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.

Further resolution of the low density lipoprotein spectrum in normal human plasma: physicochemical characteristics of discrete subspecies separated by density gradient ultracentrifugation

The molecular basis of the heterogeneity of plasma low density lipoproteins (LDL, d 1.024-1.050 g/ml) was evaluated in 40 normolipidemic male subjects following fractionation by isopycnic density gradient ultracentrifugation into eight major subspecies. The mass profile of our subjects' LDL uniformly displayed single symmetric or asymmetric peaks as a function of density; the peak occurred most frequently (20 subjects) in subfraction 7 (d 1.0297-1.0327 g/ml). Several physicochemical properties (hydrodynamic behavior, electrophoretic mobility, chemical composition, size and particle heterogeneity, and apolipoprotein heterogeneity) of the LDL subfractions were examined. Hydrodynamic analyses revealed unimodal distributions and distinct peak Sf degree rates in individual subfractions. Such behavior correlated well with particle size and heterogeneity data, in which LDL subspecies were typically resolved as unique narrow bands by gradient gel electrophoresis. Subspecies with average densities of 1.024 to 1.0409 g/ml ranged from 229 to 214 A in particle diameter. LDL protein content increased in parallel with density while the proportion of triglyceride diminished; cholesteryl esters predominated, accounting for approximately 40% or more by weight. Distinct differences in net electric charge were demonstrated by electrophoresis in agarose gel, the subspecies with average density of 1.0314 g/ml displaying the lowest net negative charge. ApoB-100 was the major apoprotein in all subspecies, and constituted the unique protein component over the density interval 1.0271-1.0393 g/ml. ApoE and apo[a] were detected at densities less than 1.0271 and greater than 1.0393 g/ml. While apoE was evenly distributed within these two regions, representing up to 2% of apoLDL, the distribution of apo[a] was skewed towards the denser region, in which it amounted to 3-7% of apoLDL. ApoC-III was detectable as a trace component at densities greater than 1.0358 g/ml. Calculation of the number of molecules of each chemical component per LDL subspecies showed the presence of one copy of apoB-100 per particle, in association with decreasing amounts of cholesteryl ester, free cholesterol, and phospholipid. These data indicate that a similar overall molecular organization and structure is maintained in a unimodal distribution of LDL particle subspecies over the density range approximately 1.02 to 1.05 g/ml. In sum, our data may be interpreted to suggest that microheterogeneity in the physicochemical properties of human LDL subspecies reflects dissimilarities in their origins, intravascular metabolism, tissular fate, and possibly in their atherogenicity.

Pharmacology of fenofibrate

This discussion outlines the major aspects of the human pharmacology of fenofibrate, a hypolipidemic agent. In view of its short half-life, efficient absorption, and elimination, fenofibrate would not appear to accumulate in either plasma or tissues. It is extensively absorbed only in the presence of food and is transported through the bloodstream by albumin. Fenofibrate is taken up by both the liver and kidney. Except for a small percentage (about 5 percent) reduced at the ketone moiety before conjugation, most drug is excreted as a conjugate in the urine. Less than 20 percent is excreted through the bile. In normal persons, at steady state with usual doses of 100 mg three times daily, the plasma half-life approximates 30 hours. Because fenofibrate is not dialyzable, it has a markedly prolonged half life in patients with renal failure and should not be used.

Lipoprotein composition of human suction-blister interstitial fluid

Interstitial fluid (IF) was obtained in 27 apparently healthy subjects (12 males, 15 females) by applying mild suction (200-250 mm Hg) on the skin either on the midvolar forearm or on the paraumbilical region of the abdomen. The IF concentrations of lipids and apolipoproteins (apo) were studied and compared with those of serum (S). The mean ratio between interstitial fluid and serum (IF/S ratio) varied from 0.14 for forearm apoE to 0.29 for apoA-II on the abdomen. This ratio was consistently lower for apoE, C-II, C-III, and B than for apoA-I and A-II, and significantly lower on the arm than on the abdomen for all apolipoproteins studied. The IF/S ratios showed marked variations among individuals. However, interstitial fluid apolipoprotein concentrations at different blister sites were highly correlated within each individual. Studies with agarose gel electrophoresis and density gradient ultracentrifugation revealed that large triglyceride-rich particles were virtually lacking in interstitial fluid and that the relation between the low density lipoproteins (LDL) and high density lipoproteins (HDL) was shifted towards a greater proportion of HDL. The lipoprotein distribution in the HDL range of interstitial fluid differed from that of serum showing one maximum at a density of about 1.070 g/ml (serum HDL2 about 1.090 g/ml) and one at a density of 1.130-1.140 g/ml (serum HDL3, 1.110-1.120 g/ml). The former subfraction contained most of the lipoprotein-bound apoE while the latter contained the major part of apoA-I and apoA-II. Studies of the lipoproteins of interstitial fluid may add to our understanding of the development of atherosclerosis and xanthomatosis and may also provide valuable information on the permeability of the capillary membrane in normo- and pathophysiological states.

Identification of surface-exposed segments of apolipoprotein B-100 in the LDL particle

The isolation and amino acid sequence of eleven peptides liberated by tryptic treatment from surface-exposed regions of apolipoprotein B-100 in the native low-density lipoprotein particle are described. These peptides represent eight segments in the sequence of the B-100 protein, one of which was localised to the amino-terminal thrombolytic fragment T4 (1297 amino acids), four to the T3 fragment (2052 residues) and three to the carboxylterminal fragment T2 (1287 residues). An exposed segment was identified on each side of the T2/T3 cleavage site, in close proximity to two segments enriched in basic amino acids (residues 3147-3157 and 3359-3367 respectively). The surface exposure of this region is consistent with its contribution to the putative apo-B,E receptor binding domain. Four of the eight tryptic segments contribute to regions of proline-rich clusters. Homology between the sequence of the tryptic peptides and those predicted by cDNA cloning was complete.

A study of the structural heterogeneity of low-density lipoproteins in two patients homozygous for familial hypercholesterolaemia, one of phenotype E2/2

The structural heterogeneity of the low-density lipoproteins (d 1.019-1.063 g ml-1) in two female patients homozygous for familial hypercholesterolaemia, one of phenotype E2/2, has been evaluated using a new ultracentrifugal density gradient procedure. The mass distribution, chemical composition, particle size and heterogeneity, hydrated density and apolipoprotein content of 16 LDL subfractions were determined. By gradient gel electrophoresis, the lighter LDL subfractions (d 1.016-1.037 g ml-1) displayed a single particle species which progressively diminished in size from 24.8 to 22.0 nm with increase in density. By contrast, subfractions of higher density (d greater than 1.037 g ml-1) exhibited two LDL particle species of distinct size; one component decreased in size from 21.8 to 20.4 nm with increase in density, while the second maintained an essentially constant diameter (between 22.5 and 23.5 nm) across these LDL subfractions. Immunoblotting with anti-apo-B100 of LDL subspecies separated by gradient gel electrophoresis showed all particles to contain apo-B100. However, dot-blots and immunoblotting with a monoclonal antibody to lipoprotein (a) (Lp(a)) revealed that the LDL particle subspecies of greatest diameter (22.5-23.5 nm) present in the denser subfractions (d greater than 1.037 g ml-1) also contained the Lp(a) antigen. These findings, taken together with the high plasma Lp(a) levels (greater than 60 mg dl-1) in our patients, raise the possibility that Lp(a) may contribute in a significant manner to the atherogenic process in homozygous familial hypercholesterolaemia.

Dielectric behavior of water in biological solutions: studies on myoglobin, human low-density lipoprotein, and polyvinylpyrrolidone

The dielectric behavior of the aqueous solutions of three widely differing macromolecules has been investigated: myoglobin, polyvinylpyrrolidone (PVP), and human serum low-density lipoprotein (LDL). It was not possible to interpret unambiguously the dielectric properties of the PVP solution in terms of water structure. The best interpretation of the dielectric data on the myoglobin and LDL solutions was that, in both cases, the macromolecule attracts a layer of water of hydration one or two water molecules in width. For LDL, this corresponds to a hydration factor of only 0.05 g/g, whereas for myoglobin the figure is nearer 0.6 g/g. With myoglobin, part of the water of hydration exhibits its dispersion at frequencies of a few GHz, and the rest disperses at lower frequencies, perhaps as low as 10-12 MHz. The approximate constancy of the width of the hydration shell for two molecules as dissimilar in size as LDL and myoglobin confirms that the proportion of water existing as water of hydration in a biological solution depends critically on the size of the macromolecules as well as on their concentration.

A density gradient study of the lipoprotein and apolipoprotein distribution in the chicken, Gallus domesticus

Plasma lipoproteins from 5-week old male chickens were separated over the density range 1.006-1.172 g/ml into 22 subfractions by isopycnic density gradient ultracentrifugation, in order to establish the distribution of these particles and their constituent apolipoproteins as a function of density. Lipoprotein subfractions were characterized by electrophorectic, chemical and morphological analyses, and their protein moieties were defined according to net charge at alkaline pH, molecular weight and isoelectric point. These analyses have permitted us to reevaluate the density limits of the major chicken lipoprotein classes and to determine their main characteristics, which are as follows: (1) very-low-density lipoproteins (VLDL), isolated at d less than 1.016 g/ml, were present at low concentrations (less than 0.1 mg/ml) in fasted birds; their mean diameter determined by gradient gel electrophoresis and by electron microscopy was 20.5 and 31.4 nm respectively; (2) as the the density increased from VLDL to intermediate density lipoproteins (IDL), d 1.016-l.020 g/ml) and low-density lipoproteins (LDL, d 1.020-1.046 g/ml), the lipoprotein particles contained progressively less triacylglycerol and more protein, and their Stokes diameter decreased to 20.0 nm; (3) apolipoprotein B-100 was the major apolipoprotein in lipoproteins of d less than 1.046 g/ml, with an Mr of 350000; small amounts of apolipoprotein B-100 were detectable in HDL subfractions of d less than 1.076 g/ml; urea-soluble apolipoproteins were present in this density range as minor components of Mr 38000-39000, 27000-28000 (corresponding to apolipoprotein A-1) and Mr 11000-12000; (4) high density lipoprotein (HDL, d 1.052-1.130 g/ml) was isolated as a single band, whose protein content increased progressively with increase in density; the chemical composition of HDL resembled that of human HDL2, with apolipoprotein A-1 (M 27000-28000) as the major protein component, and a protein of Mr 11000-12000 as a minor component; (5) heterogeneity was observed in the particle size and apolipoprotein distribution of HDL subfractions: two lipoprotein bands which additional apolipoproteins of Mr 13000 and 15000 were detected. These studies illustrate the inadequacy in the chicken of the density limits applied to fractionate the lipoprotein spectrum, and particularly the inappropriateness of the 1.063 g/ml density limit as the cutoff for LDL and HDL particle populations in the species.

Lipid transport systems: some recent aspects in swine, cattle and trout during development

Lipids such as triacylglycerols, free and esterified cholesterol and phospholipids are essentially insoluble in water; lipoproteins, which are complex macromolecules of pseudomicellar structure, have evolved to ensure their efficient transport in the vascular system of vertebrates. Plasma lipoproteins are operationally classified according to their relative lipid and protein contents and thus according to density. The protein components, i.e. apolipoproteins, have a highly specialized structure contributing to particle stability and metabolism. The chemistry and structure of plasma lipoproteins and apoproteins is discussed and their biosynthesis, intravascular metabolism and cellular degradation briefly considered. Circulating concentrations of lipoproteins are under complex control, involving hormonal, nutritional and genetic factors; changes occurring in lipoprotein levels in pigs (Sus domesticus), cattle (Bos sp.) and rainbow trout (Salmo gairdnerii) during development are discussed. In pigs and calves, LDL predominate at the fetal stage and VLDL are absent. During suckling, this pattern is rapidly modified with HDL becoming the major class and low levels of VLDL appearing. These changes in part reflect high levels of exogenous triglyceride lipolysis consequent to the ingestion of fat-rich, maternal milk. With further growth, HDL predominate in both adult pigs and steers, although the LDL:HDL ratio is considerably lower in the latter (0.5-0.8 and approximately 0.2, respectively). Our studies in pigs suggest that the intestinal secretion of lipoproteins commences rapidly after birth since proteins akin to human apo-B48 and apo-B100 are detectable in plasma VLDL some 2-3 h after parturition. Although the trout is an oviparous vertebrate, LDL is also preponderate at the juvenile stage. With sexual maturity, LDL and VLDL levels diminish progressively, plasma HDL attaining concentrations as high as 1,500 mg/dl in adults. Our knowledge of the biochemical mechanisms at the origin of these developmental changes in lipid transport in both mammals and fish remains largely incomplete.

The lipid transport system in the mouse, Mus musculus: isolation and characterization of apolipoproteins B, A-I, A-II, and C-III

Four of the principle apolipoproteins of murine serum have been isolated and characterized. On the basis of their physicochemical properties, they are homologous with the human and rat apoA-I, A-II, B, and C-III. The group of apolipoproteins of middle to low molecular weight, i.e., A-I, A-II and C-III, were separated from the protein moiety of high density lipoproteins (HDL) by gel filtration chromatography, followed by electrophoresis in alkaline-urea polyacrylamide gel with electrophoretic elution. Murine apoA-I, the major protein of HDL (60-80%) displayed an Mr of approximately 27,000, and was polymorphic (four prominent isoproteins with isoelectric points in the range of pH 5.5-5.7). The amino acid profiles of mouse, rat, and human apoA-I generally resembled each other, the former being distinguished by a content of one isoleucine residue per mole. Amino terminal sequence analysis revealed marked homology between the mouse, rat, dog, and human proteins; mouse and rat apoA-I differed at residues 9 and 18 with potential dissimilarities at residues 5 and 15, while the murine and canine sequences were distinct at residues 6, 9, 13, 15, and 30. Apolipoprotein A-II was a monomer, exhibiting an Mr approximately 11,000 in SDS gels; in addition, it was polymorphic (three apparent isoproteins with pI in the pH range 5.05-5.2), and resembled its human and rat counterparts in amino acid composition. ApoC-III, an acidic peptide of pI 4.74 and of Mr approximately 9,600, possessed an amino acid composition very like that of the homologous human and rat proteins. The homology of mouse apoC-III with the human protein was confirmed by NH2-terminal sequence analysis, which revealed identical amino acids in six positions (1, 2, 4, 8, 9, and 13). As shown earlier (Camus et al. 1983. J. Lipid Res. 24: 1210-1228), two forms of immunologically reacting apoB predominated in mouse VLDL and LDL. After isolation of these lipoproteins in the presence of 1 mM PMSF, the apparent sizes of the high and low Mr forms, apoBH and apoBL, were in the ranges approximately 400,000-530,000 and approximately 250,000-280,000, respectively, according to the SDS gel system. We observed that inclusion of 1 mM PMSF was essential to retard degradation of the high Mr form apoBH. The murine B proteins were isolated from apoVLDL and apoLDL by gel filtration chromatography on Sephadex G150 in anionic detergent, and displayed apparent Mr values of 460,000 (apoBH) and 250,000 (apoBL) in 3% SDS gels.(ABSTRACT TRUNCATED AT 400 WORDS)

Chimpanzee serum lipoproteins. Isolation, characterisation and comparative aspects of the low density lipoprotein and apolipoprotein-BH

Evaluation of the serum lipoprotein profile in non-fasting, adult chimpanzees by analytical ultracentrifugation revealed a lower mean LDL level (269 mg/dl) than typical of man. The major molecular form(s) of low density lipoprotein (LDL) was then isolated in the density interval 1.024-1.050 g/ml by sequential ultracentrifugation. The physicochemical properties of chimpanzee LDL, including net surface charge as judged by electrophoresis, molecular size (220 A) by electron microscopy, and chemical composition closely resembled those of man. The antigenic structures of chimpanzee and human LDL were essentially indistinguishable, since immunodiffusion against antiserum to either the human or ape lipoprotein produced a precipitin reaction of complete identity between the two antigens. By micro-immunoprecipitation, the immunological cross-reactivity of LDL from the two species was in the range 85-97%, depending on the nature of the assay.

Plasma lipoprotein and apolipoprotein profile in alcoholic patients with and without liver disease: on the relative roles of alcohol and liver injury

In the present study, we report on alterations in plasma lipid, lipoprotein and apolipoprotein patterns in three separate populations of alcoholic patients, one without liver damage (Group I), a second presenting steatosis or mild alcoholic hepatitis or both (Group II) and a third with alcoholic cirrhosis (Group III), using a healthy, normolipidemic, nonalcoholic group as controls (Group C). Total plasma cholesterol levels were elevated in Groups II and III when compared with Groups I and C, while the ratio of esterified to free cholesterol was considerably lower in Group III than in the other groups. Plasma apo-AI levels were higher in Groups I and II than in Group C, but varied over a wide range in Group III. Apo-AII was present at higher concentrations in Groups I and II than in both Groups III and C. In contrast, no significant differences were detected in total apo-B levels, irrespective of the group. Modifications in the chemical composition of plasma lipoproteins primarily concerned a reduction in the cholesteryl ester content of low-density lipoproteins (LDL) and high-density lipoprotein (HDL) in Group III, this being compensated by a reciprocal increase in triglyceride. In addition, Group III lipoproteins, with the exception of HDL3 (density 1.100 to 1.140 gm per ml), exhibited a greater content of phospholipids than those of corresponding density from patients in Groups I and II. No significant differences were found in very low-density lipoprotein concentrations, while LDL levels increased in parallel with the severity of liver injury. In Groups I and II, HDL2 concentrations were elevated relative to Group C, while HDL3 decreased in parallel with the degree of impairment of liver function and thus from Group C to Group III.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma lipoprotein profile in fasted and refed chickens of two strains selected for high or low adiposity

The plasma lipoprotein profile has been determined in fasted and refed 5-week-old male broilers selected for low or high adiposity. Lipoprotein classes were subfractionated by density gradient ultracentrifugation, appearing as distinct bands with the following densities: very low density lipoprotein (VLDL), d less than 1.013 g/ml; low density lipoprotein (LDL), d 1.023-1.046 g/ml and high density lipoprotein (HDL), d 1.052-1.130 g/ml; the physiochemical characteristics (chemical composition, electrophoretic mobility and particle size) of these particles were then assessed. HDL, seen as a single band, represented 80% of total lipoproteins, with VLDL and LDL accounting for 1% and 16%, respectively, in fasted birds. Lipoprotein profiles were similar in fasted and refed animals of both lines, with the exception that VLDL levels were some 14-fold and 7-fold higher in the lean and fat lines, respectively, in the refed state. The VLDL of fasted birds of both lines were enriched in protein and relatively homogeneous in size; by contrast, VLDL in the refed state contained more triglyceride and less cholesteryl ester and protein and were larger and more heterogeneous, possibly representing a mixture of portomicrons and VLDL of hepatic origin. Birds of the fat line in both nutritional states differed from lean birds in exhibiting elevated plasma lipid and lipoprotein [VLDL, intermediate density lipoprotein (IDL) and HDL] levels, evidence that liver activity is directed toward increased lipoprotein production and secretion in that line.

Isolation, characterization and comparative aspects of the major serum apolipoproteins, B-100 and AI, in the common marmoset, Callithrix jacchus

The two major apolipoproteins of marmoset serum have been isolated and characterized, and on the basis of physicochemical and immunological criteria are homologous with the human AI and B-100 proteins. Marmoset apolipoprotein AI was the principal protein of high-density lipoproteins (HDL) and was purified by gel filtration chromatography and electrophoresis in alkaline-urea polyacrylamide gel followed by electrophoretic elution. Purified marmoset apolipoprotein AI displayed an Mr of approx. 27000, was polymorphic (five forms) on isoelectric focussing, with pI values in the range 4.8-5.0, and migrated similarly to human apolipoprotein AI in alkaline-urea gels. An overall resemblance was seen in the amino acid composition of marmoset apolipoprotein AI and that of its human counterpart with the notable exception that marmoset AI contained 1 isoleucine residue/mole. An immunological reaction of partial identity between the human and monkey proteins was seen upon immunodiffusion of their HDLs against antiserum to human apolipoprotein AI. Marmoset B-100 was the predominant apoprotein of VLDL and LDL, resembling the human protein in its elution profile on gel filtration chromatography in anionic detergent, and in its high apparent Mr (approx. 520000). The marmoset and human B-100 proteins were alike in amino acid composition and carbohydrate content. Moreover, their immunological behaviour with an antiserum to marmoset apolipoprotein B showed them to share certain antigenic determinant(s). We conclude that the physicochemical properties of the principle apolipoproteins of Callithrix jacchus, a New World primate, markedly resemble those of the human AI and B-100 proteins, suggesting therefore that they may function similarly in lipid transport and metabolism. Counterparts to human apolipoproteins AII, E, CII and CIII have also been tentatively identified.

Spur cells in patients with alcoholic liver cirrhosis are associated with reduced plasma levels of apoA-II, HDL3, and LDL

The precise nature and origin(s) of the abnormalities in lipoprotein and apolipoprotein profile associated with severe hepatic dysfunction and the presence of spur cells remain poorly defined. To shed light on this question, we have analyzed the plasma lipoprotein and apolipoprotein profiles in five patients with alcoholic cirrhosis and spur cells, and compared them with those of a group with similar hepatocellular dysfunction, but lacking spur cells, and with that of a control group. Lipoproteins were subfractionated by density gradient ultracentrifugation and their physicochemical properties were determined; apolipoprotein A-I, A-II, and B contents in plasma and the respective subfractions were quantitated by radial immunodiffusion, while the complement of low molecular weight apolipoproteins in each subfraction was analyzed by isoelectric focusing and electrophoresis in alkaline-urea polyacrylamide gels. Spur cell plasma was distinguished by reduced levels of apoA-II and elevated ratios of apoA-I/apoA-II (approximately 13:1 as compared to 3.3-3.9:1 in the other two groups), and by reduced concentrations of HDL3. Gradient fractionation showed the apoA-II content of HDL3 to be dramatically and significantly diminished in spur cell plasma; in addition, apoA-II content was reduced relative to apoA-I in this subclass (4.7:1 as compared to 1:1 in cirrhotics lacking spur cells and 1.9:1 in controls). Spur cell HDL2 was similarly deficient in apoA-II, with elevated ratios of apoA-I:apoA-II (9.8:1 in comparison with 1.9-2.5:1 in the two other groups). Nonetheless, high HDL2 concentrations were seen in both series of cirrhotic patients, irrespective of red cell morphology. Spur cell HDL2 thus appears to consist primarily of particles possessing only apoA-I, with a minor population containing both apoA-I and apoA-II. The free cholesterol content of all lipoprotein subfractions from spur cell plasma was increased, as indeed was the molar ratio of free cholesterol to phospholipid, in comparison with that of corresponding fractions from alcoholic cirrhotics lacking spur cells and of control subjects. LDL levels were reduced in spur cell plasma, thereby distinguishing this group from the cirrhotics without spur cells who displayed elevated LDL levels. Markedly reduced plasma levels of apoA-II, HDL3, and LDL appear characteristic of alcoholic cirrhotics presenting with spur cells. Our findings suggest that apoA-II may be essential to the normal function and metabolism of HDL, one aspect of which may be the transport of free cholesterol and thereby the direct or indirect maintenance of red cell morphology.

Secondary structure and thermal behavior of trypsin-treated low-density lipoproteins from human serum, studied by circular dichroism

Low-density lipoproteins (LDL) were prepared from the serum of normolipidemic men on normal diets with or without supplemental beta-carotene. LDL were subjected to limited hydrolysis (5 h at 37 degrees C) with trypsin (enzyme:protein, 1:40 w/w), and their digested products separated by gel filtration. The trypsin-treated LDL contained about 80% of the original protein and essentially all of the original lipids of native LDL. The circular dichroic spectrum of trypsin-treated LDL below 240 nm resembled that of native LDL, except that the magnitudes of the ellipticity were smaller, corresponding to 25 and 33% helical content, respectively. The lower content of helix in trypsin-treated LDL suggests that certain helical regions in apolipoprotein B are sensitive to tryptic attack; however, a major portion of the helical structure of the apolipoprotein is resistant. The thermal stability of helix in trypsin-treated LDL resembled that of native LDL, suggesting that removal of the trypsin-accessible regions of the apolipoprotein has little or no effect on the forces stabilizing the remaining helices. Data on the induced circular dichroism of beta-carotene, an intrinsic probe of the neutral lipid core, showed a reduced transition temperature for cholesteryl esters after trypsin treatment. This finding suggests that the trypsin-accessible regions of apolipoprotein B may influence the fluidity of the core.

Distribution and characterization of the serum lipoproteins and apoproteins in the mouse, Mus musculus

Murine lipoproteins were separated into nine subfractions by a density gradient ultracentrifugal procedure. They were characterized by electrophoretic, immunological, chemical, and morphological analyses, and their protein moieties were defined according to charge, molecular weight, and isoelectric point. HDL predominated (approximately 500 mg/dl serum), the mode of its distribution being situated in the d 1.09-1.10 g/ml (F 1.21 approximately 4) region. Chemical analysis showed subfractions of d 1.085-1.136 g/ml to resemble human HDL3 closely, including the presence of apoA-I (Mr 25,000-27,000) as their major apolipoprotein. An apoA-II-like protein, of Mr 8400 (in monomeric form), was also tentatively identified. In electrophoretic mobility and chemical composition, the d 1.060-1.085 g/ml subfraction (approximately 10% of total HDL) was distinct and akin to human HDL2. ApoA-I represented approximately 60% of its complement of low molecular weight apoproteins. The density range used for separation of human HDL2 (d 1.066-1.100 g/ml) by gradient ultracentrifugation is inadequate in the mouse, and the d 1.060-1.085 g/ml interval is more appropriate. The 1.063 g/ml boundary for separation of mouse LDL from HDL was unsuitable. Immunological and electrophoretic studies revealed that alpha-migrating lipoproteins were present in the d 1.046-1.060 g/ml range, a finding consistent with their enrichment in apoA-I; apoE-, apoA-II-, and apoC-like proteins were also detected. These findings indicate the presence of HDL1 particles. Murine apoA-I and apoB-like proteins of higher (apoBH) and lower (apoBL) molecular weight were constituents of the d 1.033-1.046 g/ml fraction. Alternative techniques, such as electrophoresis in starch block, are therefore a prequisite for separation of apoB from alpha-migrating, apoA-I-containing lipoproteins in the low density range in mouse serum. The LDL class (d 1.023-1.060 g/ml) amounted to only approximately 20% of the total murine lipoproteins of d less than 1.188 g/ml (65-70 mg/dl serum). Particles were richer In triglyceride, larger in diameter (mean 244 A), and more heterogeneous than typical of man. VLDL (40-80 mg/dl serum) was triglyceride-rich (66% by weight) and similarly heterogeneous in size (mean diameter 494 A; range 270-750 A). ApoBH and apoBL were prominent in murine VLDL, and cross-reacted with an antiserum to human apoB. ApoE- and apoA-I-like proteins were also detectable in apoVLDL, as was a protein of 70,000-75,000 mol wt. The presence of murine apolipoproteins analogous to human apoB and apoE was confirmed by the immunological cross-reactivities of VLDL and LDL with monospecific antisera to the human proteins. The marked similarity of lipoprotein and apolipoprotein profile in the mouse and rat is notable. Since murine VLDL contains apoE and apoBL, this resemblance may extend to the metabolism of chylomicron remnants and hepatic VLDL in the two species.

Density distribution, characterization and comparative aspects of plasma lipoproteins in the salamander, Pleurodeles waltii

Blood plasma from a Urodele amphibian, Pleurodeles waltii, has been found to contain very-low density, low-density and high-density lipoproteins (VLDL, LDL and HDL, respectively). HDL and LDL predominated (concentration range 72-114 and 51-101 mg/dl) with lesser quantities of VLDL (23-51 mg/dl). Following isolation by density gradient ultracentrifugation, the physicochemical properties (morphology, particle size, hydrated density, electrophoretic mobility, and chemical composition) of the three major classes resembled, but were not identical with, those of the corresponding fractions in normal human plasma. The protein moieties of VLDL and LDL consisted mainly of polypeptides of high molecular weight (Mr greater than 60,000), which in their solubility were akin to human apo-B; smaller amounts of another protein migrated to the position of human apo-C-III in basic polyacrylamide gels. The major HDL apolipoproteins were of Mr 31,000 and 27,000, with minor amounts of 13,000, 61,000 and 76,000 components; in alkaline, urea-containing polyacrylamide gels, these major proteins migrated faster than the major components (apo-AI and apo-AII) of human HDL. Immunochemically, lipoproteins with pre-beta-, beta- and alpha-mobility were detected in the VLDL, LDL and HDL, respectively. Salamander VLDL and LDL were found to share an antigenic site(s) with the LDL of trout, chicken and guinea-pig, suggesting the presence of an apo-B-like protein. The chemical compositions and immunochemical reactions of salamander lipoproteins indicate that their structure and metabolism resemble that of other amphibia, reptiles and fish rather more closely than that of most mammals and birds.

A simplified and efficient procedure for the purification of apolipoprotein AI from human serum high-density lipoprotein-3 by preparative isoelectric focussing on polyacrylamide gel beads

An improved method is described for the purification of milligram amounts of apolipoprotein AI from serum apo-HDL3 by isoelectric focussing on polyacrylamide gel beads. The procedure involves a single focussing over a narrow (1.3 unit) pH gradient, and permits isolation of apo-AI of exceptional purity and in high yield (75% recovery of HDL3 protein, ca. 50% corresponding to pure apo-AI). The electrophoretic mobility, pI values, molecular weight, antigenicity and amino acid composition of such apo-AI were indistinguishable from those reported in the literature. A rabbit antiserum to apo-AI isolated by focussing exhibited similar immunological reactivity to one prepared from an antigen isolated by gel filtration chromatography; moreover, apo-AI purified by the respective procedures reacted identically with both antisera. We conclude that isoelectric focussing on a support of polyacrylamide gel beads (as Bio-Gel P60) presents certain advantages for the isolation of highly purified apo-AI over both conventional chromatographic procedures and isoelectric focussing on a Sephadex support.

Tryptic modification of human serum low-density lipoprotein leads to altered rates of turnover and intravascular transformation in the guinea pig

The metabolism of human serum low-density lipoprotein (LDL) and its trypsin-treated counterpart have been compared in the guinea pig in vivo. Removal of surface-exposed protein from the lipoprotein particle in this way resulted in significant modification of its metabolism in guinea pigs in vivo. Limited trypsinisation of LDL permitted removal of 20-25% of its protein moiety; trypsinised LDL was deficient in lysine and arginine residues (25-30% of each removed). The modified particle retained its basic structural features, such as internal molecular architecture, but displayed an elevated net negative surface charge and diminished immunological reactivity. Following intravascular injection of iodinated LDL (131I) and trypsinised LDL (125I) into the same animal, the two lipoproteins displayed biexponential decays; the rate constants for the plasma turnover of LDL and trypsinised LDL were significantly different (P less than 0.05), trypsinised LDL exhibiting a slower disappearance from the circulation. Density-gradient ultracentrifugation revealed marked elevation in the modal densities of both LDL and trypsinised LDL upon metabolism in vivo, although the rate of increase was greater for trypsinised LDL than LDL in each case (average increment 0.022 g/ml and 0.014 g/ml at 24 h respectively). The diminished plasma clearance of trypsinised LDL as compared to the native human and guinea pig LDL indicates that sites required for the cellular recognition and uptake of the LDL particle reside in its surface-exposed, trypsin-accessible protein. Furthermore, such protein appears to play a central role in regulating the intravascular processes by which the lipid content of LDL is diminished, and by which it is transformed to a particle of higher density.

Role of the carbohydrate moiety in the antigenic site(s) of human serum low-density lipoprotein

Radioimmunoassay techniques have been used to evaluate the contribution of the carbohydrate moiety to the immunological reactivity of human serum low-density lipoprotein (LDL). Low-density lipoprotein (d = 1.024--1.045 g/mL) was isolated from normolipidemic serum by ultracentrifugal flotation. Radioimmunoassay was performed with 125I-labeled LDL and several homologous antisera, each corresponding to different periods (1--18 weeks) of immunization and thus containing various antibody populations. Unlabeled LDL and different monosaccharides characteristic to this particle, i.e., mannose, sialic acid, glucose, N-acetylglucosamine, galactose, N-acetylgalactosamine, and fucose, were used as competitors in the binding of the labeled antigen with antibody. In the reaction with antisera corresponding to the highest antibody titer, unlabeled LDL, sialic acid, and mannose inhibited the binding of labeled LDL up to 62%, 25%, and 16%, respectively; a low degree of inhibition (some 13%) was occasionally obtained with glucose. Galactose, galactosamine, glucosamine, and fucose failed to compete with labeled LDL. Studies with antisera corresponding to different periods of immunization (2, 4, and 8 weeks) indicated that antibodies reacting with mannose appeared early (maximum 31% inhibition at 2 weeks), disappearing at 6--8 weeks; in contrast, antibodies reacting with sialic acid augmented progressively (10% inhibition at 2 weeks, 20% at 4 weeks, and 35% at the end of the immunization). These data are consistent with the conclusion that sialic acid and mannose, the terminal residues of LDL glycopeptides I and II [Swaminathan, N., & Aladjem, F. (1976) Biochemistry 15, 1516--1621], are implicated in the antigenic site(s) of LDL.

A density gradient ultracentrifugal procedure for the isolation of the major lipoprotein classes from human serum

A density gradient ultracentrifugal procedure is described for the rapid and reproducible isolation of the major lipoprotein classes, VLDL, LDL, HDL2, and HDL3, from human serum. A step gradient is constructed from four NaCl/KBr solutions varying in density from 1.006 to 1.24 g/ml and from 3 ml of serum adjusted to d 1.21 g/ml. Separation is achieved after a single ultracentrifugation for some 56 x 10(7) gavg min at 15 degrees C in a swinging bucket rotor, at which time the lipoproteins band isopycnically and albumin and other serum proteins are sedimented. Densitometric scanning of gradients revealed a lipoprotein mass profile distinguished by four absorption maxima which fell within the hydrated density ranges of VLDL (d less than 1.016 g/ml), LDL (1.028-1.050 g/ml), HDL2 (1.066-1.100 g/ml), and HDL3 (1.100-1.153 g/ml). Fractionation of gradients on the basis of band distribution, followed by chemical, physical, and immunological analyses of the four principal fractions (i.e., bands) provided data on their electrophoretic mobility, chemical composition, morphology and size distribution, immunological reactivity and apolipoprotein content, thereby confirming their identities as VLDL, LDL, HDL2, and HDL3. The validity of this separation was supported by the quantitative distribution of apo B and apo A-I as assessed by radial immunodiffusion. Lipoprotein quantitation based on chemical analysis of gradient fractions was compared with that by analytical ultracentrifugation for a group of normolipidemic males; results concorded well, giving a similar HDL2:HDL3 ratio (0.35-0.36). Our procedure thus provides a simple and precise manner in which to assess the lipoprotein and apolipoprotein profile of human serum quantitatively and qualitatively.

A comparative dielectric study of human serum low density lipoprotein before and after partial digestion by trypsin

The relative permittivity of aqueous solutions of human serum low density lipoprotein (LDL) and partially trypsin digested lipoprotein (T-LDL) has been determined for various concentrations at 20 degrees C over the frequency range 0.15-100 MHz. Comparison of the dielectric dispersion curves for the digested lipoprotein with those for the native preparation revealed a larger low-frequency dielectric increment, which may be attributed to an increase in the number of counterions moving over the surface of the molecule. An explanation of this observation is an elevation of 70% in the net negative charge on the surface of the trypsin-treated particle as compared to its native counterpart.

Density distribution, characterization, and comparative aspects of the major serum lipoproteins in the common marmoset (Callithrix jacchus), a New World primate with potential use in lipoprotein research

Qualitative, quantitative, and comparative aspects of the serum lipoprotein profile in the Common marmoset (Callithrix jacchus), a New World primate, are described. Density gradient ultracentrifugation was used to evaluate lipoprotein distribution and to establish criteria for isolation of discrete molecular fractions. The major lipoprotein classes banded isopycnically on the gradient with the following hydrated densities: VLDL, d less than 1.017 g/mL; LDL, d = 1.027--1.055 g/mL; HDL fraction I, d = 1.070--1.127 g/mL; and HDL fraction II, d = 1.127--1.156 g/mL. Electrophoretic, immunological, and electron microscopic analyses attested to the purity of these fractions: the characteristics of each were assessed by chemical analysis, electron microscopy, immunological techniques, and polyacrylamide gel electrophoresis of their protein moieties. Marmoset VLDL and LDL were closely akin to those of man in size and chemical composition, although the former were richer in triglyceride; electrophoretic and immunological data showed the major protein component of VLDL and LDL to be a counterpart to human apo-B. The two HDL subfractions, i.e., HDL-I and HDL-II, corresponded in size and chemical composition to human HDL2 and HDL3, respectively, although slight differences in neutral lipid content were detected. By immunological and electrophoretic criteria, the major apolipoprotein of marmoset HDL was analogous to human apo-AI. In contrast, marked dissimilarities were evident in the complements of low molecular weight, tetramethylurea-soluble polypeptides of marmoset and human lipoproteins. Quantitatively, the human and marmoset lipoprotein profiles were not dissimilar, although HDL was the major class (approximately 50%); in fasting animals, serum concentrations of VLDL, LDL, and HDL were 50--90, 170--280, and 338--408 mg/dL, respectively. C. jacchus was distinct from man in displaying a greater proportion of its total HDL in the less dense (HDL-II) subfraction (marmoset HDL-I/HDL-II = approximately 4:1; human HDL2/HDL3 = approximately 1:3). These data indicate that, as an experimental animal for lipoprotein research, the Common marmoset combines the advantages of ready availability and maintenance with a serum lipoprotein profile which resembles, in many qualitative and quantitative aspects, that found in man.

Stimulation of triacylglycerol synthesis in rat adipocytes by plasma very-low-density lipoproteins

The effect of human plasma lipoproteins on lipogenesis from glucose has been studied in isolated rat adipocytes. The very-low-density lipoproteins increased lipogenesis specifically, whereas low-density lipoproteins and high-density lipoproteins were without effect. Such stimulation could be reproduced with partially delipidated very-low-density lipoproteins. Nod-esterified fatty acids and glycerol were also without effect. Pretreatment of the adipocytes with trypsin did not alter the effect of very-low-density lipoprotein. The presence of Ca2+ was required for the full activation of lipogenesis. The synthesis of acylglycerol fatty acids and of acylglycerol glycerol were equally increased. The effect of very-low-density lipoprotein was not additive to that of insulin. It is suggested that very-low-density lipoprotein may directly stimulate lipogenesis in fat-cells, particularly in states when the lipoproteins are present at high concentration in the circulation.

Limited tryptic digestion of human serum low-density lipoprotein: isolation and characterisation of the protein-deficient particle and of its apoprotein

Limited tryptic digestion of human serum low-density (LD) lipoprotein (rho 1.024-1.045 g/ml) under defined conditions permitted isolation by gel filtration chromatography of a stable, protein-deficient lipoprotein; the liberated protein was separated as a mixture of peptides of low molecular weight (less than 5000). Comparison of the chemical, physical and immunological characteristics of the trypsin-treated LD-lipoprotein with those of the native preparation revealed several differences, including (a) a diminished protein content (loss of some 20-25% of the total protein of LD-lipoprotein) and increased proportions of the various lipid components, except for triglyceride (probably resulting from a loss of bound free fatty acids with the liberated peptides); (b) a greater heterogeneity in particle size and slightly larger mean diameter; (c) a lower hydrated density and greater peak sf rate than the native LD-lipoprotein (d) an increased net negative charge; and (e) a partial immunological identity between LD-lipoprotein and the corresponding trypsin-treated fraction. While the amino acid compositions of the protein moieties of LD-lipoprotein and of trypsin-treated LD-lipoprotein were essentially identical, trypsin-treated apo-LD-lipoprotein was distinct in its complete solubility in urea-containing buffers at high concentrations, and also in its partial solubility in buffers lacking denaturing agents. Comparison of the apoproteins of the native and trypsin-treated LD-lipoproteins by electrophoretic techniques based on molecular weight revealed a transformation of the high-molecular weight material (greater than 250 000) characteristic of apo-LD lipoprotein into several polypeptide species (10 major forms) ranging in size from 161 500 to about 10 000. The largest of these (band b1: 161 500) could be completely dissociated into smaller components (b2: 93 500 and b3: 77 000) upon extensive heat treatment at 90 degrees C. Electrophoresis of the soluble fraction of apo-LD-lipoprotein and of that from its trypsin-treated counterpart in polyacrylamide gels containing urea at basic pH showed the disappearance of the small amounts (less than 5%) of C apoproteins of apo-LD-lipoprotein upon tryptic treatment. These results, which were highly reproducible in LD-lipoprotein preparations from different individuals, suggest that trypsin-treated LD-lipoprotein may provide a model for investigation of the organisation and structural role of the prinicipal apoprotein (apolipoprotein-B) in the LD-lipoprotein molecule.

Enhanced degradation of trypsin-treated low density lipoprotein by fibroblasts from a patient with homozygous familial hypercholesterolemia

When 125I-labeled native low density lipoprotein was incubated with skin fibroblasts from a patient with homozygous familial hypercholesterolemia, the observed rate of degradation of the protein moiety was less than 5% the rate observed with normal fibroblasts, in agreement with previous studies. When the low density lipoprotein had been first treated with trypsin, with release of about 20% of the protein, its degradation by the patient's fibroblasts was markedly increased 8-20-fold. In contrast, the rate of degradation of the trypsin-treated lipoprotein by normal fibroblasts was, if anything, slightly reduced. In neither the normal cells nor the patient's cells was binding to the cell surface appreciably altered by trypsin treatment of the lipoprotein. Prior incubation with cholesterol and 7-ketocholesterol reduced binding of trypsin-treated low density lipoprotein to normal cells by 67% but did not affect its binding to the patient's cells. The results show that the structural modifications induced by trypsin do not interfere with binding of low density lipoprotein to its normal high affinity receptor nor its degradation by normal cells. However, the modified lipoprotein is much more readily internalized and degraded by cells from the patient with homozygous familial hypercholesterolemia.

Characterization of the serum lipoproteins and their apoproteins in hypercholesterolaemic guinea pigs

1. Hypercholesterolaemia was induced in male guinea pigs after 6 days on a chow diet supplemented with 1.6% (w/w) cholesterol and 15% (w/w) corn oil. Both the VLD (very-low-density) and LD (low-density) lipoproteins were increased in cholesterol-fed animals, although the low concentrations of HD (high-density) lipoproteins remained essentially unchanged. LD lipoproteins of d 1.019-1.100 were the major class, accounting for 74% of the total substances of d less than 1.100. 2. Both VLD and LD lipoproteins exhibited alterations in their chemical composition, physical properties and apolipoprotein content. The VLD lipoproteins in cholesterolaemic animals were rich in cholesterol (25.9%), deficient in protein (4.9%) and exhibited electrophoretic mobility greater than that of beta-globulin; their average particle size (64.5 nm) was larger than that in controls (46.3 nm). The LD lipoproteins in animals fed on the experimental diet were also richer in cholesterol (53.1%) and of larger diameter (24.3 nm) than in the control group (41.1% and 21.4 nm respectively). 3. The apolipoprotein-B content of both VLD and LD lipoproteins was elevated in cholesterolaemic animals, particularly in the VLD class, where it represented 74.8% of the total protein moiety. 4. Apo-VLD lipoprotein exhibited an increase from 6 to 19% in its complement of tetramethylurea-soluble apolipoproteins with low electrophoretic mobility (relative mobility less than 0.29); this was primarily accounted for by apolipoproteins characterized by high arginine (7.2 and 6.4% respectively) and glutamic acid (20.1 and 20.0% respectively) contents. 5. By contrast, there was little change in the soluble apolipoproteins of LD lipoproteins in hypercholesterolaemic animals.6. These studies show the response of the guinea pig to dietary fat and cholesterol to be distinct from that elicited by similar stimuli in the rabbit, rat, pig and dog.

Biochemical and immunological evidence for the presence of an apolipoprotein B-like component in the serum low-density lipoproteins of several animal species

The major component of the protein moiety of human LDL, i.e. apolipoprotein B, has been compared biochemically and immunologically with its counterpart in the LDL of several groups of animals (mammals, birds, snakes and fish). A marked resemblance was found in the amino acid composition of the apo-B fractions from all the phylogenetic groups, although immunological cross-reactivity with human apolipoprotein B occurred only in the case of non-human primate (Old World monkey), non-primate mammalian (pig and guinea pig) and bird (chicken) apo-B components (63%, 24% and about 8% respectively). The cross-reactivity of each animal apo-B component with its human counterpart was 7-14% lower than that observed between the parent LDL's. The resemblance in amino acid composition between the various apo-B preparations suggests that certain structural characteristics are required in this protein in order for it to bind and stabilise the lipid complement of serum LDL.

Characterization of serum lipoproteins of the shark Centrophorus squamous

1. Blood serum from the shark Centrophorus squamosus (Bonnaterre) was shown to contain VLD (very-low-density), LD (low-density) and HD (high-density) lipoproteins. 2. In shape, size and general physical properties, these lipoproteins were very similar to those described for other animals. The VLD lipoproteins were the major components of the mixture, and HD lipoproteins were present at the lowest amount. 3. In addition to the usual lipid components, the shark lipoproteins also contain substantial amounts of hydrocarbon, probably mainly squalene, and monoalkyldiacylglycerols. Only trace amounts of wax ester were detected. 4. The protein moiety of the VLD and LD lipoproteins contained a component which, in its solubility and electrophoretic properties, molecular weight and amino acid composition, resembled the B apolipoprotein of man and other mammals. This accounted for a large part of the total shark apolipoprotein. 5. There were also present smaller amounts of proteins which were soluble in 8M-urea. In their electrophoretic mobility on basic polyacrylamide gel, some of these were like the A and C apoproteins of man. 6. The electrophoretic distribution of the soluble proteins from the VLD and LD lipoproteins resembled that in higher mammals, but in the HD lipoproteins the similarity was less.

Comparative immunochemical studies of the serum low-density lipoprotein in several animal species

The antigenic reactivities of the serum LDLs of several groups of animals (mammals, birds, snakes, and fish) have been compared by qualitative and quantitative immunological techniques. The cross-reactivities of the various LDLs were evaluated qualitatively by gel diffusion techniques and quantitatively by the precipitation and inhibition of precipitation tests. The degrees of cross-reaction between human and animal LDLs ranged from 80-88% in the Old World monkeys to 36-58% in the pig, 26-37% in the guinea pig, 1-10% in the chicken, and about 1% in the snakes and fish. These antigenic relationships are consistent with the taxonomic classification of these species, but are relatively closer than those observed for other serum proteins (e.g., albumin, transferrin, and hemoglobin). These results indicate a relatively high conservation of LDL structure through evolution, and are consistent with a common function for this lipoprotein in the transport and metabolism of cholesterol in the higher animals.

Comparison of the serum low density lipoprotein and of its apoprotein in the pig, rhesus monkey and baboon with that in man

The principal form of the serum low density lipoprotein (LDL) in man, baboon, rhesus monkey and pig was isolated by preparative ultracentrifugation in the density interval 1.024-1.045 g/ml. The physicochemical characteristics of pig LDL most closely resembled those of man; thus, electrophoretic studies suggested that both baboon and rhesus LDL have a greater surface charge than that of their human counterpart, and electron-microscopic investigations showed baboon LDL (245 A) to be larger and rhesus LDL (205 A) smaller than those of man (217 A) and pig (228 A). In contrast, the immunological relationship between LDL from the two Old World monkeys and that of man was much closer (80-85% cross-reactivity by micro-immunoprecipitation) than that between pig and man (35% cross-reactivity). The principal difference between pig and human LDL appeared to reside in their protein and carbohydrate moieties. There was a marked resemblance between the protein moieties (apo-LDL) of LDL from the four species. The principal component of each animal apo-LDL was separated by gel-filtration chromatography and amounted to greater than 95% of the total protein; it exhibited a high molecular weight (greater than 250,000) upon SDS-polyacrylamide-gel electrophoresis and was indistinguishable from human apolipoprotein B in amino acid composition. Differences both between the apo-LDL and between the apo-B preparations from the four species, however, were detectable by immunological procedures. Such studies revealed inter-species relationships which were essentially the same as those observed between the respective native LDL preparations. The soluble apolipoproteins, present as minor components (less than 5%) of each apo-LDL, were compared by their electrophoretic mobility in polyacrylamide gel; the pattern seen in baboon and rhesus apo-LDL appeared to be most closely akin to that typical of their human counterpart. It is apparent that many characteristics typical of human serum LDL are found in those of the pig, rhesus monkey and baboon. Moreover, in view of the striking relationship existing between the immunological properties and apo-protein components of the LDL of the two Old World monkeys and that of man, these subhuman primates appear to be highly suitable as animal models for experimental atherosclerosis.

Low-density lipoproteins in patients homozygous for familial hyperbetalipoproteinaemia

1. The low-density lipoproteins (LDL; density 1-007-1-063 g/ml) from two patients homozygous for familial hyperbetalipoproteinaemia have been submitted to chemical and physicochemical analysis. 2. The presence of an anomalous lipoprotein with a low proportion of triglyceride and a raised proportion of cholesterol has been confirmed. 3. In one patient, this lipoprotein accounted for about 85% of the LDL, but in the second, the amount varied from about 85% to a point at which it could not be detected among the coexisting normal lipoproteins. 4. The protein moiety of this anomalous LDL has effectively the same amino acid composition as that derived from the LDL of healthy subjects. 5. The proportions of carbohydrate, phospholipid and fatty acids could not be reliably distinguished from those of normal LDL. 6. The molecular weight and diffusion constant of the abnormal lipoprotein, even in the purest preparation, were close to the values determined for normal LDL of similar flotation rate.

The distribution and partial characterization of the serum apolipoproteins in the guinea pig

1. Very-low-density (VLD), low-density (LD) and high-density (HD) lipoproteins were isolated by sequential ultracentrifugation from the serum of male guinea pigs fed on a diet containing 3--4% fat. The apoproteins of these lipoproteins (apo-VLD, apo-LD and apo-HD lipoproteins) were studied after delipidation with organic solvents or extraction with tetramethylurea. 2. The major apolipoprotein of LD lipoprotein isolated by gel filtration was found to closely resemble apolipoprotein B of human serum in its chemical and physical properties. Electrophoresis in sodium dodecyl sulphate-polyacrylamide gel showed that this apoprotein consisted of a number of polypeptides. 3. Tetramethylurea precipitated an apoprotein from guinea-pig serum lipoproteins that is probably the apolipoprotein B-like component. This apoprotein accounted for about 80% of the apo-LD lipoprotein, about 55% of the apo-VLD lipoprotein and about 50% of the apo-HD lipoprotein. 4. The distribution of apolipoproteins soluble in tetramethylurea was determined by densitometric scanning of stained polyacrylamide disc gels. 5. A glycine-rich component of high electrophoretic mobility (band I) and a triplet of soluble apolipoproteins (bands II-IV) were present in both VLD and LD lipoprotein classes. These components constituted a higher proportion of the tetramethylurea-soluble apoproteins of VLD lipoprotein (60--80%) than of LD lipoprotein (40--55%). 6. Small amounts (10--15%) of a component of intermediate mobility, which contained traces of half-cystine, were also present in both VLD and LD lipoproteins. 7. A group of soluble components of basic character (bands VI-X), present as minor components of VLD lipoprotein (10--20%), constituted a major proportion (30--45%) of the soluble apoproteins of LD lipoprotein. Two of these apoproteins were rich in lysine, and two of lower electrophoretic mobility were rich in arginine. 8. The pattern of tetramethylurea-soluble apoproteins in HD lipoprotein was distinguished by the presence of two polypeptides of low electrophoretic mobility as its predominant components. One of these components, band VI, resembled the A-I apolipoprotein of man in both its amino acid profile and in its electrophoretic mobility. The second major component, band VI-B, was rich in lysine and resembled the C-I apolipoprotein of man in amino acid composition. 9. The soluble components of bands I and IX were analogous in physicochemical properties to the R-X1 and R-X2 (high-arginine polypeptide) peptides of human serum lipoproteins respectively.

Localization of the enzymes of ketogenesis in rat liver mitochondria

The localization of the enzymes of ketogenesis in isolated rat liver mitochondria has been investigated. Mitochondrial subfractions were isolated after disruption of this subcellular organelle by (a) hypotonic lysis in water, which permitted the ultracentrifugal separation of the soluble and membranous compartments of the mitochondrion, or by (b) a procedure involving swelling, contraction, and ultrasonic treatment, which permitted the isolation from discontinuous sucrose gradients of subfractions rich in intermembrane space protein, outer membrane, and inner membrane-matrix particles. Two membrane subfractions were invariably present as distinct bands at the lower interface of the discontinuous gradient. The upper of these two bands was found to be a highly purified preparation of outer mitochondrial membrane. Subfractions rich in matrix and in inner membrane were isolated from inner membrane-matrix particles after hypotonic treatment. The content of the various mitochondrial compartments in all subfractions was assessed from their enzymic and electron microscopic characteristics. The ketogenic activity of each subfraction was determined by measuring its capacity to form ketone bodies from acetyl CoA. The activity of this process was markedly enhanced by dithiothreitol. These measurements of ketone body formation, together with assays of individual enzymes of the ketogenic pathway, show that thiolase, HMGCoA synthase, and HMGCoA cleavage enzyme are localized in the matrix of the inner membrane-matrix particles. The rates of ketone body formation indicate that the HMGCoA synthase is the rate-limiting enzyme of the pathway in subfractions of high matrix content. Studies with sodium chloride indicate that a large portion of the HMGCoA synthase, which remains present in membrane subfractions derived from water-treated mitochondria, is bound by ionic interaction to component(s) of the membrane.

The effect of a lipid-rich diet on the properties and composition of lipoprotein particles from the Golgi apparatus of guinea-pig liver

1. A cell fraction rich in Golgi apparatus was isolated from the livers of guinea pigs fed on a lipid-rich diet (1.6% cholesterol, 15% corn oil). 2. The Golgi cisternae and secretory vesicles contained electron-dense particles which were tentatively identified as VLD (very-low-density) and LD (low-density) lipoproteins. Particles of moderate electron density, 150-500nm in diameter, were seen associated with membranous elements of the Golgi-apparatus cell fraction. Disruption of this cell fraction permitted the release of these three species of particles, which were separated into particulate lipid, and VLD and LD lipoproteins. 3. The large particles of moderate electron density, isolated as particulate lipid, were distinct from both species of Golgi particles in their chemical composition and in possessing an immunochemically unreactive apolipoprotein(s). Morphological observations suggest that the particulate lipid arose from cytoplasmic lipid droplets which were present as contaminants of the Golgi-rich fraction. 4. The chemical and immunochemical results are consistent with the suggestion that the Golgi LD particles are precursors of the VLD particles, into which they may be transformed by the addition of both triglyceride and cholesteryl ester. The present results provide further support for the proposal that the Golgi VLD particles are precursors of the serum VLD lipoproteins in the guinea pig. 5. Hepatic Golgi VLD particles isolated from guinea pigs fed on the lipid-rich diet contained significantly higher molar amounts (relative to protein) of both cholesteryl ester and triglyceride than similar particles from animals fed on a normal diet. These results suggest that the type of Golgi VLD particle produced from the LD particle is a direct consequence of the amount and composition of the dietary lipid. 6. Hepatic Golgi LD particles isolated from guinea pigs fed on different diets were similar in chemical composition and contained approx. 50% by weight of phospholipid. We conclude that the Golgi LD particle is normally present in the Golgi-apparatus cell fraction from guinea-pig liver, and may represent the end product of lipoprotein biosynthesis in the smooth endoplasmic reticulum. 7. The serum LD lipoproteins and Golgi LD particles were quite distinct in chemical composition. However, these two lipoprotein species were immunochemically identical and exhibited a similar range of flotation rate. It appears unlikely that the Golgi LD particles are secreted as the precursors of the serum LD lipoproteins.

Lipoprotein particles from the Golgi apparatus of guinea-pig liver

1. A cell fraction has been isolated from guinea-pig liver and shown to be rich in Golgi apparatus by electron microscopy. The activity of UDP-d-galactose-N-acetylglucosamine galactosyltransferase was over 100-fold greater in this cell fraction than in the liver homogenate. These data support the conclusion that the fraction was enriched in Golgi apparatus. 2. The Golgi cisternae and secretory vesicles contained electron-dense particles of 10-80nm diameter. Disruption of the Golgi apparatus cell fraction released these particles, which were separated into VLD (very-low-density) and LD (low-density) species on the basis of their density. 3. The Golgi VLD particles possessed morphological, flotational, chemical and immunochemical properties which closely resembled those of the serum VLD lipoproteins from the same animals. 4. The Golgi LD particles were rich in phospholipid, containing 48.1% by weight. The chemical composition of these particles was quite distinct from that of the serum LD lipoproteins, but did, however, show some similarity to that of the serum VLD lipoproteins. A marked resemblance was noted in the chemical characteristics of the Golgi LD and VLD particles (with the exception of triglyceride content). In addition, these two species of Golgi particles possessed the same antigenic determinant. 5. The results suggest that the Golgi VLD particles are the precursors of the serum VLD lipoproteins. On the basis of similarities in gross chemical composition and in the antigenic determinant of the Golgi LD and VLD particles, we conclude that the LD particles are probably the precursors of the VLD particles. In view of the marked differences in gross chemical composition of the Golgi LD particles and serum LD lipoproteins, it appears unlikely that the LD particles are directly secreted into the serum pool.