Apolipoproteins are the protein constituents of the lipoproteins. The lipoproteins are classified according to their ultracentrifugal flotation density. Apolipoprotein A‑I is the major protein constituent of high - density lipoproteins (HDL). HDL are synthesized by the intestines and the liver. They transport excess cellular cholesterol from extrahepatic tissue and peripheral cells to the liver. Additionally, apolipoprotein A‑I activates the enzyme lecithin - cholesterol - acyltransferase (LCAT), which catalyzes the esterification of cholesterol, thereby enhancing the lipid‑carrying capacity of the lipoproteins. Apolipoprotein A‑I levels increase in liver disease, pregnancy and as a result of estrogen administration (e.g. oral contraceptives). Apolipoprotein A‑I levels decrease in inherited hypo - α - lipoproteinemia (e.g. Tangier disease), cholestasis, sepsis and atherosclerosis. The liver also synthesizes very low density lipoproteins (VLDL) which mainly contain triglycerides and cholesterol. In the presence of lipoprotein lipase the triglycerides are hydrolyzed and LDL - particles with a high proportion of cholesterol are formed. Apolipoprotein B is the main constituent of LDL.
The combined determination of apolipoprotein A‑I/apolipoprotein B and the calculation of the apolipoprotein B : apolipoprotein A‑I ratio can reflect a lipid metabolism disorder and the risk of developing atherosclerosis or coronary heart disease particularly well, thus providing an excellent addition to the classical HDL/LDL - cholesterol determination. A high level of apolipoprotein A‑I (HDL) and a low level of apolipoprotein B (LDL) correlate best with a low risk for these diseases.