Study of the components of reverse cholesterol transport in lecithin:cholesterol acyltransferase deficiency

Enzymatic and lipid transfer reactions involved in reverse cholesterol transport were studied in healthy and lecithin:cholesterol acyltransferase (LCAT), deficient subjects. Fasting plasma samples obtained from each individual were labeled with [3H]cholesterol and subsequently fractionated by gel chromatography. The radioactivity patterns obtained corresponded to the elution volumes of the three major ultracentrifugally isolated lipoprotein classes (very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL)). In healthy subjects, the LCAT activity was consistently found in association with the higher molecular weight portion of HDL. Similar observations were made when exogenous purified LCAT was added to the LCAT-deficient plasma prior to chromatography. Incubation of the plasma samples at 37 degrees C resulted in significant reduction of unesterified cholesterol (FC) and an increase in esterified cholesterol (CE). Comparison of the data of FC and CE mass measurements of the lipoprotein fractions from normal and LCAT-deficient plasma indicates that: (i) In normal plasma, most of the FC for the LCAT reaction originates from LDL even when large amounts of FC are available from VLDL. (ii) The LCAT reaction takes place on the surface of HDL. (iii) The product of the LCAT reaction (CE) may be transferred to either VLDL or LDL although VLDL appears to be the preferred acceptor when present in sufficient amounts. (iv) CE transfer from HDL to lower density lipoproteins is at least partially impaired in LCAT-deficient patients. Additional studies using triglyceride-rich lipoproteins indicated that neither the capacity to accept CE from HDL nor the lower CE transfer activity were responsible for the decreased amount of CE transferred to VLDL and chylomicrons in LCAT-deficient plasma.     

Factors affecting the esterification of lipoprotein cholesterol by lecithin: cholesterol acyl transferase.

Lecithin: Cholesterol Acyltransferase (LCAT) esterified relatively small amounts of cholesterol from very low density lipoproteins (VLDL), low density lipoproteins (LDL) or high density lipoproteins (HDL) in the presence of 5% human serum albumin (HSA). On the other hand, in the presence of very high density (>1.225 g/ml) plasma fraction (F-4), the enzyme esterified cholesterol from VLDL at considerably higher rates than from LDL or HDL. VLDL together with some component present in the very high density plasma fraction (F-4) may thus provide a highly efficient complex resulting in a favorable configuration of substrate lipids for the enzyme.     

A dual role for lecithin:cholesterol acyltransferase (EC 2.3.1.43) in lipoprotein oxidation

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme involved in lipoprotein metabolism. It mediates the transesterification of free cholesterol to cholesteryl ester in an apoprotein A-I-dependent process. We have isolated purified LCAT from human plasma using anion-exchange chromatography and characterized the extracted LCAT in terms of its molecular weight, molar absorption coefficient, and enzymatic activity. The participation of LCAT in the oxidation of very low density lipoproteins (VLDL) and low-density lipoproteins (LDL) was examined by supplementing lipoproteins with exogenous LCAT over a range of protein concentrations. LCAT-depleted lipoproteins were also prepared and their oxidation kinetics examined. Our results provide evidence for a dual role for LCAT in lipoprotein oxidation, whereby it acts in a dose-responsive manner as a potent pro-oxidant during VLDL oxidation, but as an antioxidant during LDL oxidation. We believe this novel pro-oxidant effect may be attributable to the LCAT-mediated formation of oxidized cholesteryl ester in VLDL, whereas the antioxidant effect is similar to that of chain-breaking antioxidants. Thus, we have demonstrated that the high-density lipoprotein-associated enzyme LCAT may have a significant role to play in lipoprotein modification and hence atherogenesis.     

Lipid transport in the developing bovine follicle: messenger RNA expression increases for selective uptake receptors and decreases for endocytosis receptors

Differences in rates of steroid production and secretion will, eventually, determine the developmental rates of ovarian follicles. The major supply of cholesterol, the precursor for steroid and androgen biosynthesis, to ovarian cells is from circulating lipoproteins via membrane receptors from the low density lipoprotein receptor (LDL) superfamily. This occurs by either endocytosis, which has been described for very low density lipoprotein receptors (VLDLr), for LDL receptors (LDLr), and by the selective uptake pathway described for the scavenger receptor class B type 1 receptor (SRB1) and the recently described ovarian receptor, lipoprotein receptor-related protein 8 (LRP8). In this study, the mRNA expression of these four cholesterol receptors in bovine ovarian cells was determined at different stages of follicular development. In small antral follicles, mRNA expression of the endocytosis receptors was higher than in large antral follicles. Expression of LRP8 mRNA increased linearly with follicular size together with an increase in LDL, VLDL, and cholesterol concentrations in the follicular fluid. SRB1 mRNA expression tended to increase with follicular diameter. Because different mRNA expression patterns were found for the two types of receptor, this may imply different regulation of cholesterol supply at different stages of follicular development. Accumulation of LDL and VLDL particles in the follicular fluid of large antral follicles may enhance cholesterol availability for the intense steroidogenic activity that is essential at these stages.     

On the metabolic function of heparin-releasable liver lipase

Intravenous administration of specific antibody against heparin-releasable liver lipase (liver lipase) induced a 75% inhibition of the enzyme activity $\text{in situ}$. Administration of the antibody resulted in an increase of high density lipoprotein (density range 1.050–1.13 g/ml; HDL₂) phospholipid levels (20% after 1 h; 54% after 4 h). Short-term (1 h) treatment with antibody had no significant effect on any of the other lipoprotein components. After long-term (4 h) treatment the free cholesterol level of HDL₂ and all components in the very low density lipoprotein (VLDL) + intermediate density lipoprotein (IDL) fraction were elevated (1.5–2.0 fold). In the low density lipoprotein (LDL) fraction only the phospholipid level was affected (increased by 72%). All lipid components in the HDL₃ fraction were decreased by the antibody treatment, but this decrease was only statistically significant for the cholesterolesters. The rate of removal of iodine-labeled high density lipoprotein (HDL) and LDL from serum was not affected by the antibody treatment.These results suggest that liver lipase may promote phospholipid removal $\text{in vivo}$ and show that a lowering of liver lipase $\text{in situ}$ has profound consequences for serum lipoprotein metabolism.     

A new serum lipoprotein found in many rhesus monkeys.

A new serum lipoprotein was found in about 10 out of 30 rhesus monkeys ($\text{Macaca}\text{mulatta}$) which contained 28% by weight of protein, 42% total cholesterol, 22% phospholipid, and 8% triglyceride. This is in contrast to LDL (which the ten monkeys also contained) which had 24% protein, 46% total cholesterol, 24% phospholipid and 7% triglyceride. An S_{f, 1.063} in KBr of 3.0 to 3.7 and molecular weight of 3.5 – 3.7 million were observed compared to means of 8.1 and 3.0 million for normal rhesus LDL. The lower S_f was caused by its higher density. This new lipoprotein was most easily demonstrated and isolated by preparative ultracentrifugation of all serum lipoproteins at density 1.22 g/ml, followed by 6% agarose gel filtration at 6°. The new lipoprotein appeared as a distinct peak eluting before LDL.     

Effect of growth hormone replacement therapy on plasma lecithin:cholesterol acyltransferase and lipid transfer protein activities in growth hormone-deficient adults

The effects of growth hormone (GH) replacement on plasma lecithin:cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP), and phospholipid transfer protein (PLTP), factors involved in high density lipoprotein (HDL) metabolism, are unknown. We carried out a 6 months study in 24 GH-deficient adults who were randomized to placebo (n = 8), low dose GH (1 U daily, n = 8), and high dose GH (2 U daily, n = 8), followed by a 6 months open extension study with high dose GH (1 drop-out). No significant changes in plasma lipoproteins, LCAT, CETP, and PLTP activities, cholesterol esterification (EST) and cholesteryl ester transfer (CET) were observed after placebo. After 6 months of GH (combined data, n = 24), very low + low density lipoprotein (VLDL + LDL) cholesterol (P < 0.05) and apolipoprotein B (P < 0.05) decreased, whereas HDL cholesterol and HDL cholesteryl ester increased (P < 0. 05). Prolonged treatment showed comparable effects. Plasma apolipoprotein A-I and Lp[a] remained unchanged. Plasma LCAT (P < 0. 01) and CETP activities (P < 0.01), as well as EST (P < 0.01) and CET decreased (P < 0.01) after 12 months of GH (n = 15), but PLTP activity did not significantly change. Changes in EST and CET after 12 months of treatment were independently related to changes in plasma LCAT (P = 0.001 and CETP activity (P = 0.01). In conclusion, GH replacement therapy improves the lipoprotein profile in GH-deficient adults. Chronic GH replacement lowers plasma LCAT and CETP activities, contributing to a decrease in cholesterol esterification and cholesteryl ester transfer. These effects may have consequences for HDL metabolism and reverse cholesterol transport.     

Dissociation of high density lipoprotein precursors from apolipoprotein B-containing lipoproteins in the presence of unesterified fatty acids and a source of apolipoprotein A-I

Incubation of low (LDL), intermediate (IDL), or very low density lipoproteins (VLDL) with palmitic acid and either high density lipoproteins (HDL), delipidated HDL, or purified apolipoprotein (apo) A-I resulted in the formation of lipoprotein particles with discoidal structure and mean particle diameters ranging from 146 to 254 A by electron microscopy. Discs produced from IDL or LDL averaged 26% protein, 42% phospholipid, 5% cholesteryl esters, 24% free cholesterol, and 3% triglycerides; preparations derived from VLDL contained up to 21% triglycerides. ApoA-I was the predominant protein present, with smaller amounts of apoA-II. Crosslinking studies of discs derived from LDL or IDL indicated the presence of four apoA-I molecules per particle, while those derived from large VLDL varied more in size and contained as many as six apoA-I molecules per particle. Incubation of discs derived from IDL or LDL with purified lecithin:cholesterol acyltransferase (LCAT), albumin, and a source of free cholesterol produced core-containing particles with size and composition similar to HDL2b. VLDL-derived discs behaved similarly, although the HDL products were somewhat larger and more variable in size. When discs were incubated with plasma d greater than 1.21 g/ml fraction rather than LCAT, core-containing particles in the size range of normal HDL2a and HDL3a were also produced. A variety of other purified free fatty acids were shown to promote disc formation. In addition, some mono and polyunsaturated fatty acids facilitated the formation of smaller, spherical particles in the size range of HDL3c. Both discoidal and small spherical apoA-I-containing lipoproteins were generated when native VLDL was incubated with lipoprotein lipase in the presence of delipidated HDL. We conclude that lipolysis product-mediated dissociation of lipid-apoA-I complexes from VLDL, IDL, or LDL may be a mechanism for formation of HDL subclasses during lipolysis, and that the availability of different lipids may influence the type of HDL-precursors formed by this mechanism.     

Lipoprotein abnormalities associated with lipopolysaccharide-induced lecithin: cholesterol acyltransferase and lipase deficiency

Density gradient ultracentrifugation was used to isolate and characterize the plasma lipoproteins from African green monkeys before and 24 and 48 h after subcutaneous injection of 300 micrograms/kg lipopolysaccharide (LPS) to induce an acute phase response. Compared with 0 h values, reductions occurred in plasma cholesterol (39%), high density lipoprotein (HDL) cholesterol (54%), lecithin:cholesterol acyltransferase (LCAT) activity (55%), and post-heparin plasma lipase activity (68%) 48 h after LPS injection while plasma triglyceride concentrations increased 700%. Cholesterol distribution among lipoproteins shifted from 7 to 41% in very low density lipoproteins (VLDL), 65 to 38% in low density lipoproteins (LDL), and 28 to 21% in HDL after LPS injection. At 48 h after LPS injection, all lipoprotein classes were relatively enriched in phospholipid and triglyceride and depleted of cholesteryl ester. The plasma concentration of all chemical constituents in VLDL was increased 3-9-fold within 48 h after LPS injection. By negative stain electron microscopy, HDL were discoidal in shape while VLDL and LDL appeared to have excess surface material present. Even though total HDL protein concentration in plasma was unaffected, the plasma mass of the smallest HDL subfractions (HDL3b,c) doubled while the mass of intermediate-sized subfractions (HDL3a) was dramatically decreased within 24 h after treatment. HDL became enriched in apoE, acquired apoSAA, and became depleted of apoA-I, A-II, and Cs by 48 h after LPS injection while apoB-100 remained the major apoprotein of VLDL and LDL. We conclude that administration of LPS to monkeys prevents normal intravascular metabolism of lipoproteins and results in the accumulation of relatively nascent forms of lipoproteins in plasma. These immature lipoproteins resemble those isolated from the recirculating perfusion of African green monkey livers, which are relatively deficient of LCAT activity and those isolated from the plasma of patients with familial LCAT deficiency.     

Preferential enrichment of large-sized very low density lipoprotein populations with transferred cholesteryl esters

The effect of lipid transfer proteins on the exchange and transfer of cholesteryl esters from rat plasma HDL2 to human very low (VLDL) and low density (LDL) lipoprotein populations was studied. The use of a combination of radiochemical and chemical methods allowed separate assessment of [3H]cholesteryl ester exchange and of cholesteryl ester transfer. VLDL-I was the preferred acceptor for transferred cholesteryl esters, followed by VLDL-II and VLDL-III. LDL did not acquire cholesteryl esters. The contribution of exchange of [3H]cholesteryl esters to total transfer was highest for LDL and decreased in reverse order along the VLDL density range. Inactivation of lecithin: cholesterol acyltransferase (LCAT) and heating the HDL2 for 60 min at 56 degrees C accelerated transfer and exchange of [3H]cholesteryl esters. Addition of lipid transfer proteins increased cholesterol esterification in all systems. The data demonstrate that large-sized, triglyceride-rich VLDL particles are preferred acceptors for transferred cholesteryl esters. It is suggested that enrichment of very low density lipoproteins with cholesteryl esters reflects the triglyceride content of the particles.     

Understanding the mechanism of LCAT reaction may help to explain the high predictive value of LDL/HDL cholesterol ratio.

Traditionally, lecithin:cholesterol acyltransferase (LCAT) role in the reverse cholesterol transport (RCT) has been considered "antiatherogenic" as the cholesterol esterification is the prerequisite for the formation of mature high density lipoprotein (HDL) particles and may create a gradient necessary for the flow of unesterified cholesterol (UC) from tissues to plasma. However, newer data suggest that a higher esterification rate is not necessarily protective. Here we review the available data on the role of LCAT in RCT and propose that the LCAT-mediated esterification of plasma cholesterol promotes RCT only in the presence of sufficient concentrations of HDL2 while this reaction may be atherogenic in the presence of high concentration of plasma low density lipoprotein (LDL) cholesterol Thus, the "protective" or potentially "atherogenic" role of LCAT depends on the quality of HDL and concentration of LDL. This hypothesis is consistent with the known high predictive value of LDL/HDL cholesterol ratio.     

Binding affinity and reactivity of lecithin cholesterol acyltransferase with native lipoproteins.

The first step in the reaction of lecithin cholesterol acyltransferase (LCAT) with lipoproteins is the interfacial binding of the enzyme to the lipid surfaces. In this study the equilibrium dissociation constants (Kds) for the interaction of pure human plasma LCAT with LDL, HDL2, HDL3, and a reconstituted discoidal HDL (rHDL) were determined by the activity-inhibition method. In addition, enzyme kinetics were measured with each of the lipoprotein substrates. Based on phospholipid concentrations, the Kd values (0.9 x 10(-5) to 4.6 x 10(-5) M) increased in the order rHDL = HDL3 相似文献   

Endotoxin-lipoprotein complex formation as a factor in atherogenesis: associations with hyperlipidemia and with lecithin:cholesterol acyltransferase activity

A potential role of endotoxin–lipoprotein (bacterial lipopolysaccharide–lipoprotein, LPS–LP) complex formation as a pathogenic factor for atherosclerosis has not been studied yet. The aim of this study was to test the hypothesis that in endotoxinemia in humans hyperlipidemia associated with atherosclerosis development can favor an excessive LPS–LP complex formation, and endotoxin presented in blood can inhibit lecithin:cholesterol acyltransferase (LCAT), one of the key enzymes of reverse cholesterol transport. Endotoxin-binding capacity of lipoproteins (LP) in patients with normolipidemia and hyperlipidemia types IIa and IV was estimated from label incorporation into different LP fractions isolated by means of sequential ultracentrifugation following serum preincubation with Salmonella minnesota R595 ¹²⁵I-labeled LPS. The effect of varied concentrations of S. minnesota R595 LPS on LCAT activity was evaluated from the overall esterifying activity of serum using [1,2-³H₂]cholesterol-labeled substrate. The elevation of low density LP (LDL) and very low density LP (VLDL) contents in blood serum in hyperlipidemia types IIa and IV, respectively, resulted in significant elevation of LPS binding to these fractions. LPS added to the blood serum leads to the dose-dependent decrease in LCAT activity. The revealed phenomena of elevated LPS binding to atherogenic LP fractions in hypercholesterolemia and endotoxin-induced LCAT inhibition suggest the pathogenic role of LPS–LP complexes in atherogenesis.     

Changes in lipoprotein binding and uptake by hepatocytes during rat liver regeneration

The binding and uptake of cholesterol enriched lipoproteins by isolated hepatocytes was decreased at 16 hours after partial hepatectomy, with a tendency to return to control values as the regeneration proceeds. The number of lipoprotein binding sites of total cellular membranes remained similar to control at 16 and 24 hours. The plasma lipoprotein pattern, determined by electrophoretic analysis, showed a lower per cent of very low density lipoproteins (VLDL) and a higher per cent of low density lipoproteins (LDL) at 16 and 24 hours post-partial hepatectomy. At these times, plasma lecithin: cholesterol acyltransferase (LCAT) activity was decreased. It is intriguing to suggest that the regenerating liver could regulated the blood lipoprotein pattern and the uptake of lipoproteins by modulating the surface expression of the receptors.     

Molecular etiology of a dominant form of type III hyperlipoproteinemia caused by R142C substitution in apoE4

We have used adenovirus-mediated gene transfer in apolipoprotein (apo)E^−/− mice to elucidate the molecular etiology of a dominant form of type III hyperlipoproteinemia (HLP) caused by the R142C substitution in apoE4. It was found that low doses of adenovirus expressing apoE4 cleared cholesterol, whereas comparable doses of apoE4[R142C] greatly increased plasma cholesterol, triglyceride, and apoE levels, caused accumulation of apoE in VLDL/IDL/LDL region, and promoted the formation of discoidal HDL. Co-expression of apoE4[R142C] with lecithin cholesterol acyltransferase (LCAT) or lipoprotein lipase (LPL) in apoE^−/− mice partially corrected the apoE4[R142C]-induced dyslipidemia. High doses of C-terminally truncated apoE4[R142C]-202 partially cleared cholesterol in apoE^−/− mice and promoted formation of discoidal HDL. The findings establish that apoE4[R142C] causes accumulation of apoE in VLDL/IDL/LDL region and affects in vivo the activity of LCAT and LPL, the maturation of HDL, and the clearance of triglyceride-rich lipoproteins. The prevention of apoE4[R142C]-induced dyslipidemia by deletion of the 203-299 residues suggests that, in the full-length protein, the R142C substitution may have altered the conformation of apoE bound to VLDL/IDL/LDL in ways that prevent triglyceride hydrolysis, cholesterol esterification, and receptor-mediated clearance in vivo.     

Lipolytic degradation of human very low density lipoproteins by human milk lipoprotein lipase: the identification of lipoprotein B as the main lipoprotein degradation product

Although the direct conversion of very low density lipoproteins (VLDL) into low density (LDL) and high density (HDL) lipoproteins only requires lipoprotein lipase (LPL) as a catalyst and albumin as the fatty acid acceptor, the in vitro-formed LDL and HDL differ chemically from their native counterparts. To investigate the reason(s) for these differences, VLDL were treated with human milk LPL in the presence of albumin, and the LPL-generated LDL1-, LDL2-, and HDL-like particles were characterized by lipid and apolipoprotein composition. Results showed that the removal of apolipoproteins B, C, and E from VLDL was proportional to the degree of triglyceride hydrolysis with LDL2 particles as the major and LDL1 and HDL + VHDL particles as the minor products of a complete in vitro lipolysis of VLDL. In comparison with native counterparts, the in vitro-formed LDL2 and HDL + VHDL were characterized by lower levels of triglyceride and cholesterol ester and higher levels of free cholesterol and lipid phosphorus. The characterization of lipoprotein particles present in the in vitro-produced LDL2 showed that, as in plasma LDL2, lipoprotein B (LP-B) was the major apolipoprotein B-containing lipoprotein accounting for over 90% of the total apolipoprotein B. Other, minor species of apolipoprotein B-containing lipoproteins included LP-B:C-I:E and LP-B:C-I:C-II:C-III. The lipid composition of in vitro-formed LP-B closely resembled that of plasma LP-B. The major parts of apolipoproteins C and E present in VLDL were released to HDL + VHDL as simple, cholesterol/phospholipid-rich lipoproteins including LP-C-I, LP-C-II, LP-C-III, and LP-E. However, some of these same simple lipoprotein particles were present after ultracentrifugation in the LDL2 density segment because of their hydrated density and/or because they formed, in the absence of naturally occurring acceptors (LP-A-I:A-II), weak associations with LP-B. Thus, the presence of varying amounts of these cholesterol/phospholipid-rich lipoproteins in the in vitro-formed LDL2 appears to be the main reason for their compositional difference from native LDL2. These results demonstrate that the formation of LP-B as the major apolipoprotein B-containing product of VLDL lipolysis only requires LPL as a catalyst and albumin as the fatty acid acceptor. However, under physiological circumstances, other modulating agents are necessary to prevent the accumulation and interaction of phospholipid/cholesterol-rich apolipoprotein C- and E-containing particles.     

兔主动脉平滑肌细胞低密度脂蛋白受体相关蛋白(LRP)的诱导表达调节

在兔主动脉平滑肌细胞 ( SMC)培养基中分别加入正常低密度脂蛋白 ( N- LDL)、氧化低密度脂蛋白 ( ox- LDL)、正常极低密度脂蛋白 ( N- VLDL)、氧化极低密度脂蛋白 ( ox- VLDL)和 β-极低密度脂蛋白 (β- VLDL )培养 2 4 h后 ,用定量 RT- PCR和配体结合实验检测平滑肌细胞 LRP的m RNA和蛋白质水平的表达 .结果表明 :五种脂蛋白均能在转录和翻译水平诱导兔主动脉平滑肌细胞的 LRP表达 ,尤以富含胆固醇的 N- LDL ,ox- LDL和β- VLDL的刺激作用更明显 .用胆固醇单独或与脂蛋白共同温育 SMC后 ,发现胆固醇本身可促进 SMC的 LRP蛋白水平的表达 ,脂蛋白与胆固醇的共同刺激作用更为显著 .结果提示 :上述五种脂蛋白对 SMC上 LRP的表达有上调作用 ,其机制可能主要是通过其中的胆固醇来实现的 .     

Serum activity and hepatic secretion of lecithin:cholesterol acyltransferase in experimental hypothyroidism and hypercholesterolemia

Lecithin:cholesterol acyltransferase (LCAT), the major cholesterol esterifying enzyme in plasma, plays an important role in the removal of cholesterol from peripheral tissues. This study in rat focuses upon the effects of hypothyroidism and cholesterol feeding on serum activity and hepatic LCAT secretion. To obviate the effect that inclusion of high concentrations of cholesterol in the rat serum may have on the proteoliposome used in the assay of LCAT, very low and low density lipoproteins (VLDL and LDL) were removed by ultracentrifugation at d 1.063 g/ml. The molar esterification rate in the euthyroid VLDL + LDL-free serum was found to be 0.94 +/- 0.06 compared to 0.67 +/- 0.05 in hypothyroid rats and 1.56 +/- 0.14 in hypercholesterolemic rats. LCAT secretion by suspension cultures of hepatocytes from hypercholesterolemic rats was found to be significantly depressed when compared to that for euthyroid and hypothyroid animals. Secretion by hepatocytes from hypothyroid rats was depressed for the first 0-4 hr, but rapidly recovered. The depressed secretion of LCAT by hepatocytes from hypercholesterolemic rats correlates with the appearance in the media of apoE-rich, discoidal HDL. Discoidal HDL was six times more effective as a substrate for purified human LCAT than HDL from hypercholesterolemic serum, and twice as effective as serum and nascent HDL from euthyroid animals. It is concluded that the depressed LCAT activity in serum from hypothyroid rats is due to a depressed hepatic secretion of the enzyme and that the elevated serum activity of hypercholesterolemic rats may be related to a defect in LCAT clearance. Finally, the appearance of discoidal HDL in the medium upon culture of hepatocytes from hypercholesterolemic rats appears to be due to an inhibition of LCAT secretion by these cells.     

Influence of progesterone on the initial rate of cholesterol esterification in rat plasma

The effect of progesterone on the initial rate of cholesterol esterification in rat plasma was measured after daily injections of the hormone or following addition of progesterone $\text{in}$$\text{vitro}$. The administration of progesterone did not modify the lecithin:cholesterol acyltransferase (LCAT) activity nor the progress of the enzyme reaction with time. When increasing concentrations of progesterone were added to the medium the percentage of cholesterol esterified per minute decreased progressively. The addition of progesterone also decreased the slope of the time-course reaction. It is suggested that the inhibition of the LCAT activity due to the presence of the hormone would be masked by an increased hepatic production of the enzyme and/or by the alterations that the hormonal treatments produced in the plama lipid levels.     

Dynamic structure of the lower density lipoproteins. I. Incorporation of high levels of labelled lipids into very low and low density lipoproteins

Selectively labelled lipids have been incorporated into the surface monolayer of human serum low density lipoprotein (LDL) and very low density lipoprotein (VLDL). From 3 to 17 mol% of phosphatidylcholine, selectively deuterated at various positions along the sn-2-acyl chain, was transferred from unilamellar vesicles to VLDL using a partially purified phosphatidylcholine transfer protein. Selectively deuterated palmitic acids were incorporated into LDL (6-20 mol%) and into VLDL (7-10 mol%). Electron microscopy, light scattering, and 31P nuclear magnetic resonance indicated that particle size remained unchanged. Gel exclusion chromatography and chemical analysis showed no difference in hydrodynamic properties and only slight alteration to particle component ratios. Biological activity of labelled VLDL was measured from the rate of cholesterol esterification by cultured J774A.1 cells. Effect of labelling LDL was evaluated by monitoring LDL uptake and degradation by cultured human skin fibroblasts. In all cases the lipoproteins containing labels were indistinguishable from their native counterparts.