Apolipoprotein B-100 (apo B-100) (molecular weight 500 kDa) is necessary for the assembly and secretion of VLDL. It contains several very hydrophobic areas that serve as strong lipid-binding domains. It also has several domains that could serve as binding sites for heparin-like molecules and form the basis for some of the cell surface interactions of the apo B-containing lipoproteins. In addition, apo B-100 contains an LDL receptor binding domain (amino acids 3100–3400), which allows the specific uptake of LDL by the LDL receptor.
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John D. Brunzell, in Clinical Lipidology, 2009
Familial Defective Apolipoprotein B-100
A mutation in apoB-100 that inhibits its binding to the LDL receptor is another genetic cause of elevations in LDL cholesterol. The prevalence of this disorder is unknown but is estimated to be 5% to 10% that seen in FH. The LDL receptor is normal. A full-length apoB-100 molecule is produced with a single amino acid substitution at residue 3500; this results in apoB that binds poorly to the LDL receptor, leading to LDL accumulation in the plasma. Affected individuals are clinically indistinguishable from patients with heterozygous FH: they may present with severe hypercholesterolemia, tendon xanthomas, and premature atherosclerosis. Treatment is similar to that for patients with LDL receptor mutations.
Manisha Chandalia, Nicola Abate, in Encyclopedia of Gastroenterology, 2004
Metabolism of Apolipoprotein B-100-Containing Lipoproteins
The metabolism of apoB-100-containing particles is depicted in Fig. 1. The initial apoB-100-containing lipoprotein is VLDL. These particles are synthesized by the hepatocyte. They contain a core of triglycerides (60% by mass) and cholesterol esters (20% by mass). In addition to apoB-100, the surface apolipoproteins for VLDL include apoC-II, which acts as a cofactor for lipoprotein lipase (LPL), apoC-III, which inhibits this enzyme, and apoE, which serves as ligand for the apolipoprotein B/E (LDL) receptor in the liver. Some newly synthesized VLDL particles are in fact taken up again by the liver through this mechanism, soon after secretion. The triglyceride core of nascent VLDL particles is hydrolyzed by LPL in the microcirculation. During lipolysis, the core of the VLDL particle is reduced, generating VLDL remnant particles (also called IDL). Some of the phospholipid, unesterified cholesterol, and apolipoproteins A, C, and E of VLDL remnants are transferred to HDL. VLDL remnants can either be cleared from the circulation by the apoB/E (LDL) receptor or be further depleted in triglycerides by lipases and become LDL particles. LDL particles contain a core of cholesterol esters and lesser amounts of triglyceride. LDL particles should be taken up by the LDL (apoB/E) receptor in the hepatocyte. However, if the LDL particle becomes modified LDL, i.e., glycated or oxidized, the scavenger LDL receptor of macrophages will recognize these particles and increase the formation rate of foam cells in the arterial intima, leading to the formation of atherosclerotic lesions. This process is physiologically counteracted by the apoA-containing lipoproteins (HDL).
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FIGURE 1. Metabolism of apolipoprotein B-100-containing lipoproteins: LPL, lipoprotein lipase; FFA, free fatty acid; Ox, oxidation; HTLP, hepatic lipase; SR-B1, scavenger receptor class B type 1; CETP, cholesterol ester transfer protein. See text for details.