Lp(a) is composed of an LDL-like core and the glycoprotein apo(a). Current evidence strongly suggests that the assembly of this atherogenic lipoprotein proceeds outside the liver cells in a two-step fashion. In the first step, a loose complex is formed involving kringle-4 motifs in apo(a) and one or more Lys side chains in apoB-100. In the second step, this complex is stabilized by a disulfide bridge. Indications are that Lp(a) assembly is critical in the determination of plasma apo(a) concentrations. Therefore, we searched for substances that interfere with the first step of Lp(a) assembly. epsilon-Aminohexoic acid (epsilon-AHA), known as an inhibitor from earlier assembly studies, had an IC50 of 4.8 mmol/L. The IC50 of Pro, HO-p-aminobenzene sulfonamide, Lys, N-epsilon-acetyl-Lys, taurine, Glu, serotonin, and benzamidine were all > 20 mmol/L. gamma-Aminobutyric acid, spermine, and spermidine exhibited IC50 on the same order of magnitude as epsilon-AHA. The substances with the highest inhibitory action were tranexamic acid and delta-aminovaleric acid. Seven of eight patients treated in a pilot study with tranexamic acid (Cyclocapron) responded with a decrease of plasma apo(a) of 18.5 +/- 8.2%. We suggest that substances that interfere with the Lp(a) assembly are worth pursuing further for their usefulness as therapeutic agents in reducing high plasma Lp(a) concentrations.

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)

IntroductionThe composition of lipoproteins in the plasma of patients with LCAT deficiency (LCAT- D

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Lipoprotein-a [Lp(a)], one of the most atherogenic lipoproteins, is composed of a low-density lipoprotein (LDL) core in addition to an apo-a of variable size which is linked to apoB by a disulphide bridge. Lp(a) synthesized in vitro by incubation of recombinant apo-a (r-apo-a) with LDL is physico-chemically indistinguishable from native Lp(a). The synthesis of Lp(a) in vitro proceeds in two steps. In the first step, one of the unique kringle-IVs (K-IVs) in apo-a binds to a Lys residue of apoB; in the second step, Cys-4057 of K-IV type-9 (T-9) forms a disulphide bridge with Cys-3734 of LDL. Here we have produced r-apo-a with different combinations of unique K-IVs and shown that K-IV T-6 is required for the first step of Lp(a) assembly. For the second step not only is K-IV T-9 essential, but also the distance between T-6 and T-9 requires a length of two K-IVs. These findings give additional insight into the mode of Lp(a) assembly and are of relevance in the search for apo-a mutants influencing Lp(a) levels and for the development of Lp(a)-lowering medications.