31P nuclear magnetic resonance spectroscopy of native and recombined lipoproteins

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.

Current concepts of the molecular structure and metabolism of human apolipoproteins and lipoproteins

During the last few years major advances have occurred in our knowledge of the structure, function, and metabolism of the plasma lipoproteins. Twelve human apolipoproteins have been isolated and characterized. The primary structure of apolipoproteins A-I, A-II, C-I, C-II, and C-III have been elucidated. The primary structure of these apolipoproteins contain no unique sequences, however the primary structure of several of the apolipoproteins contain segments which can be modeled into amphipathic helices. The helical segments may be important in protein-protein as well as protein-lipid interactions. The molecular properties of the apolipoproteins have been investigated and shown to undergo self-association with major increases in conformation. The molecular organization of the plasma lipoprotein particle has been studied, and an iceberg-sea model has been proposed. This model emphasizes the micellar organization of the phospholipids, and the possibility of secondary, tertiary as well as quaternary structure of the apolipoprotein associated with the lipoprotein particle. The metabolism of plasma lipoproteins has been extensively analyzed over the last several years. Two general types of apolipoprotein-lipoprotein particle interactions have been recognized. The first type involves a "quasi-irreversible" interaction between the apolipoprotein and lipoprotein particle, and is exemplified by apolipoprotein b. The second type of interaction is a "reversible" apolipoprotein-lipoprotein particle interaction. Apolipoproteins a-I, A-II, C-I, C-II, C-III, and E are examples of the reversible interaction. Within this framework two major apoB-lipoprotein particle cascades have been proposed. ApoB-triglyceride rich lipoproteins including chylomicrons and hepatic VLDL undergo sequential triglyceride hydrolysis. Following triglyceride hydrolysis chylomicrons are converted to remnants with hydrated densities principally of VLDL and IDL. Liver apoB-VLDL is converted initially to IDL and finally to LDL. Apolipoproteins which undergo reversible interactions are present in virtually all density fractions and the distribution of these apolipoproteins is determined by the laws of mass action. With these concepts rapid progress has been made in our understanding of apolipoprotein-lipoprotein biochemistry, physiology, and clinical disorders of lipoproteins and atherosclerosis. The next several years will undoubtedly provide further insights into the structure, function, and metabolism of plasma lipoproteins.

Location and interactions of phospholipid and cholesterol in human low density lipoprotein from 31P nuclear magnetic resonance

The major phospholipids, phosphatidylcholine and spingomyelin, of low density lipoprotein (LDL) are accessible to small amounts of Pr3+, suggesting that the head groups of all mobile phospholipids are on the surface of the particle in contact with the aqueous medium. The major source of the nuclear Overhauser effect enhancement of 31P resonances is the N-methyl proton of the choline moiety, indicating close N-methyl phosphate group interactions, probably similar to those found previously in phospholipid vesicles. This behavior of the phospholipid head groups in LDL is similar to that in small vesicles without cholesterol, suggesting that in LDL most of the cholesterol is not associated with mobile, surface phospholipids. In contrast to LDL, where the presence of a large protein immobilizes some phospholipid head groups, immobilization does not occur in high density lipoprotein, consistent with occurrence of smaller peptides in the latter.

A nuclear magnetic resonance study of sphingomyelin in bilayer systems

The physical properties of small single-walled vesicles composed of the zwitterionic phospholipid sphingomyelin have been studied using 1H and 31P nuclear magnetic resonance spectroscopy. The temperature variation of proton line widths and spin-lattice relaxation times and the chemical shift behavior for sphingomyelin vesicles are compared with results previously determined for phosphatidylcholine vesicles. Differences between the two systems are interpreted as indications of the presence of both inter- and intramolecular hydrogen bonding in sphingomyelin bilayers.

31P nuclear magnetic resonance evidence for polyphosphoinositide associated with the hydrophobic segment of glycophorin A

Glycophorin A, the major human erythrocyte sialoglycoprotein, contains a significant amount of phosphorus when isolated by the lithium diiodosalicylate-phenol procedure. Only a small percentage (approximately 1%) of this phosphorus is phosphoprotein. 31P nuclear magnetic resonance (NMR) analysis of glycophorin A has identified the remaining phosphorus content as phospholipid in origin. From the 31P chemical shifts, the phospholipid has been identified as diphosphoinositide. 31P NMR spectra of the peptides produced by trypsin hydrolysis of glycophorin A reveal that all the diphosphoinositide is closely associated with the hydrophobic region of the protein, suggesting that there is a specific affinity between this phospholipid and the intramembranous portion of glycophorin A.

[Lipid-protein-interactions of human apolipoproteins-structural aspects and models of lipoproteins (author's transl)]

The plasma lipoproteins are complex macromolecular structures which play an essential role in fat transport and in energy and membrane metabolism of higher organized organisms. Much has been learned in the last decade about the structural and functional interrelationships of the different lipoprotein classes. Their protein moieties, the so-called apolipoproteins, have been purified and characterized, the primary structure of four of them is known. Initial recombination experiments showed a considerable potential of the (unfractionated) lipoprotein protein to bind to lipids and to form particles similar to native lipoproteins. Further binding experiments performed in several laboratories with the purified A- and C-apolipoproteins and different physico-chemically well defined lipids have lead to the identification of lipid binding sites within the protein molecules and the formation of amphipathic helices upon and during lipid binding. This possible common mechanism of lipid-protein fractions forms the basis of a recently proposed model of one lipoprotein class, namely the high density lipoproteins (HDL). The significance of protein-protein-interactions in the formation and maintenance of these lipoprotein particles is still unknown. Whether disturbed lipid protein interactions lead to structural and/or functional alterations of the corresponding lipoproteins is a topic of discussion. The pertinent literature is listed in this paper and the physiological relevance of these studies and their clinical aspects will be presented.

Analysis of living tissue by phosphorus-31 magnetic resonance

Nuclear magnetic resonance is a new method for assaying the content of phosphate metabolites in intact tissues. Its nondestructive nature allows simultaneous and repeated determinations of these compounds with a minimum perturbation of tissue. Changes in the concentrations of the phosphates as a function of time characterize the metabolic machinery of the tissue and reveal alterations in enzymic activity that result from drug treatment or disease. The entire phosphate profile shows differences between normal and diseased muscle which should be of diagnostic value. Further, by examining phosphate profiles we detected a family of chemical compounds that were not previously known to exist as major constituents in muscle. Of these, two have been isolated and one has been identified as glycerol 3-phosphorylcholine. Finally, shifts in the positions of resonances monitor the internal environment of the living system, its hydrogen ion concentration, the complexing of alkaline earth metals with ATP, and compartmentalization within the cell.

Interaction of apoprotein from porcine high-density lipoprotein with dimyristoly lecithin. 2. Nature of lipid-protein interaction

The detailed molecular structure of the complex formed by the apoprotein from porcine high density lipoprotein and dimyristoly phosphatidylcholine (lecithin) has been investigated by a range of physical techniques. The complex, an oblate ellipsoid with major axis 11.0 nm and minor axis 5.5 nm (see the accompanying paper), is comprised of a section of lecithin bilayer with apoprotein at the surface. The main site of interaction between protein and lipid is in the lipid glycerophosphorylcholine group region; as with native high density lipoprotein the surface of the particle consists of a mosaic of lecithin polar groups and protein. The formation of this mosaic reduces the cooperativity of the lecithin chain motions and changes the curvature of the lipid-water interface, as compared to a bilayer. Otherwise, there are no major changes in lecithin motions indicating that no strong binding of lipid to protein occurs. The interaction involves the intercalation of amphipathic, 60% alpha-helical, apoprotein molecules among the lecithin molecules so that the protein residues at the lipid-water interface. The apoprotein has a high affinity for the lipid-water interface but specific lipid-protein interactions are not involved.

Phosphorus-31 nuclear magnetic resonance spectroscopy of extracellular, yeast O-phosphonohexoglycans

P nuclear magnetic resonance spectra of a number of purified yeast O-phosphonohexoglycans were recorded. The data therefrom were correlated with established chemicals aspects of individual and collective polymer structures, permitting (a) conclusions to be drawn regarding the chemical environment of the phosphate groups of these polymers, and (b) assignment of anormeric configurations to the hexosyl phosphate residues.

13C nuclear magnetic resonance spectroscopy of native and recombined lipoproteins

(13)C nuclear magnetic resonance data on native and recombined lipoproteins are reported. [Methyl-(13)C]phosphatidylcholine, [methyl-(13)C]sphingomyelin, 1,2-[dioleoyl-1-(13)C]-sn-phosphatidylcholine and cholesteryl-[1-(13)C]oleate were enriched with 90% carbon-13 in respective molecules by chemical synthesis and used for recombination experiments with high density lipoprotein apoproteins. Relaxation times for these specifically enriched lipids in organic solvents, (2)H(2)O, and lipid-protein complexes isolated by ultracentrifugal flotation, were measured. The results show that both the phosphatidylcholine and sphingomyelin polar headgroups have the same hydrophilic environment in either sonicated lipid particles or reassembled lipoproteins, and suggest that ionic interaction of lipid and apolipoproteins is of minor importance in the formation of plasma lipoprotein complexes. Our experiments indicate that (13)C nuclear magnetic resonance spectroscopy will contribute to the understanding of lipidprotein interaction in lipoproteins and membranes.

A molecular model of high density lipoproteins

Based on the analysis of recombined lipidapoprotein complexes by C-13 and P-31 nuclear magnetic resonance spectroscopy and circular dichroism [Assmann, G., Sokoloski, E. A. & Brewer, H. B., Jr. (1974) Proc. Nat. Acad. Sci. USA 71, 549-553; Assmann, G., Highet, R. J., Sokoloski, E. A. & Brewer, H. B., Jr. (1974) Proc. Nat. Acad. Sci. USA 71, in press; Assmann, G. & Brewer, H. B., Jr. (1974) Proc. Nat. Acad. Sci. USA 71, 989-993] and the identification of conformational amphipathic regions in apoproteins, a new model for human high density lipoproteins is proposed. This model is analogous to membrane models proposed by Singer, in that protein "icebergs" are embedded in a "sea" of lipid.

Lipid-protein interactions in high density lipoproteins

Delipidated high density lipoprotein (apo-HDL), isolated apolipoproteins apoA-I and apoA-II, S-carboxymethylated apoA-II, apoC-III, the NH(2)- and COOH-terminal CNBr peptides of apoA-II, and the COOH-terminal CNBr peptide of apoA-I were recombined in vitro with [N-C(3)H(3)-choline]phosphatidylcholine (PC) and [N-(14)CH(3)-choline]sphingomyelin (SPM). The lipid-protein complexes were analyzed by ultracentrifugal flotation, agarose gel chromatography and circular dichroism. ApoHDL, apoA-II, and S-carboxymethylated apoA-II readily recombined with PC or SPM to form particles that were similar in size to native HDL. The COOH- but not the NH(2)-terminal CNBr peptide of apoA-II recombined with lipid. ApoA-I and the COOH-terminal CNBr peptide of apoA-I, however, recombined with PC or SPM to only a limited extent, suggesting that protein-protein interactions between apoA-I and apoA-II are important in the integration of apoA-I into recombined lipoprotein particles. Analysis of the recombined lipid-protein complexes by circular dichroism indicated that there was an increase in helical structure concomitant with lipid-protein binding. The reconstituted particles had many of the physical and chemical properties of the native lipoprotein.