As a major lipoprotein in mammals, high-density lipoprotein (HDL) originates in the liver and small intestine as small, protein-rich particles that contain relatively little cholesterol and no cholesteryl esters. HDLs contain primarily apoA-I and other apolipoproteins. They also contain the enzyme lecithincholesterol acyl transferase (LCAT), which catalyzes the formation of cholesteryl esters from lecithin (phosphatidylcholine) and cholesterol.

LCAT on the surface of nascent (newly forming) HDL particles converts the cholesterol and phosphatidylcholine of chylomicron and VLDL remnants encountered in the bloodstream to cholesteryl esters, which begin to form a core, transforming the disk-shaped nascent HDL to a mature, spherical HDL particle. Nascent HDL can also pick up cholesterol from cholesterol-rich extrahepatic cells including macrophages and foam cells, formed from macrophages.

Mature HDL then returns to the liver, where the cholesterol is unloaded via the scavenger receptor SR-BI. Some of the cholesteryl esters in HDL can also be transferred to LDL by the cholesteryl ester transfer protein. The HDL circuit is reverse cholesterol transport.

Much of this cholesterol is converted to bile salts by enzymes sequestered in hepatic peroxisomes; the bile salts are stored in the gallbladder and excreted into the intestine when a meal is ingested. Bile salts are reabsorbed by the liver and recirculate through the gallbladder in this enterohepatic circulation.