HDL (High Density Lipoproteins)

HDL acts as a scavenger. It transports cholesterol from extra hepatic tissues to the liver.

In the liver this cholesterol will be converted to bile acids or excreted as such in the bile.

There is reverse transport of cholesterol by HDL as compared to VLDL,  Chylomicrons and  LDL.

Increased HDL is good for health. Less than 35 mg%  is atherogenic and greater than 60 mg%  is good for health.

HDL is synthesized in intestine and liver.

It is small sized, disc like in shape and is known as  Nascent HDL.

Core is made up of cholesterol ester and TAG, outer side is lipid bilayer made of phospholipids and proteins.

Proteins present in HDL are ApoA-1 (major protein 70%), ApoA-2,  ApoC-1, ApoC-2, ApoC-3, ApoD and ApoE.

About 55% of HDL is protein.

When HDL comes in circulation it takes up LCAT enzyme that is activated by ApoA-I.

HDL takes cholesterol from extrahepatic tissues and converts them to cholesterol ester.

Cholesterol ester is more hydrophobic than free cholesterol.

Cholesterol ester forms the inner core of HDL.

HDL increases in size becomes spherical called HDL3.

It also takes some cholesterol from circulating VLDL and chylomicron remnants.

It gives cholesterol ester to them and takes up more TAG from the VLDL remnants and further increases in size called HDL 2.

HDL2 goes to the liver where there are protein receptors SR-B1 receptors.

These receptors recognize ApoA1, lipids are unloaded and HDL2 converts back to HDL3.

HDL3 comes back in the circulation.

This cycle is repeated again and again.

In the liver cholesterol ester is converted to free cholesterol and fatty acids.

Fatty acids can be used for fats synthesis, β-oxidation for energy.

Free cholesterol will be converted to bile acids and excreted.

Extrahepatic tissues have SR-B1 receptors, they recognize ApoA1.

HDL is endocytosed, these tissues give their load to HDL, it comes out and takes it to the liver.

There are some tissues that require cholesterol for the synthesis of steroid hormones (like adrenal cortex) which synthesizes glucocorticoids and mineralocorticoids.

These also have receptors that recognize ApoA1 and HDL combines with them but, there is no endocytosis, only passive transfer of cholesterol to cells for the synthesis of steroids.