Fig. 1. High-density lipoprotein (HDL) metabolism. HDL are a heterogeneous lipoprotein family and plasma interconversion within different subclasses is due to action of various enzymes. Lecithin:cholesterol acyltransferase (LCAT) esterifies free cholesterol located on the surface of pre-β-HDL. The cholesterol esters generated by LCAT activity are more hydrophobic than free cholesterol and they migrate into the hydrophobic core of the lipoprotein, with the resulting conversion of pre-β-HDL into α-HDL₃. The α-HDL₃ produced by LCAT interact with cholesteryl ester transfer protein (CETP), that exchanges cholesteryl esters for triglycerides generating large cholesteryl ester-poor and triglyceride-rich HDL particles (α-HDL₂). α-HDL₂ can be converted back to pre-β-HDL through a variety of lipases activities that hydrolyze triglycerides and phospholipids on HDL. CE, cholesteryl esters; TG, triglyceride; HL, hepatic lipase; EL, endothelial lipase; PLTP, phospholipid transfer protein.