apolipoprotein A-I, a protein component of lipoprotein complexes found in high-density lipoprotein (HDL) and chylomicrons. It is an activator of lecithin-cholesterol acyltransferase, which forms cholesteryl esters in HDL. All apoproteins of plasma lipoproteins bind and transport lipid in the blood. A deficiency of apolipoprotein A-I is associated with low HDL levels and Tangier disease.
