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PubMedDate not supplied

Structural aspects of bile salt-lecithin mixed micelles.

Mixed micells formed by the major constituents of native bile, i.e., bile salts and lecithin, were studied by X-ray scattering differential scanning calorimetry and electron spin resonance spectroscopy. The aim of this study was to find differences in micellar structure and thermodynamic properties which might explain the different abilities of various biles to keep cholesterol in solution at comparable degrees of oversaturation. The data indicate not only that different types of micelles exist but also that different types of complex bile salt/phospholipid arrangements exist within the micelles. It is speculated that the stabilizing and destabilizing factors found by other authors influence the balance between the different micellar structures and thereby the cholesterol solubilization power of bile.

Bile Acids and Salts↗
PubMed1999

Hepatic regulation of platelet-activating factor acetylhydrolase and lecithin:cholesterol acyltransferase biliary and plasma output in rats exposed to bacterial lipopolysaccharide.

Normal rat bile contains secretory platelet-activating factor acetylhydrolase (PAF-AH), the enzyme capable of hydrolyzing the inflammatory mediator platelet-activating factor (PAF), and phospholipids containing oxidized truncated fatty acids. Because lecithin:cholesterol acyltransferase (LCAT) possesses intrinsic PAF-AH-like activity, it also may represent a potential anti-inflammatory enzyme. The behavior of PAF-AH and LCAT in hepatobiliary inflammatory responses in vivo has not been characterized. We therefore investigated the biliary and plasma secretion and pharmacological characteristics of these enzymes in rats subjected to intraportal bacterial endotoxin exposure (lipopolysaccharide [LPS], Escherichia coli, 055:B5). Portal vein LPS infusion (1 mg/kg, bolus) resulted in a maximal 4- to 5-fold increase in bile PAF-AH-specific activity with a gradual decline to baseline by 18 hours. Biliary PAF-AH hydrolyzed also the truncated sn-2-succinoyl and sn-2-glutaroyl analogs of PAF, indicating a broader activity of PAF-AH in bile toward byproducts of glycerophospholipid peroxidation. Plasma PAF-AH activity was not altered 5 hours after LPS injection compared with saline injection, but it was significantly elevated 18 hours after endotoxin exposure. The levels of LCAT in bile were low and declined to nearly undetectable values by 5 hours after cannulation in both control and LPS-exposed rats. Plasma LCAT activity was significantly increased after 5 hours and decreased 18 hours after LPS injection. In summary, hepatic exposure to endotoxin results in a rapid increase in biliary secretion of PAF-AH followed by elevation of LCAT and PAF-AH levels in plasma. We propose that biliary secretion of PAF-AH may be involved in the hepatic response to endotoxic insult by counteracting potential inflammatory damage in the biliary tree and gastrointestinal tract, whereas plasma increases in LCAT and PAF-AH may promote elimination of excess PAF and oxidized phospholipids in the circulation.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗
PubMed1991

Lecithin: cholesterol acyltransferase activity in familial hypercholesterolemia treated with simvastatin and simvastatin plus low-dose colestipol.

In 19 patients with heterozygous familial hypercholesterolemia (FH), the effects of simvastatin (S) 20 mg/d, 40 mg/d, and 40 mg/d plus low-dose colestipol (10 g/d) on plasma lipids, plasma lipoproteins, and plasma lecithin: cholesterol acyltransferase (LCAT) activity were investigated after an original dose-range escalation/descalation design. The drug regimen was changed every 8 weeks. A significant reduction in total cholesterol and LDL-cholesterol was observed, reaching 39% and 54% for the drug combination (week 28), and total apoprotein B and LDL-apoprotein B were reduced by 39% and 50%, respectively. Triglycerides were significantly lowered by S alone (up to 29% with 40 mg/d). HDL-cholesterol increased during therapy but the cholesterol content in HDL2-HDL3 fractions (isolated by ultracentrifugation) did not change significantly during the different steps. The ratio LDL-C/HDL-C fell by 57% at week 28. Plasma LCAT activity expressed as FER was significantly enhanced by S alone (+33%), and a further increase on drug combination regimen (+58%) was observed. This effect could be considered as a consequence of the increased fractional clearance of LDL-C. It tended to be sustained during the descalation part of the study. Biochemical adverse effects were scarce and transient. In conclusion, the combination therapy increased the plasma LCAT/FER activity without a preferential enhancement in HDL2-C concentration. This original design allowed to define the most appropriate individual cholesterol-lowering drug dosage in FH patients.

Adolescent↗
PubMed1995

Inhibition of lecithin: cholesterol acyltransferase activity in human blood plasma by cigarette smoke extract and reactive aldehydes.

Cigarette smoking is a risk factor for atherosclerosis. It is conceivable that reactive chemical components in cigarette smoke may adversely affect reverse cholesterol transport at the level of lecithin: cholesterol acyltransferase (LCAT) and promote atherogenesis. Hence, the effect of cigarette smoke extract (CSE) on the activity of LCAT in human plasma was studied. When incubated with plasma, CSE caused both concentration- and time-dependent losses of LCAT activity. Addition of glutathione, but not ascorbate, to plasma prevented loss of LCAT activity caused by CSE. Incubation of plasma with some reactive aldehydes known to be present in cigarette smoke also inhibited LCAT activity. Among five aldehydes tested, acrolein was the strongest inhibitor of LCAT, with complete enzyme inhibition occurring at 1 mM. Acetaldehyde was the weakest inhibitor of LCAT, with 85% enzyme inhibition at 50 mM. Hexanal, formaldehyde, and malondialdehyde completely inhibited LCAT activity at 10, 50, and 50 mM, respectively. When plasma was incubated with 1 mM acrolein in the presence of 2.5 mM glutathione or dihydrolipoic acid, 100 and 57% of LCAT activity, respectively, remained after incubation. This finding suggest that reactive aldehydes may form adducts with certain free sulfhydryl groups functioning in the active site of LCAT to inhibit enzyme activity. It is concluded that reactive aldehydes are at least partially responsible for the reduction in LCAT activity in plasma treated with CSE.

Adult↗
PubMedDate not supplied

Sonographically detected free-floating particles in amniotic fluid predict a mature lecithin-sphingomyelin ratio.

Amniocentesis for the determination of fetal lung maturity is associated with some morbidity, and so the prediction of a mature fetus by noninvasive means could be useful in managing certain high-risk patients. In the first portion of this study, 467 amniotic fluid specimens were classified into one of five groups based on the turbidity and particulate matter present. The mean lecithin: sphingomyelin (L/S) ratios and the proportion of mature L/S values increased with increasing turbidity of the amniotic fluid. In the second portion of this study, the presence of amniotic fluid free-floating particles (FFPs) detected by real-time ultrasound was correlated with fetal lung maturity. When FFPs were present, the L/S was uniformly mature, but when FFPs were absent, the L/S was mature in 74% of patients (P less than 0.01). Among the patients in whom the L/S ratios were mature, 39/110 were associated with positive FFPs (sensitivity = 35%). This preliminary study suggests that the presence of FFPs on real-time ultrasound could be used to confirm fetal lung maturity.

Amniotic Fluid↗
PubMed2002

Amphotericin B in oil-water lecithin-based microemulsions: formulation and toxicity evaluation.

A novel lecithin-based microemulsion containing AmB was developed to reduce the toxic effects of the drug, comparing it with the commercial formulation Fungizone. Phase diagrams containing the microemulsion region were constructed for pseudoternary systems composed of isopropil myristate (IPM)/Brij((R)) 96V/lecithin/water. The incorporation of AmB to the microemulsions was done following the Phase Inversion Temperature (PIT) method or by diluting the drug in the aqueous phase of the disperse system before forming the microemulsion. The percentage of drug entrapped in the microemulsion was analyzed by an HPLC method obtaining recoveries > 98%. Mean droplet size of the microemulsions chosen for the acute toxicity evaluation was of 45 nm, and the rheological studies showed that those microemulsions mentioned followed a Newtonian behavior. Different studies are described in this work to prove the stability of these new dosage forms. Acute toxicity results, determined by a graphic method, the probit binary model and the Reed and Muench method showed that lethal dose 50 (LD(50)) for AmB microemulsions was of 2.9 mgkg(-1) compared to 1.4 mgkg(-1) for the commercial deoxycholate suspension, Fungizone. The overall results indicate that treatment with AmB microemulsions was less toxic than Fungizone, suggesting a potential therapeutic application.

Amphotericin B↗
PubMed2002

Transport across 1,9-decadiene precisely mimics the chemical selectivity of the barrier domain in egg lecithin bilayers.

The barrier domain solubility-diffusion theory of lipid bilayer permeability relates the permeability coefficient (P(m)) to the solute's partition coefficient (PC(barrier/w)) and diffusion coefficient (D(barrier)) in the ordered chain region of the bilayer that serves as the barrier region for polar permeants. To select the best solvent to mimic the barrier domain, permeability coefficients across a layer of 1,9-decadiene were compared with permeability coefficients from bilayer transport. Rate constants for transport, k, of alpha-methyl substituted analogues of p-toluic and p-methylhippuric acid were measured across a layer of 1,9-decadiene embedded in a PTFE filter membrane placed between two aqueous solutions in side-by-side diffusion cells. Permeability coefficients (P(1,9-decadiene)) were normalized to that obtained for p-toluic acid, which was included in donor solutions. The correlation of log(P(bilayer)) versus log(P(1,9-decadiene)) was linear with a slope of 0.99 +/- 0.02 SD, indicating that 1,9-decadiene precisely mimics the egg lecithin bilayer barrier domain in its chemical selectivity. Using the decadiene membrane transport method to indirectly estimate partition coefficients for similarly sized permeants extended the range of measurable values beyond those readily attainable by the traditional shake-flask method, allowing measurement of 1,9-decadiene/water PCs as low as 3 x 10(-7).

Algorithms↗
PubMed2003

Lecithin-based oil-in-water microemulsions for parenteral use: pseudoternary phase diagrams, characterization and toxicity studies.

Pseudoternary phase diagrams have been constructed to evaluate the phase behavior of systems containing water/lecithin/polysorbate 80/isopropyl myristate at different polysorbate 80:lecithin weight ratios (K(m)). Oil-in-water microemulsion regions were accurately determined and the influence of the K(m) on the area of existence of such disperse systems was also examined. Viscosity studies as well as particle size analysis by dynamic light scattering were carried out on oil-water microemulsions, and the influence of the oil phase content, the total amount of surfactants and K(m) on the rheological behavior, viscosity, and droplet size of such disperse systems was evaluated. All systems studied showed a water-rich isotrope region (oil-in-water microemulsion area), that was seen to be highly dependent upon the surfactant/cosurfactant weight ratio. Most of the microemulsions analyzed showed a non-Newtonian rheological behavior and both, droplet size, and viscosity of the disperse systems, were found to be much more influenced by the total content of oil phase and surfactants present in the microemulsion than by the K(m). The selected system underwent both stability and in vivo acute toxicity studies, and seemed to be highly stable, even at extreme conditions, and very low toxic according to the results obtained.

Animals↗
PubMed1993

A rapid method for the measurement of cholesterol thermodynamic activity in bile salt-lecithin-cholesterol solutions.

Earlier work from this laboratory suggested that the cholesterol (Ch) thermodynamic activity is a more meaningful measure of the degree of Ch supersaturation in human bile than the widely known cholesterol saturation index. An early version of a method for determining thermodynamic activity based on Ch uptake from bile salt (BS)-lecithin (LE) solutions into silicone polymer particles, but requiring 12-24 h for reaching equilibrium, was considered unsatisfactory because Ch nucleation and crystal formation frequently occurred within a few hours. The aim of the present work was to develop a method that would reduce equilibration times to the order of 1 h. Changing the thickness of the silicone film alone did not result in the desired reduction of equilibration times and it was soon deduced that the uptake of Ch by the silicone film from the BS-LE solution was a surface-controlled transport process involving the transport of Ch by negatively charged BS and BS-LE micelles at the interface. Three different approaches were tried to modify the silicone film to make its surface positively charged, thereby reducing and/or eliminating the presumed electrical repulsion barrier for the interfacial transport of Ch. The film was treated with different concentrations of aminopropyl methyl-dimethylsiloxane (AMDS) in cyclohexane, octadecyldimethyl-[3-(trimethoxysilyl)-propyl] ammonium chloride (ODTOP) in methanol, and octadecylamine solution in ethanol. Films treated with 1-1.5% ODTOP and 5-10% AMDS reduced the Ch equilibration times for model BS-LE solutions to < 1 h.

Bile Acids and Salts↗
PubMed2001

Haptoglobin inhibits lecithin-cholesterol acyltransferase in human ovarian follicular fluid.

The activity of the enzyme lecithin-cholesterol acyltransferase (LCAT; E.C. 2.3.1.43) is involved in the removal of cholesterol excess from peripheral cells. This activity is stimulated by the HDL (high density lipoprotein) apolipoprotein A1 (ApoA1). Haptoglobin (Hpt) was previously found to be associated with ApoA1 in ovarian follicular fluid. LCAT activity was analyzed in follicular fluids, collected from an IVF program, containing different amounts of Hpt or Hpt/ApoA1 ratio. Addition of purified Hpt to follicular fluid caused a decrease in the enzyme activity, which was measured as the rate of synthesis of cholesteryl esters. In the fractions of fluid proteins, as obtained by gel filtration chromatography, Hpt and HDL were titrated by ELISA while the LCAT activity was assayed by using radioactive cholesterol and purified HDL. When isolated LCAT was incubated with fractions containing different Hpt/ApoA1 ratios, the enzyme activity was found negatively correlated with the Hpt/ApoA1 ratio (P < 0.01). LCAT kinetic parameters were measured in two fractions with the same amount of ApoA1 (5 microg/ml) but different amounts of Hpt (0.69 or 3.77 microg/ml): the V(max) did not change while the K(m) values were 24.1 or 78.6 microM in the presence of the low or high Hpt level, respectively. The analysis of fluids associated with cytoplasmically mature MII oocytes, in a cross-sectional study, confirmed that a negative correlation exists between the Hpt/ApoA1 ratio and the LCAT activity (P < 0.01). The results suggest that Hpt inhibits the reverse transport of cholesterol by preventing ApoA1 stimulation of the LCAT activity.

Apolipoprotein A-I↗
PubMed1995

Site-specific detection and structural characterization of the glycosylation of human plasma proteins lecithin:cholesterol acyltransferase and apolipoprotein D using HPLC/electrospray mass spectrometry and sequential glycosidase digestion.

Site-specific structural characterization of the glycosylation of human lecithin:cholesterol acyltransferase (LCAT) was carried out using microbore reversed-phase high performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC/ESIMS). A recently described mass spectrometric technique involving monitoring of carbohydrate-specific fragment ions during HPLC/ESIMS was employed to locate eight different groups of glycopeptides in a digest of a human LCAT protein preparation. In addition to the four expected N-linked glycopeptides of LCAT, a di-O-linked glycopeptide was detected, as well as three additional glycopeptides. Structural information on the oligosaccharides from all eight glycopeptides was obtained by sequential glycosidase digestion of the glycopeptides followed by HPLC/ESIMS. All four potential N-linked glycosylation sites (Asn20, Asn84, Asn272, and Asn384) of LCAT were determined to contain sialylated triantennary and/or biantennary complex structures. Two unanticipated O-linked glycosylation sites were identified at Thr407 and Ser409 of the LCAT O-linked glycopeptide, each of which contain sialylated galactose beta 1-->3N-acetylgalactosamine structures. The three additional glycopeptides were determined to be from a copurifying protein, apolipoprotein D, which contains potential N-linked glycosylation sites at Asn45 and Asn78. These glycopeptides were determined to bear sialylated triantennary oligosaccharides or fucosylated sialylated biantennary oligosaccharides. Previous studies of LCAT indicated that removal of the glycosylation site at Asn272 converts this protein to a phospholipase (Francone OL, Evangelista L, Fielding CJ, 1993, Biochim Biophys Acta 1166:301-304). Our results indicate that the carbohydrate structures themselves are not the source of this functional discrimination; rather, it must be mediated by the structural environment around Asn272.

Amino Acid Sequence↗
PubMed1998

A proposed architecture for lecithin cholesterol acyl transferase (LCAT): identification of the catalytic triad and molecular modeling.

The enzyme cholesterol lecithin acyl transferase (LCAT) shares the Ser/Asp-Glu/His triad with lipases, esterases and proteases, but the low level of sequence homology between LCAT and these enzymes did not allow for the LCAT fold to be identified yet. We, therefore, relied upon structural homology calculations using threading methods based on alignment of the sequence against a library of solved three-dimensional protein structures, for prediction of the LCAT fold. We propose that LCAT, like lipases, belongs to the alpha/beta hydrolase fold family, and that the central domain of LCAT consists of seven conserved parallel beta-strands connected by four alpha-helices and separated by loops. We used the conserved features of this protein fold for the prediction of functional domains in LCAT, and carried out site-directed mutagenesis for the localization of the active site residues. The wild-type enzyme and mutants were expressed in Cos-1 cells. LCAT mass was measured by ELISA, and enzymatic activity was measured on recombinant HDL, on LDL and on a monomeric substrate. We identified D345 and H377 as the catalytic residues of LCAT, together with F103 and L182 as the oxyanion hole residues. In analogy with lipases, we further propose that a potential "lid" domain at residues 50-74 of LCAT might be involved in the enzyme-substrate interaction. Molecular modeling of human LCAT was carried out using human pancreatic and Candida antarctica lipases as templates. The three-dimensional model proposed here is compatible with the position of natural mutants for either LCAT deficiency or Fish-eye disease. It enables moreover prediction of the LCAT domains involved in the interaction with the phospholipid and cholesterol substrates.

Amino Acid Sequence↗
PubMed1988

Lecithin:cholesterol acyltransferase activation by synthetic amphipathic peptides.

The amphipathic helical theory of Segrest and colleagues (FEBS Lett.:38:247-253, 1974) proposes that the lipid-binding segments of serum apolipoproteins are in an alpha helical conformation. Furthermore the helices have a hydrophobic face and a hydrophilic face with a specific distribution of positively and negatively charged residues. The importance of the pattern of the charged residues in the lipid binding and lecithin:cholesterol acyltransferase (LCAT) activation by the segments is still debated. We designed a 30-residue peptide, GALA, which in the alpha helical conformation has a hydrophilic face composed of glutamic acid residues (Sabbarao et al.: Biochemistry 26:2964-2972, 1987). GALA behaves like the serum apolipoproteins in its interaction with dimyristoylphosphatidylcholine (DMPC) at neutral pH; the amino terminal tryptophan of GALA undergoes a blue shift in its fluorescence emission spectrum, and the circular dichroism (CD) spectrum indicates that GALA acquires alpha helical structure in the presence of DMPC. A DMPC-GALA:19/1 (molar ratio) complex can be isolated by gel-permeation chromatography. This complex has a discoidal structure with the approximate dimensions of 44-A edge thickness and a 170- to 350-A diameter. GALA activates LCAT with DMPC but not with unsaturated phospholipids as the substrate. The apparent partition coefficient of GALA into DMPC vesicles is 100-fold larger than into egg phosphatidylcholine vesicles. The interaction of GALA with unsaturated lipids at neutral pH is so weak that no detectable change in the spectroscopic properties of GALA or the structure of the liposomes can be detected under the conditions used here. The sequence of GALA differs from previously studied model Apo A1 peptides by the absence of positively charged residues on the hydrophilic face. This indicates that positive charges in Apo A1-like peptides are not required in order to form discoidal structures with saturated phospholipids or to activate LCAT with such lipid substrates.

Chromatography, Gel↗
PubMed1993

Regulation of hepatic lecithin: retinol acyltransferase activity by retinoic acid.

The activity of the hepatic enzyme lecithin: retinol acyltransferase (LRAT), thought to catalyze the esterification of retinol for storage, was previously shown to vary directly with the vitamin A (retinol) status of the rat [Randolph, R. K., and Ross, A. C. (1991) J. Biol. Chem. 266, 16453-16457]. The present studies were designed to determine whether liver LRAT activity is regulated in vivo by retinoic acid, a principal active metabolite of retinol. LRAT activity was negligible in the livers of vitamin A-deficient rats. Following treatment with a single 2-micrograms dose of retinoic acid, LRAT activity increased significantly while, following treatment with a single 20-micrograms dose, liver LRAT activity equalled that of vitamin A-sufficient adult rats. Retinoic acid was more effective than an equimolar quantity of retinol in restoring LRAT activity in the vitamin A-deficient rat liver. The increase in hepatic LRAT activity after administration of retinoic acid occurred rapidly, reaching a maximum within 12-16 h but declining again after 48-72 h. Studies were conducted in vivo to gain insight into the level of regulation of LRAT by retinoic acid. The increase in LRAT activity by retinoic acid in the vitamin A-deficient rat was blocked completely by both actinomycin D and cycloheximide. The ability of liver to esterify retinol in vivo was correlated with the in vitro activity of LRAT after retinoic acid induction. We conclude that retinoic acid, an important end product of retinol metabolism, regulates a key aspect of hepatic retinol metabolism through its regulatory activity on liver LRAT.

Acyltransferases↗
PubMed1996

Reaction of phosphatidylcholine hydroperoxide in human plasma: the role of peroxidase and lecithin:cholesterol acyltransferase.

In order to elucidate the reason why phosphatidylcholine hydroperoxide is unstable in human plasma, 1-palmitoyl-2-linoleoylphosphatidylcholine hydroperoxide (PLPC-OOH) was incubated aerobically in human plasma at 37 degrees C, and its decomposition products were measured. The major product was the corresponding alcohol (PLPC-OH) and this reduction probably occurred by an enzymatic process since no acceleration in ascorbate depletion and no significant decrease in other plasma antioxidants were observed upon addition of PLPC-OOH. Cholesteryl linoleate hydroperoxide and its alcohol (Ch18:2-OH) were also detected as minor products. Similarly, 1-stearoyl-2-arachidonoylphosphatidylcholine hydroperoxide gave its alcohol (SAPC-OH) as a major product and cholesteryl arachidonate hydroperoxide and its hydroxide (Ch20:4=OH) as minor products. These oxidized cholesteryl esters are likely to be produced by the action of lecithin:cholesterol acyltransferase (LCAT) present in high-density lipoprotein (HDL) since (a) incubation of PLPC-OH and SAPC-OH in human plasma gave Ch18:2-OH and Ch20:4-OH, respectively, (b) isolated human HDL converted PLPC-OH to Ch18:2 OH and SAPC-OH to Ch20:4-OH while isolated human low-density lipoprotein was inactive for this conversion, and (c) formation of oxidized cholesteryl esters in plasma and HDL was inhibited by the LCAT inhibitor 5,5'-dithiobis(2-nitrobenzoic acid). A possible beneficial role of LCAT for converting phosphatdylcholine hydroperoxide to cholesteryl ester hydroperoxide is also discussed.

Adult↗
PubMed2001

Probing the 121-136 domain of lecithin:cholesterol acyltransferase using antibodies.

Lecithin:cholesterol acyltransferase (LCAT) catalyzes the esterification of plasma lipoprotein cholesterol in mammals as part of the reverse cholesterol transport pathway. Studies of the natural mutations of LCAT revealed a region that is highly sensitive to mutations (residues 121-136) and it is highly conserved in six animal species. The purpose of these studies was to investigate the reactivity of wild type and several mutated forms of LCAT, with a series polyclonal antibodies to further characterize this specific domain (residues 121-136). Two polyclonal antibodies directed against the whole enzyme, one against human plasma LCAT and the other against purified recombinant LCAT, and one site specific polyclonal antibody, directed against the 121-136 region of LCAT, were employed. All three antibodies reacted with a recombinant form of purified LCAT; however, only the polyclonal antibodies directed against the whole enzyme were able to recognize the LCAT when it was adsorbed to a hydrophobic surface in a solid phase immunoassay, or when bound to HDL in a sink immunoassay. These findings indicate that the epitope(s) of the 121-136 region are not accessible to antibodies under these conditions. Three mutant forms of LCAT, representing alterations in the 121-136 region, were also examined for their immunoreactivity with the same panel of antibodies and compared to the wild-type enzyme. These studies demonstrate that in its native configuration the 121-136 region of LCAT is likely to reside on a surface of LCAT. Furthermore, mutations within this region appear to markedly impact the exposure of epitopes at additional sites. These findings suggest that the 121-136 region could play an important role in enzyme interaction with its hydrophobic lipoprotein substrates as mutations within this region appear to alter enzyme conformation, catalytic activity, and the specificity of LCAT.

Adsorption↗
PubMed1997

Activation of lecithin cholesterol acyltransferase by a disulfide-linked apolipoprotein A-I dimer.

Apolipoprotein A-IMilano is a molecular variant of apoA-I, containing the Arg173-->Cys substitution that forms a disulfide linked homodimer (A-IM/A-IM). To assess the effect of this structural modification on a major function of the apolipoprotein, its activation of lecithin cholesterol acyltransferase (LCAT), we prepared well-defined complexes of A-IM/A-IM and apoA-I with phospholipids and cholesterol and compared their reactivities with LCAT. Particles with A-IM/A-IM had very similar diameters to apoA-I particles (7.8 and 12.5 nm) but had distinct apolipoprotein and phospholipid contents and protein secondary structures; they bound LCAT with comparable affinities, but were less efficient substrates for the enzyme (40 to 70% less reactive). We conclude that the local structural constraints in A-IM/A-IM do not prevent the formation of well-defined complexes with phospholipids and do not influence the binding of the enzyme to the particles, but have an inhibitory effect on LCAT activation.

Apolipoprotein A-I↗
PubMed1997

Identification of a domain of lecithin-cholesterol acyltransferase that is involved in interfacial recognition.

Lecithin-cholesterol acyltransferase (LCAT) is an interfacial enzyme that acts on lipid substrates on the surface of high density lipoproteins (HDL). Based on observations with other interfacial lipases, we propose that LCAT contains a surface region of 25 amino acids linked by a disulfide bond (C50-C74) that is involved in the binding of LCAT to lipoproteins. Using LCAT cDNA, we have deleted most of this region (delta 53-71) and expressed the mutant enzyme (LCAT delta 53-71) in COS-1 cells. The deletion mutant is expressed and secreted at levels similar to wildtype LCAT, suggesting that the deleted region is located on the surface of the enzyme and is not required for folding. The enzymatic activity of the mutant was tested using two interfacial substrates, reconstituted HDL (rHDL) and low density lipoprotein (LDL), as well as a water soluble substrate, p-nitrophenyl butyrate (PNPB). There was no reaction with rHDL and LDL, but 30% of the activity with PNPB was retained. This suggests that the deleted region plays a role in interfacial binding, while the active site core is not disrupted. We thus conclude that this region (C50-C74) forms part of the interfacial binding domain of LCAT.

Amino Acid Sequence↗