High-density lipoproteins: Biochemical and metabolic factors
The high-density lipoproteins (HDL) are the smallest of the plasma lipoprotein families and are approximately one half lipid and one half protein by weight. The major lipid constituent of HDL is phosphatidylcholine, with sphingomyelin second, followed by cholesterol and cholesteryl ester. The principal protein constituent of HDL is apolipoprotein A-I. Nascent or newly secreted HDLs, appear to be secreted by the gut, lymph and liver. The nascent HDL are converted into the native particle by the action of lecithin:cholesterol acyltransferase. The transfer of a fatty acid moiety from phosphatidylcholine to cholesterol provides a core of cholesteryl ester for HDL and facilitates formation of a stable spherical particle. Another source of nascent HDL is hydrolysis of the triglyceride-rich lipoprotein particles, which results in the formulation of HDL2. HDL2 are larger than HDL3 and contain more lipid-rich particles, whereas HDL3 are relatively protein-rich, lipid-poor, and dense. The strong inverse relation between HDL and coronary artery disease is believed to be most closely related to the HDL2 subfraction.