Figure 2.HDL metabolism. B-100, A-I and A-II indicate specific apolipoproteins; ABCA1, ATP-binding cassette A1; CETP, cholesteryl ester transfer protein; HL, hepatic lipase; LDLR, LDL receptor. LCAT (lecithin:cholesterol acyltransferase) is a key enzyme of reverse cholesterol transport, and esterifies all circulating unesterified cholesterol molecules. SR-BI (scavenger receptor class B, type I) is a HDL receptor that mediates selective cholesteryl esters uptake by cells.