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The protein, apolipoprotein E, has an important role in lipid metabolism; principally, it serves as a carrier protein.  ApoE combines with lipids in the body to form lipoprotein particles '<ref>Genetics Home Reference. 2013. APOE gene. Ghr.hlm.nih.gov/gene/APOE.</ref>' '<ref>Reynolds, Dawn.  Apolipoprotein E.  www.chem.cuteu.edu/chem4400/sjbr/dan971.htm.</ref>'.  These lipoprotein particles have hydrophobic lipids situated at the core and hydrophilic side chains made of amino acids '<ref>Reynolds, Dawn.  Apolipoprotein E. www.chem.cuteu.edu/chem4400/sjbr/dan971.htm.</ref>'.  Specifically, apoE is responsible for packaging cholesterol,other lipids, and fat soluble vitamins and transporting them systemically '<ref>Han X.  2010.  The pathogenic implication of abnormal interaction between apolipoprotein E isoforms, amyloid-beta peptides, and sulfatides in Alzheimer's disease.  Mol Neurobiol 41(2-3): 97-106.</ref>''<ref>Genetics Home Reference. 2013. APOE gene. Ghr.hlm.nih.gov/gene/APOE.</ref>'.   

Synthesis of this protein primarily occurs in the liver; approximately three-fourths of plasma apoE is generated in the liver by hepatic parenchymal cells '<ref>Genetics Home Reference. 2013. APOE gene. Ghr.hlm.nih.gov/gene/APOE.</ref>'.  In this location, apoE is included as a major component of very-low density lipoproteins (VLDL) '<ref>Genetics Home Reference. 2013. APOE gene. Ghr.hlm.nih.gov/gene/APOE.</ref>' '<ref>Reynolds, Dawn.  Apolipoprotein E.  www.chem.cuteu.edu/chem4400/sjbr/dan971.htm.</ref>'.  VLDL serves to remove excess cholesterol from the blood and to carry it to the liver for processing.  Thus, apoE acts as a cholesterol chaperone '<ref>Arold, S. et al.  2012.  Apolipoprotein E level and cholesterol are associated with reduced synaptic amyloid beta in Alzheimer's disease and apoE TR mouse cortex.  Acta Neuropathol 123(1):39-52.</ref>'.  Maintaining optimal levels of cholesterol is crucial for the prevention of disorders that affect the heart and blood levels '<ref>Genetics Home Reference. 2013. APOE gene. Ghr.hlm.nih.gov/gene/APOE.</ref>'.  Triglycerides are also transported to the liver tissue with the aid of VLDL '<ref>Reynolds, Dawn.  Apolipoprotein E.  www.chem.cuteu.edu/chem4400/sjbr/dan971.htm.</ref>'.  ApoE is essential for the normal catabalism of triglyceride-rich lipoprotein constituents '<ref>Arold, S. et al.  2012.  Apolipoprotein E level and cholesterol are associated with reduced synaptic amyloid beta in Alzheimer's disease and apoE TR mouse cortex.  Acta Neuropathol 123(1):39-52.</ref>'.  In the plasma, apoE will associate with most lipoproteins, however, n the central nervous system, it mainly associates with high-density lipoproteins (HDL) '<ref>Han X.  2010.  The pathogenic implication of abnormal interaction between apolipoprotein E isoforms, amyloid-beta peptides, and sulfatides in Alzheimer's disease.  Mol Neurobiol 41(2-3): 97-106.</ref>'.   

Another way in which apoE is involved in the metabolsim of lipids is through binding to the low denstiy lipoprotein receptor; apoE assists in both the transportation of lipids and the facilitation of their uptake.  ApoE mediates the receptor binding of apoE lipoprotein particles to the LDL recptor, an action that initiates the cellular uptake of lipoproteins '<ref>Reynolds, Dawn.  Apolipoprotein E.  www.chem.cuteu.edu/chem4400/sjbr/dan971.htm.</ref>'.  Following the receptor-mediated endocytosis of apoE-containing lipoprotein particles by LDL receptor family member, apoE may undero one of two fates.  ApoE can either be degraded, or it can be recycled back to the cell surface '<ref>Han X.  2010.  T he pathogenic implication of abnormal interaction between apolipoprotein E isoforms, amyloid-beta peptides, and sulfatides in Alzheimer's disease.  Mol Neurobiol 41(2-3): 97-106.</ref>'.  One other function that apoE plays with regards to lipid metabolism takes place within the intestinal tract.  ApoE has the ability to incorporate into intestinally synthesized chylomicrons.  By doing so, apoE can transport dietary triglycerides and cholesterol '<ref>Reynolds, Dawn.  Apolipoprotein E.  www.chem.cuteu.edu/chem4400/sjbr/dan971.htm.</ref>'.