ELISA measurement of LDL receptors

An enzyme-linked immunosorbent assay was developed for measurement of low density lipoprotein (LDL) receptors. A monospecific polyclonal antibody to LDL receptor purified from rat liver that reacted with rat, mouse, canine, and human LDL receptor was used. With this assay, LDL receptors could be measured on 2-4 x 10(5) adherent cells and 1.0 x 10(5) cells in suspension, although results were more variable with cell suspensions. Membranes from a variety of receptor-rich and receptor-poor tissues could be assayed directly after adherence of the membranes to the ELISA plate by an overnight incubation. In some instances, the quality of the assay was improved by first solubilizing the membranes. The sensitivity of the assay is such that between 0.15 and 2 micrograms of membrane protein is required. This could be obtained from leukocytes in a modest (20-30 ml) quantity of human blood. The assay was used to demonstrate the rapid down-regulation of LDL receptors in human mononuclear leukocytes in response to a cholesterol-containing meal. Overall, the results support the use of ELISA technology to measure LDL receptors, particularly for physiologic studies.