The catabolism of very low density lipoproteins

The lipolysis of very low density lipoproteins (VLDL) in vitro is a useful model for the study of the process of conversion of this triacylglycerol rich lipoprotein into low (LDL) and high (HDL) density lipoproteins. Data is reviewed to show that a portion of surface cholesterol and phospholipid which becomes redundant during lipolysis is lost from the lipoprotein. In the absence of HDL, the material forms lipoprotein-X (LpX) like vesicles which are not readily disrupted by HDL once they are formed. In the presence of HDL during lipolysis, the material is largely incorporated into HDL. The data is used to suggest a mechanism of formation of LpX-like vesicles in conditions where the production of surface remnants exceeds the capacity of HDL to disrupt them. Evidence is also provided to show that apolipoproteins (apo) C-II, C-III, and E are lost from VLDL and that this loss is primarily in association with a neutral core particle of HDL size which can subsequently exchange lipids and apolipoproteins with plasma HDL. Such a mechanism could account for the removal of apo E and excess cholesteryl ester which is necessary for conversion of VLDL to LDL. The role of hepatic lipase in this process remains speculative. Recent evidence is reviewed and used to propose that the enzyme may serve to rearrange the neutral core and surface composition of LDL and HDL subfractions to allow for the packaging of cholesteryl esters and the cycling of apolipoproteins.