Apolipoprotein B, Human (Apo B)

Lipids (triglyceride (TG), cholesterol (Chol), phospholipids (PL), cholesteryl esters (CE)) are insoluble in plasma. These lipids combine with apolipoproteins to form lipoproteins. This process solubilises lipids in plasma. Lipoproteins provide a transport system for lipids. They also play an important role in lipoprotein receptor recognition and regulation of certain enzymes in lipoprotein metabolism. Apolipoprotein B plays an essential role in lipid transport and metabolism. Apo B may regulate cholesterol synthesis through its interaction with specific cell membrane receptors and by inhibition of HMG Co A reductase. This enzyme has been identified as the rate controlling enzyme in cholesterol biosynthesis. Apo B may be important in the genesis of atherosclerosis. Apo B is the major protein moiety of all lipoproteins other than high density lipoprotein (HDL) in plasma. Increased levels of apo B100 are associated with an increased risk of coronary artery disease. Apo B exists in human plasma as two isoforms: apo B48 and apo B100. Apo B100 is the major physiological ligand for the LDL receptor. It contains 4536 amino acid residues (Chen et al., 1986; Law et al., 1986). Apo B100 is the full length protein. Its gene has been mapped on the short arm of chromosome 2, with an approximate length of 43 kilobases and 29 exons (Ludwig, et al., 1987). The LDL-binding domain of the molecule is proposed to be located between the aa3129-3532 (Knott, et al., 1986). Apo B100 is synthesized in the liver. It is required for the assembly of very low density lipoproteins (VLDL). It does not interchange between lipoprotein particles, as do the other lipoproteins. It is found in IDL and LDL particles after the removal of the apolipoproteins A, E and C (Young, 1990). Apo B48 is present in chylomicrons and chylomicron remnants. It plays an essential role in the intestinal absorption of dietary fats (Kane, 1983). Apo B48 is synthesized in the small intestine. It comprises the N-terminal 48% of apo B100.Apo B48 is the degraded protein. Apo B48 lacks the LDL receptor-binding domain of Apo B100. It is produced due to post-transscriptional Apo B100 mRNA editing at codon 2153, which creates a stop codon in the intestine instead of a glutamine in the liver (Chen et al., 1987). Apo B is purified from LDL isolated by density gradient ultracentrifugation (1.03-1.05g/ml density range) followed by size exclusion chromatography in 10mM deoxycholate, 50mM NaCl, 50mM Na2CO3, pH 10 (1). Apo B, which has a Mr of 550kD, is the major protein found in LDL and is responsible for the cellular uptake of LDL particles from the plasma (3). The Apo B prepared by this procedure is aggregated. The aggregates are not retained by a 0.45 um filter, but migrate only a short distance into a 5% SDS PAGE gel. In the isolation buffer, Apo B gives a stable solution that is slightly hazy.