E₂)-sn-glycero-3-phosphocholine. These products were also formed in incubations where the neutrophils were replaced by mononuclear cells, and the amounts produced per million cells were similar. These results show that following oxidative attack by phagocytes stimulated by PMA, intact phospholipid oxidation products can be detected. The identification of an epoxyisoprostane phospholipid as the major product of phagocyte-induced phospholipid oxidation is novel, and in view of its inflammatory properties has implications for phagocyte involvement in atherogenesis.     

Three-dimensional models of HDL apoA-I: implications for its assembly and function

The purpose of this review is to highlight recent advances toward the refinement of a three-dimensional structure for lipid-bound apolipoprotein A-I (apoA-I) on recombinant HDL. Recently, X-ray crystallography has yielded a new structure for full-length, lipid-free apoA-I. Although this approach has not yet been successful in solving the three-dimensional structure of lipid-bound apoA-I, analysis of the X-ray structures has been of immense help in the interpretation of structural data obtained from other methods that yield structural information. Recent studies emphasize the use of mass spectrometry to unambiguously identify cross-linked peptides or to quantify solvent accessibility using hydrogen-deuterium exchange. The combination of mass spectrometry, molecular modeling, molecular dynamic analysis, and small-angle X-ray diffraction has provided additional structural information on apoA-I folding that complements previous approaches.     

Identification of the nitration site of insulin by peroxynitrite.

Our previous investigation indicated that insulin can be nitrated by peroxynitrite in vitro. In this study, the preferential nitration site of the four tyrosine residues in insulin molecule was confirmed. Mononitrated and dinitrated insulins were purified by RP-HPLC. Following reduction of insulin disulfide bridges, Native-PAGE indicated that A-chain was preferentially nitrated. Combination of enzymatic digestion of mononitrated insulin with endoproteinase Glu-C, mass spectrometry confirmed that Tyr-A14 was the preferential nitration site when insulin was treated with peroxynitrite. Tyr-A19, maybe, was the next preferential nitration site. According to the crystal structure, Tyr-B26 between the two tyrosine residues in insulin B-chain was likely easier to be nitrated by peroxynitrite.     

Selective association of protein molecules followed by mass spectrometry.

Nanoflow electrospray mass spectrometry was used to monitor the formation of protein heterodimers of HU proteins from Bacillus stearothermophilus and Bacillus subtilis. This has enabled us to analyze both thermodynamic and kinetic features associated with the dissociation of homodimeric HU proteins. The results obtained correlate well with the kinetics of the protein dissociation process and the free energy difference between homo- and heterodimeric species anticipated from other studies. We suggest that this approach will have general applicability in studying protein association and dissociation under near-equilibrium conditions and will be relevant to a wide range of biological systems.     

Structural modification of plasma HDL by phospholipids promotes efficient ABCA1-mediated cholesterol release

It has been suggested that ABCA1 interacts preferentially with lipid-poor apolipoprotein A-I (apoA-I). Here, we show that treatment of plasma with dimyristoyl phosphatidylcholine (DMPC) multilamellar vesicles generates prebeta(1)-apoA-I-containing lipoproteins (LpA-I)-like particles similar to those of native plasma. Isolated prebeta(1)-LpA-I-like particles inhibited the binding of (125)I-apoA-I to ABCA1 more efficiently than HDL(3) (IC(50) = 2.20 +/- 0.35 vs. 37.60 +/- 4.78 microg/ml). We next investigated the ability of DMPC-treated plasma to promote phospholipid and unesterified (free) cholesterol efflux from J774 macrophages stimulated or not with cAMP. At 2 mg DMPC/ml plasma, both phospholipid and free cholesterol efflux were increased ( approximately 50% and 40%, respectively) in cAMP-stimulated cells compared with unstimulated cells. Similarly, both phospholipid and free cholesterol efflux to either isolated native prebeta(1)-LpA-I and prebeta(1)-LpA-I-like particles were increased significantly in stimulated cells. Furthermore, glyburide significantly inhibited phospholipid and free cholesterol efflux to DMPC-treated plasma. Removal of apoA-I-containing lipoproteins from normolipidemic plasma drastically reduced free cholesterol efflux mediated by DMPC-treated plasma. Finally, treatment of Tangier disease plasma with DMPC affected the amount of neither prebeta(1)-LpA-I nor free cholesterol efflux. These results indicate that DMPC enrichment of normal plasma resulted in the redistribution of apoA-I from alpha-HDL to prebeta-HDL, allowing for more efficient ABCA1-mediated cellular lipid release. Increasing the plasma prebeta(1)-LpA-I level by either pharmacological agents or direct infusions might prevent foam cell formation and reduce atherosclerotic vascular disease.     

Metabolomic analysis and identification of a role for the orphan human cytochrome P450 2W1 in selective oxidation of lysophospholipids

Human cytochrome P450 (P450) 2W1 is still considered an "orphan" because its physiological function is not characterized. To identify its substrate specificity, the purified recombinant enzyme was incubated with colorectal cancer extracts for untargeted substrate searches using an LC/MS-based metabolomic and isotopic labeling approach. In addition to previously reported fatty acids, oleyl (18:1) lysophosphatidylcholine (LPC, lysolecithin) was identified as a substrate for P450 2W1. Other human P450 enzymes tested showed little activity with 18:1 LPC. In addition to the LPCs, P450 2W1 acted on a series of other lysophospholipids, including lysophosphatidylinositol, lysophosphatidylserine, lysophosphatidylglycerol, lysophosphatidylethanolamine, and lysophosphatidic acid but not diacylphospholipids. P450 2W1 utilized sn-1 18:1 LPC as a substrate much more efficiently than the sn-2 isomer; we conclude that the sn-1 isomers of lysophospholipids are preferred substrates. Chiral analysis was performed on the 18:1 epoxidation products and showed enantio-selectivity for formation of (9R,10S) over (9R,10S). The kinetics and position specificities of P450 2W1-catalyzed oxygenation of lysophospholipids (16:0 LPC and 18:1 LPC) and fatty acids (C16:0 and C18:1) were also determined. Epoxidation and hydroxylation of 18:1 LPC are considerably more efficient than for the C18:1 free fatty acid.     

Analysis of bisdesmosidic saponins in Saponaria vaccaria L. by HPLC-PAD-MS: identification of new quillaic acid and gypsogenin 3-O-trisaccharides

A high-performance liquid chromatographic method using photodiode array and single quadrupole electrospray mass detection for analysis and profiling of bisdesmosidic saponins in Saponaria vaccaria seed was developed. Profiles of seed extract from three different plant sources were obtained and found to contain the same saponins, albeit in different proportions. Several known saponins were identified by selected ion extraction of quasi-molecular ions from the total ion chromatogram and confirmed by their mass spectra. Application of high cone voltages afforded mass spectra containing key diagnostic fragments and relatively strong singly charged quasi-molecular ions. In addition to previously identified saponins, several new quillaic acid and gypsogenin bisdesmosides could be detected via mass spectral analysis. Five of these were tentatively identified as pentose homologues of known saponins, having an added xylosyl residue linked to the 3-O-glucuronyl group (1 --> 3). The stereochemistry and identity of the xylosyl linkage in the new saponins was determined by chemical means. Previously reported vaccaric or segetalic acid-type bisdesmosides could not be detected in any of the extracts.     

Characterization and direct quantitation of sphingoid base-1-phosphates from lipid extracts: a shotgun lipidomics approach

Here, we have extended shotgun lipidomics for the characterization and quantitation of sphingosine-1-phosphate (S1P) and dihydrosphingosine-1-phosphate (DHS1P) in crude lipid extracts in the presence of ammonium hydroxide by using precursor ion scanning of m/z 79.0 (corresponding to [PO(3)](-)) in the negative-ion mode. It is demonstrated that a broad linear dynamic range for the quantitation of both S1P and DHS1P and a detection limit at low amol/mul concentration are achieved using this approach. The developed method for the quantitation of sphingoid base-1-phosphates is generally simpler and more efficient than other previously published methods. Multiple factors influencing the quantitation of sphingoid base-1-phosphates, including ion suppression, extraction efficiency, and potential overlapping with other molecular species, were examined extensively and/or are discussed. Mass levels of S1P and DHS1P in multiple biological samples, including human plasma, mouse plasma, and mouse brain tissues (e.g., cortex, cerebellum, spinal cord, and brain stem), were determined by the developed methodology. Accordingly, this technique, as a new addition to shotgun lipidomics technology, will be extremely useful for understanding the pathways of sphingolipid metabolism and for exploring the important roles of sphingoid base-1-phosphates in a wide range of physiological and pathological studies.     

Heme catalyzes tyrosine 385 nitration and inactivation of prostaglandin H2 synthase-1 by peroxynitrite

The mechanism by which the inflammatory enzyme prostaglandin H(2) synthase-1 (PGHS-1) deactivates remains undefined. This study aimed to determine the stabilizing parameters of PGHS-1 and identify factors leading to deactivation by nitric oxide species (NO(x)). Purified PGHS-1 was stabilized when solubilized in beta-octylglucoside (rather than Tween-20 or CHAPS) and when reconstituted with hemin chloride (rather than hematin). Peroxynitrite (ONOO(-)) activated the peroxidase site of PGHS-1 independently of the cyclooxygenase site. After ONOO(-) exposure, holoPGHS-1 could not metabolize arachidonic acid and was structurally compromised, whereas apoPGHS-1 retained full activity once reconstituted with heme. After incubation of holoPGHS-1 with ONOO(-), heme absorbance was diminished but to a lesser extent than the loss in enzymatic function, suggesting the contribution of more than one process to enzyme inactivation. Hydroperoxide scavengers improved enzyme activity, whereas hydroxyl radical scavengers provided no protection from the effects of ONOO(-). Mass spectral analyses revealed that tyrosine 385 (Tyr 385) is a target for nitration by ONOO(-) only when heme is present. Multimer formation was also observed and required heme but could be attenuated by arachidonic acid substrate. We conclude that the heme plays a role in catalyzing Tyr 385 nitration by ONOO(-) and the demise of PGHS-1.     

Modeling and optimization of phospholipase A1‐catalyzed hydrolysis of phosphatidylcholine using response surface methodology for lysophosphatidylcholine production

Modeling the phospholipase A₁ (PLA₁)‐catalyzed partial hydrolysis of soy phosphatidylcholine (PC) in hexane for the production of lysophosphatidylcholine (LPC) and optimizing the reaction conditions using response surface methodology were described. The reaction was performed with 4 g of PC in a stirred batch reactor using a commercial PLA₁ (Lecitase Ultra) as the biocatalyst. The effects of temperature, reaction time, water content, and enzyme loading on LPC and glycerylphosphorylcholine (GPC) content in the reaction products were elucidated using the models established. Optimal reaction conditions for maximizing the LPC content while suppressing acyl migration, which causes GPC formation, were as follows: temperature, 60°C; reaction time, 3 h; water content, 10% of PC; and enzyme loading, 1% of PC. When the reaction was conducted with 40 g of PC under these conditions, the reaction products contained 83.7 mol % LPC and were free of GPC. LPC had a higher total unsaturated fatty acid content than original PC had and was mainly composed of linoleic acid (78.0 mol % of the total fatty acids). © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:35–41, 2015     

Affinities of Shiga toxins 1 and 2 for univalent and oligovalent Pk-trisaccharide analogs measured by electrospray ionization mass spectrometry

The binding stoichiometry and affinities of the Shiga toxins, Stx1 and Stx2, for a series of uni- and oligovalent analogs of the Pk-trisaccharide were measured using the direct electrospray ionization mass spectrometry (ES-MS) assay. Importantly, it is shown that, for a given ligand, Stx1 and Stx2 exhibit similar affinities. The binding data suggest a high degree of similarity in the spatial arrangement and structural characteristics of the Pk binding sites in Stx1 and Stx2. The results confirm that both toxins recognize the alpha-D-Galp(1-->4)-beta-D-Galp(1-->4)-beta-D-Glcp carbohydrate motif of the cell surface glycolipid Gb3. This, taken together with the results of the chemical mapping study, suggests that the nature of the Pk binding interactions with Stx1 and Stx2 are similar. The affinities of Stx1-B(5) and Stx2 for the multivalent ligands reveals that site 2 of Stx2, which shares the same spatial arrangement as site 2 in Stx1, is the primary Pk binding site and that site 1 of Stx1 and of Stx2 can also participate in Pk binding.     

Multiple equilibria of the Escherichia coli chaperonin GroES revealed by mass spectrometry

Nanospray time-of-flight mass spectrometry has been used to study the assembly of the heptamer of the Escherichia coli cochaperonin protein GroES, a system previously described as a monomer-heptamer equilibrium. In addition to the monomers and heptamers, we have found measurable amounts of dimers and hexamers, the presence of which suggests the following mechanism for heptamer assembly: 2 Monomers <--> Dimer; 3 Dimers <--> Hexamer; Hexamer + Monomer <--> Heptamer. Equilibrium constants for each of these steps, and an overall constant for the Monomer <--> Heptamer equilibrium, have been estimated from the data. These constants imply a standard free-energy change, DeltaG(0), of about 9 kcal/mol for each contact surface formed between GroES subunits, except for the addition of the last subunit, where DeltaG(0) = 6 kcal/mol. This lower value probably reflects the loss of entropy when the heptamer ring is formed. These experiments illustrate the advantages of electrospray mass spectrometry as a method of measuring all components of a multiple equilibrium system.     

Apolipoprotein A-I promotes the formation of phosphatidylcholine core aldehydes that are hydrolyzed by paraoxonase (PON-1) during high density lipoprotein oxidation with a peroxynitrite donor

High density lipoprotein (HDL) is rich in polyunsaturated phospholipids that are sensitive to oxidation. However, the effect of apolipoprotein A-I and paraoxonase-1 (PON-1) on phosphatidylcholine oxidation products has not been identified. We subjected native HDL, trypsinized HDL, and HDL lipid suspensions to oxidation by the peroxynitrite donor, 3-morpholinosydnonimine. HDL had a basal level of phosphatidylcholine mono- and di-hydroperoxides that increased to a greater extent in HDL, compared with either trypsinized HDL or HDL lipid alone. Phosphatidylcholine core aldehydes, which were present in small amounts, increased 10-fold during oxidation of native HDL, compared with trypsinized HDL (p = 0.004), and 4-fold compared with HDL lipid suspensions (p = 0.0021). In addition, the content of lysophosphatidylcholine increased 300% during oxidation of native HDL, but only 80 and 25%, respectively, during oxidation of trypsinized HDL and HDL lipid suspensions. Phosphatidylcholine isoprostanes accumulated in comparable amounts during the oxidation of all three preparations. Incubation of apolipoprotein A-I with 1-palmitoyl-2-linoleoyl glycerophosphocholine proteoliposomes in the presence of 3-morpholinosydnonimine or apoAI with phosphatidylcholine hydroperoxides resulted in a significant increase in phosphatidylcholine core aldehydes with no formation of lysophosphatidylcholine. We propose that apolipoprotein A-I catalyzes a one-electron oxidation of alkoxyl radicals. Purified PON-1 hydrolyzed phosphatidylcholine core aldehydes to lysophosphatidylcholine. We conclude that, upon HDL oxidation with peroxynitrite, apolipoprotein AI increases the formation of phosphatidylcholine core aldehydes that are subsequently hydrolyzed by PON1.     

Characterization of monomeric and dimeric forms of recombinant Sml1p-histag protein by electrospray mass spectrometry.

Sml1p is small protein that binds to and inhibits the activity of ribonucleotide reductase (RNR)3, a protein enzyme complex that controls the balance and level of the cellular deoxynucleotide diphosphate pools that are critical for DNA synthesis and repair. In this respect, Sml1p is a checkpoint protein whose function is to regulate the activity of the large subunit of RNR (Rnr1p). Sml1p is thought to be regulated by the MEC1/RAD53 cell cycle checkpoint pathway. Neither the structure of Sml1p nor its complex to Rnr1p is well known. In this report, we describe how a recombinant Sml1p-histag protein (in both monomeric and dimeric forms) can be characterized with electrospray mass spectrometry. Mass spectrometry can play a vital role in the study of the Sml1p-Rnr1p complex by: (1) confirming the identities and purities of recombinant proteins such as Sm1lp-histag (with mass accuracy and resolution far superior to SDS-PAGE) and (2) verifying the presence or absence of PTM, chemical modifications, or metal-ion binding to the protein species, which may alter the function and binding of the protein partners.     

Characterization of a noncovalent lipocalin complex by liquid chromatography/electrospray ionization mass spectrometry.

Nanoscale liquid chromatography coupled to electrospray ionization mass spectrometry was used to identify the nature of the ligand that binds noncovalently to siderocalin (lipocalin 2). The folded state siderocalin-ligand complex was separated from free, unfolded siderocalin using reversed phase chromatography, and the molecular weight of the siderocalin ligand was then determined from the deconvoluted molecular weights of the complex and of the free protein. The ligand was identified as dihydroxybenzoyl-serine, a breakdown product of enterobactin, an iron-chelating compound ("siderophore") synthesized in bacteria. These results demonstrate that, in some cases, electrostatic noncovalent protein complexes can survive the denaturing conditions of reversed phase liquid chromatography and the gas phase transfer occurring during electrospray ionization.     

On the hepatic mechanism of HDL assembly by the ABCA1/apoA-I pathway

The mechanism for the assembly of HDL with cellular lipid by ABCA1 and helical apolipoprotein was investigated in hepatocytes. Both HepG2 cells and mouse primary culture hepatocytes produced HDL with apolipoprotein A-I (apoA-I) whether endogenously synthesized or exogenously provided. Probucol, an ABCA1 inactivator, inhibited these reactions, as well as the reversible binding of apoA-I to HepG2. Primary cultured hepatocytes of ABCA1-deficient mice also lacked HDL production regardless of the presence of exogenous apoA-I. HepG2 cells secreted apoA-I into the medium even when ABCA1 was inactivated by probucol, but it was all in a free form as HDL production was inhibited. When a lipid-free apoA-I-specific monoclonal antibody, 725-1E2, was present in the culture medium, production of HDL was suppressed, whether with endogenous or exogenously added apoA-I, and the antibody did not influence HDL already produced by HepG2 cells. We conclude that the main mechanism for HDL assembly by endogenous apoA-I in HepG2 cells is an autocrine-like reaction in which apoA-I is secreted and then interacts with cellular ABCA1 to generate HDL.