Apolipoprotein CII (ApoCII) is a 79-amino acid protein encoded by the APOC2 gene, serving as an essential cofactor for lipoprotein lipase (LPL) activation. This protein circulates in plasma as a component of triglyceride-rich lipoproteins (TRLs), including very low-density lipoproteins (VLDL) and chylomicrons, with normal concentrations ranging from 3–8 mg per 100 mL. ApoCII binds to LPL via its C-terminal α-helix (residues 58–79), stabilizing the enzyme’s catalytic domain and facilitating the hydrolysis of VLDL triglycerides into free fatty acids and glycerol, a process critical for generating intermediate-density lipoproteins (IDL). This lipolytic activity enables cellular energy uptake and regulates plasma lipid homeostasis.