VLDL = very low density lipoprotein; IDL = intermediate density lipoprotein; LDL = low density lipoprotein; HDL = high density lipoprotein.

FIGURE 15.3 Metabolism of cholesterol. Dietary cholesterol enters the circulation in chylomicrons (C). After removal of triacylglycerols from the chylomicrons through the action of the lipoprotein lipase (LPL) enzyme, chylomicron remnants (CR) are taken up by the liver. The liver can resecrete cholesterol, together with endogenously synthesized cholesterol, into the circulation in very low density lipoproteins (VLDL). As for chylomi-crons, most of the triacylglycerol is hydrolyzed into free fatty acids (FFA) and glycerol and taken up by peripheral tissues. The remaining particles are intermediate density lipo-proteins (IDL) that can be taken up by the liver. IDL particles that are not taken up are converted into cholesterol-rich low density lipoproteins (LDL). LDL particles are also cleared from the circulation by LDL receptors. Part of the LDL is also removed by the scavanger pathway. Cholesterol from extrahepatic tissues can be removed via high density lipoproteins (HDL) that transfer it to the liver or with the help of cholesterol ester transfer protein (CETP) to LDL and VLDL particles.

The particles formed by the gradual delipidation are relatively rich in cholesterol and are now called chylomicron remnants. They are taken up by the liver, which can store the cholesterol as cholesterol esters through the action of ACAT or resecrete it along with endogenously synthesized cholesterol into the circulation in very low density lipoprotein (VLDL) particles (Figure 15.3).

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