Cholesterol, triglycerides and phospholipids are insoluble in aqueous media and are transported in the plasma in the form of lipoproteins. Lipoproteins are composed of a core of esterified cholesterol and triglyceride surrounded by an external layer of phospholipid (and free cholesterol) and proteins. Five major groups of lipoproteins are recognized.
- have a diameter ranging from 0.03 – 0.5μm. They are produced by epithelial cells in the intestine. The triglyceride/cholesterol ratio is 10:1. Chylomicrons are composed of the following apoproteins: mainly apoA-I, apoB, apoC, and apoE. Chylomicrons carry absorbed lipids from the intestinal tract to the blood stream.
Very low-density lipoproteins (VLDL)
- have a diameter ranging from 25 – 75nm. They contain mostly triglycerides manufactured by the liver plus small amounts of phospholipids and cholesterol. The triglyceride/cholesterol ratio is 5:1. VLDLs are built up of apoB, apoC and apoE. VLDLs are produced in the liver and are responsible for the transport of triglycerides to peripheral tissues.
Intermediate-density lipoproteins (IDL, not depicted here)
- As the name of IDL already suggests, they are intermediate in size and lipid composition between VLDLs and low-density lipoproteins (LDLs). IDLs contain smaller amounts of triglycerides than do VLDLs and an increasing concentration of cholesterol. The triglyceride/cholesterol ratio is 1:1. IDLs contain apoproteins apoB, apoC and apoE.
Low-density lipoproteins (LDL)
- are composed of large amounts of cholesterol and lesser phospholipids. LDLs measure about 25 nm in diameter. The major protein content of LDL is formed by apoB. The main function of LDL is to deliver cholesterol to peripheral tissues. Because of their role in arterial plaques, LDLs are often called "bad cholesterol".
High-density lipoproteins (HDL)
- are the smallest of the lipoproteins with a diameter of 10 nm. They contain equal amounts of protein and lipid (mainly cholesterol and phospholipids). The significant apoproteins in HDL are mainly apoA and small amounts of apoC and apoE. HDLs are produced by the intestine, liver and in the plasma. The function of HDL is to transport excess cholesterol from the peripheral tissue back to the liver for storage or excretion in bile. Since HDL returns cholesterol from peripheral tissues and will not cause vascular problems, it is called the "good cholesterol".
For more information on hyperlipidemia and lipid lowering agents check: Blood lipid lowering agents
Cholesterol is required for the formation of all of the following compounds EXCEPT:
Extra info: Bile acids, vitamin D and corticosteroids require cholesterol as precursor. Remember the steroid-skeleton.
Which lipoproteins carry the most cholesterol in the plasma? What is the right order of lipoproteins containing cholesterol in the plasma from high to low?
Extra info: Cholesterol is transported through the circulation in LDL (75%), HDL (20%), and VLDL (5 %).