Lipid Metabolism

Lipoproteins (Apolipoprotein-Lipid Complexes)

The term lipoprotein refers, unsurprisingly, to molecules of lipids attached proteins. Confusingly, it is also used to refer to complexes that use proteins to carry lipids in the blood. These are not simple molecules but a collection of triglycerides and cholesterol surrounded by a shell of phospholipids and lipoproteins.



These complexes are divided into groups based on the density of apolipoproteins in the shell (Chylomicrons, Very Low Density, Low Density and High Density). The larger the "lipoprotein" the lower the density of Apolipoproteins.

Lipoprotein Types

Chylomicrons (CHY) - Carry digested lipids to blood stream via lymphatics
VLDL - Endogenous production in the liver - converted to LDL in the blood
LDL - Transport lipids to cells in the body - fatty acids liberated by Lipoprotein Lipase (LPL)
HDL - Have "spare capacity" and so absorb free lipids



LDL - Associated with increased risk of atherosclerosis
HDL - Lowers risk of atherosclerosis as removes

The following levels are generally regarded as desirable:

• Total cholesterol (TC) - 5.0 mmol/l or less. However, about 2 in 3 adults in the UK have a total cholesterol level of 5.0 mmol/l or above.
• Low-density lipoprotein (LDL) cholesterol after an overnight fast: 3.0 mmol/l or less.
• High-density lipoprotein (HDL) cholesterol: 1.2 mmol/l or more.
• TC/HDL ratio: 4.5 or less. That is, your total cholesterol divided by your HDL cholesterol. This reflects the fact that for any given total cholesterol level, the more HDL, the better.

Apolipoproteins

The lipoproteins in the complex shell varry in distribution and function. Here are some of the main highlights of these Apolipoproteins.

Apo A1 - Found in HDL
Apo B - Found in LDL, Chylomicrons
Apo CII - Causes release of FA in Capillaries
Apo CIII - Inhibits breakdown of plasma Triglycerides
Apo D - HDL

Apo E - Facilitates uptake in liver

Apo B/Apo A1 is possibly a better indicator of risk of vascular disease than total cholesterol/HDL

Statins

Statins reduces the endogenous production of cholesterol in the liver. This causes the hepatocytes to up-regulate their intake of existing cholesterol from LDL - thus reducing the level of circulating LDL.