Lipoproteins are spherical particles. They transport triglycerides and cholesterol esters through the plasma. Both are non-polar hydrophobic lipids. They form the core of it, which is surrounded by a hydrophilic surface coat of phospholipids, unesterified cholesterol and apoproteins.
Due to hydrophilic surface coat, it becomes soluble in plasma and acts as an interface between the plasma and the core components. Apoproteins also help in lipoprotein metabolism as well as in the regulation of lipid transport exchange.
Lipoproteins have been classified on the basis of their densities as under:
• Very low density (VLDL)
• Intermediate density (IDL)
• Low density (LDL)
• High density (HDL)
• Lipoprotein-A [LP (a)] similar to LDL with additional apoprotein-A besides apoprotein-B 100. It is a homologue of plasminogen but does not have plasminogen’s enzymatic activity. It is a competitive inhibitor of plasminogen and has tendency to increase thrombosis due to its preferential deposition in atherosclerotic tissues.
Apoproteins (Apos) are the surface proteins of various lipoproteins. They provide structural stability to the lipoproteins and play an important role in determining the metabolic fate of the lipoproteins on which they reside. There are 9 types of Apos, viz. Apo-A-1, ApoA-il, Apo-A-IV, Apo-B-48, Apo-B-100, Apo-CI, Apo-C-II, Apo-C-III, and Apo-E.