Relationship between structure and biochemical phenotype of lecithin:cholesterol acyltransferase (LCAT) mutants causing fish-eye disease

A systematic review of the natural history and biomarkers of primary Lecithin:Cholesterol Acyltransferase (LCAT) deficiency

Syndromes associated with LCAT deficiency, a rare autosomal recessive condition, include fish-eye disease (FED) and familial LCAT deficiency (FLD). FLD is more severe and characterized by early and progressive chronic kidney disease (CKD). No treatment is currently available for FLD, but novel therapeutics are under development. Furthermore, although biomarkers of LCAT deficiency have been identified, their suitability to monitor disease progression and therapeutic efficacy is unclear, as little data exist on the rate of progression of renal disease. Here, we systematically review observational studies of FLD, FED, and heterozygous subjects, which summarize available evidence on the natural history and biomarkers of LCAT deficiency, in order to guide the development of novel therapeutics. We identified 146 FLD and 53 FED patients from 219 publications, showing that both syndromes are characterized by early corneal opacity and markedly reduced HDL-C levels. Proteinuria/hematuria were the first signs of renal impairment in FLD, followed by rapid decline of renal function. Furthermore, LCAT activity toward endogenous substrates and the percentage of circulating esterified cholesterol (EC%) were the best discriminators between these two syndromes. In FLD, higher levels of total, non-HDL, and unesterified cholesterol were associated with severe CKD. We reveal a nonlinear association between LCAT activity and EC% levels, in which subnormal levels of LCAT activity were associated with normal EC%. This review provides the first step toward the identification of disease biomarkers to be used in clinical trials and suggests that restoring LCAT activity to subnormal levels may be sufficient to prevent renal disease progression.

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.

A retractable lid in lecithin:cholesterol acyltransferase provides a structural mechanism for activation by apolipoprotein A-I

Lecithin:cholesterol acyltransferase (LCAT) plays a key role in reverse cholesterol transport by transferring an acyl group from phosphatidylcholine to cholesterol, promoting the maturation of high-density lipoproteins (HDL) from discoidal to spherical particles. LCAT is activated through an unknown mechanism by apolipoprotein A-I (apoA-I) and other mimetic peptides that form a belt around HDL. Here, we report the crystal structure of LCAT with an extended lid that blocks access to the active site, consistent with an inactive conformation. Residues Thr-123 and Phe-382 in the catalytic domain form a latch-like interaction with hydrophobic residues in the lid. Because these residues are mutated in genetic disease, lid displacement was hypothesized to be an important feature of apoA-I activation. Functional studies of site-directed mutants revealed that loss of latch interactions or the entire lid enhanced activity against soluble ester substrates, and hydrogen-deuterium exchange (HDX) mass spectrometry revealed that the LCAT lid is extremely dynamic in solution. Upon addition of a covalent inhibitor that mimics one of the reaction intermediates, there is an overall decrease in HDX in the lid and adjacent regions of the protein, consistent with ordering. These data suggest a model wherein the active site of LCAT is shielded from soluble substrates by a dynamic lid until it interacts with HDL to allow transesterification to proceed.

The Effect of Natural LCAT Mutations on the Biogenesis of HDL

We have investigated how the natural LCAT[T147I] and LCAT[P274S] mutations affect the pathway of biogenesis of HDL. Gene transfer of WT LCAT in LCAT(-/-) mice increased 11.8-fold the plasma cholesterol, whereas the LCAT[T147I] and LCAT[P274S] mutants caused a 5.2- and 2.9-fold increase, respectively. The LCAT[P274S] and the WT LCAT caused a monophasic distribution of cholesterol in the HDL region, whereas the LCAT[T147I] caused a biphasic distribution of cholesterol in the LDL and HDL region. Fractionation of plasma showed that the expression of WT LCAT increased plasma apoE and apoA-IV levels and shifted the distribution of apoA-I to lower densities. The LCAT[T147I] and LCAT[P274S] mutants restored partially apoA-I in the HDL3 fraction and LCAT[T147I] increased apoE in the VLD/IDL/LDL fractions. The in vivo functionality of LCAT was further assessed based on is its ability to correct the aberrant HDL phenotype that was caused by the apoA-I[L159R]FIN mutation. Co-infection of apoA-I(-/-) mice with this apoA-I mutant and either of the two mutant LCAT forms restored only partially the HDL biogenesis defect that was caused by the apoA-I[L159R]FIN and generated a distinct aberrant HDL phenotype.

Association of lecithin-cholesterol acyltransferase activity measured as a serum cholesterol esterification rate and low-density lipoprotein heterogeneity with cardiovascular risk: a cross-sectional study

The cholesterol-esterifying enzyme, lecithin-cholesterol acyltransferase (LCAT), is believed to play a key role in reverse cholesterol transport. However, recent investigations have demonstrated that higher LCAT activity levels increase the formation of triglyceride (TG)-rich lipoproteins (TRLs) and atherogenesis. We hypothesized that higher LCAT activity measured as a serum cholesterol esterification rate by the endogenous substrate method might increase the formation of TRLs and thereby alter low-density lipoprotein (LDL) heterogeneity. The estimated LDL particle size [relative LDL migration (LDL-Rm)] was measured by polyacrylamide gel electrophoresis with the LipoPhor system (Joko, Tokyo, Japan) in 538 consecutive patients with at least risk factor for atherosclerosis. Multivariate regression analysis after adjustments for traditional risk factors identified elevated TRL-related marker (TG, remnant-like particle cholesterol, apolipoprotein C-II, and apolipoprotein C-III) levels as independent predictors of smaller-sized LDL particle size, both in the overall subject population and in the subset of patients with serum LDL cholesterol levels of <100 mg/dL. Area under the receiver operating characteristic curve of the LCAT activity (0.79; sensitivity 60 %; specificity 84.8 %) was observed for the evaluation of the indicators of an LDL-Rm value of ≥0.40, which suggests the presence of large amounts of small-dense LDL. The results lend support to the hypothesis that increased LCAT activity may be associated with increased formation of TRLs, leading to a reduction in LDL particle size. Therefore, to reduce the risk of atherosclerotic cardiovascular disease, it may be of importance to pay attention not only to a quantitative change in the serum LDL-C, but also to the LCAT activity which is possibly associated with LDL heterogeneity.

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.

Lipid oxidation in carriers of lecithin:cholesterol acyltransferase gene mutations

OBJECTIVE

Lecithin:cholesterol acyltransferase (LCAT) has been shown to play a role in the depletion of lipid oxidation products, but this has so far not been studied in humans. In this study, we investigated processes and parameters relevant to lipid oxidation in carriers of functional LCAT mutations.

METHODS AND RESULTS

In 4 carriers of 2 mutant LCAT alleles, 63 heterozygotes, and 63 family controls, we measured activities of LCAT, paraoxonase 1, and platelet-activating factor-acetylhydrolase; levels of lysophosphatidylcholine molecular species, arachidonic and linoleic acids, and their oxidized derivatives; immunodetectable oxidized phospholipids on apolipoprotein (apo) B-containing and apo(a)-containing lipoproteins; IgM and IgG autoantibodies to malondialdehyde-low-density lipoprotein and IgG and IgM apoB-immune complexes; and the antioxidant capacity of high-density lipoprotein (HDL). In individuals with LCAT mutations, plasma LCAT activity, HDL cholesterol, apoA-I, arachidonic acid, and its oxidized derivatives, oxidized phospholipids on apo(a)-containing lipoproteins, HDL-associated platelet-activating factor-acetylhydrolase activity, and the antioxidative capacity of HDL were gene-dose-dependently decreased. Oxidized phospholipids on apoB-containing lipoproteins was increased in heterozygotes (17%; P<0.001) but not in carriers of 2 defective LCAT alleles.

CONCLUSIONS

Carriers of LCAT mutations present with significant reductions in LCAT activity, HDL cholesterol, apoA-I, platelet-activating factor-acetylhydrolase activity, and antioxidative potential of HDL, but this is not associated with parameters of increased lipid peroxidation; we did not observe significant changes in the oxidation products of arachidonic acid and linoleic acid, immunoreactive oxidized phospholipids on apo(a)-containing lipoproteins, and IgM and IgG autoantibodies against malondialdehyde-low-density lipoprotein. These data indicate that plasma LCAT activity, HDL-associated platelet-activating factor-acetylhydrolase activity, and HDL cholesterol may not influence the levels of plasma lipid oxidation products.

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.

Depletion of high-density lipoprotein and appearance of triglyceride-rich low-density lipoprotein in a Japanese patient with FIC1 deficiency manifesting benign recurrent intrahepatic cholestasis

OBJECTIVE

Lipoprotein metabolism in FIC1 deficiency due to ATP8B1 mutations has never been studied sufficiently. This study was performed to investigate the detailed lipoprotein metabolism in benign recurrent intrahepatic cholestasis (BRIC) caused by FIC1 deficiency.

PATIENTS AND METHODS

Lipoprotein profile and major lipoprotein regulators such as lecithin:cholesterol acyltransferase (LCAT), hepatic triglyceride lipase (HTGL), lipoprotein lipase, and cholesteryl ester transfer protein in a Japanese patient with BRIC were serially examined during a bout of cholestasis. Liver expression of farnesoid X receptor (FXR), which suppresses high-density lipoprotein (HDL) generation, was also examined.

RESULTS

Hypercholesterolemia and lipoprotein X accumulation were never observed throughout this study. When the cholestasis was severe, triglyceride-rich low-density lipoprotein (LDL) accounted for most of the plasma lipoproteins whereas HDL was hardly detectable. Concurrently, activities of all regulators were decreased, together with decreases of the serum parameter for liver protein synthesis. In particular, suppressions of LCAT and HTGL activities were severe and greatly contributed to the appearance of triglyceride-rich LDL. As the cholestasis improved, this LDL gradually transformed into normal LDL with the recoveries of LCAT and HTGL activities. The activities of all regulators for the last 1 to 2 months were normal but HDL remained depleted. His liver showed low FXR expression compared with control livers.

CONCLUSIONS

The present study showed an appearance of triglyceride-rich LDL due to suppressions of LCAT and HTGL activities and a depletion of HDL that is not able to be explained by lipoprotein regulators or FXR in our patient.

Compound heterozygosity (G71R/R140H) in the lecithin:cholesterol acyltransferase (LCAT) gene results in an intermediate phenotype between LCAT-deficiency and fish-eye disease

The esterification of free cholesterol (FC) in plasma, catalyzed by the enzyme lecithin:cholesterol acyltransferase (LCAT; EC 2.3.1.43), is a key process in lipoprotein metabolism. The resulting cholesteryl esters (CE) represent the main core lipids of low (LDL) and high density lipoproteins (HDL). Primary (familial) LCAT-deficiency (FLD) is a rare autosomal recessive genetic disease caused by the complete or near absence of LCAT activity. In fish-eye disease (FED), residual LCAT activity is still detectable. Here, we describe a 32-year-old patient with corneal opacity, very low LCAT activity, reduced amounts of CE (low HDL-cholesterol level), and elevated triglyceride (TG) values. The lipoprotein pattern was abnormal with regard to lipoprotein composition and concentration, but distinct lipoprotein classes were still present. Despite of typical features of glomerular proteinuria, creatinine clearance was normal. DNA sequencing and restiction fragment analyses revealed two separate mutations in the patient's LCAT gene: a previously described G to A transition in exon 4 converting Arg140 to His, inherited from his mother, and a novel G to C transversion in exon 2 converting Gly71 to Arg, inherited from his father, indicating that M.P. was a compound heterozygote. Determination of enzyme activities of recombinant LCAT proteins obtained upon transfection of COS-7 cells with plasmids containing G71R-LCAT or wild-type LCAT cDNA revealed very low alpha- and absence of beta-LCAT activity for the G71R mutant. The identification of the novel G71R LCAT mutation supports the proposed molecular model for the enzyme implying that the "lid" domain at residues 50-74 is involved in enzyme:substrate interaction. Our data are in line with the hypothesis that a key event in the etiology of FLD is the loss of distinct lipoprotein fractions.

LCAT-dependent conversion rate is a determinant of plasma prebeta1-HDL concentration in healthy Japanese

BACKGROUND

Prebeta1-HDL acts as a primary acceptor of cellular cholesterol. Prebeta1-HDL is converted into alpha-migrating high-density lipoprotein (HDL) by lecithin/cholesterol acyltransferase (LCAT). We examined whether the LCAT-dependent conversion rate of prebeta1-HDL is a determinant of the plasma prebeta1-HDL concentration in healthy Japanese.

METHODS

We measured the conversion half time (CHT(prebeta1)), the time required for 50% of baseline prebeta1-HDL to be changed into alpha-migrating HDL by LCAT, in 100 healthy Japanese (47 men, 53 women, 22-88 years).

RESULTS

Prebeta1-HDL concentration, as determined by immunoassay, was significantly lower in younger women (<50 years, n=24) than in older women (>or=50 years, n=29) (16.8+/-3.3 vs. 21.7+/-8.0 mg/l apolipoprotein AI (apoAI), p<0.01). There was no significant difference in prebeta1-HDL concentration between younger (n=24) and older (n=23) men (21.2+/-6.8 vs. 22.5+/-6.6 mg/l apoAI). The mean CHT(prebeta1) for all subjects was 47.4+/-13.0 min, and was not influenced by gender or age. Prebeta1-HDL concentration was positively correlated with CHT(prebeta1) in both men and women, suggesting that high prebeta1-HDL levels may reflect delayed conversion of prebeta1-HDL.

CONCLUSION

LCAT-dependent conversion rate is a determinant of plasma prebeta1-HDL concentration in healthy Japanese. We speculate that prebeta1-HDL concentration may be used as a metabolic marker for HDL maturation.

T13M mutation of lecithin-cholesterol acyltransferase gene causes fish-eye disease

BACKGROUND

Lecithin-cholesterol acyltransferase (LCAT) esterifies free cholesterol (FC) in plasma and plays a crucial role in the maturation of prebeta1-HDL (lipid-poor HDL) into alpha-migrating HDL (spherical HDL). Natural mutations of LCAT gene cause familial LCAT deficiency (FLD) or fish-eye disease (FED). The relationship between mutations and their phenotypes gives important clues to the functions of specific regions of LCAT. We investigated the first homozygous case with a substitution of threonine to methionine at codon 13 (T13M) of LCAT gene.

METHODS

We evaluated LCAT activity, LCAT distribution among HDL subfractions and conversion of prebeta1-HDL to alpha-migrating HDL by native two-dimensional gel electrophoresis (N-2DGE).

RESULTS

The proband had corneal opacity, severe hypo-alpha-lipoproteinemia, half-normal LCAT activity and near normal cholesteryl ester/total cholesterol (TC) ratio in plasma. These features were characteristic of FED. Plasma prebeta1-HDL concentration was near normal, but not converted to alpha-migrating HDL during 37 degrees C incubation. As expected, alpha-migrating HDL (especially large particles) was markedly reduced. In the immunoblot against LCAT, the small alpha-migrating HDL from the proband had much less LCAT in this patient than in controls.

CONCLUSION

T13M mutation of LCAT gene causes FED.

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.

Deletion of specific glycan chains affects differentially the stability, local structures, and activity of lecithin-cholesterol acyltransferase

The enzymatic and interfacial binding activity of lecithin-cholesterol acyltransferase (LCAT) is affected differentially by the location and extent of its glycosylation. Two LCAT glycosylation-deficient mutants, N84Q and N384Q, were constructed, permanently expressed in Chinese hamster ovary cells, and purified to determine the effects of deleting individual glycan chains on its stability, structure, and function. These purified mutants were studied by spectroscopic structural methods and enzymatic and binding assays to develop a molecular rationale for the relationship between LCAT glycosylation and activity. The N84Q LCAT mutant did not possess measurable enzymatic activity or interfacial binding affinity for reconstituted high-density lipoproteins. In addition, in thermal and chemical denaturation studies, N84Q LCAT was found to be significantly less stable than wild-type LCAT. The N384Q variant was initially more enzymatically active than wild-type LCAT, but gradually lost activity within months; however, it retained full interfacial binding activity. Significant changes were detected over time by circular dichroism in the alpha-helical content of N384Q LCAT and in the beta-sheet content of N84Q LCAT, compared with wild-type LCAT. Fluorescence measurements with the probe 1-anilinonapthalene-8-sulfonate suggested an alteration of the active site cavity in both mutants. In conclusion, both mutants lost catalytic activity, N84Q shortly after purification and N384Q more gradually, and were destabilized, probably because the deletion of the glycan chains altered local structural elements near the active site cavity and/or the interfacial binding regions.

Detailed molecular model of apolipoprotein A-I on the surface of high-density lipoproteins and its functional implications

The major apolipoprotein (apo) A-I containing lipoprotein, high- density lipoprotein, is a negative risk factor for cardiovascular disease. An atomic resolution molecular model for lipid-associated apo A-I was recently proposed in which two apo A-I molecules are wrapped beltwise around a small discoidal patch of phospholipid bilayer. Because of its detailed predictions of lipid-associated apo A-I structure, this molecular belt model, if confirmed, provides a blueprint for understanding the molecular mechanisms of reverse cholesterol transport, and thus for the rational design of new classes of drugs for reversal of atherosclerosis and cardiovascular disease. The details and implications of the model are currently being explored by site-directed mutagenesis.

Headgroup specificity of lecithin cholesterol acyltransferase for monomeric and vesicular phospholipids

In this study, we investigated how the nature of the phospholipid head group and the macromolecular structure of the phospholipid, either as a monomer or incorporated into a lipid matrix, influence the activity of lecithin cholesterol acyltransferase (LCAT). As substrates we used 1,2-bis-(1-pyrenebutanoyl)-phosphatidylcholine, 1, 2-bis-(1-pyrenebutanoyl)-phosphatidylethanolamine and 1, 2-bis-(1-pyrenebutanoyl)-phosphatidyl-alcohols, either as monomers or incorporated into small unilamellar vesicles consisting of dipalmitoylphosphatidylcholine ether. The rate of hydrolysis of the pyrene-labeled phospholipids was determined both by fluorescence and by high performance liquid chromatography. V(max) and K(m) were calculated for the different substrates. The data show that V(max) is 10- to 30-fold higher for the hydrolysis of monomeric phosphatidylcholine (PC) compared to phosphatidylethanolamine (PE) and the phosphatidylalcohols, while K(m) values are comparable. When the fluorescent substrates were incorporated into dipalmitoylphosphatidylcholine ether vesicles, we observed a 4- to 10-fold increase of V(max) for PE and the phosphatidylalcohols, and no significant change for K(m). V(max) for PC remained the same. Natural LCAT mutants causing Fish-Eye Disease (FED) and analogues of these mutants expressed in Cos-1 cells, had similar activity on monomeric PC and PE. These data suggest that the activity of LCAT is determined both by the molecular structure of the phospholipid and by its macromolecular properties. The LCAT activity on monomeric substrates decreases as: phosphatidylcholine&z. Gt;phosphatidylethanolamine congruent withphosphatidylpropanol congruent withphosphatidylethanol congruent withphosphatidylethyleneglycol. The incorporation of PE and the phosphatidylalcohols into a matrix of dipalmitoylphosphatidylcholine decreases the specificity of the phospholipid head group.