Lp(a) is a plasma lipoprotein particle consisting of a plasminogenlike protein [apo(a)

A B S T R A C T An elevated concentration of lipoprotein (a) [Lp(a)] in the serum has been considered a risk factor for coronary heart disease by various investigators. In the present study, the turnover of Lp(a) was investigated in nine individuals with serum Lp(a) levels ranging from 1 to 68 mg/100 ml. After intravenous injection of radioiodinated Lp(a), the radioactivity time-curve of the serum and the specific activitity time-curves of the isolated Lp(a) and Lp(a) apolipoproteins were measured for 14 d. More than 97% of the label was found in the protein moiety of Lp(a). During the entire study period, the serum radioactivity remained with Lp(a), only insignificant amounts of radioactivity were detectable in other lipoprotein fractions. The serum radioactivity timecurves and the specific activity time-curves of the isolated Lp(a) and Lp(a) apolipoproteins were identical.The kinetic parameters of Lp(a) turnover were calculated in terms of a two-compartment model.

A B S T R A C T The binding of '25I-lipoprotein (a)[Lp(a)] to cell surface receptors was studied on cultured human fibroblasts. The results were compared with corresponding data obtained with 1251-low density lipoproteins (LDL). Equilibrium binding studies showed that Lp(a) is bound with high affinity by the cell surface receptors. The maximum binding capacity for Lp(a) was 37% lower than for LDL. For Lp(a) and LDL, the Scatchard plots displayed linearity, indicating a single category of binding sites. Half-maximal saturation occurred at a concentration of 9.52±1.04 nM for Lp(a) and 7.76±1.29 nM for LDL. Competition binding experiments revealed that Lp(a) and LDL are nearly equally potent in competing each other for the binding sites. Binding of Lp(a) and LDL were followed by suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. Cyclohexanedione treatment of Lp(a) and LDL completely abolished receptor binding. Neither Lp(a) nor LDL were specifically bound by fibroblasts obtained from a patient with homozygous familial hypercholesterolemia (FH).The removal mechanisms for Lp(a) and LDL were further compared by in vivo studies. Radioiodinated Lp(a) and LDL [1431][1432][1433][1434][1435][1436][1437][1438][1439][1440][1441] LDL are nearly identical (6-8). The hexose, hexosamin, and sialic acid content, however, are significantly higher in Lp(a) (9). The main protein constituent of both lipoproteins is apolipoprotein B. Lp(a) has an additional apoprotein that has been termed "specific Lp(a) antigen" (9-11). On agarose gel or cellulose acetate Lp(a) migrates somewhat faster than LDL and has therefore also been described as -pre-fl-lipoprotein" (3,4,12).In spite of these remarkable similarities between Lp(a) and LDL, Lp(a) is not a metabolic product of very low density lipoproteins (VLDL), LDL, or METHODS Binding studiesIsolation of Lp(a) and LDL. The details of the method for isolation of Lp(a) and LDL were described recently (14). Plasma was obtained by plasmapheresis from healthy volunteers who had high levels of Lp(a) as checked by doubleimmunodiffusion using monospecific anti-Lp(a) antibodies (14). The plasma was subjected to sequential ultracentrifugation to obtain density fractions from 1.006 to 1.055 g/ml and from 1.055 to 1.110 g/ml. The densities were adjusted by addition of solid NaCl and checked with a density meter (Anton Paar K. G., Graz). All centrifugal procedures were performed in a Beckman L8-70 centrifuge using a Ti 50.2 rotor (Beckman Instruments, Inc., Fullerton, CA). The supernatants, containing the lipoproteins of the d 1.006-1.055 g/ml and 1.055-1.110 g/ml, were collected by tube slicing. These fractions were concentrated to a volume of -5 ml by dialysis against polyethyleneglycol and then applied to an agarose column (Bio Gel A-Sm, Bio-Rad Laboratories, Richmond, CA). Elution of the lipoproteins was performed with 0.15 M NaCl, pH adjusted to 8.5 by addition of NH40H. During all steps of the isolation procedure, Na2EDTA and NaN3 were present in a concentration of 1 mg/ml...

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