Lipids (triglyceride (TG), cholesterol (Chol), phospholipids (PL), cholesteryl esters (CE)) are insoluble in plasma. These lipids combine with special proteins called apoproteins - apolipoproteins - to form lipoproteins and this process solubilises lipids in plasma. Therefore, the lipoproteins provide a transport system for lipids. Apoproteins are important not only in maintaining the structural integrity of lipoproteins and thereby facilitating the solubilisation of lipids, but they also play an important role in lipoprotein receptor recognition and regulation of certain enzymes in lipoprotein metabolism.
Function of some of the individual lipoproteins is summarised below:
- Apo A1: binds PL on lipoproteins; activates cholesterin acyl transferase (LCAT: esterified chol in plasma)
- reference range:
- male: 94-178 mg/dL
- female: 101-199 mg/dL
- Apo B - this is the apolipoprotein that is involved in chylomicron, very low density lipoprotein (VLDL) and LDL metabolism.
- apoB100: produced in liver; component of VLDL, intermediate density lipoprotein (IDL) and low density lipoproteins (LDL); contributes to hepatic and peripheral tissue uptake of LDL by receptor recognition
- apoB48:produced in intestinal mucosal cells; an important component of chylomicrons (CM); contributes to hepatic uptake of chylomicron remnant by receptor recognition. Gut apolipoprotein B has a molecular weight that is 48% of hepatic apolipoprotein. The shorter gut apolipoprotein B lacks the part of the apoB100 that is recognised by the LDL receptor - this is probably why chylomicron remnants are largely cleared by the liver, whereas LDL is free to provide cholesterol to the peripheral tissues
- reference range:
- male: 55-140 mg/dL
- female: 55-125 mg/dL
- C-CII: component of high density lipoprotein (HDL), after transfer to CM and VLDL it activates the enzyme lipoprotein lipase (situated on the endothelium) which removes TG from CM and VLDL
- D: component of HDL
- E: involved in receptor recognition of IDL and CM remnant by the liver