Apolipoprotein A-I is the major apoprotein of HDL and is a relatively abundant plasma protein with a concentration of 1.0-1.5 mg/ml. It is a single polypeptide chain with 243 amino acid residues of known primary amino acid sequence. ApoA-I is a cofactor for LCAT, which is responsible for the formation of most cholesteryl esters in plasma. ApoA-I also promotes efflux of cholesterol from cells. The liver and small intestine are the sites of synthesis of apoA-I.
Decreased serum high density lipoprotein cholesterol levels have been reported to correlate with increased risk of coronary artery disease. However, Apo A-I has been suggested as a better discrimination of coronary artery disease than HDL. In some populations, mainly Caucasian, the pattern of apolipoprotein a polymorphism determines lipoprotein A levels and coronary heart disease risk. Apo A-I levels also correlate with survival rates or risk factors for patients with myocardial infarction and peripheral vascular disease. Measurement of Apo A-I may also aid in the diagnosis of Tangier disease (absence of alpha-lipoprotein).