Apolipoproteins are the protein constituents of the lipoproteins. The lipoproteins are classified according to their ultracentrifugal flotation density. The liver 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 major protein constituent of LDL. About one third of the LDL particles provide cholesterol to peripheral cells. The other two thirds are metabolized by the liver. LDL‑uptake in all of these tissues occurs via LDL receptors. Apolipoprotein B levels increase in pregnancy, hypercholesterolemia, LDL receptor defects, bile obstruction, type II hyperlipidemia and nephrotic syndrome. Apolipoprotein B levels decrease during liver disease, α‑β lipoproteinemia, sepsis and estrogen administration.
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