Characterization of baboon plasma high-density lipoproteins and of their major apoproteins

Sphingomyelin in high-density lipoproteins: structural role and biological function

High-density lipoprotein (HDL) levels are an inverse risk factor for cardiovascular diseases, and sphingomyelin (SM) is the second most abundant phospholipid component and the major sphingolipid in HDL. Considering the marked presence of SM, the present review has focused on the current knowledge about this phospholipid by addressing its variable distribution among HDL lipoparticles, how they acquire this phospholipid, and the important role that SM plays in regulating their fluidity and cholesterol efflux from different cells. In addition, plasma enzymes involved in HDL metabolism such as lecithin-cholesterol acyltransferase or phospholipid transfer protein are inhibited by HDL SM content. Likewise, HDL SM levels are influenced by dietary maneuvers (source of protein or fat), drugs (statins or diuretics) and modified in diseases such as diabetes, renal failure or Niemann-Pick disease. Furthermore, increased levels of HDL SM have been shown to be an inverse risk factor for coronary heart disease. The complexity of SM species, described using new lipidomic methodologies, and their distribution in different HDL particles under many experimental conditions are promising avenues for further research in the future.

Sequence of horse (Equus caballus) apoA-II. Another example of a dimer forming apolipoprotein

Apolipoprotein A-II, the second major apolipoprotein of human HDL, also has been observed in a variety of mammals; however, it is either present in trace amounts or absent in other mammals. In humans and chimpanzee, and probably in other great apes, apoA-II with a cysteine at residue 6 is able to form a homodimer. In other primates as well as other mammals, apoA-II, lacking a cysteine residue, is monomeric. However, horse HDL has been reported to contain dimeric apoA-II that following reduction forms monomers. In this report, we extend these observations by reporting on the first complete sequence for a horse apolipoprotein and by demonstrating that horse apoA-II also contains a cysteine residue at position 6. Both the intact protein and its enzymatic fragments were analyzed by chemical sequence analysis and time-of-flight MALDI-MS (matrix assisted laser desorption ionization mass spectrometry). We also obtained molecular mass data on dimeric and monomeric apoA-II using electrospray-ionization mass spectrometry (ESI-MS). The data are compared with other mammalian sequences of apoA-II and are discussed in terms of resulting similarities and variations in the primary sequences.

Purification and characterization of recombinant human apolipoprotein A-II expressed in Escherichia coli

We have expressed recombinant human apolipoprotein A-II (apoA-II) in Escherichia coli, as a fusion protein with Schistosoma japonicum glutathione-S-transferase (GST). The GST-AII fusion protein was recovered by affinity chromatography using glutathione as a ligand. After thrombin cleavage and removal of the GST carrier, recombinant apoA-II was obtained in a highly purified form and was exclusively composed of dimeric apoA-II. Kinetics of association to dimyristoylglycerophosphocholine (Myr2GroPCho) vesicles showed that recombinant apoA-II exhibited the same pattern of association as human plasma apoA-II. Electron microscopic analysis of the complexes showed a typical pattern of rouleaux, characteristic of stacked discs, with a diameter similar to that determined by gradient-gel electrophoresis. Circular dichroism measurements showed that the alpha-helical content of both plasma and recombinant apoA-II increased similarly when the proteins associated with Myr2GroPCho vesicles, at the expense of a random-coil structure. Lipid-bound apoA-II consisted of 70-72% alpha helices, suggesting the presence of three 18-residue alpha helices/apoA-II monomer. Cross-linking experiments indicated that Myr2GroPCho complexes contained two molecules dimeric apoA-II/vesicle. Recombinant apoA-II was as efficient as plasma apoA-II in associating with HDL subclasses, and in displacing apoA-I from dipalmitoylglycerophosphocholine/cholesterol/apoA-I complexes, most likely due to its highly ordered secondary structure when associated with Myr2GroPCho vesicles. These findings demonstrate that recombinant apoA-II exhibits the same structural and functional properties as human plasma apoA-II. Thus, the expression system utilized is appropriate to produce mutagenized forms to further structure/function analysis.

Characterization and amino-terminal sequence of apolipoprotein AI from plasma high density lipoproteins in the preruminant calf, Bos spp

The major apolipoprotein of calf plasma high-density lipoproteins, apo-AI, has been isolated and characterized. Apolipoprotein AI (apo-AI) was separated from the protein moiety of high-density lipoproteins (d 1.090-1.180 g/ml) by preparative electrophoresis in SDS-polyacrylamide gels followed by electrophoretic elution. Purified calf apo-AI had an Mr of approx. 27,000-28,000 in SDS-polyacrylamide gels, resembling human apo-AI. The amino acid composition of calf apo-AI displayed an overall similarity to that of its human and other mammalian counterparts (baboon, dog, badger, rabbit, rat and mouse), but differed in having higher proportions of glutamic acid, alanine and isoleucine. Amino-terminal amino acid sequence analysis up to the 47th residue showed close homology between calf apo-AI and those of the mammals with which it was compared. However, residues 2, 7, 20 and 22 in calf AI (i.e. aspartic acid, serine, glutamic acid and isoleucine, respectively) were substituted by glutamic acid, proline or glutamine, aspartic acid, and valine or leucine respectively, in the other mammals.

Homologues of the human C and A apolipoproteins in the Macaca fascicularis (cynomolgus) monkey

We used antisera to human A and C apolipoproteins to identify homologues of these proteins among the high-density lipoprotein apoproteins of Macaca fascicularis (cynomolgus) monkeys, and NH2-terminal analysis was used to verify the homology. The NH2-terminal sequence of the M. fascicularis apoA-I is identical with that of another Old World species, Erythrocebus patas, and differs from human apoA-I at only 4 of the first 24 residues. M. fascicularis apoA-II contains a serine for cysteine replacement at position 6 and is therefore monomeric like the apoA-II from all species below apes. Human and monkey apoA-II are not otherwise different through their first 25 residues. About 20% of M. fascicularis apoC-I aligns with human apoC-I through residue 22, and 80% lacks an NH2-terminal dipeptide. Otherwise, the monkey apoC-I differs from the human protein at only 2 of 25 positions. Two forms of M. fascicularis apoC-II were identified. ApoC-II1 is highly homologous with human apoC-II, whereas an NH2-terminal hexapeptide is absent from apoC-II2. ApoC-II2 was the predominant species, and apoC-II1 appears to represent a propeptide from which a hexapeptide prosegment is cleaved at a Gln-Asp bond. Both forms of monkey apoC-II are potent activators of lipoprotein lipase. There are two polymorphic forms of M. fascicularis apoC-III, and their electrophoretic mobilities become identical after treatment with neuraminidase. Except for a glycine for serine substitution at position 10, the first 15 NH2-terminal residues of M. fascicularis and human apoC-III are the same.

Plasma lipoproteins of free-ranging howling monkeys (Alouatta palliata)

1. Plasma lipids and lipoproteins of free-ranging howling monkeys from Costa Rica (Alouatta palliata), aged 5 months to 23 years, were characterized. 2. High density lipoproteins were lipid-rich, similar to HDL2 of human plasma. 3. Fatty acid compositions of major lipid classes of very low, low and high density lipoproteins differed among social groups, possibly due to both dietary and genetic factors. 4. Low and high density lipoprotein phospholipids were enriched in phosphatidylethanolamine. 5. Howler plasma cross reacted with antihuman apoA-I antibodies but not with antihuman LDL antibodies. 6. No dimeric form of apoA-II was present, unlike human apoA-II.

Lipoprotein patterns in nondiabetic, borderline diabetic, and diabetic Macaca nigra

Lipoproteins were isolated by sequential ultracentrifugation, and the concentrations and compositions were determined in nondiabetic (ND), borderline diabetic (BD), and diabetic (D) Macaca nigra males consuming a chow ration. The total concentrations and components of the VLDL and IDL increased significantly with metabolic deterioration (P less than 0.01). Concentrations and components of LDL increased in the BD and D monkeys, but changes were not statistically significant. The HDL2 and HDL3 particles were virtually unchanged among the three different metabolic groups. The VLDL was the major carrier of the triglycerides, especially in D monkeys. Cholesterol was present predominantly in the LDL. The LDL-cholesterol to HDL-cholesterol ratio increased in the BD and D monkeys, owing mainly to increases in the LDL-cholesterol content. Apoprotein antisera showed apoprotein B in the VLDL, IDL, and LDL, apoprotein E in the VLDL and IDL, and apoprotein A-I in the HDL2 and HDL3 fractions. Because Macaca nigra consume a nonatherogenic, low-cholesterol, low-fat ration, the changes in lipoproteins, particularly in VLDL and IDL, are attributable to metabolic alterations associated with diabetes.

Isolation, characterization and comparative aspects of the major serum apolipoproteins, B-100 and AI, in the common marmoset, Callithrix jacchus

The two major apolipoproteins of marmoset serum have been isolated and characterized, and on the basis of physicochemical and immunological criteria are homologous with the human AI and B-100 proteins. Marmoset apolipoprotein AI was the principal protein of high-density lipoproteins (HDL) and was purified by gel filtration chromatography and electrophoresis in alkaline-urea polyacrylamide gel followed by electrophoretic elution. Purified marmoset apolipoprotein AI displayed an Mr of approx. 27000, was polymorphic (five forms) on isoelectric focussing, with pI values in the range 4.8-5.0, and migrated similarly to human apolipoprotein AI in alkaline-urea gels. An overall resemblance was seen in the amino acid composition of marmoset apolipoprotein AI and that of its human counterpart with the notable exception that marmoset AI contained 1 isoleucine residue/mole. An immunological reaction of partial identity between the human and monkey proteins was seen upon immunodiffusion of their HDLs against antiserum to human apolipoprotein AI. Marmoset B-100 was the predominant apoprotein of VLDL and LDL, resembling the human protein in its elution profile on gel filtration chromatography in anionic detergent, and in its high apparent Mr (approx. 520000). The marmoset and human B-100 proteins were alike in amino acid composition and carbohydrate content. Moreover, their immunological behaviour with an antiserum to marmoset apolipoprotein B showed them to share certain antigenic determinant(s). We conclude that the physicochemical properties of the principle apolipoproteins of Callithrix jacchus, a New World primate, markedly resemble those of the human AI and B-100 proteins, suggesting therefore that they may function similarly in lipid transport and metabolism. Counterparts to human apolipoproteins AII, E, CII and CIII have also been tentatively identified.

Dissociation of apolipoprotein AI from apoprotein-lipid complexes and from high-density lipoproteins. A fluorescence study

The dissociation of the apoprotein AI (apoAI) from the apoAI-dimyristoylglycerophosphocholine complex and from human high-density lipoproteins (HDL), was induced by incubation with guanidinium hydrochloride at concentrations between 0 M and 7 M. The kinetics and extent of denaturation were followed by monitoring both the fluidity of the lipid phase by fluorescence polarization measurements and the protein conformation by measuring the tryptophanyl fluorescence emission. The association with lipids protects apoAI against denaturation both in HDL and in the apoAI-phospholipid complex. The results of the kinetic and end-point measurements suggest that the denaturing effect of guanidine hydrochloride on the apoAI-lipid complex and on HDL is a two-step process. It involves the dissociation of the apoAI-phospholipid bond, as evidenced by fluidity measurements: this effect is maximal between 3 M and 4 M guanidine hydrochloride. The conformational change of the apoAI protein into a randomly coiled structure with the tryptophanyl residues exposed to the solvent is maximal between 4 M and 6 M guanidine hydrochloride. HDL and the apoAI-phospholipid complex have a closely similar behaviour towards denaturation by guanidine hydrochloride indicating that the phospholipid-apoAI association in HDL is primarily responsible for the stability of the lipoprotein molecule.

In vitro interaction of human HDL with human apolipoprotein A-II. Synthesis of apolipoprotein A-II-rich HDL

The aim of this study was to define the specific affinity of human apolipoproteins A-I and A-II for HDL lipids and to investigate the possible transfer of apolipoproteins from the HDL molecule. For this purpose we incubated human HDL with increasing amounts of isolated apolipoprotein A-II. After incubation the reaction products were separated by gel chromatography and apolipoproteins A-I and A-II were quantified separately by immunonephelometry and HDL lipids by thin-layer chromatography. According to our results, apolipoprotein A-II progressively displaces apolipoprotein A-I to generate an HDL-like particle with identical lipid composition, hydrodynamic properties and lipid fluidity. These data indicate that apolipoprotein A-II is able to displace quantitatively apolipoprotein A-I from HDL in vitro, and that such a mechanism might contribute to the regulation of the HDL2 in equilibrium or formed from HDL3 distribution in plasma.

Reassembly of human apoproteins A-I and A-II with unilamellar phosphatidylcholine-cholesterol liposomes. Association kinetics and characterization of the complexes

The kinetics of association between the human apoprotein A-I and apoprotein A-II and cholesterol dimyristoyl phosphatidylcholine (DMPC) vesicles are compared in this study and the lipid-apoprotein complexes are characterized. The association kinetics are followed by turbidity measurements monitoring the decrease of the vesicular size and by fluorescence polarization measurements monitoring the decrease in the mobility of the phospholipid acyl chains during complex formation. The influence of the incubation temperature and of the cholesterol/DMPC ratio has been studied by both techniques. Under all incubation conditions the apoprotein A-II associates more readily with cholesterol-DMPC vesicles than apoprotein A-I, as the kinetics are faster and the complex yield larger. With both apoproteins optimal complex formation takes place around the phospholipid transition temperature and around 10 mol% cholesterol. The apoprotein A-I/lipid association seems restricted to this narrow range for the temperature and the cholesterol/DMPC ratio, while the apoprotein A-II still associates with vesicles containing 20 mol% cholesterol and at temperatures up to 32 degrees C. The lipid-apoprotein complexes were isolated by gradient ultracentrifugation and by gel chromatography. According to these data the apoprotein A-II associates more readily than apoprotein A-I with cholesterol-DMPC vesicles to form protein-rich complexes, whilst the optimal apoprotein A-I-lipid association requires a more disordered lipid structure.

Detection of immunological heterogeneity of an isolated, purified protein (vervet apolipoprotein A-I)

Using a single goat antiserum, we have identified immunological heterogeneity of purified apolipoprotein A-I from high density lipoprotein of vervet monkeys. We examined whether the apparent heterogeneity was due to separate antigenic sites within the polypeptide sequence or rather on the different isoproteins, which result in charge heterogeneity of this protein. The apolipoprotein A-I was cleaved with cyanogen bromide and the resulting three fragments were purified and characterized. By using immunodiffusion, each of the fragments was found to show a characteristic and different reaction to the antiserum. By contrast, apparent identity was found by immunodiffusion among the separate isoprotein forms of apolipoprotein A-I. We have concluded that the immunological heterogeneity of apolipoprotein A-I was due to different antigenic sites within the primary sequence of apolipoprotein A-I.

Density distribution, characterization, and comparative aspects of the major serum lipoproteins in the common marmoset (Callithrix jacchus), a New World primate with potential use in lipoprotein research

Qualitative, quantitative, and comparative aspects of the serum lipoprotein profile in the Common marmoset (Callithrix jacchus), a New World primate, are described. Density gradient ultracentrifugation was used to evaluate lipoprotein distribution and to establish criteria for isolation of discrete molecular fractions. The major lipoprotein classes banded isopycnically on the gradient with the following hydrated densities: VLDL, d less than 1.017 g/mL; LDL, d = 1.027--1.055 g/mL; HDL fraction I, d = 1.070--1.127 g/mL; and HDL fraction II, d = 1.127--1.156 g/mL. Electrophoretic, immunological, and electron microscopic analyses attested to the purity of these fractions: the characteristics of each were assessed by chemical analysis, electron microscopy, immunological techniques, and polyacrylamide gel electrophoresis of their protein moieties. Marmoset VLDL and LDL were closely akin to those of man in size and chemical composition, although the former were richer in triglyceride; electrophoretic and immunological data showed the major protein component of VLDL and LDL to be a counterpart to human apo-B. The two HDL subfractions, i.e., HDL-I and HDL-II, corresponded in size and chemical composition to human HDL2 and HDL3, respectively, although slight differences in neutral lipid content were detected. By immunological and electrophoretic criteria, the major apolipoprotein of marmoset HDL was analogous to human apo-AI. In contrast, marked dissimilarities were evident in the complements of low molecular weight, tetramethylurea-soluble polypeptides of marmoset and human lipoproteins. Quantitatively, the human and marmoset lipoprotein profiles were not dissimilar, although HDL was the major class (approximately 50%); in fasting animals, serum concentrations of VLDL, LDL, and HDL were 50--90, 170--280, and 338--408 mg/dL, respectively. C. jacchus was distinct from man in displaying a greater proportion of its total HDL in the less dense (HDL-II) subfraction (marmoset HDL-I/HDL-II = approximately 4:1; human HDL2/HDL3 = approximately 1:3). These data indicate that, as an experimental animal for lipoprotein research, the Common marmoset combines the advantages of ready availability and maintenance with a serum lipoprotein profile which resembles, in many qualitative and quantitative aspects, that found in man.

Fluorescence depolarization studies and phase transition in human apoprotein . phospholipid complexes

The microviscosity of unilamellar vesicles of dimyristoyl-3-sn-phosphatidylcholine and that of phosphatidylcholine . apoprotein complexes was followed by fluorescence depolarization after labeling with 1,6-diphenyl-1,3,5-hexatriene. The transition temperature from gel-crystalline to liquid-crystalline phase in 24 degrees C for the dimyristoyl-phosphatidylcholine vesicles and is shifted to around 30 degrees C in the complexes between phosphatidylcholine and apoA-I, apoA-II, apoC-I, apoC-III proteins while the cooperativity of the transition is decreased. At temperatures below the transition of the phospholipid, the microviscosity of the complexes of phosphatidylcholine with apoA-I, apoA-II and apoC-I proteins is lower than that of the phosphatidylcholine, while the opposite effect is observed above 30 degrees C. The phosphatidylcholine . apoprotein complexes isolated on a Sepharose 6B column have a molecular weight around 100 000 and a phosphatidylcholine/apoprotein ratio of 2--2.6 (w/w). The microviscosity measurments at 35 degrees C performed after elution of the column enable the complex to be detected. The size and microviscosity of the apoprotein . phosphatidylcholine complex is compatible with a model where the vesicular structure has disappeared and the amino acid side chains present hydrophobic interaction with the phosphatidylcholine acyl chains.

Ionization behaviour of native apolipoproteins and of their complexes with lecithin. 1. Calorimetric and potentiometric titration of the native apoA-I protein and of the apoA-I protein-dimyristoyl lecithin complex

The ionization behaviour of native apoA-I protein is compare to that of its complex with synthetic dimyristoyl lecithin in studies using calorimetric, potentiometric and spectrophotometric titration. In the presence of phospholipids, 10 out of 21 lysines together with 22 acidic residues are masked in the complex. All tyrosines remain accessible to titration below pH 13. The apparent ionization enthalpy of the 11 lysine residues is not affected by the presence of phospholipids. These data are consistent with discrete binding sites located in the apoprotein helical segments as suggested by the model of Segrest et al. [FEBS Lett. 38, 247-253 (1974)]. A tentative localisation of lysine, arginine, aspartic acid and glutamic acid residues directly involved in phospholipid binding is suggested, assuming that such helical regions are involved in apoprotein-phospholipid association.

Ionization behaviour of native apolipoproteins and of their complexes with lecithin. 2. Potentiometric titration of the native apo-A-II, apoC-I, apoC-III proteins and of their complexes with dimyristoyl lecithin

A comparison of the ionization behaviour of the human apoA-II, apoC-I, apoC-III proteins and of their complexes with dimyristoyl lecithin is based on potentiometric titration of the basic and acidic residues and spectrophotometric titration of the phenolic groups. Experimental data suggest that a number of lysine, arginine, aspartic acid and glutamic acid residues are masked in the complexes. For each of these amino acids and in all three proteins the number of masked residues is consistent with the content of those regions predicted to be involved in lipid binding by the model of Segrest et al. [FEBS Lett. 38, 247-253 (1974)]. These data taken together with the results of calorimetric and titration experiments with the apoA-I protein reported in the accompanying article [Rosseneu et al. (1977) Eur. J. Biochem. 79, 251-257] strongly support the general nature of the proposed model and further suggest that ionic interactions have some role in the formation of the dimyristoyl lecithin/apolipoprotein complexes.