Lipoproteins are important protein-lipid assemblies that are responsible for the transport of fats to different parts of the body via the bloodstream. The five major groups of lipoproteins are very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), intermediate-density lipoprotein (IDL), high-density lipoprotein (HDL), and chylomicrons. LDL and HDL are of particular interest due to their impact on human health. For example, studies have shown that higher levels of LDL promote health problems and cardiovascular disease. On the other hand, those with higher levels of HDL seem to correlate with a lower risk of cardiovascular disease. Therefore LDL is commonly referred to as bad cholesterol, while HDL is referred to as good or healthy cholesterol.
Lipoproteins are composed of lipids and proteins which are held together by noncovalent forces. There is an outer layer of phospholipids, unesterified cholesterol, and proteins, with a core of neutral lipids predominately cholesteryl esters and triacylglycerols (TAG). Apolipoproteins are the proteins that, along with other amphipathic molecules, surround the lipids to make up the lipoproteins. This association allows lipoproteins to be carried through water-based circulation, specifically blood and lymph.
Apolipoproteins can be grouped into two classes: the nonexchangeable apolipoproteins (ApoB-100 and ApoB-48), and the exchangeable apolipoproteins (ApoA-I, ApoA-II, ApoA-IV, ApoC-I, ApoC-II, ApoC-III, and ApoE). Exchangeable apolipoproteins are able to interact with a range of macromolecular lipid assemblies, from large VLDL droplets to tiny HDL particles. In contrast to exchangeable apolipoproteins, the non-exchangeable apolipoproteins remain on the same lipoprotein particle from biosynthesis to breakdown.
Tests have been developed that measure apolipoproteins found in VLDL, LDLs, IDL, HDL, and chylomicrons. In general, VLDL, IDL, and LDL contain mostly ApoB particles, while ApoAI is the primary protein constituent of HDL. ApoC and ApoE are found mainly in chylomicrons, VLDL, and HDL. A recent study indicated that determining the ratio of ApoB to ApoAI is the best predictor of those at risk of heart attack. For example, some people who have normal levels of LDL may have higher-than-normal levels of ApoB (usually there's only one ApoB molecule on each LDL), and this higher level may indicate greater risk of cardiovascular disease.