This chapter highlights the complete structure of human apolipoprotein B (ApoB)-100 and its messenger RNA. ApoB is an important component in the system of plasma lipoproteins. It functions as the ligand for the low-density lipoprotein (LDL) receptor in peripheral cells, and is a component of plasma chylomicrons (CM), very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low-density lipoproteins (LDL). ApoB-100 is synthesized by the liver and is an obligatory constituent of VLDL, IDL, and LDL. Apo-B is insoluble in both 4.2 M tetramethyl urea and aqueous buffers after extraction with organic solvents. The composition of ApoB following partial tryptic digestion and cyanogen bromide cleavage are all similar to one another. Human plasma LDL contains approximately 80% lipid and 20% protein by weight. About 4–10% of the mass of apoB consists of carbohydrate chains containing galactose, mannose, N-acetylglucosamine, and sialic acid residues. ApoB-100 in LDL is highly immunogenic, and high-titer antibodies against human LDL can be consistently produced by injection of LDL into rabbits.
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