No abstract

Characterization of the assembly of lipoprotein(a) [Lp(a)] is of fundamental importance to understanding the biosynthesis and metabolism of this atherogenic lipoprotein. Since no established cell lines exist that express Lp(a) or apolipoprotein(a) [apo(a)], a "transferrinfection" system for apo(a) was developed utilizing adenovirus receptor-and transferrin receptor-mediated DNA uptake into cells. Using this method, different apo(a) cDNA constructions of variable length, due to the presence of 3, 5, 7,9, 15, or 18 internal kringle IV sequences, were expressed in cos-7 cells or C H O cells. All constructions contained kringle IV-36, which includes the only unpaired cysteine residue (Cys-4057) in apo(a). r-Apo(a) was synthesized as a precursor and secreted as mature apolipoprotein into the medium. When medium containing r-apo(a) with 9, 15, or 18 kringle IV repeats was mixed with normal human plasma LDL, stable complexes formed that had a bouyant density typical of Lp(a). Association was substantially decreased if Cys-4057 on r-apo(a) was replaced by Arg by site-directed mutagenesis or if Cys-4057 was chemically modified. Lack of association was also observed with r-apo(a) containing only 3, 5, or 7 kringle IV repeats without "unique kringle IV Sequences", although Cys-4057 was present in all of these constructions. Synthesis and secretion of r-apo(a) was not dependent on its sialic acid content. r-Apo(a) was expressed even more efficiently in sialylation-defective C H O cells than in wild-type C H O cells. In transfected C H O cells defective in the addition of N-acetylglucosamine, apo(a) secretion was found to be decreased by 50%. Extracellular association with LDL was not affected by the carbohydrate moiety of r-apo(a), indicating a protein-protein interaction between r-apo(a) and apoB. These results show that, besides kringle IV-36, other kringle IV sequences are necessary for the extracellular association of r-apo(a) with LDL. Changes in the carbohydrate moiety of apo(a), however, do not affect complex formation.Lipoprotein(a) [Lp(a)] is currently the subject of intensive investigation because elevated plasma concentrations of Lp-(a) represent an independent risk factor for myocardial infarction and stroke (Scanu & Fless, 1990;Utermann, 1989Utermann, , 1990Edelberg & Pizzo, 1991). Despite its clinical importance, the physiological function of Lp (a)

Summary: Lipoprotein(a) (Lp(a)) is a strong independent risk factor for premature atherosclerosis. Structurally, Lp(a) closely resembles LDL. Its protein moiety contains apolipoprotein B-100 and apolipoprotein(a). We evaluated two commercial enzyme immunoassays (ElAs) and an immunoradiometric assay (IRMA) for Lp(a). The three assays differed in their design and they used different antibodies. In the immunoradiometric assay, two different monoclonal antibodies were used. In the first EIA, monoclonal anti-apolipoprotein(a) was bound to the solid phase and Lp(a) was detected with polyclonal anti-apolipoprotein B (Lp(a):B-EIA). In the second EIA, polyclonal anti-apolipoprotein(a) was used as capturing antibody and as detecting antibody (apo(a)-EIA). Ninety three plasma samples were assayed for Lp(a) with the three methods. The best correlation was obtained between the IRMA and the Lp(a):B-EIA (r = 0.971). Correlations between the apo(a)-EIA on the one hand and the IRMA or the Lp(a):B-EIA on the other hand were 0.889 and 0.836, respectively. The methods significantly differed in their calibration. This resulted in different mean Lp(a) concentrations. When tested against purified Lp(a), the apo(a)-EIA appeared accurately calibrated, whereas the IRMA and the Lp(a):B-EIA overestimated Lp(a) by approximately twofold. In the Lp(a):B-EIA, the detecting antibody is directed against apolipoprotein B. The Lp(a):B-EIA is, therefore, not affected by the apolipoprotein(a) size polymorphism. This allows expression of the concentration of Lp(a):B complexes on a molar basis. In contrast, the polyclonal antibody-based apo(a)-EIA measures the concentration of apolipoprotein(a) antigen, and may, therefore, be susceptible to inter-and intra-individual polydispersity of apolipoprotein(a) and Lp(a) particles. The data underline that both design and calibration of Lp(a) immunoassays are crucial.

We have characterized the molecular defect causing lecithin:cholesterol acyltransferase (LCAT)-deficiency (LCAT-D) in the LCAT gene in three siblings of Austrian descent. The patients presented with typical symptoms including corneal opacity, hemolytic anemia, and kidney dysfunction. LCAT activities in the plasma of these three patients were undetectable. DNA sequence analysis of polymerase chain reaction (PCR)-amplified DNA of all six LCAT exons revealed a new point mutation in exon IV of the LCAT gene, i.e., a G to A substitution in codon 140 converting Arg to His. This mutation caused the loss of a cutting site for the restriction endonuclease HhaI within exon IV: Upon digestion of a 629-bp exon IV PCR product with HhaI, the patients were found to be homozygous for the mutation. Eight of 11 family members were identified as heterozygotes. Transfection studies of COS-7 cells with plasmids containing a wild-type or a mutant LCAT cDNA revealed that, in contrast to the cell medium containing wild-type enzyme, no enzyme activity was detectable upon expression of the mutant protein. This represents strong evidence for the causative nature of the observed mutation for LCAT deficiency in affected individuals and supports the conclusion that Arg140 is crucial for the structure of an enzymatically active LCAT protein.

Summary: The serum concentration of several lipids, including high-density lipoprotein-cholesterol (HDL-C) and the HDL subfractions, HDL-2-C and HDL-3-C, were measured in 44 male and 26 female survivors of myocardial infarction and compared with those of a control group matched for age, sex, and body weight. Serum concentrations of total cholesterol (TC) and low-density lipoprotein (LDL-C) were significantly increased in patients as compared to control individuals. The total HDL-C concentration was lower in patients than in controls. By differential quantitation of HDL subfractions with a new precipitation method using polyethylene glycol, it was found that HDL-3-C was not significantly different between female patients and controls. The reduction of HDL-3-C in male patients was only of borderline significance. HDL-2-C in contrast was highly significantly reduced in both male and female patients. The greatest difference between patients and controls was found in the HDL-2/HDL-3-C ratio. It is therefore concluded that HDL-2-C quantitation is a valuable risk indicator for myocardial infarction yielding a better discrimination of patients from controls than total HDL-C quantitation.