Low density lipoprotein receptor determination in peripheral blood mononuclear cells: Influence of differences in cell concentration

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Abstract
Low density lipoprotein (LDL) receptor determination in peripheral blood mononuclear cells (PBMCs) is influenced by differences in cell concentration. As the cell concentration increases, measured LDL receptor activity decreases. This inter-relationship is caused by a PBMC-induced modification of 125I-LDL. The PBMC-modified 125I-LDL results from shedding of polyanionic cell membrane constituents that subsequently bind to 125I-LDL, and has reduced capacity of binding to the LDL receptors, to the cell membrane independent of the receptors and even to plastic. The cell membrane constituents contain sulphate, have a MW=200,000-300,000, are heat stable and are rapidly released at 37 °C as well as at 4 °C. They probably represent a heterogenous group of proteoglycans, glycoproteins and glycolipids. The higher the cell concentration is, the more polyanionic cell membrane constituents are released, and at high concentrations they may even form aggregates of LDL. We conclude that differences in PBMC concentration interfere with LDL receptor analyses through shedding of different amounts of polyanionic cell membrane constituents into the medium. Thus, standardisation of the experimental procedures with respect to cell number is of great importance in LDL receptor determination in PBMCs.

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