Isolation, characterization, and uptake in human fibroblasts of an apo(a)-free lipoprotein obtained on reduction of lipoprotein(a).
Treatment of native human Lp(a) under nondenaturing conditions with dithiothreitol yielded both a lipoprotein particle and a lipid-free protein component that could be separated by either ultracentrifugation at d 1.063 g/ml or heparin-Sepharose chromatography. The protein component only showed antigenicity against anti-Lp(a) but not against anti-B. It was heterogeneous according to SDS polyacrylamide gel electrophoresis (PAGE) consisting of two bands, a major band with molecular weight similar to apoB and a minor band with slightly lower molecular weight. The lipoprotein particle was similar to LDL with regard to its electrophoretic mobility, lipid-protein composition, its apparent molecular weight according to gel-exclusion chromatography, and its apoprotein content; only apoB was found to be present by SDS-PAGE and immunochemical analysis. This lipoprotein also proved to be identical to LDL in its uptake by the receptor-mediated LDL-pathway in cultured human fibroblasts as shown by the similarity of the concentration-dependent binding, internalization, and degradation curves at 37 degrees C of the 125I-labeled lipoproteins. Normal Lp(a) was not taken up as readily as either its reduced lipoprotein component or LDL in the various steps of the receptor-mediated pathway. The maximal capacity for Lp(a) in the degradation assay was only 25% of that of LDL and it had a fourfold higher Km. It is therefore probable that the LDL-receptor-mediated pathway is not a major route for the clearance of Lp(a) in vivo. These studies suggest that Lp(a) is, in essence, an LDL-particle to which the protein (a) is attached through disulfide bonds to apoB.
D002846:Chromatography, Affinity; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D004229:Dithiothreitol / Q000494:pharmacology; D004587:Electrophoresis, Agar Gel; D005347:Fibroblasts / Q000378:metabolism*; D006801:Humans; D005779:Immunodiffusion; D007122:Immunoelectrophoresis; D008055:Lipids / Q000032:analysis; D017270:Lipoprotein(a); D008074:Lipoproteins / Q000302:isolation & purification* / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000378:metabolism; D011506:Proteins / Q000032:analysis; D011956:Receptors, Cell Surface / Q000378:metabolism; D018110:Receptors, Lipoprotein
D008055:Lipids; D017270:Lipoprotein(a); D008074:Lipoproteins; D008077:Lipoproteins, LDL; D011506:Proteins; D011956:Receptors, Cell Surface; D018110:Receptors, Lipoprotein; D004229:Dithiothreitol