Probucol normalizes cholesteryl ester transfer in type 2 diabetes

AIMS

Accelerated cholesteryl ester transfer (CET) protein (CETP) activity is believed to promote macrovascular disease in patients with type 2 diabetes (T2D) by increasing the cholesterol burden of the apoB - containing triglyceride-rich lipoprotein (TGRLP) CE acceptors and promoting small dense LDL formation. While previous studies have shown that this same abnormality is present in patients with type 1 diabetes (T1D) and was normalized by the anti-oxidant drug probucol, its effects on CET in T2D are unknown.

PATIENTS AND METHODS

The net mass transfer of CE from HDL to the apoB lipoproteins (VLDL+LDL) was studied in intact plasma from seven T2D patients before and two months after treatment with probucol (1g/day).

RESULTS

Before treatment, CET was significantly greater than controls at 1 and 2h (p<.005). Recombination studies showed that this disturbance was attributable to dysfunction of VLDL and not due to altered behavior of HDL or CETP. Probucol treatment normalized CET in all subjects and significantly lowered plasma cholesterol (pre-Rx: 197±4.5 vs post-Rx: 162±27.1mg/dL; mean±S.D.; p<.025) and HDL-C (pre-Rx: 46.4±7.5 vs post-Rx: 39.1±4.0; p<.025) without changing glycemic control.

CONCLUSIONS

By normalizing CET in T2D, probucol likely reduces the formation of atherogenic lipoproteins. This effect on CET is achieved through qualitative alterations in CETP's lipoprotein substrates and not through changes in CETP or HDL. Since probucol also has potent anti-oxidative and anti-inflammatory properties, it may have a new role to play in lipoprotein remodeling that reduce cardiovascular risk in T2D.

Molecular dynamic simulation study of cholesterol and conjugated double bonds in lipid bilayers

Conjugated linoleic acids (CLA) are found naturally in dairy products. Two isomers of CLA, that differ only in the location of cis and trans double bonds, are found to have distinct and different biological effects. The cis 9 trans 11 (C9T11) isomer is believed to have anti-carcinogenic effects, while the trans 10 cis 12 (T10C12) isomer is believed to be associated with anti-obesity effects. In this paper we extend earlier molecular dynamics (MD) simulations of pure CLA-phosphatidylcholine bilayers to investigate the comparative effects of cholesterol on bilayers composed of the two respective isomers. Simulations of phosphatidylcholine lipid bilayers in which the sn-2 chains contained one of the two isomers of CLA were performed in which, for each isomer, the simulated bilayers contained 10% and 30% cholesterol (Chol). From MD trajectories we calculate and compare structural properties of the bilayers, including areas per molecule, thickness of bilayers, tilt angle of cholesterols, order parameter profiles, and one and two-dimensional radial distribution function (RDF), as functions of Chol concentration. While the structural effect of cholesterol is approximately the same for both isomers, we find differences at an atomistic level in order parameter profiles and in two-dimensional radial distribution functions.

Conjugated double bonds in lipid bilayers: a molecular dynamics simulation study

Conjugated linoleic acids (CLA) are found naturally in dairy products. Two isomers of CLA, that differ only in the location of cis and trans double bonds, are found to have distinct and different biological effects. The cis 9 trans 11 (C9T11) isomer is attributed to have the anti-carcinogenic effects, while the trans 10 cis 12 (T10C12) isomer is believed to be responsible for the anti-obesity effects. Since dietary CLA are incorporated into membrane phospholipids, we have used Molecular Dynamics (MD) simulations to investigate the comparative effects of the two isomers on lipid bilayer structure. Specifically, simulations of phosphatidylcholine lipid bilayers in which the sn-2 chains contained one of the two isomers of CLA were performed. Force field parameters for the torsional potential of double bonds were obtained from ab initio calculations. From the MD trajectories we calculated and compared structural properties of the two lipid bilayers, including areas per molecule, density profiles, thickness of bilayers, tilt angle of tail chains, order parameters profiles, radial distribution function (RDF) and lateral pressure profiles. The main differences found between bilayers of the two CLA isomers, are (1) the order parameter profile for C9T11 has a dip in the middle of sn-2 chain while the profile for T10C12 has a deeper dip close to terminal of sn-2 chain, and (2) the lateral pressure profiles show differences between the two isomers. Our simulation results reveal localized physical structural differences between bilayers of the two CLA isomers that may contribute to different biological effects through differential interactions with membrane proteins or cholesterol.

Sphingomyelin: a natural modulator of membrane homeostasis and inflammation

Although membrane sphingomyelin (SPH) serves as the precursor for many signaling molecules, its presence in large amounts, and its specific localization in the outer monolayer of the plasma membrane suggest that it may have a cytoprotective function. We propose that SPH helps maintain the integrity of the plasma membrane by protecting phosphatidylcholine (PC) against oxidative damage and phospholipase degradation. Since it contains mostly saturated longchain hydrocarbon groups, we postulate that SPH impedes the lateral propagation of the lipid peroxides by decreasing membrane fluidity, while also acting as an 'insulating' molecule. By virtue of its structural similarity to PC, it acts as a competitive inhibitor of the phospholipases, which may otherwise hydrolyze PC excessively. Because phospholipase reaction is the rate-limiting step in eicosanoid synthesis, SPH may serve as an endogenous anti-inflammatory molecule.

Importance of the free sulfhydryl groups of lecithin-cholesterol acyltransferase for its sensitivity to oxidative inactivation

Lecithin-cholesterol acyltransferase (LCAT) of human plasma is known to be highly susceptible to oxidative inactivation, although the mechanism of this inactivation is unknown. We tested the hypothesis that the high sensitivity of the enzyme is due to the derivatization of its two free SH groups flanking the active site pocket. Modification of the SH groups with a reversible inhibitor protected the enzyme against oxidative inactivation. Mutagenesis of either of the cysteines to glycine increased the resistance of the enzyme, which retained 46% of activity in presence of 150 microM Cu(2+), compared to only 27% of the activity retained by the wild type enzyme (WT). Replacement of both the cysteines with glycines resulted in retention of over 65% activity. Cysteine replacement similarly protected the enzyme from inactivation by the oxidized substrate. Chicken LCAT, which has only one cysteine (Cys(26)), was more resistant than the human enzyme. Introduction of an additional cysteine corresponding to the second cysteine in human LCAT (N184C) resulted in increased susceptibility of chicken enzyme (87% loss of activity in presence of 150 microM Cu(2+), compared to 55% loss in WT). Substitution of the lone cysteine with glycine (C26G) resulted in a more resistant enzyme, which lost <40% activity under the same conditions. These results show that the primary targets of the oxidizing agents or the products of oxidation are the SH groups of the enzyme, whose derivatization leads to steric inhibition of the activity.

Novel physiological function of sphingomyelin in plasma. Inhibition of lipid peroxidation in low density lipoproteins

Although sphingomyelin (SPH) is a major constituent of all lipoproteins, its physiological function in plasma is not known. In this study, we tested the hypothesis that SPH inhibits lipid peroxidation in low density lipoproteins (LDL) because of its effects on surface fluidity and packing density and that the relative resistance of the buoyant LDL to oxidation, compared with the dense LDL, is partly due to their higher SPH content. Depletion of SPH by treatment with SPHase resulted in shortened lag times and increased rates of oxidation in both LDL subfractions, as measured by the conjugated diene formation in the presence of Cu(2+). Oxidation of LDL by soybean lipoxygenase was similarly stimulated by the degradation of SPH. Oxidation-induced fluorescence decay of diphenylhexatriene-labeled phosphatidylcholine (PC), equilibrated with LDL-PC, was accelerated significantly by the enzymatic depletion of SPH from the lipoprotein. Oxidation of 16:0-18:2 PC in the proteoliposomes was inhibited progressively by the incorporation of increasing amounts of egg SPH into the liposomes. Treatment of SPH-containing proteoliposomes with SPHase reversed the effect of SPH, showing that the presence of intact SPH is necessary for the inhibition of oxidation. Although the incorporation of SPH into the same liposome as the PC (intrinsic SPH) protected the PC against oxidation, the addition of SPH liposomes to PC liposomes (extrinsic SPH) was not effective. Oxidation of 16:0-18:2 PC in liposomes was also inhibited by the incorporation of dipalmitoyl-PC, but not by free cholesterol. These results suggest that SPH acts as a physiological inhibitor of lipoprotein oxidation, possibly by modifying the fluidity of the phospholipid monolayer and thereby inhibiting the lateral propagation of the lipid peroxy radicals.

Role of lecithin-cholesterol acyltransferase in the metabolism of oxidized phospholipids in plasma: studies with platelet-activating factor-acetyl hydrolase-deficient plasma

To determine the relative importance of platelet-activating factor-acetylhydrolase (PAF-AH) and lecithin-cholesterol acyltransferase (LCAT) in the hydrolysis of oxidized phosphatidylcholines (OXPCs) to lyso-phosphatidylcholine (lyso-PC), we studied the formation and metabolism of OXPCs in the plasma of normal and PAF-AH-deficient subjects. Whereas the loss of PC following oxidation was similar in the deficient and normal plasmas, the formation of lyso-PC was significantly lower, and the accumulation of OXPC was higher in the deficient plasma. Isolated LDL from the PAF-AH-deficient subjects was more susceptible to oxidation, and stimulated adhesion molecule synthesis in endothelial cells, more than the normal LDL. Oxidation of 16:0-[1-14C]-18:2 PC, equilibrated with plasma PC, resulted in the accumulation of labeled short- and long-chain OXPCs, in addition to the labeled aqueous products. The formation of the aqueous products decreased by 80%, and the accumulation of short-chain OXPC increased by 110% in the deficient plasma, compared to the normal plasma, showing that PAF-AH is predominantly involved in the hydrolysis of the truncated OXPCs. Labeled sn-2-acyl group from the long-chain OXPC was not only hydrolyzed to free fatty acid, but was preferentially transferred to diacylglycerol, in both the normal and deficient plasmas. In contrast, the acyl group from unoxidized PC was transferred only to cholesterol, showing that the specificity of LCAT is altered by OXPC. It is concluded that, while PAF-AH carries out the hydrolysis of mainly truncated OXPCs, LCAT hydrolyzes and transesterifies the long-chain OXPCs.

All ApoB-containing lipoproteins induce monocyte chemotaxis and adhesion when minimally modified. Modulation of lipoprotein bioactivity by platelet-activating factor acetylhydrolase

Mildly oxidized LDL has many proinflammatory properties, including the stimulation of monocyte chemotaxis and adhesion, that are important in the development of atherosclerosis. Although ApoB-containing lipoproteins other than LDL may enter the artery wall and undergo oxidation, very little is known regarding their proinflammatory potential. LDL, IDL, VLDL, postprandial remnant particles, and chylomicrons were mildly oxidized by fibroblasts overexpressing 15-lipoxygenase (15-LO) and tested for their ability to stimulate monocyte chemotaxis and adhesion to endothelial cells. When conditioned on 15-LO cells, LDL, IDL, but not VLDL increased monocyte chemotaxis and adhesion approximately 4-fold. Chylomicrons and postprandial remnant particles were also bioactive. Although chylomicrons had a high 18:1/18:2 ratio, similar to that of VLDL, and should presumably be less susceptible to oxidation, they contained (in contrast to VLDL) essentially no platelet-activating factor acetylhydrolase (PAF-AH) activity. Because PAF-AH activity of lipoproteins may be reduced in vivo by oxidation or glycation, LDL, IDL, and VLDL were treated in vitro to reduce PAF-AH activity and then conditioned on 15-lipoxygenase cells. All 3 PAF-AH-depleted lipoproteins, including VLDL, exhibited increased stimulation of monocyte chemotaxis and adhesion. In a similar manner, lipoproteins from Japanese subjects with a deficiency of plasma PAF-AH activity were also markedly more bioactive, and stimulated monocyte adhesion nearly 2-fold compared with lipoproteins from Japanese control subjects with normal plasma PAF-AH. For each lipoprotein, bioactivity resided in the lipid fraction and monocyte adhesion could be blocked by PAF-receptor antagonists. These data suggest that the susceptibility of plasma lipoproteins to develop proinflammatory activity is in part related to their 18:1/18:2 ratio and PAF-AH activity, and that bioactive phospholipids similar to PAF are generated during oxidation of each lipoprotein. Moreover, LDL, IDL, postprandial remnant particles, and chylomicrons and PAF-AH-depleted VLDL all give rise to proinflammatory lipids when mildly oxidized.

Modulation of the positional specificity of lecithin-cholesterol acyltransferase by the acyl group composition of its phosphatidylcholine substrate: role of the sn-1-acyl group

Human lecithin-cholesterol acyltransferase (LCAT), which is normally specific for the sn-2 position of phosphatidylcholine (PC), derives a significant percentage of acyl groups from the sn-1 position, when sn-2 is occupied by 18:0, 20:4, or 22:6. We investigated the relative importance of the two acyl groups of PC in determining the positional specificity by first analyzing the cholesteryl esters formed in the presence of symmetric PCs labeled at sn-2. Both human and rat LCATs transferred exclusively the sn-2-acyl group from all symmetric PCs, including 18:0-18:0, and 20:4-20:4, showing that the presence of these fatty acids at sn-2 does not automatically alter the positional specificity. The role of the sn-1-acyl group was then tested by using PCs containing 20:4 or 18:0 at sn-2 and fatty acids of various chain lengths and unsaturation at sn-1. With 20:4 at sn-2 and saturated fatty acids of various chain lengths at sn-1, human and rat LCATs derived, respectively, 5-72% and 1-20% of the total acyl groups from the sn-1 position. However, the chain length of the sn-1-acyl did not correlate with its utilization by either enzyme. Various unsaturated fatty acids at sn-1 also were transferred by human LCAT at a higher rate (5-75% of total) than they were transferred by rat LCAT (0-21%). With sn-2-18:0 PCs, however, rat LCAT exhibited greater alteration in positional specificity (30-95% from sn-1) than human LCAT (15-83% from sn-1). These results show that while the primary determinant of positional specificity is the sn-2-acyl group of PC, the structure of sn-1-acyl significantly modifies it.

Trans unsaturated fatty acids inhibit lecithin: cholesterol acyltransferase and alter its positional specificity

Although dietary trans unsaturated fatty acids (TUFA) are known to decrease plasma HDL, the underlying mechanisms for this effect are unclear. We tested the hypothesis that the decreased HDL is due to an inhibition of lecithin:cholesterol acyltransferase (LCAT), the enzyme essential for the formation of HDL, by determining the activity of purified LCAT in the presence of synthetic phosphatidylcholine (PC) substrates containing TUFA. Both human and rat LCATs exhibited significantly lower activity (-37% to -50%) with PCs containing 18:1t or 18:2t, when compared with the PCs containing corresponding cis isomers. TUFA-containing PCs also inhibited the enzyme activity competitively, when added to egg PC substrate. The inhibition of LCAT activity was not due to changes in the fluidity of the substrate particle. However, the inhibition depended on the position occupied by TUFA in the PC, as well as on the paired fatty acid. Thus, for human LCAT, 18:1t was more inhibitory when present at sn-2 position of PC, than at sn-1, when paired with 16:0. In contrast, when paired with 20:4, 18:1t was more inhibitory at sn-1 position of PC. Both human and rat LCATs, which are normally specific for the sn-2 acyl group of PC, exhibited an alteration in their positional specificity when 16:0-18:1t PC or 16:1t-20:4 PC was used as substrate, deriving 26-86% of the total acyl groups for cholesterol esterification from the sn-1 position. These results show that the trans fatty acids decrease high density lipoprotein through their inhibition of lecithin: cholesterol acyltransferase (LCAT) activity, and also alter LCAT's positional specificity, inducing the formation of more saturated cholesteryl esters, which are more atherogenic.

Impaired function of lecithin:cholesterol acyltransferase in atherosclerosis-susceptible White Carneau pigeons: possible effects on metabolism of oxidized phospholipids

Although White Carneau (WC) pigeons are known to be more susceptible to atherosclerosis than Show Racer (SR) pigeons, the reasons for this difference are not fully understood. While no major differences are known in the lipoprotein composition, a difference in the cholesteryl ester (CE) composition was reported. However, there is little information on the activity or specificity of lecithin:cholesterol acyltransferase (LCAT), the major source of plasma CE. In order to determine whether the esterification of cholesterol or other functions of LCAT are compromised in WC pigeons, we studied the various reactions catalyzed by LCAT in the two groups. The cholesterol esterification was found to be significantly lower in WC pigeons, whether assayed with exogenous or endogenous substrates. Furthermore, lyso phosphatidylcholine (PC) esterification and oxidized PC hydrolysis, two other reactions carried out by LCAT, were also lower in WC. We found evidence for the presence of an active lysophospholipase in pigeon plasma, and this activity was also lower in WC compared to SR. A significant increase in the FC/PC ratio, another reported atherogenic risk factor, was found in WC. plasma. Because of the absence of other hydrolytic enzymes in pigeon plasma, LCAT may play an important role in the metabolism of oxidized PC generated during lipoprotein oxidation, and therefore a decrease in its activity in White Carneau pigeons may contribute to increased risk of atherosclerosis.

Novel function of lecithin-cholesterol acyltransferase. Hydrolysis of oxidized polar phospholipids generated during lipoprotein oxidation

Although the major function of lecithin-cholesterol acyltransferase (LCAT) is cholesterol esterification, our previous studies showed that it can also hydrolyze platelet-activating factor (PAF). Because of the structural similarities between PAF and the truncated phosphatidylcholines (polar PCs) generated during lipoprotein oxidation, we investigated the possibility that LCAT may also hydrolyze polar PCs to lyso-PC during the oxidation of plasma. PAF acetylhydrolase (PAF-AH), which is known to hydrolyze polar PCs in human plasma, was completely inhibited by 0.2 mM p-aminoethyl benzenesulfonyl fluoride (Pefabloc), a new serine esterase inhibitor, which had no effect on LCAT at this concentration. On the other hand, 1 mM diisopropylfluorophosphate (DFP) completely inhibited LCAT but had no effect on PAF-AH. Polar PC accumulation during the oxidation of plasma increased by 44% in the presence of 0.2 mM Pefabloc and by 30% in the presence of 1 mM DFP. The formation of lyso-PC was concomitantly inhibited by both of the inhibitors. The combination of the two inhibitors resulted in the maximum accumulation of polar PCs, suggesting that both PAF-AH and LCAT are involved in their breakdown. Oxidation of chicken plasma, which has no PAF-AH activity, also resulted in the formation of lyso-PC from the hydrolysis of polar PC, which was inhibited by DFP. Polar PCs, either isolated from oxidized plasma or by oxidation of labeled synthetic PCs, were hydrolyzed by purified LCAT, which had no detectable PAF-AH activity. These results demonstrate a novel function for LCAT in the detoxification of polar PCs generated during lipoprotein oxidation, especially when the PAF-AH is absent or inactivated.

Analysis of human lecithin-cholesterol acyltransferase activity by carboxyl-terminal truncation

Lecithin-cholesterol acyltransferase (LCAT) is a key enzyme in reverse cholesterol transport and catalyzes the esterification of cholesterol in human plasma. Human LCAT is a glycosylated protein, containing 416 amino acids and a proline-rich region at the C-terminus. To address the function of the C-terminal region of LCAT as well as that of the proline-rich region, we constructed and expressed LCAT mutants with C-terminal truncations at different positions. The expression of wild-type LCAT in COS-1 cells resulted in an enzymatically active protein that was secreted by the cells. The mutants lacking the proline-rich region at the C-terminus were expressed and secreted at levels comparable to those of wild-type (approximately 50% of wild-type concentrations in cell media). The proline-deletion mutants were similar to wild-type LCAT in terms of phospholipase or transferase activities with various interfacial substrates, including reconstituted HDL, proteoliposomes, LDL, and micelles of platelet activating factor. Thus, the binding of LCAT to the diverse interfaces is not affected by the removal of its C-terminal region. Also, the activation by apolipoproteins and access of water-insoluble substrates to the active site are not significantly affected by the deletion of the proline-rich region. However, deletions of the proline-rich region, including the five amino acids nearest to the C-terminus, resulted in approximately an 8-fold increase in the specific activity of LCAT towards the water-soluble substrate, p-nitrophenylbutyrate. This suggests that the C-terminal proline-rich region may interfere with the access of this water-soluble substrate to the active site of LCAT, and may form part of a protective covering of the active site of LCAT while in solution. Further deletions at the C-terminus, beyond the proline-rich region, impaired the secretion of the enzyme, implying that this region may play a critical role in either the secretion or folding of LCAT in COS-1 cells.

Impaired cholesterol esterification in the plasma in patients with breast cancer

An important factor which determines the movement of cholesterol in and out of the cells is the free cholesterol (FC)/esterified cholesterol (EC) ratio in the plasma. Although this ratio has been shown to be increased in several types of malignancies in humans as well as experimental animals, it is not known whether such an abnormality is found in breast cancer patients. Furthermore, the reasons for such an increase in cancer patients are unknown. We studied the plasma lipid composition and the activity of lecithin-cholesterol acyltransferase (LCAT), the enzyme responsible for the formation of most of EC in human plasma, in 12 women with breast cancer and 9 age-matched control women. The plasma EC concentration was found to be significantly decreased in cancer patients, whereas the FC concentration was unchanged, leading to increased FC/EC ratios (P < 0.05). The concentration of phosphatidylcholine, the acyl donor in the LCAT reaction, was decreased significantly, whereas all other phospholipids were unaffected. The cholesterol-esterifying activity of LCAT was significantly lower in cancer patients, whether assayed with endogenous substrates (P < 0.05), or with an exogenous substrate (P < 0.01). However, another function of the enzyme, namely the lysolecithin acyltransferase activity, was increased (P < 0.02), indicating that the enzyme concentration in plasma may not be decreased. These results show that the increase in the FC/EC ratio in cancer patients is due to an impaired esterification of cholesterol by plasma LCAT, probably due to an alteration in the composition of substrate lipoproteins, or the presence of an inhibitory factor.

Disparate effects of oxidation on plasma acyltransferase activities: inhibition of cholesterol esterification but stimulation of transesterification of oxidized phospholipids

Oxidation of lipoproteins results in the formation of several polar phospholipids with pro-inflammatory and pro-atherogenic properties. To examine the possible role of lecithin/cholesterol acyltransferase (LCAT) in the metabolism of these oxidized phospholipids, we oxidized whole plasma with either Cu(2+) or a free-radical generator, and determined the various activities of LCAT. Oxidation caused a reduction in plasma phosphatidylcholine (PC), an increase in a short-chain polar PC (SCP-PC), and an inhibition of the transfer of long-chain acyl groups to cholesterol (LCAT activity) or to lyso PC (lysolecithin acyltransferase (LAT) I activity). However, the transfer of short-chain acyl groups from SCP-PC to lyso PCLAT II activity) was stimulated several fold, in direct correlation with the degree of oxidation. LAT II activity was not stimulated by oxidation in LCAT-deficient plasma, showing that it is carried out by LCAT. Oxidized normal plasma exhibited low LCAT activity even in the presence of exogenous proteoliposome substrate, indicating that the depletion of substrate PC was not responsible for the loss of activity. Oxidation of isolated LDL or HDL abolished their ability to support LCAT and LAT I activities of exogenous enzyme, but promoted the LAT II activity. Purified LCAT lost its LCAT and LAT I functions, but not its LAT II function, when oxidized in vitro. These results show that while oxidation of plasma causes a loss of LCAT's ability to transfer long-chain acyl groups, its ability to transfer short-chain acyl groups, from SCP-PC is retained, and even stimulated, suggesting that LCAT may have a physiological role in the metabolism of oxidized PC in plasma.

Marine lipids normalize cholesteryl ester transfer in IDDM

Patients with insulin-dependent diabetes mellitus (IDDM) have a pathological increase in cholesteryl ester transfer (CET) that enriches the apolipoprotein B-containing lipoproteins with cholesteryl ester and increases their atherogenicity. Since we have shown earlier that omega-3 (n-3) fatty acids present in marine lipids normalize both CET and lipoprotein composition in non-diabetic patients with hypercholesterolaemia, we sought to determine whether the same beneficial effects could be achieved in nine normolipidaemic (triglycerides 1.10; cholesterol 4.94, high density lipoprotein 1.10 mmol/l) IDDM patients (fructosamine 424 +/- 156; normal 174-286 mumol/l) treated for 2 months with n-3 fatty acids (4.6 g/day). Before treatment, CET measured by both mass and isotopic assays was abnormally accelerated (p < 0.001). While marine lipids modestly decreased triglyceride levels (-14%; p < 0.05 ), CET fell dramatically in all subjects (mass assay: -97% at 1 h; isotopic assay: -58%; p < 0.001) to below control levels with no change in glycaemic control (fructosamine 408 +/- 103 mumol/l). The mass of cholesteryl ester transfer protein paradoxically increased significantly (pre-treatment: 2.04 +/- 0.86 vs post-treatment 2.48 +/- 0.97 micrograms/ml; p < 0.05). Since it is believed that accelerated CET promotes the formation of atherogenic cholesteryl ester-enriched apo B-containing lipoproteins, the capacity of marine lipids to reverse this functional abnormality without altering glycaemic control suggests that these agents may have an adjunctive role to play in the nutritional therapy of IDDM.

Comparative studies on the substrate specificity of lecithin:cholesterol acyltransferase towards the molecular species of phosphatidylcholine in the plasma of 14 vertebrates

Comparative studies indicate that plasma cholesteryl ester (CE) composition is correlated with susceptibility to atherosclerosis. We previously showed that the lecithin:cholesterol acyltransferases (LCATs) of susceptible species such as rabbit, pig, and chicken (group I) differ in their substrate and positional specificities from the LCATs of resistant species such as rat and mouse (group II). However, the relative importance of enzyme specificity and substrate phosphatidylcholine (PC) composition in determining the CE composition is not known. To address this, we analyzed the molecular species composition of plasma PC in the same 14 vertebrates in which we previously studied the CE composition and LCAT specificity. The utilization of native PC species by LCAT was studied by determining the loss of each PC after incubation of plasma at 37 degrees C. The major contributor for LCAT reaction was either 16:0-18:2 PC or 18:0-18:2 PC in all species except dog, in which it was 18:0-20:4 PC. The formation of 20:4 CE correlated more with the consumption of 18:0-20:4 PC in group I, and with the consumption of 16:0-20:4 PC in group II. The group II enzymes exhibited higher selectivity for sn-2-20:4 PCs, whereas the group I enzymes showed preference for sn-2-18:2 PCs. The synthesis of high percentage of 20:4 CE in dog plasma was found to be due to the presence of unusually high concentration of 18:0-20:4 PC, rather than due to enzyme selectivity. These results show that the PC molecular species composition, especially the concentrations of sn-2-20:4 phosphatidylcholines has profound influence on plasma CE composition, and possibly on atherogenic risk.

Specificity of lecithin:cholesterol acyltransferase and atherogenic risk: comparative studies on the plasma composition and in vitro synthesis of cholesteryl esters in 14 vertebrate species

To determine whether the specificity of lecithin: cholesterol acyltransferase (LCAT) influences the susceptibility to atherosclerosis, we compared the composition and in vitro synthesis of cholesteryl ester (CE) in the plasmas of 14 vertebrate species with varying predisposition to atherosclerosis. The susceptible species (Group I) had significantly higher ratios of 16:0 CE/20:4 CE in their plasma than the resistant species (Group II). The in vitro formation of labeled CE species in native plasma from labeled cholesterol correlated highly with the mass composition, showing that the LCAT reaction is the predominant source of plasma CE in all the animal species examined. Isolated LCATs from Group I species also synthesized CE with higher ratios of 16:0/20:4 than LCATs from Group II when egg phosphatidylcholine (PC) was used as the acyl donor. In addition, the Group I LCATs exhibited lower specificity towards sn-2-20:4 and sn-2-22:6 PCs, and higher specificity towards sn-2-18:2 PC species than Group II LCATs. With 16:0-20:4 PC as the substrate, all Group I LCATs synthesized more 16:0 CE than 20:4 CE, whereas all Group II LCATs, with the exception of dog enzyme, synthesized predominantly 20:4 CE, showing that the two types of LCAT have different positional specificities towards this PC. These results suggest that there are two classes of LCAT in nature that differ from each other in their substrate and positional specificities, possibly because of differences in their active-site architectures. We propose that the presence of one type of LCAT, which cannot efficiently transfer certain long chain polyunsaturated acyl groups and which consequently synthesizes more saturated CE, may increase the risk of atherosclerosis.

Differential effects of eicosapentaenoic acid and docosahexaenoic acid on human skin fibroblasts

To better understand the mode of action of omega 3 fatty acids in cell membranes, human foreskin fibroblasts were grown in serum-free medium supplemented with 50 microM oleic acid linoleic acid, eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA), and the effects on membrane composition, fluorescence polarization and enzyme activities were followed. The cells were enriched with EPA and DHA up to 7 and 13% of total lipids, respectively, of which > 95% was associated with phospholipids. In addition, the concentration of 22:5n-3 increased with both EPA and DHA to 7.5, and 2.1% of the total fatty acids, respectively. When compared to controls (oleic acid), cells treated with DHA showed a decrease in cholesterol, phospholipids, arachidonic acid (AA) and free cholesterol/phospholipid ratio (P < 0.05). In the presence of EPA, only decreases in AA and cholesterol were significant (P < 0.05). Membrane fluidity, assessed by fluorescence anisotropy, was increased 16% in cells enriched with DHA (P < 0.05), but showed no change with EPA or linoleic acid. There was an increase in membrane-associated 5'-nucleotidase (+27%) and adenylate cyclase (+19%) activities (P < 0.05), in DHA-enriched, but not in EPA-enriched cells, when compared with oleate controls. The studies show that incorporation of DHA, but not EPA, into cell membranes of fibroblasts alters membrane biophysical characteristics and function. We suggest that these two major n-3 fatty acids of fish oils have differential effects on cell membranes, and this may be related to the known differences in their physiological effects.

Diversity in the ability of cultured cells to elongate and desaturate essential (n-6 and n-3) fatty acids

Essential fatty acids (EFAs) cannot be synthesized by mammalian cells. Once taken in with the diet, they can undergo desaturations/saturations and chain elongations/shortenings to yield a variety of polyunsaturated fatty acids of the same family. Cells in vitro from a variety of tissues are capable of processing EFAs to varying extents. Conversion of the parent EFAs, linoleic (LA, n-6) and alpha-linolenic (LNA, n-3) acids, to the 20-carbon polyunsaturated fatty acids, arachidonic (AA, n-6) and eicosapentanoic (EPA, n-3), requires chain elongation and delta 6 and delta 5 desaturations. AA and EPA are required by many tissues for optimal biological function and are precursors of biologically active eicosanoid hormones. All cultured cells are able to elongate exogenous LA and LNA, and most can perform delta 5 desaturation, so delta 6 desaturation is the limiting step in AA and EPA production. Longer fatty acids that have more double bonds than AA or EPA are less frequently produced due to a deficiency in delta 4 desaturating ability. The process of retroconversion (chain shortening) is less extensively studied, but evidence from a variety of cells suggests that this type of metabolic conversion is normally active. The example of MCF-7 (human breast cancer cell line) and MCF-10A cells (human noncancerous breast cell line) is discussed in order to emphasize the diversity in EFA processing ability of cultured cells. Under identical culture conditions, MCF-10A cells perform extensive desaturations, elongations, and retroconversions, whereas MCF-7 cells can only elongate and retroconvert exogenous EFAs. Given the great diversity in the ability of cultured cells to process EFAs, no conclusions can be drawn regarding the mechanisms responsible for the effects of exogenous EFAs on a particular cell until that cell's EFA processing patterns have been evaluated.

Substrate and positional specificities of human and mouse lecithin-cholesterol acyltransferases. Studies with wild type recombinant and chimeric enzymes expressed in vitro

Human lecithin-cholesterol acyltransferase (LCAT) preferentially attacks sn-1 position of 16:0-20:4 phosphatidylcholine (PC), producing more 16:0 cholesteryl ester (CE) than 20:4 CE. In contrast, rat and mouse LCATs produce mostly 20:4 CE from the same PC. To understand the structural basis for this difference in positional specificity, we studied the specificities of recombinant mouse and human LCATs and several chimeric constructs of the two. The rLCATs retained the substrate and positional specificities of the plasma enzymes when expressed in COS-1 cells. Human and mouse LCAT cDNAs were each cleaved into three fragments, recombined in various combinations, and the chimeric products were analyzed for their specificities. When the N-terminal, or (and) C-terminal segments of human LCAT were replaced by the corresponding mouse LCAT segments, the chimeric products exhibited the specificity of intact human enzyme. However, when the middle segment, containing the residues 130-306 was replaced by the corresponding mouse LCAT segment, the enzyme exhibited the specificity of mouse LCAT. Similarly, the mouse rLCAT exhibited the specificity of human enzyme when its central segment, but not its N-terminal or C-terminal segment was replaced by the corresponding segment from human LCAT. These results show that the substrate and positional specificities of LCAT are controlled by the central domain of LCAT protein, corresponding to the amino acid residues 130-306.

Role of sn-2 acyl group of phosphatidylcholine in determining the positional specificity of lecithin-cholesterol acyltransferase

Although human plasma lecithin-cholesterol acyltransferase (LCAT) is believed to be specific for the sn-2 position of phosphatidylcholine (PC), our recent studies showed that it derives a significant percent of acyl groups from the sn-1 position of certain PC species. To understand the physicochemical basis for this altered positional specificity, we determined the effect of sn-2 acyl group of PC on the enzyme activity and utilization of 16:0 from the sn-1 position by purified human and rat LCATs. Positional isomers of PC containing 16:0 at sn-2 were better substrates for human LCAT than the corresponding sn-1-16:0 isomers, whereas the reverse was true for rat LCAT. The positional specificity of human LCAT varied greatly depending on the nature of the acyl group at sn-2. The sn-1 contribution from various sn-1-16:0-2-acyl PCs for cholesteryl ester (CE) synthesis was 1.0% from 16:0-16:0, 1.4% from 16:0-20:5, 7.3% from 16:0-18:1, 47.0% from 16:0-20:3, 49.9% from 16:0-20:4, 54.9% from 16:0-22:6, and 72.3% from 16:0-18:0. There was a linear relationship between the percentage of 16:0 CE formed (from sn-1 position) and the acyl chain length at sn-2 position (r = 0.94). Rat LCAT also transferred some 16:0 from sn-1 position of 16:0-22:6, 16:0-20:3, and 16:0-18:0 PCs, but not from the other natural PCs tested. The phospholipase A activity of both LCATs in the presence of 16:0-20:4 PC showed the same positional specificity as CE synthesis, indicating that the specificity is determined at the formation of acyl-enzyme intermediate. These results show that the positional specificity of LCAT is influenced by the structure of PC, especially the chain length of the sn-2 acyl group.

Abnormal acyltransferase activities and accelerated cholesteryl ester transfer in patients with nephrotic syndrome

To determine the effects of the nephrotic syndrome (NS) on atherogenic risk, we studied the lipoprotein composition and the activities of lecithin-cholesterol acyltransferase (LCAT), lysolecithin acyltransferase (LAT), and cholesteryl ester transfer (CET) in the plasma of 11 NS patients and 10 control subjects. NS plasma had lower ratios of high-density lipoprotein (HDL) to low-density lipoprotein (LDL) and HDL2/HDL3 and an elevated free cholesterol (FC) to phosphatidyl choline (PC) ratio (1.09 +/- 0.27 in NS and 0.72 +/- 0.21 in controls, P < .02), all of which indicate an increased atherogenic potential. LCAT activity was normal in NS plasma when assayed with an exogenous substrate, but was 40% lower than in control plasma when assayed with the endogenous substrates. However, in vitro addition of serum albumin to NS plasma failed to normalize the LCAT activity. The LAT reaction, which is catalyzed by LCAT protein in the presence of LDL, was 60% to 80% higher in NS plasma, and consequently the ratio of LAT/LCAT activities was increased twofold. CET activity was significantly increased (+160% of control), and this abnormality was attributable to changes in both the acceptor (very-low-density lipoprotein [VLDL] + LDL) and donor (HDL) lipoproteins and possibly in CET protein. These results suggest that the NS may increase the risk of atherosclerosis not only by adversely affecting the concentrations of lipoproteins, but also by altering their composition and function.

n-3 and n-6 fatty acid processing and growth effects in neoplastic and non-cancerous human mammary epithelial cell lines

The type rather than the amount of dietary fat may be more important in breast carcinogenesis. While animal studies support this view, little is known about the effects of essential fatty acids (EFAs) at the cellular level. The MCF-7 breast cancer and the MCF-10A non-cancerous human mammary epithelial cell lines are compared in terms of growth response to EFAs and ability to incorporate and process the EFAs. Eicosapentaenoic (EPA, n-3) and docosahexaenoic (DHA, n-3) acids, presented bound to albumin, inhibited the growth of MCF-7 cells by as much as 50% in a dose-dependent manner (6-30 microM) in medium containing 0.5% serum. alpha-Linolenic (LNA, n-3) and arachidonic (AA, n-6) acids inhibited growth less extensively, while linoleic acid (LA, n-6) had no effect. In contrast, MCF-10A cells were not inhibited by any of the EFAs at levels below 24 microM. The differential effects of AA, EPA and DHA on MCF-7 and MCF-10A cells support a protective role of highly unsaturated essential fatty acids against breast cancer. The EFAs were primarily incorporated into phosphoglycerides. MCF-7 cells showed chain elongations and possibly delta 8 desaturation, but no AA was formed from LA, nor EPA or DHA from LNA. In contrast, MCF-10A cells desaturated and elongated the exogenous EFAs via all the known pathways. These findings suggest defects in the desaturating enzymes of MCF-7 cells. LNA, DHA and AA presented to MCF-7 cells in phospholipid liposomes inhibited growth as extensively as albumin-bound free acids, but were less extensively incorporated, suggesting different mechanisms of inhibition for the two methods.

Hydrolysis and transesterification of platelet-activating factor by lecithin-cholesterol acyltransferase

Purified lecithin-cholesterol acyltransferase (LCAT, EC 2.3.1.43) from human plasma was found to hydrolyze platelet-activating factor (PAF) to lyso-PAF and acetate. In addition, it catalyzed the transfer of the acetate group from PAF to lysophosphatidylcholine, forming lyso-PAF and a 1-acyl analog of PAF. In contrast to the cholesterol-esterification reaction carried out by the enzyme, the hydrolysis and transacetylation of PAF by LCAT did not require an apoprotein activator and were not inhibited by sulfhydryl inhibitors but were inhibited by serum albumin. When added to a proteoliposome substrate of LCAT or to whole plasma, PAF inhibited cholesterol esterification by LCAT competitively. PAF acetylhydrolase (EC 3.1.1.47), purified from human plasma, also catalyzed the transfer of acetate from PAF to lysophosphatidylcholine. However, the LCAT-catalyzed reactions of PAF were not due to contamination with PAF acetylhydrolase, since the ratio of acetyl transfer to acetyl hydrolysis was 3 times greater for LCAT, when compared with PAF acetylhydrolase under identical conditions. Furthermore, recombinant human LCAT secreted by baby hamster kidney cells also catalyzed the hydrolysis and transacetylation of PAF. These results demonstrate that LCAT can inactivate PAF in plasma by transacetylation and suggest that it may have a role in the metabolism of PAF, and possibly of oxidized phospholipids, in plasma.

Diverse effects of essential (n-6 and n-3) fatty acids on cultured cells

Fatty acids (FAs) have long been recognized for their nutritional value in the absence of glucose, and as necessary components of cell membranes. However, FAs have other effects on cells that may be less familiar. Polyunsaturated FAs of dietary origin (n-6 and n-3) cannot be synthesized by mammals, and are termed 'essential' because they are required for the optimal biologic function of specialized cells and tissues. However, they do not appear to be necessary for normal growth and metabolism of a variety of cells in culture. The essential fatty acids (EFAs) have received increased attention in recent years due to their presumed involvement in cardiovascular disorders and in cancers of the breast, pancreas, colon and prostate. Many in vitro systems have emerged which either examine the role of EFAs in human disease directly, or utilize EFAs to mimic the in vivo cellular environment. The effects of EFAs on cells are both direct and indirect. As components of membrane phospholipids, and due to their varying structural and physical properties, EFAs can alter membrane fluidity, at least in the local environment, and affect any process that is mediated via the membrane. EFAs containing 20 carbons and at least three double bonds can be enzymatically converted to eicosanoid hormones, which play important roles in a variety of physiological and pathological processes. Alternatively, EFAs released into cells from phospholipids can act as second messengers that activate protein kinase C. Furthermore, susceptibility to oxidative damage increases with the degree of unsaturation, a complication that merits consideration because lipid peroxidation can lead to a variety of substances with toxic and mutagenic properties. The effects of EFAs on cultured cells are illustrated using the responses of normal and tumor human mammary epithelial cells. A thorough evaluation of EFA effects on commercially important cells could be used to advantage in the biotechnology industry by identifying EFA supplements that lead to improved cell growth and/or productivity.

Accelerated cholesteryl ester transfer in noninsulin-dependent diabetes mellitus

Alterations in core lipid composition of lipoproteins in noninsulin-dependent diabetes mellitus (NIDDM) patients have suggested that the heteroexchange of neutral lipids between HDL and the apo B-containing lipoproteins may be enhanced. For this reason, we studied cholesteryl ester transfer (CET) in ten sulfonylurea-treated patients with stable NIDDM. CET measured in all NIDDM subjects with an assay of mass transfer was significantly greater than that of controls at 1 and 2 h (P < 0.001); the transfer of radiolabeled CE also was increased in a subset of four of the NIDDM group (NIDDM k = 0.21 +/- 0.04 vs. control k = 0.10 +/- 0.05; P < 0.05). A weak correlation was demonstrable between the mass of CE transferred at 1 h and diabetic control expressed as plasma fructosamine (r = 0.58, P < 0.09). To characterize this disturbance in CET further, the donor (HDL + VHDL) and acceptor (VLDL + LDL) lipoprotein fractions were isolated by ultracentrifugation at d 1.063 g/ml from NIDDM and control plasma and a series of recombination experiments were performed. Combining NIDDM acceptor with control donor fractions that contained HDL and CETP and not the combination of NIDDM donor and control acceptor lipoproteins resulted in an accelerated CET response identical to that observed in NIDDM whole plasma. This observation indicated that the abnormality in CET in NIDDM was associated with the VLDL + LDL fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of sphingomyelin in the regulation of cholesterol esterification in the plasma lipoproteins. Inhibition of lecithin-cholesterol acyltransferase reaction

In order to determine whether sphingomyelin (SPH) affects the rate of cholesterol esterification by plasma lecithin-cholesterol acyltransferase (LCAT), we studied the effects of its incorporation in to defined proteoliposome substrates containing phosphatidyl choline (PC), unesterified cholesterol, and apoprotein A-I, on the activity of purified LCAT. Cholesterol esterification was inhibited by up to 90% in the presence of SPH, and this inhibition was reversed by treatment with bacterial sphingomyelinase. The inhibition could be overcome by increasing the concentration of PC, but not unesterified cholesterol or apoprotein A-I, in the substrate. The effect of SPH was not related to the alterations in the size of the substrate particle and was not dependent on the type of acyl donor or apoprotein activator employed. The lysolecithin acyltransferase and phospholipase reactions carried out by LCAT were also inhibited by SPH. Kinetic studies suggested that: 1) LCAT binds better to substrate vesicles which contain SPH; 2) SPH competes with PC in binding to the active site of the enzyme; and 3) SPH is a more powerful competitive inhibitor than a diether analog of PC. The ability of various lipoproteins to act as substrates for purified LCAT varied inversely with the SPH/PC ratio. Treatment of the lipoproteins with sphingomyelinase activated the LCAT reaction, the percent activation being directly proportional to the SPH concentration in the native lipoprotein. Enrichment of high density lipoproteins with SPH inhibited cholesterol esterification in them by 50%, and this inhibition could be reversed by the degradation of SPH. These results show that SPH is a physiological inhibitor of cholesterol esterification in the plasma, by virtue of its competition with PC, the acyl donor for the reaction.

Incorporation of dietary n-3 fatty acids into molecular species of phosphatidyl choline and cholesteryl ester in normal human plasma

To understand the differences in the antiatherogenic actions of eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 22:6), we determined their incorporation into molecular species of phosphatidylcholine (PC) and cholesteryl ester (CE) after feeding 12 g marine lipid concentrate/d to six normolipidemic males for 28 d. The time course of incorporation of EPA into plasma PC and CE showed a precursor-product relationship. In contrast, the DHA concentration of CE was markedly lower than that in PC, and the EPA-DHA ratio was 2-6-fold higher in CE than in PC at all time intervals. Three PC species--16:0-20:5, 16:0-22:6, and 18:0-20:5--increased, whereas 18:1-18:2, 18:0-18:2, and 16:0-20:3 decreased. In vitro formation of CE species in plasma by lecithin-cholesterol acyltransferase (LCAT) showed an increased formation of 20:5 CE but not 22:6 CE, indicating that DHA is a poor substrate for LCAT. These results demonstrate a differential incorporation of EPA and DHA into plasma lipids, which may be related to the differences in their biological effects.

Activation of plasma lysolecithin acyltransferase reaction by apolipoproteins A-I, C-I and E

The lysolecithin acyltransferase (LAT) activity of lecithin-cholesterol acyltransferase (LCAT) converts lysophosphatidylcholine (lyso PC) to PC, and requires low-density lipoproteins (LDL). To determine whether LDL can be replaced by defined substrates, we tested proteoliposomes containing egg PC, labeled lyso PC and apoprotein (apo) A-I at molar ratios of 250:12.5:0.8, as substrate for purified enzyme. A significant percent of lyso PC in this substrate was acylated to PC, indicating that apo A-I can substitute for apo B in LAT reaction, and that PC is the acyl donor in the reaction. Apo C-I and apo E were, respectively, 70% and 40% as effective as apo A-I. When both lyso PC and free cholesterol (FC) were incorporated into the same proteoliposome, they competed with each other for the acyl groups, with 72% of the total acylation being directed to FC and 28% to lyso PC, at equimolar concentrations. With the native lipoproteins, the esterification of lyso PC was dominant in LDL, whereas FC esterification was dominant in high-density lipoproteins (HDL). Albumin inhibited LAT and activated LCAT in both lipoproteins, but its effects were more pronounced in HDL. These results indicate that the esterification of lyso PC and FC involve the same mechanism, and that the relative predominance of LAT on LDL is due to higher affinity of lyso PC to LDL, compared to HDL.

Effect of marine lipids on cholesteryl ester transfer and lipoprotein composition in patients with hypercholesterolemia

While the effects of omega-3 (n-3) fatty acids present in marine lipids on plasma lipoprotein levels have been intensively studied, less is known about their impact on reverse cholesterol transport. For this reason, for a 3-month period we studied the effects of the administration of n-3 fatty acids (6 g/day) as a dietary supplement on cholesteryl ester transfer (CET), a key step in this process, and lipoprotein composition in 12 outpatients with genetically heterogeneous forms of hypercholesterolemia. Before treatment, CET in hypercholesterolemic patients, estimated as the mass of cholesteryl ester (CE) transferred from high density lipoprotein (HDL) to very low density lipoprotein (VLDL) plus low density lipoprotein (LDL), was markedly accelerated, peaking after only 1-2 hours of incubation of whole plasma; this response differed significantly (p < 0.001) from the initial delayed curvilinear response of control subjects. Consistent with the accelerated CET occurring in vivo, their triglyceride to esterified cholesterol core lipid ratio before treatment was reduced in the intact VLDL fraction and VLDL1 but not in VLDL2 or VLDL3 and was reciprocally increased in HDL. In addition, the free (unesterified) cholesterol to lecithin ratio of VLDL1 was abnormally increased. Recombination experiments performed with individual lipoprotein fractions revealed that accelerated CET was specifically associated with the VLDL1 subfraction and not LDL, HDL, and cholesteryl ester transfer protein (CETP), although pretreatment levels of CETP were significantly increased (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Altered positional specificity of human plasma lecithin-cholesterol acyltransferase in the presence of sn-2 arachidonoyl phosphatidyl cholines. Mechanism of formation of saturated cholesteryl esters

The positional specificity of purified human lecithin-cholesterol acyltransferase (LCAT) was studied by analyzing the labeled cholesteryl ester (CE) species formed in the presence of proteoliposome substrates containing mixed chain phosphatidylcholine (PC) species, labeled cholesterol and apoprotein A-I. Whereas over 90% of the acyl groups used for CE synthesis were derived from the sn-2 position of most of the naturally occurring PC substrates, about 75% of the CE species formed in the presence of sn-1-myristoyl 2-arachidonoyl PC, sn-1-palmitoyl-2-arachidonoyl (PAPC) and sn-1-palmitoyl 2-docosahexaenoyl PC were derived from the sn-1-position. On the other hand, rat LCAT utilized mostly sn-2-acyl group from either PAPC or from sn-1-palmitoyl 2-linoleoyl PC. The positional specificity of the human enzyme was not affected by the alteration in the matrix fluidity, type of the apoprotein activator used, or by the free cholesterol/PC ratio in the substrate. These results show that the positional specificity of human plasma LCAT is altered in the presence of sn-2-arachidonoyl PC, or sn-2-docosahexaenoyl PC, probably due to steric restrictions at the active site, and this may account for the formation of disproportionately high concentrations of saturated CE, and low concentrations of long-chain polyunsaturated CE in human plasma, relative to the composition of sn-2-acyl groups in plasma PC.

Effect of apoprotein B conformation on the activation of lysolecithin acyltransferase and lecithin: cholesterol acyltransferase. Studies with subfractions of low density lipoproteins

In order to determine the role of apoprotein (apo) B conformation in the activation of the lysolecithin acyl-transferase reaction, we studied the activation of purified enzyme by various subfractions of low density lipoprotein (LDL), isolated by density gradient centrifugation. The activation of LAT correlated positively with the density of LDL and negatively with cholesterol/protein and triglyceride (TG)/protein ratios. The enzyme activation was also positively correlated with the number of trinitrobenzenesulfonic acid-reactive lysine amino groups, which increased with increasing density of LDL. The immunoaffinity of the LDL subfractions for B1B6, a monoclonal antibody directed to the receptor-binding region of apoB, increased with increasing density, while the affinity toward C1.4, a monoclonal antibody directed to the amino-terminal region of apoB, was not altered. Enrichment of normal whole LDL with TG resulted in a 45% reduction in enzyme activation, a 27% decrease in the number of trinitrobenzenesulfonic acid-reactive lysine groups, and a marked reduction in the immunoaffinity for B1B6. All these parameters reversed to normal when the TG-enriched LDL was treated with milk lipoprotein lipase, which specifically reduced the TG content of LDL. The LDL subfractions also supported cholesterol esterification by the purified enzyme, in parallel with lysolecithin esterification, indicating that apoB can also serve as an activator of the lecithin-cholesterol acyltransferase reaction. These results strongly suggest that the localized conformational change of apoB which occurs during the TG depletion of the precursor particle is critical for its activation of acyltransferase reactions, in a manner analogous to its interaction with the cellular receptors.

Effect of apolipoprotein activators on the specificity of lecithin:cholesterol acyltransferase: determination of cholesteryl esters formed in A-I/C-III deficiency

Although it is known that plasma lecithin:cholesterol acyltransferase (LCAT) is activated by several apolipoproteins (apo) including A-I, C-I, D, A-IV, and E, it is not clear what the physiological importance of having different apolipoprotein activators is. One possible explanation is that the activation by different apolipoproteins may result in the utilization of different species of phosphatidylcholine (PC), leading to the formation of different species of cholesteryl esters (CE). In order to determine this possibility, we analyzed the molecular species composition of PC and CE in two patients with familial deficiency of apoA-I and apoC-III. The LCAT activity, assayed by three different procedures, was found to be 36-63% of the control value. The lower LCAT activity, however, was due to deficiency of the enzyme rather than the absence of apoA-I. The patients' plasma was relatively enriched with sn-2 18:2 PC species reflecting the partial deficiency of LCAT activity. The fatty acid composition of plasma CE was not significantly different from that of controls. HPLC analysis of labeled CE formed after incubation of plasma with [C14]cholesterol showed no significant difference in the species of CE synthesized by the LCAT reaction. The transfer of pre-existing as well as newly formed CE from HDL to the apoB-containing lipoproteins was accelerated compared to control plasma. These results show that the absence of apoA-I does not significantly affect either the activity or the specificity of LCAT, and that the other apolipoprotein activators can substitute adequately for it.

Marine oil capsule therapy for the treatment of hyperlipidemia

Because marine oil capsules may vary widely in their content of omega-3 fatty acids, saturated fat, and cholesterol composition and, therefore, their biologic potency, we compared the lipid-lowering effects of three representative preparations in patients with different forms of hyperlipidemia. The ester and triglyceride forms of marine oil both effectively lowered triglyceride, but the response of low-density lipoprotein cholesterol was variable; it declined modestly in patients with hypercholesterolemia and was either unchanged or increased in those with hypertriglyceridemia. The saturated fat and cholesterol content of the marine oil preparation appeared to influence the low-density lipoprotein cholesterol response. Therefore, marine oil capsules are useful for lowering levels of very-low-density lipoprotein cholesterol, but the large dose required to achieve and sustain this effect (4.5 g of omega-3 fatty acids, or nine to 18 capsules daily) may limit long-term compliance.

Lipoprotein composition and HDL particle size distribution in women with non-insulin-dependent diabetes mellitus and the effects of probucol treatment

To further characterize the spectrum of potentially atherogenic disturbances in lipoprotein composition in non-insulin-dependent diabetes mellitus (NIDDM), we have studied a subset of women with NIDDM before and after treatment with the lipophilic lipid-lowering drug probucol (1 gm day), which we have shown corrects certain compositional abnormalities these women share with subjects who have hypercholesterolemia. Before treatment, the NIDDM group had a somewhat higher plasma triglyceride level (154 +/- 58.3 mg/dl, vs control, 80.0 +/- 21 mg/dl [mean +/- SD]; p less than 0.025) than controls but their cholesterol and high-density lipoprotein cholesterol (HDL-C) levels did not differ from control levels. A number of significant disturbances, however, were present in the surface and core lipid composition of their lipoproteins. Although the cholesterol content of NIDDM low-density lipoprotein (LDL) was similar to that of controls, its content of sphingomyelin and phosphatidylinositol plus phosphatidylserine and sphingomyelin-to-lecithin ratio all were significantly reduced. Moreover, their very-low-density lipoprotein (VLDL) and HDL2 tended to have reduced amounts of free (unesterified) cholesterol (FC) relative to lecithin, and their HDL2 and HDL3 tended to be triglyceride enriched. Probucol therapy resulted in significant decreases in total plasma cholesterol (-15%), FC (-28%), HDL-C (-22%), and triglyceride (-16%) and in apoproteins A-I, B, and E (apo A-I, B, and E), without changing diabetic control (before probucol: hemoglobin A1, cholesterol, 10.7% +/- 2.7%; after probucol: 10.9% +/- 3.0%).(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated cholesteryl ester transfer in patients with insulin-dependent diabetes mellitus

Abnormalities in cholesteryl ester transfer (CET) may play a role in the development of diabetic arterial vascular complications. To assess this important step systematically in reverse cholesterol transport, we have studied 20 treated, clinically stable, normolipidaemic patients. Contrary to the impairment in CET described previously in NIDDM, the mass of CE transferred from HDL to VLDL + LDL was significantly greater in IDDM patients than in controls at 1,2, and 4 h (P less than 0.001). When the d less than 1.063 plasma fractions from IDDM subjects were combined with controls d less than 1.063 fractions, an accelerated CET response was observed which was identical to that found in intact IDDM plasma. This finding, which indicates that this disturbance in CET was associated with the acceptor lipoproteins, was confirmed when we found that it was reproduced by the addition of IDDM VLDL and not LDL to control d greater than 1.063 fractions. Changes observed in lipoprotein core lipid composition were consistent with accelerated CET occurring in IDDM in vivo: the TG/CE core lipid ratio was decreased in VLDL from six subjects (diabetic 9.5 +/- 0.8 vs control 12.9 +/- 3.4; P less than 0.1) and increased in their HDL (diabetic 0.55 +/- 0.11 vs control 0.42 +/- 0.04; P less than 0.025). No correlation was demonstrable between estimates of diabetic control (glycoalbumin, fasting glucose) and CET. These data indicate that CET may be abnormally increased in normolipidaemic IDDM patients. A defect of this type may be atherogenic because it increases the number of lipoprotein particles in plasma which resemble cholesteryl ester-enriched chylomicron and VLDL remnants but whose normal receptor-mediated catabolism may be altered.

Accelerated cholesteryl ester transfer in plasma of patients with hypercholesterolemia

To discern the mechanism(s) that underlie abnormal cholesteryl ester transfer (CET) in patients with hypercholesterolemia, we have studied this dysfunctional step in reverse cholesterol transport in 13 subjects with genetically heterogeneous forms of hypercholesterolemia (HC). In all HC patients, the mass of CE transferred in whole plasma from HDL to VLDL and LDL increased rapidly initially and was significantly greater than in controls at 1, 2, and 4 h (P less than 0.005). To further characterize this disturbance, we performed a series of recombination experiments. Combining HC d less than 1.063 containing acceptor VLDL + LDL with the d greater than 1.063 fraction from controls containing donor HDL + CE-transfer protein (CETP) and not the converse combination showed the same characteristics of accelerated CET noted with intact HC plasma, indicating that abnormal transfer was associated with the HC acceptor lipoproteins. When HC VLDL and its subfractions and LDL were isolated separately and then combined with control d greater than 1.063 fractions, accelerated CET was only associated with VLDL1. Consistent with an acceleration of the neutral lipid transfer reaction occurring between HDL and VLDL1 in HC in vivo, we found that the triglyceride/CE ratio was decreased in HC VLDL1 (P less than 0.001), and increased in HDL (P less than 0.25). CETP mass was significantly increased in HC plasma (HC 2.3 +/- 4 micrograms/ml vs. control 1.3 +/- 0.3 micrograms/ml; mean +/- SD; P less than 0.025). This series of observations demonstrate that CET is accelerated in the plasma of HC patients, and this disturbance results from dysfunction of the VLDL1 subfraction rather than an elevation of CETP levels. Since an abnormality of this type in vivo can lead to the accumulation of potentially atherogenic CE-enriched apoB-containing lipoproteins in plasma, it may be an additional previously unrecognized factor that increases cardiovascular risk in HC patients.

Effects of estropipate treatment on plasma lipids and lipoprotein lipid composition in postmenopausal women

It is generally believed that the cardioprotective benefit of long term treatment of postmenopausal women with estrogen results in part from its capacity to increase high density lipoprotein (HDL) and lower low density lipoprotein (LDL) concentrations. The extent to which the various estrogens employed in replacement treatment affect the composition of lipoproteins, however, is not known. For this reason, we have examined the impact of one such preparation, the synthetic estrone estropipate (1.25 mg/day), on lipoprotein levels and composition in six postmenopausal women. After 6 months of treatment, whole plasma triglyceride (pretreatment, 135 +/- 63; posttreatment, 143 +/- 56 mg/dL), cholesterol (pretreatment, 232 +/- 14; posttreatment, 216 +/- 29 mg/dL), and HDL-C (pretreatment, 57.8 +/- 14.8; posttreatment, 55.6 +/- 13.2) were unchanged. However, plasma free (unesterified) cholesterol (FC) fell (pretreatment, 73.4 +/- 6.2; posttreatment, 53.7 +/- 9.3 mg/dL; P less than 0.05) and lecithin (L) rose significantly (pretreatment, 2.12 +/- 0.29; posttreatment, 2.47 +/- 0.34 mumol/mL; P less than 0.01). The consequence of these changes was a significant decline in the plasma FC/L ratio (pretreatment, 0.91 +/- 0.17; posttreatment, 0.68 +/- 0.12; P less than 0.01) to levels observed in healthy menstruating women. The calculated lipoprotein particle size was unchanged in very low density lipoproteins and increased significantly (P less than 0.05) in LDL after estropipate therapy. Since qualitatively altered lipoproteins enriched in FC and an increased FC/L ratio in plasma are both associated with increased coronary risk, the improvement noted in these parameters after estropipate therapy indicates that its use may be beneficial despite the lack of change in whole plasma lipids.

A high-performance liquid chromatographic procedure for the determination of disaturated phosphatidylcholine in human plasma

Plasma disaturated phosphatidylcholine (DSPC) concentration has been implicated as a risk factor for atherosclerosis. However, suitable methods for the estimation of these compounds in plasma are not available. In this paper, a method for the estimation of DSPC using argentation thin-layer chromatography and high-performance liquid chromatography is described. It is quantitative for the measurement of individual and total DSPC species and is not dependent on fatty acid chain length. The method employs hydrolysis of total plasma phosphatidyl choline by phospholipase C, followed by benzoylation of the diacylglycerols. The benzoates are then fractionated on silver nitrate-impregnated silica gel thin-layer chromatography plates, and the disaturated species separated and quantitated by high-performance liquid chromatography. The method is sensitive and reproducible and allows many samples to be done at once. With this method, the amounts of DSPC were found to be significantly higher in a group of normolipidemic diabetic subjects, compared to age-matched controls.

Probucol treatment in hypercholesterolemic patients: effects on lipoprotein composition, HDL particle size, and cholesteryl ester transfer

Despite probucol's capacity to induce regression of tendinous xanthomata and reduce whole plasma and LDL cholesterol (LDL-C) in patients with hypercholesterolemia, its therapeutic use in the United States has been limited because of concern about its HDL-lowering effects. To assess the possibility that probucol might facilitate mobilization of tissue cholesterol in the presence of low HDL levels as a consequence of favorable changes in lipoprotein composition and function, we have analyzed lipoproteins and studied cholesteryl ester transfer (CET) in hypercholesterolemic patients before and after treatment. Prior to treatment, the free cholesterol (FC)/lecithin (L) ratio in plasma, a new index of cardiovascular risk, and the mass of cholesteryl ester transferred from HDL to the apo B-containing lipoproteins (CET) both were significantly increased (P less than 0.001). As previously shown, plasma cholesterol, LDL-C, HDL-C, HDL2, and apolipoproteins A-I, A-II, and B all fell significantly following probucol treatment. The FC/L ratio in plasma (P less than 0.01) and HDL2 (P less than 0.01) both fell significantly also, as did the sphingomyelin/lecithin ratio in VLDL + LDL (P less than 0.001) which is typically increased in untreated patients with hypercholesterolemia. Nondenaturing gradient gel electrophoresis in 6 patients revealed that the quantitative changes in HDL were associated with a redistribution of particles characterized by a decrease in the prevalence of the largest (HDL2b) and a relative increase in the number of the smallest (HDL3b) particles. Moreover, CET following probucol therapy returned to levels which were indistinguishable from those of normolipidemic controls. These results indicate that untreated patients with hypercholesterolemia have abnormalities in (1) lipoprotein composition which have been shown to retard the movement of cholesterol from tissues to HDL, and in (2) CET which is accelerated and can potentially lead to the formation in plasma of atherogenic CE-enriched apo B-containing lipoproteins. Probucol's capacity to reverse these specific alterations suggests that it may have beneficial effects on cholesterol transport in patients with hypercholesterolemia.

Persistent abnormalities in lipoprotein composition and cholesteryl ester transfer following lovastatin treatment

Optimally effective lipid-lowering agents should not only restore plasma lipids to normal levels but also correct potentially atherogenic alterations in lipoprotein composition and function often present in hyperlipidemic patients. Lovastatin, a competitive inhibitor of cholesterol biosynthesis, clearly lowers plasma cholesterol levels. Its effects on lipoprotein composition and cholesteryl ester transfer (CET), a key step in reverse cholesterol transport, however, are not known. Since abnormalities in CET and lipoprotein composition are present in patients with hypercholesterolemia, we studied these parameters of plasma lipoprotein transport in twelve hypercholesterolemic (HC; Type IIa) subjects (six male, six female) before and 2 months after lovastatin treatment (20 mg qd). Before lovastatin, the free cholesterol (FC)/lecithin (L) ratio in plasma, a new index of cardiovascular risk that reflects lipoprotein surface composition, was abnormally increased (1.18 +/- 0.26 vs controls 0.83 +/- 0.14; P less than 0.001) in very low density lipoproteins (VLDL) and high density lipoprotein-3 (HDL3), and remained so after treatment despite significant declines in whole plasma cholesterol (311.7 +/- 68.2 vs 215.6 +/- 27.2 mg/dl; P less than 0.001), low density lipoprotein (LDL)-cholesterol (206.3 +/- 47.9 vs 146.8 +/- 29.4; P less than 0.001), and apolipoprotein B (149 +/- 30 vs 110 +/- 17; P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular species of cholesteryl esters formed in abetalipoproteinemia: effect of apoprotein B-containing lipoproteins

In order to study the effects of very low density (VLDL) and low density (LDL) lipoproteins on the activity and specificity of lecithin:cholesterol acyltransferase (LCAT), we determined the molecular species of cholesteryl esters (CE) synthesized in the plasma from three abetalipoproteinemic (ABL) patients, before and after supplementation with normal VLDL or LDL. The patients' plasma had significantly lower concentration of 18:2 CE and higher concentrations of 16:0 CE and 18:1 CE compared to normal plasma. Incubation of ABL plasma with [4-14C]cholesterol at 37 degrees C and the subsequent analysis of labeled CE formed by high performance liquid chromatography revealed that the major species formed was 16:0 CE (34% of total label), whereas similar incubation of the d greater than 1.063 g/ml fraction of normal plasma resulted in the formation of predominantly 18:2 CE (45% of total label). Addition of normal VLDL or LDL to ABL plasma stimulated the total LCAT activity by 30-80% and normalized the CE species synthesized. The LCAT activity of a normal d greater than 1.063 g/ml fraction also was stimulated by the normal VLDL or LDL, but there was no alteration in the species of CE formed. Most of the CE synthesized was found in the added VLDL or LDL with both ABL and normal plasma, indicating that the CE transfer (CET) activity was not affected in ABL plasma. These results suggest that while the VLDL and LDL are required for the maximal activity of LCAT, the species of CE formed are primarily determined by the molecular species composition of phosphatidylcholine in the plasma.

Effects of tamoxifen treatment on plasma lipids and lipoprotein lipid composition

Concern has been raised that long term treatment with the antiestrogen tamoxifen might predispose women to the rapid development of cardiovascular disease. Since estrogen-induced changes in plasma lipids confer protection to females from coronary heart disease, we have examined the impact of tamoxifen on lipoprotein levels and composition on eight posmenopausal women. After 3 months of tamoxifen treatment (10 mg, twice daily), no significant changes were observed in either whole plasma triglyceride (pre-Rx, 137 +/- 59; post-Rx, 157 +/- 110 mg/dL) or cholesterol (pre-Rx, 193 +/- 23; post-Rx, 204 +/- 14 mg/dL); plasma free (unesterified) cholesterol (FC), however, fell significantly (pre-Rx, 66.5 +/- 6.5; post-Rx, 59.6 +/- 4.6 mg/dL; P less than 0.05). Since plasma lecithin (L) was unchanged, the FC/L ratio declined significantly to levels observed in healthy menstruating women (pre-Rx, 95 +/- 0.16; post-Rx, 0.74 +/- 0.12 molar ratio; P less than 0.025). In low density lipoprotein (LDL), the concentrations of cholesterol and FC and the FC/L ratio all fell significantly (P less than 0.025, P less than 0.05, and P less than 0.025, respectively). Despite a tendency for high density lipoprotein2 cholesterol (HDL2-C) to increase (pre-Rx, 9.7 +/- 3.6; post-Rx, and 14.4 +/- 13.3 mg/dL; P less than 0.4) and phosphoinositol to fall, there were few clear-cut alterations in either HDL2 or HDL3 surface or core lipid composition. The combination of reduced HDL3 lysolecithin (P less than 0.025) associated with a posttreatment trend toward increased triglyceride/cholesterol esters ratios in both HDL subfractions are findings consistent with tamoxifen-induced inhibition of hepatic lipase. These changes in lipoprotein composition together with the fall in LDL cholesterol and increase in sex hormone-binding globulin (P less than 0.005) indicate that tamoxifen acts as an estrogen agonist on the liver. Since elevated LDL cholesterol levels and qualitatively altered lipoproteins enriched in FC are both associated with increased coronary risk, the improvement noted in these parameters after tamoxifen should allay to some degree anxiety about its use with regard to cardiovascular risk.

Effects of omega-3 fish oils on plasma lipids, lipoprotein composition, and postheparin lipoprotein lipase in women with IDDM

Because the apparent reduction in cardiovascular risk noted in nondiabetic populations that ingest diets rich in marine lipids containing omega-3 fatty acids is believed to result in part from their capacity to modify the composition and physicochemical behavior of lipoproteins, we sought to determine whether dietary supplementation with marine lipids might favorably affect lipoprotein composition in insulin-dependent diabetes mellitus (IDDM). Eight normolipidemic IDDM women (mean +/- SD age 29.8 +/- 4.7 yr) were studied before and 3 mo after receiving a marine-lipid concentrate (Super-EPA) containing 6 g omega-3 fatty acids and a total of 12 mg of cholesterol daily. Weight, insulin requirements, and glycosylated hemoglobin remained stable. After treatment, mean +/- SD plasma triglyceride (TG) levels fell (before, 81.7 +/- 22 mg/dl; after, 69.19 +/- 17; P less than 0.025). High-density lipoprotein2 (HDL2) cholesterol (before, 10.98 +/- 5.45 mg/dl; after, 18.43 +/- 7.93; P less than 0.01), its major apolipoprotein A-I (apoAI), and the major phospholipids (sphingomyelin and lecithin) all rose significantly. ApoB and plasma and low-density lipoprotein cholesterol levels and HDL3 composition were unchanged. Postheparin hepatic and lipoprotein lipase activities were unaffected by marine lipids. These data indicate that women with IDDM experience apparently beneficial effects on TG and HDL2 from dietary supplementation with omega-3 fatty acids administered in a low-cholesterol-containing oil without adversely affecting overall diabetes management. If these changes in lipoprotein concentration and composition prove to have antiatherogenic consequences and are free of long-term toxicity, these agents may have a role in the therapy of IDDM patients.

Molecular species of phosphatidylcholine in abetalipoproteinemia: effect of lecithin:cholesterol acyltransferase and lysolecithin acyltransferase

In order to study the role of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) in determining the molecular species composition of phosphatidylcholine (PC) and the specificity of lecithin:cholesterol acyltransferase (LCAT) in human plasma, we studied the PC species composition in plasma from abetalipoproteinemic (ABL) and control subjects before and after incubation at 37 degrees C. The ABL plasma contained significantly higher percentages of sn-2-18:1 species (16:0-18:1, 18:0-18:1, and 18:1-18:1) and lower percentages of sn-2-18:2 species (16:0-18:2, 18:0-18:2, and 18:1-18:2) as well as sn-2-20:4 species (16:0-20:4, 18:0-20:4, and 18:1-20:4). Similar abnormalities were found in the PC of ABL erythrocytes, while the PE of the erythrocytes was less affected. The relative contribution of various PC species towards LCAT reaction in ABL plasma was significantly different from that found in normal plasma. Thus, while 16:0-18:2 and 16:0-18:1 contributed, respectively, 43.8% and 15.9% of the total acyl groups used for cholesterol esterification in normal plasma, they contributed, respectively, 21.5% and 37.9% in ABL plasma. The relative contribution of 16:0-20:4 was also significantly lower in ABL plasma (4.7% vs. 9.0% in normal), while that of 16:0-16:0 was higher (6.4% vs. 0.5%). However, the selectivity factors of various species (percent contribution/percent concentration) were not significantly different between ABL and normal plasma, indicating that the substrate specificity of LCAT is not altered in the absence of VLDL and LDL. Incubation of ABL plasma in the presence of normal VLDL or LDL resulted in normalization of its molecular species composition and in the stimulation of its LCAT activity. Addition of LDL, but not VLDL, also resulted in the activation of lysolecithin acyltransferase (LAT) activity. The incorporation of [1-14C]palmitoyl lysoPC into various PC species in the presence of LDL was similar to that observed in normal plasma, with the 16:0-16:0 species having the highest specific activity. These results indicate that the absence of apoB-containing lipoproteins significantly affects the molecular species composition of plasma PC as well as its metabolism by LCAT and LAT reactions.

Effects of dietary supplementation with marine lipid concentrate on the plasma lipoprotein composition of hypercholesterolemic patients

Although the triglyceride-lowering actions of n-3 fatty acids of marine lipids are now well-recognized, their effects on plasma lipoproteins have not been studied systematically in patients with hypercholesterolemia. To address this question, we supplemented the Phase 1 American Heart Association diets of 14 hypercholesterolemic ambulatory outpatients with a commercially available preparation of marine lipid concentrate (SuperEPA) containing 7.5 g n-3 fatty acids per day and studied their plasma lipids and lipoproteins before and after 30 days of treatment. Both plasma triglyceride and cholesterol levels fell uniformly in all patients while the mean VLDL- and LDL-cholesterol decreased by 58% (P less than 0.005) and 13% (P less than 0.025) respectively. The decrease in whole plasma cholesterol was significantly correlated with the fall in LDL-cholesterol (r = 0.85, P less than 0.01), and not VLDL-cholesterol (r = 0.39, NS). Among the other potentially beneficial actions observed was an increase in HDL2 in all patients (mean increment 41%, P less than 0.005), and an increase in the HDL2/HDL3 ratio (+46%, P less than 0.001) and decreases in the LDL/HDL ratio (-14%, P less than 0.005) and in the unesterified cholesterol/lecithin ratio (-17%; P less than 0.001) in plasma. The increase in the unesterified cholesterol/esterified cholesterol ratio in VLDL and HDL3 suggested that marine lipid therapy resulted in a reduction in the size of lipoprotein particles in these fractions. Since these changes may reduce cardiovascular risk, these findings suggest that marine lipids may prove useful in the treatment of certain patients with hypercholesterolemia.

Whole-plasma and high-density lipoprotein subfraction surface lipid composition in IDDM men

To determine whether compositional abnormalities are present in high-density lipoprotein (HDL) in patients with insulin-dependent diabetes mellitus (IDDM) that might negate its putatively protective cardiovascular effects, we studied the plasma lipoproteins of 12 men with varying degrees of clinical control (mean fasting glucose 193 +/- 10 mg/dl, mean glycoalbumin greater than 73% above control mean). The diabetic patients' basal plasma triglyceride, total- and free- (unesterified) cholesterol, HDL cholesterol (HDL-chol), and apolipoprotein AI, AII, and B concentrations were similar to those of control subjects, but the free-cholesterol-to-lecithin ratio, a new index of cardiovascular disease risk, was significantly increased in their plasma (0.97 +/- 0.14 vs. 0.88 +/- 0.07, P less than .02) and their very-low-density lipoprotein (VLDL)-low-density lipoprotein (LDL) subfraction (1.50 +/- 0.51 vs. 1.08 +/- 0.15, P less than .005). Although HDL2-chol was similar in diabetic and control groups, the HDL2-chol-to-free-cholesterol ratio (diabetic vs. control, 4.64 +/- 1.7 vs. 1.96 +/- 1.0 mumol/ml, P less than .025) and the sphingomyelin-to-lecithin ratio (0.23 +/- 0.08 vs. 0.20 +/- 0.09, P less than .025) were both significantly increased in the IDDM group. HDL3-chol was higher in the IDDM than in the control subjects (diabetic vs. control, 38.6 +/- 5.2 vs. 32.7 +/- 2.7 mg/dl, P less than .005). In contrast to whole plasma and the VLDL + LDL subfraction, the free-cholesterol-to-lecithin ratio of IDDM and control HDL subfractions were similar.(ABSTRACT TRUNCATED AT 250 WORDS)