Apolipoprotein A-I (apo A-I) is a protein that has specific roles in the transportation and metabolism of lipids and is the main protein component in high-density lipoprotein (HDL, the “good cholesterol”). This test measures the amount of apo A-I in the blood.
Lipids alone cannot dissolve in the blood; they are like oil that floats on water. Apolipoproteins are the proteins that combine with lipids to make lipoprotein particles that can transport lipids throughout the bloodstream. Apolipoproteins provide structural integrity to lipoproteins and shield the water-repellent (hydrophobic) lipids at their center.
Most lipoproteins are cholesterol- or triglyceride-rich (two main lipids) and carry them throughout the body for uptake by cells. HDL, however, is like an empty taxi. It goes out to the tissues and picks up excess cholesterol, then transports it back to the liver. In the liver, the cholesterol is either recycled for future use or excreted in bile. HDL’s reverse transport is the only way that cells can get rid of excess cholesterol. This reverse transport helps protect the arteries and, if there is enough HDL present, it can even reverse the build-up of fatty plaques, deposits resulting from atherosclerosis that can lead to cardiovascular disease (CVD).
Apolipoprotein A is the taxi driver. It activates the enzymes that load cholesterol from the tissues into HDL and allows HDL to be recognized and bound by receptors in the liver at the end of the transport. There are two forms of apolipoprotein A: apo A-I and apo A-II. Apo A-I is found in greater proportion than apo A-II (about 3 to 1). The concentration of apo A-I can be measured directly and tends to rise and fall with HDL levels. Deficiencies in apo A-I correlate with an increased risk of developing CVD. Apo A-I levels provide more information to help evaluate CVD risk, especially when HDL levels are low.