Apolipoprotein E (APOE, A704Bo) is a key lipid-binding protein mainly synthesized in the liver, brain, and macrophages. Encoded by the APOE gene on human chromosome 19, it exists in three major isoforms: APOE2, APOE3, and APOE4. As a critical component of chylomicrons, VLDL, and HDL particles, APOE primarily mediates the transport and redistribution of cholesterol, triglycerides, and other lipids in the circulatory and central nervous systems. It acts as a ligand for cell-surface lipoprotein receptors, facilitating the uptake and clearance of lipoprotein particles from plasma. Beyond lipid metabolism, APOE participates in neuronal repair, immune modulation, and amyloid-beta clearance in the brain, making it closely linked to cardiovascular diseases and neurodegenerative disorders such as Alzheimer’s disease. APOE specifically binds to LDLR (B008Hu) on cell membranes, enabling receptor-mediated endocytosis of lipoprotein complexes. Briefly, LDLR was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μL were then transferred to APOE-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-LDLR pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 µL stop solution to the wells and read at 450/630nm immediately. Measured by its binding ability in a functional ELISA. When recombinant cattle APOE is lmmobilized at 2 µg/mL(100 µLwell), the concentration of LDLR that produces 50% optimal bindingresponse is found to be approximately 0.033 µg/mL.