Immunochemical characterization and quantitation of lipoprotein (a) in baboons. Development of an assay depending on two antigenically distinct proteins

The ins and outs of lipoprotein(a) assay methods

Pathophysiological, epidemiological and genetic studies convincingly showed lipoprotein(a) (Lp(a)) to be a causal mediator of atherosclerotic cardiovascular disease (ASCVD). This happens through a myriad of mechanisms including activation of innate immune cells, endothelial cells as well as platelets. Although these certainties whether or not Lp(a) is ready for prime-time clinical use remain debated. Thus, remit of the present review is to provide an overview of different methods that have been employed for the measurement of Lp(a). The methods include dynamic light scattering, multi-angle light scattering analysis, near-field imaging, sedimentation, gel filtration, and electron microscopy. The development of multiple Lp(a) detection methods is vital for improved prediction of ASCVD risk.

Structure, function, and genetics of lipoprotein (a)

Lipoprotein (a) [Lp(a)] has attracted the interest of researchers and physicians due to its intriguing properties, including an intragenic multiallelic copy number variation in the LPA gene and the strong association with coronary heart disease (CHD). This review summarizes present knowledge of the structure, function, and genetics of Lp(a) with emphasis on the molecular and population genetics of the Lp(a)/LPA trait, as well as aspects of genetic epidemiology. It highlights the role of genetics in establishing Lp(a) as a risk factor for CHD, but also discusses uncertainties, controversies, and lack of knowledge on several aspects of the genetic Lp(a) trait, not least its function.

Improved method for making nondenaturing composite gradient gels for the electrophoretic separation of lipoproteins

Nondenaturing gradient gel electrophoresis continues to be used widely for resolution and characterization of lipoprotein subclasses. Methods for making such gels in the laboratory have been published, but occasionally samples do not display uniform mobilities for all lanes in a laboratory-made gel. To help overcome this limitation, we recommend a modification

Locus controlling LDL cholesterol response to dietary cholesterol is on baboon homologue of human chromosome 6

OBJECTIVE

Cholesterolemic responses to dietary lipids are known to be heritable, but the genes that may affect this response have yet to be identified. Using segregation analysis, we previously detected a potential quantitative trait locus (QTL) in baboons that influenced low density lipoprotein cholesterol response to dietary cholesterol. We performed linkage analyses to locate this QTL by using data on the baboon genetic linkage map.

METHODS AND RESULTS

We obtained evidence for linkage of this potential QTL to the same locus (D6S311) on the baboon homologue of human chromosome 6 by using variance components and parametric linkage analysis methods (2-point lod scores 4.17 [genomic probability value 0.008] and 2.81 [genomic P=0.10], respectively). Linkage analyses of serum levels of apolipoprotein B dietary response, a correlated trait, also gave weak suggestive evidence of linkage to this chromosomal region (maximum 2-point lod score 1.91). Although the LPA locus is nearby, we found no evidence of linkage with LPA.

CONCLUSIONS

This report is the first to localize, in any primate species, a potential QTL that influences low density lipoprotein cholesterol response to dietary cholesterol.

Functional analysis of the chimpanzee and human apo(a) promoter sequences: identification of sequence variations responsible for elevated transcriptional activity in chimpanzee

Lp(a) concentrations vary considerably among individuals and are primarily determined by the apo(a) gene locus. We have previously shown that mean plasma Lp(a) levels in the chimpanzee are significantly higher than those observed in humans (Doucet, C., Huby, T., Chapman, J., and Thillet, J. (1994) J. Lipid Res 35, 263-270). To evaluate the possibility that this difference may result from a high level of expression of chimpanzee apo(a), we cloned and sequenced 1.4 kilobase (kb) of the 5'-flanking region of the gene and compared promoter activity to that of its human counterpart. Sequence analysis revealed 98% homology between chimpanzee and human apo(a) 5' sequences; among the differences observed, two involved polymorphic sites associated with Lp(a) levels in humans. The TTTTA repeat located 1.3 kb 5' of the apo(a) gene, present in a variable number of copies (n = 5-12) in humans, is uniquely present as four copies in the chimpanzee sequence. The second position concerns the +93 C>T polymorphism that creates an additional ATG start codon in the human apo(a) gene, thereby impairing translation efficiency. In chimpanzee, this position did not appear polymorphic, and a base difference at position +94 precluded the presence of an additional ATG. In transient transfection assays, the chimpanzee apo(a) promoter exhibited a 5-fold elevation in transcriptional activity as compared with its human counterpart. This marked difference in activity was maintained with either 1.4 kb of 5' sequence or the minimal promoter region -98 to +141 of the human and chimpanzee apo(a) genes. Using point mutational analyses, nucleotides present at positions -3, -2, and +8 (relative to the start site of transcription) were found to be essential for the high transcription efficiency of the chimpanzee apo(a) promoter. High transcriptional activity of the chimpanzee apo(a) gene may therefore represent a key factor in the elevated plasma Lp(a) levels characteristic of this non-human primate.

Evidence that multiple genes influence baseline concentrations and diet response of Lp(a) in baboons

We investigated the response of lipoprotein(a) [Lp(a)] levels to dietary fat and cholesterol in 633 baboons fed a series of 3 diets: a basal diet low in cholesterol and fat, a high-fat diet, and a diet high in fat and cholesterol. Measurement of serum concentrations in samples taken while the baboons were sequentially fed the 3 diets allowed us to analyze 3 Lp(a) variables: Lp(a)(Basal), Lp(a)(RF) (response to increased dietary fat), and Lp(a)(RC) (response to increased dietary cholesterol in the high-fat environment). On average, Lp(a) concentrations significantly increased 6% and 28%, respectively, when dietary fat and cholesterol were increased (P<0.001). As expected, most of the variation in Lp(a)(Basal) was influenced by genes (h(2)=0.881). However, less than half of the variation in Lp(a)(RC) was influenced by genes (h(2)=0.347, P<0. 0001), whereas the increase due to dietary fat alone was not significantly heritable (h(2)=0.043, P=0.28). To determine whether Lp(a) phenotypic variation was due to variation in LPA, the locus encoding the apolipoprotein(a) [apo(a)] protein, we conducted linkage analyses by using LPA genotypes inferred from the apo(a) isoform phenotypes. All of the genetic variance in Lp(a)(Basal) concentration was linked to the LPA locus (log of the odds [LOD] score was 30.5). In contrast, linkage analyses revealed that genetic variance in Lp(a)(RC) was not linked to the LPA locus (LOD score was 0.036, P>0.5). To begin identifying the non-LPA genes that influence the Lp(a) response to dietary cholesterol, we tested, in bivariate quantitative genetic analyses, for correlation with low density lipoprotein cholesterol [LDLC; ie, non-high density lipoprotein cholesterol less the cholesterol contribution from Lp(a)]. LDLC(Basal) was weakly correlated with Lp(a)(Basal) (rho(P)=0.018). However, LDLC(RC) and Lp(a)(RC) were strongly correlated (rho(P)=0. 382), and partitioning the correlations revealed significant genetic and environmental correlations (rho(G)=0.587 and rho(E)=0.251, respectively). The results suggest that increasing both dietary fat and dietary cholesterol caused significant increases in Lp(a) concentrations and that the response to dietary cholesterol was mediated by a gene or suite of genes that appears to exert pleiotropic effects on LDLC levels as well. The gene(s) influencing Lp(a) response to dietary cholesterol is not linked to the LPA locus.

Stability of apolipoprotein(a) isoform phenotype to postmortem conditions

Studies, such as the one entitled Pathobiological Determinants of Atherosclerosis in Youth or PDAY, have investigated the relationship of atherosclerotic lesions with lipid and lipoprotein risk factors. However, it is possible that proteolytic hydrolysis during the postmortem period could alter protein components of lipoproteins. Apo(a) is the defining protein of Lp(a), a lipoprotein associated with risk of cardiovascular disease. Apo(a) proteins are highly polymorphic for size and we simulated, using baboon tissues, the postmortem conditions for the PDAY study in order to determine their effects on apo(a) phenotypes. The bulk of the postmortem samples (93%) were correctly phenotyped and showed little or no evidence of degradation. Thus, apo(a) isoform size phenotypes can be determined in postmortem samples, although with caution.

Lipoprotein(a) as a risk factor for coronary artery disease

Since its identification by Kåre Berg in 1963, lipoprotein(a) [Lp(a)] has become a focus of research interest owing to the results of case-control and prospective studies linking elevated plasma levels of this lipoprotein with the development of coronary artery disease. Lp(a) contains a low-density lipoprotein (LDL)-like moiety, in which the apolipoprotein B-100 component is covalently linked to the unique glycoprotein apolipoprotein(a) [apo(a)]. Apo(a) is composed of repeated loop-shaped units called kringles, the sequences of which are highly similar to a kringle motif present in the fibrinolytic proenzyme plasminogen. Variability in the number of repeated kringle units in the apo(a) molecule gives rise to different-sized Lp(a) isoforms in the population. Based on the similarity of Lp(a) to both LDL and plasminogen, it has been hypothesized that the function of this unique lipoprotein may represent a link between the fields of atherosclerosis and thrombosis. However, determination of the function of Lp(a) in vivo remains elusive. Although Lp(a) has been shown to accumulate in atherosclerotic lesions, its contribution to the development of atheromas is unclear. This uncertainty is related in part to the structural complexity of the apo(a) component of Lp(a) (particularly apo(a) isoform size heterogeneity), which also poses a challenge for standardization of the measurement of Lp(a) in plasma. The fact that plasma Lp(a) levels are largely genetically determined and vary widely among different ethnic groups adds scientific interest to the ongoing study of this enigmatic particle. Most recently, the identification of proteolytic fragments of apo(a) in both plasma and urine has fueled speculation about the origin of these fragments and their possible function in the atherosclerotic process.

Insulin and 2-hour glucose levels are inversely related to Lp(a) concentrations controlled for LPA genotype

In this study we assessed the relationship of lipoprotein(a) [Lp(a)] with diabetes status and with measures of glucose and insulin in a population of Mexican Americans having a high prevalence of non-insulin-dependent diabetes mellitus (NIDDM). Because of enormous allelic diversity at LPA [the locus encoding the apo(a) protein] that directly influences Lp(a) concentrations, it was first necessary to adjust for the large effects of variation at LPA. We calculated residual Lp(a) concentration as the difference between observed and expected; expected Lp(a) concentration was based on information from all family members sharing each identical-by-descent (IBD) allele. We found significant effects of sex and age on residual Lp(a) concentrations that increased with age (P=0.0004) and in females (P=0.0034). Although diabetes status per se was not related to residual Lp(a) concentrations (P=0.097), we found that residual Lp(a) concentrations were inversely correlated with fasting insulin (P=0.0017) and with insulin (P=0.0028) and glucose (P=0.0429) concentrations measured 2 hours after a glucose challenge. Furthermore, significant inverse correlations with the 2 insulin measures were observed for a subgroup of nondiabetic individuals. Inclusion of 2 lipid measures (plasma concentrations of cholesterol and triglycerides) in the models showed that the correlations with insulin and glucose were independent of the relationship between Lp(a) concentrations and the lipid measures. Also, we determined the residual size for each apo(a) isoform by adjusting for the IBD isoform group average. Although not related to diabetes status, residual apo(a) isoform size was positively correlated with fasting insulin (P=0.0013) and with 2-hour glucose (P=0.0246) and 2-hour insulin (P=0.0182) concentrations. In addition, significant correlations for all 4 measures were found for the subgroup of nondiabetic individuals. Thus, the results demonstrate that glucose-intolerant individuals have significantly lower residual Lp(a) concentrations and a significant increase of residual apo(a) size.

Biogenesis of Lp(a) in transgenic mouse hepatocytes

Lipoprotein(a) [Lp(a)] biogenesis was examined in primary cultures of hepatocytes isolated from mice transgenic for both human apolipoprotein(a) [apo(a)] and human apoB. Steady-state and pulse-chase labeling experiments demonstrated that newly synthesized human apo(a) had a prolonged residence time (approximately 60 min) in the endoplasmic reticulum (ER) before maturation and secretion. Apo(a) was inefficiently secreted by the hepatocytes and a large portion of the protein was retained and degraded intracellularly. Apo(a) exhibited a prolonged and complex folding pathway in the ER, which included incorporation of apo(a) into high molecular weight, disulfide-linked aggregates. These folding characteristics could account for long ER residence time and inefficient secretion of apo(a). Mature apo(a) bound via its kringle domains to the hepatocyte cell surface before appearing in the culture medium. Apo(a) could be released from the cell surface by apoB-containing lipoproteins. These studies are consistent with a model in which the efficiency of post-translational processing of apo(a) strongly influences human plasma Lp(a) levels, and suggest that cell surface assembly may be one pathway of human Lp(a) production in vivo. Transgenic mouse hepatocytes thus provide a valuable model system with which to study factors regulating human Lp(a) biogenesis.

Identification of cryptic peanut agglutinin-reactive sites in human lipoprotein(a)

After treatment of human lipoprotein(a) (Lp(a)) with neuraminidase, formerly cryptic sites became available for binding to peanut agglutinin (PNA) lectin and a Thomsen-Friedenreich antigen (T-antigen)-specific monoclonal antibody. The PNA-reactive sites were localized to the apo(a) moiety of Lp(a) and O-specific carbohydrate side chains. Lp(a) with larger isoforms of apo(a) contained more potential PNA reactivity per molecule of Lp(a) apoB than did smaller isoforms. Very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL) did not contain comparable amounts of the cryptic PNA-reactive sites.

Genetic basis for multimodal relationship between apolipoprotein (a) size and lipoprotein (a) concentration in Mexican-Americans

The reported general inverse relationship between apolipoprotein (a) (apo(a)) size and plasma lipoprotein (a) (Lp(a)) concentrations was further characterized using 927 samples taken from members of 42 Mexican-American families. When all samples were displayed in a scatter plot of apo(a) size versus natural log of Lp(a) concentration, the expected inverse relationship was observed (r2 = 0.24). However, the scatter plot revealed a multimodal pattern with at least two distinct modes of the inverse relationship between apo(a) size and Lp(a) concentration. Plots of 148 single-banded samples also showed the multimodal pattern, indicating that this pattern did not result from a difference between double- and single-banded samples. Also measured was the Lp(a) concentration associated with each of 508 apo(a) isoforms in samples from 254 double-banded phenotype individuals. These separated isoforms also showed the multimodal pattern. Using pedigree information, 29 different alleles were identified which were found in three or more family members. About 85% of variation in Lp(a) was explained by allele information, suggesting a genetic basis for the multimodal pattern. Thus, the data demonstrate at least two series of apo(a) alleles that have distinct relationships between apo(a) size and Lp(a) concentration.

Lipid and lipoprotein factors associated with variation in Lp(a) density

To determine the facets of lipoprotein metabolism associated with variation in lipoprotein(a) [Lp(a)] density, we examined by density gradient ultracentrifugation 246 plasma samples exhibiting single apo(a) isoform band phenotypes. Estimated molecular weights of apo(a) isoforms ranged from 488 to 874 kD, and they accounted for approximately 80% of variation in Lp(a) density. After adjustment for variation in the protein composition, we found in univariate analyses that variation in residual Lp(a) density was associated with 10 different measures of lipoprotein size and concentration. Residual Lp(a) density was positively correlated with measures of apoB-containing lipoproteins and negatively correlated with measures of HDL. HDL size phenotypes were based on nondenaturing gradient gel electrophoresis fractions after staining for esterified cholesterol with Sudan black B and for apoA-I with immunoblotting methods. The HDL size variables in each case had higher correlations with residual Lp(a) density than did the HDL concentration measures. Stepwise regression analyses selected two lipoprotein variables (LDL density and HDL size) that were significantly correlated with residual Lp(a) density, and they accounted for approximately 35% of variation in density. The densities of LDL and Lp(a) were highly correlated, and additional stepwise regression analyses showed that they were similarly correlated with triglyceride concentration and with a measure of HDL size. Thus, with respect to residual Lp(a) density, the results show that small, dense Lp(a) particles are found under conditions leading also to small, dense LDL particles and to small, dense HDLs.

Intracellular processing of apo(a) in primary baboon hepatocytes

We have developed a serum-free medium for the long-term culture of highly differentiated primary baboon hepatocytes. Hepatocytes isolated from animals with defined plasma Lp(a) levels and apo(a) glycoprotein phenotypes were used to study the assembly of Lp(a). A combination of steady-state and pulse-chase labeling studies and endoglycosidase digests demonstrated that apo(a) was synthesized as a lower molecular weight precursor. After a prolonged period of time in the endoplasmic reticulum, apo(a) was converted to a mature form and secreted. A proportion of mature apo(a) also had a prolonged residence time in the trans Golgi apparatus. In all experiments, apoB co-immunoprecipitated with apo(a) from the culture medium but not from the cell lysates, supporting an extracellular association of the proteins for the formation of Lp(a). Analysis of hepatic RNA from 29 'null' Lp(a) phenotype baboons revealed that one-third of the animals had detectable apo(a) transcripts, whereas the remainder had no detectable apo(a) mRNA. The baboon hepatocyte system therefore represents a valuable model to examine the effect of allelic variation at the apo(a) locus on Lp(a) assembly.

Automated latex nephelometric immunoassay for the measurement of serum lipoprotein (a)

A sensitive immunoassay based on latex particle agglutination has been developed for measuring lipoprotein Lp(a) concentrations in serum or plasma. Carboxylated latex particles (diameter 240 nm) covalently coated with F(ab')2 fragments of anti-lipoprotein Lp(a) antibodies are incubated with diluted sample (400-fold) for 12 min at room temperature, with the resulting agglutination quantified by measuring the change of light-scatter produced. The assay has been automated on the Behring nephelometer analyzer with a sampling rate of 150 samples/hour. This assay generates a standard curve in the range of 27 to 1750 mg/L, showing inter-assay precision of less than 8%. There were no interferences from plasminogen, bilirubin, Intralipid, haemoglobin, rheumatoid factor, and apolipoprotein B. No significant differences were observed when fresh and frozen samples were compared. Sample pretreatment with "Lipoclean" clearing agent and sample lyophilization decreased the agglutinating reaction. In two separate studies using 77 and 112 patient sera the Lp(a) values, determined by the latex nephelometric method, the Terumo Macra Lp(a) ELISA test, and the Pharmacia Apo(a) radioimmunoassay method, gave correlation coefficients of 0.948 and 0.974, respectively. Physiological lipoprotein (a) values were determined in a blood donor group, with the distribution of serum Lp(a) highly skewed, with a mean (SD) and median values of 213(236) mg/L and 116 mg/L, respectively. Concentrations of Lp(a) were found to be age- and sex-independent. This latex nephelometric procedure is a convenient method and an interesting alternative to other immunoassays for routine measurement of human lipoprotein (a).

Dramatic differences in lipoprotein composition among gray short-tailed opossums (Monodelphis domestica) fed a high cholesterol/saturated fat diet

Lipoproteins of gray short-tailed opossums (Monodelphis domestica) were characterized to determine the basis of differences among individuals in response to a challenge diet enriched in saturated fat and cholesterol. Animals were selected from two phenotypic groups (high and low plasma cholesterol response to the challenge diet). Half of the animals in each group were fed basal diet (8.1% fat and 0.04% cholesterol by weight), and the remainder were fed challenge diet (17.7% fat and 0.61% cholesterol). The plasma cholesterol values of both groups fed the basal diet and of low responders fed the challenge diet were similar. In addition, both very-low-density and low-density lipoproteins (VLDL+LDLs) and high-density lipoproteins (HDLs) were similar among these groups in density and in lipid and apolipoprotein compositions. In contrast, the high responders fed the challenge diet showed a 7-fold increase in total plasma cholesterol, which was primarily a consequence of increases in the VLDL+LDL cholesterol component defined by heparin-Mn precipitation. Moreover, the VLDL+LDLs were more heterogeneous and were characterized by decreased densities. The VLDL+LDLs of the high-responding group had higher levels of apolipoprotein (apo) B and apoE than the other groups. Plasma apoB concentrations estimated by dot blotting techniques increased by 3-fold, and apoE by 44-fold in the high responding group. Understanding the factor(s) mediating responder phenotype in this new model species will expand our knowledge of the regulation of lipemic response to diet.

Optimization and characterization of capture ELISA methodology for Lp(a) lipoprotein quantification

In order to better characterize and optimize a typical capture ELISA system for Lp(a) lipoprotein, we have analysed kinetic details of the reaction. Plate coating with polyclonal antibody, recognition of captured analyte with monoclonal antibody, and detection of monoclonal antibody with alkaline phosphatase-labeled antiglobulin were essentially complete after one hour, probably being driven forward by a relative excess of reagent. However, complete capture of the Lp(a) analyte required about 6 h at low input concentrations. Shorter time periods for capture might therefore result in decreased sensitivity and reproducibility. Deviations from linearity in the assay dose response were associated with incomplete capture of Lp(a) and significant depletion of the monoclonal recognition antibody. With the final reaction conditions described, no significant differences in immunochemical reactivity between samples were found by analysis of dose response slopes. Finally, interferences from plasminogen, -20 degrees C storage, anticoagulants, LDL, haemolysis, and bilirubin were minimal.

Apolipoprotein B-32: a new truncated mutant of human apolipoprotein B capable of forming particles in the low density lipoprotein range

We have identified a new species of apolipoprotein (apo) B in an individual with heterozygous hypobetalipoproteinemia. The new apo B (apo B-32) is the result of a single point mutation (1450 Gln----Stop) in the apo B gene that prevents full length translation. Apo B-32 is predicted to contain the 1449 amino-terminal amino acids of apo B-100 and is associated with a markedly decreased low density lipoprotein (LDL) cholesterol level. The density distribution of apo B-32 in the plasma lipoproteins makes it unique amongst other truncated apo B species. Normally, apo B-100 is found in both very low density lipoprotein (VLDL) and LDL particles. However, the majority of the apo B-32 protein was found in the high density lipoprotein (HDL) and lipoprotein-deplete (d greater than 1.21 g/ml) fractions, suggesting that it was mainly assembled into abnormally dense lipoprotein particles. A small amount of apo B-32 was also found in the LDL, making it the shortest known apo B variant capable of forming particles in this density range. Apo B-32 was undetected in VLDL. The apo B-32 mutation further defines the minimum length of the apo B protein that is required for the assembly of LDL.

Automated measurement of lipoprotein(a) by immunoturbidimetric analysis

Immunoturbidimetric analysis of lipoprotein(a) in plasma or serum was developed for use on the Roche COBAS FARA II and COBAS MIRA clinical chemistry analyzers. The components of the assay are: (1) buffer consisting of 2.25% polyethylene glycol in phosphate-buffered saline, 0.2% gelatin, and a surfactant; (2) fractionated goat anti-human lipoprotein(a) IgG; (3) five standards with lipoprotein(a) concentrations ranging from 0.05 to 1.0 g/l; (4) two controls with concentrations of approximately 0.2 and 0.5 g/l. The analyzer delivers sample and buffer, incubates the reaction mixture at 37 degrees C for 5 min, delivers neat lipoprotein(a) antibody, and incubates for an additional 10 min. The lipoprotein(a) concentration of samples is calculated by the COBAS DENS (Data Evaluation for Non-linear Standard Curves) option by fitting the standard curve values to a four-parameter logit-log curve model. Total imprecision results (CV%) for the FARA II and MIRA were under 11% (NCCLS protocol EP5-T). The assay is linear beyond the highest calibrator to 2.6 g/l. No interference was observed for plasminogen up to 2.3 g/l, apolipoprotein B up to 4.36 g/l, hemoglobin up to 10 g/l, bilirubin up to 4.0 g/l, and triglycerides up to 4.36 g/l. Comparison with a double monoclonal ELISA used at the Northwest Lipid Research Laboratories yielded: R = 0.970, slope = 1.013, and y-intercept = 0.00009 (n = 37). Comparison with a commercially available ELISA kit for lipoprotein(a) yielded: r = 0.987, slope = 1.243, and y-intercept = 0.024 (n = 40). This assay provides rapid, accurate, and precise screening of lipoprotein(a) in serum or plasma.

A competitive ELISA for lipoprotein(a)

A competitive ELISA for lipoprotein(a) (Lp(a)) is described. The method uses a commercially available polyclonal anti-Lp(a) antibody and an IgG biotinstreptavidin-horseradish peroxidase detection system. The method is simple and robust with an assay sensitivity of 0.7 ng/well (1.4 micrograms/l). The antibody cross-reactivity was 0.14% against LDL and 0.70% against plasminogen. The coefficients of variation obtained with control sera of 266 and 552 mg/l were: 5.0% and 4.6% (n = 6), respectively for the intraassay; and 10.8% and 9.5% (n = 16), respectively for the interassay. The method showed an excellent correlation with a commercial immunoradiometric assay (IRMA), y (ELISA) = 0.94x (IRMA) - 8, (r = 0.98). A recovery study in which a 200 mg/L standard and four plasma samples were diluted with different proportions of a low plasma sample, gave linear relationships and also confirmed the specificity of the antibody.

Linkage of plasminogen (PLG) and apolipoprotein(a) (LPA) in baboons

Four allelic forms of serum plasminogen (PLG) were detected in baboons (Papio hamadryas Linneaus 1758) by isoelectric focusing and were determined to be inherited as autosomal codominant traits. Linkage analysis of data from 179 progeny and their parents revealed that PLG is tightly linked (lod score = 30.20) to the gene encoding apolipoprotein(a) (LPA), as in humans. No recombinant individuals were identified. This is the first linkage detected between PLG and LPA in any species other than humans and is the first genetic linkage identified in a nonhuman primate species by family studies.

Lipoprotein(a): nonhuman primate models

Lipoprotein(a) [Lp(a)] is a low density lipoprotein which has apo(a) disulfide-linked to apoB100. Apo(a) has recently been shown to have a striking homology with plasminogen, a knowledge that has stimulated a lot of interest in the mechanism of atherogenicity and thrombogenicity of this lipoprotein particle. Several studies have documented the presence of Lp(a) in nonhuman primates with particular reference to the rhesus monkeys and baboons. The Lp(a) of rhesus monkey is structurally very similar to that of humans, except for the absence of kringle V and the amino acid composition of the catalytic region. The Lp(a) of nonhuman primates, like their human counterparts, exhibit a wide range of interindividual plasma levels and also a wide size polymorphism of apo(a). Nonhuman primates appear to represent a good model for the study of the structure and biology of Lp(a).

Lipoprotein(a) and apolipoprotein(a) in a New World monkey, the common marmoset (Callithrix jacchus). Association of variable plasma lipoprotein(a) levels with a single apolipoprotein(a) isoform

In an earlier report (Chapman et al, Biochemistry 1979;18:5096-5108), we suggested that the common marmoset may represent an important model for the study of human plasma lipoprotein metabolism. We now extend the interest of this monkey model to the study of lipoprotein(a) (Lp[a]) and apolipoprotein(a) (apo[a]). Density gradient ultracentrifugal fractionation of marmoset plasma revealed a bimodal distribution of Lp(a), with one peak of concentration occurring in association with very low density lipoproteins (VLDLs) and a second in the density range 1.040-1.080 g/ml. The dense Lp(a) subspecies displayed physicochemical properties (chemical composition, particle size, and electrophoretic mobility) that closely resembled those of its counterpart in humans and baboons but that were distinct from those of low density lipoprotein (LDL). Furthermore, the particle size of marmoset Lp(a) was invariant (31 nm) over the density interval 1.040-1.080 g/ml, whereas that of LDL decreased progressively with an increase in density (approximately 26-25.2 nm). Use of polyclonal and monoclonal antibodies to human apo(a) and of a polyclonal antibody to marmoset Lp(a) allowed immunologic identification of a single apo(a) isoform in the marmoset whose size was similar to that of apo B-100; apo(a) and apo B-100 were associated in Lp(a) particles by a disulfide linkage. The total protein mass of apo-Lp(a) was estimated to be 800,000 or more by electrophoresis in sodium dodecyl sulfate-polyacrylamide-agarose gels. The amino acid compositions of marmoset and human apo(a) resembled each other but were distinct from those of the corresponding forms of apo B-100. Immunologic evidence is provided for a high degree of cross reactivity between apo(a) in marmosets, baboons, and humans, supporting the idea of the existence of a marked degree of structural homology between these proteins. In addition, electroimmunoblotting of marmoset apo(a) and marmoset plasminogen showed that these proteins shared certain epitopes in common, suggesting that marmoset apo(a) may possess kringle-like structural features. Finally, despite possession of a single apo(a) isoform, marmoset Lp(a) levels varied over a 100-fold range (0.5-49 mg/dl plasma). Considered together, our present findings suggest that the common marmoset monkey constitutes a unique model in which to study the regulation of apo(a) gene expression and the posttranslational processing of apo(a), as well as factors that modulate the synthesis, intravascular metabolism, and cellular catabolism of Lp(a).

Production of lipoprotein(a) by primary baboon hepatocytes

Primary baboon hepatocytes were cultured in a serum-free medium formulation that permitted the analysis of lipoprotein(a) (Lp(a] production by the cells. The hepatocytes were determined to synthesize Lp(a) on the basis of the following observations: (1) the culture medium reacted in an ELISA designed for detection of baboon Lp(a) in serum samples; (2) the Lp(a)-specific protein, apo(a), was detected in the culture medium by immunoblotting techniques; (3) the unique protein structure of Lp(a) was demonstrated (i.e., association of apo(a) with apoB via interchain disulfide bonds to form apoLp(a]; and (4) the Lp(a) proteins occurred in the medium at a density of about 1.05 g/ml when subjected to density gradient ultracentrifugation. De novo synthesis of Lp(a) by cultured hepatocytes was demonstrated by incorporation of [35S]cysteine. Lp(a) was produced by the hepatocytes throughout a 20 day culture period. Finally, apo(a) isoform patterns in the hepatocyte culture medium and the hepatocyte donors' serum were indistinguishable.