The interaction of prostaglandins with serum low-density lipoproteins

The interaction of human serum low-density lipoproteins (LDL) with various types of prostaglandins (PG) was studied using equilibrium dialysis, steady-state fluorescence polarization spectroscopy and photolabeling methods. Low concentrations (10(-13)-10(-9) M) of PGE1 and PGF2 alpha were shown to induce specific rearrangements of the lipids on the LDL surface, whereas the closely related PGE2 and PGF1 alpha had no effect. With fluorescent labeled LDL, the PGE1-induced changes of the steady-state fluorescence polarization (P) were shown to be time- and concentration-dependent, saturable and reversible. However, equilibrium dialysis revealed a very low binding capacity of LDL for PGE1 (approx. 1 prostaglandin molecule per 600 LDL particles). Approximately the same PGE1 concentration was sufficient to cause maximal changes of P, to enhance the binding to apolipoprotein B of a photoreactive sphingomyelin analogue inserted into the LDL surface and to alter the thermal phase behavior of the LDL surface lipids. It is proposed that the LDL surface rearrangement caused by prostaglandins is due to the interaction of prostaglandins with apolipoprotein B, resulting in formation of short-lived complexes. The mechanism of this interaction is discussed in terms of the non-equilibrium ligand-receptor interaction model proposed earlier to explain the interaction of prostaglandins with high-density lipoproteins (Bergelson, L.D. et al. (1987) Biochim. Biophys. Acta 921, 182-190). It is suggested that direct prostaglandin-lipoprotein interactions may play a role in the homeostasis of cholesterol.     

Interaction of enzymatically modified low-density lipoproteins with fibronectin

Interaction of human low-density lipoproteins (LDL) with homologous fibronectin fixed on collagen-Sepharose was studied. LDL were digested with pepsin, the degree of hydrolysis amounting to 10%. Upon passing modified LDL through a fibronectin-collagen-Sepharose column the desorption of fibronectin occurred. Addition of the increasing amount of fibronectin to the pepsin-treated LDL solution in the presence of Ca2+ ions led to the formation of LDL-fibronectin insoluble complexes. Interaction of native LDL with fibronectin was not observed. The data suggest that enzymatic modification of LDL increasing interaction of modified LDL with fibronectin, a component of extracellular matrix, could promote the accumulation of such LDL in arterial walls.     

Interaction of gangliosides with low-density proteins from human blood

Gangliosides have been shown to modulate the receptor-mediated endocytosis of low density lipoproteins (LDL). The direct interaction of LDL with various gangliosides has been studied. Binding of gangliosides to LDL immobilized on CNBr-Sepharose (LDL-Sepharose) and the influence of gangliosides on the fluorescence of LDL containing anthrylvinyl-labeled sphingomyelin were investigated. The binding of 3H-gangliosides to LDL-Sepharose, as well as fluorescence polarization of LDL were found to depend on the structure and concentration of the gangliosides in a specific saturable manner. The data obtained indicate that gangliosides interact with apolipoprotein B via specific binding sites.     

Effect of prostaglandins on lipid transfer between high- and low-density lipoproteins in human plasma

Prostaglandin (PG) E1 was shown to stimulate the transfer of phosphatidylcholine and cholesterol esters from human high density lipoproteins to low density lipoproteins. The enhancement of the interlipoprotein lipid transfer by PGE1 was observed both at low prostaglandin concentrations under conditions of spontaneous exchange as well as in the presence of the lipoprotein-depleted plasma and the partly purified lipid transfer plasma protein. At the same time PGE2 showed no significant influence on the interlipoprotein lipid transfer. It is supposed that the effect of PGE1 is due to the PGE1-induced reorganization of the high density lipoprotein surface and that the PG-lipoprotein interaction is a factor which regulates cholesterol homeostasis.     

Interaction of native and modified low-density lipoproteins with extracellular matrix

Lipoprotein-matrix interactions play an important role in arterial disease. Extracellular matrix proteoglycans bind and retain specific positively charged domains on apolipoproteins B- and E-containing lipoproteins during atherogenesis. Retained lipoproteins can undergo several modifications, which may alter their interaction with extracellular matrix molecules. Growth factors, cytokines and oxidized low density lipoproteins influence proteoglycan structure, rendering them more likely to bind and retain lipoproteins during atherogenesis. Lipoproteins, native and modified, also can modulate the expression of several of the matrix degrading enzymes present in vascular tissue, thereby influencing plaque stability. Thus, the interaction of atherogenic lipoproteins with arterial wall matrix molecules can influence the genesis and progression of atherosclerosis and its complications.     

The interaction of prostaglandins with human serum lipoproteins

Using high density and low density lipoproteins (HDL and LDL) labeled with fluorescent analogues of phosphatidylcholine or sphingomyelin it was found that low amounts (10^–12 M) of prostaglandins E₁ and F₂ induced different structural rearrangements of the lipoprotein surface, whereas prostaglandins E₂ and F₁ had no effect. The effects of prostaglandin E₁ on HDL were largely paralled by those of this prostaglandin on synthetic recombinants prepared from pure apolipoprotein A₁, phospholipids and cholesterol and were demonstrated to be caused by prostaglandin-apolipoprotein interaction. The interaction resembled that of a ligand with a specific receptor protein because it was specific, reversible, concentration and temperature dependent and saturable. However the retaining capacity of HDL or LDL for prostaglandin E₁ as determined by equilibrium dialysis was very low and a single prostaglandin E₁ molecule was able to induce structural changes in large numbers of discrete lipoprotein particles. To explain this remarkable fact a non-equilibrium model of ligand-receptor interaction is proposed. According to that model in open systems characterized by weak ligand-receptor binding, high diffusion rate of the ligand and long relaxation times which exceed the interval between two successive receptor occupations, the ligand-induced changes will accumulate, resulting in transformation of the system into a new state which may be far away from equilibrium. It is emphasized that the low mobility of lipids constituting the environment of the receptor protein plays a critcal role in this type of signal amplification.It was further demonstrated that the PGE₁-induced changes of the lipoprotein surface resulted in an enhancement of LDL-to-HDL transfer of cholesterol esters and phosphatidylcholine especially in the presence of serum lipid transfer proteins. The acceleration of the interlipoprotein transfer caused by prostaglandin E₁ in turn increases the rate of cholesterol esterification in serum. It is suggested that in such a way prostaglandin E₁ may influence the homeostasis of cholesterol.Abbreviations LDL low density lioproteins - HDL high density lipoproteins - PG prostaglandin - ASM anthrylvinyl-labeled sphingomyelin (N-12-(9-anthryl)-11-trans-dodecanoylsphingosin-1-phosphocholine - APC anthrylvinylphosphatidylcholine (1-radyl-2-[(9-anthryl)-11-transdodecanoyl)-sn-glycerophosphocholine - NAP-SM nitroazidophenyl labeled sphingomyelin (N-[N-(2-nitro-4azidophenyl)-12-aminododecanoyl]-sphingosin-1-phosphocholine) - NAP-PC adizophenyl labeled phosphatidylcholine (1-radyl-2-[N-(2-nitro-4azidophenyl)-12-aminododecanoyl]-sn-glycero-3-phosphocholine - DPPC dipalmitoylphosphatidylcholine - P fluorescence polarization - E parameter of tryptophanyl to ASM resonance energy transfer - LEP lipid-exchange protein     

Interaction of lipoproteins in blood serum with steroid hormones

Interaction of human and serum lipoproteins with steroid hormones (corticosterone and cortisol) was studied. Methods of fluorescence quenching titration and equilibrium dialysis were used for quantitative evaluation of VLDL, LDL and HDL glucocorticoid-binding ability. Association constants were found to be 0.6-2.0 x 10(6) M-1 for corticosterone and 4.0-8.0 x 10(6) M-1 for cortisol. The number of binding sites varied from 3 to 300 for different classes of lipoproteins.     

Interaction of insecticides with human plasma lipoproteins

The binding of chlorinated hydrocarbon, carbamate and organophosphate insecticides to human low density plasma lipoproteins (LDL) and high density plasma lipoproteins (HDL) was studied at pH 7.0 and 16°C and 26°C by equilibrium dialysis, difference spectra and fluorescence. The results suggest interaction to be a partitioning rather than a stoichiometric binding process. Distribution is related to lipid content and composition of the lipoproteins. The K-values vary from 3 × 10⁵ M^?1 for 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) to less than 10 M^?1 for nicotine and aldicarb, and ΔG_tr° is in the range of 7400 cal for DDT to less than 1000 cal for aldicarb and nicotine. The K and ΔG_tr° are inversely related to the water solubility of the insecticides. A significant role of plasma lipoproteins in the transport of slightly water soluble insecticides is suggested.     

Preparative isoelectric focusing of human serum very low-density and low-density lipoproteins.

Ten percent glycerol prevented the usual precipitation of human serum very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL) at their isoelectric points during their preparative isoelectric focusing (IEF), IEF separated VLDL and LDL into two major fractions. The observed optical density peaks are not artifacts caused by binding of Ampholines to VLDL or LDL since no radioactivity accumulated in the fractions containing VLDL or LDL during IEF in the presence of [¹⁴C]Ampholine, and gel filtration completely separated the lipoproteins from [¹⁴C]Ampholine. These results suggest that IEF may separate subspecies of VLDL and LDL under suitable experimental conditions.     

Lipid composition of cells and low-density lipoproteins in blood serum of human and some vertebrate species

To establish interaction of atherogenic low-density lipoproteins (LDL) with the erythrocyte membrane, the content of lipid components in blood cells and serum LDL was studied in healthy people (donors) and in 12 species of vertebrates (the mammals non-predisposed to atherosclerosis — birds and fish). Lipid composition of blood cells and LDL was also analyzed in patients with pathologies: ischemic heart disease (IHD), bronchial asthma (BA), and chronic obstructive bronchitis (COB), as well as in 2 species of mammals predisposed to atherosclerosis, in whose blood LDL predominated. The content of lipids in the blood cells and LDL of the studied vertebrates has been found to depend on their taxonomy and on the clear trends either for an increase in the cholesterol content and a decrease in the phosphatidylcholine level in patients, particularly with IHD, or for a rise of the ratio of the content of the more saturated sphingomyelin and cholesterol to the less saturated phosphatidylcholine from the lower to the higher organisms, including humans (donors). The highest levels of free cholesterol in blood cells of total cholesterol in LDL, as well as of parameters of ratio of the cholesterol/phosphatidylcholine content have been revealed in patients, especially with IHD, and in the mammals predisposed to atherosclerosis, i.e. in representatives with predominance of blood LDL, in contrast to donors and the mammals resistant to atherosclerosis. The highest parameters of lipid components were determined in blood cells and LDL in patients with IHD. The lipid LDL composition affects directly the composition and ratio of lipids in blood cells.     

Cholesterol content of red blood cells and low-density lipoproteins in hypertriglyceridemia

The red blood cells and the low-density lipoproteins in hypertriglyceridemia have a lower ratio of unesterified cholesterol to phospholipid than normal. The low-density lipoproteins are also smaller and more dense in hypertriglyceridemia, and contain only 45% of the normal unesterified cholesterol mass. The phase behavior of the lipids shows that normal red cells and low-density lipoproteins are close to saturation with cholesterol, whereas in hypertriglyceridemia less cholesterol is present. Because newly secreted triacylglycerol-rich lipoproteins are poor in cholesterol, their excess production and transport in hypertriglyceridemia may prevent maintenance of the normal cholesterol content of blood cells and low-density lipoproteins. Partitioning of cholesterol into triacylglycerol-rich lipoproteins is able to account for significant fluxes of unesterified cholesterol in the plasma compartment.     

Interaction of peroxynitrite with carotenoids in human low density lipoproteins

Interaction of peroxynitrite, the product of the reaction between nitric oxide and superoxide, with carotenes (lycopene, alpha-carotene, and beta-carotene) and oxocarotenoids (beta-cryptoxanthin, zeaxanthin, and lutein) was studied both in homogeneous solution and in human low-density lipoproteins (LDL). All carotenoids prevented the formation of rhodamine 123 from dihydrorhodamine 123 caused by peroxynitrite, suggesting that the carotenoids react with peroxynitrite. Oxocarotenoids were as effective as biothiols, known scavengers of peroxynitrite, whereas lycopene, alpha-carotene, and beta-carotene exhibited a considerably more pronounced effect. Moreover, peroxynitrite caused a loss of carotenoids in LDL as was revealed by HPLC. The concentration of peroxynitrite causing half-maximal loss of carotenoids in LDL ranged from 13 +/- 3 to 68 +/- 3 microM for lycopene and lutein, respectively. Again, oxocarotenoids were less reactive in this system. A correlation between efficiency of carotenoids in the competitive assay with dihydrorhodamine 123 and the concentration of peroxynitrite causing half-maximal loss of carotenoids in LDL was observed (r(2) = 0.91). These findings suggest that carotenoids can efficiently react with peroxynitrite and perform the role of scavengers of peroxynitrite in vivo.     

Complex-type carbohydrates of apolipoprotein-B of human plasma low-density lipoproteins

Two fractions of glycopeptides containingN-glycosidic asparagine-linked glycans were isolated by concanavalin-A-Sepharose affinity chromatography from Pronase digests of apolipoprotein-B of human low-density lipoproteins. Methylation analysis indicated that the non-binding fraction contains about 0.5 mol complex-type tri- or tetra-antennary oligosaccharides per mol lipoprotein. Structures containing a bisectingN-acetylglucosamine were also detected in this fraction. The weakly binding fraction from the chromatography contains the majority of the complex oligosaccharides of apolipoprotein-B, i.e. 5–6 mol of bi-antennary chains of the transferrin-type per mol of lipoprotein. In addition to sialylated structures, branches containing a terminalN-acetylglucosamine residue were detected on the complex-type glycans of apolipoprotein-B.     

Interaction of alpha-tocopherol with model human high-density lipoproteins.

The effects of alpha-tocopherol on the properties of model high-density lipoproteins (HDLs), composed of human apolipoprotein A-I and dimyristoylphosphatidylcholine, were investigated by physicochemical methods. The intrinsic fluorescence of alpha-tocopherol and its effects on the polarization of fluorescence of 1,6-diphenyl-1,3,5-hexatriene, which probes the hydrocarbon region of the lipids, and 4-heptadecyl-7-hydroxycoumarin, which is a probe of lipid surfaces, suggest that alpha-tocopherol is located at the lipid-water interface. Relative to cholesterol, alpha-tocopherol in lipid surfaces is virtually inert physicochemically. Incorporation of alpha-tocopherol into HDLs induces only a modest increase in particle size, no change in the transition temperature, and little change in lipid polarity and lipid-lipid interactions. Moreover, alpha-tocopherol has only a negligible effect on the kinetic parameters of the lipophilic enzyme lecithin:cholesterol acyltransferase, which binds to phosphatidylcholine surfaces and forms cholesteryl esters. However, alpha-tocopherol has a dramatic inhibitory effect on the rate of association of apolipoprotein A-I with dimyristoylphosphatidylcholine, a process that occurs through the insertion of the protein into preformed defects in the lipid surface. It is proposed that alpha-tocopherol inhibits the rate of association of apolipoprotein A-I with dimyristoylphosphatidylcholine by inserting into defects within the lipid surface, thereby reducing the size and/or number of sites for insertion of apolipoprotein A-I.     

Physical-chemical interaction of heparin and human plasma low-density lipoproteins

This study characterizes the physical-chemical interactions of heparin with human plasma low-density lipoproteins (LDL). A high reactive heparin (HRH) specific for the surface determinants of LDL was isolated by chromatography of commercial bovine lung heparin on LDL immobilized to AffiGel-10. HRH was derivatized with fluoresceinamine and repurified by affinity chromatography, and its interaction with LDL in solution was monitored by steady-state fluorescence polarization. Binding of LDL to fluoresceinamine-labeled HRH (FL . HRH) was saturable, reversible, and specific; HRH stoichiometrically displaced FL . HRH from the soluble complex, and acetylation of lysine residues on LDL blocked heparin binding. Titration of FL.HRH with excess LDL yielded soluble complexes with two LDL molecules per heparin chain (Mr 13,000) characterized by an apparent Kd of 1 microM. Titration of LDL with excess HRH resulted in two classes of heparin binding with two and five heparin molecules bound per LDL and apparent Kd values of 1 and 10 microM, respectively. At physiological pH and ionic strength, the soluble HRH-LDL complexes were maximally precipitated with 20-50 mM Ca2+. Insoluble complexes contained 2-10 HRH molecules per LDL with the final product stoichiometry dependent on the ratio of the reactants. The affinity of HRH for LDL in the insoluble complexes was estimated between 1 and 10 microM. Insoluble LDL-heparin complexes were readily dissociated with 1.0 M NaCl, and their formation was prevented by acetylation of the lysine residues on LDL.     

The immunochemical detection of apoprotein dissociation from particles of very low-density lipoproteins in human blood plasma]

The dissociation of very-low-density lipoprotein (VLDL) apoproteins was studied using immunochemical approaches. The analysis of monospecific antibody binding to apo E, C-II and C-III on VLDL surface showed low apoprotein accessibility for the antibodies while the accessibility of apo C-II and C-III in solution was complete. Lipoprotein preparation dilution resulted in increasing of apo E and C-II accessibility. It was suggested that apoprotein dissociation led to apoprotein cluster dissolving on VLDL surface and higher antigen determinant accessibility. The findings confirmed previous theoretical analysis of apoprotein dissociation.