Identification of a critical lysine residue in apolipoprotein B-100 that mediates noncovalent interaction with apolipoprotein(a)

We have previously shown that lipoprotein(a) (Lp(a)) assembly involves an initial noncovalent interaction between sequences within apolipoprotein(a) (apo(a)) kringle IV types 5-8 and the amino terminus of apolipoprotein B-100 (sequences between amino acids 680 and 781 in apoB-100), followed by formation of a disulfide bond. In the present study, citraconylation of lysine residues in apoB-100 abolished the ability of the modified low density lipoprotein to associate with apo(a), thereby demonstrating a direct role for lysine residues in apoB in the first step of Lp(a) assembly. To identify specific lysine residues in the amino terminus of apoB that are required for the noncovalent interaction, we initially used an affinity chromatography method in which recombinant forms of apo(a) (r-apo(a)) were immobilized on Sepharose beads. Assessment of the ability of carboxyl-terminal truncations of apoB-18 to bind to r-apo(a)-Sepharose revealed that a 25-amino acid sequence in apoB (amino acids 680-704) bound specifically to apo(a) in a lysine-dependent manner; citraconylation of the lysine residues in the apoB derivative encoding this sequence abolished the binding interaction. Using fluorescence spectrometry, we found that a synthetic peptide corresponding to this sequence bound directly to apo(a); the peptide also reduced covalent Lp(a) formation. Lysine residues present in this sequence (Lys(680) and Lys(690)) were mutated to alanine in the context of apoB-18. We found that the apoB-18 species containing the Lys(680) mutation was incapable of binding to r-apo(a)-Sepharose columns, whereas the apoB-18 species containing the Lys(690) mutation exhibited slightly reduced binding to these columns. Taken together, our data indicate that Lys(680) is critical for the noncovalent interaction of apo(a) and apoB-100 that precedes covalent Lp(a) formation.     

A ligand-induced conformational change in apolipoprotein(a) enhances covalent Lp(a) formation

Lipoprotein(a) (Lp(a)) assembly proceeds via a two-step mechanism in which initial non-covalent interactions between apolipoprotein(a) (apo(a)) and low density lipoprotein precede disulfide bond formation. In this study, we used analytical ultracentrifugation, differential scanning calorimetry, and intrinsic fluorescence to demonstrate that in the presence of the lysine analog epsilon-aminocaproic acid, apo(a) undergoes a substantial conformational change from a "closed" to an "open" structure that is characterized by an increase in the hydrodynamic radius (approximately 10%), an alteration in domain stability, as well as a decrease in tryptophan fluorescence. Although epsilon-aminocaproic acid is a well characterized inhibitor of the non-covalent interaction between apo(a) and low density lipoprotein, we report the novel observation that this ligand at low concentrations (100 microm-1 mm) significantly enhances covalent Lp(a) assembly by altering the conformation of apo(a). We developed a model for the kinetics of Lp(a) assembly that incorporates the conformational change as a determinant of the efficiency of the process; this model quantitatively explains our experimental observations. Interestingly, an analogous conformational change has been previously described for plasminogen resulting in an increase in the hydrodynamic radius, an increase in tryptophan fluorescence, and an acceleration of the rate of plasminogen activation. Although the functions of apo(a) and plasminogen have diverged considerably, elements of structural and conformational homology have been retained leading to similar regulation of two unrelated biological processes.     

Quantitative evaluation of the contribution of weak lysine-binding sites present within apolipoprotein(a) kringle IV types 6-8 to lipoprotein(a) assembly

During lipoprotein(a) (Lp(a)) assembly, non-covalent interactions between apolipoprotein(a) (apo(a)) and low density lipoprotein precede specific disulfide bond formation. Studies have shown that the non-covalent step involves an interaction between the weak lysine-binding sites (WLBS) present within each of apo(a) kringle IV types 6, 7, and 8 (KIV(6-8)), and two lysine residues (Lys(680) and Lys(690)) within the NH(2) terminus of the apolipoprotein B-100 (apoB) component of low density lipoprotein. In the present study, we introduced single point mutations (E56G) into each of the WLBS present in apo(a) KIV(6-8) and expressed these mutations in the context of a 17-kringle (17K) recombinant apo(a) variant. Single mutations that disrupt the WLBS in KIV(6), KIV(7), and KIV(8), as well as mutants that disrupt the WLBS in both KIV(6) and KIV(7), or both KIV(7) and KIV(8), were assessed for their ability to form non-covalent and covalent Lp(a) complexes. Our results demonstrate that both apo(a) KIV(7) and KIV(8), but not KIV(6), are required for maximally efficient non-covalent and covalent Lp(a) assembly. Single mutations in the WLBS of KIV(7) or KIV(8) resulted in a 3-fold decrease in the affinity of 17K recombinant apo(a) for apoB, and a 20% reduction in the rate of covalent Lp(a) formation. Tandem mutations in the WLBS in both KIV(7) and KIV(8) resulted in a 13-fold reduction in the binding affinity between apo(a) and apoB, and a 75% reduction in the rate of the covalent step of Lp(a) formation. We also showed that KIV(7) and KIV(8) specifically bind with high affinity to apoB-derived peptides containing Lys(690) or Lys(680), respectively. Taken together, our data demonstrate that specific interactions between apo(a) KIV(7) and KIV(8) and Lys(680) and Lys(690) in apoB mediate a high affinity non-covalent interaction between apo(a) and low density lipoprotein, which dictates the efficiency of covalent Lp(a) formation.     

Genetic linkage between lipoprotein(a) phenotype and a DNA polymorphism in the plasminogen gene

Coronary heart disease risk correlates directly with plasma concentrations of lipoprotein(a) (Lp(a)), a low-density lipoprotein-like particle distinguished by the presence of the glycoprotein apolipoprotein(a) (apo(a)), which is bound to apolipoprotein B-100 (apoB-100) by disulfide bridges. Size isoforms of apo(a) are inherited as Mendelian codominant traits and are associated with variations in the plasma concentration of lipoprotein(a). Plasminogen and apo(a) show striking protein sequence homology, and their genes both map to chromosome 6q26-27. In a large family with early coronary heart disease and high plasma concentrations of Lp(a), we found tight linkage between apo(a) size isoforms and a DNA polymorphism in the plasminogen gene; plasma concentrations of Lp(a) also appeared to be related to genetic variation at the apo(a) locus. We found free recombination between the same phenotype and alleles of the apoB DNA polymorphism. This suggests that apo(a) size isoforms and plasma lipoprotein(a) concentrations are each determined by genetic variation at the apo(a) locus.     

Effect of tranexamic acid and delta-aminovaleric acid on lipoprotein(a) metabolism in transgenic mice

The assembly of lipoprotein(a) (Lp(a)) is a two-step process which involves the interaction of kringle-4 (K-IV) domains in apolipoprotein(a) (apo(a)) with Lys groups in apoB-100. Lys analogues such as tranexamic acid (TXA) or delta-aminovaleric acid (delta-AVA) proved to prevent the Lp(a) assembly in vitro. In order to study the in vivo effect of Lys analogues, transgenic apo(a) or Lp(a) mice were treated with TXA or delta-AVA and plasma levels of free and low density lipoprotein bound apo(a) were measured. In parallel experiments, McA-RH 7777 cells, stably transfected with apo(a), were also treated with these substances and apo(a) secretion was followed. Treatment of transgenic mice with Lys analogues caused a doubling of plasma Lp(a) levels, while the ratio of free:apoB-100 bound apo(a) remained unchanged. In transgenic apo(a) mice a 1. 5-fold increase in plasma apo(a) levels was noticed. TXA significantly increased Lp(a) half-life from 6 h to 8 h. Incubation of McA-RH 7777 cells with Lys analogues resulted in an up to 1. 4-fold increase in apo(a) in the medium. The amount of intracellular low molecular weight apo(a) precursor remained unchanged. We hypothesize that Lys analogues increase plasma Lp(a) levels by increasing the dissociation of cell bound apo(a) in combination with reducing Lp(a) catabolism.     

Effect of tranexamic acid and δ-aminovaleric acid on lipoprotein(a) metabolism in transgenic mice

The assembly of lipoprotein(a) (Lp(a)) is a two-step process which involves the interaction of kringle-4 (K-IV) domains in apolipoprotein(a) (apo(a)) with Lys groups in apoB-100. Lys analogues such as tranexamic acid (TXA) or δ-aminovaleric acid (δ-AVA) proved to prevent the Lp(a) assembly in vitro. In order to study the in vivo effect of Lys analogues, transgenic apo(a) or Lp(a) mice were treated with TXA or δ-AVA and plasma levels of free and low density lipoprotein bound apo(a) were measured. In parallel experiments, McA-RH 7777 cells, stably transfected with apo(a), were also treated with these substances and apo(a) secretion was followed. Treatment of transgenic mice with Lys analogues caused a doubling of plasma Lp(a) levels, while the ratio of free:apoB-100 bound apo(a) remained unchanged. In transgenic apo(a) mice a 1.5-fold increase in plasma apo(a) levels was noticed. TXA significantly increased Lp(a) half-life from 6 h to 8 h. Incubation of McA-RH 7777 cells with Lys analogues resulted in an up to 1.4-fold increase in apo(a) in the medium. The amount of intracellular low molecular weight apo(a) precursor remained unchanged. We hypothesize that Lys analogues increase plasma Lp(a) levels by increasing the dissociation of cell bound apo(a) in combination with reducing Lp(a) catabolism.     

Abetalipoproteinemia with an ApoB-100-lipoprotein(a) glycoprotein complex in plasma. Indication for an assembly defect

Patients with autosomal recessive abetalipoproteinemia (ABL) lack in their plasma all lipoproteins containing apolipoprotein (apo)B-100 or B-48. Previous studies have suggested that this is due to the complete absence of apoB. We have investigated whether such patients (n = 10) are able to secrete the lipoprotein(a) (Lp(a] glycoprotein (apo(a] which, in normal plasma, exists as a complex with low density lipoproteins containing apoB-100 (Lp(a) lipoprotein). All 10 patients had reduced but detectable apo(a) levels in plasma (mean, 0.49 mg/dl; range, 0.2-2.03 mg/dl) but no Lp(a) lipoprotein. However, we also detected small amounts (0.2-2.8 mg/dl) of apoB in all patients with ABL. The apoB in the ABL patients had the size of apoB-100 and occurred as a lipid-poor complex with the Lp(a) glycoprotein in a fraction of density 1.22 g/ml. This material may represent partially assembled Lp(a) lipoprotein. There was also uncomplexed apo(a) and apoB-100 in the ABL plasma. The distribution and relative concentration of both proteins in the density fraction greater than 1.06 g/ml varied among patients. The data suggest that in ABL, the assembly of apoB-containing lipoproteins is defective and that apoB-100 may be secreted without its full lipid complement when complexed with apo(a).     

Mutation of lysine residues in apolipoprotein B-100 causes defective lipoprotein[a] formation

Lipoprotein[a] (Lp[a]) is assembled by a two-step process involving an initial lysine-dependent binding between apolipoprotein B-100 (apoB-100) and apolipoprotein[a] (apo[a]) that facilitates the formation of a disulphide bond between apoB-100Cys4,326 and apo[a]Cys4,057. Previous studies of transgenic mice expressing apoB-95 (4,330 amino acids) and apoB-97 (4,397 amino acids) have shown that apoB-100 amino acids 4,330-4,397 are important for the initial binding to apo[a]. Furthermore, a lysine-rich peptide spanning apoB-100 amino acids 4,372-4,392 has recently been shown to bind apo[a] and inhibit Lp[a] assembly in vitro. This suggests that a putative apo[a] binding site exists in the apoB-4,372-4,392 region. The aim of our study was to establish whether the apoB-4,372-4,392 sequence was important for Lp[a] assembly in the context of the full-length apoB-100. Transgenic mice were created that expressed a mutant human apoB-100, apoB-100K4-->S4, in which all four lysine residues in the 4,372-4,392 sequence were mutated to serines. The apoB-100K4-->S4 mutant showed a reduced capacity to form Lp[a] in vitro compared with wild-type human apoB-100. Double transgenic mice expressing both apoB-100K4-->S4 and apo[a] contained significant amounts of free apo[a] in the plasma, indicating a less-efficient assembly of Lp[a] in vivo. Taken together, these results clearly show that the apoB-4,372-4,392 sequence plays a role in Lp[a] assembly.     

Recombinant adenovirus vector mediated expression of lipoprotein (a) [Lp(a)] in rabbit plasma.

Lipoprotein (a) [Lp(a)] is a heterodimer of apolipoprotein (a) [apo(a)] and apolipoprotein B-100 (apoB-100) of low density lipoprotein linked by a disulfide bond. Apo(a) and apoB-100 are synthesized by the liver and covalently associate or couple to form Lp(a) extracellularly. Elevated plasma Lp(a) is an independent risk factor for vascular injury disorders such as restenosis after balloon angioplasty and accelerated graft atherosclerosis following heart transplantation. Lp(a) is not expressed in laboratory animals making studies of its pathophysiology difficult. To overcome this problem, we explored the possibility of generating Lp(a) in rabbit plasma using replication-deficient adenovirus vector mediated gene delivery. Rabbits were chosen because of their large vessels and unlike mouse or rat, rabbit apoB-100 could interact with apo(a) to generate Lp(a). The recombinant (r) adenovirus vector construct used encoded a 200 kDa apo(a) [Ad-apo(a)]. Ad-apo(a) injection into the rabbit marginal vein caused the appearance of plasma rLp(a). Injection of a r adenovirus vector expressing the bacterial LacZ gene (Ad-LacZ) or PBS (vehicle) did not result in detectable plasma rLp(a). These are the first results to demonstrate plasma expression of rLp(a) in rabbits using adenovirus vector mediated gene transfer. Therefore, this system may be suitable for investigating Lp(a)'s role in the development of vascular injury diseases in a rabbit model.     

An analysis of the interaction between mouse apolipoprotein B100 and apolipoprotein(a)

The assembly of lipoprotein(a) (Lp(a)) involves an initial noncovalent interaction between apolipoprotein (apo) B100 and apo(a), followed by the formation of a disulfide bond between apoB100 cysteine 4326 and apo(a) cysteine 4057. The structural features of apoB100 that are required for its noncovalent interaction with apo(a) have not been fully defined. To analyze that initial interaction, we tested whether apo(a) could bind noncovalently to two apoB proteins that lack cysteine 4326: mouse apoB100 and human apoB100-C4326G. Our experiments demonstrated that both mouse apoB and the human apoB100-C4326G bind noncovalently to apo(a). We next sought to gain insights into the apoB amino acid sequences required for the interaction between apoB100 and apo(a). Previous studies of truncated human apoB proteins indicated that the carboxyl terminus of human apoB100 (amino acids 4330-4397) is important for Lp(a) assembly. To determine whether the carboxyl terminus of mouse apoB100 can interact with apo(a), transgenic mice were produced with a mutant human apoB gene construct in which human apoB100 amino acids 4279-4536 were replaced with the corresponding mouse apoB100 sequences and tyrosine 4326 was changed to a cysteine. The mutant apoB100 bound to apo(a) and formed bona fide disulfide-linked Lp(a), but Lp(a) assembly was less efficient than with wild-type human apoB100. The fact that Lp(a) assembly was less efficient with the mouse apoB sequences provides additional support for the notion that sequences in the carboxyl terminus of apoB100 are important for Lp(a) assembly.     

A quantitative assay for the non-covalent association between apolipoprotein[a] and apolipoprotein B: an alternative measure of Lp[a] assembly

Increasing evidence suggests that the assembly of lipoprotein[a] (Lp[a]) proceeds in two steps. In the first step, non-covalent interactions between apolipoprotein[a] (apo[a]) and apolipoprotein B (apoB) of low density lipoprotein (LDL) form a dissociable apo[a]:LDL complex. In the second step, a covalent disulfide linkage forms the stable Lp[a] particle. Several methods are currently used to study the assembly of Lp[a], however, these methods are laborious, time-consuming, and not suitable for a high throughput screening. We report here the development of a rapid and simple assay based on the binding of labeled LDL to a Lp[a]/apo[a] substrate which is immobilized on the surface of a microtiter plate. Quantification of bound LDL provides a measure of the extent of complex formation. Labeled LDL bound to both Lp[a] and apo[a] substrates with similar affinity. Plasma lipoproteins containing apoB as well as free apo[a] were capable of competing with LDL binding. The binding of LDL to Lp[a]/apo[a] was inhibited by L-proline and lysine analogs, which are known to inhibit the non-covalent association between apo[a] and apoB. Using this method we have found that nicotinic acid and captopril are able to inhibit the association of apo[a] with apoB. This method is compatible with automation and can be applied to a high throughput screening of inhibitors of Lp[a] formation.     

Interaction of apolipoprotein[a] with apolipoproteinB-100 Cys3734 region in lipoprotein[a] is confirmed immunochemically

Monospecific polyclonal antibodies (MPAbs) to apoB-100 regions Cys3734 and Cys4190 were isolated by affinity chromatography using the synthetic polypeptides, Q₃₇₃₀VPSSKLDFREIQIYKK₃₇₄₆ and G₄₁₈₂IYTREELSTMFIREVG₄₁₉₈, respectively, coupled to a hydrophilic resin. Molecular modeling and fluroescence labeling studies have suggested that Cys67 located in kringle type 9 (LPaK9, located between residues 3991 and 4068 of the apo[a] sequence inferred by cDNA) of the apo[a] molecule is disulfide linked to Cys3734 of apoB-100 in human lipoprotein[a] (Lp[a]). This possibility has been further explored with MPAbs. Four species of MPAbs directed to a Cys3734 region of apoB-100 (3730–3746) were isolated from goat anti-human LDL serum by a combination of synthetic peptide (Q₃₇₃₀VPSSKLDFREIQIYKK₃₇₄₆) affinity chromatography and preparative electrophoresis (electrochromatography). MPAbs to the Cys4190 region of apoB-100, a second or alternative disulfide link-site between apo[a] and apoB-100, were also isolated using a synthetic peptide (G₄₁₈₂IYTREELSTMFIREVG₄₁₉₈) affinity resin. Results of immunoassays showed that binding of these four MPAbs to Lp[a] was significantly lower than to LDL. In contrast, MPAbs to the apoB-100 region 4182–4198 which contains Cys4190, a second or alternative disulfide link-site between apo[a] and apoB-100, displayed a less significant difference in binding to Lp[a] and LDL. These results provide additional evidence that the residues 3730–3746 of apoB-100 interact significantly with apo[a] in Lp[a], and that Cys3734 is a likely site for the disulfide bond connecting apo[a] and apoB-100.Abbreviations amino acids single letter, e.g., alanine, A, etc. - BSA bovine serum albumin - d density (g/ml) - aca -aminocaproic acid - ELISA enzyme-linked immunosorbant assay - DTT dithiothreitol - HRP horseradish peroxidase - MAb monoclonal antibody - MPAb monospecific polyclonal antibody - PAGE polyacrylamide gel electrophoresis - PMSF phenylmethylsulfonyl fluoride - SDS sodium dodecyl sulfate - Na₂EDTA sodium ethylenediaminetetraacetate - NaN₃ sodium azide - TRIS (hydroxymethyl)aminomethane     

Lipoprotein(a) binds to fibronectin and has serine proteinase activity capable of cleaving it.

The plasma concentration of human lipoprotein(a) [Lp(a)] is correlated with the risk of heart disease. A distinct feature of the Lp(a) particle is the apolipoprotein (a) [apo(a)], which is associated with apoB-100, the main protein component of low-density lipoprotein. We now report that apo(a), which has extensive homology to plasminogen, binds to immobilized fibronectin. The binding of Lp(a) was localized to the C-terminal heparin-binding domain of fibronectin. Incubation of Lp(a) with fibronectin resulted in fragmentation of fibronectin. The cleavage pattern, as visualized by gel electrophoresis and immunoblotting, was reproducibly obtained with Lp(a) purified from five different individuals and was distinct from that obtained upon proteolysis of fibronectin by plasmin or kallikrein. The use of synthetic peptide substrates demonstrated that the amino acid specificity for Lp(a) was arginine rather than lysine. The proteolytic activity of Lp(a) was localized to apo(a) and experiments with inhibitors indicated that the proteolytic activity was of serine proteinase-type.     

Identification of sequences in apolipoprotein(a) that maintain its closed conformation: a novel role for apo(a) isoform size in determining the efficiency of covalent Lp(a) formation

We have previously demonstrated that, in the presence of the lysine analogue epsilon-aminocaproic acid, apolipoprotein(a) [apo(a)] undergoes a conformational change from a closed to an open structure that is characterized by a change in tryptophan fluorescence, an increase in the radius of gyration, an alteration of domain stability, and an enhancement in the efficiency of covalent lipoprotein(a) [Lp(a)] formation. In the present study, to identify sequences within apo(a) that maintain its closed conformation, we used epsilon-aminocaproic acid to probe the conformational status of a variety of recombinant apo(a) isoforms using analytical ultracentrifugation, differential scanning calorimetry, intrinsic fluorescence, and in vitro covalent Lp(a) formation assays. We observed that the closed conformation of apo(a) is maintained by intramolecular interaction(s) between sequences within the amino- and carboxyl-terminal halves of the molecule. Using site-directed mutagenesis, we have identified the strong lysine-binding site present within apo(a) kringle IV type 10 as an important site within the C-terminal half of the molecule, which is involved in maintaining the closed conformation of apo(a). Apo(a) exhibits marked isoform size heterogeneity because of the presence of varying numbers of copies of the kringle IV type-2 domain located within the amino-terminal half of the molecule. Using recombinant apo(a) species containing either 1, 3, or 8 copies of kringle IV type 2, we observed that, while apo(a) isoform size does not alter the affinity of apo(a) for low-density lipoprotein, it affects the conformational status of the protein and therefore influences the efficiency of covalent Lp(a) assembly. The inverse relationship between apo(a) isoform size and the efficiency of covalent Lp(a) formation that we report in vitro may contribute to the inverse relationship between apo(a) isoform size and plasma Lp(a) concentrations that has been observed in vivo.     

Protein composition of Lp(a) lipoprotein from human plasma

The apolipoprotein composition of purified human Lp(a) lipoprotein was investigated by SDS--polyacrylamide gel electrophoresis and immunochemically. The lipoprotein contains two different polypeptides. One is identical by its app. Mr of approximately 250 000 and immunologically with apolipoprotein B of LDL (B-100). The other polypeptide has a higher app. Mr (approximately 350 000) and stains strongly with the periodate-Schiff's reagent. This high-Mr glycoprotein contains the specific Lp(a) immunoreactivity but does not react with antibodies against apo B. Apo B and Lp(a)-protein seem to be linked by disulfide bonds in the native lipoprotein. The unreduced detergent delipidized protein moiety from Lp(a) lipoprotein shows a single band of Mr approximately 700 000 in SDS--polyacrylamide gel electrophoresis and the immunoprecipitates formed against anti-Lp(a) and anti-apo B by the unreduced protein show a reaction of immunological identity.     

High-level lipoprotein [a] expression in transgenic mice: evidence for oxidized phospholipids in lipoprotein [a] but not in low density lipoproteins

Efforts to elucidate the role of lipoprotein [a] (Lp[a]) in atherogenesis have been hampered by the lack of an animal model with high plasma Lp[a] levels. We produced two lines of transgenic mice expressing apolipoprotein [a] (apo[a]) in the liver and crossed them with mice expressing human apolipoprotein B-100 (apoB-100), generating two lines of Lp[a] mice. One had Lp[a] levels of approximately 700 mg/dl, well above the 30 mg/dl threshold associated with increased risk of atherosclerosis in humans; the other had levels of approximately 35 mg/dl. Most of the LDL in mice with high-level apo[a] expression was covalently bound to apo[a], but most of the LDL in the low-expressing line was free. Using an enzyme-linked sandwich assay with monoclonal antibody EO6, we found high levels of oxidized phospholipids in Lp[a] from high-expressing mice but not in LDL from low-expressing mice or in LDL from human apoB-100 transgenic mice (P <0.00001), even though all mice had similar plasma levels of human apoB-100. The increase in oxidized lipids specific to Lp[a] in high-level apo[a]-expressing mice suggests a mechanism by which increased circulating levels of Lp[a] could contribute to atherogenesis.     

Evaluation of Lp[a] and other independent risk factors for CHD in Asian Indians and their USA counterparts

Conventional risk factors for coronary heart disease (CHD) do not completely account for the observed increase in premature CHD in people from the Indian subcontinent or for Asian Indians who have immigrated to the USA. The objective of this study was to determine the effect of immigration to the USA on plasma levels of lipoprotein [a] (Lp[a]) and other independent risk factors for CHD in Asian Indians. Three subject groups were studied: group 1, 57 subjects living in India and diagnosed with CHD (CHD patients); group 2, 46 subjects living in India and showing no symptoms of CHD (control subjects); group 3, 206 Asian Indians living in the USA. Fasting blood samples were drawn to determine plasma levels of triglyceride (TG), total cholesterol (TC), low density lipoprotein [LDL cholesterol (LDL-Chol)], high density lipoprotein [HDL cholesterol (HDL-Chol)], apolipoprotein B-100 (apoB-100), and Lp[a]. Apolipoprotein [a] (apo[a]) size polymorphism was determined by immunoblotting. Plasma TG, apoB-100, and Lp[a] concentrations were higher in CHD patients than in control and USA groups. CHD patients had higher levels of TC and LDL-Chol and lower HDL-Chol than control subjects. However, the USA population had higher levels of TC, LDL-Chol, and apoB-100 and lower HDL-Chol than control subjects. Plasma Lp[a] levels were inversely correlated with the relative molecular weight of the more abundant of each subject's two apo[a] isoforms (MAI), and CHD patients showed higher frequencies of lower relative molecular weights among MAI. Our observed changes in lipid profiles suggest that immigrating to the USA may place Asian Indians at increased risk for CHD. This study suggests that elevated plasma Lp[a] confers genetic predisposition to CHD in Asian Indians, and nutritional and environmental factors further increase the risk of CHD. This is the first report implicating MAI size as a predictor for development of premature CHD in Asian Indians. Including plasma Lp[a] concentration and apo[a] phenotype in screening procedures may permit early detection and preventive treatment of CHD in this population.     

Rhesus monkey apolipoprotein(a). Sequence, evolution, and sites of synthesis

Human lipoprotein(a) is a low density lipoprotein-like lipoprotein whose concentration in plasma is correlated with atherosclerosis. The characteristic protein component of lipoprotein(a) is apolipoprotein(a) (apo(a)) which is disulfide-linked to apolipoprotein B-100. Sequencing of rhesus monkey apo(a) cDNA suggests that this protein, like human apo(a), is highly similar to plasminogen. Sequence data suggests that a plasminogen-like protease activity and kringle 1-, 2-, 3-, and 5-like domains are unnecessary for apo(a) function, but a highly repeated kringle four-like domain is important. Liver is the major site of apo(a) RNA synthesis; reduced amounts of message were also found in testes and brain. Co-expression with apoB-100 and plasminogen in rhesus tissues is not mandatory.     

Abnormal in vivo metabolism of apoB-containing lipoproteins in human apoE deficiency

The present study was undertaken to elucidate the metabolic basis for the increased remnants and lipoprotein(a) [Lp(a)] and decreased LDL apolipoprotein B (apoB) levels in human apoE deficiency. A primed constant infusion of (13)C(6)-phenylalanine was administered to a homozygous apoE-deficient subject. apoB-100 and apoB-48 were isolated, and tracer enrichments were determined by gas chromatography-mass spectrometry, then kinetic parameters were calculated by multicompartmental modeling. In the apoE-deficient subject, fractional catabolic rates (FCRs) of apoB-100 in VLDL and intermediate density lipoprotein and apoB-48 in VLDL were 3x, 12x, and 12x slower than those of controls. On the other hand, the LDL apoB-100 FCR was increased by 2.6x. The production rate of VLDL apoB-100 was decreased by 45%. In the Lp(a) kinetic study, two types of Lp(a) were isolated from plasma with apoE deficiency: buoyant and normal Lp(a). (125)I-buoyant Lp(a) was catabolized at a slower rate in the patient. However, (125)I-buoyant Lp(a) was catabolized at twice as fast as (131)I-normal Lp(a) in the control subjects. In summary, apoE deficiency results in: 1) a markedly impaired catabolism of VLDL/chylomicron and their remnants due to lack of direct removal and impaired lipolysis; 2) an increased rate of catabolism of LDL apoB-100, likely due to upregulation of LDL receptor activity; 3) reduced VLDL apoB production; and 4) a delayed catabolism of a portion of Lp(a).