The protein components of lipoproteins are known as apolipoproteins or, simply, apoproteins. They perform the following functions:
The comparative characteristic features of different lipoproteins with regard to electrophoretic patterns, size composition, etc., are given in Table 4.4. Table 4.4 Characteristics of Human Plasma Lipoproteins
HDL is synthesised and secreted from both liver and intestinal mucosa. It contains the maximum amount of protein cholesterol, and phospholipids are the main lipids present in HDL. HDL from intestine contains apo-A and that synthesised in liver contains apo-C.
They are formed from VLDL in the blood circulation. They are particularly rich in cholesterol. Approximately, 70% of the plasma cholesterol is present in the LDL fraction. The main apoprotein of LDL is apo-B100. The LDL level in plasma is clinically important as high level is indicative of susceptibility to ischemic heart disease (IHD).
VLDL are rich in Triglycerides (TG) and are derived mainly from liver and secreted by the intestines in a manner similar to that of chylomicrons. The particle size is smaller than chylomicrons (CM), and the apoproteins of VLDL include apo-B48 and apo-C.
Chylomicrons are rich in triglycerides and are derived from intestinal absorption of lipids. They are secreted from the intestinal mucosal cells into the intracellular fluid. Chylomicrons are responsible for transport of all dietary lipids into circulation. The principal apoprotein component of chylomicron in apo-B-48, which is incorporated into the chylomicron in the intestine itself. This…