α-Lecithin: Cholesterol Acyltransferase Deficiency

Substrate specificity of plasma lecithin: cholesterol acyltransferase in abetalipoproteinemia

The lecithin: cholesterol acyltransferase (LCAT) activity of lipoprotein-depleted plasma from a patient with abetalipoproteinemia has been assayed in a modified Glomset-Wright incubation system with three different normal lipoprotein substrates consisting of an authentic mixture of very low (VLDL), low (LDL) and high (HDL) density lipoproteins for the assay of total LCAT activity, HDL to assay alpha-LCAT activity and combined VLDL and LDL to assay beta-LCAT activity, respectively. Although reduced to about half the normal control values, both alpha- and beta-LCAT activities were present in the patient's plasma. It has been shown earlier that secretion of LCAT is linked to that of VLDL, but since patients with abetalipoproteinemia cannot form either chylomicrons or VLDL, our results suggest that a secretion of these triglyceride-rich lipoproteins do not seem to be a prerequisite for a basal secretion of beta-LCAT.

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).

Deletion of N-terminal amino acids from human lecithin:cholesterol acyltransferase differentially affects enzyme activity toward alpha- and beta-substrate lipoproteins

Lecithin:cholesterol acyltransferase (LCAT) is the enzyme responsible for generation of the majority of the cholesteryl esters (CE) in human plasma. Although most plasma cholesterol esterification occurs on high-density lipoprotein (HDL), via alpha-LCAT activity, esterification also occurs on low-density lipoprotein (LDL) via the beta-activity of the enzyme. Computer threading techniques have provided a three-dimensional model for use in the structure-function analysis of the core and catalytic site of the LCAT protein, but the model does not extend to the N-terminal region of the enzyme, which may mediate LCAT interaction with lipoprotein substrates. In the present study, we have examined the functional consequences of deletion of the highly conserved hydrophobic N-terminal amino acids (residues 1-5) of human LCAT. Western blot analysis showed that the mutant proteins (Delta 1-Delta 5) were synthesized and secreted from transfected COS-7 cells at levels approximately equivalent to those of wild-type hLCAT. The secreted proteins had apparent molecular weights of 67 kDa, indicating that they were correctly processed and glycosylated during cellular transit. However, deletion of the first residue of the mature LCAT protein (Delta 1 mutant) resulted in a dramatic loss of alpha-LCAT activity (5% of wild type using reconstituted HDL substrate, rHDL), although this mutant retained full beta-LCAT activity (108% of wild-type using human LDL substrate). Removal of residues 1 and 2 (Delta 2 mutant) abolished alpha-LCAT activity and reduced beta-LCAT activity to 12% of wild type. Nevertheless, LCAT Delta 1 and Delta 2 mutants retained their ability to bind to rHDL and LDL lipoprotein substrates. The dramatic loss of enzyme activity suggests that the N-terminal residues of LCAT may be involved in maintaining the conformation of the lid domain and influence activation by the alpha-LCAT cofactor apoA-I (in Delta 1) and/or loss of enzyme activity (in Delta 1-Delta 5). Since the Delta 1 and Delta 2 mutants retain their ability to bind substrate, other factor(s), such as decreased access to the substrate binding pocket, may be responsible for the loss of enzyme activity.

Phospholipase A2: its usefulness in laboratory diagnostics

Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of membrane phospholipids. This article reviews the source and structure of PLA2, the involvement of the enzyme in various biological and pathological phenomena, and the usefulness of PLA2 assays in laboratory diagnostics. Of particular importance is the role of PLA2 in the cellular production of mediators of inflammatory response to various stimuli. Assays for PLA2 activity and mass concentration are discussed, and the results of enzyme determinations in plasma from patients with different pathological conditions are presented. The determination of activity and mass concentration in plasma is particularly useful in the diagnosis and prognosis of pancreatitis, multiple organ failure, septic shock, and rheumatoid arthritis. A very important result is the demonstration that PLA2 is an acute phase protein, like CRP. Indeed, there is a close correlation between PLA2 mass concentration and CRP levels in several pathological conditions. Although the determination of C-reactive protein is much easier to perform and is routinely carried out in most clinical laboratories, the assessment of PLA2 activity or mass concentration has to be considered as a reliable approach to obtain a deeper understanding of some pathological conditions and may offer additional information concerning the prognosis of several disorders.

In vitro expression of structural defects in the lecithin-cholesterol acyltransferase gene

Classic LCAT deficiency (CLD) and fish eye disease (FED) are two clinically distinct syndromes, associated with defects in the lecithin-cholesterol acyltransferase (LCAT) gene resulting in total (CLD) or partial (FED) enzyme deficiency. In order to investigate the underlying molecular mechanisms that lead to different phenotypic expression in CLD and FED, LCAT mutants associated with either CLD (LCAT147, LCAT156, and LCAT228) or FED (LCAT10, LCAT123, LCAT158, LCAT293, LCAT300, and LCAT347) were expressed in vitro in human embryonic kidney 293 cells and characterized with respect to LCAT expression and enzyme activity. Evaluation of mutant LCAT gene transcription by Northern blot analysis demonstrated LCAT mRNA of normal size and concentration. Although all constructs gave rise to similar intracellular LCAT mass, the amount of enzyme present in the media for LCAT147, LCAT156, and LCAT300 was reduced to less than 10% of normal, suggesting that these mutations disrupted LCAT secretion. Western blot analysis of cell culture media containing wild type or mutant LCAT demonstrated the presence of a single normal-sized band of 67 kDa. The ability of the different enzymes to esterify free cholesterol in high density lipoprotein-like proteoliposomes (alpha-LCAT-specific activity) was reduced to less than 5% of normal for CLD mutants LCAT147 and LCAT228 and FED mutants LCAT10, LCAT123, LCAT293, and LCAT347, whereas that of LCAT156, LCAT158, and LCAT300 ranged from 45 to 110% of control. Although most FED mutant LCAT enzymes retained the ability to esterify free cholesterol present in alpha- and beta-lipoproteins of heat-inactivated plasma, esterification was undetectable in all CLD mutants (LCAT147, LCAT156, and LCAT228). In contrast, all mutant enzymes retained the ability to hydrolyze the water soluble, short-chained fatty acid substrate p-nitrophenolbutyrate. In summary, our studies establish the functional significance of nine LCAT gene defects associated with either FED or CLD. Characterization of the expressed LCAT mutants identified multiple, overlapping functional abnormalities that include defects in secretion and/or disruption of enzymic activity. All nine LCAT mutants retained the ability to hydrolyze the water-soluble PNPB substrate, indicating intact hydrolytic function. Based on these studies we propose that mutations in LCAT residues 147, 156, 228 (CLD) and 10, 123, 158, 293, 300, and 347 (FED) do not disrupt the functional domain mediating LCAT phospholipase activity, but alter structural domains involved in lipid binding or transesterification.

Phospholipases in biology and medicine

Phospholipases, a group of enzymes that catalyze the hydrolysis of membrane phospholipids, are classified according to the bond cleaved in a phospholipid into PLA1 (EC 3.1.1.3), PLA2 (EC 3.1.1.4), PLB (EC 3.1.1.5), PLC (EC 3.1.4.3), and PLD (EC 3.1.4.4). This paper reviews source and structure of PLA2 and the involvement of PLA2 and PLC in several biological phenomena, such as, signal transduction, photoreception, biosynthesis of lung surfactant, sperm motility, and fertilization. New assays for PLA2 activity and concentration in biological fluids are discussed. Phospholipases are involved in many inflammatory reactions by making arachidonate available for eicosanoid biosynthesis. The determination of PLA2 activity and mass concentration in plasma is useful in the diagnosis and prognosis of pancreatitis and of septic shock. Naturally occurring phospholipase inhibitors, such as lipocortins act as second messengers in the anti-inflammatory response to steroids. Lipocortins may be valuable therapeutic agents, because they are more specific in their anti-inflammatory action than glucocorticoids; therefore, they are less likely to produce harmful side effects.