Purification of recombinant apolipoproteins A-I and A-IV and efficient affinity tag cleavage by tobacco etch virus protease.

The expression of recombinant apolipoproteins provides experimental avenues that are not possible with plasma purified protein. The ability to specifically mutate residues or delete entire regions has proven to be a valuable tool for understanding the structure and function of apolipoproteins. A common feature of many recombinant systems is an affinity tag that allows for straightforward and high-yield purification of the target protein. A specific protease can then cleave the tag and yield the native recombinant protein. However, the application of this strategy to apolipoproteins has proven somewhat problematic because of the tendency for these highly flexible proteins to be nonspecifically cleaved at undesired sites within the native protein. Although systems have been developed using a variety of proteases, many suffer from low yield and, especially, the high cost of the enzyme.We developed a method that utilizes the tobacco etch virus protease to cleave a histidine-tag from apolipoproteins A-I and A-IV expressed in Escherichia coli. This protease can be easily and inexpensively expressed within most laboratories. We found that the protease efficiently cleaved the affinity tags from both apolipoproteins without nonspecific cleavage. All structural and functional measurements showed that the proteins were equivalent to native or previously characterized protein preparations. In addition to cost-effectiveness, advantages of the tobacco etch virus protease include a short cleavage time, low reaction temperature, and easy removal using the protease's own histidine-tag.

Serum prostacyclin stabilizing factor is identical to apolipoprotein A-I (Apo A-I). A novel function of Apo A-I.

Serum PGI2 stabilizing factor (PSF) was purified from human serum to a single protein with a molecular weight of 28,000 D by SDS-PAGE. Analyses of NH2-terminal sequence (32 residues), COOH-terminal sequence (3 residues) and the composition of amino acids disclosed its homology with human apolipoprotein A-I (Apo A-I), a major apolipoprotein of HDL. Apolipoprotein A-II, C-I, C-II, C-III, D and E, as well as LDL, and VLDL did not possess this activity. The alpha-helix structure of Apo A-I is necessary for the binding of PGI2. HDL and nascent HDL reconstituted from Apo A-I and phospholipid significantly prolonged the half-life of PGI2. PGI2 stabilization by HDL and Apo A-I may be an important protective action against the accumulation of platelet thrombi at sites of vascular damage. The beneficial effect of HDL in the prevention of coronary artery disease may be partly due to this action.

Studies on the in vivo and in vitro distribution of apolipoprotein A-IV in human plasma and lymph.

To investigate the unique distribution in plasma of apolipoprotein A-IV (apo A-IV) we have determined, in a series of in vitro and in vivo studies, the redistribution among lipoproteins of 125I-apo A-IV. Free 125I-apo A-IV associated predominantly with high density lipoprotein (HDL) (72 +/- 3.5%) in incubations with plasma, and with triglyceride-rich lipoproteins (TRL) (65 +/- 3.0%) in incubations with lymph, rather than with the lipoprotein-deficient fraction (LDF) where greater than 90% of apo A-IV resides. Incubations with 125I-apo A-IV (incorporated within HDL or TRL) also resulted in similar redistributions of label. Specific radioactivities of apo A-IV in HDL and in TRL were of a similar order and 15-fold higher than those in LDF. However, when 125I-apo A-IV in LDF was incubated with plasma, 57 +/- 2.6% of label remained in the LDF, though the specific radioactivity of apo A-IV in HDL was 15-fold higher than in LDF. Thus, apo A-IV apparently exchanges freely between TRL, HDL, and a part of apo A-IV in LDF, but most of apo A-IV in LDF is refractive to free exchange or transfer. In vivo experiments carried out in five subjects, in which 125I-apo A-IV was injected within TRL, HDL, or LDF, were consistent with the in vitro data in showing rapid exchange of label among plasma apo A-IV containing fractions with much higher specific radioactivities in HDL than in LDF (10-30-fold). However, the small fraction of apo A-IV in LDF that did become labeled was removed from plasma in a biexponential fashion and at the same rate as from HDL. Thus, only a small fraction of the bulk of apo A-IV in plasma LDF exchanges freely with apo A-IV in TRL and HDL, suggesting that apo A-IV in LDF exists in at least two pools. This is consistent with our previous findings that apo A-IV in plasma is present in two distinct complexes with lipids and other peptides.

Rapid in vivo transport and catabolism of human apolipoprotein A-IV-1 and slower catabolism of the apoA-IV-2 isoprotein.

Apolipoprotein (apo) A-IV is a polymorphic, intestinally derived apolipoprotein that is genetically linked to and similar in structure to apoA-I, the major apolipoprotein in high density lipoproteins (HDL). ApoA-IV plays a potentially important role in lipoprotein metabolism and reverse cholesterol transport, but its in vivo metabolism is poorly understood. In order to gain insight into factors modulating apoA-IV metabolism in humans, the in vivo kinetics of the two major human apoA-IV isoproteins apoA-IV-1 and apoA-IV-2 were investigated in normolipidemic human subjects. 131I-apoA-IV-1 and 125I-apoA-IV-2 were reassociated with autologous plasma and injected into study subjects. Analysis of the kinetic data revealed a rapid mean fractional catabolic rate (FCR) for apoA-IV-1 of 2.42 +/- 0.11 d-1. The mean production, or transport, rate of apoA-IV-1 was 16.3 +/- 1.4 mg/kg per d. Plasma apoA-IV concentrations were highly correlated with apoA-IV production rate (r = 0.84, P < 0.001) and not correlated with apoA-IV fractional catabolic rate (r = 0.25, P = NS). The mean FCR of apoA-IV-2 was 2.21 +/- 0.10 d-1. In the ten subjects in whom 131I-apoA-IV-1 and 125I-apoA-IV-2 were simultaneously injected, the FCR of apoA-IV-2 was significantly slower by paired t test (P = 0.003). The FCR of apoA-IV-2 in an apoA-IV-2/2 homozygote was only 1.49 d-1, substantially slower than in all other subjects. We conclude that: (a) apoA-IV is a rapidly catabolized apolipoprotein in humans, with a fractional catabolic rate more than 10 times greater than that of apoA-I; (b) apoA-IV has a high absolute transport rate similar to that of apoA-I; (c) plasma levels of apoA-IV are primarily determined by apoA-IV production rate in normolipidemic subjects; and (d) the fractional catabolic rate of the common variant apoA-IV-2 is slower than that of the wild-type apoA-IV-1.

[A synthetic peptide selected by bioinformatics inhibits mouse corneal neovascularization].

OBJECTIVE: To identify the active anti-angiogenic region in the amino acid sequence of human apolipoprotein (a) [apo (a)] kringle V (KV), and to evaluate the role of this synthetic peptide on VEGF-induced angiogenesis of mouse cornea in vivo. METHODS: The characterization of the structure and biological activity of the amino acid sequence of apo (a) KV was analyzed using the bioinformatic methods which included sequence alignment, analysis of antigenicity, surface accessibility and hydrophilicity, and then a peptides was selected. The peptide was synthesized with a high efficiency solid-phase method. Corneal neovascularization was induced with a pellet containing 160 ng vascular endothelial growth factor (VEGF) in a mouse corneal micropocket model. 40 C57BL/6 mice (40 eyes) were divided randomly into 4 groups (10 eyes per group). Four kinds of pellets were made containing 160 ng VEGF plus the dose range of 0.0, 0.5, 1.0 and 1.5 microg synthetic peptide for control group, group A, group B and group C, respectively. Neovascularization was observed biomicroscopically on day 7 after the operation, and the corneas were then examined histologically. RESULTS: The result of bioinformatic analysis showed that the peptide contained a majority of conservative residues and possessed fine properties of antigenicity, surface accessibility and hydrophilicity. The synthetic peptide at the doses of 1.0 microg and 1.5 microg showed significant inhibition of mouse corneal neovascularization induced by VEGF in the parameters of vessel length, clock hours and area compared with the control group on day 7 after the operation (P < 0.01). There was no difference in the two doses (1.0 microg and 1.5 microg peptide) in the inhibition of the neovascularization. The dose of 0.5 microg peptide did not show any significant inhibition of the neovascularization compared with the control group (P > 0.05). CONCLUSIONS: The peptide, selected from the amino acid sequence of apo (a) KV by bioinformatics, appears to inhibit VEGF-induced angiogenesis in a mouse corneal micropocket assay in vivo, therefore, the study suggest that this amino acid sequence may locate at the active anti-angiogenic region of apo (a) KV.

Glycated Apolipoprotein A-IV Induces Atherogenesis in Patients With CAD in Type 2 Diabetes.

BACKGROUND: Nonenzymatic glycation of apolipoproteins plays a role in the pathogenesis of the vascular complications of diabetes. OBJECTIVES: This study investigated whether apolipoprotein (apo) A-IV was glycated in patients with type 2 diabetes mellitus (T2DM) and whether apoA-IV glycation was related to coronary artery disease (CAD). The study also determined the biological effects of glycated apoA-IV. METHODS: The authors consecutively enrolled 204 patients with T2DM without CAD (Group I), 515 patients with T2DM with CAD (Group II), and 176 healthy subjects (control group) in this study. ApoA-IV was precipitated from ultracentrifugally isolated high-density lipoprotein, and its glycation level was determined based on Western blotting densitometry (relative intensity of apoA-IV glycation). ApoA-IV NƐ-(carboxylmethyl) lysine (CML) modification sites were identified by mass spectrometry in 37 control subjects, 63 patients in Group I, and 138 patients in Group II. Saline or glycated apoA-IV (g-apoA-IV) generated by glyoxal culture was injected into apoE-/- mice to evaluate atherogenesis, and was also used for the cell experiments. RESULTS: The relative intensity and the abundance of apoA-IV glycation were associated with the presence and severity of CAD in patients with T2DM (all p < 0.05). The experiments showed that g-apoA-IV induced proinflammatory reactions in vitro and promoted atherogenesis in apoE-/- mice through the nuclear receptor NR4A3. G-apoA-IV with mutations (K-A) at high-frequency glycation sites exhibited more weakened proinflammatory and atherogenic effects than did g-apoA-IV both in vitro and in vivo. CONCLUSIONS: ApoA-IV glycation is associated with CAD severity in patients with T2DM, and g-apoA-IV induces atherogenesis through NR4A3 in apoE-/- mice.

Characterization of complexes of egg yolk phosphatidylcholine and apolipoprotein A-II prepared in the absence and presence of sodium cholate.

Complexes of apolipoprotein A-II and egg yolk phosphatidylcholine were prepared in mixtures of different composition in the absence and presence of sodium cholate. By gradient gel electrophoresis, complex preparations were polydisperse and particle size distributions were influenced by the composition of the reconstitution mixture. Complexes generally exhibited a discoidal morphology by electron microscopy, but showed increased formation of vesicular complexes at elevated levels of egg yolk PC in the mixtures. By chemical crosslinking, complexes formed in the absence of cholate were shown to consist primarily of discoidal species with three apolipoprotein A-II molecules per particle in the mixtures investigated; complexes formed in the presence of cholate included species ranging from three to five apolipoprotein A-II per particle. The number of apolipoprotein A-II per particle and the sizes of the complexes, prepared in cholate, increased with increase of egg yolk PC in the reconstitution mixture. Relative to the particle size distribution of discoidal complexes formed in the absence of cholate, those prepared in cholate showed a distribution shifted to larger particle sizes. Complexes of similar particle size distribution formed in the presence or absence of cholate showed similar physical-chemical properties. Discoidal complexes with the same number of apolipoprotein A-II per particle but of different size and composition were observed, suggesting the possibility of some conformational adaptation of apolipoprotein A-II leading to stabilization of egg yolk PC bilayers of different diameter. Properties of particle size distributions of discoidal complexes prepared in cholate of apolipoprotein A-II and egg yolk PC were compared with those of complexes of apolipoprotein A-I previously reported (Nichols, A.V., Gong, E.L., Blanche, P.J. and Forte, T.M. (1983) Biochim. Biophys. Acta 750, 353-364).

Structure of the apolipoprotein A-IV/lipid discoidal complexes: an attenuated total reflection polarized Fourier transform infrared spectroscopy study.

Discoidal lipid particles were prepared from a reaction mixture containing apo A-IV and dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) in the molar ratio of 185:1 (lipid/protein). The complexes were isolated by gel filtration and characterized in terms of composition and size. Infrared attenuated total reflection spectroscopy was used to estimate the secondary structure of apolipoprotein A-IV and the orientation of its amphipathic alpha-helices with respect to the lipid hydrocarbon chains. In addition, infrared spectra were analyzed in terms of the conformation and organization of different regions of the lipid molecules in the particles. This approach has been applied successfully to reconstituted HDL particles prepared from a reaction mixture containing DPPC and apo A-I in the molar ratio of 150:1 (Wald, J.H., Goormaghtigh, E., De Meutter, J., Ruysschaert, J.M. and Jonas, A. (1990) J. Biol. Chem. 265, 20044-20050). Apo A-IV helicity increased for the protein bound to DMPC or DPPC but the increase was more pronounced for the apo A-IV/DMPC particles. In both complexes, the alpha helical amphipathic segments of the protein were parallel to the lipid acyl chains and no significant modification of the overall organization of the lipid molecules in the lipid bilayer was observed. The presence of apo A-IV seems only to affect the conformation of the lipid hydrocarbon chains in close contact with the protein in the discoidal particles.

Amplification of human APO(a) kringle 4-37 from blood lymphocyte DNA.

We have been able to amplify the lysine binding pocket region of human apo(a) kringle type 5 starting from the DNA isolated from peripheral blood lymphocytes. This development now permits the identification of Lp(a) mutants that by lacking their ability to bind to lysine/fibrin would have a lesser thrombogenic potential.

Tangier disease apolipoprotein A-I compared with normal plasma A-I using monoclonal antibodies.

The molecular defect in Tangier disease is unknown. We have compared the electrophoretic and immunoreactive properties of Tangier disease and normal apolipoprotein A-I using four monoclonal antibodies. We verified that the molecular weight, pI and CNBr-cleaved fragments of Tangier disease and normal apolipoprotein A-I were not different, excluding the possibility that dimers, aggregates or fragments of apolipoprotein A-I could be responsible for its rapid catabolism in this disease.

Putative mechanisms of action of probucol on high-density lipoprotein apolipoprotein A-I and its isoproteins kinetics in rabbits.

Probucol is a widely prescribed lipid-lowering agent, the major effects of which are to lower cholesterol in both low- and high-density lipoproteins (LDL and HDL, respectively). The mechanism of action of probucol on HDL apolipoprotein (apo) A-I kinetics was investigated in rabbits, with or without cholesterol feeding. 125I-labeled HDL was injected intravenously, and blood samples were taken periodically for 6 days. Kinetic parameters were calculated from the apo A-I-specific radioactivity decay curves. Fractional catabolic rate (FCR) and synthetic rate (SR) of apo A-I in rabbits fed a normal chow and normal chow with 1% probucol were similar. Apo A-I FCR of the rabbits fed 0.5% cholesterol was significantly increased but there were no changes in SR, compared to findings in the normal chow-fed group. Apo A-I FCR of the rabbits fed 1% probucol with 0.5% cholesterol (both 1 month and 2 months) was significantly increased compared to findings in rabbits fed the normal chow as well as 0.5% cholesterol diet group, while SR of apo A-I was significantly reduced in the former groups. Kinetics at 1 month after discontinuation of 1% probucol (under cholesterol feeding) showed a similar FCR of HDL-apo A-I to that of the rabbits fed 0.5% cholesterol, but the SR of apo A-I remained lower. Apo A-I isoproteins kinetics assessed by autoradiography of isoelectric focusing slab gels showed that the synthesis of proapo A-I was significantly reduced in the 1% probucol with 0.5% cholesterol administered, compared to the 0.5% cholesterol group. Thus, the action of probucol on HDL apo A-I kinetics was only prominent in case of higher serum cholesterol levels. The decreased HDL or apo A-I seen with probucol was apparently the result of an increase in FCR and a decrease in SR of HDL-apo A-I. A decreased synthesis of apo A-I remained evident even 1 month after discontinuing probucol. The action of probucol on the intracellular synthetic processes of apo A-I was revealed by the reduced synthesis of proapo A-I.

Immunoaffinity fractionation of high-density lipoprotein subclasses 2 and 3 using anti-apolipoprotein A-I and A-II immunosorbent gels.

High-density lipoprotein (HDL) subclasses 2 and 3 prepared by density gradient ultracentrifugation have been further fractionated by immunoaffinity chromatography using antibody affinity gels targetting the major HDL apolipoproteins, A-I and A-II. Fractions containing A-I without A-II (AI w/o AII) and A-I with A-II (AI w AII) were isolated from both density ranges. Whereas there were similar concentrations of the major subfraction (HDL3(AI w AII] in both males and females, the remaining subfractions were present in higher concentrations in females as compared to males, in the order HDL3 (AI w/o AII) less than HDL2(AI w AII) less than HDL2(AI w/o AII). The difference was most marked for HDL2 (AI w/o AII), where plasma concentrations in females were almost 3-fold greater than in males. Compositional analyses indicated that the plasma concentrations of the fractions, rather than their compositions, were the major determinants of male-female differences in HDL levels. In contrast, fractions defined by similar apolipoprotein criteria and isolated from different density subclasses (i.e., HDL2(AI w/o AII) vs. HDL3(AI w/o AII) and HDL2(AI w AII) vs. HDL3(AI w AII] showed major compositional differences. This is suggestive of distinct lipoprotein particles.

Quantitative determination of apolipoprotein A-I in high-density lipoproteins and 'free' apolipoprotein A-I by two-dimensional agarose gel lipoprotein-'rocket' immunoelectrophoresis of human serum.

A method has been developed for quantitative analysis of 'free' apolipoprotein A-I and apolipoprotein A-I associated with high-density lipoprotein (HDL) in serum. The method utilizes the difference between the rate of electrophoretic migration of apolipoprotein A-I associated with HDL (alpha) and 'free' apolipoprotein A-I (pre-beta) in agarose gel. Apolipoprotein A-I is subsequently quantitated by electrophoresis in a second dimensional gel containing anti-apolipoprotein A-I antibodies. Using this method all apolipoprotein A-I of normal fasting serum was found associated with HDL (n = 16). By contrast, 'free' apolipoprotein A-I accounted for up to 12% of the total in the serum of patients with isolated hypertriglyceridemia (n = 8) or mixed hyperlipoproteinemia (n = 8). Between 30 and 35% of 'free' apolipoprotein A-I was found in one patient afflicted with the apolipoprotein C-II deficiency syndrome. Also, 'free' apolipoprotein A-I could be detected in normal postabsorptive serum. 30 and 90 min following heparin-enhanced lipolysis 'free' apolipoprotein A-I accounted for 23 and 20%, respectively, of the total apolipoprotein A-I of serum. Apolipoprotein A-I associated with HDL remained unaltered. It appears, therefore, that 'free' apolipoprotein A-I is liberated from triglyceride-rich lipoproteins during lipolysis.

Interaction of apolipoprotein A-II with recombinant HDL containing egg phosphatidylcholine, unesterified cholesterol and apolipoprotein A-I.

The preparation of discoidal, recombinant HDL (r-HDL) containing various phospholipids, apolipoproteins and a range of concentrations of unesterified cholesterol has been reported by several investigators. The present study describes the preparation of r-HDL containing both apolipoprotein (apo) A-I and apo A-II. r-HDL with 100:1 (mol:mol) egg PC.apo A-I and 0 (Series I), 5 (Series II) or 10 (Series III) mol% unesterified cholesterol were prepared by the cholate dialysis method. The resulting complexes had a Stokes' radius of 4.7 nm and contained two molecules of apo A-I per particle. When the r-HDL (2.0 mg apo A-I) were supplemented with 1.0 mg of apo A-II, one of the apo A-I molecules was replaced by two molecules of apo A-II. This modification was not accompanied by a loss of phospholipid, nor by major change in particle size. The addition of 2.5 or 4.0 mg of apo A-II resulted in the displacement of both apo A-I molecules from a proportion of the r-HDL and the formation of smaller particles (Stokes' radius 3.9 nm), which contained half the original number of egg PC molecules and three molecules of apo A-II. The amount of apo A-I displaced was dependent on the concentration of unesterified cholesterol in the r-HDL: when 2.5 mg of apo A-II was added to the Series I, II and III r-HDL, 44, 60 and 70%, respectively, of the apo A-I was displaced. Addition of 4.0 mg of apo A-II did not promote further displacement of apo A-I from any of the r-HDL. By contrast, the association of apo A-II with r-HDL was independent of the concentration of unesterified cholesterol and was a linear function of the amount of apo A-II which had been added. It is concluded that (1), the structural integrity of egg PC.unesterified cholesterol.apo A-I r-HDL, which contain two molecules of apo A-I, is not affected when one of the apo A-I molecules is replaced by two molecules of apo A-II; (2), when both apo A-I molecules are replaced by apo A-II, small particles which contain three molecules of apo A-II are formed; and (3), the displacement of apo A-I from r-HDL is facilitated by the presence of unesterified cholesterol in the particles.

Changes in apolipoprotein A-I mRNA level in the liver of rats with experimental nephrotic syndrome.

In previous studies we had shown that: one of the most specific feature of hyperlipoproteinemia found in rats with experimental nephrotic syndrome is the accumulation of apolipoprotein A-I-rich HDL in plasma and this disorder is associated with an overproduction of apolipoprotein A-I by the liver. The present study was designed to investigate whether the increased hepatic synthesis of apolipoprotein A-I was due to an accumulation of functionally active apolipoprotein A-I mRNA in liver of nephrotic rats. Hepatic mRNA was translated in vitro by rabbit reticulocyte lysate in the presence of [35S]methionine and in vitro synthesized apolipoprotein A-I, albumin and apolipoprotein E were immunoprecipitated by specific rabbit IgG. In nephrotic rats the amount of in vitro synthesized apolipoprotein A-I was almost twice that found in the controls, suggesting that functionally active apolipoprotein A-I mRNA was increased in liver of nephrotic rats. To confirm that this difference in apolipoprotein A-I mRNA activity was due to an actual increase of hepatic apolipoprotein A-I mRNA sequences, we performed nucleic acid hybridization experiments (northern blot) using several cloned cDNA probes (rat and human apolipoprotein A-I, rat apolipoprotein E and apolipoprotein A-II). The results indicate that in nephrotic rats the amount of hybridizable apolipoprotein A-I mRNA sequences was about 3-fold higher than that in controls. In contrast, there was no difference in the amount of hybridizable apolipoprotein A-II and apolipoprotein E mRNA sequences, indicating that the change in apolipoprotein A-I mRNA induced by the nephrotic state was specific for this mRNA.

Tangier disease: isolation and characterization of LpA-I, LpA-II, LpA-I: A-II and LpA-IV particles from plasma.

Tangier disease (TD) is characterized by extremely low plasma levels of HDL, apoA-I and apoA-II due to very rapid catabolism. However, the risk of premature coronary heart disease (CHD) is not markedly increased in TD. In order to gain insight into reverse cholesterol transport in TD, we isolated LpA-I, LpA-I:A-II, LpA-II and LpA-IV particles from fasting plasma of 5 TD patients. LpA-I composition was similar to control LpA-I, but TD LpA-I had more LCAT and CETP activity (respectively, 0.35 +/- 0.14 and 0.14 +/- 0.04 mumol of cholesterol esterified/h/micrograms of protein, and 7 +/- 2.5 and 1.4 +/- 0.3 mumol of cholesteryl ester transferred/h/micrograms of protein). In contrast, TD LpA-I:A-II had abnormal composition, with a low molar ratio of apoA-I to apoA-II (0.2-1.33). In addition, LpA-I:A-II in TD contained a substantial amount of apoA-IV compared with control, making this particle an LpA-I:A-II:A-IV complex. LpA-I:A-II from normal plasma do not promote cholesterol efflux from adipocytes cells, whereas TD LpA-I:A-II:A-IV complexes promoted cholesterol efflux from these cells. Moreover LpA-I:A-II:A-IV complexes have more LCAT and CETP activity than control (respectively 1.2 +/- 0.16 and 0.05 +/- 0.01 mumol of cholesterol esterified/h/micrograms of protein and, 41 +/- 3.7 and 1 +/- 0.4 mumol of cholesteryl ester transferred/h/micrograms of protein). The LpA-II particle in TD represented in fact an LpA-II:A-IV complex (75% mol apoA-II and 22% mol apoA-IV).(ABSTRACT TRUNCATED AT 250 WORDS)

Different Functional and Structural Characteristics between ApoA-I and ApoA-4 in Lipid-Free and Reconstituted HDL State: ApoA-4 Showed Less Anti-Atherogenic Activity.

Apolipoprotein A-I and A-IV are protein constituents of high-density lipoproteins although their functional difference in lipoprotein metabolism is still unclear. To compare anti-atherogenic properties between apoA-I and apoA-4, we characterized both proteins in lipid-free and lipid-bound state. In lipid-free state, apoA4 showed two distinct bands, around 78 and 67 Å on native gel electrophoresis, while apoA-I showed scattered band pattern less than 71 Å. In reconstituted HDL (rHDL) state, apoA-4 showed three major bands around 101 Å and 113 Å, while apoA-I-rHDL showed almost single band around 98 Å size. Lipid-free apoA-I showed 2.9-fold higher phospholipid binding ability than apoA-4. In lipid-free state, BS3-crosslinking revealed that apoA-4 showed less multimerization tendency upto dimer, while apoA-I showed pentamerization. In rHDL state (95:1), apoA-4 was existed as dimer as like as apoA-I. With higher phospholipid content (255:1), five apoA-I and three apoA-4 were required to the bigger rHDL formation. Regardless of particle size, apoA-I-rHDL showed superior LCAT activation ability than apoA-4-rHDL. Uptake of acetylated LDL was inhibited by apoA-I in both lipid-free and lipid-bound state, while apoA-4 inhibited it only lipid-free state. ApoA-4 showed less anti-atherogenic activity with more sensitivity to glycation. In conclusion, apoA-4 showed inferior physiological functions in lipid-bound state, compared with those of apoA-I, to induce more pro-atherosclerotic properties.

Isoforms of rat apolipoprotein A-I isolated from the lipoproteins of hepatic Golgi apparatus and plasma.

We compared apo A-I isolated from the lipoproteins of the Golgi apparatus of rat liver with apo A-I found in plasma lipoproteins. Golgi apo A-I consists of 3 main isoforms with a molecular weight of approximately 28000 and isoelectric points (pI) of 5.97, 5.88 and 5.76, respectively. Plasma apo A-I consists of 4 major and 3 minor isoforms with a molecular weight of 27000. The pI of the major isoforms (numbered 4-7) is 5.88, 5.80, 5.70 and 5.60, respectively. In order to investigate which of the plasma isoforms derived directly from Golgi apo A-I, [35S]methionine was injected into the portal vein and Golgi and plasma apo A-I were isolated shortly thereafter. While all Golgi isoforms were labelled only 3 isoforms of plasma apo A-I (namely isoforms 5, 6 and 7) were found to be labelled. The major plasma isoform (isoform 4 which accounts for more than 60% of apo A-I mass of plasma HDL) was found to be unlabelled. However, when 35S plasma lipoproteins newly secreted by the liver were incubated in vitro in the presence of heparinized plasma, labelled isoform 4 appeared suggesting that heparinized plasma contained some factor capable of converting isoforms 5-7 into isoform 4. This plasma factor appears to be a protease as the in vitro formation of isoform 4 is prevented by protease inhibitors.

Genetic but not diet-induced hypercholesterolemia causes low apolipoprotein A-IV level in rabbit sera.

The present report describes a competitive enzyme immunoassay for rabbit apolipoprotein A-IV (apo A-IV). This assay was applied to the determination of its concentration and distribution in sera from normolipidemic and hyperlipidemic rabbits. The assay was sufficiently sensitive to study this 42-kDa protein in lipoproteins fractionated from 200 microliters of serum by FPLC gel filtration. In normolipidemic sera (n = 8), apo A-IV concentration was 5.32 +/- 0.76 mg/dl. A diet rich in cholesterol (0.5%), which induced an 18-fold increase in serum cholesterol, did not significantly alter apo A-IV concentration (6.65 +/- 1.52 mg/dl, n = 8). By contrast, genetically induced hypercholesterolemia (Watanabe heritable hyperlipidemia, WHHL mutation) caused a significantly reduced level of apo A-IV (3.8 +/- 1.14 mg/dl, n = 7). In each of the groups studied, apo A-IV was distributed in two distinct pools; a high-density lipoprotein-(HDL) associated pool and a lipoprotein-free pool. However, compared to normal, the distribution of apo A-IV in WHHL rabbit sera was shifted towards the lipoprotein-free pool. Consistent with previously reported observations on apo A-I, these results are compatible with the hypothesis of an impaired reverse transport of cholesterol in WHHL rabbits, an animal model for familial hypercholesterolemia.

Quantification of plasma lipids and apolipoproteins in British Halflop rabbits. A comparison between normocholesterolemic rabbits, hypercholesterolemic rabbits (modified WHHL rabbits) and rabbits fed an atherogenic diet.

We have established isolation methods and developed electroimmunoassays for rabbit apolipoprotein A-I (apo A-I), apo B, apo C-III and apo E. The assays were used to characterize a hyperlipidemic strain of the British Halflop rabbits (BHL rabbits), obtained after cross-breeding with WHHL rabbits and referred to as modified WHHL rabbits, and to investigate the changes in the apolipoprotein levels induced by feeding normal BHL rabbits an atherogenic diet (0.25% cholesterol and 3% coconut oil). The modified WHHL rabbits were characterized by increased levels of apo B, apo C-III and apo E as well as cholesterol, phospholipids and triacylglycerol as compared to chow-fed BHL rabbits, while the apo A-I levels were only half of those found in the chow-fed animals. The modified WHHL rabbits had virtually no low density lipoprotein (LDL) receptor activity and a low fractional catabolic rate (FCR) of LDL. These results indicate that the modified WHHL rabbit has the homozygous form of the LDL receptor deficiency. The BHL rabbits fed the atherogenic diet showed increased levels of cholesterol, triacylglycerol, apo B, apo C-III and apo E, as compared to those of the chow-fed BHL rabbits. The apo E and apo C-III reached levels in the range of or even higher than those of the modified WHHL rabbits. The apo A-I levels on the other hand did not differ from those of the chow-fed rabbits. Feeding an atherogenic diet led to a decrease in the FCR of LDL to a level similar to that found in the modified WHHL rabbits.