Interaction of human plasma high density lipoprotein HDL2 with synthetic saturated phosphatidylcholines

The interaction of human plasma high density lipoprotein HDL₂ (d 1.063–1.125 g/ml) with sonicated dispersions of synthetic saturated phosphatidylcholines, dipalmitoyl- (diC₁₆PC), dimyristoyl- (diC₁₄PC), didodecanoyl- (diC₁₂PC), didecanoyl- (diC₁₀PC), and dioctanoyl- (diC₈PC) L-alpha phosphatidylcholine, was investigated. Incubation (4.5 hr, 37 C) of HDL₂ with diC₁₄PC, diC₁₂PC, diC₁₀PC and diC₈PC followed by gradient gel electrophoresis or preparative ultracentrifugation resulted in a redistribution of apolipoprotein A-I (apoA-I). The extent of redistribution depended on the molar ratio of the phospholipid to HDL₂ in the incubation mixture. Redistributed apoA-I occurred as lipid-free apoA-I and/or as complexes of apoA-I with phosphatidylcholine. Increasing the length of time of ultracentrifugation of the interaction mixtures did not increase the extent of redistribution. No redistribution of apoA-I was detected following incubation and gradient gel electrophoresis or preparative ultracentrifugation of mixtures of HDL₂ with dispersions of diC₁₆PC. Presented in part at the Joint Meeting of the American Oil Chemists' Society and the Japan Oil Chemists' Society, 1979.     

Compositional changes and apoprotein A-I metabolism of plasma high density lipoprotein in estrogenized chicks

The effect of estrogen on compositional changes, apolipoprotein (apo) A-I metabolism and the morphology of plasma high density lipoprotein (HDL) were investigated in chicks. The administration of 17β-estradiol (25 mg/kg body weight) to growing male chicks (8-week-old) markedly reduced the concentrations of plasma HDL components, except for triglyceride (TG). At the same time, levels of TG, total cholesterol (TC) and phospholipid (PL) in plasma were greatly elevated. The respective values for TG, TC, PL and protein in HDL were 13.9, 89.3, 154.1 and 231.7 (mg/dL) in the control, and 39.0, 35.1, 113.8 and 160.0 (mg/dL) in chicks upon estrogen treatment for one day.In vivo kinetic studies showed that the fractional catabolic rate of HDL apo A-I was significantly higher (p<0.05) in estrogen-treated chicks than in control birds, indicating an increased efficiency of HDL removal in the former. The production rate of HDL apo A-I also was significantly lower (p<0.05) in estrogen-treated chicks. Sodium dodecyl sulfate-acrylamide gel electrophoresis followed by laser scanning densitometry of HDL apolipoproteins in estrogen-treated chicks revealed a reduction of apo A-I and the occurrence of new apolipoproteins which had been absent in HDL of untreated birds. The HDL particles showed that the mean particle size of HDL became larger upon estrogen treatment. Particles with diameters between 70 and 123 Å were predominant in HDL of control chicks, while particles with diameters between 97 and 143 Å were most abundant in HDL of estrogen-treated chicks.     

Interaction of plasma high density lipoprotein HDL2b (d 1.063–1.100 g/ml) with single-bilayer liposomes of dimyristoylphosphatidylcholine

Incubation of a major subfraction, HDL_2b (d 1.063–1.100 g/ml), of human plasma high density lipoproteins, HDL (d 1.063–1.21 g/ml), with single-bilayer liposomes of dimyristoylphosphatidylcholine (DMPC) resulted in uptake of DMPC by the HDL_2b and dissociation of lipid-free apolipoprotein A-I (apoA-I). In the presence of excess DMPC, the dissociated apoA-I was also incorporated with DMPC into discoidal complexes. Preliminary studies with model apoA-I-DMPC complexes indicated that they also can interact with native HDL_2b with the resultant transfer of their DMPC to HDL_2b and the concomitant release of their apoA-I. After interaction of HDL_2b with DMPC liposomes, the DMPC-enriched HDL_2b product showed a lower hydrated density and a larger particle size than the control HDL_2b. The molecular properties of the lipoprotein product suggest that stabilization of the apoA-I-depleted HDL_2b probably occurred via substitution of DMPC for the apoA-I at the HDL_2b surface rather than by fusion of the apoA-I-depleted HDL_2b. The above interactions of HDL_2b with single-bilayer liposomes and discoidal complexes indicate pathways of phospholipid transfer relevant to the possible role of HDL in the metabolism of lipoprotein surface components in vivo.     

Evidence for the lipoprotein heterogeneity of human plasma high density lipoproteins isolated by three different procedures

Quantitative electroimmunoassays of apolipoproteins in ultracentrifugally isolated high density lipoproteins (HDL) of normolipidemic subjects showed that A-I and A-II are the major (80–85% of total HDL protein) and B, C-III, E, D and F are the minor protein constituents of this density class. A comparison between the apolipoprotein composition of ultracentrifugally isolated HDL and heparin-Mn⁺⁺ supernates showed no significant difference in the levels of A-I and C-III. However, the concentration of ApoE in the heparin-Mn⁺⁺ supernates was almost twice as high as that in the ultracentrifugally isolated HDL; ApoB was only detectable in trace amounts in the heparin-Mn⁺⁺ supernates. To establish whether these apolipoproteins are parts of a single macromolecular complex or form separate, discrete lipoprotein particles, the high density lipoproteins were isolated by three different procedures including ultracentrifugation, heparin-Mn⁺⁺ precipitation and agarose column chromatography. The double diffusion analyses of each of these HDL preparations with antisera to A-I, A-II, ApoB, C-III, ApoD, ApoE, and ApoF showed nonidentity reactions between each possible combination of these antisera. The only exception was a reaction of partial identity between antisera to A-I and A-II polypeptides indicating the occurrence of two types of lipoprotein particles, a major one (LP-A) containing both polypeptides and a minor one (LP-A-I) containing A-I as the sole protein constituent. These findings indicate that high density lipoproteins, regardless of the manner of isolation, do not consist of a single macromolecular complex, but represent a mixture of several, discrete lipoprotein families. Presented at the AOCS Meeting, St. Louis, May 1978.     

Subfractionation and characterization of native and incubation enlarged human plasma high density lipoprotein particles by high performance gel filtration

Incubation of human plasma in vitro at 37 C results in an increase of the mean particle size of the high density lipoproteins (HDL) accompanied by an almost complete disappearance of the original particles present prior to incubation. A rapid high performance gel filtration technique has been developed in order to study the chemical composition of subfractions of native and incubation enlarged HDL particles as a function of particle size. Subfractionation of HDL isolated by preparative ultracentrifugation from 3 normal human plasmas incubaed in vitro at 0 and 37 C for 24 hr have been performed using a 150 cm long TSK-G 3000 SW column. The separation time was less than 65 min. The curves obtained at high performance gel filtration of HDL, by monitoring the effluents from the column at 280 nm, agreed well both in positions of peak maxima and relative peak intensities with the particle distribution patterns observed at polyacrylamide gradient gel electrophoresis of the corresponding HDL preparations run in parallel. The different HDL particle subfractions of the effluents from the gel filtration column have been characterized by quantification of free and esterified cholesterol, total phospholipids and apolipoprotein A-I and A-II. The incubation enlarged HDL particles, subfractionated by the high performance gel filtration technique, were found to have a composition which differed from that of native HDL particles of corresponding size. Incubation enlarged HDL had a generally higher and almost constant relative cholesteryl ester content over the whole particle range compared to native HDL in which a continuous increase in relative cholesteryl ester content could be observed when going from large to small particles. The molar ratio of phospholipids to free cholesterol was higher in small native HDL particles than in the corresponding large ones. The relation between apolipoprotein A-I and A-II remained nearly constant between small and large HDL particles in each subfractionation experiment. The results demonstrate that the high performance gel filtration technique is a rapid and reproducible means for studying the composition of subfractions of HDL particle populations.     

Normalization of high density lipoprotein in fish eye disease plasma by purified normal human lecithin: Cholesterol acyltransferase

Plasma from a patient with fish eye disease has been enriched with autologous high density lipoproteins (HDL) and supplemented with highly purified normal human plasma lecithin:cholesterol acyltransferase (LCAT). Incubation of such plasma at 37 C in vitro resulted in normalization of its low HDL cholesteryl ester percentage, from 23% to 79%, associated with a two-fold increase in both the cholesteryl ester and triglyceride contents of the HDL fraction, as compared to incubation experiments with absent or heat-inactivated purified normal LCAT. The normalization of the HDL cholesteryl ester percentage induced by incubation with purified normal LCAT also was accompanied by an increase in the size of the original fish eye disease HDL particles, which had a mean mass of 115 kd, to HDL particle populations with mean particle masses ranging from 130–220 kd, depending on the concentration of purified LCAT in the incubate. Both HDL cholesterol esterification and particle enlargement were abolished completely by the LCAT inhibitor DTNB and by heat inactivation of the purified normal LCAT. The results give further evidence that fish eye disease is an α-LCAT deficiency.     

Comparative study of the consumption of virgin olive oil or seje on lipid profile and oxidation resistance of high density lipoprotein (HDL) of rat plasma

We compared the effect of the consumption of seje oil (Oenocarpus bataua), with that of olive oil, on plasma lipids and susceptibility in vitro to oxidation of high density lipoprotein (HDL) in the rat. Two groups often male Sprague Dawley rats were fed ad libitum, for a lapse of eight week, with a purified diets with 10g de seje oil or olive oil/100 g of diet (GS y GO respectively). The animals were exsanguinated at the end of the experimental after a 14 hour fast. Plasma was isolated by centrifugation, and the fractions of lipoproteins were separated from the plasma by sequential ultracentrifugation. Rats of GO had a statistically significant lower in concentration of TG (p < 0.05) compared with GS group. HDL fractions in both groups were oxidatively modified by incubation with copper ions. Differences in the fractions susceptibilities to peroxidation were studied by measuring the formation of thiobarbituric acid reactive substance (TBARS) for 3 hours. HDL in GS had a statistically significant decrease in TBARS formation (p < 0.05) relative to HDL of GO. This may be explained by the lower concentration of polyunsaturated fatty acids of HDL in GS compared with HDL in GO.     

Intercorrelations among plasma high density lipoprotein,obesity and triglycerides in a normal population

The interrelationships among fatness measures, plasma triglycerides and high density lipoproteins (HDL) were examined in 131 normal adult subjects: 38 men aged 27–46, 40 men aged 47–66, 29 women aged 27–46 and 24 women aged 47–66. None of the women were taking estrogens or oral contraceptive medication. The HDL concentration was subdivided into HDL_2b, HDL_2a and HDL₃ by a computerized fitting of the total schlieren pattern to reference schlieren patterns. Anthropometric measures employed included skinfolds at 3 sites, 2 weight/height indices and 2 girth measurements. A high correlation was found among the various fatness measures. These measures were negatively correlated with total HDL, reflecting the negative correlation between fatness measures and HDL₂ (as the sum of HDL_2a and_2b). Fatness measures showed no relationship to HDL₃. There was also an inverse correlation between triglyceride concentration and HDL₂. No particular fatness measure was better than any other for demonstrating the inverse correlation with HDL but multiple correlations using all of the measures of obesity improved the correlations. Partial correlations controlling for fatness did not reduce any of the significant correlations between triglycerides and HDL₂ to insignificance. The weak correlation between fatness and triglycerides was reduced to insignificance when controlled for HDL₂. Presented (in part) at the Annual Meeting of the Oil Chemists' Society in St. Louis, MO, May 1978.     

Effect of dietary palm oil and its fractions on rat plasma and high density lipoprotein lipids

Male Sprague Dawley rats were fed semipurified diets containing 20% fat for 15 weeks. The dietary fats were corn oil, soybean oil, palm oil, palm olein and palm stearin. No differences in the body and organ weights of rats fed the various diets were evident. Plasma cholesterol levels of rats fed soybean oil were significantly lower than those of rats fed corn oil, palm oil, palm olein or palm stearin. Significant differences between the plasma cholesterol content of rats fed corn oil and rats fed the three palm oils were not evident. HDL cholesterol was raised in rats fed the three palm oil diets compared to the rats fed either corn oil or soybean oil. The cholesterol-phospholipid molar ratio of rat platelets was not influenced by the dietary fat type. The formation of 6-keto-PGF_1α was significantly enhanced in palm oil-fed rats compared to all other dietary treatments. Fatty acid compositional changes in the plasma cholesterol esters and plasma triglycerides were diet regulated with significant differences between rats fed the polyunsaturated corn and soybean oil compared to the three palm oils.     

The composition and metabolism of high density lipoprotein subfractions

The composition and metabolism of high density lipoprotein (HDL) subfractions were investigated in seven nomal individuals. Mean HDL₂ (d, 1.063–1.125 g/ml) composition (by weight) was 43% protein, 28% phospholipid, 23% cholesterol, and 6% triglyceride, and mean HDL₃ (d, 1.125–1.21 g/ml) composition was 58% protein, 22% phospholipid, 14% cholesterol, and 5% triglyceride. The mean apoA-I; apoA-II weight ratio was 4.75 for HDL₂ and 3.65 for HDL₃.HDL₂ protein was proportionally slightly richer in C apolipoproteins and higher molecular weight constituents (including apoE) than HDL₃. Kinetic studies utilizing radiolabeled HDL_A (d, 1.09–1.21 g/ml), HDL₂, and HDL₃ demonstrated rapid exchange of apoA-I and apoA-II radioactivity among HDL subfractions, similar fractional rates of catabolism of apoA-I and apoA-II within HDL, and similar radioactivity decay within HDL subfractions. Mean plasma residence time was 5.74 days for radiolabeled HDL₂ and 5.70 days for radiolabeled HDL₃. Differences in HDL protein mass among individuals were largely due to alterations in catabolism, and in general both HDL₂ and HDL₃ were catabolized via a plasma and a nonplasma pathway. Data from simultaneous radiolabeled very low density lipoprotein and HDL studies in 2 individuals are consistent with the concept that apoC-II and apoC-III are catabolized at a different rate than are apoA-I and apoA-II within the HDL density range.     

Alterations of high density lipoprotein subclasses in obese subjects

The object of this study was to investigate the characteristics of lipid metabolism in obese subjects, with particular emphasis on the alteration of HDL subclass contents and distributions. A population of 581 Chinese individuals was divided into four groups (25 underweight subjects, 288 of desirable weight, 187 overweight, and 45 obese) according to body mass index (BMI). Apoprotein A-I (apoA-I) contents of plasma HDL subclasses were determined by 2-D gel electrophoresis associated with an immunodetection method. The concentrations of TG and the apoA-I content of pre-α₁-HDL were significantly higher (P<0.01 and P<0.01, respectively), but the levels of HDL cholesterol, and the apoA-I contents of HDL_2a and HDL_2b were significantly lower (P<0.01, P<0.05, and P<0.01, respectively) in obese subjects than in subjects having a desirable weight. Moreover, with the elevation of BMI, small-sized pre-α₁-HDL increased gradually and significantly, whereas large-sized HDL_2b decreased gradually and significantly. Meanwhile, the variations in HDL subclass distribution were more obvious with the elevation of TG levels in obese as well as overweight subjects. In addition, Pearson correlation analysis revealed that BMI and TG levels were positively correlated with pre-α₁-HDL but negatively correlated with HDL_2b. Multiple regression analysis also showed that TG concentrations were associated independently and positively with high pre-α₁-HDL and independently and negatively with low HDL_2b in obese and overweight subjects. The HDL particle size was smaller in obese and overweight subjects. The shift to smaller size was more obvious with the elevation of BMI and TG, especially TG levels. These observations, in turn, indicated that HDL maturation might be abnormal, and reverse cholesterol transport might be impaired. The first two authors contributed equally to this study.     

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 S_f ^o values 9.49, 7.94, 6.42, 5.17, and 3.71 determined by sedimentation equilibrium were 2.97×10⁶; 3.13×10⁶; 2.89×10⁶; 2.45 ×10⁶; and 2.61×10⁶ 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 these fractions determined by flotation velocity at 37,020 rpm were 2.57×10⁶; 2.37×10⁶; 2.09×10⁶; 1.94×10⁶; and 1.81 ×10⁶ daltons; whereas similar molecular weights determined at 52,640 rpm were 2.53×10⁶; 2.27 ×10⁶; 1.99×10⁶; 1.86×10⁶; and 1.74×10⁶ 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 frictional 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.     

HDL3-mediated cholesterol efflux from cultured enterocytes: The role of apoproteins A-I and A-II

High density lipoproteins (HDL) were recently demonstrated in an enterocyte model (CaCo-2 cells) to mediate reverse cholesterol transport by retroendocytosis. The present study was carried out to define the role of the major HDL apoproteins (apo) A-I and apo A-II in this pathway. HDL₃ was fractionated by heparin affinity chromatography into the two main fractions containing either apo A-I only (fraction A) or both apo A-I and apo A-II (fraction B). In addition, liposomes were reconstituted from purified apo A-I or apo A-II and dimyristoyl phosphatidylcholine. The cell binding properties and cholesterol efflux potential were studied in the lipoprotein fractions and the liposomes. Both fractions exhibited similar maximal binding capacities of 4427 (A) and 5041 (B) ng/mg cell protein, but their dissociation constants differed (40.5 and 167.7 μg/mL, respectively). Fraction A induced cholesterol efflux and stimulated cholesterol synthesis more than did fraction B. Fraction A mobilized both cellular free and esterified cholesterol, whereas fraction B preferentially mobilized cholesteryl esters. Liposomes, containing either apo A-I or apo A-II, showed specific binding, endocytosis and endosomal transport, and were released as intact particles. Apo A-I liposomes also mediated cholesterol efflux. In conclusion, there is evidence that the HDL₃ subfractions A and B, as well as reconstituted liposomes containing either apo A-I or apo A-II, were specifically bound and entered a retroendocytosis pathway which was directly linked to cholesterol efflux. Quantitatively, the apo A-I subfraction appeared to play the dominant role in normal enterocytes. The apo A-II content of fraction B was related to the mobilization of cholesteryl esters.     

The role of high density lipoprotein apolipoprotein CII in triglyceride metabolism

The purpose of these studies was (a) to examine the relationship between total plasma triglycerides (TG) and the amount of apolipoprotein CII (apo CII) in triglyceride rich lipoproteins (TRL), and (b) to determine whether TRL could be enriched with apo CII in vitro. In 13 patients with primary endogenous hypertriglyceridemia, (log₁₀) total plasma TG correlated inversely with the amount of apo CII per unit very low density lipoprotein (VLDL) protein (r=−0.76;p<0.005) and VLDL TG (r=−0.75; p<0.005). The potency of VLDL to activate milk lipoprotein lipase (LPL) in hydrolyzing triolein was studied in vitro. LPL activator potency per unit VLDL protein or VLDL TG correlated inversely with (log₁₀) total plasma TG (r=−0.86 and r=−0.76, respectively; p<0.005). LPL activator potency per nM VLDL apo CII also correlated inversely with (log₁₀) total plasma TG (r=−0.49; p<0.01). In seven patients with familial type V hyperlipoproteinemia, the average amount of apo CII in TRL protein was subnormal (5.86±0.62% vs 10.0±0.51% in normal subjects). The higher the (log₁₀) total plasma TG, the lower was the apo CII content in TRL protein (r=−0.93; p<0.01). To determine the factors governing the distribution of apo CII between lipoproteins and whether TRL could be enriched with apo CII, five approaches were undertaken: (a)¹²⁵I apo CII was added to mixtures of VLDL and HDL. The amount of labelled apo CII in VLDL was proportional to the ratio of VLDL to HDL. (b) TRL from four patients with familial type V hyperlipoproteinemia was incubated with high density lipoprotein (HDL) from a normal subject. An increase in the TRL/HDL ratio was associated with transfer of apo CII from HDL to TRL and a reciprocal transfer of non-apo CII protein from TRL to HDL. Net apo CII enrichment of TRL protein was possible below a HDL/TRL protein ratio of ca. 6 under the experimental conditions. (c) A fixed amount of normal plasma feed of TRL was incubated with different amounts of TRL from two patients with familial type V hyperlipoproteinemia. The amount of apo CII that transferred from normal TRL free plasma to the patient’s TRL was proportional to the amount of TRL in the mixture. (d) A doubling and tripling in the amount of apo CII in TRL was found when apo CII was added directly to TRL from a normal subject and TRL from a patient with familial type V hyperlipoproteinemia, respectively. (e) When apo CII was added directly to normal plasma and plasma from a patient with primary type IV hyperlipoproteinemia, the peptide was taken up mainly by VLDL and HDL, indicating enrichment of these fractions. The distribution of the added apo CII in each lipoprotein fraction resembled the distribution in the native plasma. TRL was isolated after addition of apo CII to plasma from two patients with familial types IV and V, respectively. Enrichment of TRL with apo CII was associated with an approximate 1.5-fold increase in the LPL activator potency per unit TRL protein. These studies suggest that firstly, the amount of apo CII in TRL is inversely related to the severity of hypertriglyceridemia. Secondly, the distribution of apo CII between TRL and HDL is governed by the mass ratios of these two lipoprotein classes. Thirdly, plasma TRL and HDL have a reserve binding capacity of apo CII and fourthly, it is possible to enrich these lipoproteins with this functionally important peptide. Whether net enrichment of TRL with apo CII and also an increase in its biological activity to activate LPL in vitro is related to increased in vivo catabolic rate requires to be determined.     

The distribution of serum high density lipoprotein subfractions in non-human primates

The ultracentrifugal flotation patterns in 1.2 g/ml solvent and ultracentrifugal gradient distribution of high density lipoproteins (HDL) from the primates-human, apes and monkeys-were determined, with emphasis on the gorilla species of apes and rhesus monkeys. Diets for non-human primates were commercial chow, which is low in cholesterol. Molecular weights and protein, cholesterol, phospholipid and triglyceride compositions of various density fractions were determined on human, gorilla and rhesus HDL. The HDL₂/HDL₃ ratio was determined from the two peaks observed upon flotation in high salt in the analytical ultracentrifuge. The HDL₂ of all three species of apes-gorillas (Gorilla gorilla), chimpanzees (Pan troglodytes) and orangutans (Pongo pygmaeus)—was always greater than HDL₃, while that of all six species of Old World monkeys-Rhesus (Macaca mulatta), sooty mangabeys (Cercocebus atys), cynomolgus (Macaca fascicularis), stumptails, (Macaca arctoides) patas (Erythrocebus patas) and African greens (Cercopithecus aethiops)—was less. In addition, the HDL₃ concentration in five gorillas was about 15 mg/dl as cholesterol while the HDL₂ concentration was 92 mg/dl, much lower and higher, respectively, than humans. HDL₂ of gorillas was similar in density and molecular weight to that of humans. The distribution of densities in gorilla HDL was predominantly in HDL₂, while rhesus HDL usually, but not always, was unimodal, having a density distribution similar in heterogeneity to human HDL₃, but somewhat less dense (peaking at 1.109 vs 1.129 g/ml). The molecular weight of rhesus HDL was about the same as human HDL₃ in all three density subfractions and at the peak density. Likewise, the chemical compositions were similar for the subfractions 1.10–1.125 and>1.125 g/ml for rhesus HDL and human HDL₃. Consequently most but not all chow-fed rhesus HDL was very similar to human HDL₃, but lighter in density. A preliminary report of this study was given at the American Society for Biological Chemists Meeting in New Oreleans in April 1982.     

The evidence for the antiatherogenicity of high density lipoprotein in man

It has long been recognized that patients with clinical coronary heart disease (CHD) have, on average, higher concentrations of plasma very low density and low density lipoproteins than do healthy subjects. The same, studies clearly demonstrated that coronary victims tend also to have low plasma concentrations of high density lipoprotein (HDL). It is only recently, however, that the possible significance of this second observation has been examined. Direct evidence for an inverse relationship between HDL cholesterol concentration and the prevalence of clinical CHD, independent of other plasma lipoproteins, has been provided by the Honolulu Heart and Cooperative Lipoprotein Phenotyping Studies. The Tromsø Heart and Framingham Studies subsequently demonstrated that this relationship precedes the clinical manifestation of coronary disease. More recently, angiographic studies have confirmed that the severity of existing coronary atherosclerosis is inversely related to HDL cholesterol concentration. Other investigations have shown that coronary victims also have low mean concentrations of apolipoproteins AI and AII (the major protein components of HDL), although the reduction of apoAI concentration may be less marked than that of HDL cholesterol, and preliminary findings from Tromsø have suggested that apolipoprotein AI may be less powerful than HDL cholesterol as a predictor of CHD. Such observations have supported the proposal that HDL may exert a protective effect against coronary atherosclerosis. Final confirmation (or otherwise) of this hypothesis, however, must await the results of carefully controlled animal experiments and of regression studies in patients with angiographically defined atherosclerosis.     

Comparison of the structural and functional effects of monomeric and dimeric human apolipoprotein A-II in high density lipoprotein particles

High density lipoprotein (HDL) is throught to play a significant role in the process of reverse cholesterol transport. It has become clear that the apolipoprotein (apo) composition of HDL is important in determining the metabolic fate of this particle. The major proteins of human HDL are apoAI and APOAII; the latter protein is a disulfide-linked dimer in humans and higher primates but monomeric in the other species. The consequences of the apo Cys6-Cys6 disulfide bridge in apoAII for human HDL structure and function are not known. To address this issue, the influence of the Cys6-Cys6 disulfide bridge on the interaction of human apoAII with palmitoyl-oleoyl phosphatidylcholine has been studied. The size and valence of a series of homogeneous discoidal complexes containing either monomeric (reduced and carboxymethylated) or dimeric apoAII have been determined, and their ability to remove cholesterol from rat Fu5AH hepatoma cells grown in culture has been compared. The apoAII dimer and monomer form discoidal complexes of similar size, with twice as many of the latter molecule required per disc. Removal of the disulfide bond influences the stability of the helical segments around the edge of the disc as seen by a decrease in α-helix content of the monomeric protein. The discoidal particles containing the monomeric form of apoAII are somewhat more effective than particles containing either dimeric apoAII or apoAI in removing cellular cholesterol. Overall, reduction of the disulfide bridge of apoAII probably does not have a major effect in the determination of HDL particle sizein vivo. It follows that the evolution of the Cys6-Cys6 disulfide bond in higher primates probably has not had a major effect on the function of the apoAII molecule.     

Susceptibility of serum lipids to copper-induced peroxidation correlates with the level of high density lipoprotein cholesterol

As a first step in evaluating the significance of our recently developed method of monitoring the kinetics of copper-induced oxidation in unfractionated serum, we recorded the kinetics of lipid oxidation in the sera of 62 hyperlipidemic patients and analyzed the correlation between oxidation and lipid composition of the sera [high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, and triglycerides]. We used six factors to characterize the kinetics of oxidation, namely, the maximal absorbance of oxidation products (OD_max), the maximal rate of their production (V _max), and the time at which the rate was maximal (t _max) at two wave-lengths (245 nm, where 7-ketocholesterol and conjugated dienic hydroperoxides absorb intensely, and 268 nm, where the absorbance is mostly due to dienals). The major conclusions of our analyses are that: (i) Both OD_max and V _max correlate positively with the sum of concentrations of the major oxidizable lipids, cholesterol, and cholesteryl esters. (ii). The value of t _max, which is a measure of the lag preceding oxidation and therefore reflects the resistance of the serum lipids to copper-induced oxidation, exhibits a negative correlation with HDL cholesterol. Although this finding accords with the observation of shorter lags for HDL than for LDL, it is apparently inconsistent with the role of HDL as an antirisk factor in coronary heart diseases.