As discussed in section 22.214.171.124, newly absorbed fatty acids are re-esterified to form triacylglycerol in the intestinal mucosal cells, then assembled into chylomicrons, which enter the lacteal of the villus (see Figure 4.2), then the lymphatic system; they enter the bloodstream (the subclavian vein) at the thoracic duct. Chylomicrons begin to appear in the bloodstream about 60 minutes after a fatty meal, and have normally been cleared within 6-8 hours. In the bloodstream they acquire three additional proteins from HDL:
- Apoprotein C-II, which activates lipoprotein lipase at the cell surface, permitting uptake of fatty acids from chylomicron triacylglycerol. The triacylglycerols are hydrolysed extracellularly, and the free fatty acids are then taken into the cell and re-esterified to triacylglycerol. In the fed state the major site of lipoprotein lipase activity is adipose tissue, but other tissues can also hydrolyse chylomicron triacylglycerol as required.
- Apoprotein C-III, which activates lecithin-cholesterol acyltransferase, permitting cells to take up cholesterol from chylomicron cholesteryl esters.
- Apoprotein E, which binds to hepatic receptors for uptake of lipid-depleted chylomicron remnants. Chylomicron remnants are taken into the liver by receptor-mediated endocytosis, followed by hydrolysis of the proteins and the residual lipids.
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