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Indium-111-Labeled Low-Density Lipoprotein Binds with Higher Affinity to the Human Liver as Compared to Iodine-123-Low-Density-Labeled Lipoprotein

Wilhelm Auerswald Atherosclerosis Research Group (ASF), Vienna
Department of Nuclear Medicine, University of Vienna
Ludwig Boltzmann Institute for Nuclear Medicine
Chemical Institute, Research Center Seibersdorf
Department of General Biochemistry and Ludwig Boltzmann Forschungsstelle for Biochemistry, University of Vienna, Vienna, Austria

Correspondence: For reprints contact: Dr. Irene Virgolini, Dept. of Nuclear Medicine, University of Vienna, Garnisongasse 13, A-1090 Vienna, Austria.

ABSTRACT

The interaction of ¹¹¹In-low-density lipoprotein (LDL) and ¹²³I-LDL with human liver-plasma membranes was investigated and compared. LDLs were isolated by sequential ultracentrifugation and radiolabeled either with ¹²³I (using lodogen or iodine-monochloride) each followed by purification with gelchromatography or dialysis) or ¹¹¹In (using cyclic DTPA-anhydride). LDL concentrations of 0.1 to 32 µg protein/ml were used for direct binding assays investigating the specific binding of labeled LDL (in the presence of a 50-fold excess of unlabeled LDL) to human liver apoB-receptors. In separate experiments, displacement of bound ¹¹¹In-(¹²³I)-LDL by unlabeled LDL was studied. Human liver plasma membranes bound 239 ± 26 ng protein of ¹¹¹In-LDL/mg protein and 148 ± 18 ng protein of ¹²³I-LDL/mg protein specifically (p < 0.001). The corresponding dissociation constants were 0.6 ± 0.2 and 1.2 ± 0.7 µg protein/ml, respectively (p < 0.001). The capacity of unlabeled LDL to displace bound ¹¹¹In-LDL was four times higher than that for ¹²³I-LDL (IC₅₀: 1.7 ± 0.7 versus 7.7 ± 1.0 µg protein/ml). No significant differences among the different methods of iodination of LDL were found. The findings show that ¹¹¹In-labeled lipoproteins might be a better ligand for lipoprotein-receptor binding studies as compared to radioiodinated lipoprotein products.

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