Fate of apolipoproteins C-II, C-III, and E during lipolysis of human very low density lipoproteins in vitro

The apoprotein and lipid composition of HDL-like products arising from lipolysis of human VLDL was studied. The VLDL was perfused through beating rat hearts in the absence of serum to avoid possible alteration of the primary products of lipolysis due to apoprotein exchange with serum lipoproteins. The lipolytic products were separated by gel filtration to obviate possible losses of apoproteins from the lipoproteins during ultracentrifugation. Apoprotein B, E, C-II, and C-III were quantitated by electroimmunoassay and lipids were determined by chemical methods. During perfusion, a 50% hydrolysis of the VLDL triacylglycerols was associated with the appearance of 11% of the apoC-II, 30% of the apoC-III, and 20% of apoE of the VLDL in HDL-like particles as isolated by agarose gel filtration. The shift of the apoproteins to these particles was associated with a similar redistribution of cholesterol, phospholipid, and cholesteryl ester. The lipid composition of the HDL-like particles was cholesterol (15.1±4.0%), phospholipid (37.8±3.2%), cholesteryl ester (34.2±2.6%). and trigylceride (12.9±3.2%). The particles possessed a hydrated density of 1.063-1.21 g/ml and were spherical, with particle diameters (mean 124±36Å, range 50-160Å) that were comparable to the diameter (140{) estimated from calculations of the surface to volume ratio, assuming a spherical particle consisting of a neutral lipid core surrounded by cholesterol, phospholipid, and protein. No discoidal forms or rouleau structures were observed in the HDL-sized fraction isolated by gel filtration. The HDL-like fraction could be resolved further by heparin-Sepharose chromatography into an unretained fraction containing predominantly apoC-III with apoE and apoC-II, and a retained fraction containing apoC-III and apoE. Small amounts of apoE were also recovered in extremely small particles that are normally observed in the D>1.21 g/ml fraction after ultracentrifugation. No apoC-II or C-III was observed in this fraction. Incubation of VLDL with lipoprotein lipase, mobilized from hearts by heparin perfusion, yielded results that were similar to those with the perfused heart. Hearts perfused with VLDL removed apoB but not apoC-II, C-III, E, or phospholipids from the perfusate. It is concluded that the initial products of VLDL catabolism include spherical particles, similar in size to HDL, that contain apoC-II, C-III, E, cholesterol, cholesteryl esters, and phospholipids. ApoE is also released as a small extensively delipidated particle.