high density lipoprotein (HDL, alpha-lipoprotein)

Classification

• HDL is usually divided into 2 density classes, HDL2 & HDL3 (precursor of HDL2)
• HDL2 is the larger lipoprotein particle with MW = 360 kD vs. 175 kD for HDL3
• 60% of HDL2 is lipid vs. 45% for HDL3
• another form of HDL, HDLc is thought to develop after ingestion of cholesterol
• HDLc is particularly abundant in apo E, thus binds to LDL receptor

Function

• HDL plays roles in:
  • returning cholesterol from the periphery to the liver for removal as bile acids
  • scavenging lipids & lipoproteins during catabolism of chylomicrons & VLDL
  • reservoir for plasma apo C2
• HDL may play a role in reducing inflammation
  • HDL induces ATF3 in macrophages
  • this reduces expression of Toll-like receptor-induced proinflammatory cytokines

Structure

• ~50% of mass of HDL is protein, with apo A1 & apo A2 constituting 90%
• the A1/A2 ratio is approximately 3:1
• 30% of HDL is phospholipid & 20% cholesterol
• the phospholipid/sphingomyelin ratio is 5:1 & esterified/free cholesterol about 3:1
• some HDL particles contain only apo A1, while others contain apo A1 & apo A2
  • HDL particles containing only apo A1 appear to stimulate cholesterol efflux from cells

Expression

• HDL is secreted by liver & intestine, in the form of a disc-shaped precursor containing apo A1, apo A2, lecithin & free cholesterol
• transformation to the spherical HDL3 depends upon LCAT activity
• little is known of HDL catabolism, although the liver & kidneys are probably involved

More general terms

• alpha globulin
• lipoprotein
• multisubunit protein
• macromolecular complex

Additional terms

• cholesterol HDL in serum
• scavenger receptor