Lipoproteins arise from enterocytes in the intestine, forming chylomicrons, and from hepatocytes in the liver, forming, VLDL, HDL, and IDL.
The assembly of chylomicrons within the enterocyte and the subsequent secretion of these lipoprotein particles into the lymph is a complex, multi-step process that includes :
- The absorption of lipids by the enterocytes [Dietary fats must be broken down into into triacylglycerol via bile salts produced in the liver; triacylglycerides are emulsified into two fatty acids and a monoacylglycerol via pancreatic lipases];
- Cellular lipid (re)synthesis [As the two fatty acids move into the enterocytes, they chemically react with coenzyme A (CoA) to form two fatty acyl coenzyme A molecules (fatty acyl-CoA); these two molecules combines with the monoacylglycerol that was also broken down and absorbed earlier to form a triacylglycerol molecule once again; the combination of various triacylglycerides, cholesterol, and other absorbed lipids come together with apolipoproteins to form a nascent chylomicron];
- Synthesis and post-translational modification of various apolipoproteins within the enterocytes;
- The assembly of lipid and lipoprotein components into a chylomicron.
The key process in chylomicron synthesis is the intracellular association of apolipoprotein ApoB48, the structural protein of chylomicrons, with lipids .