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PubMed1986

Anion effects on the reaction of lecithin-cholesterol acyltransferase with discoidal complexes of phosphatidylcholines . apolipoprotein A-I . cholesterol.

Discoidal complexes of phosphatidylcholine (PC) . apolipoprotein A-I . cholesterol were prepared with egg PC, palmitoyloleoylPC, dipalmitoylPC, or dimyristoylPC, and were used as substrates of purified lecithin-cholesterol acyltransferase to investigate the effects of neutral salts on the enzymatic reaction. Sodium fluoride, chloride and bromide concentrations up to 1 M, did not affect the properties of the substrate particles, but caused marked and distinct changes in the activity of the enzyme with the various PC particles. The effects of salts were largely due to the anions, which followed the order of the lyotropic series in their inactivating capacity: F- less than Cl- less than Br- less than NO3- less than I- less than SCN-. Sodium salts (F-, Cl-, and Br-) produced a very large increase in the pH optimum of the enzymatic reaction (7.4 to at least 8.5) essentially obliterating the ionization of a functional group with pK of 8.1. The kinetics of the enzymatic reaction revealed major differences among the PC particles, and different responses of their kinetic parameters with increasing salt concentrations. The conclusions reached in this work are the following: (1) The relative reactivity of PC substrates, in discoidal particles, with lecithin-cholesterol acyltransferase depends strongly on the concentration and type of salts in the medium. (2) Anions (in lyotropic series) rather than cations affect the enzymatic reaction. (3) There are functional groups with pK of 8.1 which are affected markedly in their ionization behavior by anion binding. (4) The active site of lecithin-cholesterol acyltransferase and its interaction with anions are affected by the exact nature of the PC-apolipoprotein interface.

Apolipoprotein A-I↗
PubMed1986

Synthesis and secretion of lecithin-cholesterol acyltransferase by the human hepatoma cell line HepG2.

The human liver cell line HepG2 was investigated for its synthesis and secretion of lecithin-cholesterol acyltransferase. The cells were grown to confluency in Eagle's minimal essential medium plus 10% fetal bovine serum. At the onset of the study, fetal bovine serum was removed and cells were grown in minimal essential medium only. At 6, 12, 24, and 48 h the cells were harvested, and the culture medium collected at each time point was assayed for lecithin-cholesterol acyltransferase mass and activity, cholesterol esterification rate, and apolipoprotein A-I mass. The rate of the enzyme secretion measured by both mass and activity was linear over 24 h of culture. The enzyme mass by radioimmunoassay was 1.7, 4.1, 7.9 and 13.7 ng/ml culture medium (or 8.3, 19.9, 38.5 and 66.7 ng/mg cell protein), respectively, and enzyme activity using an exogenous source of phosphatidylcholine/cholesterol liposomes containing apolipoprotein A-I as substrate was 85, 170, 315, and 402 pmol cholesterol esterified/h per ml culture medium (or 414, 828, 1534 and 1957 pmol cholesterol esterified/h per mg cell protein) for 6, 12, 24, and 48 h of culture, respectively. The endogenous cholesterol esterification rate of the culture medium was 47, 104, 224 and 330 pmol/h per ml and apolipoprotein A-I mass was 305, 720, 2400 and 3940 ng/ml culture medium over the same time frame. In contrast to culture medium, low levels of enzyme activity (approximately 10% of that in culture medium at 24 and 48 h) were observed in the extracts of HepG2 cells. The enzyme secreted by HepG2 was found to be similarly activated by apolipoprotein A-I, apolipoprotein E, or apolipoprotein A-IV, and was similarly inhibited by phenylmethylsulfonyl fluoride, dithiobisnitrobenzoate, p-hydroxymercuribenzoate, or iodoacetate as compared to human plasma enzyme. High-performance gel filtration of the culture medium revealed that the HepG2-secreted enzyme was associated with a fraction having a mean apparent molecular weight of approximately 200,000. We concluded that human hepatoma HepG2 cells synthesize and secrete lecithin-cholesterol acyltransferase, which is functionally homologous to the human plasma enzyme.

Apolipoproteins↗
PubMed1986

Antigenic relatedness between human lecithin-cholesterol acyltransferase and phospholipases of the A2 family.

A monoclonal antibody, B10, generated against pure human lecithin-cholesterol acyltransferase (EC 2.3.1.43) caused the inhibition of the esterolytic and cholesterol esterifying activities of the enzyme. This antibody also reacted with a number of pancreatic and snake venom phospholipases A2 species but not phospholipase A1. A concentration-dependent inhibition of phospholipase A2 was also seen in the presence of B10. Treatment of lecithin-cholesterol acyltransferase or B10-reacting phospholipases with phenacyl bromide, a reagent known to interact with the active site of phospholipase A2, inhibited both their esterolytic activity and their capacity to bind to B10. A dimeric phospholipase A2 species with a known occluded active site did not cross-react with B10. Thus, lecithin-cholesterol acyltransferase and some enzymes of the phospholipase A2 family share a common antigenic determinant which is probably located near or at their esterolytic active site.

Animals↗
PubMed1988

Substrate specificity of human plasma lecithin-cholesterol acyltransferase towards molecular species of phosphatidylcholine in native plasma.

The specificity of human plasma lecithin-cholesterol acyltransferase for molecular species of phosphatidylcholine (PC) was studied by determining the molecular species composition of whole plasma before and after incubation at 37 degrees C. Since the disappearance of PC under the conditions employed is entirely due to the activity of lecithin-cholesterol acyltransferase, its specificity can be determined from the decrease in the concentration of each species after the reaction. The selectivity factor for each species was calculated by dividing its observed contribution by its concentration at zero time. The major species contributing to cholesterol esterification in whole plasma were 16:0-18:2 (46%), 18:0-18:2 (16%), 16:0-18:1 (15%), 16:0-20:4 (10%), 18:0-20:4 (5%) and 18:1-18:2 (5%). The specificity, as determined from the selectivity factors for whole plasma, was in the order: 16:0-18:2 greater than 18:1-18:2 greater than 16:0-18:1 greater than 18:0-18:2 greater than 16:0-22:6 greater than 18:0-20:4 greater than 16:0-20:4. The high-density lipoproteins (HDL) contained a significantly higher percentage of 16:0-20:4 and 18:0-20:4 and a lower percentage of 16:0-18:1 and 18:0-18:1 compared to the very-low and low-density lipoproteins. These differences disappeared after incubation of the plasma for 24 h. Using selectivity factors for HDL PCs only, the specificity of the enzyme was found to be in the order: 16:0-18:2 greater than 18:1-18:2 greater than 18:1-18:1 greater than 16:0-22:6 greater than 18:0-18:2 greater than 16:0-18:1 greater than 16:0-20:4. These results indicate that in native plasma, lecithin-cholesterol acyltransferase prefers 16:0 greater than 18:1 greater than 18:0 at the 1-position and 18:2 greater than 18:1 greater than 22:6 greater than 20:4 at the 2-position of PC.

Adult↗
PubMed1984

Interaction of calcium and neomycin with anionic phospholipid-lecithin liposomes. A differential scanning calorimetry study.

The interactions of calcium and neomycin with liposomes of various anionic phospholipids plus lecithin were studied by differential scanning calorimetry. Phosphatidylinositol bisphosphate differed from other acidic phospholipids in its interactions with both calcium and neomycin. Calcium, at concentrations as low as 1 mM, induced the appearance of a second transition peak in phosphatidylinositol bisphosphate-enriched liposomes only. Neomycin acted antagonistically and precluded this phase separation. In addition, neomycin lowered the phase transition temperature of phosphatidylinositol bisphosphate-lecithin liposomes while it raised the transition temperature of all other anionic phospholipid-lecithin liposomes tested. This fluidizing effect of neomycin and the antagonism to calcium may induce critical alterations of properties of biological membranes. The study supports and extends our previous findings and conclusions that phosphatidylinositol bisphosphate may play a crucial role in the expression of aminoglycoside toxicity.

Calcium↗
PubMed1977

A new colorimetric method for the determination of plasma lecithin-cholesterol acyltransferase activity.

A new method has been developed to improve the linearity of the plasma lecithin-cholesterol acyltransferase reaction by adding synthetic dipalmitoyl lecithin sol to the incubation medium and subsequently measuring the change in free cholesterol content after incubation by a combined enzymatic method using cholesterol oxidase and peroxidase. The method was found to be simpler than other techniques such as gas-liquid chromatography or isotope methods and therefore may be suitable for the routine assays of plasma lecithin-cholesterol acyltransferase activities in clinical laboratories.

Acyltransferases↗
PubMed1985

The generation of lysolecithin by enterokinase in trypsinogen prophospholipase A2 lecithin mixtures, and its relevance to the pathogenesis of acute necrotising pancreatitis.

The cascade enterokinase-trypsinogen-prophospholipase A2 lecithin, generating trypsin, phospholipase A2 and lysolecithin, respectively, was studied in vitro using a novel phospholipase A2 assay. The rate of enterokinase catalysed activation of trypsinogen was maximal at 4 mmol/1 glycodeoxycholic acid; higher concentrations of bile salt progressively inhibited enterokinase activity. Net phospholipase A2 activity in reaction mixtures was critically dependent on the trypsin/prophospholipase A2 molar ratio. Lecithin hydrolysis by phospholipase A2 was dependent on the bile salt/lecithin molar ratio and was optimal at 1.25 to 1. The addition of enterokinase to lecithin and bile salt mixtures, containing trypsinogen and prophospholipase A2 at presumed pathophysiological concentrations, resulted in the generation of concentrations of lysolecithin lytic for pancreatic acinar cells within 5 min. These findings would support the concept that the entry of bile containing active enterokinase into the pancreatic duct system in vivo may in some cases be involved in the initiation of necrotising acute pancreatitis in man.

Acute Disease↗
PubMed1996

DNA initiates polymorphic structural transitions in lecithin.

The inverted micellar phase, obtained by treating lecithin and Ca(2+)-DNA complex with chloroform, was used as an intermediate step in the preparation of DNA-Ca(2+)-lecithin complex. DSC analysis demonstrated the involvement of a large fraction of lipid in the interaction with DNA. Freeze-fracture electron microscopy revealed (i) rod-like structures on the hydrophobic fracture surface of membranes and (ii) regular bundles of fibrils with a repeat distance of about 6 nm, which were located free in solution. Similar regular bundles of fibrils were also revealed by staining the samples with uranyl acetate. According to the suggested model, the observed structures are hexagonally packed inverted lipid tubes, with DNA located in their central cores. The possible biological relevance of the capability of Ca(2+)-DNA to initiate polymorphic phase transitions of lecithin is discussed.

Animals↗
PubMed1977

Uptake and metabolism of 3H-fatty acid labelled lecithin by normal and atherosclerotic intima in vivo and in vitro.

Following the intravenous injection of tracer doses of 3H-fatty acid labelled lecithin into normally fed rabbits, minimal incorporation into serum cholesterol ester over a 6-hour period was observed. The labelled phospholipid was rapidly removed from the serum primarily by R.E. tissues with little uptake by the aorta. The uptake and metabolism of 3H-fatty acid labelled lecithin by explants of normal and cholesterol-fed rabbit aortas was also investigated in vitro for periods up to 8 days. Low uptake and incorporation into cholesterol ester was observed for the normal aortic explants. The labelled phospholid was taken up to a greater extent by explants from cholesterol-fed rabbit aortas and the fatty acid incorporated into both triglyceride and cholesterol ester in the explants. The incorporation of fatty acid into cholesterol ester from phospholipid was associated with hydrolysis of the phospholipid and re-incorporation of the fatty acid into cholesterol ester, rather than by the action of lecithin cholesterol acyl transferase.

Animals↗
PubMedDate not supplied

Enhancement of the response of ageing mouse lymphocytes by in vitro treatment with lecithin.

The effect of treatment with phosphatidylcholine (lecithin) has been examined on splenocytes from young (6 months) and aged (greater than 20 months) (C3H/eB X C57BL/6)F1 male mice, by probing their responsiveness to proliferative signals of mitogens and mixed lymphocyte reaction. This study was initiated since old mice have been known to manifest an increased ratio of cholesterol: phospholipids (C/PL) in their plasma membranes. Unlike the case in young mice, enhancement in proliferative responses was demonstrated on cells from the old after incubation with lecithin. This enhancement was achieved using a variety of methods by which lymphocytes were exposed to the lipids, and which were all designed to reduce the C/PL ratio in the plasma membrane. The observed differences in response to lecithin treatment between young and old did not stem from different extents of lipid incorporation, and seems unlikely to be a result of modified binding of the mitogens after treatment with the lipid. The data suggest that the normal functional performance of specific reactions, which decline with age, may be restored artificially at an appropriate age, if other complementary functions have not been damaged.

Aging↗
PubMed1985

High-dose depot-medroxyprogesterone acetate--effects on the fatty acid composition of serum lecithin and cholesterol ester.

Twenty-one women were treated with high doses of depot-medroxyprogesterone acetate (1000 mg/week im) for 6 months as part of the treatment of endometrial carcinoma. The relative fatty acid composition of serum lecithin and cholesterol ester were analyzed. In previous studies low doses of medroxyprogesterone acetate (MPA), in contrast to progestins of the 19-nor-testosterone series, have been shown not to affect the fatty acid composition of serum lecithin and cholesterol ester. However, the high doses of MPA used in this study increased linoleic acid and decreased arachidonic and di-homogammalinolenic acids in serum lecithin. The ability of a steroid to induce this shift has been ascribed to its androgenicity. MPA is considered to have weak, if any, such properties. The findings of this study emphasize the necessity to delineate effects on metabolism of different doses and administrative routes of any particular steroid.

8,11,14-Eicosatrienoic Acid↗
PubMed1979

Serum and liver lipid composition and lecithin: cholesterol acyltransferase in horses, Equus caballus.

1. The lipid composition of serum and liver and some properties of serum lecithin: cholesterol acyltransferase of the horse were investigated. 2. Phospholipids and cholesterol were the major components of serum lipids and the concentration of triglyceride was considerably low. The concentration of liver lipids was comparable with that of other mammals. 3. Fatty acid composition of serum cholesterol ester resembled that of the 2-position of lecithin, except palmitic acid. 4. The activity of serum cholesterol esterifying enzyme was found to be 0.03-0.09 mumol/hr per ml. There was an equimolar decrease in free cholesterol and lecithin during incubation, and changes in unsaturated fatty acids in these two components were in good agreement. 5. Cholesterol esterification was reversibly inhibited by 5,5'-dithiobis-(2-nitrobenzoic acid). The acyl-transferase had a specificity for linoleic acid.

Animals↗
PubMed2005

Inhibition assay of beta-hematin formation initiated by lecithin for screening new antimalarial drugs.

Measurement of heme crystallization provides a tool for screening new antimalarial drugs. Current assays for heme crystallization have employed initiators such as thermo, histidine-rich proteins, and lipids extracted from parasites and infected plasma. These initiators are unnatural or require laborious steps to prepare. In this study, we used a commercially available lipid, lecithin, a kind of phospholipid containing about 50% unsaturated fatty acids, as an initiator for heme crystal (beta-hematin) formation. We demonstrated that the inhibition of lecithin-based beta-hematin formation by antimalarial drugs is highly correlated with the preformed beta-hematin-based method. In addition, the lecithin-based assay is sensitive and convenient for large-scale screening of new novel antimalarials. We also indicated that dimethyl sulfoxide is an ideal solvent for preparation of heme stock solution, which is stable and can be used for 1 month.

Antimalarials↗
PubMed2005

Dialkylphosphatidylcholine and egg yolk lecithin for emulsification of various triglycerides.

Synthesized saturated phosphatidylcholine (PC) and egg yolk lecithin (EYL) were investigated to explore their influence on particle sizes in emulsions when dispersing various triglycerides (TG). One of four different kinds of synthesized saturated PC (DLPC, DMPC, DPPC and DSPC) or three different kinds of EYL (purified EYL (PEL) and hydrogenated purified EYL with two different iodine values (IV), R-20 and R-5), 2.5% (w/w) glycerol solution and one of four kinds of TG (tricaprylin, tricaprin, trilaurin and trimyristin) were sonicated five times for 1 min with intervals of 0.5 min. When using four kinds of synthesized saturated PCs as emulsifiers, the carbon numbers of each PC had a strong correlation with the mean diameters of the emulsion when analyzed with each of the four kinds of TG used in the study (regression function ranged from 0.811 to 0.915). The carbon numbers of the TG had less correlation with the mean diameters than the PC in simple regression analysis (regression function ranged from 0.236 to 0.875). Multiple regression analysis using the carbon numbers both of the PC and TG as independent variables was remarkably significant in the regression function (2.0 x 10(-14)) and all regression coefficients (2.7 x 10(-13), 5.8 x 10(-7) and 1.9 x 10(-9) for PC, TG and intercept, respectively). Among the regression coefficients, the contribution of the carbon number of the PC was the most significant. These results indicated that a multiple regression function should be useful to estimate the mean diameters of emulsion droplets in any combinations of PC and TG used in this study. In the experiments using three kinds of EYL, the mean diameters also tended to increase according to the order of PEL, R-20 and R-5, which corresponds to the order of degrees of saturation (IV = 75, 20 and 2, respectively). The experimental values for EYL were compared with the estimated values calculated by the multiple regression function derived from synthesized PC data using the arithmetic carbon number, based on the components of each EYL. The estimated mean diameters were at comparable levels to the corresponding experimental mean diameters in the most saturated hydrogenated lecithin (R-5), while those were larger than the experimental mean diameters in two less saturated kinds of lecithin (R-20 and purified EYL). These findings gave useful information on the mean diameters of emulsion droplets when designing an emulsion formulation using a particular combination of a phospholipid and triglyceride.

Caprylates↗
PubMed1971

A lamellar complex of lecithin and poly-L-tyrosine.

Complexes of poly-L-tyrosine (PT) with dipalmitoyllecithin, synthetic, (DPL) and with egg lecithin (EL) have been obtained by precipitation from methanol-water solutions. Chemical analysis indicates that both lecithins bind PT up to a limiting ratio of about 4 tyrosine residues/lecithin molecule. DPL-PT complexes have a lamellar structure closely resembling lecithin itself. In fact, DPL and DPL-PT lamellae have very nearly the same thickness as precipitated from methanol-water, although their swelling behavior on resuspension in pure water is different. The complexes crystallize in the form of hexagonal platelets, some monolayers and some with terraced spiral growths, with a thickness of 50-55 A. In X-ray and electron diffraction they yield sharp reflections at 4.14 A which are characteristic of hexagonal packing of phospholipid paraffinic chains. The order-disorder transition temperature of this crystalline lattice, determined by differential scanning calorimetry, is somewhat higher in the complex than in pure DPL. Physical models consistent with these observations are discussed.

Calorimetry↗
PubMed1975

Tilted hydrocarbon chains of dipalmitoyl lecithin become perpendicular to the bilayer before melting.

Differential scanning calorimetry studies of dipalmitoyl lecithin show two reversible transitions as the temperature is changed between 20 and 50 degrees C. A pretransition endotherm occurs at 35 degrees C prior to the main chain melting endotherm which occurs at 42 degrees C. X-ray diffraction studies show that below 33 degrees C the chains of the lecithin are fully extended, packed in a hexagonal crystalline lattice but tilted with respect to the plane of the bilayer. Between 35 and 42 degrees C the chains are similarly packed but oriented perpendicular to the bilayer plane. Above 44 degrees C the chains are "melted" or disordered. Monolayer studies of dipalmitoyl lecithin using continuous recording of pressure with molecular area reveal the existence of two solid condensed phases corresponding to these tilted and verticle chain structures. The tilted to perpendicular transition would account for the pretransition endotherm of the lipid; the crystalline to melted change corresponds to the larger transition observed at 42 degrees C.

Calorimetry↗
PubMed1994

Structural characterization of the micelle-vesicle transition in lecithin-bile salt solutions.

Small-angle neutron scattering (SANS) and dynamic light scattering (QLS) are used to characterize the aggregates found upon dilution of mixed lecithin-bile salt micelles. Molar ratios of lecithin (L) to taurocholate (TC) studied varied from 0.1 to 1, and one series contained cholesterol (Ch). Mixed aggregates of L and taurodeoxycholate (TDC) at ratios of 0.4 and 1 were also examined. In all cases the micelles are cylindrical or globular and elongate upon dilution. The radius of the mixed micelles varies only slightly with the overall composition of lecithin and bile salt which indicates that the composition of the cylindrical micelle body is nearly constant. The transition from micelles to vesicles is a smooth transformation involving a region where micelles and vesicles coexist. SANS measurements are more sensitive to the presence of two aggregate populations than QLS. Beyond the coexistence region the vesicle size and degree of polydispersity decrease with dilution. Incorporation of a small amount of cholesterol in the lipid mixture does not affect the sequence of observed aggregate structures.

Bile Acids and Salts↗
PubMed2003

Peroxyl radicals promoted changes in water permeability through gramicidin channels in DPPC and lecithin-PC vesicles.

Gramicidin incorporation to DPPC or lecithin-PC large unilamellar vesicles (LUVs) leads to pore formation that, under hyper-osmotic conditions, produces a noticeable increase in the rate of trans-membrane water flow. This pore formation is more efficient in the more fluid lecithin-PC LUVs. Exposure of these vesicles to peroxyl radicals generated in the aerobic thermolysis of 2,2'-azo-bis(2-amidinopropane) (AAPH), changes the physical properties of the bilayer (as sensed employing fluorescent probes), modifies gramicidin molecules (as sensed by the decrease in Trp fluorescence) and notably reduces the transbilayer rate of water outflow. In order to evaluate if this reduced water-transport capacity is due to changes in the membrane due to lipid-peroxidation and/or direct damage to gramicidin channels, results obtained in the oxidable vesicles (lecithin-PC) were compared to those obtained in DPPC vesicles. The data obtained show that most of the water transport efficiency loss can be ascribed to a direct disruption of gramicidin channels by AAPH derived peroxyl radicals.

1,2-Dipalmitoylphosphatidylcholine↗