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

Single nucleotide polymorphisms in LCAT may contribute to dyslipidaemia in HIV-infected individuals on HAART in a Ghanaian population

Highly active antiretroviral therapy (HAART) is known to cause lipid abnormalities such as dyslipidaemia in HIV-infected individuals. Yet, dyslipidaemia may not independently occur as it may be worsened by single nucleotide polymorphisms (SNPs) in lecithin cholesterol acyltransferase (LCAT) and lipoprotein lipase (LPL). This case–control study was conducted in three-selected hospitals in the Northern part of Ghana. The study constituted a total of 118 HIV-infected participants aged 19–71 years, who had been on HAART for 6–24 months. Dyslipidaemia was defined based on the NCEP-ATP III criteria. HIV-infected individuals on HAART with dyslipidaemia were classified as cases while those without dyslipidaemia were grouped as controls. Lipid profile was measured using an automatic clinical chemistry analyzer and genomic DNA was extracted for PCR (GeneAmp PCR System 2700). Overall, the prevalence of dyslipidaemia was 39.0% (46/118). High levels of low-density lipoprotein cholesterol (LDL-C), total cholesterol (TC), and reduced levels of high-density lipoprotein cholesterol (HDL-C) were observed in all cases. A total of 256 selected PCR amplicons comprising 137 LPL (exons 3, 5 and 6) and 119 LCAT (exons 1, 4, and 6) were sequenced in 46 samples (Inqaba Biotech). Six (6) clinically significant SNPs were identified in exons 1 and 4 for LCAT whereas 25 non-clinically significant SNPs were identified for LPL in exons 5 and 6. At position 97 for LCAT exon 1, there was a deletion of the nucleotide, ‘A’ in 32.5% (13/40) of the sampled population while 67.5% (27/40) of the sample population retained the nucleotide, ‘A’ which was significantly associated with dyslipidaemic outcomes in the study population (p = 0.0004). A total of 25 SNPs were identified in exons 5 and 6 of LPL; 22 were substitutions, and 3 were insertions. However, none of the 25 SNPs identified in LPL exon 5 and 6 were statistically significant. SNPs in LCAT may independently contribute to dyslipidaemia among Ghanaian HIV-infected individuals on HAART, thus, allowing genetic and/or functional differential diagnosis of dyslipidaemia and creating an opportunity for potentially preventive options.

Interaction of lecithin:cholesterol acyltransferase with lipid surfaces and apolipoprotein A-I-derived peptides

LCAT is an enzyme responsible for the formation of cholesteryl esters from unesterified cholesterol (UC) and phospholipid (PL) molecules in HDL particles. However, it is poorly understood how LCAT interacts with lipoproteins and how apoA-I activates it. Here we have studied the interactions between LCAT and lipids through molecular simulations. In addition, we studied the binding of LCAT to apoA-I-derived peptides, and their effect on LCAT lipid association-utilizing experiments. Results show that LCAT anchors itself to lipoprotein surfaces by utilizing nonpolar amino acids located in the membrane-binding domain and the active site tunnel opening. Meanwhile, the membrane-anchoring hydrophobic amino acids attract cholesterol molecules next to them. The results also highlight the role of the lid-loop in the lipid binding and conformation of LCAT with respect to the lipid surface. The apoA-I-derived peptides from the LCAT-activating region bind to LCAT and promote its lipid surface interactions, although some of these peptides do not bind lipids individually. The transfer free-energy of PL from the lipid bilayer into the active site is consistent with the activation energy of LCAT. Furthermore, the entry of UC molecules into the active site becomes highly favorable by the acylation of SER181.

Structure and function of lysosomal phospholipase A2 and lecithin:cholesterol acyltransferase

Lysosomal phospholipase A2 (LPLA2) and lecithin:cholesterol acyltransferase (LCAT) belong to a structurally uncharacterized family of key lipid metabolizing enzymes responsible for lung surfactant catabolism and for reverse cholesterol transport, respectively. Whereas LPLA2 is predicted to underlie the development of drug-induced phospholipidosis, somatic mutations in LCAT cause fish eye disease and familial LCAT deficiency. Here we describe several high resolution crystal structures of human LPLA2 and a low resolution structure of LCAT that confirms its close structural relationship to LPLA2. Insertions in the α/β hydrolase core of LPLA2 form domains that are responsible for membrane interaction and binding the acyl chains and head groups of phospholipid substrates. The LCAT structure suggests the molecular basis underlying human disease for most of the known LCAT missense mutations, and paves the way for rational development of new therapeutics to treat LCAT deficiency, atherosclerosis and acute coronary syndrome.

Activation of lecithin:cholesterol acyltransferase by HDL ApoA-I central helices

Lecithin:cholesterol acyltransferase (LCAT) is an enzyme that first hydrolyzes the sn-2 position of phospholipids, preferentially a diacylphosphocholine, and then transfers the fatty acid to cholesterol to yield a cholesteryl ester. HDL ApoA-I is the principal catalytic activator for LCAT. Activity of LCAT on nascent or lipid-poor HDL particles composed of phospholipid, cholesterol and ApoA-I allows the maturation of HDL particles into lipid-rich spherical particles that contain a core of cholesteryl ester surrounded by phospholipid and ApoA-I on the surface. This article reviews the recent progress in elucidating structural aspects of the interaction between LCAT and ApoA-I. In the last decade, there has been considerable progress in understanding the structure of ApoA-I and the central helices 5, 6, and 7 that are known to activate LCAT. However, much less information has been forthcoming describing the 3D structure and conformation of LCAT required to catalyze two separate reactions within a single monomeric peptide.

Structural differences between wild-type and fish eye disease mutant of lecithin:cholesterol acyltransferase

Fluorescence spectroscopy has been used to investigate the conformational changes that occur upon binding of wild type (WT) and mutant (Thr123Ile) lecithin:cholesterol acyltransferase (LCAT) to the potential substrates (dioleoyl-phosphatidyl choline [DOPC] and high density lipoprotein [HDL]). For a detailed analysis of structural differences between WT and mutant LCAT, we performed decompositional analysis of a set of tryptophan fluorescence spectra, measured at increasing concentrations of external quenchers (acrylamide and KI). The data obtained show that Thr123Ile mutation in LCAT leads to a conformation that is likely to be more rigid (less mobile/flexible) than that of the WT protein with a redistribution of charged residues around exposed tryptophan fluorophores. We propose that the redistribution of charged residues in mutant LCAT may be a major factor responsible for the dramatically reduced activity of the enzyme with HDL and reconstituted high density lipoprotein (rHDL).

Characterization of lecithin:cholesterol acyltransferase expressed in a human lung cell line

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme for the transfer of mammalian cholesterol from peripheral tissues to the liver. In patients deficient in LCAT, serum cholesterol levels rise and can lead to corneal opacity, proteinuria, anemia, and kidney failure. As early as 1968, relatively low volume transfusion of normal plasma was shown to temporarily correct the abnormal lipoprotein profiles in LCAT-deficient patients. However, despite the cloning, study, and extensive expression of LCAT in mammalian cell lines, there is still no viable, clinical therapy for LCAT deficiency. The current study was initiated to provide a source of recombinant human LCAT for enzyme replacement therapy. Accordingly, human LCAT has been cloned and expressed for the first time in a human cell line. The recombinant LCAT secreted by these cells was purified by phenyl-Sepharose chromatography, analyzed to determine the nature of its glycosylation, and tested for its enzymatic properties. The activity and basic kinetic parameters for the enzyme were determined using both a fluorescent water-soluble substrate and a macromolecular (proteoliposome) substrate. The enzymatic properties and the carbohydrate components of the recombinant LCAT were all sufficiently similar to those of the circulating human plasma enzyme, suggesting that this source of LCAT may be appropriate for use in some form of enzyme replacement therapy.