Very low density and low density lipoprotein subfractions in type III and type IV hyperlipoproteinemia. Chemical and physical properties

Per-Particle Triglyceride-Rich Lipoproteins Imply Higher Myocardial Infarction Risk Than Low-Density Lipoproteins: Copenhagen General Population Study

[Figure: see text].

Fractionation of cholesteryl ester rich intermediate density lipoprotein subpopulations by chondroitin sulphate

IDL is considered an atherogenic lipoprotein but little progress has been made on methods to subfractionate this density class. Furthermore, previous work suggests that lipoproteins retained by the arterial wall, of which chondoitin sulphate is the major arterial wall proteoglycan, are potentially atherogenic. The aim of this study was to assess the subfractionation of IDL particles using chondroitin sulphate (CS). Forty healthy subjects were recruited from laboratory staff and/or their partners. Fasted plasma samples were obtained and IDL (1.006 g/ml<d<1.030 g/ml) was isolated. Approximately 1mg protein of IDL was allowed to interact with CS. The unbound and bound IDL particles were eluted using a low salt and high salt buffer, respectively. On average 70% of IDL bound to CS ranging from 56 to 92%. Total, unbound and bound IDL particles were analysed for lipid composition and particle size. The unbound IDL particles were larger (32 nm) and triglyceride rich (40% versus 33%, P<0.01), whereas the bound IDL particles were smaller (26-28 nm) and cholesterol rich (21% versus 14%, P<0.01). The unbound particles contain at least double the amount of apo C-II and apo C-III per IDL particle compared with the bound IDL particles. There are specific IDL particles that bind to CS in vitro, these being the cholesterol rich IDL particles. It remains to be determined if these cholesterol rich IDL particles are potentially more atherogenic than the triglyceride rich IDL particles.

Insights from apoB: from better diagnosis & therapy to the Medusa Hypothesis

This article will review six of the most important insights that have come from the measurement of apolipoprotein B (apoB). Amongst these are critical clinical advances, which include better definition of those at high risk of vascular disease and better definition of the adequacy of statin therapy. There are also major advances in our understanding of the fundamental processes that interact to cause vascular disease. These include a more precise understanding of the determinants of lipoprotein levels and lipoprotein composition in plasma and, even more importantly, an appreciation of the pivotal role excess fatty acid intake and impaired fatty acid trapping by adipose tissue play in the generation of the complex risk profile that is so common in patients with coronary disease. We have designated the constellation of hypertriglyceridemic (hyperTg) hyperapoB, dysglycemia, and elevated levels of markers of prothrombosis and inflammation--all of which are traceable to adipose tissue dysfunction--as the Medusa Hypothesis. The Medusa Hypothesis puts forward a common pathophysiology for the complex and biologically diverse array of factors that have been associated with vascular disease.

The physiology of lipoproteins

The seminal studies of Brown and Goldstein (Science 1986;232:34-47) coupled with the findings of the Framingham study revolutionized our understanding of the metabolic basis for vascular disease. These studies led to the widespread use of the coronary risk lipid profile, which uses the total cholesterol/high-density lipoprotein (HDL) ratio (or low-density lipoprotein [LDL]/HDL ratio) in predicting risk for vascular disease and as a tool for therapeutic management of patients at risk for vascular disease. However, although these methods are predictive of coronary artery disease (CAD) in general, it is also well known that the extent of occlusive disease and CAD varies greatly between individuals with similar cholesterol and HDL lipid profiles. For this reason, the National Cholesterol Education Program Expert Panel revised these guidelines and now recommends monitoring LDL and HDL cholesterol in the context of coronary heart disease risk factors and "risk equivalents." In addition, more recent findings indicate that specific alterations in individual lipoprotein subclasses may account for the variations in CAD in subjects with similar lipid profiles. For example, a preponderance of small, dense LDL particles correlates with a marked increase in risk for myocardial infarction independent of LDL levels. In particular, the association of small, dense LDL with elevated triglycerides (large, less dense VLDL) and reduced HDL has been defined as the atherogenic lipoprotein profile, and the key metabolic defect driving this profile may be elevated levels of triglycerides, specifically large, less dense VLDL. In an attempt to explain the physiologic basis for lipoprotein variations, this review describes the basic metabolic scheme underlying the traditional view of lipoprotein metabolism and physiology. It then examines the identity and role of the various lipoprotein subfractions in an attempt to distill a working model of how lipoprotein abnormalities might account for vascular disease in general and the metabolic syndrome in particular.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Two subpopulations of intermediate density lipoprotein and their relationship to plasma triglyceride and cholesterol levels

We observed the appearance of two intermediate density lipoprotein (IDL) subfractions on gradient gel electrophoresis of lipoproteins in the density range 1.006-1.030 g/ml and estimated their approximate concentrations in plasma in subjects with a wide range of lipid levels, from 0.55 to 28.0 mmol/l plasma triglyceride and 3.75-10.0 mmol/l cholesterol. The larger species, IDL-I (31.7 +/- 0.7 nm, mean +/- SD), showed little variation in size in normal and moderate hyperlipidaemic individuals. The estimated concentration of IDL-I was positively related to plasma triglyceride (r = 0.63, P = 0.0004) and low density lipoprotein (LDL) cholesterol (r = 0.68, P = 0.0003). These findings are consistent with the view that IDL-I is a metabolic intermediate between very low density lipoprotein (VLDL) and LDL. The smaller subfraction, IDL-II (25.7 +/- 2.4 nm) was virtually the only true species observed in subjects with plasma triglyceride < 1.0 mmol/l and its estimated concentration fell as plasma triglyceride increased (r = -0.58, P = 0.0002). IDL-II particle size changed in concert with LDL particle size (r = 0.61, P < 0.0001), suggesting that they were influenced by common metabolic factors. These observations provide further support for the hypothesis outlined by Musliner et al. [1] that IDL-I was part of the delipidation chain from VLDL to LDL, whereas IDL-II arose from a separate source, possibly directly released from the liver. Hence the two subpopulations of IDL differ in their relationship to plasma triglyceride and cholesterol levels.

Apolipoprotein E polymorphism and the distribution profile of very low density lipoproteins; an influence of the E4 allele on large (Sf > 60) particles

Very low density lipoprotein (VLDL) distribution and composition have been examined as a function of apo E genotype (E2/2 + E2/3 vs. E3/3 vs. E3/4 + E4/4) in healthy, normolipaemic subjects. Apo E genotype had a marked impact on plasma concentrations of apo E rich VLDL, but no influence on concentrations of apo E free particles. Thus, there was a trend to lower concentrations of apo E rich total VLDL in apo E4 carriers (mg/dl; E2, 49.1 +/- 35.2; E3, 52.5 +/- 30.9; E4 35.2 +/- 22.3; ANOVA P = 0.16; when comparing E4 with E2 + E3, P = 0.06). Consequently, there were highly significant differences between apo E-defined subgroups in terms of the percentage distribution of bound and non-bound fractions (% total VLDL non-bound to apo E: E2, 44.0 +/- 12.7%; E3, 39.7 +/- 8.7%; E4 51.0 +/- 12.2%; ANOVA P = 0.007). Subfractionation of VLDL into density subclasses revealed that genotype differences were restricted to large VLDL (Sf > 60). Significantly lower concentrations of apo E-rich particles were observed in E4 carriers for VLDL-1 Sf 400-100 (ANOVA P = 0.004) and VLDL-2 (P = 0.009) but not for small VLDL-3 Sf 60-20 (P = 0.34). No differences in plasma concentrations of apo E free VLDL were observed between genotype subclasses across the density spectrum. Compositional differences between the apo E defined VLDL were also evident for the core lipids. Apo E containing VLDL was enriched in esterified cholesterol and depleted in triglycerides compared to apo E poor VLDL: the difference became more marked with increasing density of the particles. Lipoprotein composition was not modulated to any great extent by apo E genotype. In patients with familial hypercholesterolaemia, relative concentrations of apo E rich, large VLDL were significantly higher than in controls. Treatment lowered concentrations of both apo E rich and apo E free VLDL but led to a greater relative enrichment of large VLDL in apo E containing particles. Apo E polymorphism appears to influence plasma concentrations of VLDL particles. The data are consistent with more pronounced receptor-mediated elimination of apo E4 containing VLDL. This may be a contributory factor to the down regulation of receptor activity which is suggested to be of major importance in provoking higher cholesterol levels associated with the apo E4 isoform.

Lipoprotein heterogeneity and apolipoprotein B metabolism

The apolipoprotein B containing lipoproteins VLDL, IDL, and LDL exhibit variation in their structure, function, and metabolism. These major lipoprotein classes can be fractionated into apparently discrete components by density gradient centrifugation or affinity chromatography. Examination of the behavior of subfractions in vivo reveals the presence of metabolic channels within the VLDL-LDL delipidation cascade so that the pedigree of a lipoprotein in part determines its metabolic fate. Evidence from VLDL and LDL apoB turnovers together with epidemiological data allows the construction of a quantitative model for the generation of small, dense LDL. This lipoprotein subspecies is one component of the dyslipidemic syndrome known as the atherogenic lipoprotein phenotype, a common disorder in those at risk for coronary heart disease. Understanding lipoprotein heterogeneity is an essential step in the further discovery of the pathogenesis of atherosclerosis and in the tailoring of pharmacologic treatment for subjects at risk.

Increased low density lipoprotein oxidation in stable kidney transplant recipients

We studied factors that may add to the high risk of atherosclerosis in kidney transplant recipients. Plasma lipoprotein concentrations and parameters of low density lipoprotein (LDL) oxidation were determined in 19 clinically stable kidney recipients and 19 healthy controls. Plasma triglycerides and total cholesterol were increased in the patients. High density lipoprotein-cholesterol (HDL-c) was in the normal range. The mean LDL diameter was smaller in patients than in controls (236.5 +/- 7.3 A vs. 247.8 +/- 11.6 A, P < 0.002), which was due to a higher frequency of the LDL subclass pattern B in the patients than in controls (58% vs. 28%). The lag time of copper-induced in vitro LDL oxidation was shorter in patients than in controls (101 +/- 23 min vs. 148 +/- 81 min, P = 0.02). The titer and concentration of autoantibodies against malondialdehyde-modified (MDA-LDL) determined by ELISA were higher in the patients than in the controls. This difference was found in both IgG (titer + 9%, concentration + 75%; P < 0.05) and IgM (titer + 35%, concentration + 102%; P < 0.001). Based on these results, we propose that there is in vitro and in vivo evidence of enhanced LDL oxidation in patients post-renal transplantation. This might represent one cause for the clinical finding of advanced atherosclerosis in these patients.

Autoantibodies against malondialdehyde-modified LDL are elevated in subjects with an LDL subclass pattern B

Small low density lipoproteins (LDL) are more susceptible to in vitro oxidation than larger LDL. To study whether this leads to more oxidation of small LDL in vivo, we determined the level of autoantibodies against malondialdehyde-modified LDL (MDA-LDL) in subjects with small or large LDL (LDL subclass pattern B or A) by ELISA. The study group consisted of 92 subjects with coronary heart disease without severe hypercholesterolemia (mean total plasma cholesterol 5.9 +/- 0.8 mM), 46 with an LDL subclass pattern A and 46 with an LDL subclass pattern B. In the subjects with LDL subclass pattern B the titre of autoantibodies of the IgM class against MDA-LDL was 29% higher than in the subjects with LDL subclass pattern A (P < 0.0001). The concentration of the anti-MDA-LDL autoantibodies of the IgM class was 58% higher in the patients with the pattern B than in the patients with the pattern A (P < 0.0001). There was no statistically significant difference in the titre or concentration of autoantibodies of the IgG class between subjects with LDL subclass patterns A and B. Besides plasma triglyceride and HDL cholesterol, the titre and concentration of the IgM autoantibodies were found to be independent predictors of the LDL subclass pattern. These results show that small LDL are associated with higher autoantibody levels than large LDL. Based on the assumption that the level of autoantibodies against MDA-LDL represents the rate of LDL oxidation in vivo, we conclude that in vivo small LDL is more readily oxidised than larger LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic lipase and lipoprotein lipase are not major determinants of the low density lipoprotein subclass pattern in human subjects with coronary heart disease

The influence of hepatic lipase (HL) and lipoprotein lipase (LPL) activity on the low density lipoprotein (LDL) subclass pattern was studied in a population of males with coronary heart disease and without severe hypercholesterolemia. LDL subclass patterns, lipases and plasma lipoproteins were determined in 326 patients. In part of the study population, fasting insulin and glucose levels were also determined. The LDL subclass pattern was determined by gradient gel electrophoresis (GGE) and classified according to Austin et al. (J. Am. Med. Assoc. 260 (1988) 1917 (predominantly large LDL = A-pattern, predominantly small LDL = B-pattern). An LDL subclass A-pattern was exhibited by 199 subjects; 108 exhibited a B-pattern. In 19 subjects no distinctive A- or B-pattern was present (A/B-pattern). Hepatic and lipoprotein activities differed significantly between patients with the A- or B-pattern. The median hepatic lipase activity was lower (384 vs. 417 mU/ml, P = 0.006), and the lipoprotein lipase activity higher (122 vs. 101 mU/ml, P = 0.001) in the A-pattern subjects than in the B-pattern subjects. In subjects with the A/B pattern the lipase activities were intermediate between the values in the A- and B-pattern subjects (HL 408 +/- 87 mU/ml, LPL 115 +/- 55 mU/ml). Plasma triglyceride, very low density lipoprotein (VLDL)-triglyceride, intermediate density lipoprotein (IDL)-triglyceride and LDL-triglyceride were higher in the patients with a B-pattern (+84%, +171%, +10% and +16%, respectively). Total plasma cholesterol was not different between A- and B-pattern subjects. VLDL- and IDL-cholesterol were higher in the B-pattern group (+174% and +66%, respectively), while LDL- and HDL-cholesterol were higher in the A-pattern group (+2 and +24%, respectively). In univariate analysis HL, LPL, plasma (and VLDL) triglyceride, HDL-cholesterol and IDL-cholesterol were each significantly associated with the LDL subclass pattern. In multivariate analysis plasma triglyceride (or VLDL-triglyceride) and HDL-cholesterol appeared to be independently associated with the LDL subclass pattern. No additional discriminative value of HL or LPL was found. Similar results were obtained if the patients with or without beta blocker were evaluated separately. An estimate of insulin resistance (EIR), calculated from plasma insulin and glucose in part of the study population (n = 145), was significantly higher in the subjects with a B-pattern than in those with an A-pattern (3.12 vs. 2.00, P < 0.003). EIR correlated positively with plasma triglyceride (P < 0.0001), but not with HL or LPL.(ABSTRACT TRUNCATED AT 400 WORDS)

Re-evaluation of the structure of low density lipoproteins

Low density lipoprotein (LDL) is an established atherogenic factor. Much effort has therefore been devoted to elucidation of its structure, yielding the generally accepted model according to which the neutral lipids (cholesterol ester and triglycerides) form a lipid core emulsified by phospholipids, cholesterol and the amphipathic Apolipoprotein B. Yet, the detailed structure of LDL is not clear. The present work was carried out with the aim of re-evaluating the LDL structure using the minimal number of assumptions: in view of the previously noted surface deficit (lack of sufficient PL and cholesterol to cover the surface of the lipid core) we have assumed that polar head groups are not covered by apo B. Other than that, we have 'allowed' Apo B to penetrate into the PL monolayers and the lipidic core and to pertrude into the solution (be elevated above the PL head group level). We have also 'allowed' neutral lipid penetration into the monolayer and variation of the thickness of the phospholipid monolayers within reasonable boundaries. Based on the established values of relevant constants (molecular weights and volumes, densities and surface areas) we have computed the radius of the particle, the penetration of Apo B into lipidic milieus and the fraction of the surface area covered by Apo B as functions of the LDL composition, the monolayer thickness and the 'elevation' of Apo B above this monolayer. These computations show that at least 40% of the LDL surface must be covered by protein and that the protein penetrates, on the average, only about a half of the PL monolayer. Thus it is not very likely to penetrate into the lipid core. These general features are preserved in the smaller LDL particles of hypertriglyceridemic patients. Assuming that no PL head group is covered by Apo B, the previously described immobilization of 20% of the phospholipids is likely to result from the interaction of Apo B with neighboring PL. According to our computations this can be regarded consistent with the previously proposed arrangement of the apo B as a '3-4 domain structure' or a long string configuration but inconsistent with 'one domain' or 'twenty domain' structures.

Levels and physicochemical properties of lipoprotein subclasses in moderate hypertriglyceridemia

Moderate hypertriglyceridemia is associated with several abnormalities of the plasma lipoprotein particles and it may be a risk factor for atherosclerotic vascular diseases. Plasma lipid, lipoprotein and apolipoprotein levels, as well as lipoprotein composition and physical properties, were examined by ultracentrifugation in a zonal rotor and by gradient gel electrophoresis in 14 patients with moderate hypertriglyceridemia (plasma triglycerides 4.00 +/- 0.32 mmol/l, mean +/- S.D.) and in 14 control subjects. Based on zonal ultracentrifugation hypertriglyceridemic patients have higher levels of cholesterol in all VLDL subclasses (Sf > 200, 100-200, 60-100 and 20-60), in IDL and in small and dense LDL. Both HDL2 and HDL3 cholesterol levels are reduced. The LDL flotation rate is inversely related to plasma triglyceride levels, thus indicating that the higher the plasma triglycerides the smaller and/or denser the LDL are. The triglyceride percent content of LDL2 and HDL3 is increased, while that of esterified cholesterol is reduced in hypertriglyceridemic patients. Gradient gel electrophoresis shows that the LDL peak size is lower (25.2 +/- 0.5 nm, mean +/- S.D.) in hypertriglyceridemic than in control subjects (27.1 +/- 0.4 nm; P < 0.0001). Considering both hypertriglyceridemic and control subjects the LDL peak effluent volume from the zonal rotor (which reflects the LDL flotation rate) is inversely related to the LDL peak size determined by gradient gel electrophoresis (r = -0.71; P = 0.0006) and the plasma triglyceride levels are related to LDL peak effluent volume (r = 0.74; P = 0.0002) and to LDL peak size (r = -0.95; P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution profiles of very low density and low density lipoproteins in poorly-controlled male, type 2 (non-insulin-dependent) diabetic patients

The distribution and composition of lipoproteins spanning the very low density and low density lipoprotein spectra have been analysed in ten poorly-controlled, male, Type 2 (non-insulin-dependent), diabetic patients pre-disposed to mild, secondary hypertriglyceridaemia. As compared to age-matched control subjects, the diabetic patients displayed grossly modified, distinctly atherogenic lipoprotein profiles. Modifications were not limited to the very low density lipoprotein profile, as would be expected from the pre-treatment hypertriglyceridaemia. There was also an aberrant low density lipoprotein profile, which was not evident from plasma cholesterol measurements, especially as the diabetic patients at entry were well matched to control subjects with respect to plasma levels of this lipid. Compositional abnormalities were also observed in the poorly-controlled diabetic group, although these were less marked than the distributional changes. There were substantial improvements of the abnormalities detailed above, even over a short treatment period (two weeks), with therapy designed primarily to ameliorate metabolic control. The data suggest that, in the presence of poor metabolic control and hypertriglyceridaemia, occult, atherogenic modifications of low density lipoproteins can occur. The results argue in favour of strict control of triglyceride levels even in diabetic patients with apparently acceptable cholesterol levels.

Effect of combined therapy with bezafibrate and cholestyramine on low-density lipoprotein metabolism in type IIa hypercholesterolemia

This study was designed to examine the influence of combined therapy with bezafibrate and cholestyramine on plasma lipids and on the metabolism of low-density lipoprotein (LDL). Twenty-one type II hyperlipidemic subjects were treated with bezafibrate alone or in combination with cholestyramine. A 17% fall in plasma cholesterol was seen with bezafibrate, and addition of cholestyramine produced an additional 9% reduction in this lipid. The effectiveness of the combination therapy was mediated through a 47% decrement in very-low-density lipoprotein (VLDL) cholesterol, a 37% reduction in LDL cholesterol, and a 15% increase in the level of that lipid in high-density lipoprotein (HDL). Plasma triglyceride fell 43% when bezafibrate was given alone, and did not change further when cholestyramine was added. The metabolism of LDL was examined in nine individuals to determine the mechanism underlying these changes. No significant modification in LDL synthetic rate was incurred with either drug regimen, whereas the fractional catabolic rate of LDL via the receptor pathway rose by 66% with bezafibrate alone and by 79% (compared to baseline) following the addition of cholestyramine. Plasma HDL rose during bezafibrate therapy due to an increase in the HDL3 subfraction. Compositional analysis of LDL showed a reduction in cholesterol ester and an increase in triglyceride and phospholipid during combined drug therapy. These results demonstrate that combined therapy with bezafibrate and cholestyramine markedly improves the lipoprotein profile in type II hyperlipidemia. The drugs appear to be complementary in their actions upon the LDL receptor pathway.

The influence of metabolic control on very low density lipoprotein composition in hypertriglyceridemic type II diabetics. A study using heparin-sepharose chromatography

The composition and heparin-binding capacity of very low density lipoproteins (VLDL) from type II diabetics prone to develop secondary hypertriglyceridemia have been examined. Twelve diabetic patients whose triglyceride levels normalized during short-term treatment for hyperglycemia were studied. Normalization of triglyceride values reduced plasma levels of VLDL without modifying its relative lipid composition. There was, however, an increase in its relative apoprotein (apo) content, to which apo B made a greater percentage contribution. The calculated average particle diameter of posttreatment VLDL was reduced. The fraction of VLDL binding to heparin increased after treatment. Binding was strongly correlated to the apo B content and, to a lesser extent, to the apo E content. The data suggest that treatment of hyperglycemia favorably modified VLDL such that they more closely resemble VLDL from normolipidemic subjects, with potentially beneficial physiological consequences.

Further resolution and comparison of the heterogeneity of plasma low-density lipoproteins in human hyperlipoproteinemias: type III hyperlipoproteinemia, hypertriglyceridemia and familial hypercholesterolemia

The heterogeneity of the plasma low-density lipoproteins (LDL) in subjects with type III hyperlipoproteinemia (3 cases), with hypertriglyceridemia (4 cases) and with the heterozygous form of familial hypercholesterolemia (FH, 4 cases) has been evaluated using a new, high resolution equilibrium density gradient ultracentrifugation procedure. The mass distribution profile, physicochemical properties, particle heterogeneity and apoprotein B content of a series of 13 LDL subfractions was examined in the 3 hyperlipidemic groups and the data were compared with those reported earlier in normolipidemic subjects. In FH, LDL mass was distributed as a narrow peak of d approximately 1.031-1.034 g/ml, whereas the distribution in hypertriglyceridemia was markedly asymmetric with a single peak of elevated density (d approximately 1.037-1.043 g/ml); the distribution in type III subjects was distinguished by its bi- or trimodal nature and broad profile. The chemical composition of LDL gradient subfractions in FH and in hypertriglyceridemia markedly resembled that of the respective parent LDL of d = 1.019-1.063 g/ml, displaying elevated proportions of cholesteryl ester in FH and of protein in hypertriglyceridemia. LDL subfractions in type III disease were enriched in free cholesterol. The Stokes diameters of LDL particles in corresponding subfractions from the 3 hyperlipidemic states were similar; however, whereas a single particle species was characteristic of each LDL subfraction in both FH and in our normolipidemic group, 2 species were frequently present in each subfraction in both type III and type IV diseases; in addition, subfractions from type III subjects occasionally exhibited 3 size species. Apolipoprotein B-100 was the predominant protein component in LDL subfractions from all 3 hyperlipidemic groups. Plasma LDL consist then of multiple particle species which constitute a particularly complex spectrum in type III hyperlipoproteinemia and in hypertriglyceridemia. The origin(s) of such particle subspecies is indeterminate at present; moreover, they may differ in their intravascular metabolism, in their degradation in tissues and in their relative atherogenicities.

Metabolism of apolipoprotein B in large triglyceride-rich very low density lipoproteins of normal and hypertriglyceridemic subjects

The metabolic fate of very low density lipoprotein can be examined by following the transit of its apolipoprotein B moiety through the delipidation cascade, which leads to low density lipoprotein. In this study we have used cumulative flotation ultracentrifugation to follow the metabolism of various lipoprotein subclasses that participate in this process in normal, hypertriglyceridemic (Type IV), and dysbetalipoproteinemic (Type III) subjects. Large triglyceride-rich very low density lipoproteins of Svedberg units of flotation (Sf) 100-400 were converted virtually quantitatively in normal subjects to smaller Sf 12-100 remnant particles. Only a minor fraction appeared thereafter in low density lipoproteins (Sf 0-12), most being removed directly from the plasma. Type IV hyperlipoproteinemic individuals converted the larger Sf 100-400 very low density lipoproteins to intermediate particles at approximately 50% of the control rate but thereafter their metabolism was normal (fractional clearance of Sf 12-100 particles in controls, 1.29 +/- 0.23 pools/d; in Type IV hypertriglyceridemics, 1.38 +/- 0.23 pools/d; n = 4 in each case). Since the apolipoprotein B in large triglyceride-rich particles did not contribute significantly to the mass of the low density lipoprotein apoprotein pool, the latter must come largely from another source. This was examined by following the metabolic fate of small very low density lipoproteins of Sf 20-60 or of the total lipoprotein spectrum of d less than 1.006 kg/liter (approximate Sf 20-400). The small particles were rapidly and substantially converted to low density lipoproteins, suggesting that the major precursor of the latter was to be found in this density range. Whereas only 10% of apolipoprotein B in Sf 100-400 lipoproteins reached the low density lipoprotein flotation range, greater than 40% of Sf 20-100 B protein eventually appeared in Sf 0-12 particles; and when very low density lipoprotein of d less than 1.006 kg/liter is used as a tracer of apolipoprotein B metabolism it is primarily this population of small very low density lipoprotein particles in the Sf 12-100 flotation range that is labeled. A detailed examination was made of apolipoprotein B metabolism in three dysbetalipoproteinemic subjects. The plasma clearance curves of their Sf 100-400 lipoproteins were distinctly biphasic. The quickly decaying component converted rapidly into remnants of Sf 20-60 at a near normal rate (0.56 vs. 0.62 pools/d in normal subjects). Its subsequent processing, however, was retarded. The more slowly catabolized fraction, comprising 30% of the total apolipoprotein B radioactivity, had no counterpart in normal or Type IV hyperlipoproteinemic individuals. These data, taken together, suggest that the very low density lipoprotein consists of a complex mixture of particles with different origins and fates. Within the Sf 20-100 flotation range there are at least two subcomponents. One represents remnants of larger triglyceride-rich particles which are catabolized slowly and feeds little apolipoprotein B into low density lipoprotein. The other is apparently secreted directly into this flotation interval and transfers significant amounts of B protein rapidly into Sf 0-12 lipoproteins.

Plasma triglyceride determines structure-composition in low and high density lipoproteins

Because the association of hypertriglyceridemia and premature atherosclerosis is not due to the direct effects of the triglyceride molecule itself, we studied the effects of increased plasma triglyceride-rich lipoproteins on the composition and structure of low density lipoprotein (LDL) and high density lipoprotein (HDL). We found profound changes in the core and surface domains of both lipoproteins with increasing triglyceridemia. Core cholesterol esters were progressively depleted and replaced by triglyceride molecules. Highly significant negative correlations were found between cholesterol ester/protein ratios (r = -0.64 for LDL and -0.58 for HDL (p less than 0.001); positive correlations were found for triglyceride/protein ratios (r = 0.62 for LDL and 0.58 for HDL) and for triglyceride/cholesterol ester ratios (r = 0.70 for LDL and 0.83 for HDL) when these variables were assayed as a function of plasma triglyceride concentrations. With severe hypertriglyceridemia, triglyceride/cholesterol ester ratios of more than 1.0 were consistently observed (normal, less than 0.02). This leads to an underestimation of LDL and HDL levels when cholesterol alone is measured. At the surface, LDL and HDL were depleted of phospholipid and free cholesterol, with a relative enrichment of protein. These changes can be explained on the basis of high levels of plasma triglyceride-rich lipoproteins serving as acceptors for cholesterol esters and other constituents from LDL and HDL. Concomitantly, triglycerides are transferred to LDL and HDL. These transfer processes are likely to be mediated by the activity of lipid transfer proteins present in human plasma.

Interactions of ionic and non-ionic surfactants with plasma low density lipoprotein

Surfactants might be useful in facilitating the replacement of the interior of low density lipoprotein (LDL2) (p = 1.02-1.063 g ml-1) with drug molecules. We have used photon correlation spectroscopy, supported by sedimentation velocity measurements to study the effects of surfactants on LDL2. Sodium dodecyl sulphate, cetrimide, and all non-ionic surfactants studied caused rapid increases of ca 50% in the Stokes' radius up to surfactant/LDL2 molar ratios of ca 1000:1. This was interpreted as due to partial unfolding of the LDL2 protein and intercalation of surfactant with the LDL2 surface layer. At higher concentrations, ionic surfactants and non-ionics with HLB values less than 14.6 decreased the Stokes' radius due to delipidation of LDL2. These interactions are similar to those between surfactants and biological membranes, thus LDL2 might be a useful model system to study surfactant structure-activity relationships.

Differential transfer of C and E apolipoproteins from very-low-density lipoprotein to lysophosphatidylcholine-phosphatidylcholine micelles

Rat plasma VLDL was incubated with lysoPC/PC micelles consisting of 45-90 mol% lysoPC at micelle/VLDL phospholipid ratios of 0.33-6. Following incubation, the VLDL and micellar particles were reisolated by ultracentrifugation and the lipid and apolipoprotein composition determined by high-temperature gas chromatography and polyacrylamide gel electrophoresis, respectively. Lysophosphatidylcholine was found to equilibrate between the very-low-density lipoproteins and micellar fraction without transfer of phosphatidylcholine or sphingomyelin between these fractions. This produced reisolated VLDL and micellar particles with nearly identical lysoPC/PC ratios. Only apolipoprotein E transferred to reisolated micellar fractions with less than 35 mol% lysoPC. The C apolipoproteins were also transferred to the micellar fraction when the reisolated micelles contained more than 35 mol% lysoPC. It is concluded that apolipoproteins E and C can bind to HDL-size micellar particles of appropriate composition. The differential transfer of apolipoproteins E and C indicates that fundamental differences exist between these apolipoproteins in their interaction with lipid interfaces.

Effects of bezafibrate on receptor-mediated and receptor-independent low density lipoprotein catabolism in type II hyperlipoproteinaemic subjects

This study examines the effects of bezafibrate (200 mg t.i.d.) on LDL metabolism in 7 type II hyperlipoproteinaemic subjects. Eight weeks of treatment lowered plasma cholesterol and triglyceride by 10% and 30%, respectively (P less than 0.02). These reductions were associated with a fall in circulating VLDL (31%, P less than 0.02) and LDL (11%, P less than 0.05), while HDL cholesterol stayed the same. LDL metabolism changed during therapy. The plasma fractional clearance rate (FCR) of autologous [125I]LDL normalized from a low value of 0.256 +/- 0.048 (mean +/- SD) to 0.298 +/- 0.040 pools/day (P less than 0.001). This was attributable to a 65% increase (P less than 0.01) in receptor-mediated LDL catabolism since the clearance of simultaneously injected 1,2-cyclohexanedione-modified [131I]LDL, which measures the receptor-independent pathway, was unaltered (FCR of [131I]cyclohexanedione/LDL in control phase = 0.194 +/- 0.030 pools/day; during drug treatment = 0.194 +/- 0.024 pools/day). We conclude that bezafibrate lowers plasma LDL in type II hyperlipoproteinaemia by promoting its degradation via high affinity receptors.

Increased lipoprotein-remnant formation in chronic renal failure

Since accelerated atherosclerosis may be induced by excess circulating remnants of triglyceride-rich lipoprotein catabolism, we looked for evidence of remnant particle accumulation in the lipoproteins of 11 patients on long-term dialysis. We found several abnormalities in lipoprotein protein and lipids: enrichment of intermediate-density lipoproteins (ILD) and low-density lipoproteins (LDL) with triglyceride; the presence of apoprotein B48 (a "marker" for intestinal lipoproteins) in very-low-density lipoproteins (VLDL); an increased concentration of apoprotein AIV (a protein related to chylomicron transport); the presence of AIV in VLDL, IDL, and LDL; and the presence in LDL of apoproteins C and E (proteins not normally found in LDL). These findings strongly suggest accumulation of remnants of triglyceride-rich lipoproteins in patients with chronic renal failure who are undergoing peritoneal dialysis or hemodialysis, and may explain in part the increased incidence of coronary deaths among these patients.

Lipid class and molecular species interrelationships among plasma lipoproteins of type III and type IV hyperlipemic subjects

As a further appraisal of lipoprotein interconversion and equilibration of lipid components a detailed examination was made of the chemical class and molecular species interrelationships among the major fasting plasma lipoprotein fractions within each of six male Type IIi and Type IV hyperlipemic subjects subsisting on free choice diets. The lipoprotein fractions were prepared by conventional ultracentrifugation and the lipid class and molecular species composition of the corresponding lipoprotein fractions were determined by gas chromatography of the intact glycerol esters and ceramides. In general, each lipoprotein fraction possessed a well defined lipid class composition, which was characterized by a dramatically decreasing triacylglycerol and increasing phospholipid and cholesteryl ester content, when progressing from the very low (VLDL) to the low (LDL) and high (HDL) density lipoproteins, as already established for normolipemic subjects. Likewise, the LDL1, and LDL2 of the hyperlipemic subjects contained about two times higher proportion of total phospholipid as sphingomyelin than VLDL and HDL. Furthermore, the sphingomyelins of the HDL fraction contained about 30% more of the higher and 30% of the lower molecular weight species than the sphingomyelins of the VLDL. Smaller differences were seen in the molecular species composition of the phosphatidylcholines, cholesteryl esters and triacylglycerols among the corresponding lipoproteins. In comparison to normolipemic subjects analyzed previously, the hyperlipemic subjects showed greater individual variability. Despite this variability the lipid class and molecular species composition in the hyperlipemic subjects was again incompatible with the hypothesis which postulates direct VLDL conversion into LDL nd HDL under the influence of lipoprotein lipase and lecithin: cholesterol acyltransferase. The main differences between normolipemic and hyperlipemic plasma were found to reside in the number of the VLDL and LDL, lipoprotein particles and not in their chemical composition or physical structure, or in the apparent mechanism of their metabolic interconversion.

Sedimentation equilibrium of human low density lipoprotein subfractions

The molecular weights of low density lipoprotein (LDL) subfractions were determined precisely by meniscus depletion sedimentation equilibrium. Equilibrium speeds ranged from 9743 to 5896 rpm. The average molecular weights of various LDL subfractions of Sf values 9.49, 7.94, 6.42, 5.17, and 3.71 determined by sedimentation equilibrium were 2.97 X 10(6) ; 3.13 X 10(6); 2.89 X 10(6); 2.45 X 10(6); and 2.61 X 10(6) daltons, respectively; and their respective densities were 1.0267, 1.0306, 1.0358, 1.0422, and 1.0492 g/ml. Minimal hydrated molecular weights for this fractions determined by flotation velocity at 37,020 rpm were 2.57 X 10(6); 2.37 X 10(6); 2.09 X 10(6); 1.94 X 10(6); and 1.81 X 10(6) daltons; whereas similar molecular weights determined at 52,640 rpm were 2.53 X 10(6); 2.27 X 10(6); 1.99 X 10(6); 1.86 X 10(6); and 1.74 X 10(6) daltons for the respective LDL subfractions. Higher molecular weights of fractions 2 and 5 compared to their adjacent fractions 1 and 4 by sedimentation equilibrium are of great interest. The calculated fractional ratio f/f O from sedimentation equilibrium and flotation velocity data ranges from 1.10 to 1.31, suggesting complexity and asymmetry of LDL subfraction molecules. There is also evidence that compressibility of LDL molecules may be different than that for the salt solution under high g-force. Assuming that redistributed LDL molecules at equilibrium under low g-force are spherical, it is possible that the shape of LDL molecules undergoing flotation velocity determinations may be distorted in high g-force conditions. Such distortion may be consistent with the high f/f O values obtained and may also be a basis for structural rearrangement and/or lipoprotein degradation with prolonged preparative ultracentrifugation at high g-force and pressure.

High-performance liquid chromatography of human serum lipoproteins: selective detection of triglycerides by enzymatic reaction

A simple and convenient method for detection and quantitation of triglycerides in each lipoprotein fraction (chylomicron, VLDL, LDL and HDL) has been developed by high-performance liquid chromatography followed by enzymatic reaction using a high-speed reaction type chromatograph. Triglycerides in serum lipoproteins eluted from the gel permeation column (TSK GEL) could be sensitively and selectively detected by the absorbance at 550 nm using a commercial reagent kit. The distribution of triglycerides in each lipoprotein fraction could be examined with a small amount of serum (10-50 microliters) in less than 50 min by this method. Moreover, free glycerol could be detected as a sharp peak at the elution volume of total permeation of the column. This technique was found to be suitable for the study of triglyceride-rich lipoproteins such as chylomicron and VLDL.

Effect of core composition and particle size of lipid emulsions on apolipoprotein transfer of plasma lipoproteins in vivo

Lipid emulsions corresponding in size and surface composition to chylomicron and low density lipoprotein but differing in lipid core composition were injected into rats a tail vein cannula and 10 min later blood was withdrawn via the abdominal aorta. Plasma lipoproteins were isolated by ultracentrifugation and the lipid and apolipoprotein composition was determined by gas-liquid chromatography and gel electrophoresis, respectively. It was found that apolipoprotein C peptides were transferred to Sf greater than 400 particles of large diameter regardless of whether their cores contained cholesteryl esters or triacylglycerols, but not to Sf greater than 400 particles of small diameter. Apolipoprotein A-I transferred to both large and small diameter cholesteryl ester core particles but only to larger diameter triacylglycerol core particles. In contrast, apolipoprotein E became associated with small and large diameter particles regardless of core composition. In addition, all Sf greater than 400 fractions contained significant amounts of apopeptides with molecular weights greater than 45 000, which did not correspond to the common apolipoproteins. This is the first study to prove systematically the effects of lipid composition and particle size on apolipoprotein transfer in vivo.

Concentration and distribution of human plasma apolipoprotein E

(1) A sensitive, specific radioimmunoassay for human apolipoprotein E is described. (2) The method gave a plasma apolipoprotein E concentration of 0.107 +/- 0.051 g/l n = 55) in normal subjects. This value increased significantly (to 0.179 +/- 0.088 g/l) in type II hyperlipoproteinaemia (n = 31, p less than 0.01). Type III subjects formed a distinct group whose mean plasma apolipoprotein E concentration was increased four fold over normal (p less than 0.001). The level in type IV subjects (0.136 +/- 0.056, n = 30) did not differ significantly from the normal value. (3) When plasma was subjected to gel filtration on columns of 6% agarose, apolipoprotein E was located in three regions. Peak I corresponded to very low density lipoproteins, Peak II to the area between very low density and low density lipoproteins and Peak III to the leading edge of the high density lipoprotein fraction. No E protein eluted from the column after high density lipoproteins. (4) In contrast, ultracentrifugation of plasma resulted in up to 40% of apolipoprotein E appearing in the lipoprotein-free (d greater than 1.21 kg/l) fraction. This protein appears to hae been stripped largely from high density lipoprotein during ultracentrifugation. (5) Compositional analysis of very low density, intermediate density and low density lipoproteins isolated by a cumulative flotation procedure showed that the apoE/apoB ratio was the same in the first two species but fell to a very low value in low density lipoproteins.

Plasma lipoproteins of leukemic guinea pigs (L2C) can regulate cholesterol biosynthesis by lymphocytes of normal guinea pigs. A comparative study of plasma lipoproteins of normal and neoplastic animals

The defect of regulation of cholesterol biosynthesis by leukemic (L2C) guinea pig lymphocytes is not a consequence of serum lipoprotein modifications which would make them unable to participate in the regulatory process. Low density lipoprotein of leukemic animals, in parallel to normal low density lipoprotein, can inhibit the cholesterol biosynthesis by normal cells. Surprisingly, very low density lipoprotein of leukemic animals have the same inhibitory property. Analyses of serum of leukemic animals showed a larger amount of the different lipoprotein fractions (+323% very low density, +27% low density lipoproteins, the high density lipoprotein staying undetectable in control and leukemic sera) than in normal serum. L2C leukemia produces low density lipoprotein slightly richer in unesterified cholesterol and very low density lipoprotein markedly modified by an increased proportion of unesterified cholesterol, phospholipids and apoprotein B. The inhibitory power of leukemic very low density lipoprotein is discussed by analogy with corresponding power of normal low density lipoprotein which can operate either by the way of binding to the low density lipoprotein receptor or by exchange of unesterified cholesterol between the lipoprotein and the cell.

Serum lipoprotein structure: resonance energy transfer localization of fluorescent lipid probes

The location of several fluorescent chromophores in lipoproteins has been determined by using resonance energy transfer. The primary acceptor is 5-(N-hexadecanoylamino)fluorescein whose chromophore is shown to reside at the lipoprotein surface at pH 7.4. Polar donors include cis-parinaric acid (cis,trans,trans,cis-9,11,13,15-octadecatetraenoic acid), trans-parinaric acid (all-trans-9,11,13,15-octadecatetraenoic acid), and 16-(9-anthroyloxy)palmitic acid; nonpolar donors are parinaric acid methyl ester, parinaric acid cholesteryl ester, and 1,6-diphenyl-1,3,5-hexatriene. The polar donors transfer more efficiently than the nonpolar donors in several classes of lipoprotein particles. The data are analyzed by a simple mathematical model from which it is concluded that the polar donors are localized in the putative lipoprotein surface monolayer; the possibility that nonpolar donors are partitioned between the surface and core of lipoproteins is considered.