| Functions of lipoproteins
|
| Chylomicrons transport dietary triacylglycerols (triglycerides) from the intestine to the peripheral tissues. Their main apoprotein is apoB48, which is synthesized in the intestine. Chylomicrons also contain apoproteins A, C, and E. After hydrolysis of chylomicrons by the enzyme lipoprotein lipase (LDL), their remnants are taken up by the liver. Their uptake is mediated by apoE binding to the LDL receptor and to the LDL receptor-related protein (LRP).
|
| VLDL transport triacylglycerols from the liver to the periphery. The main apoprotein of VLDL is apoB100. They also contain apoC and apoE. VLDL loses triacylglycerols through the action of the LPL while it transforms into IDL (a remnant particle), which in turn is either taken up by the liver or, by losing more triacylglycerols, transforms further into LDL. These consecutive transformations result in a loss of all the apoproteins except apoB100. The size of particles also successively decreases, and this changes the conformation of apoproteins on their surface. In the relatively large VLDL particles apoB100 and apoE remain 'extended'. Such conformation does not allow binding to the apoB/E receptor. As VLDL shrinks during the transformation into a remnant particle, apoE assumes a conformation that allows its binding to the receptor and liver uptake. Some remnants, instead of being taken up, transform into LDL; in LDL it is apoB100 which assumes receptor-friendly conformation. LDL are the main carrier of cholesterol in plasma.
|
| HDL transport cholesterol centripetally (from the periphery to the liver). This pathway is known as reverse cholesterol transport. The main apoproteins in HDL are apoAI and apoAII. They are synthesized in the intestine and liver. HDL also contains apoC and apoE. HDL participates in the metabolism of other particles (chylomicrons, VLDL, and remnants) through component exchange, exchanging apoproteins, phospholipid, triacylglycerol and cholesteryl ester.
|
|
