Cholesterol in plasma membrane

Role of HDL in peripheral cholesterol removal. Free cholesterol in plasma membranes of peripheral tissues is transferred to apo A-I containing pre-/ -HDL (nascent HDL), via an ATP-binding cassette transporter 1 (ABCl). Cholesterol is esterified by LCAT and stored in the core of the HDL particle. In the presence of suitable acceptor lipoproteins (VLDL or LDL), cholesteryl esters are transferred from HDL via apo D and CETP to the lower density lipoproteins, thereby shortening the half-life of plasma cholesterol since VLDL and LDL have a much faster turnover time than HDL. PL = Phospholipid. 

Excess cellular cholesterol in plasma membranes, or cholesterol released from the breakdown of cells, enters another type of lipoprotein, high-density lipoprotein (HDL). 

Sphingomyelin (SM) A Hpid with a phosphocholine group linked to a ceramide shares the same head group as phosphatidylcholine (PC) but displays different physicochemical properties. SM and PC do not normally mix in biological membranes, because SM is associated with cholesterol in plasma membrane microdomains such as lipid rafts. 

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