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. Both proapolipoprotein C-II and apolipoprotein C-II can activate lipoprotein lipase. Proapolipoprotein C-II is synthesized as a sialic acid containing glycoprotein which is subsequently desialylated prior to its proteolytic processing. Proapolipoprotein C-II, the major form found in plasma undergoes proteolytic cleavage of its N-terminal hexapeptide to generate apolipoprotein C-II, which occurs as the minor form in plasma. Belongs to the apolipoprotein C2 family.