Rhesus monkey lipoprotein(a) binds to lysine Sepharose and U937 monocytoid cells less efficiently than human lipoprotein(a). Evidence for the dominant role of kringle 4(37).

Rhesus lipoprotein(a) (Lp[a]) binds less efficiently than human Lp(a) to lysine-Sepharose and to cultured U937 cells. Studies using elastase-derived plasminogen fragments indicated that neither kringle 5 nor the protease domain of Lp(a) are required in these interactions pointing at an involvement of the K4 region. Comparative structural analyses of both the human and simian apo(a) K4 domain, together with molecular modeling studies, supported the conclusion that K4(37) plays a dominant role in the lysine binding function of apo(a) and that the presence of arginine 72 rather than tryptophan in this kringle can account for the functional deficiency observed with rhesus Lp(a). These in vitro results suggest that rhesus Lp(a) may be less thrombogenic than human Lp(a).

Atherogenic diets and neutral-lipid organization in plasma low density lipoproteins.

The plasma low density lipoproteins (LDL) of rhesus monkeys fed 3 atherogenic diets exhibited thermal transitions at temperatures much higher (37--43 degrees C) than those observed in control animals or in normal humans (20--33 degrees C). The same differences were noted in the neutral lipids (cholesteryl esters and triglycerides) which were isolated from the respective lipoproteins. In particular, the difference in thermal properties between the normal and abnormal LDLs was attributable to subtle differences in their cholesteryl ester compositions (mainly an increase in the saturated and monounsaturated fatty acid moieties), with altered triglyceride contents playing only a minor role. Thus, at body temperature, the hyperlipidemia that follows the administration of atherogenic diets is associated with a high degree of order of the neutral lipids in the core of the LDL particle. This, in turn, may be related to the atherogenicity of the abnormal lipoprotein species.

Serum low density lipoproteins with mitogenic effect on cultured aortic smooth muscle cells.

Low density lipoprotein (LDL) subspecies of different size and lipid mass were isolated by density gradient ultracentrifugation from the serum of male rhesus monkeys (Macaca mulatta) fed both a low fat, low cholesterol commercial primate ration, and cholesterol-supplemented high-fat diets, as well as from the serum of human donors. The mitogenic effect of these lipoproteins was examined using primary cultures of rhesus aortic smooth muscle cells. It was observed that the smaller LDL (molecular weight 2.7 X 10(6) from normolipidemic monkeys and a small LDL (molecular weight 2.6 X 10(6) occurring in some normal human subjects exhibited no mitogenic action. In turn, the larger LDL subspecies (molecular weight greater than 3.0 X 10(6), and buoyant density less than 1.030 g/ml), whether from normolipidemic or hyperlipidemic monkeys, or from some normal human subjects, had a marked proliferative action. The results indicate that both hyperlipidemic and normal sera (both human and rhesus) contain mitogenic LDL species although in different amounts. LDL-III, the rhesus equivalent of human Lp(a) was not mitogenic despite its similarity on size and lipid composition to the stimulating particles. However, on the removal of most of its large sialic acid moiety, a clear mitogenic action was observed. The mechanisms responsible for the proliferative effect are unclear and may involve LDL mass, lipid composition, and surface charge although other speculations cannot at present be ruled out. Furthermore, since the small LDL subspecies of either rhesus or human origin were nonmitogenic and similar in mass to the LDL found in calf serum, the mitogenic response of the smooth muscle cells to large LDLs may depend on their early conditioning with the LDL of calf serum.

Plasma lipoprotein (a) protein concentration and coronary artery disease in black patients compared with white patients.

PURPOSE: This study examines the relation between lipoprotein (a) protein levels and other lipid parameters and coronary artery disease in white and black patients. PATIENTS AND METHODS: Plasma lipoprotein (a) protein levels were measured prior to coronary angiography in a population of 127 white and 111 black patients. Each angiogram was given a total coronary artery disease score based on the number and severity of atherosclerotic coronary lesions. RESULTS: White and black patients exhibited no differences in total plasma cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and triglycerides. Black patients had higher lipoprotein (a) protein levels than white patients (8.6 versus 4.0 mg/dL; p < 0.0001). The extent and severity of coronary artery disease was the same in white and black patients. White and black patients with coronary artery disease had higher lipoprotein (a) levels than patients without coronary lesions (4.37 versus 1.99 mg/dL, p = 0.027 for white; 9.23 versus 6.87 mg/dL, p = 0.072 for black). In both groups of patients, there was a weak but significant positive correlation between lipoprotein (a) protein levels and coronary artery disease score. CONCLUSION: Lipoprotein (a) is higher in patients with coronary artery disease. Black patients have higher plasma lipoprotein (a) protein levels than white patients and a comparable degree of coronary artery disease. It follows that the cardiovascular pathogenicity of lipoprotein (a) is not significantly greater in black patients despite higher lipoprotein (a) levels.

Phospholipase A2 modification enhances lipoprotein(a) binding to the subendothelial matrix.

Lipoprotein(a), Lp(a), is found in the extracellular matrix in atherosclerotic plaques, but with a different localization than LDL. A two-compartment system, with a monolayer of endothelial cells forming a barrier, was used to compare the transport, cell binding, and retention of Lp(a) and LDL into the subendothelial matrix. Baseline values for transport and retention of Lp(a) and LDL were not significantly different. Incubation with lipoprotein lipase or sphingomyelinase caused modest and similar increases in transport and retention of the two lipoproteins. In contrast, incubation with phospholipase A2 (PLA2) resulted in a marked (4-fold) increase in retention of Lp(a) on the subendothelial matrix, but a lesser (2-fold) increase in LDL retention. Moreover, PLA2 treatment of Lp(a) enhanced its binding to individual matrix proteins (fibronectin, laminin, or collagen) by 4-10 times above that of LDL. The enzymatic activity of PLA2 was responsible for its effect on Lp(a) binding. The lysine binding sites of Lp(a) contributed to the increased binding of PLA2-modified Lp(a) to the matrix, and the enhanced lysine binding functions of PLA2-modified Lp(a) was demonstrated by two independent approaches. Thus, PLA2 modification leads to enhanced interactions of lipoproteins with the extracellular matrix, and this effect is more pronounced with Lp(a).

Interaction of Lp(a) with plasminogen binding sites on cells.

Lp(a) competes with plasminogen for binding to cells but it is not known whether this competition is due to the ability of Lp(a) to interact directly with plasminogen receptors. In the present study, we demonstrate that Lp(a) can interact directly with plasminogen binding sites on monocytoid U937 cells and endothelial cells. The interaction of Lp(a) with these sites was time dependent, specific, saturable, divalent ion independent and temperature sensitive, characteristics of plasminogen binding to these sites. The affinity of plasminogen and Lp(a) for these sites also was similar (Kd = 1-3 microM), but Lp(a) bound to fewer sites (approximately 10-fold less). Both gangliosides and cell surface proteins with carboxy-terminal lysyl residues, including enolase, a candidate plasminogen receptor, inhibited Lp(a) binding to U937 cells. Additionally, Lp(a) interacted with low affinity lipoprotein binding sites on these cells which also recognized LDL and HDL. The ability of Lp(a) to interact with sites on cells that recognize plasminogen may contribute to the pathogenetic consequences of high levels of circulating Lp(a).

Postprandial lipoprotein(a) response to a single meal containing either saturated or omega-3 polyunsaturated fatty acids in subjects with hypoalphalipoproteinemia.

We have recently reported that the apolipoprotein (apo) B-100-apo(a) complex, the protein moiety of lipoprotein(a) [Lp(a)], has a high affinity for triglyceride(TG)-rich particles (TRP) and that this complex can affiliate with endogenous TG-rich lipoproteins. To shed more light on the apo B-100-apo(a) complex associated with plasma TRP during postprandial lipidemia, we fed five male subjects presenting with primary hypoalphalipoproteinemia (HP) and four male controls a single fat meal (60 g/m2) containing saturated fatty acids (SFA) and, 6 weeks later, an isocaloric meal containing omega-3 polyunsaturated fatty acids. The subjects were phenotyped for plasma Lp(a) and apo C-III levels, apo(a) and apo E isoforms, and lipoprotein lipase and hepatic lipase activities. Vitamin A was included in the meal as a marker of intestinally derived TRP. Following the SFA meal, three of the HP subjects showed a decrease in plasma levels of Lp(a) that lasted 10 to 12 hours in the presence of an increased hypertriglyceridemic response. Two HP subjects who had low preprandial lipoprotein lipase activity and elevated plasma apo C-III levels showed an increase in plasma Lp(a) levels along with the hypertriglyceridemic excursion. However, in all cases, inclusive of the controls, there was an elevation in plasma levels of TRP of Sf greater than 1,000 that contained apo B-100-apo(a) 6 to 8 hours after the meal. This TRP excursion appeared not to be related to the basal levels of plasma Lp(a), high-density lipoprotein (HDL) cholesterol, TGs, or apo(a) and apo E isoforms, and it did not coincide with the retinyl ester peak.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular binding and degradation of lipoprotein (a).

Lp(a) is an important contributing factor to the development of atherosclerosis, and in structure is similar to LDL. Given the central role of the LDL receptor (LDL-R) in the metabolism of LDL, we felt that a study of the binding and degradation of Lp(a) facilitated by the LDL-R of human monocyte derived macrophages (HMDM) would be of value in understanding its pathological nature. In this study we compared equimolar amounts of Lp(a) and LDL and found that nearly equal amounts of Lp(a) and LDL bound to the LDL-R of HMDM at 4 degrees C, however the affinity of both lipoproteins was much lower than has been observed for the LDL-R of fibroblasts, being 0.80 muM for Lp(a) and 0.23 muM for LDL. The binding of Lp(a) to HMDM could be competed by 63% with a 50-fold excess of LDL. Degradation of Lp(a) at 37 degree C, unlike 4 degrees C binding, was mainly nonspecific (75% of total Lp(a) degradation) and when compared on an equimolar basis, nearly 6 times more LDL than Lp(a) was processed by the LDL-R pathway in 5 hr. Lower degradation of Lp(a) appears to be the result of lower binding at 37 degree C and a lower degradation rate when compared to LDL. It was not caused by increased intracellular accumulation or retroendocytosis. Degradation of both lipoproteins was only modestly affected by up and down regulation of the LDL-R. Because the binding of LDL at 4 degrees C and degradation at 37 degree C is mainly LDL-R specific, whereas only the 4 degree C binding of Lp(a) is so, suggests that the poor LDL-R dependent degradation of Lp(a) at 37 degree C is caused by a conformational change that is inducted in Lp(a) upon lowering the temperature to 4 degree C which allows better recognition of Lp(a) by the HMDM LDL-R.

Polymorphic forms of Lp(a) with different structural and functional properties: cold-induced self-association and binding to fibrin and lysine-Sepharose.

Two different Lp(a) polymorphs were isolated from the same individual and shown to have important differences both in their solution properties and in interaction with lysine Sepharose and fibrin. One Lp(a) particle (d-Lp(a)) with a large apo(a) isoform had a density of 1.087 g/ml and a molecular weight of 3.17 million, while the other Lp(a) particle with a small apo(a) isoform having a mobility faster than that of apoB was larger and had a molecular weight of 3.75 million and a density of 1.054 g/ml. D-Lp(a) underwent cold-induced self-association and also had a higher affinity for lysine Sepharose, whereas the other Lp(a) polymorph did not. Both Lp(a) particles bound fibrin via two different binding sites, one of which involved fibrin lysine residues which are also recognized by plasminogen. Lysine-mediated binding of d-Lp(a) by fibrin was ten times stronger than that of the other Lp(a) particle, whereas non-lysine-mediated binding of either Lp(a) species by fibrin was of equal strength. At saturation, 80% of d-Lp(a) bound fibrin at sites that did not involve lysine residues, whereas only 33% of the other Lp(a) polymorph bound to these sites. These findings indicate that the binding of Lp(a) to fibrin is more complex than previously thought and imposes another layer of difficulty on our understanding of how Lp(a) regulates and/or impairs fibrinolysis.

Lipoprotein (a) displays increased accumulation compared with low-density lipoprotein in the murine arterial wall.

Lipoprotein (a) (Lp(a)) is known to be an independent risk factor for cardiovascular disease, but the mechanisms by which it contributes to this disease remain unclear. Current evidence indicates that the closely related plasma particle, low-density lipoprotein (LDL), may initiate atherosclerosis through deposition in the arterial wall. This study has compared the ability of both lipoproteins to enter and accumulate within the arterial wall. Experiments were conducted in vivo with animals from two strains of mice: C57BL/6 mice, which develop fatty streak lesions upon challenge by a high-fat diet, and C3H/HeJ mice, which are resistant to lesion formation. Animals from both strains were maintained up to 16 weeks either on chow or high-fat diet. The mice were intravenously injected with 125I-labeled human Lp(a) or 125I-labeled human LDL in equimolar amounts and the lipoprotein allowed to circulate in vivo for 2 or 24 h. Transverse sections of the aortic root including sites of predilection for lesion formation at the commissures of the valve were prepared and examined after autoradiography. The autoradiographic grains over lesions and histologically uninvolved areas were enumerated and compared after normalization. Both Lp(a) and LDL demonstrated nearly ten times greater accumulation in lesions compared with histologically uninvolved areas from C57BL/6 mice. Analyses of histologically uninvolved areas from both strains of mice showed a significantly higher accumulation of Lp(a) than LDL. Finally, significantly higher accumulations of both Lp(a) and LDL occurred in the histologically uninvolved intima and subintima of lesion-prone C57BL/6 mice as compared with lesion-resistant C3H/HeJ mice after 5 weeks on the diets. We propose that enhanced accumulation of Lp(a) in the arterial wall accounts, in part, for the increased risk of cardiovascular disease.

Evaluation of common electrophoretic methods in determining the molecular weight of apolipoprotein(a) polymorphs.

Five different gel systems were evaluated for their utility in determining the molecular weights of apolipoprotein(a) (apo(a)) polymorphs by SDS polyacrylamide or agarose gel electrophoresis. Three linear polyacrylamide gradient gels (2-16% from Isolab (Akron, OH), 4-15% from Pharmacia (Piscataway, NJ), and 2.5-6% homemade), a 4% polyacrylamide, and a 1.5% agarose gel were examined. Crosslinked phosphorylase B oligomers served as molecular weight standards. Molecular weights of four different apo(a) polymorphs were determined in each gel system and compared to values measured previously by sedimentation equilibrium. The results indicate that molecular weights obtained by gradient polyacrylamide gel electrophoresis were within 10% and often not statistically different from values acquired by sedimentation equilibrium. The use of homogenous 4% polyacrylamide and 1.5% agarose gels led to molecular weights that were overestimated by 20 and 60-70%, respectively. ApoB100, which is a commonly used molecular weight marker, was found to have anomalously fast mobility in each of the four polyacrylamide gel systems. Because its use would lead to overestimated apo(a) molecular weights, it was not useful as a molecular weight standard. Our results indicate that SDS-gradient polyacrylamide gel electrophoresis with cross-linked phosphorylase B as standard is a suitable gel system for evaluating apo(a) molecular weights.

Molecular weight determination of lipoprotein(a) [Lp(a)] in solutions containing either NaBr or D2O: relevance to the number of apolipoprotein(a) subunits in Lp(a).

Molecular weight determination of low-density lipoprotein (LDL) is usually performed in solutions containing high concentrations of salt (up to 13.4 M NaBr) by sedimentation velocity and diffusion experiments, because it does not preferentially bind salt or water. Considering that lipoprotein(a) [Lp(a)] is structurally similar to LDL, differing only by the presence of Apo(a), the molecular weight, M, of Lp(a) has also been measured in solutions containing high concentrations of NaBr. We questioned the suitability of this practice by comparing the apparent molecular weight, Mapp, and partial volume, phi', of Lp(a) determined by sedimentation and flotation equilibrium in a three-component system containing NaBr with the analogous parameters, M and partial specific volume, v, determined in a two-component system containing D2O. LDL served as a control. In agreement with previous findings obtained with different methods, our results indicate no significant differences in M and v of four different LDL samples and apparently no significant preferential binding of solvent components. In contrast, values of Mapp and phi' of Lp(a) evaluated in NaBr are significantly greater than M and v. Preferential binding of solvent components appeared to be a function of Apo(a) mass or the number of kringle IV domains, as expressed by increasing percentage differences between the two sets of parameters, ranging from 4 to 13% in M and 0.2 to 0.5% in v of Lp(a) species having Apo(a) with 15-27 kringle IV domains. Furthermore, our results indicate that the variable Apo(a) kringle IV domains are more involved in this process than the constant domain of Apo(a). These findings indicate that the Lp(a) molecular weight should be determined in D2O and that high concentrations of NaBr should be avoided as their use would lead to overestimated molecular weights and partial specific volumes. Application of this method to the question of how much Apo(a) is released upon the reduction of Lp(a) led to the conclusion that Lp(a) contains only one Apo(a) molecule.

Comparative study of density distribution of plasma lipoproteins of normo- and hypercholesterolemic rhesus monkeys and humans.

Density gradient centrifugation was used to characterize the lipoprotein distribution of rhesus monkeys and human subjects with normal and elevated cholesterol levels. The lipoprotein profile of control monkeys differed from that of normal humans in that their density distribution as a whole was shifted to lower density. The most striking difference was that both rhesus high density lipoprotein (HDL) subspecies had densities lower than human HDL3, with one component having a lower, and the other a higher, density than human HDL2. Rhesus monkeys fed a diet supplemented with 0.5% cholesterol and 15% lard were divided into two groups. Those animals with cholesterol levels less than 435 mg/dl had a high apo A-I concentration and HDL subspecies similar to human HDL2 and HDL3, whereas those with concentrations greater than 435 mg/dl had a low apo A-I concentration and HDL species with a density either similar to, or exceeding that, of human HDL3. Low density lipoprotein (LDL) density decreased in both groups of hypercholesterolemic monkeys and was particularly pronounced in animals with cholesterol levels above 435 mg/dl. When the lipoprotein profiles of normal humans were compared with those having hypercholesterolemia, increases in the density of both HDL2 and HDL3 in hypercholesterolemic men and to a lesser extent in the density of HDL2 in women were detected. The results indicate that the overall density distribution of plasma lipoprotein is different between rhesus monkeys and humans, and may vary within each species as a function of the nutritional status. It follows that it is difficult to define lipoprotein classes of one species by using density intervals determined for another. Furthermore, these intervals can be inadequate in identifying lipoprotein classes within an individual before and after dietary manipulations.

Physicochemical characterization of Rhesus low density lipoproteins.

The serum low density lipoprotein (LDL; p 1.019-1.050 g/ml) of the normal Macaca mulatta monkey (rhesus), kept on a low-fat Purina primate chow diet, was isolated by ultracentrifugal flotation, and its physicochemical properties were compared with those previously reported for human LDL. Rhesus LDL was found to be chemically similar to human LDL. The values for the sedimentation (S25, w-O) and diffusion (D25,w-O) coefficients were 7.09 S and 2.50 times 10- minus-7 cm-2 sec- minus-1, respectively. The intrinsic viscosity was 3.40 ml g- minus-1. The partial specific volume of rhesus LDL, determined in an Anton Paar precision density meter, was 0.960 ml g- minus-1. Molecular weights, calculated from a combination of S-O and D-O and of S-O and [n], were in agreement with the weight-average molecular weight, Mw, of 2.29 times 10-6 obtained by high-speed sedimentation equilibrium. In addition, a Z-average molecular weight, Mz, of 2.73 times 10-6 was calculated because curvature in the graphs of log c vs. r-2 indicated that rhesus LDL was heterogeneous. From the frictional ratio of 1.02, a maximum hydration of 0.1 g of H2O/g of lipoprotein was obtained. On electron micrographs, rhesus LDL appeared spherical with a mean diameter of 196 A, which was substantiated by hydrodynamic analysis.

Enzyme-linked immunoassay for Lp[a].

Based on our findings that rabbit antisera raised against human Lp[a] or apo[a] have the potential to cross-react with plasminogen, and in some cases have nearly equal affinities for plasminogen and Lp[a], we have developed an assay for plasma Lp[a] based on a "sandwich" ELISA that is insensitive to the presence of plasminogen. This was accomplished through the use of anti-apo[a] as a capture antibody and quantitation of the bound Lp[a], i.e., the apoB-100-apo[a] complex, with an anti-apoB antibody. Although apo[a] is heterogeneous in size, all Lp[a] particles tested, either in pure form or contained in whole plasma, gave parallel dose-response curves and were immunologically equivalent. However, when purified Lp[a] particles with different apo[a] isoforms were studied, those having larger isoforms were, on a weight basis, less reactive than those having a smaller size. Nearly equivalent reactivity was observed when protein concentration was expressed on a molar basis. The distribution of Lp[a] in a population of 84 subjects was skewed with one-third of the individuals having less than 1 mg/dl Lp[a] protein. All subjects tested had measurable concentrations of Lp[a] with a lower limit of detection of 0.030 mg/dl Lp[a] protein. The mean level was 3.2 mg/dl with a range of 0.045 to 13.3 mg/dl. These studies demonstrate the successful development of an ELISA for Lp[a] protein that is insensitive to the presence of plasminogen; that heterogeneity of Lp[a] and apo[a] are an important source of variation in the assay; and the need for an appropriate Lp[a] standard in order to minimize this variation.

Isolation of apolipoprotein(a) from lipoprotein(a).

An easy method was developed for the rapid and selective isolation of apo(a) from human plasma Lp(a). This procedure was applied to a "low density" Lp(a) subspecies (usually found in the density interval 1.050 to 1.070 g/ml) from a single individual whose apo(a) was of a size smaller than apoB-100. After reduction with 0.01 M dithiothreitol, apo(a) was separated from the Lp(a) particle by rate zonal centrifugation on a 7.5-30% NaBr density gradient. Two completely water-soluble products were recovered: apo(a), which contained less than 1% each of phospholipid and cholesterol, remained at the bottom of the gradient, and a lipid-rich floating LDL-like particle which contained apoB but not apo(a) and which we referred to as Lp(a-). The separation of these two components was also achieved by subjecting reduced Lp(a) to electrophoresis on 2.5-16% polyacrylamide gradient gels. However, dissociation of reduced Lp(a) could not be achieved by gel filtration in either low or high salt solutions. These observations indicate that apo(a) is associated to Lp(a) by non-covalent interactions in addition to its disulfide linkage to apoB. The latter is sensitive to chemical reduction whereas the former are broken through the action of a gravitational or electrical field.

Heterogeneity of human plasma lipoprotein (a). Isolation and characterization of the lipoprotein subspecies and their apoproteins.

Lipoprotein (a) (Lp(a] from the plasma of normolipidemic human donors was isolated by rate zonal and isopycnic density gradient ultracentrifugation. The final preparations usually contained varying amounts of isopycnic low-density lipoproteins (LDL), which were totally removed either by heparin-Sepharose column chromatography or by chromatofocusing. The Lp(a) preparations exhibited both inter- and intraindividual density heterogeneity which was accounted for by the differences in their protein and lipid composition. In addition, there was heterogeneity in the size of apoprotein (a) (apo(a] which was found to be linked to apoprotein B (apo-B) through disulfide bonds. Three different apo(a) species were obtained; they had a size either smaller, equal to, or larger than apo-B-100, the protein moiety of LDL. The apo(a) that was smaller than apo-B resided in a low-density Lp(a) particle whose peak was in the 1.019-1.063 g/ml density range. The larger apo(a) was a component of the dense Lp(a) particle and was responsible for the increased density in this Lp(a) species. The third apo(a) which was equivalent in size to apo-B resided in a density range intermediate between the other two Lp(a)s. It is concluded that Lp(a) may differ not only from one individual to another, but also within the same individual who may have more than one Lp(a) species. Part of this heterogeneity may be accounted for by differences in the (a) polypeptide.

Physicochemical properties of apolipoprotein(a) and lipoprotein(a-) derived from the dissociation of human plasma lipoprotein (a).

Chemical reduction of human plasma lipoprotein(a) (Lp(a)) yielded two water-soluble products which were separated by rate zonal ultracentrifugation. Apolipoprotein(a) (apo(a)) was completely recovered from the bottom of the gradient, whereas lipoprotein(a-) (Lp(a-)), which contained all of the lipids and apo-B100 of Lp(a), floated. By the techniques of circular dichroism and viscometry Lp(a-) was identical to low density lipoprotein (LDL). Lp(a-) was slightly larger in mass than autologous LDL and contained proportionally more triglyceride. The difference in mass between Lp(a) and Lp(a-) was accounted for by the loss of 2 molecules of apo(a) from the Lp(a) particle. The molecular weight of reduced and carboxymethylated apo(a) was 281,000 as determined by sedimentation equilibrium in 6 M guanidine HCl. By circular dichroism the structure of apo(a) was mostly random (71%) with the remainder representing 8% alpha-helix and 21% beta-sheet; its intrinsic viscosity, 28.3 cm3/g, was consistent with an extended flexible coil. The amino acid composition was characterized by an unusually high content of proline (11.4 mol %) as well as tryptophan, tyrosine, arginine, threonine, and a low amount of lysine, phenylalanine, and isoleucine. Apo(a) contained 28.1% carbohydrate by weight represented by mannose, galactose, galactosamine, glucosamine, and sialic acid in an approximate molar ratio of 3:7:5:4:7, respectively. Overall, the structure of Lp(a) appears to be consistent with a rigid spherical LDL-like core particle which, as a consequence of its association with a flexible glycoprotein such as apo(a), favors the entrapment of significant amounts of hydrodynamically associated solvent. Furthermore, the Lp(a-) remnant generated by the removal of apo(a) from Lp(a) was similar in structure but not identical to autologous LDL.