The lipids absorbed from the intestine is incorporated into chylomicrons. They reach the blood stream through the thoracic duct after passing through the lymphatic systm. Within the peripheral circulation, the triglycerides in the chylomicrons are hydrolysed by lipoprotein lipase present in the endothelium of muscle and adipose tissue. This release free fatty acids and glycerol. The free fatty acids are taken up by muscles for energy production and by adipose tissue for storage. The glycerol is taken up by liver for gluconeogeneis and synthesis of triglycerides.
The chlyomicrons after loss of triglycerides (TG’s) are converted into chylomicron remnants composed of mainly cholesteryl esters (CHE). These are taken up by liver where cholesteryl esters are hydrolysed to produce cholesterol, which is either stored in liver, incorporated into lipoproteins and secreted in blood as VLDL (very low density lipoprotein) or secreted in bile.
The VLDL secreted by liver is composed of mainly TG’s and CHE’s. The TG’s are hydrolysed by lipoprotein lipase present in the endothelium similar to that occuring in the case of chylomicrons. The VLDL is now converted into IDL (intermediate density lipoprotein) which contains more of CHE’s. IDL may be taken up by liver or get converted to LDL after losing most of its TG’s. LDL circulated in the blood and is taken up by tissues expressing the LDL receptor when there is a requirement for Cholesterol. Cholesterol is necessary for the formation of cell membranes.
The cholestrol formed by the destruction of cell membranes is released into blood, where it is taken up by HDL (high density lipoprotein) and converted to CHE by action of Lecithin Cholestorl Acyl Transferase (LCAT). HDL is taken up by liver.
The excess lipoproteins present in the blood are phagocytosed by macrophages. When great quantities of lipoproteins are phagocytosed, they may get deposited in the intima of arteries forming atheromas and in skin forming xanthomas.
LDL, IDL and VLDL are considered atherogenic; whereas HDL is considered non atherogenic.