Component of chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) in plasma. Plays an important role in lipoprotein metabolism as an activator of lipoprotein lipase. Adult and fetal liver, intestine and peritoneal macrophages. Belongs to the apolipoprotein C2 family.